As a 60 year old physicist, I can say that this is the best demonstration of the twin's paradox I have seen, and I saw the first one when I was 15. I also love your enthusiasm. Bravo!
This seems promising, let's watch! I watched it and the flag is not giving me a better perspective, the 5 or 35 seems arbitrary to me. It is still a paradox in my opinion.
But it's completely wrong. It is all about acceleration. He replaced one twin and not the other with two spaceships. That completely begs the question.
The twin's paradox has three velocities: the velocity of the Earth, the velocity of the outbound trip, and the velocity of the return trip. That means there are three inertial frames, one for each velocity. And in special relativity, an inertial frame is an observer. There are three observers but only two people, the twins. This is what confuses people. Try drawing the space-time diagrams for each of the frames. In the first one, the Earth is not moving and the spaceships are. Time for the outbound trip is slower than Earth time and time for the return trip is also slower. We have: Earth's time > outbound time + return time. This is the part everyone agrees on. From the point of view of the outbound trip, the Earth is moving away from the ship, so its time is slower. But to return to Earth, the ship has to leave this frame faster than the Earth or it will never catch up to it. Time for the return trip is even slower than Earth's time. We get: slow Earth time > outbound regular time + even slower return trip. And from the point of view of the return trip, we get a similar result: slow Earth time > even slower outbound time + return regular time. I think the space-time diagrams make things a lot clearer. PS: I have made the space-time diagrams. photos.app.goo.gl/duHuAZtMhMTkuDDw7
@silverrahul I don't understand why "laymen" should be scared of "spacetime diagams". Haven't they been taught in school to draw simple x against t graphs to solve problems about speeding cars and trains?
No, you're simply ignoring the problem completely. The entire point is that the twin on the spaceship should have just as much right to perceive himself as "at rest" as anyone else. To him, the other twin speeds away and returns. What breaks the symmetry is only absolute acceleration. The rockets on the ship.
@@MrCmon113 how is that ignoring the problem? I feel like his explanation very much makes sense. But I kinda agree, judging by the video, when the twin "changes direction" the Earth twin suddenly "gains age" and it is directly correlated with the required acceleration.
@@AbsoluteHumanI kinda agree and want to add: Mahesh never said it's completely unrelated to the acceleration needed to jump between frames, he just said acceleration is not the root cause but rather it is relativity of simultaneity. And that it gives you a better understanding if you think of it this way.
@@Grecks75 This video is nonsense. "You will see for yourself... I know you have many questions". People who don't understand space-time diagrams can not understand relativity. Everyone who thinks he learned anything from this video is deluding himself.
School taught me how to do maths in physics like memorizing it... but mahesh taught me how to visualize it .. every inch of my understanding of physics has a contribution of him
@@Mahesh_Shenoyisn't this change in direction the same thing as acceleration? it's not physical change in acceleration, but a directional change in acceleration. i think this language of physics is causing a lot of problems. as in logic, it's called the fallacy of equivocation. when we think the 1 word means the same thing but it actually means different things in the sentences. ps. your videos are by far the best on the twin paradox. the others don't even mention Relativity of simultaneity or the different perspectives of the ships/twins. but it is still very hard for me to grasp as someone who didn't do higher maths.
@@Mahesh_Shenoyp.s.i don't know if this is an accurate metaphor. but i like to think of the blue spaceship as running away from the light of the earth. so it's like a earth light being a tv monitor that is going from 60 frames per second to 30 frames per second. as the space ship is running away from the light of the earth. like doppler effect. while the red spaceship by moving towards the earth its fast forwarding through time. so all of a sudden the light or frames per second coming from earth doubles from 60 per second to 120 per second. as you race towards them. running into all of them faster. like doppler effect. so blue space ship is running away from earth, so from its perspective earth is running slow, like a monitor tv screen film running slow at half speed. and then when it becomes the red ship, its like the earth's light, earth's monitor suddenly speeds up x2. like how you can change the speeds of the video on RU-vid. not sure if this is a good way of understanding the time jump, time dilation and Relativity of simultaneity. . but it seems to help. i think. one direction you see earths frame rate slow down. the other direction you see all of tbose delayed earth frame rate suddenly speed up and bombard you with it. like doppler effect. you ran away from your wave first ship. then you turned around and ran towards it second ship. maybe you should change the color of your ships engine lol. 😂😂😂 I'm no expert. but thanks for the video.
17:22 "Acceleration causes change of frame. Change of frame causes relativity of simultaneity. Relativity of simultaneity causes the resolution to the paradox." Based on definition of root cause, this would actually mean the acceleration is the root cause, not relativity of simultaneity. You are correct in saying that relativity of simultaneity is the correct explanation and not general relativity because the thought experience is done in flat space time. To again ignore the role of acceleration in breaking the symmetry is still an incomplete solution. The explanation in the video shows it is the spaceship that is changing frames. This modification of the original paradox has already assumed it is the spaceship that is acceleraing. But if you do not consider acceleration and the fact that it's absolute, the people on spaceship can say it is in fact the people on Earth who are changing frames. Then you can construct an symmetrical situation with this scenario (i.e. the original eEarth is always moving to the left, then a second earth moves to the right during the change in reference frame, there is only 1 spaceship that is stationary and remains in the same frame, and the ropes are attached to the earths instead of the spaceships) and the math all works out to be the same and the paradox would still be there. Ultimately, you can explain the discrepancy between the twins' age due to their relative movement in variety of ways, such as through relativity of simultaneity, change in frames, doppler, space time diagrams etc, but all these explanation are incomplete without incorporating acceleration to break the symmetry, i.e. it is the spaceship that is accelerating and not the Earth.
It depends on how you define root cause. To me (and to Mohesh) the definition that is the most useful is that the root cause is the deepest cause that is present in any scenario, and is still explanatory. Acceleration is only present in some versions of the paradox and not others, therefore it can't be the root cause by this definition. It's like if you're trying to explain why people get colds. The root cause of getting a cold is that someone is exposed to and infected by a virus. Getting exposed and infected has its own causes of course. Maybe *you* got a cold by going to the movies, and for your individual case it is valid to define root cause this way; but saying that that root cause of getting a cold *in general* is going to the movies makes no sense at all. So if you are trying to understand colds in general, it makes sense to say that the root cause is exposure and infection by virus, because those are true in every case. Of course, it is still helpful to understand how exposure and infection happen, because understanding the causes of those things is still helpful in preventing colds. Similarly, the root cause of the twin paradox is changing reference frames. But it's still helpful to understand the cause of that cause, which in many (but not all!) cases is acceleration.
