To clarify: In order for two observers to measure the passage of time and confidently compare those measurements, they must share _two_ events (the starting of the clocks _and_ the stopping of the clocks). Without having both events in common, the comparison between the clocks is meaningless. On the space station, I'm traveling along the spacetime path that _maximizes_ the time between those events. All other paths, including Rocket Clone's, _must_ be shorter.
@@chonchjohnch A "clock" is any device that measures time between two events. It doesn't really matter what it is, but you can imagine a fancy stopwatch (if that helps).
Indeed, anything that evolves over time counts as a "clock". It could be a burning candle, a chemical reaction, or a person counting "One one thousand, two one thousand, three one thousand...".
*Sid Knight* said: _is it fair to say that time passage is relative to the speed of light?_ I'd say so. Everything at our macro level that presents the passage of time either involves the interaction of fields at light speed or the movement of sub-light speed particles which are subject to SR effects because they move through a space that is subject to light speed. And at the micro level this still seems to hold true, even for a point particle like the muon whose lifetime becomes extended when it is moving through space. So, yes, I imagine that if you could find a piece of space where the speed of light was different then the flow of time there should be different by the same amount. There is an old saying that _time is what stops everything from happening at once._ I guess we could modify that to say that it's the speed of light in a vacuum, combined with a space through which things must travel, that stops everything from happening at once! :-)
The universe described by the equations below has no problem... c = speed of light Distance of earth to star (as measured from earth or star) = 8 light years te = gamma [ tr - xr(v/c^2) ] tr = gamma [ te - xe(v/c^2) ] xr = gamma [ -v*te + xe ] xe = gamma [ -v*tr + xr ] te = t_earth (passage of earth time for the round trip as measured on earth or earth time as predicted from rocket) tr = t_rocket (passage of rocket time for the round trip as measured on rocket or rocket time as predicted from earth) xe = x_earth (distance of round trip as measured from earth or earth distance predicted from rocket) xr = x_rocket (distance of round trip as measured from rocket or rocket distance predicted from earth) beta = v/c = 0.55 gamma = 1/sqrt(1-beta^2) = 1.19737 Step 1: We are on earth and we measure/calculate te. Step 2: We are on earth and we predict tr. Step 3: We are on the rocket and we predict te. Step 4: We compare te from step 1 and te from step 3. Step 1: Time we measure on earth (with earth clock) xe = 2*8 light years = 16c v = 0.55c te = xe/v = 16/.55 = 29.09 te = 29.1 years Step 2: Rocket time we measure on rocket (with rocket clock) or rocket time we predict on earth (with relativity) tr = gamma [ te - xe(v/c^2) ] tr = 1.19737 [ 29.09 - 16c(0.55c/c^2) ] = 24.29 tr = 24.3 years Step 3: Earth time we predict on rocket (with relativity) Substitute xr = gamma [ -v*te + xe ] into te = gamma [ tr - xr(v/c^2) ] te [1 - (gamma*beta)^2] = gamma*tr - (xe/c)*(gamma/c)^2 te = [gamma*tr - (xe*v)*(gamma/c)^2 ] / [1 - (gamma*beta)^2] te = [gamma*tr - (16c*0.55c)*(gamma/c)^2 ] / [1 - (gamma*beta)^2] te = [29.091 - 12.217] / [1 - 0.434] = 29.1 te = 29.1 years Step 4: te from step 1 (earth time we measure on earth) = 29.1 years te from step 3 (earth time we predict from rocket) = 29.1 years
The trouble with UA-cam is that the articles and comments aren’t vetted by a competent authority so a lot of them are cockeyed nonsense put up by idiots. Wikipedia is a better source of information.
I have watched Matt O'dowd explain this exact same thing, and I could never really understand it. After watching this video, I understand it completely. Thank you! You have an incredible ability to simplify complex ideas. Honesty the best physics channel on youtube.
I'm sticking with my 4D look at reality. By looking at the 4D view of reality, one sees that all objects move with the c magnitude of motion, and they all do so within that 4D environment known as Space-Time. The only change still possible is changing the direction of travel within Space-Time. 4D rotation also takes place while doing so. If you then create a simple geometric representation of this, you can use it to derive all of the Special Relativity mathematical equations, along with the Lorentz Transformation equations, and you can do so even if you have never seen these equations beforehand. This also eliminates any trouble resolving the Twin Paradox.
NEWKNOWLEDGE : Sir this one debunked the acceleration point of view that bothered me.I haven't solved lorentz equations as I am a high school student but I did create a better scenario for twin paradox and solved it using them and got a better understanding.This video made me do that. 4D spacetime is impossible to imagine for me.I stick with 3d version with colour differences but the concrete way is u pick up a pencil and do the necessary math...You will learn much more than from a video.
I made my own Special Relativity UA-cam videos. They are a step by step analysis of motion which then led me to the cause of the special relativity(SR) phenomena. The next step was to convert the understanding into a geometric representation. This then made it an easy breezy job to derive the SR mathematical equations, and resolve the twin paradox etc. So my method was to first figure out what is what, and thus understand what special relativity was all about, and then use that understanding to create the equations. Plus you only need to view one dimension of space of the 4D space-time to get the job done. But the main point of my videos is to show that by analyzing motion, just about anybody can discover special relativity on their own, and derive all of the SR mathematical equations on their own as well.
This started as a confusing entangled mess and you weave it all out beautifully. It's amazing how your brain manages to deconstruct every part of the paradox, anticipate the viewers' objections and give them a satisfactory answer. I am a big fan of Fermi lab Dr Lincoln's use of equations to explain relativity. But you just beat everyone when it comes to manipulating the Space time diagrams!
I'll watch this again, because it's not yet 100% clear, but I think I've been finally put on the correct track. Thank you Nick for making relativity more "human" and less of "a mathematical abstraction we can experimentally check", so to speak.
True. But it was just a passing comment... and he then went on to explain how it really did matter. Which differs from the FermiLab video which fundamentally claims acceleration doesn't matter and is just wrong!
@@corwin-7365 My take is that he was saying acceleration matters, but it isn't responsible for the paradox, it is just one relative term that you can ignore and still show that it isn't a paradox.
You can imagine all paths are straight lines. When go away from the space station, rocket clone instantly reaches 55% c, and you can assume when he reaches wolf 359, he turn around but keep velocity constant until he get back to the station
This is the best explanation of the twins paradox. Other explanations alludes to the same solutions and misconceptions but left me more confused. The last part about needing 2 shared events to make actual measurement rather than simply making a prediction was very insightful.
'Paradoxes are not a problem with the Universe, they're a problem of the human mind'. Thanks Nick. You've given me more to think about than just paradoxes.
Early enough to make a compliment. Been here since 5000 sub. Sooooo glad that you got so many subscribers. You even answered a question of mine in an older video. Thank you and keep up !!!
Christian Gingras Nice attempt to explain the paradox. Here is a simpler explanation: When the Sarah is flying away from Earth at 95% the speed of light, the radio communication is impaired because of a Doppler shift. Both sister need to tune their 1 GHz frequency to 50 MHz, which electrical engineers call carrier frequency. The AM or FM modulated signal, a slower wave which modify slightly the carrier frequency amplitude or frequency is equally slower. All embedded binary code are also streaming 20 times slower. The decoded voice of Sarah is deeper, like playing a vinyl disc or magnetic cassette in slow motion and the video are almost still pictures. A year later, Sarah turn around and come back toward Earth at 95% the speed of light, the following happen: That day, one year after their separation, Alice on Earth keep receiving the slow stream of radio at 50 MHz. She just received the video from Sarah telling how boring it is now, after 2 weeks of travel. She think her sister is crazy, talking about 2 weeks when in reality she his gone for a year and she is supposed to turn around now. Sarah also receive old news from her sister on Earth. She see the video telling her that she already miss her after two weeks of separation. However, the radio is fucked up because Sarah at the switch frequency from 50 MHz (for the 1 GHz carrier frequency) to 20 GHz. Also, the voice sound like chip monks and the videos are 20 times too fast. As the time pass between 12 month and 24 month of the total duration, the video that Sarah receive from Alice catch-up with the real time. Everything synchronize just right so that the last day of this 2 years voyage, Sarah can see a video of her sister recorded only 20 days earlier. For Alice, the second year events are a little more complicated to explain. She keep receiving outdated video on the radio station tuned at 50 MHz. Suddenly, 2 weeks before the end of the 2 years trip, the frequency jump from 50 MHz to 20 GHz. The video from Sarah describe that she now prepare to turn around. Alice think that she is crazy to announce that after 23.5 months. Then, during these last 2 weeks, Alice receive all the videos from Sarah describing that entire year where she is flying back to Earth. Finally, both sisters re-unite and they both aged by exactly 2 years. End of paradox.
Wrong. What they'll see in received signals is described well, but you completely ignored time dilation that will make Sarah actually younger. You completely missed the relativity part.
@@thedeemon The time dilation is carefully documented by the carier frequency of the radio communication. It occurs at different absolute time for both sisters. Note that for 2 weeks, there is a length extension while the rest of the 23 month and 2 weeks, there is length contraction.
If in your scenario sisters re-unite aged exactly the same, it's a direct contradiction to what special relativity predicts and what many experiments show.
