Hi again Prof. Greene, I would just like to say that your story of the 2 Presidents on the train, is one of the best & easiest-to-understand explanations of Simultaneity I have heard. It is both simple and enlightening simultaneously ! Thanks again.
Brian Greene, you’ve done as much for science as Albert himself. You inspire millions to follow suit with your ability to communicate complex topics in such an easy to understand and enticing fashion. Keep it up my dude, you may even inspire the next Einstein.
@@nickbros If you're talking about him educating more people about science then I'd 100% agree. If you believe he's done more for science in general compared to Einstein, well... No.
Brian your smart. I listened to Lenny Leonard Susskin. But Len said he proved Hawkins's wrong about information can't be lost. But you're my favorite. You have a good calm pace. I listen to you because there is not enough information. But you teach as much as possible. I want to learn new thing's about the universe and I wish we could learn something spectacular and new everyday. With the tools we have limits us but I could only amagin 400 years from now.
Hey Brian, I just want to thank you for taking the time out of your day to create these videos. They deepen my understanding and nurture my curiosity. I have the utmost respect and appreciation for all the work you do. I hope to see you at World Science Festival one day!
Dr.Brian you've got no idea how much this Biologist/Relativity enthusiast appreciates these equations of relativity which are immaculately explained by high school level Mathematics !
Dr. Green, I wanted to thank you for your incredible explanations and taking the time to post these videos. This is the most access I have had to most of these enlightening equations and your teaching is second to none. I’m only on number 4 of your daily equation videos but I’m hooked. Thanks again.
I really appreciate you putting these out at a time when amateurs like myself have time to really dig into what you are saying. COVID-19 is terrible, but at least many of us are getting a chance to expand our scientific understanding thanks to you. Excellent work!
Thanks Brian for your efforts in this. FYI...I am 68 years old. Have a BSC in Math and computer but have worked in the area of broadcast TV engineering for what seems like a millenium. I read a book years ago called “ 1-2-3 Infinity” that really captured my interest. One concept that was presented in the book was...as an observer on Earth, a photon leaving the sun would take 8ish minutes to arrive to visit me. However...if I were that photon...at the moment I decided to visit earth, because I am travelling at the speed of light, this visit would happen immediately. No time would pass. Ie...no mass, at the speed of light. I’d love to hear your thoughts.
This remarkable fact about the lack of simultaneity seems to be a fitting episode as a follow up of the first three in special relativity. I am curious to see where we will go from then - to the General Relativity/equivalence principle, or elsewhere?
Excellent... wow... getting it; that helped and brought recall... exciting stuff, because you see the world is not just ideas or limited to your eye's witness, but is in a phenomenal cause and effect motion... love it...thank you
I suspect that if the speed of light were much lower (say 50 mph) we would have evolved so that these concepts would be intuitive to us. Our intuition tells us that if we see something happen in the distance we are seeing it as it happens and that is what makes these ideas difficult to comprehend even if logically we can understand that they are correct. I love the way you make these ideas as clear as they can be from an experince perspective.
I have a feeling that people say, wow cool and move on without contemplating the real life implications of the relativity of simultanity. this idea helped me deal with loss of loved ones and my own anxiety. Once you realize that the future and past are actually more real than the illusion of "now" that you experience, greef is much more manageable. As Einstein himself said to a widow, "It's not at all like your loved one was yanked out of existence by death. He is still there over a hill. You just can't see him"
Say we have detectors at both ends of the train instead of heads of state. The detectors do two things the moment they detect the light beam coming from centre: stop the train and shoot a beam of light back at the centre. In the centre there is a third detector that turns a signal on but only if it is hit by two opposite beams at the same instant. So from the perspective of someone in the train the centre detector will receive the beams at the same time and turn on the signal but from the perspective of someone outside the signal stays off. What we have here is that the signal is both on and off at the same time and the train (of thought) collapses when someone measures the result :)
The signal from the perspective of someone outside the train will see it on because from the perspective of someone outside the Train the detector only turns on if the signals come at different times.
16:24 Two additional points. (1) Typically, we replace c/v with the lower-case Greek letter beta. (2) the equation can also be written using gamma with gamma being equal to 1/sqrt(1 - beta^2) so we would have gamma squared.
Thank you, Brian. This has been a roadblock for me for a long time and I now better understand :) Your example is much better than the normal train/platform/lightning strikes example which completely sidetracked me. Thank you for these *excellent* videos, I appreciate your time and efforts.
Thanks for giving the actual equations in a simple way! looking forward to see how you will do this for Einstein's field equations and Maxwell's equations
As v approaches c, then time difference becomes indeterminate, i.e. L/0, as does the gamma factor. What's more, from my understanding, as v approaches c, then length contracts to zero and time dilates to zero as well. It's a mind-bending notion!
This video reveals what happens to simultaneity under constant velocity (and Dr. Greene mentioned what happens for the special constant velocity condition of delta(v)=0), my question: does the same relationship of simultaneity hold for constant acceleration, or even any pattern of motion?
Thank you for sharing 6:35 Don't you think there is an inconsistency in the reasoning for the two perspectives? In the first scenario, we believe that the light emitter is on the train, moving with the train. In the second scenario, we kind of consider the two light photons coming out of nowhere, as if it suddenly appears in the train and emits the photons. However, the reality should be that it is always on the train. Or, at least, the light emitter is either moving with the train or not moving with the train, it should not be changed because of different perspectives. ?
The photons, once left the light bulb, travel at a constant speed regardless how fast the train is moving. It is indeed true that the photons travel a short distance to reach the forward facing president. But the observer in the train do not get to see it immediately. It takes time for the photons bounced back from the forwarding-facing president and their pen to travel to the eyes of the observer in the train. Now don’t forget that the observer is moving forward with the train, so these photons will have to play catch-up to the observer, and it will take more time than if the observer was stationary. Now if you consider the round-trip, the effects cancel out. So somebody standing equally far from the two presidents in the training will end up seeing the signing events at the same time. To other people in the train who are closer to one president than another, they would SEE the events in different orders. In the extreme case, to either of the president, they’d both see themselves signing first. I think these details are omitted in the video, but it is totally true that observers won’t agree on the order of events depending on their position and frame of reference.
