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Science: Only photons can travel at c because they have no mass. If you could travel at c, time would stop. Also science: You are travelling at c right now.
..... please do not become one of those low hanging fruit mid-video non-adsense channels :( In *mutual respect,* plenty people watch non-adsense if at very front/back/both, rather than skip/click off =/
The Speed of Light is extremely slow, when viewed from the vast distances even across our Solar System, let alone Milky Way distances. Scale the Sun-Neptune distance down to the length of a football field. Here an ant will walk faster than the speed of light.
Another fun way to think about it: your movement through spacetime can be thought of as a 4-dimensional vector with magnitude of c. If you're sitting perfectly still you have zero motion in the space dimensions and your motion through time is just c, while if you have some significant motion through the space dimensions your motion through time must be reduced so that the magnitude of the vector is still c. Hence, time dilation.
Almost correct. Remember time and space are one. It’s hard to us to wrap our brains around this concept because if we sit still we still in the same place on earth, our reference point was only the Earth, that would be true. But just rewind back to earlier to January of this year. The earth isn’t in the same place it was around the sun as now. Thus we moved in space and in time if the Sun was our reference point. It gets even crazier. We are getting close to starting a new year. So that means Earth will be the same place it was when it was last year in January. Yes, but only if you’re looking at the Sun as reference point. Zoom out to the galaxy as a reference point. The sun is moving around the Milky Way so the also is moving through space and time. That means that the earth is never in the same place in space time. It has moved with the sun around the Milky Way since creation. The same thing happens when you zoom out to universe as a reference point where time and space between galaxies is expanding. Your example only sees earth as reference point but not the sun, or the galaxy which at those reference point totally shows that we are always in motion in time and space. It’s only when we look at the universe as a reference point do we realize everything moves at the speed of light.
@@JasonB808 Almost correct. 😉 There is no absolute universal reference frame as you're describing. Motion and position only have meaning in relation to other objects, and the Earth or your toilet are as valid as any of them.
There's a mistake near the end (the bit starting around 9:40), as some others have mentioned. Yes, the observer will measure a larger amount of movement through space, but there's a minus sign in front of that term! So the amount of time they measure has to be higher, not lower.
💯 Though it gets confusing, because we're talking about "distance in time per unit of proper time." And, in terms of intuition, what ends up happening is that the relationship gets inverted. That is, the core observation is: *the faster you observe something moving through space, the **_slower_** you observe their clock will be ticking.*
@@kylelochlann5053 Yup. If you define an inertial reference from for yourself where your velocity is zero, and then conclude your velocity is the speed of light, you automatically contradicted yourself. Your velocity can't be both zero and the speed of light in the same sense at the same time.
It's so cool hearing about all these other people involved in the process. It's a reminder that even people who we think of as lone, super geniuses in pop culture still stood up on the shoulders of those before them in order to make their contributions
@@culwin I guess ya didn't look. Lorentz came up with transformation in special theory of relativity. Poincare then developed a way to synchronize clocks on opposite sides of oceans using principle relativity. Einstein got credit for not using the Ether as a model for lights travel in space. Before that it was called Lorentz-Einstein special relativity. Sorry you don't know how to use Google, I just typed the last name of each one followed by relativity and got all that.
1:24 OK, but what if I’m sitting on the toilet watching this on my Vision Pro, and I have the immersive environment of the Vision Pro set to the surface of the moon, which is one of the options? Yes, one of the benefits of having a Vision Pro is that you can take a dump on the moon!
"Now the universe itself keeps on expanding and expanding, in all of the directions it can whiz! As fast as it can go, the speed of light you know, 12 million miles a minute and that's the fastest speed there is...." -- "The Galaxy Song" written by Eric Idle of Monty Python (it was in "The Meaning of Life" and has a really cute video of it on UA-cam). The song is remarkably accurate, actually. He rounded up to the 12 million miles a minute, however (it's 11-something million really).
I'm pretty sure I've seen the "butthurt version" with Brian Cox or Dara O'Brien or Neil Tyson, singing along and being "scientifically accurate." and it was funny as hell. :D EDIT: I remember I'm on youtube, found it: ua-cam.com/video/_IFGH11uxaI/v-deo.html
A relevant fact is that the expansion of the universe is actually much faster than the speed of light. This is because the "expansion" isn't actually physical objects moving apart in space; rather, it's the space itself which is expanding. It's like if I have two marbles sitting on a piece of clay, and I flatten the clay so it gets wider. The marbles (planets, stars, etc.) will end up further apart, but they won't have actually moved relative to the clay itself (spacetime). Distance is a geometric property of spacetime which we call the metric (really a pseudometric because of the sign difference). A change in the metric doesn't involve anything actually moving, because spacetime isn't an object, it's a space that has objects inside it. What we call "expansion" is really a change in the metric that increases distances between things. This isn't a particle moving around; it's a change in the laws of motion themselves.
It's funny, how we load ourselves with mind blowing ideas about how universe works, while we still have no idea about the connection between the Universe and life, and particularly our thinking and idea formulation processes. Sometimes I feel that instead of studying a great being we only study how it's nails grow...
9:42 I get you're trying to simplify it, but an increase in distance in space must be accompanied by an increase in distance in time, not a decrease. It's subtractive, not additive. If t - x = s then (t + 1) - (x + 1) = s t + 1 - x - 1 = s t - x = s t goes up, x goes up.
I'm replying to boost this comment higher. I've just said the same thing before checking to see if anyone else had already said it. (BTW, you meant "t - x = s" in the last line, not "t + x = s".)
Yeah, they showed a formula where something on one side of the equation is inversely related to the other thing on the same side of the equation. That works in multiplication but not in addition/subtraction. If add to the amount you subtract, the other value has to put back what's being subtracted by also going up. I don't know what they were thinking and trying to say.
They're squaring both components. This turns a quantity (x + a)² into x² + 2ax + a² This would turn the quantity (x - a)² into x² - 2ax + a² So an increase in distance correlates to adding 2ax + a² to the original value. To conserve the quantity, the time² must decrease by an amount that offsets this to make it the same.
@@raymondwall8187 I had believed "c" is for "constant". Kindly correct me if I'm mistaken. And while I'm not proposing any hypothesis, I wouldn't be surprised if we discover that, as it seems with Hubble constant, it varies throughout the universe. (disclaimer: not a real physicist)
Been there done that for 2 years down to exit 15 then up to Philly for my second stop and final stop in Trenton NJ. 2 years was all I could handle of it, and I usually got in there before morning rush hour.
I almost had a two second period in which I understood the space-time equation. Then it faded and I’m confused again. But it gives me hope that with enough study, I COULD one day understand it. 😁
The explanation of time dilation at 9:45 is completely wrong. An external "stationary" observer measures a LARGER time interval than the "moving" observer measures. For the "moving observer", in their own reference frame, ds^2 = (cdt)^2, as previously explained correctly in the video. But for the external observer, dx^2 + dy^2 + dz^2 is non-zero. For ds^2 to remain the same, the external observer's (cdt)^2 must INCREASE by the value of dx^2 + dy^2 + dz^2.
Had this exact conversation in Physics 2. I asked my professor if there was such a thing as a rate of travel through time and he disagreed with me because he didn't think there were units that could describe such a thing. I'm like, "dude, the universe doesn't even care if we have the units figured out."
Sometimes actual profs are wiser than ten youTubers who think that the phrase "travel through time" is even semantically, let alone physically, cogent.