@@thedeemon That is the point, Einstein pulled science from the dark age by proving the atom theory, calculating the size of water molecule from Brownian motion. He also made mistakes. He revised the equation for relativity years after years until nobody found further errors. Einstein never promoted the incoherent aging of the twin paradox. He probably knew it was wrong. His other work eclipse so much these detail, who knew people would still resurrect the worst of the work of a genius.
You are the first person I know who could ever clarify so brilliantly this age old "Paradox"! THANKS! Yes -- it is caused by doing a simplistic math of just the time -- ignoring the core reason -- longer space-time path! Remember, I was so frustrated after a third failed attempt of otherwise sensible professional that I almost pleaded you to jump into this subject and explain in your 100% articulate Style! I always like the explanation when you plug in numbers in those equations, and actually draw the consequences! It removes any confusion!
Very intelligently explained! I really appreciate how you steadily kept on building the common misconception path (others routinely follow), and then pin pointing exactly what was wrong with that line of reasoning -- it made me really happy! I appreciate your efforts and want you to come up with videos with similar level of numerical/graphical clarity on other relativistic/quantum topics!
With his explanation and knowing its a "mistake" with the transformation and not a paradox at all now makes the real (relative) scenario more comprehensible than the paradox. Lol :D
Does it matter whether you or Rocket Clone is the one actually moving/accelerating with respect to space/time? Rocket Clone could perceive you accelerating away and back toward you while he is stationary-but does it matter, in terms of relative clock-speed, which one is actually stationary?
Well, I have an understanding problem: This transformation thing seems to work only, when one part is already considered as the "moving" one and the other one as "stationary", doesn't it? If Rule One is valid, then for Space Clone, it must seem, as if Earth Guy speeds up away and then flew back to Space Clone. This means, that in Space Clone's perception, he himself would be the stationary one, whose path is projected on the y-axis. According to this perception, Earth Guy would be the one, who is travelling through space. Consequently, now Earth Guy's path should have to be projected as the bulking out one space-x-axis-wise "to the right" in the diagram and then back to the stationary Space Clone. From the perspective of Space Clone, the grid-squeezing-thingy should then also have to be applied, with the consequence, that from his view, Earth Guy should be the space travelling one, whose time was slower. Earth Guy would have to be younger. In conclusion, the grid squeezing illustrates only, HOW the travelling part should be the younger one. But it seems for me as if it does not explain, which guy should be considered as the travelling part, when speed and acceleration of one of them is only measured relative to the other one.
Younger should be, whoever is considered as "moving", the paradox doesn't seem to be solved by this video: In the space-time-diagram, one of the twins is displayed stationary without moving through space, his graph in stays on the y-axis. The other one is displayed as moving through space. Because of the diagram shifting, the moving one has to be younger than stationary one. But since the motion is only relative, for both of them it seems like the other one would be the moving one. Consequently, for both of them the diagram would have to be drawn with the other one as the moving one. The result of the diagram shifting would then be that for both of them, the moving other one should be younger. Paradoxically. How can we decide, whose diagram is objectively right? Because ScienceGuy made that decision by drawing the Earth Person as stationary and Space Clone as moving. At least it seems like that for me, who has no deeper understanding of the matter.
Yes, exactly this: the traveling part I've never had a problem with, but the whole paradox is that either of them could be considered moving, so A and B are both younger than each other. The paradox is 'which do we consider moving' and why.
@@quantisedspace7047 Absolutely. Well summarized. And now, three years later, I still didn't get a satisfactory answer. Although this video and many others try to give the impression to have an answer.
Omg! You are a life saver! I have been searching around the internet to find an answer to this question that satisfies me and this is by far the most satisfying! Time to sit down, roll up the sleeves, and do some Lorentz transformations myself!
Maaaaaaannnnnnn......... AT LAST!!! Finnally the twin paradox makes sense. Why can't everyone else explain the things like you do?! Thanks a lot (really)!!!
Sir, I am satisfied with this explanation. Seriously, your work truly is amazing. You take so much time and you put so much work into every of your videos... it's so amazing
I like this video not because I now understand everything but because a lot of thing I thought I knew about it was wrong (oversimplified). In textbooks I jst assumed the twin that stayed on earth was in a prefered reference frame.
Much better then that first one! This video would have to be THE best one ever made, explaining the notion of "relative time" .While doing special relativity calculations, it is SO easy to get lost in the math to forget "whose time delta are we calculating and in whose reference frame?". This just nails that, Great. Except what about the "Twin's Paradox" which this video was supposed to explain? In Twin's paradox, the "rocket twin" IS supposed to come back to meet his stationary twin/clone again. So when they meet again, they both are at rest at the same place and time!! No ambiguity about frames of reference there. WHAT do they perceive at end, is the whole question!! Yes, Nick does come up with the nice thought experiment of "what if the travelling twin was already in motion at the point when the FIRST depart?" Great beginning!. Just complete that though experiment, and bring back the travelling twin back (that would have to involve acceleration, IMO, but at this point, i don't freaking care!!) Just SOMEHOW/anyhow bring him back, at rest with his stationary twin, and explain the timeline PLEASE!!
Thanks a lot!!! I've been asking this question for twenty years and this is the first time I am satisfied with the answer (even though I'll have to watch this video several times, pausing, drawing spacetime diagrams and doing some calculations in order to fully understand it, since I'm not very good at Physics). I'd just like to know what would happen if the universe was finite but borderless? (i.e. it is possible for someone to go back to the starting point always walking in the same direction). In this case, would a non-accelerating rocket twin still see an older resting twin when he came back to the starting point? (the rocket twin is always traveling in the same direction, without any acceleration)
Hmmm... Well, first, even if the universe was finite and borderless, the clone/twin would never be able to get back here that way because the universe is expanding too quickly. Buuuuuuuuut, assuming it wasn't expanding the fast? I'd need to think about it a while.
Wrap-around universes don't work nicely with Special Relativity (which doesn't mean they couldn't exist... just that physics would be more yukky). For example, a wrap-around universe (like on the old Asteroids game) has a defined width. But in SR observers at different velocities would see a different width! Basically, so long as you don't send messages all the way around the universe to behind you, the universe would look like Special Relativity. If you could send a message all the way around, you'd be able to detect how fast you were moving relative to an absolute space!
Excellent question, which I have been trying to figure out for a while. If we assume that this is a wrap around universe, and that the above rules for relativity still all hold true, then geometrically this would mean that as rocket twin's path goes off diagonally to the right, he wraps around the time axis cylindrically, as an extra dimension, and comes back and meets up with earth twin at some future point. Since the stationary paths always take maximum time, the earth twin must be older. Problem is, due to the rule about all observers are equal and can consider themselves stationary, this reasoning also applies to rocket twin, which appears paradoxical. Another problem is that the light signal of the initial event (rocket twin's departure) comes back and meets the stationary observer before the actual meeting, producing a hall of mirrors type situation. This is assuming that general relativity doesn't come into play, which it probably will, since we have warped the spacetime into a cylinder.
After all this time dwelling on this, I think I want to see the version of this accelerationless scenario where the two send wireless photos of themselves and their clocks to each other the whole trip. I know it'll get a lot more complicated, but I'm only on the very cusp of understanding this and why one's coordinate system ends up less tall than the other. I can't quite cross that finish line...
Hello. I explained the answer to the paradox like this. That a person who has traveled must show less time in his watch. Therefore, this difference is enough for the explanation. As you said, acceleration solves some of the problems, but not all of them. Is my thinking correct in your opinion? Please answer.
Ive searched for videos all day on this problem and not one of them was able to answer the question in a way that couldnt function. This video however was the most helpful and i think i finally understand
That was an excellent explanation, much better this time. I'm glad you used space-time diagrams and you also did a great job showing the wiggle-room involved when the events we're trying to talk about are at different locations. -Matt
1:02 creo que es importante indicar que hay que imaginar un reloj en cada punto espacial. Para cada sistema inercial, los relojes se pueden sincronizar y podria hablarse de un unico tiempo "global" para ese sistema. Pero al analizar un sistema inercial en movimiento respecto al nuestro, hay que imaginar que las reglas están contraidas en el eje x, y que los relojes de cada plano yz (mismo valor x) marcan diferente, pero...todos van al mismo ritmo (mismos incrementos temporales)
3:50 No, accelerated points of view do not require general relativity. They do require some of the math typically associated with general relativity (specifically, general coordinate transformations rather than for some arbitrary reason limiting yourself to Lorentz transformations), but physics in Minkowski space is still special relativity, regardless of whether you put curved coordinate grids on it.
@@watertommyz I usually do that. It's been a while, so I did it again just to be safe. And no. He doesn't correct that mistake at all. He just ignores acceleration altogether. So I stand by my criticism: You can deal with accelerated frames of reference in special relativity. He says you can't. That's wrong.
Saying that “The Twins Paradox exists without acceleration” is a bit strange, because in the situation you described, there’s no paradox. Both twins see each other as younger than themselves, but they never meet again, because their worldliness can only intersect once. That’s strange, but not a paradox anymore. Acceleration is required for the twins paradox because only curved worldlines can intersect at multiple locations in flat space. However, it’s not the acceleration of the rocket that causes the paradox. It’s the acceleration of the reference frame of the rocket, because there is no Lorenz transformation that can make Rocket Clone’s worldline straight.