Thanks for this great video series! In 16:30, I am a little confused by whether the length L is the one that contracted or not. Should we put "length contraction" into account in this derivation?
Sir, thank you for this series. I am late and try to catch-up, but I have a question on this: To determine the middle of the train I place one mirror at the front and one at the back end of the train. I use an apparatus that emits light flashes in both directions and rings a bell when it receives the reflections simultaneously. When people on the train position the device so that they can hear the bell ring, will the people on the platform disagree and don’t hear the bell ring? Obviously not. But we’re is my mistake? Any comments are welcome. Thank you!
Navier Stokes!!! please!!! Brian i have a long time to go until i reach your level and become one of your peers. Im a self taught graphics software engineer, and i just LOVE the complexity of modeling fluids on computers. After i get my finances in check, Im going to study physics more :) I LOVE PHYSICS & MATH!!! SOO COOOOOL!!!!!
In general, one cannot claim to be the first, as someone else in a different inertial frame can also claim to be the first, and it all depends on your point of view!
Thanks for the video Brian. One thing I do not get though: should one not take relativistic lenghts into account, so L would be L/gamma due to contraction of the length of the train as observed from the platform?
This is the difference in time for the people on the platform (assuming actual simultaneity for people on the train). My question is: does the gamma factor appear (when comparing the difference in simultaneities ) IF simultaneity is assumed for people on the platform and the difference in time for simultaneity is calculated for people on the train?
It does not take relativity of simultaneity to have constant speed of light. We can take the equations that physicists threw away in 1887 and show constant speed of light. x'=x-vt y'=y z'=z t'=t If there is a slower clock in a moving frame of reference, as Einstein describes, all we have to do to show constant speed of light in both frames of reference is to use another set of Galilean transformation equations with different variables for time and velocity. So the inverse equations as seen from the frame of reference with the slower clock would be x = x' - (-vt/n')n' y = y' z = z' n = n' n' is the time of the slower clock. (-vt/n') is the velocity of frame of reference S(x,y,z,n) relative to frame of reference S'(x',y',z',n'). n = n' shows that the time of the slower clock in S' is being used in both frames of reference. We can show that these equations agree with the first set of Galilean transformation equations by cancelling out the (n')'s. x = x' - (-vt/n')n' x = x' + vt t = t' So now all we have to do to show constant speed of light is to say x = ct and x' = cn'. cn' = ct - vt n' = t - vt/c This value for n' is actually the same as the value of the numerator in Lorentz's equation for t'. t - vt/c = t - vct/c^2 = t - vx/c^2 The x in this expression is unnecessary in the Galilean transformation equations because there is no length contraction in the Galilean transformation equations.
Question: Thought experiment...What if instead of measuring the 2 differing perspectives, they each sent a live video(at light speed) of said view point(s). Would they still arrive & agree on the relativity of simultaniety?? Or would things somehow be self consistent for both observers?
THE PARADOX THAT DISPROVES RELATIVITY: There is a triangle of lights which we will call A, B, and C. They flash simultaneously in the frame of reference that is at rest relative to these lights. There is someone moving at a high rate of speed from B to A. There is someone else moving at a high rate of speed from C to B. There is someone else moving at a high rate of speed from A to C. So A flashes first and then B flashes and then C flashes and then A flashes again. How can A flash twice? When A flashes has B already flashed or not yet flashed? or B flashes first and then C flashes and then A flashes and then B flashes again. How can B flash twice? When B flashes has C already flashed or not yet flashed? or C flashes first and then A flashes and then B flashes and then C flashes again. How can C flash twice? When C flashes has A already flashed or not yet flashed?
I understand that there can be no agreement on the “perception” of simultaneity, but does that really eliminate the idea of universal time? This is similar to the Doppler effect… a musician plays a C note, the stationary audience hears a C note, a driver approaching the musician will hear a D, and driver heading away hears a B note. But the actual note is a C. Does that analogy not apply to simultaneity as well? In “universal time” two things happen at the same time, but observers will have varied perceptions. With the example of space traveler moving close to the speed of light who only ages 5 years while his family on Earth has aged 50. At some point he returns to Earth to meet his family - that moment happens simultaneously for both family and traveler. Can we not say that on the “universal clock” during the time he was away, 1 year on the spaceship happened simultaneously with 10 years on Earth, 2 years simultaneously with 20 years, etc.?
Interesting observation in the opening comments regarding how intuitive (or not) the concept of relativity is. The same question came up in a recent podcast I listened to with Sam Harris and Sean Carroll that involved discussion of quantum mechanics and the question of “free will” (vs determinism). Cogent arguments can be made that relativity and quantum mechanics are valid, useful concepts (mathematically, empirically, predictively, etc.) and that the human mind creates merely the _illusion_ of free will, without our actually possessing it. However none of these three concepts are available via intuition alone. All three require a HUGE concatenation of prior knowledge before the can even BEGIN to seem plausible. Arguably, the only aspects of human cognition that are available via intuition are those that provided a selective advantage in the context of human evolution, and by this criterion, an intuitive concept of quantum mechanics and relativity simply were not needed. Arguably the ONLY facets of human cognition that were actually needed for human survival and genetic “fitness” were: 1) those capabilities that enabled us to form cohesive, effective extended social groups and 2) those skills that enabled rational inductive reasoning. (Actually, abductive reasoning represents the actual application of our capacity for inductive inference. (Note that of these two, CLEARLY #1 takes precedence in most circumstances.) ONE aspect of human cognition that can come ONLY via intuition, and can be proved in no other way is the concept that there is a single external objective reality that we all share. Without this, language, communication, and scientific (that is, empirical) collaboration would not be possible. Personal aside: Speaking ONLY from intuition, certain concepts in quantum mechanics are incompatible with our intuitive understanding that there is a single shared external objective reality. Therefore, i conclude there is something fundamentally wrong in quantum mechanics.... Although i admit i don’t know what it is. I suspect it’s that there is a higher order deterministic control that we are completely unable to detect (as it was not necessary for our survival on the African Savanah).