Your prof is right, and you are not. Dimensional analysis, which is about considering units, is a very important tool in physics. From a mathematical perspective, it's unclear what "rate of travel through time" is even supposed to mean. Rate of travel through space is the distance travelled relative to time. If you used the same logic for rate of travel through time, it would always be 1. A better thing to consider would be the angle between your trajectory, the space component, and the time axis. This isn't Lorentz invariant, but it's still interesting.
@@dorongrossman-naples9207 That's fair. Imagine if humanity had developed different units for travel in north/south vs east/west directions. It would be an additional layer of complexity in converting units but would still produce accurate results when calculated. If we really want to consider time as a 4th dimension, you could say waiting one second is the same as moving 300 million meters along the temporal axis. Interesting, but not really useful in real world applications I guess.
@@BrackenStrike people actually do do this! It's equivalent to setting c to 1, which is part of something called "natural units". The issue is that you still need to divide distance by distance to get a unitless quantity.
A visual way to express your speed thru space-time is to imagine a circle centered at the origin in 2-D with a radius of c. Now only consider the arc in the first quadrant (upper right). The x-coordinate is your speed thru space. The y-coordinate is your speed thru time. (Speed is the mangnitude of your velocity vector. Direction doesn't matter for space.) The points along the arc are the corresponding speeds thru space-time allowed (which is fixed at c since that's the radius of the circle). Notice: if you know your speed thru space, it's easy to find your speed thru time. If you are massless, your x coordinate would max out at c and your y-coordinate would be zero, indicating you do not experience time at all. If your speed thru space is zero (you are stationary and not inside a gravity well), then you travel at c thru the time dimension. So it's easy to visualise that every particle/object with a real, non-negative mass travels at c thru space-time.
Careful here: space and time don't have any independent existence. Your interpretation of the axes is not correct. The y-axis is the distance along your world-line, the derivative (spacetime gradient) of which is your speed through your own defined time. The x-axis defines the set of locations (spatial hypersurface) at the start of your stopwatch, t=0.
@kylelochlann5053 I wasn't referring to a Minkowski diagram. I was describing a 2-dimentional plot of Speed through Space (condensed horizontal) vs Time (vertical) to help visualize why it's always c.
@@seanspartan2023 What does "speed through time" even mean? And isn't the speed through space always zero as you're taking your reference frame with you? Also keep in mind that there's no independent existence of either space or time, hence "spacetime" (which itself has no independent existence).
@@kylelochlann5053 but light is massless ...doesn't time only bear upon entities of mass? (I'm not a scientist, but awfully curious ....please be gentle)
@@horustwohawks The are few ways to think about all this that I've learned from others. The first comes from Lewis Epstein: What we have is 4-dimensional space (no time) filled with particles that have mass and thus all moving at the speed of light. Because they move at the speed of light, the space along their direction of motion is length contracted to zero (which is why we can't access that past or future). The only "time" is the distance along each individual world-line (particle path). The second comes from Willis Lamb and Geoff Jones where "photon" is word that describes the exchange of quantum state and momentum between matter particles that is instantaneous (instantaneous in spacetime). That is, to question what it means to even exist between emission and absorption. So there is no sense in which "time" or "the distance along the path of a massive particle" can be applied to a photon. The massive particles (observers) have time and so measure the interval between emission and absorption.
Someone I once talked with years ago had this theory called "the stillness of light." His theory was light didn't move at all, but somehow everything else moved slower than light did. That's all I remember about it, and I don't believe it, but I still think about that from time to time.
I am really enjoying these newer episodes (such as the one on optics) that go a bit deeper into the science than most of the past couple years of this channel!
In college, I took an advanced physics class unintentionally, due to a clerical error at the registrar. The only thing I actually understood from that class was special relativity... and that professor still never explained the whole "time dilation happens because we're all moving at the same speed through spacetime" concept. I only learned about it a few years ago, from another UA-cam video, and my mind? Fully blown 🤯 Science is so cool!
So the best and easiest way to explain it is you’re always gonna go C (remember that time and the space you move through are intrinsically linked and inseparable) at all times. If you have 0 (relative) motion, you’re going at what we can call a reference frame for time of 1. Now let’s say you go to half of the velocity of “the speed of light” (calling it that for simplicity). You have to still end at the same point of C, so your extra velocity means time is at half its usual reference frame (it’s now 0.5) because the other 0.5 for a whole reference frame comes from your velocity. The best way to think of it is “velocity + time = C”. Increase speed and your time has to slow to keep C consistent. It’s why from its perspective, light doesn’t experience time. The velocity half of its equation is 1, so as a balance time has to be 0. This is also where the paradox of FTL travel goes. If you go faster than that 1 velocity of light that makes time = 0, time HAS to be a negative number to compensate. If you go double that C velocity, time will be at a negative integer because it MUST be balanced for C at the end (based on physics as we know it). And insofar as said knowledge goes, time can’t be a negative value just as velocity can’t be a negative value. There are theoretical workarounds like the Alcubierre Drive, but those are really manipulations of the rule rather than a violation of them (I can explain that further if you want).
I started on a physics major path in college. It was mostly equations and details, without reference to how it would play out in the natural world. I lost interest and lost track of what they were talking about and switched majors.
I don't know why everyone always says that reference frames are weird to think about and hard to grasp. It makes perfect sense to me. If I'm in the passenger seat of a car going down the highway, it doesn't matter to me that my phone is also going at 70mph. I can reach over and pick it up no problem. If someone on the side of the highway wanted to snatch my phone as I went past, it would be an entirely different thing. Why is that hard to grasp?
Perfect understanding of Galilean relativity, which is an important step to understanding special relativity. You are 100% correct that physicists and mathematicians often make things sound more complicated than they are. In this case, the part that could be tricky is the fact that the speed of light is a finite constant no matter what speed you're going, and everyone would measure an identical speed for the same photon. Reconciling this with being able to trivially handle a phone moving at 70mph by also moving at 70mph is what leads to time dilation. It's possible to derive the Lorentz equations from these two facts, plus a few ones generally considered too obvious to mention, like how moving at 70mph means everything else looks like it moves backwards at 70mph.
This is the best video version of this topic that I have ever come across. I love that you show the math at a level where anyone that works numbers to solve problems can understand the topic in it's intended content.
6:10 train the scientists in literary skills so they can translate their findings for the press, and train writers in the sciences so they _stop clickbaiting with titles that say the opposite of the research findings_
9:50 isn't quite right. If S=T-X and X goes up by an amount y, then T also needs to go up by y if S is to remain constant: S=(T+y)-(X+y)=T-X. In terms of interpretation, the distant observer is using their own clock. Yours is slower relative to theirs, but theirs is equivalently faster relative to yours, meaning they measure more ticks of the clock and record a higher T value than you do any time they record a higher X value.
Basically the entire video is wrong. For example he sets dx=0 to get "time dilation" but if dx=0 then the traveler isn't in relative motion, so there's no time dilation.
My thought experiment for this is to define three frames of reference. First to fire a photon/lightwave along a vector/numberline X starting from X=0. This is reference 1. From a Stationary perspective, reference 0, the Lightwave is moving some speed in a postive X direction. The Lightwave arguably can view itself traveling in a positive X direction. That Lightwave, immediately once fired, fires it's own Lightwave behind itself, creating reference 2, in the X vector in a negative X direction. So from the stationary reference 0, Lightwave 1 is traveling +X and Lightwave 2 is traveling -X. From Lightwave 1's perspective while traveling positive, Lightwave 2 is at X=0. And from Lightwave 2's perspective while traveling negative, Lightwave 1 is at X=0. As such, the only way to rectify how both photons can 'see' the other as stationary is to apply time dilation to such a degree that time has effectively 'stopped' for the other in their perspective. In other words, Photons travelling at the speed of light do not 'age' in reference to other Photons. So I ask the question. From the persepective of itself, does a Photon age? and if so, what does it experience? Does it travel to the infinite 'end' of the X vector in one infantesimially small duration of time? A photon from a distant star travels a vacant path till it happens to excite an electron in the cluster of mass that is a Human eye in a degree of time? If a Photon does not 'age', then the distance between the star and the human eye is a non-factor and from a photon's perspective, we still exist inside the singularity?