But acceleration isn't the _only_ way to get the clones back together. The traveling clone could also travel in a straight line around a curved universe arriving back where it started, never having accelerated. The paradox is caused by them coming back together, _not_ by the acceleration.
Fermilab also coincidentally posted two videos on the Twins Paradox recently that also explained why acceleration is not the cause of the paradox. In the Fermilab video he considered a thought experiment where there are three observers: one on Earth, one on a ship flying past the Earth toward a star at high speed and another on a ship flying past the star at the same high speed toward the Earth. None of the observers experience any acceleration, they all have constant speeds relative to each other. As the outbound ship passes the Earth they start a timer. When the outbound ship and the inbound ship pass each other at the halfway point the outbound ship holds up a big sign showing the time the outbound ship claims has passed. The inbound ship starts its own timer and as it passes the Earth it holds up a sign showing the time the outbound ship experienced plus the time the inbound ship experienced since that event. As expected when you do the correct transformations the total of the two times from the ships is less than the total time the Earth observer experienced.
This is the best way to resolve the paradox if one wants to avoid accelerations. However, the problem then becomes an "information transfer" issue, and not a matter of which twin actually ages differently. An asymmetry is introduced between the original two twins, as one transfers info from one particular set of clock/coordinates to another, distinct set of clocks/coordinates, and the other twin has no such transfer of information. Personally, I like the acceleration explanation much better than the non-acceleration one, because I understand better what an acceleration is compared to "transfer of information".
The video you refer to is completely correct in everything it says. But it does not describe the twin paradox. And not only because there are three (and not two) observers, it doesn't even describe a "triplet paradox". In this video the observers are never sharing a reference frame, and sharing a reference frame (that is being at rest with each other) both at the start of the trip and at the end of the trip is a vital part of defining the twin paradox. In this video the ships passing by the earth just observe time passing at a different rate than they do on the other ship and the earth. That's just time dilation, without any paradox.
@@grantgussie8768 You're missing the point, though, of the Fermilab video, which is that it creates a scenario similar to the classic Twin Paradox with similar time dilation effects but doesn't involve and acceleration in the reference frames. Thus acceleration isn't the key to understanding the asymmetry in Twin Paradox like some people try to claim.
Amazing how you touch upon so many things in just 10 minutes. I took close to 17 hours of video to build up to this finale and therefore breaking every rule of becoming a popular UA-camr :-) But boy did I have fun while doing it.
You did it and that's awesome - best twin paradox explanation out there! As a slow thinker, I watched that Hmm @ 6:57 about 10 times. Hmm, I will probably watch ten times more till this sinks in...and next, let's open that can of warms:-)
A question from the little me: When a particle is "shaking" because or the entropy, from its point of view is that entire univers is moving randomly? And what makes from the point of view of the particle the entire univers to move?
It becomes easier to show that there is NO PARADOX, if we oversimplify the narrative for a moment and say “whichever party has covered more distance (say, more kilometers) will have aged less” Now, we need two events to be able to compare the rates of aging: first, an event in spacetime where both astronauts are in the same reference frame and synchronise their clocks. Second event has to take place at a future time, at an arbitrary “same” spatial coordinate, bringing the two persons back into shared reference frame. Immediately after the relative motion begins, both onservers agree on one thing: the distance between them is increasing. For the second event to happen, at least one of them must change its trajectory and “catch” the other. This part is not relativistic. Both observers will later agree on who changed trajectory (and this is where “experiencing acceleration” plays any role). Whoever does that, and becomes the “chaser” will have traveled more in space by the time they meet again in the same reference frame. They may safely compare the number of kilometers each one has covered and agree that the one with more mileage has indeed aged less. No chance of disagreement.
I should say at this point that you r my inspiration for going into a physics major.I know I will struggle with money as u r hence this channel.But I can see the effort u put into these videos such as length contractions in the animations and I hate the system(and also kind of like it being selfish)that you ain't doing some next level physics and earning millions right now.
No matter how one looks at it, if there is a true age difference between the twins, then there must have been an asymmetry between the two twins. If there is no asymmetry between the twins, then they MUST age the same.
As always Nick a brilliant simplification, I was so engrossed that I was shouting at my phone, “No Nick you’ve done the maths wrong”, How stupid of me to think you’d make a mistake like that, hmmmh maybe I’m just a little crazy and not stupid cos at least I spotted the maths. Oooops the flying pig that’s sat next to me has pointed out that it was ‘He’ that spotted the mistake, . . . oh dear . . . Oh dear . . . maybe I’m past being a little crazy . . . Lol
Hey, long time subscriber, and my classes get a lot from you too! I am preparing to teach special relativity and had a question - in the twins paradox, does the rocket twin come back shorter? High school books emphasise a real, notable difference in time (since the twin comes back younger), but only an apparent difference in length which is only observable under the conditions talked about in special relativity. So when the frames of reference once again come together for the two individuals, there is a permanent difference in time, but length remains unchanged - is this correct?
Great question! No, the astronaut doesn't come back shorter... but they _do_ come back having traveled a different _distance_ than the Earth observer thinks they did. The "permanent differences" are the ones that accumulated over the entire trip. The length of the astronaut is only a local instantaneous observation.
I understood better than the last explanation you did of this, but I'm still a little confused about why you need two shared events and what exactly a coordinate transformation IS. can you clarify?
Say we measured the time with some fancy stop watches. We need two shared events because we have to agree on when to start our stop watches _and_ when to stop them. We can't do that unless we're at the same place and the same time for each one of those events. A coordinate transformation lets you take measurements in one coordinate system and see what those same measurements look like in a different coordinate system. Simple example: You're sitting across a table from your friend facing them. You measure where the kitchen is relative to you. Now you want to know where the kitchen is relative to your friend. What do you do? You have to shift your measurement by the width of the table and rotate everything by 180 degrees (so that now you're looking through their eyes instead of yours).
The Science Asylum Ok, now I get what you mean by coordinate transformation. I was thinking it was something like converting between cartesian and polar coordinates in trigonometry. Keeping the same type of units but looking from a different perspective makes a lot more sense. Thanks!
The Science Asylum I think I get what you mean about agreeing when to start measuring and when to stop, but I still feel a little confused. Could you give another example? And thanks for the response. I appreciate how well thought your videos and how you take viewer comments into account. :)
So, you and your friend decide to measure how much time passes for yourselves after lunch (at the table from my earlier example). While you're sitting at the table, you can easily start and stop your watches at the same time. Instead, let's say you start your watches together just before you leave the table. No problem so far... but then you both leave and drive to your own homes. How do you know when to stop your watches and make sure you stop them _at the same time_ if you're in different places? You could call each other on the phone, but that signal takes a little bit of time to travel between your phones (maybe half of a second). It'll be close, but it won't be perfect.
Thank you for explaining. To me the thing is more intuitive with the "cutting a baguette" example: Depending on the direction a moves relative to b he would cut the bread in another angel. You showed this in your diagramme, too (the space time is the bread, and the "nows" are the cut area of bread). But if you had added another clone c meeting b abd flying to a they could habe synchronised clocks without accelaration. And just by c cutting bread in another angel one sees that there is no paradox when c later compares his clock with a.
The simplest way to measure the speed of light (event propagation) is basically a light (event) emitter, a far-away (known-distance) reflector, and an eyeball positioned in the same place as the emitter. The light makes the round trip, and then you have the speed of light. That round trip seems very fundamental who the whole thing. You can't just put Janice at one point, and Maria at another, and have both scientists hash out how long it takes for light to travel from one point to another, because Einstein established that synchronizing watches between observers who are very far away from each other is a logistical and philosophical impossibility. The twin paradox also relies on a round trip. I feel like there's a "there" there that I'm not fully equipped to articulate.
As a final episode on Lorentz transformations, you should do a fun video about a space ship going around the earth at 99% speed of light. Space ship is equipped with a radio and a telescope so it can look down the earth and communicate with earthlings. Makes you wonder would they see things on Earth speed up as if someone speed up the video. Or when communicating with Earth, would they be able to talk, since one side would hear speech slowed down, while the other would hear it speed up, i imagine. Doh! This mind experiment opens a can of worms.
Yes, they'll see Earth sped up. If they use Skype of Facetime, it would be like the connection is really shitty in rocket-Earth direction (too much lags!) and superfast in Earth-rocket direction (packet overload!).
I was hopeful to see a better approach than the usual and while this adds some detail, I think it could have been clearer by taking the first spacetime diagram for the unaccelerated clone and rotating it so their worldline was vertical, then add a horizontal to show the elapsed time on Earth was 10 years, reduced by the same gamma factor. Then add another clone to create the "triplets" version. Rotate to his frame and add a horizontal. Return to the Earth frame and you get the extra 7 years in the middle which is due to "relativity of simultaneity", the feature that most beginners forget. Overall though, it's a step in the right direction.
No one can ever explain better than u Lucid!!! Great Job... But.. wait a minute.. wont that star u mentioned is also moving in galatic plane? If that too considered.. it would be awesome..
Thank you! The common explanation of change of frame of reference was bothering me since we should still have a difference in time due to their relative difference in motion even if acceleration was not involved. Thank you for the synthesis of the rules.