@@erinm9445 "Acceleration is only present in some versions of the paradox and not others, therefore it can't be the root cause by this definition." The problem is in this statement. Acceleration is in fact stil in the paradox, albeit hidden. The change from the original version with the 2 twins to this modified version with has already used the absoluteness of acceleration to break the symmetry in the first place. Let me explain why this is: In the original version, the earth person (A) sees the spaceship person (B) leaving and then returning. There is a change in reference frame for B, so in this modified version, a third person (C) is introduced to eliminate the acceleration of B, but instead the two different frames are represented by B and C. I will call this scenario 1. However, in the original version, B also sees A (and the earth) leaving and return. From his point of view, he is stationary, and A is the one who is apparently accelerating. If B were to modify the thought experiment into this modified version without acceleration, he would be completely stationary throughout the entire thought experiment, A and earth travel constantly to the left, and a third person (C) would be travel towards the right, passing A and syncing clocks with A when he passes A, and then meet up with B to compare clocks at the end. In this modified setup, B is in a single reference frame and A and C are representing the two different frames. All the math is the same as in the video and B would conclude that C's time would be younger. I will call this scenario 2. If we were to ignore acceleration or to say the paradox does not have any acceleration, there is no basis to choose scenario 1 over scenario 2 and vice versa for the modified version. The only reason that scenario 1 correctly represents the original paradox and not scenario 2 is because in the original paradox, A is truly not accelerating and B truly is accelerating. In other words, when this video is selecting scenario 1 instead of scenario 2 to illustrate the solution, it has already used the fact that acceleration is absolute when doing so. The rest of the analysis no longer requires acceleration, but the initial set up of this modified paradox did require it.
@@ToboGamers I honestly couldn't follow everything you were saying here (not a criticism, I can't really parse dense physics arguments in paragraph form). But if what you're trying to say is "changing reference frame *is* acceleration, by definition, so Mahesh's example here actually does have acceleration, just not the kind we're used to thinking about. There aren't any twins acceleration, but the time information *is* accelerated when it moves from one ship to another", then yes, I think that is probably right. But that is not the definition of acceleration that most people use, even someone with an undergraduate physics degree would not use that definition. So if you want to claim that the clock information is in fact accelerated, then that is a pretty technical definition that needs to be called out, and recognized as a very specialized definition of acceleration.
@@erinm9445 That's not actually what I'm saying, but I'll say what I said in my previous comment a bit simpler. As demonstrated in this video and as you have stated, by using two different persons to substitute for the accelerating twin in the original version, acceleration was removed in the modified version. But deciding on which twin to replace with the two different persons depends on which twin is accelerating in the original version. Twin A would want to replace twin B with two persons, but twin B would also want to replace twin A with two persons. The correct analysis is to replace with accelerating twin, as this video did. The only justification for this that he was the one who is truly accelerating. Therefore, acceleration is still required in the complete answer. The video simply ignores this part when he replaces the original paradox with the modified one.
@@wesjohnson6833Einstein moved faster than the speed of light (which he told impossible cuz he was scared of future) and talked with this guy and traveled at 0 velocity
This is such a fantastic video, well explained! It seems to me that the explanation for the "lost 30 years" is essentially the Andromeda Paradox. It would be interesting to do a video on the Andromeda Paradox too, and then link it back to the twin paradox explanation here. There is an interesting technicality that a few people are pointing out in the comments, which I think is worth considering: that your example here does in fact include acceleration, but that acceleration itself has a deeper and more subtle meaning than what we are used to. Essentially, the meaning of acceleration *is* a change in reference frames. Therefore, to say that the change of reference frame is the cause, is equivalent to saying that acceleration is the cause. In the example here, none of the twins are accelerating, but the clock information does accelerate--first it was moving away from earth, then it was moving towards earth, and that is still a symmetry breaking and is, by definition, an acceleration. I like the idea that the argument over reference-frame-change vs acceleration is meaningless, because they are two ways of expressing the same thing.
Yes, nature doesn't care what we call it. Yet there is one difference: We can always accelerate -- it is local and it is a choice --- but without the consistency of light the other aspects fad away. Great point about the "information" undergoing acceleration. Information may be the only real thing out there.
Well said! What causes the 'paradox' to be paradox-y is that the theory of relativity seems to point to absolute acceleration. Which is confusing since space, time and velocity are all relative in relativity. I think this bothered Einstein at the time, because he was reading philosophers like Mach who said that all physical quantities (like acceleration) should be defined relationally, not absolutely. I think one of the goals of general relativity was to try to make acceleration relative.
I used to tell people you can’t resolve the twin paradox because it is the andromeda paradox, once you get the trivial parts out of the way…and they would give me 💩, but minesh is on the 💰 here.
Thank you sir for also explaining the photon part and the age disagreement, it answers my question in previous video about "seeing the future" related to andromeda paradox
You did it. You don't leave room for any "but what if" and now my brain rest at last. The explanations with spacetime graphs never left me 100 percent satisfied, because for me there was still "symmetry" but with your explanation there is clearly and visually no symmetry.
What if instead of staying on earth, the other twin travels at the same speed and distance with the first twin, but to the opposite direction? What about perpendicular direction?
If I understand correctly, on the returning spaceship they would calculate earth's clock to be running slow but if they looked out the window they would see it running fast! Earth's clock measures 5 years (from 35 to 40) while ship clock measures 10 (from 10 to 20). However, since the image they see at the red planet is from time 2.7 they will see the remaining 37.3 years of Earth's history sped up (and blue shifted) during their 10 year return trip.
But they know it’s Doppler shifted. When JWST looks at a spectral line in a receding galaxy, we don’t say time is running extra slow there and the atom’s orbitals are running slow…we take part of the Doppler out… but not all of it. Now I confused myself. It is running slow…and helium atoms are flat, not spherical like they are here. It’s all relative
This is by far the best explanation for the twin paradox I’ve seen, and I have watched a lot of RU-vid videos on this spanning close to a decade. This holds true for basically all of your videos. Thank you!!
That was such a good experience. I'm simultaneously Happy, impressed, dazzled, and awakened. I understand others comparing this to other explanations, but I want to say this is the first time it's ever really been explained. Your approach of having a conversation and asking all those questions is so incredibly effective. Thank you so much. Special relativity is itself an amazing leap of imagination. And then on top of that solving the paradoxes are a bunch of other giant leaps.