One thing that fascinates me about special relativity is that there's nothing there that say Galileo or Newton couldn't have figured out, if they had known about the constant speed of light. The math is really not difficult.
Q&A: If you were to place a mechanism on the train that could detect the treaty signing, and this mechanism were constructed and calibrated by the people on the train such that it sounded the train's whistle if they were signed at the same time, would both the people on the train and off the train at least agree that the whistle sounded?
Thank you prof. Green, for this video series and all the others too, that you have set up in youtube. Does relativity of simultaneity also explain for example why the "delayed choice quantum eraser" version on the double slit experiment seems so weird? And if it doesn't explain it, can you explain why it doesn't? It seems (after years of studying in the university of youtube) somewhat quantum-logical that photons, going at the speed of light, splitting up in an entangling way, going through slits and mirrors to separate detectors, only seem to do so at different instants of time. From the photons frame of reference the distance they travel goes to 0 because of length contraction, right? And the time it takes them to go through the apparatus goes to 0 because of time dilation, doesn't it? So the weirdness of the "measurement of the second photon affecting how the first photon reacted in the past" is only weird because simultaneous things happening to the photons just don't seem simultaneous to us as observers?
It would be neat to show what happens if we assume that the speed of light is not constant. That is, the pulse moves at c + v to the right and c-v to the left. In this case, if I did the math right, even the stationary observers would see the light arrive at each president at the same time; tf-tb = 0. Did I get that right?
I have a strange question ... Suppose people outside the train have a button, that when they press it they pass a light electric current to the hand of their head of state, and suppose they press it the second they see the head of state seal the deal - will the electric current get to the head of "forward" before signing and to the head of "back" after he signed?
Great video/explanation. Maybe somebody else could explain something to me.. I always struggle with relativity when it comes to time. So in this example, since the observers saw the light take longer to get to the ‘backward’ passenger, does that also imply that the backward passenger has aged slightly longer from the bystander perspective? What am I missing?
I do not have a problem with understanding the experience of the people on the station. I do however have a problem with understanding what happens within the train. Here is why: In a stationary train coupe, A throws a ball to B. It does not matter if the train coupe moves, both A and B as well as the ball gets part of that movement. If the train is standing still and A throws the ball at 20 km/h, the ball will travel at 20 km/h. If the train moves at 50 km/h, both A and B will travel at 50 km/h. When A throws the ball at what he considers 20 km/h the trains veloicity will be added to the ball and the ball will actually fly at 70 km/h. However, the above does NOT apply if we replace the ball with a photon. The photon will have to chase the escaping B. The light source moves at the speed of the train, but that speed is not added to the photon. The only reference the photon has is "the center of the universe" and moves in its 300,000 km/s no matter how much we try to push it faster. It is as if by default, the photon always will be outside the system. Please enlighten me where my thoughts go wrong.
A question revolves around my mind and that is, whether the 'tick' of a moving horizontal light clock is longer than its 'tock', for those who are stationary? (Although it's clear that the time elapsed on moving vertical and horizontal light clock is equal). You drew such figure in the derivation of the Length contraction examples (World Science U).
If the assumption is that the moving clock is travelling at a constant rate, then by Pythagoras, the tick and tock will be equal. why would there be an asymmetry? You would therefore be saying that the speed of light in one direction, the Tick is different to the speed in the Tock direction, as measured by the static observer. Which directly contradicts Special Relativity.
@@timbeaton5045 Yes, the tick and tock will be equal, when the motion is perpendicular. But, when the motion is parallel, the tick will take much longer than the tock, precisely because the speed of light is constant.
I found this wall of text to related question; "Are moving clocks really running at different rhythms just because of velocity?": Well of course they do, there are 2 effects, one due to gravity and one due to speed.Considering for instance the GPS satellites, the clocks onboard the GPS satellites experiences relativistic shifts, which have both a constant and time varying components. This constant component is compensated for by introducing a fixed offset, which lowers the frequency of the on-board oscillator. The fact that there is some orbital eccentricity and the higher order terms (quadropole) of the Earth’s gravity field are primarily responsible for the components that vary with time. For instance, if two clocks are located on an equipotential surface, the rates of the clocks will be the same. If however one clock is moved to a height of 1 km, their rates would differ by about 1 x10^-13. Earth’s quadrupole’s effect on the potential at the GPS satellite is approximately one part in 10^14 , so in the case of the potential at the height of the GPS satellite, the contribution of the quadrupole can be ignored in most cases (GPS orbits are high enough to be nearly Keplerian); there is also no centrifugal component so that the gravitational potential at the GPS satellite can be easily calculated.The total gravitational frequency shift of the clock onboard the GPS satellite can then be calculated as 45.688 microseconds per day. If you ignored this, you would have a one way navigational error of 13.697 km per day. Secondly when we consider the speed of a GPS satellite, (~3874 m/s), Einstein's special theory of relativity needs to be applied. The time dilation effect causes the GPS satellite to appear to run slow by about 7 microseconds per day. The GPS satellite clocks are adjusted for three constant rate corrections before launching them into orbit, the satellite clock has a higher frequency in orbit than on the ground (read geoid, where the clock frequency should be 10.23 MHz) and its proper frequency is therefore reduced to 10.229 999 995 43 Mhz. So the nett effect is that between the faster clock in the weaker gravity field due to orbital height, and the slower clock due to the clock's in orbit speed, the gravity effect wins. One can calculate this very accurately, and it works perfectly, otherwise we would not have sub-cm accuracy in positioning using geodetic quality GPS receivers.
That means that for the observers on the platform the speed of light propagating in the forward direction is =c+v, which is greater than c. But this should not be possible.
Sir you mention in your talks that theory of General relativity is kind of tough and complex . Will you present that complexity in simpler ways . I am talking about einstein field equation . Love you sir love from india ❤️❤️😊😊
If, in the expt, you replaced the light source with a sound source, I would expect it demonstrates the relative nature of simultaneity just the same? So speed of light being constant bears no significance in simultaneity?