It's much more easy if we consider only one half of the trip. We can place a clock at the distant star wihich is synchronized to our clock on Earth. We observe, that the traveling clock shows less time. If the traveling twin moves back, we can simply repeat the same consideration. The moving clock will be delayed again by the same amount.
There's a simpler explanation. Ignoring acceleration, there are three paths (which are straight lines). That means there are three observers since in Special Relativity, each straight line has an unique observer. If you draw the light-cone diagrams for each observer, that is, you draw three diagrams, then you realize that there is no paradox. From the point of view of the Earth, there is the outbound trip, where time is running slower and the return trip, where time is running slower. Slow time plus slow time is less than Earth time. For the out bound trip, Earth time is running slower because the Earth is moving away from the rocket. But the return trip must move away from the outbound trip faster than the Earth in order for it to catch up with the Earth. That means its time is even slower. So, normal time for the outbound trip plus even slower time for the return trip is less than the Earth's slow time. And similarly for the return trip. The Earth is approaching, so its time is slower. But the outbound trip has to have travelled faster than the Earth go get away from it. So its time is even slower than the Earth's. Normal time for the return trip plus even slower time for the outbound trip is less than the Earth's slow time.
That doesn't really address the problem though because the paradox is there even of the rocket doesn't turn around. If that's the case, there aren't "three observers." There are only two observers and, in that case, the paradox is still there.
Exactly!! This video nails the first part, it is the SECOND shared event (when the twin meet again), which remains unexplained!!. Please see my comment/question as well.
Man I wish I could help you, get more subscribers and build your channel as fast as possible... having said that I am 100% confident you are going to make it big.... I'm talking millions of subscribers... you're totally unique and excellent at your brand of humour/education.. flawless... I really really hope people manage to find out about your channel, you deserve the success. I've only just subscribed but seen all of your videos at least twice over if not more... and you've shot to the top of my favorite science channel list straight away.. we're talking over the likes of Veritasium, PBS space time, Issac Arthur.... the big hitters... you're just better and I'm pretty sure it's because of the comedy element... but your content isn't bad either (I'm being modest) I've been watching UA-cam videos on GR and quantum mechanics for over 10 years.. and I've only just started to see GR in a more coherent light thanks to your videos.. keep it up my man and best of luck with the channel ;-) (Could easily see this show being on TV) oh one thing I wanted to ask... so your real name is Nick Lucid?
Thanks! I'm hoping my visit to VidCon U.S. this coming June is going to be my big break. I'm involved in some official stuff, so lots of "networking" opportunities and stuff.
Please answer: Less Time passed by rocket in relative to space station. So Rocket had gone in the past of space station or spacestation had gone in the future of Rocket. So, when the Rocket returned, it should have returned to the past of Spacestation and they should have not meet again unless the Rocket TimeTravell forward or the Space ship TimeTravell backward. How "Now" for both become same again though the passenger of the Rocket look younger than the person in Space station.
What about a completely symmetric version of the paradox? The only way out of the paradox is that motion itself doesn't affect the flow of time as it is relative. Even in your example if you change the reference frame and remove acceleration, then everything is symmetric.
Riddle me this all those who claim to understand this: Say the two participants accelerate away from eachother equally in a totally symmetric fashion. Then they turn around and accelerate back. When they meet each other again, who will be older?
It's not actually "everyone has their own clock" - but every tiny particle or bundle of energy (quanta) has It's own clock. That we are made up of such a colossal number of these tiny things, and can move about more or less as separate entities, makes it more comprehensible to us.
You said that explanation by acceleration is completly wrong but ,it seems correct, without throwing 1st law,just by including gravitational time dilation Can you explain more
As we've learned with a lot of upper-level physics, just because it _seems_ correct, it doesn't mean it _is_ correct. The twins paradox is nuanced. 1. What you get when you calculate gravitational time dilation depends on the size and duration of the acceleration, which adds unnecessary complication to the scenario. 2. Including gravitational time dilation doesn't fix the 4-year disagreement. 3. You can't really use gravitational time dilation to compare time intervals that are this far apart in space any more than you can Lorentz time dilation.
I am going to have to watch this again (again). I understand the premise at the start and the conclusion at the end but I think I zoned out in the middle. TBH, I think I might just have to take your word for it. You are quite right, for years I had been under the illusion that the twin paradox is solved because: acceleration. I will need to prime my consciousness with a couple more of your related videos if only to get my head into the correct reference frame. That and hit the pause button more often to let things sink in.
The difference is more fundamental than acceleration. Time dilation is all about measurements. To compare measurements of time, you need to have two events in common. There are many ways that spacetime paths can have two events in common, only one of which is acceleration. (To be fair, the way the paradox is traditionally stated and the way it's stated in this video, acceleration _is_ at play. My main point is just that it doesn't _have_ to be acceleration, so the paradox isn't actually _about_ the acceleration.)
Yeah, I’m in the same boat. The explanation travels closer to light speed than my brain, so I’m waiting for the two to cross paths again with each other and compare notes. 🙂
@@F16_viper_pilot The fundamental thing that breaks the symmetry and that's responsible for actually disagreeing clocks (clocks in same place disagreeing after formerly having agreed) is acceleration. There is simply nothing else it could be.
Omg...wow. This was brilliant. And it explains the apparent paradox. I have always heard/read that it was acceleration or apparent gravitational fields that caused the difference/paradox. Which always seems a bit...ad hoc.
@@ScienceAsylum Let's us use the equations of general relativity to simulate the video we would see as we jump between the two different frames of reference. That would be worth seeing.
A large problem is the simplified models used to explain things which cut ever so small corners. When stacking multiple models, those cut corners start adding up fast. For example, speed is stated to be relative. It's really not... If it was, both twins would experience the same thing. Acceleration is absolute and different for each twin since it can be measured within the local frame. And yet, acceleration is not involved in the time difference. Now, to come up with an intuitive model that doesn't cut corners...
To clarify: In order for two observers to measure the passage of time and confidently compare those measurements, they must share _two_ events (the starting of the clocks _and_ the stopping of the clocks). Without having both events in common, the comparison between the clocks is meaningless. On the space station, I'm traveling along the spacetime path that _maximizes_ the time between those events. All other paths, including Rocket Clone's, _must_ be shorter.
The Science Asylum what’s the general equivalent of the “clock” here?
@@chonchjohnch A "clock" is any device that measures time between two events. It doesn't really matter what it is, but you can imagine a fancy stopwatch (if that helps).
Indeed, anything that evolves over time counts as a "clock". It could be a burning candle, a chemical reaction, or a person counting "One one thousand, two one thousand, three one thousand...".
Soooooooo... is it fair to say that time passage is relative to the speed of light?
*Sid Knight* said: _is it fair to say that time passage is relative to the speed of light?_
I'd say so. Everything at our macro level that presents the passage of time either involves the interaction of fields at light speed or the movement of sub-light speed particles which are subject to SR effects because they move through a space that is subject to light speed. And at the micro level this still seems to hold true, even for a point particle like the muon whose lifetime becomes extended when it is moving through space.
So, yes, I imagine that if you could find a piece of space where the speed of light was different then the flow of time there should be different by the same amount.
There is an old saying that _time is what stops everything from happening at once._ I guess we could modify that to say that it's the speed of light in a vacuum, combined with a space through which things must travel, that stops everything from happening at once! :-)
Best quote ever:
“The universe doesnt have a problem, we do”
... but I am a part of the universe.
PB P Still. it's only your problem.
The universe described by the equations below has no problem...
c = speed of light
Distance of earth to star (as measured from earth or star) = 8 light years
te = gamma [ tr - xr(v/c^2) ]
tr = gamma [ te - xe(v/c^2) ]
xr = gamma [ -v*te + xe ]
xe = gamma [ -v*tr + xr ]
te = t_earth (passage of earth time for the round trip as measured on earth or earth time as predicted from rocket)
tr = t_rocket (passage of rocket time for the round trip as measured on rocket or rocket time as predicted from earth)
xe = x_earth (distance of round trip as measured from earth or earth distance predicted from rocket)
xr = x_rocket (distance of round trip as measured from rocket or rocket distance predicted from earth)
beta = v/c = 0.55
gamma = 1/sqrt(1-beta^2) = 1.19737
Step 1: We are on earth and we measure/calculate te.
Step 2: We are on earth and we predict tr.
Step 3: We are on the rocket and we predict te.
Step 4: We compare te from step 1 and te from step 3.
Step 1:
Time we measure on earth (with earth clock)
xe = 2*8 light years = 16c
v = 0.55c
te = xe/v = 16/.55 = 29.09
te = 29.1 years
Step 2:
Rocket time we measure on rocket (with rocket clock) or rocket time we predict on earth (with relativity)
tr = gamma [ te - xe(v/c^2) ]
tr = 1.19737 [ 29.09 - 16c(0.55c/c^2) ] = 24.29
tr = 24.3 years
Step 3:
Earth time we predict on rocket (with relativity)
Substitute xr = gamma [ -v*te + xe ] into te = gamma [ tr - xr(v/c^2) ]
te [1 - (gamma*beta)^2] = gamma*tr - (xe/c)*(gamma/c)^2
te = [gamma*tr - (xe*v)*(gamma/c)^2 ] / [1 - (gamma*beta)^2]
te = [gamma*tr - (16c*0.55c)*(gamma/c)^2 ] / [1 - (gamma*beta)^2]
te = [29.091 - 12.217] / [1 - 0.434] = 29.1
te = 29.1 years
Step 4:
te from step 1 (earth time we measure on earth) = 29.1 years
te from step 3 (earth time we predict from rocket) = 29.1 years
Your face is wrinkled up with the effort to make people "get it." Just short of frustration, lol.