Great video. Simultaneity in relativity is often overlooked. Nice explanation of why you can't ignore it. And it was great to highlight that what you measure using scientific apparatus, clocks, rulers etc is not the same thing as what you a see using your eyes or cameras. Nice.
Absolutely..Too many explainers just handwave away the idea that an 'oberver will see ..' without bothering to consider such things as light takes time for seeing to occur.
I liked your explanations in the first 18 minutes of the video, but I felt like you were missing a key part of the explanation that would make it more intuitive. But then I saw the final section starting at 18:20, and I was very happy to see that you covered the key part. Your explanation was great!
Excellent explanation. Thanks again Mahesh. There are very few explanations of twins that even mention simultaneity, you have once again cracked the case by starting with intuition. I would love to help you with the animations, let me know if you would like some after effects help to really bring your message home. Keep it up!!
Hi, Mahesh. Will you make videos about general relativity too? I really hope so. General relativity is something I never understood, but I am very confident you could make us understand. Congratulations for these amazing videos..
That is how I thought you should explain in the first video.(the rope one) But because of my SSC exams I don't want to use my brain in understanding this video. But this video looks great. The hardest part in understanding physics is using imagination. This video makes us to them imagine easier.
Very good, lengthening the spaceships with the rods is a very good way to show how the asymmetries created by the length contraction affects spacetime. No other video on this topic that I have seen on RU-vid has gone into this in such detail... if at all. The age difference between the twins is less due to time dilation, but simply due to the fact that the traveling twin arrives at the destination at a different time than the twin on Earth observes. As you move quickly in one direction, events in the direction of movement move closer to you from the future, while events in the opposite direction of movement move away from you into the future.
@@trevoro.9731 Things in the direction of movement really do happen earlier, which is why the traveling twin ages less: he arrives at his destination earlier. This event occurs later for the twin on Earth because he has a different perspective on space-time. Events are therefore not moved within space-time, but by speed or acceleration you get a different perspective. Imagine you have a magazine with a picture of a person on it. If you turn the magazine slightly so that you are looking at it from the side, the photo will become smaller for you... although it is still exactly the same size as before. This can be compared to length contraction. One side of the magazine will simultaneously turn towards you and the other away from you, making one side appear larger than the other in perspective. This can be compared to the relativity of simultaneity.
@@TingleCowboy I understand it as slow down of energy interaction for one twin, the time doesn't really slow down and remains the same. The perception of the time is different, and that it it. I don't get why adopt such a complex way of thinking. The perceived time or the time when you get information about the event doesn't matter. For me there is no paradox once you accept that the time is absolute, but the speed of energy interaction and therefore perceived time may differ. Even if one twin aged by 2 days, while the other aged by 20 years, the time relatively to the other one is still 20 years, just the perceived time in the exact spot is different. The c "constant" is different locally for both of the twins, and that is it.
@@TingleCowboy Also, I understand the aging of the traveling twin as a result of gaining energy relatively to his inert mass, the more energy he has, the greater the energy interaction is slowed for him at his moving point. But the time remains the same no matter how fast he travels.
Relativity is the very engaging topic possible only when mahesh sir turns around.Keep doing your good work sir. First view sir. Thank you 🎉for your videos expecting this long time.
Sir, what about quantum? And electromagnetism? I also want vids on it!!! @@Mahesh_Shenoy Also would u explain everything from basics in quantum mechanics like u made the series of relativity , including works of shrodinger, broglie, and maths behind quantum? Ik it will take a long time.. and m ready to wait.. but I would like to see ur response on this!
I thought of this with your last video on the triplet paradox, and realized that changing reference frames is the only reason the turnaround matters. If we turn the accelerating frame into 2 non-accelerating ships, then that twin’s “acceleration” just moves her into the first non-accelerating ship’s reference frame, and the turnaround “acceleration” moves her into the other ship’s reference frame. In your last video, the radio tower between the planets helped me see why those reference frames have the opposite planet clock 7.5 years ahead of the close-planet clock. Those reference frames exist regardless of any acceleration, as discussed in this video you can move a clock into that reference frame by beaming a signal, but acceleration is just a different way for someone to move their clock into each of those reference frames. I finally intuitively understand the twin paradox after years and dozens of videos, thank you!!
Hi Mahesh! Excellent work and real mind-bender of a concept. If I may offer a suggestion, I think the point would have really been hammered if you showed the length contracted version of spaceship 1 from the frame of spaceship 2. The reason is that there is a third spatio-temporal simultanety event that appears to be violated (i.e. the simultaneity of the ends of the two ropes being at earth at the same time (at time 20). A visualization of this simultaenity in the three frames of reference would have made a lot of sense.
Another way to think about it is if you don’t have a frame of reference switch and stay in one frame of reference the whole time. When the twin flies away from earth the twin is at rest. if we stay in this frame of reference when the twin turns around, the twin agrees that she is moving left with respect to her initial frame. And she will say that she is moving left at twice the speed than what she would be moving left at from earth’s perspective. So I suspect that the increased time dilation and length contraction she accounts for then what earth accounts for will make up for the time dilation and length contraction she didn’t have from her frame of reference in the first leg of the trip.
Nifty. For anyone interested, it's pretty old now but Brian Green's free special relativity course on world science u website was really some next level stuff, especially at the time. It's quite comprehensive regardless & there are interactive demos with every chapter. Not sure where the actual demos stand now BC they had to update them for browser compatibility a few times. Also there was a long comprehensive version & a short(mathless) version called spacetime & Einstein.
I am delighted to be finally able to understand the Twins Paradox. Thank you. This is awesome. (All your videos are awesome, but I've been struggling with this basic question for fifty years. You nailed it for me. Thank you.)
For me it gets more intuitive and clear when looking at a spacetime chart and drawing planes of simultaneity for different observers, seeing how they rotate by Lorentz transformation. We all live in the same 4D spacetime, we just slice it differently with our different coordinate systems.
Yes, this is the most intuitive to me as well. But it does leave me wondering how meaningful the idea of that coordinate system slicing is, outside of the math. It is an invaluable accounting tool, but to me it says that the idea of "now" is a fiction, unless you're talking about things that are happening locally; the meaningful thing is only what can reach you. So in the video, receiving the photo from a 2.7 years older earth is meaninful.