It is a difficult question and has no answer in relativity, other than to say massless particles have NO concept of time or time dilation according to Mr Albert :) Time dilation can only be applied to particles and objects of mass. > So officially does a photon experience zero time or does it experience time is an invalid question. . It is an interesting thought though and even bothered old Mr Albert somewhat when he was working it out, and even later in life :)
@@axle.student There is so much to say and think about this, but then again this is YT comments, lol. I can't say I agree that it's an 'invalid question' considering photons ARE effected by objects that exist within the dimension of time, which means that Time has at least an indirect effect on massless objects. But I do agree that whether or not a photon in isolation experiences time is basically unknowable/invalid without a radically new understanding of physics.
@@bosslca9630 Kool kool I did a number of thought experiments where I gave a photon minimal mass so it could behave a little more like other particles, reduced its velocity by 0.0...1% from 'c' and allowed it to have time dilation. . It goes from a particle that experiences Zero time on its internal clock, to it's clock moving so slow that it experiences infinite time instead. That is 0 = never existed by its clock, to infinitesimally slow clock and existing for an infinite time internally. It's a big difference from such a small change :)
@@kylelochlann5053 That's the standard mathematical answer derived from relativity. But the question is more related to the definition/structure of relativity itself. . So based around the misconception that photons experience no time. Although there is an issue within the fundamental definition of a photon and how it behaves. I guess it will always be a point of ambiguity :)
😂😂😂😂 Neil has actually covered this multiple times. He presents it differently. By saying there's a way of interpretation of the space time interval like this.
Gentlemen, they said it wasn't possible, yet here we are piloting our porcelain thrones at the speed of light. Godspeed my fellow travelers. May your journey bear fruit and your destination be all that you hoped and more.
It does not slow down… it excites and de-excites in the electron clouds of atoms and that creates a latency or delay that depends of the relaxation time of excited vibrational states of the electrons.
When you look at the sky, you can only see the past because of the light, but you can't see what's happening in the present. Only the tangled communication could allows us to see the present.🙃
As has been pointed out, there is a mistake at 9:40. The time the Andromedan measures needs to be larger to make invariant interval the same. This makes sense if you recall the classic thought experiment of someone observing a person on a fast moving train: they will see the watch of the person on the train run slower than their own. In other words, the observer is measuring a larger time change in their own reference frame than the person in the train is measuring in their reference frame.
This really helps to understand why photons don't "experience" time so to speak. For anything, like a photon, to travel only through the 3 dimensions of space at a velocity of C, it must necessarily have a speed of 0 in the time dimension. It may take around 10 minutes from our perspective for a photon to reach us from the Sun, but to the photon the trip is instantaneous, along with all the rest of its entire existence. A difficult concept to make sense of, but thinking of it in terms of C being a value that is distributed amongst the 4 dimensions of spacetime, and photons allocating 100% of that value to only the 3 dimensions of space makes it a bit easier to understand. When we are at rest in space, we are moving through time at the fastest rate possible. When a photon is moving through space at the fastest rate possible, it is at rest in time, i.e. not moving through time at all.
No, that's not right, but a nice attempt. Photons don't travel any distance through spacetime (they're restricted to the null structure of the gravitational field, i.e. ds=0) so there's no meaningful proper time affine parameterization of a null curve (no way to measure out a length when there isn't any). There isn't any such thing as "space" or "time" for anything to move through.
If you travel against the rotation of something pulling on time like a a planet or galaxy itself flying against the rotation of the galaxy . at light speed . You'd travel as far back on time as time is moving forward.
There is a very large problem with this that I see people making. Consider the following. If a spacecraft sitting on Earth instantly leaves Earth travelling at 99% the speed of light away from Earth for about 6 months and then instantly reversed direction to come back to the same spot on Earth approximately 6 months in return time, then stopped at the starting point on Earth, the amount of time that past on the spacecraft will have been significantly slower than the time that past on Earth. Thus about 1 year will have past on Earth where as about 14% of 1 year will have past on the spacecraft. But, if this is truly all relative, the the spacecraft can say it is the one standing still and not moving. So the Earth is the one which moved at 99% the speed of light for 6 months in one direction then 6 months coming back, thus about 14% of 1 year will have past on the Earth while 1 year past on the spacecraft. See any problem with this?
I think the main difference is that the spacecraft is dealing with acceleration when it instantly turns around and the earth is not and so they are two different situations.
@@MrMoose1347 this paradox is a misunderstanding of how it works. It does not matter whether you take the point of view from being on Earth or on the spacecraft, the time slow down happens to the spacecraft. This is because within the whole of the space that both Earth and the spacecraft exist within as they move relative to each other, there is a relative common slowest moving region of space where time is pulsing forward the fastest. Thus, that relatively becomes the local non-relative non-moving center. That is why the paradox does not happen in real life.
@@jaredf6205 Those trying to push the purely relative frame will say that since we are talking about instant acceleration at the end points, you could argue it is everything except the spacecraft moving from the point of view of the spacecraft. A different version of this could look at a rogue planet traveling at of 99% the speed light since it was created and it is the same age as the Earth. As it passes through the solar system in reality the time on that rogue planet would be pulsing forward more slowly than on Earth. This is because while things are relative, there is in fact a relative non-moving (slowest moving) common center for both the Earth and the rogue planet, compared to which both are pulsing forward in time slower than. That is why we do not get the time paradox happening in real life.
Ugh... I shouldn't have clicked on this one... It's definitely interesting!! But listening to just the first 2:39 of this video has me nauseous from vertigo..... 😢
You know when you're having a really bad week, and your brain hurts super easily and Murphy's law seems to have somehow gotten mixed up with Twitter to produce the most hateful things you've ever read and making you fear how much evil is in the world? And then your best friend comes over, and says 'check this out, it's so cool!' and instead of a nice simple pun or fun analysis of which dinosaur pooped the most, they start talking about relativity, the speed of light, relative motion, space time, and the time dilation side effect and then they REALLY get going. I love you SciShow but you're making my head hurt so much and giving me existential dread!
Elsewhere I've heard it said by physicists that c is more correctly the speed of causality. P.S. for those who don't seem to grasp what is being discussed - ua-cam.com/video/msVuCEs8Ydo/v-deo.htmlsi=Q_b0mPVxijA3FuoP
@@Zebred2001 Yeah, it's wrong. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light. And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously. You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
@@kylelochlann5053 No less sensible than applying a pseudo spacial speed to time in relativity. To say time travels at the speed of light in a spacial dimension at 'c' in m/s unfortunately implies distance in time "Speed" :)
@@axle.student It's not about being sensible, it's about being wrong. Time-like curves are causal, i.e. there's an infinite number of speeds of causality.
@@kylelochlann5053 "Furthermore, it's not even *sensible* to say that causality can have a speed." "It's not about being *sensible* , ..." Back on topic. Any way, to talk about the max "speed" of light and max speed of causality are typically considered the same. It's just a time/period measured by a clock. To talk about the speed of time as a physical motion in space is blurring a measurement with reality. Time has no physical motion. So no, we don't physically move at the speed of light :)
Excellent explanations of everything - especially a simple explanation of the mathematics involved in four dimensions, which is what spacetime is based on.