Indeed! It's a problem of a (tiny) _part_ of the universe, then.
1 Dislike?! They probably tried to hit the "Like" button from someone else's coordinate system.
Or a few likes quantum tunneled to the dislikes
These clicks were travelling through mood field and interacted with it by absorbing grumpy bosons, flipping their like charge.
You would make Albert happy!!
Oh, hahaha!!😂
Those are just likes that flipped their spin.
This channel doesn't just make videos, you touch every aspect of the problem which i haven't seen in other channels
jai ho yes indeed.
That's it! I was wondering what it was about this goofball that I enjoy his videos as I do.
The trouble with UA-cam is that the articles and comments aren’t vetted by a competent authority so a lot of them are cockeyed nonsense put up by idiots. Wikipedia is a better source of information.
exactly!!
I have watched Matt O'dowd explain this exact same thing, and I could never really understand it. After watching this video, I understand it completely. Thank you! You have an incredible ability to simplify complex ideas. Honesty the best physics channel on youtube.
Thanks! Glad it helped 🤓
This channel is so underrated !!
Agree.
It does not only touch interesting problems but it also has its own valuable characteristic like the famous French Once upon a Time... series.
Overrated. The explanation is wrong.
u r övrratd
I rate it as my favorite channel tho… hmmm. :)
Minutephysics didn't do justice to this paradox and I was searching for a better explanation.This one was much better.
I'm sticking with my 4D look at reality. By looking at the 4D view of reality, one sees that all objects move with the c magnitude of motion, and they all do so within that 4D environment known as Space-Time. The only change still possible is changing the direction of travel within Space-Time. 4D rotation also takes place while doing so.
If you then create a simple geometric representation of this, you can use it to derive all of the Special Relativity mathematical equations, along with the Lorentz Transformation equations, and you can do so even if you have never seen these equations beforehand. This also eliminates any trouble resolving the Twin Paradox.
NEWKNOWLEDGE : Sir this one debunked the acceleration point of view that bothered me.I haven't solved lorentz equations as I am a high school student but I did create a better scenario for twin paradox and solved it using them and got a better understanding.This video made me do that.
4D spacetime is impossible to imagine for me.I stick with 3d version with colour differences but the concrete way is u pick up a pencil and do the necessary math...You will learn much more than from a video.
I made my own Special Relativity UA-cam videos. They are a step by step analysis of motion which then led me to the cause of the special relativity(SR) phenomena. The next step was to convert the understanding into a geometric representation. This then made it an easy breezy job to derive the SR mathematical equations, and resolve the twin paradox etc. So my method was to first figure out what is what, and thus understand what special relativity was all about, and then use that understanding to create the equations. Plus you only need to view one dimension of space of the 4D space-time to get the job done. But the main point of my videos is to show that by analyzing motion, just about anybody can discover special relativity on their own, and derive all of the SR mathematical equations on their own as well.
NEWKNOWLEDGE:That is obvious if you are a genius.
I shall watch ur videos some day.Thanks!!!
This started as a confusing entangled mess and you weave it all out beautifully.
It's amazing how your brain manages to deconstruct every part of the paradox, anticipate the viewers' objections and give them a satisfactory answer.
I am a big fan of Fermi lab Dr Lincoln's use of equations to explain relativity. But you just beat everyone when it comes to manipulating the Space time diagrams!
Truly wonderful. In decades of being bothered about explanations of this, this is the only accessible one that makes sense. Thank you!
You're welcome :-)
I'll watch this again, because it's not yet 100% clear, but I think I've been finally put on the correct track. Thank you Nick for making relativity more "human" and less of "a mathematical abstraction we can experimentally check", so to speak.
this is the first time I feel I've ever properly understood this!! And boy have I read a number of explanations! PS your physics book is awesome :-)
He does a book?
This video should be a standard in special relativity textbooks.I mean Wow.
how do you put a video in a textbook?
And also, he said the problem doesn't need acceleration which is incorrect.
True. But it was just a passing comment... and he then went on to explain how it really did matter. Which differs from the FermiLab video which fundamentally claims acceleration doesn't matter and is just wrong!
@@corwin-7365 My take is that he was saying acceleration matters, but it isn't responsible for the paradox, it is just one relative term that you can ignore and still show that it isn't a paradox.
@@sugarfree4073 - he might be saying that... but I hope not, since the paradox (and resolution) completely arise from the nature of the acceleration.
Everybody gets a clock 😂
The true way to reconcile the 'paradox'! :-)
You can imagine all paths are straight lines. When go away from the space station, rocket clone instantly reaches 55% c, and you can assume when he reaches wolf 359, he turn around but keep velocity constant until he get back to the station
This is the best explanation of the twins paradox. Other explanations alludes to the same solutions and misconceptions but left me more confused. The last part about needing 2 shared events to make actual measurement rather than simply making a prediction was very insightful.
Glad I could help! The two shared events is the main point of this video.
'Paradoxes are not a problem with the Universe, they're a problem of the human mind'. Thanks Nick. You've given me more to think about than just paradoxes.
Early enough to make a compliment. Been here since 5000 sub. Sooooo glad that you got so many subscribers. You even answered a question of mine in an older video. Thank you and keep up !!!
Christian Gingras
Nice attempt to explain the paradox. Here is a simpler explanation:
When the Sarah is flying away from Earth at 95% the speed of light, the radio communication is impaired because of a Doppler shift. Both sister need to tune their 1 GHz frequency to 50 MHz, which electrical engineers call carrier frequency.
The AM or FM modulated signal, a slower wave which modify slightly the carrier frequency amplitude or frequency is equally slower. All embedded binary code are also streaming 20 times slower.
The decoded voice of Sarah is deeper, like playing a vinyl disc or magnetic cassette in slow motion and the video are almost still pictures.
A year later, Sarah turn around and come back toward Earth at 95% the speed of light, the following happen:
That day, one year after their separation, Alice on Earth keep receiving the slow stream of radio at 50 MHz. She just received the video from Sarah telling how boring it is now, after 2 weeks of travel. She think her sister is crazy, talking about 2 weeks when in reality she his gone for a year and she is supposed to turn around now.
Sarah also receive old news from her sister on Earth. She see the video telling her that she already miss her after two weeks of separation. However, the radio is fucked up because Sarah at the switch frequency from 50 MHz (for the 1 GHz carrier frequency) to 20 GHz. Also, the voice sound like chip monks and the videos are 20 times too fast.
As the time pass between 12 month and 24 month of the total duration, the video that Sarah receive from Alice catch-up with the real time. Everything synchronize just right so that the last day of this 2 years voyage, Sarah can see a video of her sister recorded only 20 days earlier.
For Alice, the second year events are a little more complicated to explain. She keep receiving outdated video on the radio station tuned at 50 MHz. Suddenly, 2 weeks before the end of the 2 years trip, the frequency jump from 50 MHz to 20 GHz. The video from Sarah describe that she now prepare to turn around. Alice think that she is crazy to announce that after 23.5 months. Then, during these last 2 weeks, Alice receive all the videos from Sarah describing that entire year where she is flying back to Earth.
Finally, both sisters re-unite and they both aged by exactly 2 years. End of paradox.
Wrong. What they'll see in received signals is described well, but you completely ignored time dilation that will make Sarah actually younger. You completely missed the relativity part.
@@thedeemon The time dilation is carefully documented by the carier frequency of the radio communication.
It occurs at different absolute time for both sisters.
Note that for 2 weeks, there is a length extension while the rest of the 23 month and 2 weeks, there is length contraction.
If in your scenario sisters re-unite aged exactly the same, it's a direct contradiction to what special relativity predicts and what many experiments show.
@@thedeemon That is the point, Einstein pulled science from the dark age by proving the atom theory, calculating the size of water molecule from Brownian motion.
He also made mistakes. He revised the equation for relativity years after years until nobody found further errors.
Einstein never promoted the incoherent aging of the twin paradox. He probably knew it was wrong. His other work eclipse so much these detail, who knew people would still resurrect the worst of the work of a genius.
You are the first person I know who could ever clarify so brilliantly this age old "Paradox"! THANKS!
Yes -- it is caused by doing a simplistic math of just the time -- ignoring the core reason -- longer space-time path!
Remember, I was so frustrated after a third failed attempt of otherwise sensible professional that I almost pleaded you to jump into this subject and explain in your 100% articulate Style!
I always like the explanation when you plug in numbers in those equations, and actually draw the consequences! It removes any confusion!
Glad you liked it! This video took a lot of planning.
Very intelligently explained! I really appreciate how you steadily kept on building the common misconception path (others routinely follow), and then pin pointing exactly what was wrong with that line of reasoning -- it made me really happy! I appreciate your efforts and want you to come up with videos with similar level of numerical/graphical clarity on other relativistic/quantum topics!