@@erinm9445 Well, we all can only observe something right here, right now, locally. Lorentz transformation keeps light cones intact, those form certain causal structure, but all the distant things we can call "now" are outside our light cones and can never be observed directly, so there's a lot of leeway in choosing what to consider "now", we're somewhat free to mentally project what we think "now" is at distant points... The "now" plane doesn't seem to be so rigidly defined as causally connected events. (just thoughts without a very clear conclusion)
@@thedeemon For me, the explanation in the video is more intuitive than the spacetime chart because with the spacetime chart I always have the question: Wait, wouldn't the graph be the same as the one of the ship, if the earth is the one that leaves and comes back? but with this video I have no room for doubt and my brain is at peace now.
@@PADARM The question at the end is valid point! But the reason you cannot do the spacetime diagram in which the earth goes away and comes back is that you are not allowed to treat the spaceship as an inertial reference frame for the whole trip. Because it accelerates at the turning point. In this videos scenario you cannot think about putting the "travelling twin" only in one reference frame because there are two ships that are clearly in different reference frames.
I have one more question. There is a theory that spacetime is a torus. According to that you could travel in one direction, and after a long time you would come back to the same place WITHOUT changing the velocity or changing the reference frame to the other spaceship. How does the solution looks like in a setup like this?
I watched lectures of another physics professor on the SR topic in the past (he did not have such high-quality visualzations, though), and back then I already got the impression that profoundly or intuitively understanding relativity of simultaneity is key to understanding all of SR. There isn't as much talk about it than about time dilation although it is such an important aspect or result. I would love to see another video on relativity of simultaneity with another example.
the IDEA that whether two events that are separated in space happen at the same time is not absolute, but depends on the observer's reference frame. That's the crux of the problem. You are conflating information with time. It doesn't work that way. Causality. Cause and effect. The traveling twins ship blows up. It takes 6 months for the EFFECT to reach Earth. Is the twin alive or dead? You won't know until the wavefront collapses. Aka, stops transmitting. You don't see things in real time. You are always experiencing the past. Lightning strikes a tree. You don't know that until the effect arrives at your location in space. What I don't understand is why you don't understand that light is no different than sound. They are both just carrier waves. Other than c being absolute acceleration, they are essentially the same because the laws of physics are equally applicable in ALL frames of reference. Including the frames of light and sound. E=mc. Everything is an emergent property of acceleration. F=ma. Force equals Acceleration and Acceleration equals Force. Acceleration in space equals Acceleration in Time. The inverse square law of motion. Force decreases with distance. Information about an event (when it takes place) is moved further back in time. Betelgeuse. 500 light years away. When it explodes you will think it's taking place in real time. That's the IDEA of simultanaeity. That doesn't mean they are OCCURING at the same time.
It’s frustrated me for a while that this is referred to as the “twins paradox”, because there is never a paradox. The universe always consistently works out the correct solution the same way in every such scenario, regardless of perspective. It’s only been an issue with people being able to identify and explain the misunderstanding that’s been the source of the perceived “paradox”. Thank you for finally providing a clear explanation that so many physicists have fumbled trying to explain. Check out Fermilab too, where they use two separate rockets also to explain the science.
paradox is generally a really annoying term as it refers to both contradictions and simple problems of Intuition (which can completely become Intuitive when working with relativity!)
the previous video introducing a 3rd twin and saying the sister already sees the brother already aged 7.5 years certainly did not help and was very misleading.
_"an issue with people being able to identify and explain the misunderstanding"_ - that's what makes it a paradox. The term "paradox" is used in its meaning "a seemingly contradictory statement". There is definitely an apparent contradiction, so use of the term "paradox" is appropriate. That there is no real contradiction in reality, does not mean it's not a paradox. It just means the paradox is resolved.
@@F16_viper_pilot _"A “resolved paradox” is not a paradox"_ - then you have a different definition of the term "paradox". That is fine. I was just explaining what the normal definition is.
What I find paradoxical is that in this video-which is supposed to explain advanced physics concepts intuitively and without math-there's a lot of hand-wavy _"if you do the math"_ that's doing all the heavy-lifting, but without showing how to actually do the math. Certainly there are lots of positive comments from people, claiming that they've finally understood the resolution to the twin paradox. But without the actual math, they've accepted your explanation through faith alone. Consequently, I don't think they really understand relativity, let alone the twin paradox ☹.
Alot? It's just the part about the light they see and the math isn't the point of that part. It would add unnecessary complexity to not help get an intuitive understanding of how it works. Doing and showing the Math is great for when you need to know how to do that but not when you want people to get a fundamental understanding of how it works and how to understand the paradox. The point there was how can they see the same earth and still disagree about it's current age. That question is not a math question. That is an intuition question. That is the counter argument that he was addressing there. He is not trying to show that that the math adds up but how you can resolve that question: That they disagree how old the light is they are seeing.
@@helderboymh Relativity is _not_ intuitive, and the only way to get a decent understanding of it is-IMO-to work through the math oneself. However, if Mahesh's viewers feel they understood relativity by watching videos like this, I wouldn't begrudge them their satisfaction. I mean, solving relativistic problems is not a required life skill for most people. It makes no difference whether they truly understand relativity or only think they do. I was formally taught both relativity and quantum mechanics in college. I found both very interesting. But as an engineering student, I knew even back then that I wouldn't be using them in my professional work anytime. And indeed, that's how it has been. In general, though, I find people's math phobia-coupled with the reluctance of popular educators to use math as a teaching aid-really unfortunate. It has real-world consequences, such as the inability of people to comprehend the nature of credit card debt.
As every experiment from Huygens, Newton, Young, Maxwell & Einstein only deduce its Particle/Wave Results by BEHAVIOUR... "ASSUMING" if we could see the Light in Microscopic Scale in Real Time, What would it really look like, A Particle/A Wave or Both??? Only you can justify my Curiosity sir🙏🙏🙏
Yes, I do agree, all videos I've watched is just trying to convince through Double Slit Experiment based on the RESULTS as Wave Behaviour...it's just so irritating to really visualise the True nature of light...plz MAHESH SIR, make a detailed rational video on this...I want to get over of This Double Slit Experiment...plz come with some new fresh approach
Notice that what you call ‘jumps in time’ only occurs when there is acceleration. Acceleration can then be called ‘traveling’ into the future frame of a locus of regions where the magnitude of the ‘jump’ depends on distance to regions.