I watched a video recently that discussed the fact that tone is just a result of percussion being sped up to the point that humans perceive it as as a continuous tone. Is it possible that time actually is a fourth dimension of space but we move so quickly through it that we experience it differently than the other dimensions of space?
So what happens when an extra-universal creature/ being is watching our planet from an outside perspective and makes c the expansion of space which is faster than light?
From what I learned from another Physicist's channel - C isn't the speed of light - it's the speed of causality. A photon can't travel faster than the speed of causality.
The "speed of causality" is just laughable. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light. And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously. You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
Meters per second combines both the time direction and the space direction from the reference frame of the observer. From the reference frame of a photon, the time direction is 0 and the space direction is the speed limit c. So yes, we have a very precise understanding of c for any thing in any given reference frame. Massless objects move through time at 0 speed and through space at c.
This was really well explained! I'm in that knowledge bliss state, right now, thanks to your team!!! ❤️🔥 Amazing usage of those equations and graphics, script, and presentation!
On some level every time I hear / learn more about things like relativity, the more I come to a conclusion that in the grand scheme of things none of it matters... while also being extremely important.
Woah. I've been hearing about spacetime and relativity in various science and science fiction contexts for years, and this is the first time someone has managed to present the math in a way that my intuition could glue up and connect with. Thanks, SciShow.
This is something I thought up in high school but I didn't have a good enough understanding of physics to fully explain it or justify it. It just kinda made sense to me. Glad to see this video.
c is sometimes called the speed of causality, because it's not only the upper limit for light, it's also the upper limit for any action at distance. In particular, c is also the speed of gravitational waves.
The "speed of causality" is just laughable. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light. And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously. You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
I feel like this did a good job specific to the point it was trying to make, but then completely failed to square/reconcile it with the fact that massive objects can never travel at the speed of light/causality.
To your other point, massive objects cannot move at the local vacuum speed of light through the spatial coordinates of an inertial observer in flat space. This is different than having a spacetime speed of c.
My first thought when you mentioned being still, was how the Earth is not only rotating, but also orbiting the sun, and the sun itself is actually orbiting around the galaxy, and that even the _entire milky way_ is moving through space. Is there actually _anywhere_ in the known universe where anyone _could_ truly be still? Between the myriad of gravatational forces and the expansion of the universe, I assume the answer is no.
No there is not. Everything is moving relative to everything else. Unless all the matter in the universe froze in place and the expansion of space stopped, nothing can ever be "still."
If you are traveling at the speed of light through an infinite vacuum of dark, empty space with no reference by which to gauge it, can you really prove you are moving at all?
@@MrMoose1347 So there is really no difference between traveling at the speed of light and standing when reference is removed. Like Schrodinger's cat, it seems that we are both moving at the speed of light and also not moving at all until viewed.
@ no. That has no bearing here. Energy takes time to travel. We know the limit of massless particles. Thus we know how to adjust to view the measure in any frame.
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Science: Only photons can travel at c because they have no mass. If you could travel at c, time would stop.
Also science: You are travelling at c right now.
..... please do not become one of those low hanging fruit mid-video non-adsense channels :( In *mutual respect,* plenty people watch non-adsense if at very front/back/both, rather than skip/click off =/
Finally! An adequate excuse for my partners dissatisfaction with regard to our nightly quantum entanglments ...
It's more accurate to say that c is the speed of causality.
The Speed of Light is extremely slow, when viewed from the vast distances even across our Solar System, let alone Milky Way distances. Scale the Sun-Neptune distance down to the length of a football field. Here an ant will walk faster than the speed of light.
They call me Mr Fahrenheit, I’m traveling at the speed of liiiight!
- F. Mercury
That is literally exactly all I could keep thinking of when watching this video. I think I wanna listen to some Queen now.
Freddie was way ahead oh his space and time.
@@Laochraiceann yes, but Brian May is a literal astrophysicist
Don’t. Stop. Me. NOWWEW
I wanna make a supersonic man outta you!
He knows 😳 1:19
Pink shorts, leopard print slippers. Nice choice!
💩💩💩
Lol
Is it not the best place for enlightenment?
Called! OUT!
🎶 C is for lightspeed
And that's good enough for me! 🎶
"186,272 miles/second -- it's not just a good idea, it's the law." A favorite bumpersticker of mine, from years ago.
I'm off to watch sesame street
And eat cookies love the comment❤❤
I'm from New Zealand where you
Who you calling a c😂😂
Another fun way to think about it: your movement through spacetime can be thought of as a 4-dimensional vector with magnitude of c. If you're sitting perfectly still you have zero motion in the space dimensions and your motion through time is just c, while if you have some significant motion through the space dimensions your motion through time must be reduced so that the magnitude of the vector is still c. Hence, time dilation.
Wow nice explanation
This is exactly how I picture it.
Almost correct. Remember time and space are one. It’s hard to us to wrap our brains around this concept because if we sit still we still in the same place on earth, our reference point was only the Earth, that would be true. But just rewind back to earlier to January of this year. The earth isn’t in the same place it was around the sun as now. Thus we moved in space and in time if the Sun was our reference point. It gets even crazier. We are getting close to starting a new year. So that means Earth will be the same place it was when it was last year in January. Yes, but only if you’re looking at the Sun as reference point. Zoom out to the galaxy as a reference point. The sun is moving around the Milky Way so the also is moving through space and time. That means that the earth is never in the same place in space time. It has moved with the sun around the Milky Way since creation. The same thing happens when you zoom out to universe as a reference point where time and space between galaxies is expanding.
Your example only sees earth as reference point but not the sun, or the galaxy which at those reference point totally shows that we are always in motion in time and space. It’s only when we look at the universe as a reference point do we realize everything moves at the speed of light.
@@JasonB808 Almost correct. 😉
There is no absolute universal reference frame as you're describing. Motion and position only have meaning in relation to other objects, and the Earth or your toilet are as valid as any of them.
If I understand the formula, we have (ds^2+dx^2+dy^2+dz^2)/dt^2 = c^2.
That does not make it a euclidian 4D space xyzt.
There's a mistake near the end (the bit starting around 9:40), as some others have mentioned. Yes, the observer will measure a larger amount of movement through space, but there's a minus sign in front of that term! So the amount of time they measure has to be higher, not lower.
💯 Though it gets confusing, because we're talking about "distance in time per unit of proper time." And, in terms of intuition, what ends up happening is that the relationship gets inverted.
That is, the core observation is: *the faster you observe something moving through space, the **_slower_** you observe their clock will be ticking.*
Thanks for explaining that I thought I didn't get anything by the end of the video
The entire video is misinformation.
@@kylelochlann5053is ignorant.
@@kylelochlann5053 Yup. If you define an inertial reference from for yourself where your velocity is zero, and then conclude your velocity is the speed of light, you automatically contradicted yourself. Your velocity can't be both zero and the speed of light in the same sense at the same time.
If the cops find out about this, speeding tickets are going to go crazy.
Dude, you were going 670,616,629 in a 30 zone
But if they know how fast you were going, how do they know where you were?
Excuse me sir, do you know how fast you were going?
@@theswiv
Unfortunately we are not particles…
If those cops could read, they'd be very upset.
It's so cool hearing about all these other people involved in the process. It's a reminder that even people who we think of as lone, super geniuses in pop culture still stood up on the shoulders of those before them in order to make their contributions
Hendrik Lorentz and Henri Poincaré developed principle relativity and Einstein took their research and patents and credit.