So in other words, the time difference between two events is relative but didn't minkowski prove that time diff b/w two events would always be the same for any reference frame? Please do correct me if im wrong. And im aware that this idea prove that every reference body has its own time in a heuristic way and was originally pre lorentz transformation but after knowing the transformation, wouldn't contraction of train play a role in the numerical distance the light has to travel to both the presidents with respect to the platform?
Of course it is an absolute fact that two events can truly occur simultaneously. First we start with our 4D environment that is known of as space-time. It is composed of 3 spatial dimensions, and the 1 dimension of time. Two events can take place within this one 4D space-time environment. Two events, events A & B, can be set apart from each other spatially, but they can also be located at the same location within the dimension of time. These two events would truly be simultaneous events. However, from a frame of reference other than the space-time frame of reference, it may appear as though event A occurred before event B. Also, from another frame of reference, event B seems to have occurred before event A. And so no observer can say that their conclusion is the correct one, since neither observer in their specific frame of reference, can truly know how the two events are actually physically oriented within the 4D environment known as space- time. In other words, to say that both are correct, both would have to exist within different realities, or different universes, which of course is not the case since they both exist within only one specific 4D space-time environment.
Brian is it possible that maybe the parallel universes gravity could be pulling on are Galaxys in our universe. Could it be these universes are so big we could be transferrin information through black holes to another universe.
Do these equations kinda use the same rules as classical newtonian physics does for calculating speeds that are relative to one another (same direction=subtracting, different directions=adding)? Since I did hear that in relativity no matter what speed you're moving at, if you compare it to the speed of light , the relative speed between your speed and the speed of light will always equal c, right? Does that fact matter in this example or am I tripping?
Brian, can we use Lorentz theory of light in order to explain the results of the experiments that confirm special relativity, if only we decide to believe in a more complicated theory?
I paused at 11:06 mark. I used pen and paper at home to try how would I derive it. And then compared how well I did. I was able to work this out for this equation! (i m more of an Einstein myself u know ;) )
Brian greene....... I really love physics and mathematics but I can't focus with my studies for a little amount of time....... Can I overcome this in time and become some of the names like you? ⛮☰
But what would happen if the platform is moving and the train is Stationary. Is it exactly the same ? Or is it different ? My first instinct is it is the same but I am second guessing myself I am trying to visualise it in my head and I am not sure it would be the same.
This is a problem created by a physicist. For an engineer, given that we are able to distinguish such tiny differences in time, he would have placed 2 lamps, one in front of each president and the switch at the center. In this setup the lamps should light up simultaneously and they both see the light at the same moment without any delay, thus nobody could disagree...
QUESTION... Is this the same thing as: Earth is between two stars (stars a & b) they both go supernova at the same time according to us Earthlings. There is another planet much closer to star A. They say star A went supernova first.... Is this the same thing you explained? Thank you!
1:25 It's interesting that he does not feel he has a deep intuition for Relativity. I read my first book on Relativity ("Relativity for the Million" by Martin Gardner) when I was 13 and it made perfect sense. I have always felt a deep intuition for the concepts of Relativity because it all fits together perfectly and beautifully. Of all the concepts in so-called modern Physics, it is the one that strikes me as, "Of course it must be true, how could it be anything else for the world to operate in a sane and logical manner." It is possible to understand why Special Relativity must be true without the math. Yes, yu may not be able to solve all kinds of complex problems, but you can understand why and how it works at a basic level.
No, when light starts in both directions the both presidents were at equal distance from the bulb. After the light starts in both directions, the speed of train move the system towards Backward president, including the Foreward president and the bulb. With one exception: the light who started when the both presidents were at equal distance from the bulb. That means that light will have less distance to travel to Forward president than to travel to Backward president. This is how see the observers from the train station. In the train, both presidents signed at same time.
Hi again Prof. Greene, I would just like to say that your story of the 2 Presidents on the train, is one of the best & easiest-to-understand explanations of Simultaneity I have heard. It is both simple and enlightening simultaneously ! Thanks again.
Brian Greene, you’ve done as much for science as Albert himself. You inspire millions to follow suit with your ability to communicate complex topics in such an easy to understand and enticing fashion. Keep it up my dude, you may even inspire the next Einstein.
I'd like to think he's contributed more.
@@nickbros If you're talking about him educating more people about science then I'd 100% agree. If you believe he's done more for science in general compared to Einstein, well... No.
Hi Everyone....Next episode starting shortly. I will also likely do a Q & A this week, live if bandwidth will allow. Send in your questions.
Dear prof Brian, here is my Question: if a clock is not a cyclical motion device (as used in special relativity), how can otherwise can be?
How can be the clock defined different from a cyclical motion device?
Hello sir, pardon my curiosity but what are you working on with respect to your field of study these days?
@@mahadlodhi pretty sure it's string theory calculations!
Sir, Could we say rest mass is invariant ?
I admire professor Greene and salute him for making all his knowledge available and comprehensible. 🤗
Yes, but does he know how to make guacamole?
@@aaronnatera3685 that should be the next question 🤔😂
Brian your smart. I listened to Lenny Leonard Susskin. But Len said he proved Hawkins's wrong about information can't be lost. But you're my favorite. You have a good calm pace. I listen to you because there is not enough information. But you teach as much as possible. I want to learn new thing's about the universe and I wish we could learn something spectacular and new everyday. With the tools we have limits us but I could only amagin 400 years from now.
These residents at the train station have good eyesight and reaction time!
maybe the wagon was really long ?
did you even watch the video ?
;)
Ideas: Schrödinger equation, Heisenberg uncertainty principle, Newton’s law of gravitation
I mentioned the first two in the first video
These are all things. What is the idea you have? Or is the idea to write these 3 things together?
Newton seriously?
Don't be repetitive ;-))
Hey Brian, I just want to thank you for taking the time out of your day to create these videos. They deepen my understanding and nurture my curiosity. I have the utmost respect and appreciation for all the work you do. I hope to see you at World Science Festival one day!
Dr.Brian you've got no idea how much this Biologist/Relativity enthusiast appreciates these equations of relativity which are immaculately explained by high school level Mathematics !