Great video. There is another way to look at it. If you plot the courses of the spaceships on a space/time diagram (assuming time is a dimension with the same characteristics as the three dimensions of space), you can see that the outbound spacecraft is on a trajectory with a frame of reference where the Earth is behind in time (losing time due to time dilation). The inbound spacecraft is on a trajectory with a frame of reference where the Earth is ahead in time. These trajectories are visually obvious on such a diagram. That was my epiphany with special relativity. The illustration in your video alludes to this. The observer on the outbound spaceship has to look back to see the Earth, but the observer on the inbound spaceship sees the Earth up ahead. If you extend the illustration to be a three-dimensional space/time diagram (assuming the observers can "look" in all directions of spacetime), you will see that the observer in the outbound spacecraft has to look back in time to see the earth, whereas the observer in the inbound spacecraft sees the earth ahead in time (see 13:27 in your video). When the inbound spacecraft arrives at the Earth, it arrives earlier on the time scale than the Earth; the Earth has passed through more time than the combined journeys of the spacecraft to and from the red planet. On a space/time plot, acceleration rotates your frame of reference. So, if you repeat the illustration with only two spacecraft, when the traveling spacecraft accelerates to begin its return trip, its frame of reference rotates from one where the Earth is behind in time to one where the Earth is ahead in time. In the three-spacecraft illustration, the third spacecraft is already on the return trajectory with the Earth ahead in time. It works either way.
So much explanations of twin paradox out there, but your's is the only one that gives a good explanation without that weird feeling that some things don't really make sense. Your prefious video on twin paradox I find completely right as well except that there was an example with deceleration and acceleration, while here is the same without it but with opposite directions of motion. It is still hard grasp as time here not as intuitive as we experience it differently, but still even in this example it seems that the only difference between the rocket and earth that breaks the symmetry is changing the direction of motion which could be done by either acceleration or by introduction of the second rocket. Eventually some explanations that refer to coordinate transformation and change in direction due to acceleration make much more sense now considering propper understanding principles of lenth contraction and simultaneousity. Relativity is so mind-breaking - it's fascinating. The only thing I have serious issues with now, is how comes the time dilation and length contraction calculation come from geometry of vertical clock? If we consider the information speed as C then shouldn't the passage of time depend not on vertical photon clock, but on photon moving in a circle to make more sence?
By far the best explanation of the twin paradox. However if acceleration has NOTHING to do about it, then you can attach a rope to earth instead of the spaceship, end get the exact opposite result, just using 2 earths. Hence even if the paradox is not general relativity related, still the accelerating time frame is the one that will stay young.
Very interesting video, thanks! 😊 When I think about the twin paradox, I like to imagine a ship of a certain length. (to "better visualize" the movement of the "sedentary" twin on Earth) The astronaut twin is younger than his brother when the ship reaches a star (or any point in the Earth's frame), and the "sedentary" twin is younger when he reaches the tail of the ship. As the ship moves, the distance between Earth and the star contracts. And, as the Earth moves, it is the length of the ship that contracts. We have two different "families of events", which can be simultaneous in one frame and non-simultaneous in the other.
Congratulations on the excellent presentation. The described "third spaceship" solution is a good way to understand what actually happens with perception of different clocks during the acceleration of the spaceship twin. It is good to understand this solution to get a better intuitive understanding of relativity of simultaneity. But it does NOT solve the paradox without acceleration as you claim it does. For your explanation, you have chosen a scenario that uses another spaceship traveling in the opposite direction, and the spaceship twin jumping onto that spaceship. That choice of scenario is an arbitrary choice. There is no a-priori reason to make the spaceship twin jump ships, instead of the Earth twin. You could just as well choose a scenario where the earth twin makes the jump instead, and the spaceship twin never does. Suppose, from the spaceship's perspective, the following happens: The Earth is initially moving away from the spaceship. At the turning point, the Earth meats up with a second planet that is traveling in the opposite direction (towards the spaceship). At the moment Earth reaches that other planet, the Earth twin jumps onto that other planet, and they travel towards the original spaceship. When the second planet reaches the original spaceship, they compare clocks. Nobody accelerated in this alternative scenario either, yet in this alternative scenario, less time passes for the Earth twin, than for the spaceship twin. That is the opposite from your scenario. Hence the paradox is not resolved. So which of these two scenario's must be used to solve the paradox? Does the spaceship twin jump onto another spaceship, or the Earth twin jump onto another planet? It is the ACCELERATION that decides which scenario is valid. The twin who accelerates is the one who changes reference frames, and therefore the one who jumps onto another ship/planet. It is only the acceleration that solves the paradox.
@silverrahul _"this is the same premise as the one in video"_ - exactly., it is the symmetrically opposite scenario from the one described in the video, with a symmetrically opposite result. Therefore, they cannot both be correct. Hence there is still a paradox. _"there is no acceleration involved."_ - there is acceleration involved in choosing the scenario. The acceleration determines which twin needs to jump ships/planets, and therefore which scenario to choose for the explanation.
@silverrahul _"In your example, you switched the names, so earth twin is younger."_ - I did not switch names, I chose a different scenario. In my scenario, the Earth twin ages less. In the scenario chosen by the video, the Earth twin ages more. Without referring to acceleration, there is no reason to choose one scenario over the other. Therefore, there is still a contradiction. It is very easy to understand when you don't start one twin on Earth, and the other in a spaceship, but place both of them in identical spaceships instead. One is in spaceship A and the other in spaceship B. A sees B move away and then return. B sees A move away and return. For which of the twins does the "third spaceship" need to be used in that case? _"there is no jumping ships/planets"_ - the "jumping" is just a metaphor for changing reference frames.
@@renedekker9806 "the "jumping" is just a metaphor for changing reference frames." In order for your argument to be valid, you have to explain why syncing clocks is equivalent to acceleration. There is no jumping, and using that word (which does inherently imply acceleration) is a dodge. The only argument I can see for why synching clocks implies acceleration is that for information to be sent and received, physcial processes must happen in the ships' computers, and those physical processes require acceleration of mechanical parts. But that seems very weak. That is just acceleration happening within the ship, not too the ship. The earth and ship are both experiencing all kins of internal accelerations for the entire journey, but those are meaningless to the larger problem.
@@erinm9445 _"you have to explain why syncing clocks is equivalent to acceleration. "_ - it's not. Changing reference frames is equivalent to acceleration. That's what acceleration does, it brings you from one inertial reference frame into another. You might want to watch the video again. _"...but those are meaningless to the larger problem."_ - you are missing the point. It is very easy to understand when you don't start one twin on Earth, and the other in a spaceship, but place both of them in identical spaceships instead. One is in spaceship A and the other in spaceship B. A sees B move away and then return. B sees A move away and return. For which of the twins does the "third spaceship" scenario need to be used in that case? And based on what do you decide that?
