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A video I came across in youtube recently had mentioned how Einstein perceived time. It said he thought of all moments, the present, past and future to exist together and how it is not like one follows the other. I did not quite understand the concepts mentioned in the video, although I have a basic understanding of relativistic effects. I understand that time slows down as you approach the speed of light, not to you, but to people observing you and the length contraction and everything. But I am still unable to wrap my head around concepts like light does not experience time and the past, present and future co-exist etc. Can someone give me a detailed explanation? You can include math too if required. The link to the video can be found below.

https://www.youtube.com/watch?v=86RI1G1J2p8

Edit 1 : Can someone explain the concept of block universe? Analogies will help.

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marked as duplicate by John Rennie, Ben Crowell, GiorgioP, Kyle Kanos, Dvij Mankad May 21 at 12:59

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For the idea of a block universe, consider a loaf of sliced bread. Each slice corresponds to a different time in the universe and the other spatial dimensions exist within this loaf of bread. In that sense, one has a 'God's eye' view upon the whole universe, leading to the idea of past, present and future existing always, somewhere in spacetime.

The key to the 4D spacetime model is the idea of static spacetime: events occur within spacetime but the spacetime 'block' itself is static. This view disregards the very anthropomorphic idea that the present is forever fleeting and the past is forever inaccessible.

It is true that light, photons, do not experience time. Their internal clocks are infinitely dilated such that there is no difference to them between 13 bilion years ago and today. Indeed, lengths are also infinitely contracted for them, meaning that, according to the photons, the time elapsed in travelling between two separate points in space is exactly zero.

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I don't think that time is as simple someone could imagine. If you look at relativity any observer seems to have its own timeline. There isn't one thing you could globally call "one moment". An space-time event between two things is a point at which they have an interaction at the same moment.

I believe that Einstein stated that past, future and the present exist simultaneously to make it imaginable. While the truth is that it is not. There is no strict order in which moments follow other moments. That concept also gets lost in quantum mechanics as you often don't know when something exactly happened. If there are 2 independent interactions AB and CD it is not important in which order they happened and in relativity the order can depend on the observer. In quantum mechanics the order might not be defined at all.

To give an example ( extreme ) if we would meet and I would travel away to another solar system at light speed and come back at light speed you would see me travel for thousands of years while it would only take one second on my timeline. While I accelerate to go back I see you pass thousands of years.

The best way I found to imagine it is when you look at someone from a distance; if we walk away and look at each other from a distance you would say that I look small and I would say that you look small. According to relativity the opposite happens with time; I will say that your clock runs slow, which means that 1 of your seconds looks big and you will say the same about me. Then if I turn and walk back to you this makes a huge difference; if I turn slightly I will not end up where I was; I will end up somewhere else. How farther away I was when I turned how bigger the change in the position when I arrive back.

According to relativity; if you have speed the clock of the other seems to run slower like the perspective of being farther away. Then when you accelerate you take a "turn in time" with the effect that you cross the timeline much later.

The result is that you can't put a global time label on events. The thing we call time falls apart. In the extreme you could imagine that you meet someone 4 times and that the other thinks it happened in a different order. That doesn't happen in the theory of general relativity; the order in which you interact remains the same but the time between events you experience can be completely different.

This doesn't mean future and past have to exist. It just becomes completely impossible to imagine a scene with different players at different positions and times without that. But physics happens locally ... this scene might just be imagination; an attempt to imagine the impossible. We try to model things with dimensions; you could state that we all have different dimensions and that those dimensions relate to each other following rules which depend on location and speed. My dimensions seem normal and if you have speed relative to me your dimensions seem normal to you but are kind of odd seen from my point of view.

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  • $\begingroup$ Does it really make sense for every atom to have its own timeline however? If a clock is sent away and then returns having elapsed less time on its face than the clock that stayed at home, does it make sense to argue that the traveling clock is somehow remaining further in the past than the home clock, or that the home clock has gone forward into the future, even though both clocks are currently present? It seems to me more credible to argue that neither has travelled in time - instead, they just ran at different rates for a time. $\endgroup$ – Steve May 19 at 18:34
  • $\begingroup$ Yes and no. We could rewrite things and talk about rates of time but then the vision on those rates depends on your point of view. You don't have a system in which clock A runs slower then B. A thinks the clock of B runs slower and B thinks the clock of A runs slower. $\endgroup$ – Daan May 19 at 18:44
  • $\begingroup$ But I think you are right to think of it as rates. An absolute timescale just doesn't exist. It took the photon from the big bang 0 seconds to reach my eye. So how many seconds is that on what absolute time scale? $\endgroup$ – Daan May 19 at 18:51
  • $\begingroup$ Just to clarify I didn't refer to "rates of time", I said the clocks ran at different rates for a time. We are accustomed to understanding that clocks can run fast or slow for a variety of reasons, without talking in terms of a slow clock "having its own timeline" or inferring anything more convoluted or profound from the fact. Fundamentally we cannot measure time - we measure changes in the spatial state of machines whose spatial states cycle repeatedly, and the "flow" (rather than the loop/cycle) only emerges because we also bolt on counting machinery. $\endgroup$ – Steve May 19 at 19:02
  • $\begingroup$ The problem is that time is not a "rate" in relativity, time is a place with an independent dimension separate from the 3 spatial dimensions. Although that is a useful model for certain purposes, it does not necessarily relate to any physical reality. $\endgroup$ – Steve May 19 at 19:10

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