As a mathematician, I would really like to see your perspective of the Riemann Zeta function. Thanks for the great videos by the way!
The way you make these concept so accessible to us is amazing. admirable even!
Thanks Prof Greene
Dr. Green,
I wanted to thank you for your incredible explanations and taking the time to post these videos. This is the most access I have had to most of these enlightening equations and your teaching is second to none. I’m only on number 4 of your daily equation videos but I’m hooked. Thanks again.
I'm enjoying this series very much. Thank you a lot! Greetings from Argentina!
I really appreciate you putting these out at a time when amateurs like myself have time to really dig into what you are saying. COVID-19 is terrible, but at least many of us are getting a chance to expand our scientific understanding thanks to you. Excellent work!
Thankyou Brian for taking the time to share and care.Tony.x
I love these
They are keeping me sane in this quarantine situation we live in
Thanks !
This video was very insightful. Good job!
A real privilege to be watch. Thank you Professor Green
Wow ! Brilliantly explained! Great vid! Thnx Brian 😀
i did admire Einstein's work and i consider him a pure genius, with this course, this admiration is getting higher and higher.
Fantastic how you make things understandable. Great
A succinct presentation. Thank you.
Thank you Prof. Green...a fan of you from Victoria City in Mexico
Thank you very much for beautiful lesson and demonstration.
Thank you... beautiful way of explaining in simple terms complex notions.
Thanks Brian for your efforts in this. FYI...I am 68 years old. Have a BSC in Math and computer but have worked in the area of broadcast TV engineering for what seems like a millenium. I read a book years ago called “ 1-2-3 Infinity” that really captured my interest. One concept that was presented in the book was...as an observer on Earth, a photon leaving the sun would take 8ish minutes to arrive to visit me. However...if I were that photon...at the moment I decided to visit earth, because I am travelling at the speed of light, this visit would happen immediately. No time would pass. Ie...no mass, at the speed of light. I’d love to hear your thoughts.
This remarkable fact about the lack of simultaneity seems to be a fitting episode as a follow up of the first three in special relativity. I am curious to see where we will go from then - to the General Relativity/equivalence principle, or elsewhere?
Excellent... wow... getting it; that helped and brought recall... exciting stuff, because you see the world is not just ideas or limited to your eye's witness, but is in a phenomenal cause and effect motion... love it...thank you
I suspect that if the speed of light were much lower (say 50 mph) we would have evolved so that these concepts would be intuitive to us. Our intuition tells us that if we see something happen in the distance we are seeing it as it happens and that is what makes these ideas difficult to comprehend even if logically we can understand that they are correct. I love the way you make these ideas as clear as they can be from an experince perspective.
I have a feeling that people say, wow cool and move on without contemplating the real life implications of the relativity of simultanity. this idea helped me deal with loss of loved ones and my own anxiety. Once you realize that the future and past are actually more real than the illusion of "now" that you experience, greef is much more manageable. As Einstein himself said to a widow, "It's not at all like your loved one was yanked out of existence by death. He is still there over a hill. You just can't see him"
Say we have detectors at both ends of the train instead of heads of state. The detectors do two things the moment they detect the light beam coming from centre: stop the train and shoot a beam of light back at the centre. In the centre there is a third detector that turns a signal on but only if it is hit by two opposite beams at the same instant. So from the perspective of someone in the train the centre detector will receive the beams at the same time and turn on the signal but from the perspective of someone outside the signal stays off. What we have here is that the signal is both on and off at the same time and the train (of thought) collapses when someone measures the result :)
The signal from the perspective of someone outside the train will see it on because from the perspective of someone outside the Train the detector only turns on if the signals come at different times.
I get the train story,. Thank you for putting it in such an accessible way.
16:24 Two additional points. (1) Typically, we replace c/v with the lower-case Greek letter beta. (2) the equation can also be written using gamma with gamma being equal to 1/sqrt(1 - beta^2) so we would have gamma squared.
Thank you for enlightening explanation about why there is no absolute time in terms of relativity of simultaneity.
Thank you, Brian. This has been a roadblock for me for a long time and I now better understand :) Your example is much better than the normal train/platform/lightning strikes example which completely sidetracked me. Thank you for these *excellent* videos, I appreciate your time and efforts.
Thanks for giving the actual equations in a simple way! looking forward to see how you will do this for Einstein's field equations and Maxwell's equations
Thank you for this fantastic video! You explain so clearly!
Thanks Professor. Really enjoyable and fun.
As v approaches c, then time difference becomes indeterminate, i.e. L/0, as does the gamma factor. What's more, from my understanding, as v approaches c, then length contracts to zero and time dilates to zero as well. It's a mind-bending notion!
Great explanation, Brian. Looking forward to the next one.
This video reveals what happens to simultaneity under constant velocity (and Dr. Greene mentioned what happens for the special constant velocity condition of delta(v)=0),
my question: does the same relationship of simultaneity hold for constant acceleration, or even any pattern of motion?
Please do the Euler-Lagrange Equation and the Lagrangian formalism in both Classical Mechanics and QFT
Thank you for sharing
6:35
Don't you think there is an inconsistency in the reasoning for the two perspectives?
In the first scenario, we believe that the light emitter is on the train, moving with the train.
In the second scenario, we kind of consider the two light photons coming out of nowhere, as if it suddenly appears in the train and emits the photons.
However, the reality should be that it is always on the train.
Or, at least, the light emitter is either moving with the train or not moving with the train, it should not be changed because of different perspectives. ?
The photons, once left the light bulb, travel at a constant speed regardless how fast the train is moving. It is indeed true that the photons travel a short distance to reach the forward facing president.
But the observer in the train do not get to see it immediately. It takes time for the photons bounced back from the forwarding-facing president and their pen to travel to the eyes of the observer in the train. Now don’t forget that the observer is moving forward with the train, so these photons will have to play catch-up to the observer, and it will take more time than if the observer was stationary.
Now if you consider the round-trip, the effects cancel out. So somebody standing equally far from the two presidents in the training will end up seeing the signing events at the same time.