@@renedekker9806 I run accross your comments frequently in the physics videos I watch. You are clearly really smart and knowledgeable on the subject, but there is so much unnecessary condescention in your argumentation. You often make unexplained assumptions in your writing, and then get frustrated/belittling when the person you are responding to doesn't share the same set of assumptions. Maybe those assumptions are warranted for someone who has a PhD in physics, but they are not warrented in youtube comments, where 99.9% of people do not have a PhD or even an undergrad degree in physics. I think that you'll make much more headway in your attempts to educate if you keep your audience in mind, and recognize that while your knowledge is rarely at fault, your explanatory clarity (to someone who is not an in-the-trenches expert anyway) is not.
Hey sir! Great video! I kinda have an opinion. This might be wrong but please have a look. When we are looking things from the perspective of the space ship ➡️, the space ship ⬅️ is moving left of the first one with even greater speed than the earth. So if we consider the perspective of one space ship ALONG with the movement of another space ship (length contraction, time dilation and all)in animating, maybe the "time jump" type of thing might become a lil more intuitive. :)
The intuitive explanation is that the "turn around" happens in the past relative to the Earth clock. Remember: every location in space is located in the past of every other location in space. The current "now" from any location is an expanding sphere propagating at light speed (usualy this is literally real light). The outbound spaceship is chasing the "zero" clock time that is traveling at the speed of light. It will arrive at the turn around location 2.7 years after the 0.0 light has already passed that location. The inbound spaceship started years before the outbound spaceship passed the Earth at time zero. The inbound spaceship is "swimming upstream" through the light speed "clock ticks" coming from Earth. (So the Earth is going to be blue-shifted and "sped up".)
Yaa you are correct. I got that . But what I am saying is, in the animation, while animating the perspective of first space ship we only see earth and the other planet. But what does that other spaceship look like from the first ship's perspective?
0:56 „Acceleration is absolute“ Question: In which regard is acceleration absolute? In terms of the presence of acceleration (yes or no)? Or in terms of its value (all observers agree on its value)?
You can feel acceleration. There is no ambiguity about who it is happening to. You might see earth accelerate away in the window, but you'll agree that it is you who is feeling the acceleration.
@@akaHarvesteR The reason for this is that acceleration requires a force, right? But apart from human senses, how can we know which object is accelerating? Change in frames of reference leads to emergence of forces which can be applied here too.
Very good video! The animations are vivid and very explanatory. The title is a bit misleading, though.... in reality, nothing CAN change frame of reference without being accelerated. It illustrates the steady state approximation after the acceleration part happened... and is correct, but much of the paradox's "meat" resides JUST in that phase. Even in the limit of an instantaneous frame change, with only ONE moment of infinite acceleration (= an _infinite_ force for an observer with mass, because the necessary impulse [DeltaF*Deltat ] is definite) the whole picture changes abruptly JUST there! (BTW: massless objects cannot be observers: they lack internal structure so they don't have memory, and that is equivalent to say they don't experience time, or space)
This is the best explanation I've ever heard for this paradox! Thank you! Also, does that mean the future is just as real as the past? Does that mean the future already exists since there's always a frame of reference where your future is the present? or the past? That's weird.
@silverrahul Right, that makes sense. But even with causality preserved - it still gives me a weird feeling to think that my future is already "written" so to speak. In other words, philosophically speaking here, does this mean an absence of free will?
@chrispyfrenchfries Things are happening somewhere else, and it takes time for you to find out about it. Every location in space is located in the past of every other location in space. Your feet are six nanoseconds in the past from your head, and your head is six nanoseconds in the past from your feet. Alpha Centauri is four years in the past from the Earth, and the Earth is four years in the past from Alpha Centauri. Any events happening within any three years time period on either the Earth or Alpha Centauri can be arranged so that the events at either location could occur first, or even simultaneously. All possible ordering of those events are valid. For four year periods or longer, then there will be only one possible valid order of events from cause and effect. (But for shorter periods, there is no cause and effect relationship to define a single order of events.)
Our light cones are not changed by a change of frame, Lorentz transformation, so what's in our past light cone remains in our past, what's in our future light cone remains our future, the only events that can "switch from future to past relatively to us" are ones outside our light cones, they are not directly observable anyway, so all this spacetime region outside our light cones is somewhat undefined about being future or present or past for us, the plane of simultaneity that we mentally draw remains just that, an idea.
@silverrahul Perhaps free will was a poor choice of words. I guess what I mean is best given by an example. My death has already occurred in someone's present, their "right now". I'm moving towards that event, but that event already exists out there, it just takes time for me to get to it. Do you believe in such a scenario? That means how I died, why I died, all the things surrounding that event have already been determined and have been determined for all time? In other words, time isn't something that manifests itself in the present, all of time already exists, we're just moving through it....I don't know it's hard to explain I guess.
The people in the spacecrafts-if they understand relativity-really shouldn't be fighting each other over the elapsed time on Earth. Each would know that the other sees a different picture of events, and that both are correct in their respective frames of reference. For example, the chaps flying away from Earth would say, "According to us, Earth has aged 5 years, but according to you, Earth should have aged 35 years." And-provided nobody made a mistake in calculations-the chaps in the other spacecraft would say, "That's right." Suppose you and I stood facing each other a couple of meters apart, and a dog stood between us. I would say "The dog is facing towards my right, which is your left." Of course, when I was very young, I didn't know that different people could have different perspectives. At that age, I believe I would have insisted that the dog is facing towards *the* right (not "my" right), and I would've fought you if you contradicted me. Then again, nobody let me fly spacecrafts at that age!
Thats insane! I wasn't expect to take anything more from twin paradox, but that is so cool. The best video i saw for last 2 month. I think relative simultaneity is the only reason for everything, including length contraction. Like in order to measure length you need to connect space separated points, f.e. with ruler. And that takes time to move its end to correct place.
All three effects - time dilation, length contraction, and simultaneity - can be derived from each other, all of which is a consequence of constant speed of light. I find this so wholesome (and satisfying).
