The Persistent Illusion of Past, Present, and Future

[Voidmoon]

I have been reading Brian Greene's book: The Fabric of the Cosmos and have a question. In chapter 5 Brian discusses how great distances can affect two observers conceptions of now. He uses an analogy of a loaf of bread and how two people can cut the loaf at different angles depending on their relative velocity and distance. This is known as the block-universe theory (I think).

Brian goes on to illustrate this theory with an example. Imagine a man named Chewie lives 10 billion light years away from Steve, who resides on Earth. When both of them are sitting on the couch, they both fully agree on space and time and would cut the loaf in identical ways. But if Chewie gets up and starts to walk aways from Steve, even at a low speed, this tiny shift in Chewie's notion of now is amplified by their great distance. Math is involved here but if Chewie walks away from Steve at 10 MPH, Chewie's reality is now 150 years in the past according to Steve. Um, what? I thought the loaf of bread thing was just an analogy. If Chewie walked back to the couch and sat down, would they fully agree on their notion of now again? What about if Chewie got into a plane and flew in circles at a high rate of speed. Since he would be traveling towards and away from Steve, would their versions of reality shift back and forth? Does this only happen at huge distances?

If Chewie was on Jupiter, would this still be happening, just at a smaller scale? Or does Chewie have to be light years away? I'm just having problems visualizing as to why this happens. I understand through Special Relativity, there is no absolute time and everybody can carry their own clock but I just can't seem to wrap my brain around this concept. Any help provided would be great; thanks in advance.

 

[observer7]

If you really want to warp your noggin, rean Julian Barbor's "The End of Time." He proposes a system of "Nows" which are configurations of everything in space. Every possible configuration exists, and time is just a track through these configurations based on the laws of physics.

It seems to me that his theory is an interpretation of "many worlds" that trys to incorporate the known laws of physics in a meaningful way. I like the idea because it gives substance to the many worlds interpretation of quantum mechanics and also incorporates the importance of the observer in what happens.

Sorry I can't be more helpful, but I think about things like this and get all noggined out.

Remember what Einstien said, "Time exists so everything doesn't happen at once."

 

[six_strings]

Been reading "The End of Time" for several months now. I get bored with it, confused by it, and set aside every couple days. It's a little advanced, you should minimally have some formal physics education to really be able to judge whether he's just pulling your chain. I do not have any physics classes under my belt, but do have an AS in science (majored in Respiratory Care). It's way over my head. Not a book for the layman... Just my 2 cents

 

[Voidmoon]

Einstein also said, " For we convinced physicists the distinction between past, present, and future is only an illusion, however persistent." Einstein meant all time exists exists at once. Everybody has their own sense of the past, present, and the future and they're all equally valid. I understand this concept; I just don't get how two people at distance can agree on their perception of now, but if one of them starts to move, even at a slow speed, a huge shift occurs in their realitys. I've been trying to find other on-line references to the block universe theory, but can't find another explanation that goes into this distance problem I'm encountering.

 

[nimbus]

Been reading "The End of Time" for several months now. I get bored with it, confused by it, and set aside every couple days. It's a little advanced, you should minimally have some formal physics education to really be able to judge whether he's just pulling your chain. I do not have any physics classes under my belt, but do have an AS in science (majored in Respiratory Care). It's way over my head. Not a book for the layman... Just my 2 cents

The only problem I had with it (even skipping some of the math) was the way he ties everything together with a deus ex at the end.

 

[SpeedFreek]

If Chewie was on Jupiter, would this still be happening, just at a smaller scale? Or does Chewie have to be light years away? I'm just having problems visualizing as to why this happens. I understand through Special Relativity, there is no absolute time and everybody can carry their own clock but I just can't seem to wrap my brain around this concept. Any help provided would be great; thanks in advance.

Yes, it is still happening at a smaller scale, between here and Jupiter. In fact, it happens between any two events in relative motion and is due to the relativity of simultaneity.

 

[Voidmoon]

Yes, it is still happening at a smaller scale, between here and Jupiter. In fact, it happens between any two events in relative motion and is due to the relativity of simultaneity.[/quote]

Thanks for the link, SpeedFreak. Would Chewie and Steve fall back into sync with one another if Chewie sat back down on the couch? I understand the thought experiment and the basics of the theory, but can't wrap my brain around how Chewie and Steve can be in sync one moment and then if Chewie gets up to walk around, this small movement is amplified so dramatically. So dramatically in fact that Chewie would now be 150 years in the past, according to Steve. I get the theory on a small scale, but just can't wrap my brain around it when very large distances are involved.

 

[SpeedFreek]

Thanks for the link, SpeedFreak. Would Chewie and Steve fall back into sync with one another if Chewie sat back down on the couch? I understand the thought experiment and the basics of the theory, but can't wrap my brain around how Chewie and Steve can be in sync one moment and then if Chewie gets up to walk around, this small movement is amplified so dramatically. So dramatically in fact that Chewie would now be 150 years in the past, according to Steve. I get the theory on a small scale, but just can't wrap my brain around it when very large distances are involved.

Well as you might suspect, this actually gets quite deep. What Greene is doing in the "Fabric of the Cosmos" (great book, by the way, I have it myself!) is applying the relativity of simultaneity across cosmological distances. It is an extreme and exaggerated illustration of how two observers notions of "now" can shift relative to each other due to their relative motion, applied across absolutely vast cosmological distances, in order to show how the notion of simultaneity is essentially meaningless at such distances.