With his explanation and knowing its a "mistake" with the transformation and not a paradox at all now makes the real (relative) scenario more comprehensible than the paradox. Lol :D
Does it matter whether you or Rocket Clone is the one actually moving/accelerating with respect to space/time? Rocket Clone could perceive you accelerating away and back toward you while he is stationary-but does it matter, in terms of relative clock-speed, which one is actually stationary?
It's not who is stationary but who is not accelerating.
@@joepierson3859 accelerating with respect to what?
@@cjjones999 only velocity is relative and measured relative to something on the other hand acceleration is absolute and needs no reference
@@cjjones999Nothing. I was always led to believe that acceleration was absolute, only speed was relative.
I am always very impressed by how well you do your interaction with your clones.
I think I need to watch this a few times for it to sink in... It is the "paradox" that does my head in too..
Thanks for your videos. They give a courage for young people to get into science.
Well, I have an understanding problem: This transformation thing seems to work only, when one part is already considered as the "moving" one and the other one as "stationary", doesn't it?
If Rule One is valid, then for Space Clone, it must seem, as if Earth Guy speeds up away and then flew back to Space Clone.
This means, that in Space Clone's perception, he himself would be the stationary one, whose path is projected on the y-axis. According to this perception, Earth Guy would be the one, who is travelling through space. Consequently, now Earth Guy's path should have to be projected as the bulking out one space-x-axis-wise "to the right" in the diagram and then back to the stationary Space Clone.
From the perspective of Space Clone, the grid-squeezing-thingy should then also have to be applied, with the consequence, that from his view, Earth Guy should be the space travelling one, whose time was slower. Earth Guy would have to be younger.
In conclusion, the grid squeezing illustrates only, HOW the travelling part should be the younger one.
But it seems for me as if it does not explain, which guy should be considered as the travelling part, when speed and acceleration of one of them is only measured relative to the other one.
So who is younger after rocketman returns? And why? I admit I'm lost here.
Younger should be, whoever is considered as "moving", the paradox doesn't seem to be solved by this video:
In the space-time-diagram, one of the twins is displayed stationary without moving through space, his graph in stays on the y-axis. The other one is displayed as moving through space.
Because of the diagram shifting, the moving one has to be younger than stationary one.
But since the motion is only relative, for both of them it seems like the other one would be the moving one.
Consequently, for both of them the diagram would have to be drawn with the other one as the moving one.
The result of the diagram shifting would then be that for both of them, the moving other one should be younger. Paradoxically.
How can we decide, whose diagram is objectively right? Because ScienceGuy made that decision by drawing the Earth Person as stationary and Space Clone as moving.
At least it seems like that for me, who has no deeper understanding of the matter.
Yes, exactly this: the traveling part I've never had a problem with, but the whole paradox is that either of them could be considered moving, so A and B are both younger than each other.
The paradox is 'which do we consider moving' and why.
@@quantisedspace7047 Absolutely. Well summarized. And now, three years later, I still didn't get a satisfactory answer. Although this video and many others try to give the impression to have an answer.
Omg! You are a life saver! I have been searching around the internet to find an answer to this question that satisfies me and this is by far the most satisfying! Time to sit down, roll up the sleeves, and do some Lorentz transformations myself!
Now confused on a much higher level. Thanks!
Maaaaaaannnnnnn......... AT LAST!!! Finnally the twin paradox makes sense. Why can't everyone else explain the things like you do?! Thanks a lot (really)!!!
This channel is so underrated !! i love it please share it on social media
You really gave the explanation which I was looking for all these years!!!...Glad I watched this 😌
Sir, I am satisfied with this explanation.
Seriously, your work truly is amazing. You take so much time and you put so much work into every of your videos... it's so amazing
Nick, you have a wonderful talent when it comes to explaining complex concepts, a natural teacher.
You had me at "paradox isn't a problem with the universe it's a problem with our perception"
I like this video not because I now understand everything but because a lot of thing I thought I knew about it was wrong (oversimplified). In textbooks I jst assumed the twin that stayed on earth was in a prefered reference frame.
Much better then that first one! This video would have to be THE best one ever made, explaining the notion of "relative time" .While doing special relativity calculations, it is SO easy to get lost in the math to forget "whose time delta are we calculating and in whose reference frame?". This just nails that, Great. Except what about the "Twin's Paradox" which this video was supposed to explain? In Twin's paradox, the "rocket twin" IS supposed to come back to meet his stationary twin/clone again. So when they meet again, they both are at rest at the same place and time!! No ambiguity about frames of reference there. WHAT do they perceive at end, is the whole question!! Yes, Nick does come up with the nice thought experiment of "what if the travelling twin was already in motion at the point when the FIRST depart?" Great beginning!. Just complete that though experiment, and bring back the travelling twin back (that would have to involve acceleration, IMO, but at this point, i don't freaking care!!) Just SOMEHOW/anyhow bring him back, at rest with his stationary twin, and explain the timeline PLEASE!!
Thanks a lot!!! I've been asking this question for twenty years and this is the first time I am satisfied with the answer (even though I'll have to watch this video several times, pausing, drawing spacetime diagrams and doing some calculations in order to fully understand it, since I'm not very good at Physics). I'd just like to know what would happen if the universe was finite but borderless? (i.e. it is possible for someone to go back to the starting point always walking in the same direction). In this case, would a non-accelerating rocket twin still see an older resting twin when he came back to the starting point? (the rocket twin is always traveling in the same direction, without any acceleration)
Hmmm... Well, first, even if the universe was finite and borderless, the clone/twin would never be able to get back here that way because the universe is expanding too quickly. Buuuuuuuuut, assuming it wasn't expanding the fast? I'd need to think about it a while.
Wrap-around universes don't work nicely with Special Relativity (which doesn't mean they couldn't exist... just that physics would be more yukky).
For example, a wrap-around universe (like on the old Asteroids game) has a defined width. But in SR observers at different velocities would see a different width!
Basically, so long as you don't send messages all the way around the universe to behind you, the universe would look like Special Relativity. If you could send a message all the way around, you'd be able to detect how fast you were moving relative to an absolute space!
Excellent question, which I have been trying to figure out for a while. If we assume that this is a wrap around universe, and that the above rules for relativity still all hold true, then geometrically this would mean that as rocket twin's path goes off diagonally to the right, he wraps around the time axis cylindrically, as an extra dimension, and comes back and meets up with earth twin at some future point. Since the stationary paths always take maximum time, the earth twin must be older. Problem is, due to the rule about all observers are equal and can consider themselves stationary, this reasoning also applies to rocket twin, which appears paradoxical. Another problem is that the light signal of the initial event (rocket twin's departure) comes back and meets the stationary observer before the actual meeting, producing a hall of mirrors type situation. This is assuming that general relativity doesn't come into play, which it probably will, since we have warped the spacetime into a cylinder.
unbelievable to see your channel grow this fast, been here before 1k but now it's already 70k. keep it up!
After all this time dwelling on this, I think I want to see the version of this accelerationless scenario where the two send wireless photos of themselves and their clocks to each other the whole trip. I know it'll get a lot more complicated, but I'm only on the very cusp of understanding this and why one's coordinate system ends up less tall than the other. I can't quite cross that finish line...
Hello. I explained the answer to the paradox like this. That a person who has traveled must show less time in his watch. Therefore, this difference is enough for the explanation. As you said, acceleration solves some of the problems, but not all of them. Is my thinking correct in your opinion? Please answer.
Ive searched for videos all day on this problem and not one of them was able to answer the question in a way that couldnt function. This video however was the most helpful and i think i finally understand
Glad I could help :-)
Great video. One really esoteric comment. You don’t need GR to describe relativistic accelerations. A rindler wedge only relies on special relativity.
I actually have a plan for a video about "momentarily comoving inertial frames."
That was an excellent explanation, much better this time. I'm glad you used space-time diagrams and you also did a great job showing the wiggle-room involved when the events we're trying to talk about are at different locations.
-Matt
1:02 creo que es importante indicar que hay que imaginar un reloj en cada punto espacial. Para cada sistema inercial, los relojes se pueden sincronizar y podria hablarse de un unico tiempo "global" para ese sistema. Pero al analizar un sistema inercial en movimiento respecto al nuestro, hay que imaginar que las reglas están contraidas en el eje x, y que los relojes de cada plano yz (mismo valor x) marcan diferente, pero...todos van al mismo ritmo (mismos incrementos temporales)
3:50 No, accelerated points of view do not require general relativity. They do require some of the math typically associated with general relativity (specifically, general coordinate transformations rather than for some arbitrary reason limiting yourself to Lorentz transformations), but physics in Minkowski space is still special relativity, regardless of whether you put curved coordinate grids on it.
You need to continue the video, as he intentionally mislead a portion of the video, skipping a rule entirely to prove a point.
@@watertommyz I usually do that. It's been a while, so I did it again just to be safe. And no. He doesn't correct that mistake at all. He just ignores acceleration altogether. So I stand by my criticism: You can deal with accelerated frames of reference in special relativity. He says you can't. That's wrong.
Saying that “The Twins Paradox exists without acceleration” is a bit strange, because in the situation you described, there’s no paradox. Both twins see each other as younger than themselves, but they never meet again, because their worldliness can only intersect once. That’s strange, but not a paradox anymore.
Acceleration is required for the twins paradox because only curved worldlines can intersect at multiple locations in flat space. However, it’s not the acceleration of the rocket that causes the paradox. It’s the acceleration of the reference frame of the rocket, because there is no Lorenz transformation that can make Rocket Clone’s worldline straight.