@@mikerouch416 That's not true.
@@culwin look em up even Google says it
@@mikerouch416 OK I did, and it turns out you're wrong. What could your motivation be, I wonder? j/k I'm not wondering.
@@culwin I guess ya didn't look. Lorentz came up with transformation in special theory of relativity. Poincare then developed a way to synchronize clocks on opposite sides of oceans using principle relativity. Einstein got credit for not using the Ether as a model for lights travel in space. Before that it was called Lorentz-Einstein special relativity. Sorry you don't know how to use Google, I just typed the last name of each one followed by relativity and got all that.
I'm pooping at the speed of light!
C is for poopy, that’s good enough for me.
LoL explosive diarrhea is faster than the speed of light
1:24 OK, but what if I’m sitting on the toilet watching this on my Vision Pro, and I have the immersive environment of the Vision Pro set to the surface of the moon, which is one of the options? Yes, one of the benefits of having a Vision Pro is that you can take a dump on the moon!
😅😅
You're not, you're moving away from the poop at the speed of light.
According to Pokémon Go I am always going too fast
people still play that? 😳
Deep cut!!
I'm going so fast not even the ban wave can catch me 😉
@@graphixkillzzzNo, people don't play it any more, they spoof it.
If only another 10,000,000 people had Pokemon Gone To The Polls 😕
"Now the universe itself keeps on expanding and expanding, in all of the directions it can whiz! As fast as it can go, the speed of light you know, 12 million miles a minute and that's the fastest speed there is...." -- "The Galaxy Song" written by Eric Idle of Monty Python (it was in "The Meaning of Life" and has a really cute video of it on UA-cam).
The song is remarkably accurate, actually. He rounded up to the 12 million miles a minute, however (it's 11-something million really).
I'm pretty sure I've seen the "butthurt version" with Brian Cox or Dara O'Brien or Neil Tyson, singing along and being "scientifically accurate." and it was funny as hell. :D
EDIT: I remember I'm on youtube, found it: ua-cam.com/video/_IFGH11uxaI/v-deo.html
Makes you feel so, so, unsignificant, doesn't it?
Can we have your liver then? 💀
A relevant fact is that the expansion of the universe is actually much faster than the speed of light. This is because the "expansion" isn't actually physical objects moving apart in space; rather, it's the space itself which is expanding.
It's like if I have two marbles sitting on a piece of clay, and I flatten the clay so it gets wider. The marbles (planets, stars, etc.) will end up further apart, but they won't have actually moved relative to the clay itself (spacetime).
Distance is a geometric property of spacetime which we call the metric (really a pseudometric because of the sign difference). A change in the metric doesn't involve anything actually moving, because spacetime isn't an object, it's a space that has objects inside it. What we call "expansion" is really a change in the metric that increases distances between things. This isn't a particle moving around; it's a change in the laws of motion themselves.
wtf is miles
It's funny, how we load ourselves with mind blowing ideas about how universe works, while we still have no idea about the connection between the Universe and life, and particularly our thinking and idea formulation processes. Sometimes I feel that instead of studying a great being we only study how it's nails grow...
9:42
I get you're trying to simplify it, but an increase in distance in space must be accompanied by an increase in distance in time, not a decrease. It's subtractive, not additive.
If
t - x = s
then
(t + 1) - (x + 1) = s
t + 1 - x - 1 = s
t - x = s
t goes up, x goes up.
I'm replying to boost this comment higher. I've just said the same thing before checking to see if anyone else had already said it. (BTW, you meant "t - x = s" in the last line, not "t + x = s".)
yea lol, that line confused the heck outta me.
Yeah, they showed a formula where something on one side of the equation is inversely related to the other thing on the same side of the equation. That works in multiplication but not in addition/subtraction. If add to the amount you subtract, the other value has to put back what's being subtracted by also going up. I don't know what they were thinking and trying to say.
They're squaring both components.
This turns a quantity (x + a)² into x² + 2ax + a²
This would turn the quantity (x - a)² into x² - 2ax + a²
So an increase in distance correlates to adding 2ax + a² to the original value.
To conserve the quantity, the time² must decrease by an amount that offsets this to make it the same.
@@philp4684 ope, good catch, thanks
1:24 how do you know ?
😂
bruhhhhh @codydickinson my eyes went 🫨
scishow goes hard
This is why I prefer to call c the Speed of Causality.
YES! YES! YES! YES! I have an ally here!!!!!! You're now my new friend. (EDIT: been preaching this for years)
Same
+1
Isn't that why it is "c?"
@@raymondwall8187 I had believed "c" is for "constant". Kindly correct me if I'm mistaken.
And while I'm not proposing any hypothesis, I wouldn't be surprised if we discover that, as it seems with Hubble constant, it varies throughout the universe. (disclaimer: not a real physicist)
I'm sitting in rush hour traffic on I-95 approaching Washington dc. I ain't moving at the God damn speed of light.
😂
Actually, all of you guys are _not moving_ at the speed of light.
Been there done that for 2 years down to exit 15 then up to Philly for my second stop and final stop in Trenton NJ. 2 years was all I could handle of it, and I usually got in there before morning rush hour.
And that's why it seems to take longer :D
Coming from NoVA?
I almost had a two second period in which I understood the space-time equation. Then it faded and I’m confused again. But it gives me hope that with enough study, I COULD one day understand it. 😁
"I'm sorry, officer, it is physically impossible for me to go slower than the speed of light."
The explanation of time dilation at 9:45 is completely wrong.
An external "stationary" observer measures a LARGER time interval than the "moving" observer measures.
For the "moving observer", in their own reference frame, ds^2 = (cdt)^2, as previously explained correctly in the video.
But for the external observer, dx^2 + dy^2 + dz^2 is non-zero. For ds^2 to remain the same, the external observer's (cdt)^2 must INCREASE by the value of dx^2 + dy^2 + dz^2.
He's just copying this from popular sources, he doesn't actually understand on a theoretical physics level. Just copying almost exactly
The video is almost entirely wrong.
That's the best, most easily understood explanation of relativity I've ever heard. Excellent job, SciShow!
Too bad it is wrong…
yeah was totally BS🤦
@@little_forest Really? why? Explain. I want to know...
@@MeltiahNye aliens
Had this exact conversation in Physics 2. I asked my professor if there was such a thing as a rate of travel through time and he disagreed with me because he didn't think there were units that could describe such a thing. I'm like, "dude, the universe doesn't even care if we have the units figured out."
I travel through time at the rate on one second per second. : )
Sometimes actual profs are wiser than ten youTubers who think that the phrase "travel through time" is even semantically, let alone physically, cogent.
Your prof is right, and you are not. Dimensional analysis, which is about considering units, is a very important tool in physics. From a mathematical perspective, it's unclear what "rate of travel through time" is even supposed to mean. Rate of travel through space is the distance travelled relative to time. If you used the same logic for rate of travel through time, it would always be 1.
A better thing to consider would be the angle between your trajectory, the space component, and the time axis. This isn't Lorentz invariant, but it's still interesting.
@@dorongrossman-naples9207 That's fair. Imagine if humanity had developed different units for travel in north/south vs east/west directions. It would be an additional layer of complexity in converting units but would still produce accurate results when calculated. If we really want to consider time as a 4th dimension, you could say waiting one second is the same as moving 300 million meters along the temporal axis. Interesting, but not really useful in real world applications I guess.
@@BrackenStrike people actually do do this! It's equivalent to setting c to 1, which is part of something called "natural units". The issue is that you still need to divide distance by distance to get a unitless quantity.