Yes, but it’s not that it takes time to move, it’s that if you’re in the middle of the ruler, your definition of now at the front end is in the rulers past, so it’s closer to you. Back of the ruler is in the rulers future, so it’s closer to you: boom, ruler shorted. Note that this works with atoms, without Lorentz transformations. If you solve the ground state shroedinger equation of a moving proton, with its electric and now magnetic fields and a moving electron, the answer will not be a spherical orbital…it will be squished. Length contraction is real at the at of level. Actually, it’s really at the quark level too, but that’s a longer story.
That acceleration free thought experiment is not the twin's paradox. It's simply what you get with three inertial frames of reference with their own clocks. In the case of the true twin's paradox, there are just two clocks, one with each twin. If you can work out a way in which the twin and his/her clock can jump into the frame of reference of the right bound rocket without undergoing acceleration then, on reaching the red planet, jump into to the frame of reference of the left bound rocket without also undergoing acceleration then you have the twin's paradox. As it's shown, you have just presented a different scenario altogether which is not the same problem. Nobody thinks it is. The only thing that breaks the actual symmetry of the twin's paradox are those two changes of inertial reference frame of the "travelling twin". If you know of a way of a massive body changing inertial frames of reference without undergoing acceleration, then I would be interested to hear it. So if you want to simply explain the twin's paradox as simply there are two changes in inertial reference frame, then fine, but do not claim there is no acceleration involved. There is with the problem as originally posed, but the thought experiment shown is not that problem, nor was it ever a paradox. As far as I can see, there is no way of the travelling twin and his/her clock doing those jumps in inertial frames of reference without undergoing acceleration.
I agree. Acceleration is needed to shift from one inertial frame to another. The actual resolution to the twin paradox is by keeping in mind the relativity of simultaneity as done in this video: ru-vid.com/video/%D0%B2%D0%B8%D0%B4%D0%B5%D0%BE-mZByVMQpdAI.html
This is some great video. Yet, I have a simple question. Starting from 17:21 You say, "Acceleration causes change of frame, causes relativity of simultaneity and relativity of simultaneity causes the resolution to the Paradox so the root cause is relativity of simultaneity" Let's analyze it shall we? So Acceleration causes change of frame, Which means that the acceleration is the cause to the relativity of simultaneity. Then you say, simultaneity and relativity of simultaneity causes the resolution to the Paradox, whic means the cause to the resolution of the paradox was the relativity of simultaneity right? So, if we draw the cause and effect sequence we would get, Acceleration cause relativity of simultaneity -> Relativity of simultaneity caused the resolution to the Paradox So it would be like, Acceleration -> Relativity of simultaneity -> The resolution to the Paradox If this is the cause sequence how can you conclude your statement with, "he root cause is relativity of simultaneity"?? Isn't it the "Acceleration" which is the cause of the relativity of simultaneity should be the root cause?
I feel bad for watching this now in case you think I'm only watching because you changed the thumbnail. I love your video. Thanks for explaining things simply.
Well done, but it is Einstein himself who used the acceleration in combination with GR to resolve the paradox (see: Die Naturwissenschaften, 48, p. 102-107, 1918).
Very good explanation. If you draw the timespace diagrams for the earth and for both spaceships and suppose that earth and both spaceships send regularly spaced signals, you notice that everything sets into place but if you attempts to merge the spacetime diagrams for both ships, the trajectory of the earth becomes discontinuous. Because relativity applies only to objects with constant speed…
I think this is a better explanation than the other twin paradox videos id like to hear sabina's and Dialects response to your video - what do they think?
On the "dialect" channel one of the videos has a pointer to a paper where They Did The Math. It turns out if you have gravity, you can make the person who accelerates older or younger, and have neither accelerate and be older or younger. You can have one space ship orbiting (inertial movemet) in space while the other holds itself still in a gravity field (accelerating), or you can use a "gravity slingshot" to turn around without accelerating. The only way to figure it out is calculate the path thru four-space / spacetime and see which is a longer time difference. But this gives a great insight into relativity of simultaneity.
i don't know if this is an accurate metaphor. but i like to think of the blue spaceship as running away from the light of the earth. so it's like a earth light being a tv monitor that is going from 60 frames per second to 30 frames per second. as the space ship is running away from the light of the earth. like doppler effect. while the red spaceship by moving towards the earth its fast forwarding through time. so all of a sudden the light or frames per second coming from earth doubles from 30, to suddenly 60 per second, to then 120 per second. as you race towards them. running into all of them faster. like doppler effect. so blue space ship is running away from earth, so from its perspective earth is running slow, like a monitor tv screen film running slow at half speed. and then when it becomes the red ship, its like the earth's light, earth's monitor suddenly speeds up x2. like how you can change the speeds of the video on RU-vid. not sure if this is a good way of understanding the time jump, time dilation and Relativity of simultaneity. . but it seems to help. i think. one direction you see earths frame rate slow down. the other direction you see all of tbose delayed earth frame rate suddenly speed up and bombard you with it. like doppler effect. you ran away from your wave first ship. then you turned around and ran towards it second ship. maybe you should change the color of your ships engine lol. 😂😂😂 I'm no expert. but thanks for the video.
@silverrahul how can a metaphor be wrong. its a metaphor. and you are arguing because you're insisting by saying the metaphor is wrong. when its just a metaphor. maybe you can't comprehend what a metaphor is. In physics we use metaphors a lot. Like how light is described as a wave and a particle. those are also metaphors, if you didn't realise.
You are correct in that when you're moving toward the earth, you will "see" everything moving in fast motion because the light is getting compressed and blue shifted. However, if you take into account your relative velocities and figure out how fast they are actually moving to emit the light that you're seeing, you'll calculate that they're actually moving slower, but just appear to be moving quickly. To put it another way, if there was no time dilation, you'd see them move even faster.
Am I getting this correctly? Moving to an object, accelerates your perception of time for that object whereas moving away from that same object slows down your perception of their time. This is because the speed of light is the speed of casualty. ie. when you are moving towards an object more light, therefore more events at that destination are perceived by you. Conversely, when you travel away less light/less events are perceived by you.
Yeah that's the Doppler effect. It will make distant objects appear to speed up or slow down depending on what direction stuff is going... there is another different kind of time dilation which *always* slows down moving clocks, no matter what direction they are moving in. And it is symmetric, everyone will see everyone else's clocks slow relative to their own clock. (It's not a paradox.) Anyway, you add both these two effects together to calculate what travelers will actually "see" when looking at other travelers. There's also gravitational time dilation that I'm not going to explain here.