But acceleration isn't the _only_ way to get the clones back together. The traveling clone could also travel in a straight line around a curved universe arriving back where it started, never having accelerated. The paradox is caused by them coming back together, _not_ by the acceleration.
Fermilab also coincidentally posted two videos on the Twins Paradox recently that also explained why acceleration is not the cause of the paradox. In the Fermilab video he considered a thought experiment where there are three observers: one on Earth, one on a ship flying past the Earth toward a star at high speed and another on a ship flying past the star at the same high speed toward the Earth. None of the observers experience any acceleration, they all have constant speeds relative to each other. As the outbound ship passes the Earth they start a timer. When the outbound ship and the inbound ship pass each other at the halfway point the outbound ship holds up a big sign showing the time the outbound ship claims has passed. The inbound ship starts its own timer and as it passes the Earth it holds up a sign showing the time the outbound ship experienced plus the time the inbound ship experienced since that event. As expected when you do the correct transformations the total of the two times from the ships is less than the total time the Earth observer experienced.
Link please
@@surfinch just search fermilab twins paradox
This is the best way to resolve the paradox if one wants to avoid accelerations. However, the problem then becomes an "information transfer" issue, and not a matter of which twin actually ages differently. An asymmetry is introduced between the original two twins, as one transfers info from one particular set of clock/coordinates to another, distinct set of clocks/coordinates, and the other twin has no such transfer of information. Personally, I like the acceleration explanation much better than the non-acceleration one, because I understand better what an acceleration is compared to "transfer of information".
The video you refer to is completely correct in everything it says. But it does not describe the twin paradox. And not only because there are three (and not two) observers, it doesn't even describe a "triplet paradox". In this video the observers are never sharing a reference frame, and sharing a reference frame (that is being at rest with each other) both at the start of the trip and at the end of the trip is a vital part of defining the twin paradox. In this video the ships passing by the earth just observe time passing at a different rate than they do on the other ship and the earth. That's just time dilation, without any paradox.
@@grantgussie8768 You're missing the point, though, of the Fermilab video, which is that it creates a scenario similar to the classic Twin Paradox with similar time dilation effects but doesn't involve and acceleration in the reference frames. Thus acceleration isn't the key to understanding the asymmetry in Twin Paradox like some people try to claim.
Amazing how you touch upon so many things in just 10 minutes. I took close to 17 hours of video to build up to this finale and therefore breaking every rule of becoming a popular UA-camr :-) But boy did I have fun while doing it.
You did it and that's awesome - best twin paradox explanation out there! As a slow thinker, I watched that Hmm @ 6:57 about 10 times. Hmm, I will probably watch ten times more till this sinks in...and next, let's open that can of warms:-)
This channel rocks. Much better than minute physics and all the others.
A question from the little me: When a particle is "shaking" because or the entropy, from its point of view is that entire univers is moving randomly? And what makes from the point of view of the particle the entire univers to move?
It becomes easier to show that there is NO PARADOX, if we oversimplify the narrative for a moment and say “whichever party has covered more distance (say, more kilometers) will have aged less”
Now, we need two events to be able to compare the rates of aging: first, an event in spacetime where both astronauts are in the same reference frame and synchronise their clocks. Second event has to take place at a future time, at an arbitrary “same” spatial coordinate, bringing the two persons back into shared reference frame.
Immediately after the relative motion begins, both onservers agree on one thing: the distance between them is increasing. For the second event to happen, at least one of them must change its trajectory and “catch” the other. This part is not relativistic. Both observers will later agree on who changed trajectory (and this is where “experiencing acceleration” plays any role). Whoever does that, and becomes the “chaser” will have traveled more in space by the time they meet again in the same reference frame.
They may safely compare the number of kilometers each one has covered and agree that the one with more mileage has indeed aged less. No chance of disagreement.
without that space/time graph I would've been totally lost.... thanks space/time graph!
A question
Using the graph can you show how approaching a black hole will affect the relative time to each twin ?
I should say at this point that you r my inspiration for going into a physics major.I know I will struggle with money as u r hence this channel.But I can see the effort u put into these videos such as length contractions in the animations and I hate the system(and also kind of like it being selfish)that you ain't doing some next level physics and earning millions right now.
We can always use more physicists :-)
No matter how one looks at it, if there is a true age difference between the twins, then there must have been an asymmetry between the two twins. If there is no asymmetry between the twins, then they MUST age the same.
Indeed
As always Nick a brilliant simplification, I was so engrossed that I was shouting at my phone,
“No Nick you’ve done the maths wrong”,
How stupid of me to think you’d make a mistake like that, hmmmh maybe I’m just a little crazy and not stupid cos at least I spotted the maths. Oooops the flying pig that’s sat next to me has pointed out that it was ‘He’ that spotted the mistake, . . . oh dear . . . Oh dear . . . maybe I’m past being a little crazy . . . Lol
HAHA Good thinking! Yeah, I had to do the math wrong upfront or there wouldn't have even been a paradox to talk about.
Just glad that I don't have to sit for Physics exam anymore. Awesome video as always.
Hey, long time subscriber, and my classes get a lot from you too! I am preparing to teach special relativity and had a question - in the twins paradox, does the rocket twin come back shorter?
High school books emphasise a real, notable difference in time (since the twin comes back younger), but only an apparent difference in length which is only observable under the conditions talked about in special relativity. So when the frames of reference once again come together for the two individuals, there is a permanent difference in time, but length remains unchanged - is this correct?
Great question! No, the astronaut doesn't come back shorter... but they _do_ come back having traveled a different _distance_ than the Earth observer thinks they did. The "permanent differences" are the ones that accumulated over the entire trip. The length of the astronaut is only a local instantaneous observation.
@@ScienceAsylum Brilliant, thanks so much!
What you needed was the triplet paradox version. Your version of half-way causes confusion with relativity of simultaneity.
I understood better than the last explanation you did of this, but I'm still a little confused about why you need two shared events and what exactly a coordinate transformation IS. can you clarify?
Say we measured the time with some fancy stop watches. We need two shared events because we have to agree on when to start our stop watches _and_ when to stop them. We can't do that unless we're at the same place and the same time for each one of those events.
A coordinate transformation lets you take measurements in one coordinate system and see what those same measurements look like in a different coordinate system. Simple example: You're sitting across a table from your friend facing them. You measure where the kitchen is relative to you. Now you want to know where the kitchen is relative to your friend. What do you do? You have to shift your measurement by the width of the table and rotate everything by 180 degrees (so that now you're looking through their eyes instead of yours).
The Science Asylum Ok, now I get what you mean by coordinate transformation. I was thinking it was something like converting between cartesian and polar coordinates in trigonometry. Keeping the same type of units but looking from a different perspective makes a lot more sense. Thanks!
The Science Asylum I think I get what you mean about agreeing when to start measuring and when to stop, but I still feel a little confused. Could you give another example? And thanks for the response. I appreciate how well thought your videos and how you take viewer comments into account. :)
So, you and your friend decide to measure how much time passes for yourselves after lunch (at the table from my earlier example). While you're sitting at the table, you can easily start and stop your watches at the same time. Instead, let's say you start your watches together just before you leave the table. No problem so far... but then you both leave and drive to your own homes. How do you know when to stop your watches and make sure you stop them _at the same time_ if you're in different places? You could call each other on the phone, but that signal takes a little bit of time to travel between your phones (maybe half of a second). It'll be close, but it won't be perfect.
Best explanation of the paradox
THIS CHANNEL NEEDS SUBS
Thank you for explaining. To me the thing is more intuitive with the "cutting a baguette" example: Depending on the direction a moves relative to b he would cut the bread in another angel. You showed this in your diagramme, too (the space time is the bread, and the "nows" are the cut area of bread). But if you had added another clone c meeting b abd flying to a they could habe synchronised clocks without accelaration. And just by c cutting bread in another angel one sees that there is no paradox when c later compares his clock with a.
Best explanation ever. None else could clear it for me.
The simplest way to measure the speed of light (event propagation) is basically a light (event) emitter, a far-away (known-distance) reflector, and an eyeball positioned in the same place as the emitter. The light makes the round trip, and then you have the speed of light. That round trip seems very fundamental who the whole thing. You can't just put Janice at one point, and Maria at another, and have both scientists hash out how long it takes for light to travel from one point to another, because Einstein established that synchronizing watches between observers who are very far away from each other is a logistical and philosophical impossibility.
The twin paradox also relies on a round trip. I feel like there's a "there" there that I'm not fully equipped to articulate.
As a final episode on Lorentz transformations, you should do a fun video about a space ship going around the earth at 99% speed of light. Space ship is equipped with a radio and a telescope so it can look down the earth and communicate with earthlings. Makes you wonder would they see things on Earth speed up as if someone speed up the video. Or when communicating with Earth, would they be able to talk, since one side would hear speech slowed down, while the other would hear it speed up, i imagine. Doh! This mind experiment opens a can of worms.
Yes, they'll see Earth sped up. If they use Skype of Facetime, it would be like the connection is really shitty in rocket-Earth direction (too much lags!) and superfast in Earth-rocket direction (packet overload!).