A visual way to express your speed thru space-time is to imagine a circle centered at the origin in 2-D with a radius of c. Now only consider the arc in the first quadrant (upper right).
The x-coordinate is your speed thru space. The y-coordinate is your speed thru time. (Speed is the mangnitude of your velocity vector. Direction doesn't matter for space.) The points along the arc are the corresponding speeds thru space-time allowed (which is fixed at c since that's the radius of the circle).
Notice: if you know your speed thru space, it's easy to find your speed thru time. If you are massless, your x coordinate would max out at c and your y-coordinate would be zero, indicating you do not experience time at all. If your speed thru space is zero (you are stationary and not inside a gravity well), then you travel at c thru the time dimension.
So it's easy to visualise that every particle/object with a real, non-negative mass travels at c thru space-time.
Careful here: space and time don't have any independent existence. Your interpretation of the axes is not correct. The y-axis is the distance along your world-line, the derivative (spacetime gradient) of which is your speed through your own defined time. The x-axis defines the set of locations (spatial hypersurface) at the start of your stopwatch, t=0.
@kylelochlann5053 I wasn't referring to a Minkowski diagram. I was describing a 2-dimentional plot of Speed through Space (condensed horizontal) vs Time (vertical) to help visualize why it's always c.
@@seanspartan2023 What does "speed through time" even mean? And isn't the speed through space always zero as you're taking your reference frame with you?
Also keep in mind that there's no independent existence of either space or time, hence "spacetime" (which itself has no independent existence).
@@kylelochlann5053 but light is massless ...doesn't time only bear upon entities of mass?
(I'm not a scientist, but awfully curious ....please be gentle)
@@horustwohawks The are few ways to think about all this that I've learned from others.
The first comes from Lewis Epstein: What we have is 4-dimensional space (no time) filled with particles that have mass and thus all moving at the speed of light. Because they move at the speed of light, the space along their direction of motion is length contracted to zero (which is why we can't access that past or future). The only "time" is the distance along each individual world-line (particle path).
The second comes from Willis Lamb and Geoff Jones where "photon" is word that describes the exchange of quantum state and momentum between matter particles that is instantaneous (instantaneous in spacetime). That is, to question what it means to even exist between emission and absorption.
So there is no sense in which "time" or "the distance along the path of a massive particle" can be applied to a photon. The massive particles (observers) have time and so measure the interval between emission and absorption.
Thanks!
Someone I once talked with years ago had this theory called "the stillness of light." His theory was light didn't move at all, but somehow everything else moved slower than light did. That's all I remember about it, and I don't believe it, but I still think about that from time to time.
Intelligent person.
Video: "You're moving at the speed of light."
Me: "I c."
Awful thing to say to someone who’s panic high
I'm dead.
😂😂😂😂😂😂😂😂😂😂😂😂
I am really enjoying these newer episodes (such as the one on optics) that go a bit deeper into the science than most of the past couple years of this channel!
In college, I took an advanced physics class unintentionally, due to a clerical error at the registrar. The only thing I actually understood from that class was special relativity... and that professor still never explained the whole "time dilation happens because we're all moving at the same speed through spacetime" concept. I only learned about it a few years ago, from another UA-cam video, and my mind? Fully blown 🤯 Science is so cool!
So the best and easiest way to explain it is you’re always gonna go C (remember that time and the space you move through are intrinsically linked and inseparable) at all times. If you have 0 (relative) motion, you’re going at what we can call a reference frame for time of 1. Now let’s say you go to half of the velocity of “the speed of light” (calling it that for simplicity). You have to still end at the same point of C, so your extra velocity means time is at half its usual reference frame (it’s now 0.5) because the other 0.5 for a whole reference frame comes from your velocity.
The best way to think of it is “velocity + time = C”. Increase speed and your time has to slow to keep C consistent. It’s why from its perspective, light doesn’t experience time. The velocity half of its equation is 1, so as a balance time has to be 0.
This is also where the paradox of FTL travel goes. If you go faster than that 1 velocity of light that makes time = 0, time HAS to be a negative number to compensate. If you go double that C velocity, time will be at a negative integer because it MUST be balanced for C at the end (based on physics as we know it). And insofar as said knowledge goes, time can’t be a negative value just as velocity can’t be a negative value. There are theoretical workarounds like the Alcubierre Drive, but those are really manipulations of the rule rather than a violation of them (I can explain that further if you want).
I started on a physics major path in college. It was mostly equations and details, without reference to how it would play out in the natural world. I lost interest and lost track of what they were talking about and switched majors.
The clearest explanation of this or any other difficult topic that I've seen in a long.time, maybe ever.
I don't know why everyone always says that reference frames are weird to think about and hard to grasp. It makes perfect sense to me. If I'm in the passenger seat of a car going down the highway, it doesn't matter to me that my phone is also going at 70mph. I can reach over and pick it up no problem. If someone on the side of the highway wanted to snatch my phone as I went past, it would be an entirely different thing.
Why is that hard to grasp?
Perfect understanding of Galilean relativity, which is an important step to understanding special relativity. You are 100% correct that physicists and mathematicians often make things sound more complicated than they are. In this case, the part that could be tricky is the fact that the speed of light is a finite constant no matter what speed you're going, and everyone would measure an identical speed for the same photon. Reconciling this with being able to trivially handle a phone moving at 70mph by also moving at 70mph is what leads to time dilation. It's possible to derive the Lorentz equations from these two facts, plus a few ones generally considered too obvious to mention, like how moving at 70mph means everything else looks like it moves backwards at 70mph.
This is the best video version of this topic that I have ever come across. I love that you show the math at a level where anyone that works numbers to solve problems can understand the topic in it's intended content.
As someone with existential anxiety, I don't need this right now scishow 😂
Nah, take it in. It can help
It’s not like you have agency over this.
You exist. One day, you won't. Eventually, there will be no record of your mistakes. Do your best.
@@ThraerynHow bleak 😬 but true.
How does reference frame relative to speed triggers your existential anxiety? Pretentious
probably the best host for scishow! thank you Stefan!
6:10 train the scientists in literary skills so they can translate their findings for the press, and train writers in the sciences so they _stop clickbaiting with titles that say the opposite of the research findings_
Journalist: "Scientist discovers black hole in his bathroom."
Scientist: "STOP BEING A PEEPING TOM!"
9:50 isn't quite right. If S=T-X and X goes up by an amount y, then T also needs to go up by y if S is to remain constant: S=(T+y)-(X+y)=T-X.
In terms of interpretation, the distant observer is using their own clock. Yours is slower relative to theirs, but theirs is equivalently faster relative to yours, meaning they measure more ticks of the clock and record a higher T value than you do any time they record a higher X value.
Basically the entire video is wrong. For example he sets dx=0 to get "time dilation" but if dx=0 then the traveler isn't in relative motion, so there's no time dilation.
As I recall from physics class: everything travels through spacetime at the same velocity.
My thought experiment for this is to define three frames of reference. First to fire a photon/lightwave along a vector/numberline X starting from X=0. This is reference 1. From a Stationary perspective, reference 0, the Lightwave is moving some speed in a postive X direction. The Lightwave arguably can view itself traveling in a positive X direction. That Lightwave, immediately once fired, fires it's own Lightwave behind itself, creating reference 2, in the X vector in a negative X direction. So from the stationary reference 0, Lightwave 1 is traveling +X and Lightwave 2 is traveling -X. From Lightwave 1's perspective while traveling positive, Lightwave 2 is at X=0. And from Lightwave 2's perspective while traveling negative, Lightwave 1 is at X=0.