You "see" them move in fast motion because their light emissions are all bunched together (blue shifted). However, if you then sit down and do the calculations (i.e. you note you see them moving quickly because their light emission are getting bunched together, and you figure out how fast they are actually moving, you'll calculate that they're actually moving slowly). To put it another way, if there were no time dilation, you'd see them move even faster.
You are doing great at elucidating special relativity. I understand it so much better than before. Perhaps I am still missing something, but this video in the twin paradox got me thinking that the science fiction Star Trek warp drive is already inherent in special relativity. In your example the space is warped into half its length in the direction of travel. So if I understand correctly, the spacecraft can accelerate even more, and the length in the direction of travel can contract to one-tenth, one-hundredth, one-thousandth of the lengths in the reference frame before the spaceship accelerates (given enough energy to affect such an acceleration). This at least for the proper time reference frame of the space explorers. Am I misunderstanding something here, or are warp drives inherent in special relativity?
Oh my god I'm going to have to watch these videos so many times until I fully get it, I can already tell. But this is the only video that actually resolves the paradox, which is amazing. I have been pestering GPT4 about this paradox, and it kept saying "acceleration, acceleration, acceleration" and it kept failing at the point of relativity of simultaneity. Maybe GPT5 will get it! Lol
@@Mahesh_Shenoy As of January 2024, that is definitely the truth! 😅 Hey, love your Socratic dialogue teaching style and enthusiasm! Keep making the best vids!!
Something I heard 15+ years ago was a question on if Light moves at the same speed everywhere in all directions, or slower than we expect one way and instantly the other way and how could we prove which was correct. Does this sound familiar? Have you done or would you do a video on this?
6:00 Strong disagree there. There is absolutely still acceleration. Sure, none of the ships are accelerating, but the REFERENCE FRAME from which we calculate time is accelerating, which is what really matters. Because the reference frame is accelerating, there is no Lorenz transform that can make it appear stationary, which is why the twins’ reference frames cannot be seen as equivalent.
Here is a question for you. If there is no jumping of frames, and instead one of the twins is on Earth while the other one travels close to the speed of light at a constant velocity (no turning allowed), then which twin will be younger after some period of time? In other words, I am trying to see the limits of the twin paradox. Why is it that we need a change in direction or a change in frames to resolve the paradox? And is it possible to solve it without this. Simply at constant velocity, can we still tell frames apart?
Both will consider the other to be younger. That's fine though, because without acceleration they can never meet up again to actually disagree. In the video, we introduce a third spaceship to try to meet both the moving and stationary twin. However, we see that it gets resolved because whichever one it meets first (either the twin on earth or on the ship), it will calculate that the other twin is already way older (but is still aging slowly).
I think that the case I mentioned lacked a beam of EM so that we could solve it. It seems it was not a complete question. After having watched the triplets paradox with a radio signal the answer becomes clear . Thank you!
@@Empiro3 Doesnt make sense. Yes if they look at the light coming from Earth/Spaceship. But with the math they should be able to figure out were and how old they really are. This is not really explained.
Another way of looking at it is to include the first spaceship in the second spaceship's frame. Ship 1's clock and rope will be distorted even more from Ship 2's point of view, since it is moving the opposite direction.
I'm still struggling with the concept. If the first spaceship suddenly stops moving (or from the spaceship's perspective, the Earth stops moving) when it reaches the red planet, will the Earth still be behind time-wise? But then, from Earth's perspective, when the spaceship stops moving, the spaceship should still be behind time-wise, right?
Sometime ago i saw a visual representation of relative simultainety in a random yt video. Couldnt ever find it anymore, but it sticked to me and maybe some of you may know where it does come from, and ill try to describe it. Imagine space time as a solid (like a half cilinder) represented in xy chart, where y is time, x is space, the cilinder has its radius in y and lenght in x, and simetrical in x. And imagine now as being the collections of the "now events", a slice of the solid, cutting x axis including the point in x where the reference is. The Speed of the reference change the angle of the slice with x axis. If everything has no Speed, the slice would match the x Axis. If something has a Speed to the left, It creates a positive angle, and If the speed is right, the angle is negative. It doesnt explain, but helps visualize the diference. Lets use the Mahesh frame. Using the reference as the point where red planet is. For the red planet, the "now" is the x axis, and y value is the same of earth. For the ship1, moving right, the now has a negative angle, and at the x location where earth is, the y value of the now will be lower (5 years). For the ship2, moving left, the angle is positive, so the y value for the earth location Will be higher (35). The further away the objetcs are, or the speed, the higher is the difference between the "nows". Pretty sure i didnt say everything totally accurate, but enought to picture it
I'll still say the same thing. It's the acceleration that breaks the symmetry between the twins. The exact mechanism may vary. In your explanation, its the change from one frame to another that causese the travelling twin to get two different measure of what's simaltaenous in Earth. But what causes that change of frame? It's acceleration. So, It Is the physical root cause. But the explanation directly involving acceleration and equivalence principle is also equally valid. So, it feels the relativity of simaltaenity or some special case of it is equivalent to the equivalence principle after all. But viewed this way, this relationship between acceleration/equivalence principle and relativity of simaltaenity shed a deep light on what acceleration and gravity really is.
You have it exactly backwards. What is responsible for the change in time is the change in reference frame. And acceleration is one mechanism to do this. In the example in the video there are two reference frames and no acceleration, but the result is the same. Acceleration is only necessary when you insist that one observer inhabits several inertial reference frames.
@@narfwhals7843No, you have it absolutely wrong and you obviously don't comprehend the problem to begin with. The problem is What breaks the symmetry between the twins? Saying "reference frames" is complete nonsense, because your choice of reference frames presupposes a resolution. You implicitly believe in absolute cosmic location and motion, which is why you didn't see the issue that makes the situation paradoxical in the first place.
@@MrCmon113I am not assuming absolute location and motion. Nor am I just blaming "reference frames". I am assuming that spacetime is flat and that there is an inherent difference between inertial motion and accelerated motion. Both of these are implicit in special relativity. In a flat spacetime two people moving in opposite directions can not be in the same inertial state of motion. That is enough to break the symmetry between people approaching and receding from earth because they can pass by each other. You can construct this via acceleration for a single observer but it is not necessary for the resolution of the situation.
This is one of the implications of relativity that really blows people's minds, that there's no such thing as a cosmological 'now' but only local 'now' and even different parts of the planet can have slightly different ideas about it given we're moving in different directions as the planet rotates.