To other people in the train who are closer to one president than another, they would SEE the events in different orders. In the extreme case, to either of the president, they’d both see themselves signing first. I think these details are omitted in the video, but it is totally true that observers won’t agree on the order of events depending on their position and frame of reference.
Thanks for this great video series! In 16:30, I am a little confused by whether the length L is the one that contracted or not. Should we put "length contraction" into account in this derivation?
Great and simple explanation of relativity
Sir, thank you for this series. I am late and try to catch-up, but I have a question on this: To determine the middle of the train I place one mirror at the front and one at the back end of the train. I use an apparatus that emits light flashes in both directions and rings a bell when it receives the reflections simultaneously. When people on the train position the device so that they can hear the bell ring, will the people on the platform disagree and don’t hear the bell ring? Obviously not. But we’re is my mistake? Any comments are welcome. Thank you!
Navier Stokes!!! please!!!
Brian i have a long time to go until i reach your level and become one of your peers. Im a self taught graphics software engineer, and i just LOVE
the complexity of modeling fluids on computers. After i get my finances in check, Im going to study physics more :) I LOVE PHYSICS & MATH!!! SOO COOOOOL!!!!!
I have read elegant universe. Very good one❤
Has time dilated or did I arrive here early?
XD thats a good one
being from the future
In general, one cannot claim to be the first, as someone else in a different inertial frame can also claim to be the first, and it all depends on your point of view!
For a future episode. Could you explain how the planck limit for the vacuum energy density was arrived at?
Thanks for the video Brian. One thing I do not get though: should one not take relativistic lenghts into account, so L would be L/gamma due to contraction of the length of the train as observed from the platform?
surely the length contraction applies equally in both directions so can be disregarded?
I figured that should then be the case, but to be exact, one shoul have mentioned that L is in fact L corrected with gamma
The area surrounding the president of forward land up to the light bulb is moving in slow motion, viewed from the platform?
This is the difference in time for the people on the platform (assuming actual simultaneity for people on the train). My question is: does the gamma factor appear (when comparing the difference in simultaneities ) IF simultaneity is assumed for people on the platform and the difference in time for simultaneity is calculated for people on the train?
It does not take relativity of simultaneity to have constant speed of light. We can take the equations that physicists threw away in 1887 and show constant speed of light.
x'=x-vt
y'=y
z'=z
t'=t
If there is a slower clock in a moving frame of reference, as Einstein describes, all we have to do to show constant speed of light in both frames of reference is to use another set of Galilean transformation equations with different variables for time and velocity.
So the inverse equations as seen from the frame of reference with the slower clock would be
x = x' - (-vt/n')n'
y = y'
z = z'
n = n'
n' is the time of the slower clock. (-vt/n') is the velocity of frame of reference S(x,y,z,n) relative to frame of reference S'(x',y',z',n'). n = n' shows that the time of the slower clock in S' is being used in both frames of reference. We can show that these equations agree with the first set of Galilean transformation equations by cancelling out the (n')'s.
x = x' - (-vt/n')n'
x = x' + vt
t = t'
So now all we have to do to show constant speed of light is to say x = ct and x' = cn'.
cn' = ct - vt
n' = t - vt/c
This value for n' is actually the same as the value of the numerator in Lorentz's equation for t'.
t - vt/c = t - vct/c^2 = t - vx/c^2
The x in this expression is unnecessary in the Galilean transformation equations because there is no length contraction in the Galilean transformation equations.
Question: Thought experiment...What if instead of measuring the 2 differing perspectives, they each sent a live video(at light speed) of said view point(s). Would they still arrive & agree on the relativity of simultaniety?? Or would things somehow be self consistent for both observers?
THE PARADOX THAT DISPROVES RELATIVITY:
There is a triangle of lights which we will call A, B, and C. They flash simultaneously in the frame of reference that is at rest relative to these lights. There is someone moving at a high rate of speed from B to A. There is someone else moving at a high rate of speed from C to B. There is someone else moving at a high rate of speed from A to C. So A flashes first and then B flashes and then C flashes and then A flashes again. How can A flash twice? When A flashes has B already flashed or not yet flashed?
or
B flashes first and then C flashes and then A flashes and then B flashes again. How can B flash twice? When B flashes has C already flashed or not yet flashed?
or
C flashes first and then A flashes and then B flashes and then C flashes again. How can C flash twice? When C flashes has A already flashed or not yet flashed?
I understand that there can be no agreement on the “perception” of simultaneity, but does that really eliminate the idea of universal time? This is similar to the Doppler effect… a musician plays a C note, the stationary audience hears a C note, a driver approaching the musician will hear a D, and driver heading away hears a B note. But the actual note is a C. Does that analogy not apply to simultaneity as well? In “universal time” two things happen at the same time, but observers will have varied perceptions. With the example of space traveler moving close to the speed of light who only ages 5 years while his family on Earth has aged 50. At some point he returns to Earth to meet his family - that moment happens simultaneously for both family and traveler. Can we not say that on the “universal clock” during the time he was away, 1 year on the spaceship happened simultaneously with 10 years on Earth, 2 years simultaneously with 20 years, etc.?
Interesting observation in the opening comments regarding how intuitive (or not) the concept of relativity is. The same question came up in a recent podcast I listened to with Sam Harris and Sean Carroll that involved discussion of quantum mechanics and the question of “free will” (vs determinism). Cogent arguments can be made that relativity and quantum mechanics are valid, useful concepts (mathematically, empirically, predictively, etc.) and that the human mind creates merely the _illusion_ of free will, without our actually possessing it. However none of these three concepts are available via intuition alone. All three require a HUGE concatenation of prior knowledge before the can even BEGIN to seem plausible.
Arguably, the only aspects of human cognition that are available via intuition are those that provided a selective advantage in the context of human evolution, and by this criterion, an intuitive concept of quantum mechanics and relativity simply were not needed. Arguably the ONLY facets of human cognition that were actually needed for human survival and genetic “fitness” were: 1) those capabilities that enabled us to form cohesive, effective extended social groups and 2) those skills that enabled rational inductive reasoning. (Actually, abductive reasoning represents the actual application of our capacity for inductive inference. (Note that of these two, CLEARLY #1 takes precedence in most circumstances.)