As such, the only way to rectify how both photons can 'see' the other as stationary is to apply time dilation to such a degree that time has effectively 'stopped' for the other in their perspective.
In other words, Photons travelling at the speed of light do not 'age' in reference to other Photons.
So I ask the question. From the persepective of itself, does a Photon age? and if so, what does it experience? Does it travel to the infinite 'end' of the X vector in one infantesimially small duration of time? A photon from a distant star travels a vacant path till it happens to excite an electron in the cluster of mass that is a Human eye in a degree of time? If a Photon does not 'age', then the distance between the star and the human eye is a non-factor and from a photon's perspective, we still exist inside the singularity?
It is a difficult question and has no answer in relativity, other than to say massless particles have NO concept of time or time dilation according to Mr Albert :)
Time dilation can only be applied to particles and objects of mass.
>
So officially does a photon experience zero time or does it experience time is an invalid question.
.
It is an interesting thought though and even bothered old Mr Albert somewhat when he was working it out, and even later in life :)
@@axle.student There is so much to say and think about this, but then again this is YT comments, lol. I can't say I agree that it's an 'invalid question' considering photons ARE effected by objects that exist within the dimension of time, which means that Time has at least an indirect effect on massless objects. But I do agree that whether or not a photon in isolation experiences time is basically unknowable/invalid without a radically new understanding of physics.
@@bosslca9630 Kool kool
I did a number of thought experiments where I gave a photon minimal mass so it could behave a little more like other particles, reduced its velocity by 0.0...1% from 'c' and allowed it to have time dilation.
.
It goes from a particle that experiences Zero time on its internal clock, to it's clock moving so slow that it experiences infinite time instead.
That is 0 = never existed by its clock, to infinitesimally slow clock and existing for an infinite time internally.
It's a big difference from such a small change :)
An observer is any time-like curve. Light moves along null curves and so can never be an observer.
@@kylelochlann5053 That's the standard mathematical answer derived from relativity. But the question is more related to the definition/structure of relativity itself.
.
So based around the misconception that photons experience no time. Although there is an issue within the fundamental definition of a photon and how it behaves.
I guess it will always be a point of ambiguity :)
It’s like watching SpaceTime but instead of being incredibly confused I am only partially confused 😂
It's like Pythagoras theorem but in 4D! That's mind opening 😮
I'm going to need a Space Talk with Neil and Lord Nice to discuss this.....
Good idea.😊
😂😂😂😂
Neil has actually covered this multiple times. He presents it differently. By saying there's a way of interpretation of the space time interval like this.
Chuck got knighted !?
I haven't watched them in a good while. Might as well get to it while science is still allowed 😂
Gentlemen, they said it wasn't possible, yet here we are piloting our porcelain thrones at the speed of light. Godspeed my fellow travelers. May your journey bear fruit and your destination be all that you hoped and more.
I am not ashamed to admit this one's over my head
that's on you for using derpy's pic instead of twilight's
It does not slow down… it excites and de-excites in the electron clouds of atoms and that creates a latency or delay that depends of the relaxation time of excited vibrational states of the electrons.
This doesn't look like pbs space time?
The added clip of runners and stopwatch in a hand gave a lot of depth unexpectedly!
11:00 rename c "the speed of everything" and change the symbol from "c" to "42"
I think we should honor the constant denoted by the letter "c" using its full name; celeritas.
0:36 People already keep telling me that and you too, Internet?!
This video gave a lot of explanations I have been searching for for years, thanks
"You are travelling at sea, right now." Not a controversial statement to a sailor.
When you look at the sky, you can only see the past because of the light, but you can't see what's happening in the present. Only the tangled communication could allows us to see the present.🙃
I can count on SciShow to Brighten my day 🤣
As has been pointed out, there is a mistake at 9:40. The time the Andromedan measures needs to be larger to make invariant interval the same. This makes sense if you recall the classic thought experiment of someone observing a person on a fast moving train: they will see the watch of the person on the train run slower than their own. In other words, the observer is measuring a larger time change in their own reference frame than the person in the train is measuring in their reference frame.
wat.
real
real
C
real
real
This really helps to understand why photons don't "experience" time so to speak. For anything, like a photon, to travel only through the 3 dimensions of space at a velocity of C, it must necessarily have a speed of 0 in the time dimension. It may take around 10 minutes from our perspective for a photon to reach us from the Sun, but to the photon the trip is instantaneous, along with all the rest of its entire existence. A difficult concept to make sense of, but thinking of it in terms of C being a value that is distributed amongst the 4 dimensions of spacetime, and photons allocating 100% of that value to only the 3 dimensions of space makes it a bit easier to understand.
When we are at rest in space, we are moving through time at the fastest rate possible. When a photon is moving through space at the fastest rate possible, it is at rest in time, i.e. not moving through time at all.
No, that's not right, but a nice attempt. Photons don't travel any distance through spacetime (they're restricted to the null structure of the gravitational field, i.e. ds=0) so there's no meaningful proper time affine parameterization of a null curve (no way to measure out a length when there isn't any). There isn't any such thing as "space" or "time" for anything to move through.
If you travel against the rotation of something pulling on time like a a planet or galaxy itself flying against the rotation of the galaxy . at light speed . You'd travel as far back on time as time is moving forward.
Neat!
Well, no.
There is a very large problem with this that I see people making. Consider the following.
If a spacecraft sitting on Earth instantly leaves Earth travelling at 99% the speed of light away from Earth for about 6 months and then instantly reversed direction to come back to the same spot on Earth approximately 6 months in return time, then stopped at the starting point on Earth, the amount of time that past on the spacecraft will have been significantly slower than the time that past on Earth. Thus about 1 year will have past on Earth where as about 14% of 1 year will have past on the spacecraft.
But, if this is truly all relative, the the spacecraft can say it is the one standing still and not moving. So the Earth is the one which moved at 99% the speed of light for 6 months in one direction then 6 months coming back, thus about 14% of 1 year will have past on the Earth while 1 year past on the spacecraft.
See any problem with this?
This is why the twin paradox is a paradox 😊
I think the main difference is that the spacecraft is dealing with acceleration when it instantly turns around and the earth is not and so they are two different situations.
@@MrMoose1347 this paradox is a misunderstanding of how it works. It does not matter whether you take the point of view from being on Earth or on the spacecraft, the time slow down happens to the spacecraft.
This is because within the whole of the space that both Earth and the spacecraft exist within as they move relative to each other, there is a relative common slowest moving region of space where time is pulsing forward the fastest. Thus, that relatively becomes the local non-relative non-moving center. That is why the paradox does not happen in real life.
@@jaredf6205 Those trying to push the purely relative frame will say that since we are talking about instant acceleration at the end points, you could argue it is everything except the spacecraft moving from the point of view of the spacecraft.
A different version of this could look at a rogue planet traveling at of 99% the speed light since it was created and it is the same age as the Earth. As it passes through the solar system in reality the time on that rogue planet would be pulsing forward more slowly than on Earth. This is because while things are relative, there is in fact a relative non-moving (slowest moving) common center for both the Earth and the rogue planet, compared to which both are pulsing forward in time slower than. That is why we do not get the time paradox happening in real life.
@@John_a_Technocrat it’s a paradox because relative to eachother they age differently but both can’t be older at the same time
My ADD is so bad that that ad being there totally erased everything before it. That sucks because I don't have time to watch this twice.
i'd recommend installing sponsorblock on your browser, it automatically skips ads
Great video. You made this topic so much easier to understand.
Ugh... I shouldn't have clicked on this one... It's definitely interesting!! But listening to just the first 2:39 of this video has me nauseous from vertigo..... 😢
I know, right? I'm dizzy.