I would like to hear what crazy scientist guy would say here, reply please? 🙂
You'd need a REALLY powerful engine to stay in that orbit, as you'd be orbiting several times a second! I wonder how much energy it would require...
I was hopeful to see a better approach than the usual and while this adds some detail, I think it could have been clearer by taking the first spacetime diagram for the unaccelerated clone and rotating it so their worldline was vertical, then add a horizontal to show the elapsed time on Earth was 10 years, reduced by the same gamma factor. Then add another clone to create the "triplets" version. Rotate to his frame and add a horizontal. Return to the Earth frame and you get the extra 7 years in the middle which is due to "relativity of simultaneity", the feature that most beginners forget. Overall though, it's a step in the right direction.
For my personal interests: this is your best video :) Thank you for making it.
You're very welcome :-)
No one can ever explain better than u Lucid!!! Great Job...
But.. wait a minute.. wont that star u mentioned is also moving in galatic plane? If that too considered.. it would be awesome..
Thanks for another awesome video!
Thank you! The common explanation of change of frame of reference was bothering me since we should still have a difference in time due to their relative difference in motion even if acceleration was not involved. Thank you for the synthesis of the rules.
"You're not always calculating what you think you're calculating" is a tattoo-worthy quote.
It's much more easy if we consider only one half of the trip. We can place a clock at the distant star wihich is synchronized to our clock on Earth. We observe, that the traveling clock shows less time. If the traveling twin moves back, we can simply repeat the same consideration. The moving clock will be delayed again by the same amount.
Lol the Oprah clock bit made me crack up 🤣
Best explaination I have ever come up with.
There's a simpler explanation. Ignoring acceleration, there are three paths (which are straight lines). That means there are three observers since in Special Relativity, each straight line has an unique observer. If you draw the light-cone diagrams for each observer, that is, you draw three diagrams, then you realize that there is no paradox.
From the point of view of the Earth, there is the outbound trip, where time is running slower and the return trip, where time is running slower. Slow time plus slow time is less than Earth time.
For the out bound trip, Earth time is running slower because the Earth is moving away from the rocket. But the return trip must move away from the outbound trip faster than the Earth in order for it to catch up with the Earth. That means its time is even slower. So, normal time for the outbound trip plus even slower time for the return trip is less than the Earth's slow time.
And similarly for the return trip. The Earth is approaching, so its time is slower. But the outbound trip has to have travelled faster than the Earth go get away from it. So its time is even slower than the Earth's. Normal time for the return trip plus even slower time for the outbound trip is less than the Earth's slow time.
That doesn't really address the problem though because the paradox is there even of the rocket doesn't turn around. If that's the case, there aren't "three observers." There are only two observers and, in that case, the paradox is still there.
If the rocket does not turn around, there is no paradox because, like you said in the video, there's only one shared event.
Exactly!! This video nails the first part, it is the SECOND shared event (when the twin meet again), which remains unexplained!!. Please see my comment/question as well.
Sir this is a better explanation.I did the math and it all checks out.THANK you for your comment.
What? I'll go watch it again (above sounds correct I must say ...)
0:44 Rocket Clone sees what the Borg cube has done to the fleet.
Thanks for explaining
And nice video as always
Man I wish I could help you, get more subscribers and build your channel as fast as possible... having said that I am 100% confident you are going to make it big.... I'm talking millions of subscribers... you're totally unique and excellent at your brand of humour/education.. flawless... I really really hope people manage to find out about your channel, you deserve the success. I've only just subscribed but seen all of your videos at least twice over if not more... and you've shot to the top of my favorite science channel list straight away.. we're talking over the likes of Veritasium, PBS space time, Issac Arthur.... the big hitters... you're just better and I'm pretty sure it's because of the comedy element... but your content isn't bad either (I'm being modest) I've been watching UA-cam videos on GR and quantum mechanics for over 10 years.. and I've only just started to see GR in a more coherent light thanks to your videos.. keep it up my man and best of luck with the channel ;-) (Could easily see this show being on TV) oh one thing I wanted to ask... so your real name is Nick Lucid?
Thanks! I'm hoping my visit to VidCon U.S. this coming June is going to be my big break. I'm involved in some official stuff, so lots of "networking" opportunities and stuff.
Very satisfied with the solution! And my brain thank you very much for it :)
Please answer: Less Time passed by rocket in relative to space station. So Rocket had gone in the past of space station or spacestation had gone in the future of Rocket. So, when the Rocket returned, it should have returned to the past of Spacestation and they should have not meet again unless the Rocket TimeTravell forward or the Space ship TimeTravell backward. How "Now" for both become same again though the passenger of the Rocket look younger than the person in Space station.
The science asylum is the true meaning of life
42?
The Science Asylum = 42.
What is 42
Neptune
The answer to life, universe and everything.
+Neptune
It's a number
What about a completely symmetric version of the paradox? The only way out of the paradox is that motion itself doesn't affect the flow of time as it is relative. Even in your example if you change the reference frame and remove acceleration, then everything is symmetric.
Great teacher!
Riddle me this all those who claim to understand this: Say the two participants accelerate away from eachother equally in a totally symmetric fashion. Then they turn around and accelerate back. When they meet each other again, who will be older?
It's not actually "everyone has their own clock" - but every tiny particle or bundle of energy (quanta) has It's own clock. That we are made up of such a colossal number of these tiny things, and can move about more or less as separate entities, makes it more comprehensible to us.
You are correct about each particle having it's own clock, but I feel like that makes things _less_ comprehensible.
@@ScienceAsylum Well, yesss, maybe!
I love your channel regardless ¯\(ツ)/¯
Love you please keep making videos i am in 10th grade i want to be a theoretical physicist an i 😍 physics and mathematics
1:07 hahahaha such a cute moment :)
Nick Lucid rocking the Spacetime Diagram oh yeah.
Also nicks energy is contagious
This episode was much better on wrapping up the explanation. Kudos for you! Thanks for your wonderful work.
Thank you so much! Finally it makes sense for me :-)
You're welcome! Glad I could help.
Hey Nick you should do video on time dilation and length contraction..
The Ultimate Guide to Relativity: ua-cam.com/video/FdWMM6aXpYE/v-deo.html
History of Relativity: ua-cam.com/video/a205YJsbBSQ/v-deo.html
Its OK to be a crazy
little*
Thanks for the clarification Controle Quebrado :-)
You said that explanation by acceleration is completly wrong but ,it seems correct, without throwing 1st law,just by including gravitational time dilation
Can you explain more
As we've learned with a lot of upper-level physics, just because it _seems_ correct, it doesn't mean it _is_ correct. The twins paradox is nuanced.
1. What you get when you calculate gravitational time dilation depends on the size and duration of the acceleration, which adds unnecessary complication to the scenario.
2. Including gravitational time dilation doesn't fix the 4-year disagreement.
3. You can't really use gravitational time dilation to compare time intervals that are this far apart in space any more than you can Lorentz time dilation.
Science and memes... 😃👍
I am going to have to watch this again (again). I understand the premise at the start and the conclusion at the end but I think I zoned out in the middle. TBH, I think I might just have to take your word for it. You are quite right, for years I had been under the illusion that the twin paradox is solved because: acceleration. I will need to prime my consciousness with a couple more of your related videos if only to get my head into the correct reference frame. That and hit the pause button more often to let things sink in.
The difference is more fundamental than acceleration. Time dilation is all about measurements. To compare measurements of time, you need to have two events in common. There are many ways that spacetime paths can have two events in common, only one of which is acceleration.
(To be fair, the way the paradox is traditionally stated and the way it's stated in this video, acceleration _is_ at play. My main point is just that it doesn't _have_ to be acceleration, so the paradox isn't actually _about_ the acceleration.)
Yeah, I’m in the same boat. The explanation travels closer to light speed than my brain, so I’m waiting for the two to cross paths again with each other and compare notes. 🙂
@@ScienceAsylum
Ok...
Then what is a situation in which you would get two clocks showing different times without acceleration?
@@F16_viper_pilot
The fundamental thing that breaks the symmetry and that's responsible for actually disagreeing clocks (clocks in same place disagreeing after formerly having agreed) is acceleration. There is simply nothing else it could be.
Omg...wow. This was brilliant. And it explains the apparent paradox. I have always heard/read that it was acceleration or apparent gravitational fields that caused the difference/paradox. Which always seems a bit...ad hoc.
What would be interesting would be to have a two way video link between the two frames of reference.
We could do that, but it would take time since radio (light) waves travel at a finite speed. You can account for this in a spacetime diagram 🤓
@@ScienceAsylum Let's us use the equations of general relativity to simulate the video we would see as we jump between the two different frames of reference. That would be worth seeing.
@@ScienceAsylum Just simulate what each frame of reference would see.
can we please make a video about actually sending selfies to each other. i was actualy thinking about it today. thank you.
A large problem is the simplified models used to explain things which cut ever so small corners. When stacking multiple models, those cut corners start adding up fast.
For example, speed is stated to be relative. It's really not... If it was, both twins would experience the same thing. Acceleration is absolute and different for each twin since it can be measured within the local frame. And yet, acceleration is not involved in the time difference.
Now, to come up with an intuitive model that doesn't cut corners...
Not sure why he mentioned acceration here to be honest, not needed. Guess it is down to clicks rather than clocks ?