ONE aspect of human cognition that can come ONLY via intuition, and can be proved in no other way is the concept that there is a single external objective reality that we all share. Without this, language, communication, and scientific (that is, empirical) collaboration would not be possible.
Personal aside: Speaking ONLY from intuition, certain concepts in quantum mechanics are incompatible with our intuitive understanding that there is a single shared external objective reality. Therefore, i conclude there is something fundamentally wrong in quantum mechanics.... Although i admit i don’t know what it is. I suspect it’s that there is a higher order deterministic control that we are completely unable to detect (as it was not necessary for our survival on the African Savanah).
Prof. Greene if we use lorentz transformation we get 1/(1-v²/c²)½ in the denominator
But in your calculation there is no square root why?
He hasn't used the Lorentz contraction. He has taken the derived equation LV/C^2-V^2, divided by C^2 and done a little bit of algebraic manipulation.
One thing that fascinates me about special relativity is that there's nothing there that say Galileo or Newton couldn't have figured out, if they had known about the constant speed of light. The math is really not difficult.
Q&A: If you were to place a mechanism on the train that could detect the treaty signing, and this mechanism were constructed and calibrated by the people on the train such that it sounded the train's whistle if they were signed at the same time, would both the people on the train and off the train at least agree that the whistle sounded?
Thank you prof. Green, for this video series and all the others too, that you have set up in youtube.
Does relativity of simultaneity also explain for example why the "delayed choice quantum eraser" version on the double slit experiment seems so weird? And if it doesn't explain it, can you explain why it doesn't?
It seems (after years of studying in the university of youtube) somewhat quantum-logical that photons, going at the speed of light, splitting up in an entangling way, going through slits and mirrors to separate detectors, only seem to do so at different instants of time. From the photons frame of reference the distance they travel goes to 0 because of length contraction, right? And the time it takes them to go through the apparatus goes to 0 because of time dilation, doesn't it? So the weirdness of the "measurement of the second photon affecting how the first photon reacted in the past" is only weird because simultaneous things happening to the photons just don't seem simultaneous to us as observers?
Absolutely amazing video.
Thank you for the very clear explanation sir.. it is very helpful for me to understand this topic :)
Can't wait for the next episode
Love your daily equation Sir
I'm your big fan n I also want to be an astrophysicists
Great work again, Professor Greene. Excellent point about the impact of distance on simultaneity.
anything on wave equations would be cool (totally not biased as an acoustician obviously)
Thank u sir .. i was thinking this thing from about a week.. u cleared my doubt
It would be neat to show what happens if we assume that the speed of light is not constant. That is, the pulse moves at c + v to the right and c-v to the left. In this case, if I did the math right, even the stationary observers would see the light arrive at each president at the same time; tf-tb = 0. Did I get that right?
I have a strange question ...
Suppose people outside the train have a button, that when they press it they pass a light electric current to the hand of their head of state, and suppose they press it the second they see the head of state seal the deal - will the electric current get to the head of "forward" before signing and to the head of "back" after he signed?
Anyone, even Fool thugs like me, can learn Relativity from this animation. Thank you Dr Green.
Great video/explanation. Maybe somebody else could explain something to me.. I always struggle with relativity when it comes to time. So in this example, since the observers saw the light take longer to get to the ‘backward’ passenger, does that also imply that the backward passenger has aged slightly longer from the bystander perspective? What am I missing?
nice description of the einstein idea.
I do not have a problem with understanding the experience of the people on the station.
I do however have a problem with understanding what happens within the train.
Here is why:
In a stationary train coupe, A throws a ball to B.
It does not matter if the train coupe moves, both A and B as well as the ball gets part of
that movement. If the train is standing still and A throws the ball at 20 km/h, the ball will travel at 20 km/h.
If the train moves at 50 km/h, both A and B will travel at 50 km/h. When A throws the ball at what he considers 20 km/h the trains veloicity will be added to the ball
and the ball will actually fly at 70 km/h.
However, the above does NOT apply if we replace the ball with a photon. The photon will
have to chase the escaping B.
The light source moves at the speed of the train, but that speed is not added to the photon.
The only reference the photon has is "the center of the universe" and
moves in its 300,000 km/s no matter how much we try to push it faster.
It is as if by default, the photon always will be outside the system.
Please enlighten me where my thoughts go wrong.
Time dilation is the missing key to your argument
A question revolves around my mind and that is, whether the 'tick' of a moving horizontal light clock is longer than its 'tock', for those who are stationary? (Although it's clear that the time elapsed on moving vertical and horizontal light clock is equal).
You drew such figure in the derivation of the Length contraction examples (World Science U).
If the assumption is that the moving clock is travelling at a constant rate, then by Pythagoras, the tick and tock will be equal. why would there be an asymmetry? You would therefore be saying that the speed of light in one direction, the Tick is different to the speed in the Tock direction, as measured by the static observer. Which directly contradicts Special Relativity.
@@timbeaton5045 Yes, the tick and tock will be equal, when the motion is perpendicular. But, when the motion is parallel, the tick will take much longer than the tock, precisely because the speed of light is constant.
In this blurred current of entropy we are going through, your daily equation comes to give hope:). Schrödinger equation?
is there any physical observation that can support this???
I found this wall of text to related question; "Are moving clocks really running at different rhythms just because of velocity?":
Well of course they do, there are 2 effects, one due to gravity and one due to speed.Considering for instance the GPS satellites, the clocks onboard the GPS satellites experiences relativistic shifts, which have both a constant and time varying components. This constant component is compensated for by introducing a fixed offset, which lowers the frequency of the on-board oscillator.
The fact that there is some orbital eccentricity and the higher order terms (quadropole) of the Earth’s gravity field are primarily responsible for the components that vary with time.