Just as Special Relativity does not apply when considering galaxies that are apparently receding faster than light, the same can be said to be true of any notion of simulaneity - you cannot meaningfully synchronise two observers notions of "now", if they are on different sides of the universe. This is not due to the high apparent recession speeds themselves, it is due to the the fact that there is such a large slice of space-time between events and SR only works locally - any notions of simultaneity can only be applied locally in a meaningful way.

In the famous paradox of the twins, the difference in their ages can only be meaningfully compared if one twin turns around and returns to the other. It is the act of turning round, of changing your inertial frame of reference, that can be thought to cause the simultaneity shift. Aside from the difference in relative speed, it is the distance across space-time that determines the magnitude of the simultaneity shift. The further away the twin is when he turns around, the larger the shift in the two twins notions of "now".

If both twins remain in relative motion, and both twins remain in inertial frames of reference, then time-dilation is symmetrical between them. Just as one twin calculates the others clock is running, say, at half the speed of his own, the same is true the other way round! This is what causes the supposed "paradox" - time-dilation is symmetrical between observers in inertial frames of reference and so each twin thinks the other should age slower than himself.

Special relativity tells us that, if all the twins know about each other is that the other is moving away from them, then both twins calculate the others clock to be "running slower". That's it. In order for there to be a resolution of the paradox, the twins have to meet in the same place, in the same frame of reference. From afar, each twin can only assume the other is now ageing at a slower rate than themselves.

If one twin returns to the other, then on the return journey, as long as they remain in inertial frames of reference, once again there is symmetry in the time-dilation between them! So if the time-dilation between them was symmetrical as the two of them separated, and was again symmetrical as the two of them converge, where did the difference in their ages occur? Where was the shift in simultaneity, the shift in their individual notions of "now"?

The shift in simultaneity happened when one twin changed his inertial frame, or moved between different frames. The shift occurred when one of them slowed down, turned around and sped up again.

If you consider what each twin would calculate from their observations (after subtracting out speed of light effects like Doppler or aberration) if they could see each other during the thought experiment, both would calculate that the other was ageing less than themselves during the time they were in relative inertial motion, but during the turnaround phase, the twin who was turning around would speed up tremendously to the twin back on Earth. The opposite is true from the point of view of the moving twin, who would calculate that, as he performed the turnaround, the twin back home would be ageing really slowly. Another way to think of this is that the travelling twin leaps forward in time during turnaround, so relative to him, the stay at home twin leaps backwards in time. The effect is magnified, the greater the distance the twins are apart.

So what Greene has done is taken this idea and applied it at universal scales. He is not actually concerned with the relative motions of the two distant galaxies, he is only concerned with the vast distance between them, as that alone would cause a huge shift in simultaneity for any small local movement from an occupant of one of those galaxies, relative to the other. Thus, as you pace up and down in your living room, centuries leap back and forth in a distant galaxy, but only relative to you. The same is of course true the other way around, and this shows why any notions of simultaneity are actually meaningless across large distances. So, as an alien on a planet in a distant galaxy paces up and down, the time on Earth leaps backwards and forwards by centuries, relative to him/her/it!

Of course, we can safely assume that the above is not a reflection of any true reality! It is a useful way to use Special Relativity to illustrate how time is relative and there is no absolute notion of simultaneity across the universe. It is also useful in showing how applying the postulates of SR across vast cosmological distances is an essentially meaningless exercise. In order to apply SR, you need a universe empty of any gravitational influence and path through space-time taken between events in order to compare those events. This is one of the underlying reasons why General Relativity was formulated in the first place!

 

[Voidmoon]

Thanks for the great reply, SpeedFreak! I do have a question though about the twin paradox. Why would the twin view the other as ageing less? If both twins stayed in the same reference frame, wouldn't they both age the same?

 

[darkmatter4brains]

The shift in simultaneity happened when one twin changed his inertial frame, or moved between different frames. The shift occurred when one of them slowed down, turned around and sped up again.

Good post Speedfreek. For anybody who's interested, Taylor and Wheeer's famous textbook "Spacetime physics" has an EXCELLENT mathematical description of this paradox. They workout and calculate just where all the 'time-dilation" happens during the voyage of the twin. As SF mentioned above, it's amazing to see that most of the time dilation does indeed happen during the slowing down and turning around of the twin at his destination (i.e. when he becomes a non-inertial frame).

They also work out a number of other neat paradoxes, etc. A joy to read.

 

[SpeedFreek]

Thanks for the great reply, SpeedFreak! I do have a question though about the twin paradox. Why would the twin view the other as ageing less? If both twins stayed in the same reference frame, wouldn't they both age the same?

Well, I tried to compose this post three times, and each time I found my answers lacking in clarity and perhaps misleading, so for now I will have to cop-out and refer you to the very book you are reading, The Fabric of the Cosmos. Brian Greene explains it all far better than I can.

There are relevant passages starting on page 55, and in the notes at the back check out page 533, section 8 (although I notice that section refers you to Taylor and Wheeler, Spacetime Physics for a more in depth explanation!).

If you are still having trouble, ask again and I will do my best to help, but you should expect one of my long rambling posts!