Nauseated, I am - from your nauseous grammar
Just kidding! Making a joke pls don't get offended!
Wait what? Vertigo from what?
Definitely one of the best explanations of the concept I've seen. Thank you!
Holy hell, I needed to take that edible before that one.
bro moving at any speed while pooping is terrifying thank you for putting that mental image in me brain
You know when you're having a really bad week, and your brain hurts super easily and Murphy's law seems to have somehow gotten mixed up with Twitter to produce the most hateful things you've ever read and making you fear how much evil is in the world?
And then your best friend comes over, and says 'check this out, it's so cool!' and instead of a nice simple pun or fun analysis of which dinosaur pooped the most, they start talking about relativity, the speed of light, relative motion, space time, and the time dilation side effect and then they REALLY get going.
I love you SciShow but you're making my head hurt so much and giving me existential dread!
Glad to see this video reaching the masses. Most people haven't thought this through to understand it
Elsewhere I've heard it said by physicists that c is more correctly the speed of causality.
P.S. for those who don't seem to grasp what is being discussed - ua-cam.com/video/msVuCEs8Ydo/v-deo.htmlsi=Q_b0mPVxijA3FuoP
No, that doesn't work because causal curves can be time-like. Furthermore, it's not even sensible to say that causality can have a speed.
@@kylelochlann5053 ua-cam.com/video/msVuCEs8Ydo/v-deo.htmlsi=PH9kNZ5hUJRYw1Of
@@Zebred2001 Yeah, it's wrong. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light.
And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously.
You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
What does that even mean? It's gibberish.
Ya i knewt just hope to remenber when Will be needed ...😮
Nothing like already being online listening to SciShow when they drop a new video🎉
Constantly violating the speed limits on my bicycle here.
dude, i am a theoretical mathematician and i cannot wrap my head around this part of physics 😭😭
10:59 The speed of existence
i heard the speed of light actually being the speed of causation
The speed of light is measured to just happen to be the same as the speed of causality :)
No, that doesn't work because causal curves can be time-like. Furthermore, it's not even sensible to say that causality can have a speed.
@@kylelochlann5053 No less sensible than applying a pseudo spacial speed to time in relativity.
To say time travels at the speed of light in a spacial dimension at 'c' in m/s unfortunately implies distance in time "Speed" :)
@@axle.student It's not about being sensible, it's about being wrong. Time-like curves are causal, i.e. there's an infinite number of speeds of causality.
@@kylelochlann5053 "Furthermore, it's not even *sensible* to say that causality can have a speed."
"It's not about being *sensible* , ..."
Back on topic.
Any way, to talk about the max "speed" of light and max speed of causality are typically considered the same. It's just a time/period measured by a clock.
To talk about the speed of time as a physical motion in space is blurring a measurement with reality.
Time has no physical motion.
So no, we don't physically move at the speed of light :)
make "wat." the most likes comment.
You've failed miserably in your quest.
'C' is truly the slowest, and fastest anything can be.
It’s in my name
Excellent explanations of everything - especially a simple explanation of the mathematics involved in four dimensions, which is what spacetime is based on.
I watched a video recently that discussed the fact that tone is just a result of percussion being sped up to the point that humans perceive it as as a continuous tone. Is it possible that time actually is a fourth dimension of space but we move so quickly through it that we experience it differently than the other dimensions of space?
Brilliant. One of SciShow's best!
So what happens when an extra-universal creature/ being is watching our planet from an outside perspective and makes c the expansion of space which is faster than light?
From what I learned from another Physicist's channel - C isn't the speed of light - it's the speed of causality. A photon can't travel faster than the speed of causality.
The "speed of causality" is just laughable. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light.
And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously.
You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
I appreciate that you're using meters per second so no one actually has a good understanding of how fast it is
What would you prefer? Football fields per second?
Meters per second combines both the time direction and the space direction from the reference frame of the observer. From the reference frame of a photon, the time direction is 0 and the space direction is the speed limit c. So yes, we have a very precise understanding of c for any thing in any given reference frame. Massless objects move through time at 0 speed and through space at c.
c = 1s/s
This was really well explained! I'm in that knowledge bliss state, right now, thanks to your team!!! ❤️🔥
Amazing usage of those equations and graphics, script, and presentation!
Strangely enough, this actually helped me understand black holes better
Awesome explanation. Thank you.
On some level every time I hear / learn more about things like relativity, the more I come to a conclusion that in the grand scheme of things none of it matters... while also being extremely important.
This is the first SciShow video I couldn't follow.
Woah. I've been hearing about spacetime and relativity in various science and science fiction contexts for years, and this is the first time someone has managed to present the math in a way that my intuition could glue up and connect with. Thanks, SciShow.
This is something I thought up in high school but I didn't have a good enough understanding of physics to fully explain it or justify it. It just kinda made sense to me. Glad to see this video.
From the perspective of the light, any trip, no matter how far, appears instantaneous. So crazy. I love how Interstellar explores that sort of thing.
Time seems to move faster when I'm at rest, chillin' on the sofa.
c is sometimes called the speed of causality, because it's not only the upper limit for light, it's also the upper limit for any action at distance. In particular, c is also the speed of gravitational waves.
The "speed of causality" is just laughable. Sound can carry information and electromagnetic signals are carried along wires, and they both travel at less than the speed of light.
And then there's much slower causality, chemotaxis in microorganisms, sending a letter, or just plain throwing a rock through a window, and so on. Adding to this there's quantum mechanical considerations such as the update of the wave function which happens instantaneously.
You could say that "c is the upper bound the infinite set of speeds along causal curves", but that doesn't roll of the tongue so nicely.
Bravo! Nicely formulated
I feel like this did a good job specific to the point it was trying to make, but then completely failed to square/reconcile it with the fact that massive objects can never travel at the speed of light/causality.
To your other point, massive objects cannot move at the local vacuum speed of light through the spatial coordinates of an inertial observer in flat space. This is different than having a spacetime speed of c.
My first thought when you mentioned being still, was how the Earth is not only rotating, but also orbiting the sun, and the sun itself is actually orbiting around the galaxy, and that even the _entire milky way_ is moving through space.
Is there actually _anywhere_ in the known universe where anyone _could_ truly be still? Between the myriad of gravatational forces and the expansion of the universe, I assume the answer is no.
No there is not. Everything is moving relative to everything else. Unless all the matter in the universe froze in place and the expansion of space stopped, nothing can ever be "still."
Yes, anywhere. Just attach a reference frame to yourself.
I was scoffed at for postulating this to someone long ago, so this is validating. Thanks!
If you are traveling at the speed of light through an infinite vacuum of dark, empty space with no reference by which to gauge it, can you really prove you are moving at all?
No because you have to frame your reference points to gauge a motion to prove a motion
@@MrMoose1347 So there is really no difference between traveling at the speed of light and standing when reference is removed. Like Schrodinger's cat, it seems that we are both moving at the speed of light and also not moving at all until viewed.
@ no. That has no bearing here. Energy takes time to travel. We know the limit of massless particles. Thus we know how to adjust to view the measure in any frame.
very nice explanation
Wow this was a top notch episode thank you so much.
"you're moving through spacetime"
if this was PBS Spacetime, that would've been the coldest title drop of all time.
If the d[xyz]^2 terms are all subtracted from the dt^2 term, then if d[xyz]^2 increase, doesn't dt^2 have to *increase* to keep ds^2 constant?
yeah, i was confused about that as well.
Yes, but then again, most everything in the video is wrong.
Very good . . Thank you 👍