For instance, if two clocks are located on an equipotential surface, the rates of the clocks will be the same. If however one clock is moved to a height of 1 km, their rates would differ by about 1 x10^-13. Earth’s quadrupole’s effect on the potential at the GPS satellite is approximately one part in 10^14 , so in the case of the potential at the height of the GPS satellite, the contribution of the quadrupole can be ignored in most cases (GPS orbits are high enough to be nearly Keplerian); there is also no centrifugal component so that the gravitational potential at the GPS satellite can be easily calculated.The total gravitational frequency shift of the clock onboard the GPS satellite can then be calculated as 45.688 microseconds per day. If you ignored this, you would have a one way navigational error of 13.697 km per day.
Secondly when we consider the speed of a GPS satellite, (~3874 m/s), Einstein's special theory of relativity needs to be applied. The time dilation effect causes the GPS satellite to appear to run slow by about 7 microseconds per day.
The GPS satellite clocks are adjusted for three constant rate corrections before launching them into orbit, the satellite clock has a higher frequency in orbit than on the ground (read geoid, where the clock frequency should be 10.23 MHz) and its proper frequency is therefore reduced to 10.229 999 995 43 Mhz.
So the nett effect is that between the faster clock in the weaker gravity field due to orbital height, and the slower clock due to the clock's in orbit speed, the gravity effect wins.
One can calculate this very accurately, and it works perfectly, otherwise we would not have sub-cm accuracy in positioning using geodetic quality GPS receivers.
That means that for the observers on the platform the speed of light propagating in the forward direction is =c+v, which is greater than c. But this should not be possible.
Sir you mention in your talks that theory of General relativity is kind of tough and complex . Will you present that complexity in simpler ways . I am talking about einstein field equation .
Love you sir love from india ❤️❤️😊😊
If, in the expt, you replaced the light source with a sound source, I would expect it demonstrates the relative nature of simultaneity just the same? So speed of light being constant bears no significance in simultaneity?
Thank you sir for your valuable time
The source of light, the bulb, also moves with velocity of train. Does this not have any impact on above discussion?
What about the entanglement where there is a changr in subatomic particles even though they are separated.
So in other words, the time difference between two events is relative but didn't minkowski prove that time diff b/w two events would always be the same for any reference frame? Please do correct me if im wrong. And im aware that this idea prove that every reference body has its own time in a heuristic way and was originally pre lorentz transformation but after knowing the transformation, wouldn't contraction of train play a role in the numerical distance the light has to travel to both the presidents with respect to the platform?
Of course it is an absolute fact that two events can truly occur simultaneously. First we start with our 4D environment that is known of as space-time. It is composed of 3 spatial dimensions, and the 1 dimension of time. Two events can take place within this one 4D space-time environment. Two events, events A & B, can be set apart from each other spatially, but they can also be located at the same location within the dimension of time. These two events would truly be simultaneous events. However, from a frame of reference other than the space-time frame of reference, it may appear as though event A occurred before event B. Also, from another frame of reference, event B seems to have occurred before event A. And so no observer can say that their conclusion is the correct one, since neither observer in their specific frame of reference, can truly know how the two events are actually physically oriented within the 4D environment known as space- time. In other words, to say that both are correct, both would have to exist within different realities, or different universes, which of course is not the case since they both exist within only one specific 4D space-time environment.
Brian is it possible that maybe the parallel universes gravity could be pulling on are Galaxys in our universe. Could it be these universes are so big we could be transferrin information through black holes to another universe.
Do these equations kinda use the same rules as classical newtonian physics does for calculating speeds that are relative to one another (same direction=subtracting, different directions=adding)? Since I did hear that in relativity no matter what speed you're moving at, if you compare it to the speed of light , the relative speed between your speed and the speed of light will always equal c, right? Does that fact matter in this example or am I tripping?
Brian, can we use Lorentz theory of light in order to explain the results of the experiments that confirm special relativity, if only we decide to believe in a more complicated theory?
I paused at 11:06 mark. I used pen and paper at home to try how would I derive it. And then compared how well I did. I was able to work this out for this equation! (i m more of an Einstein myself u know ;) )
You da bomb, Mr Greene
Brian greene....... I really love physics and mathematics but I can't focus with my studies for a little amount of time....... Can I overcome this in time and become some of the names like you? ⛮☰
But, two different kinds of people those sitting on the train and those standing on the flat form DO NOT agree that train in moving.
Exactly! I would appreciate seeing a walk through of these equations with the perspective of the people ON the train.
But what would happen if the platform is moving and the train is Stationary. Is it exactly the same ? Or is it different ? My first instinct is it is the same but I am second guessing myself I am trying to visualise it in my head and I am not sure it would be the same.
This is a problem created by a physicist. For an engineer, given that we are able to distinguish such tiny differences in time, he would have placed 2 lamps, one in front of each president and the switch at the center. In this setup the lamps should light up simultaneously and they both see the light at the same moment without any delay, thus nobody could disagree...
QUESTION... Is this the same thing as: Earth is between two stars (stars a & b) they both go supernova at the same time according to us Earthlings. There is another planet much closer to star A. They say star A went supernova first.... Is this the same thing you explained? Thank you!
@silverrahul Thanks. I thought so but if I didn't ask I wouldn't be sure. 👍👍
1:25 It's interesting that he does not feel he has a deep intuition for Relativity. I read my first book on Relativity ("Relativity for the Million" by Martin Gardner) when I was 13 and it made perfect sense. I have always felt a deep intuition for the concepts of Relativity because it all fits together perfectly and beautifully. Of all the concepts in so-called modern Physics, it is the one that strikes me as, "Of course it must be true, how could it be anything else for the world to operate in a sane and logical manner."
It is possible to understand why Special Relativity must be true without the math. Yes, yu may not be able to solve all kinds of complex problems, but you can understand why and how it works at a basic level.
Backwardland may have increased the distance, and forwardland streached the eather so it signed at the same time.
No, when light starts in both directions the both presidents were at equal distance from the bulb. After the light starts in both directions, the speed of train move the system towards Backward president, including the Foreward president and the bulb. With one exception: the light who started when the both presidents were at equal distance from the bulb. That means that light will have less distance to travel to Forward president than to travel to Backward president. This is how see the observers from the train station. In the train, both presidents signed at same time.
Sir, what if V approaches C ? In the derivation, should we consider about time dilation in such case?
What about the formula for the folks on the train?