PDA

View Full Version : Twin Paradox question, can anyone help?



baulch
2009-May-22, 04:50 AM
I'm not as trained or anywhere near as intelligent as most of the people on this website, and this question may have been answered before now. So I'm sorry if it has. My question has to do with the twin paradox, I understand that information would stop this from being possible because it can't travel faster than light, but lets say for one moment it could. If both twins had a video conference going while one was traveling, my question is. What would the other see? I know one would seem to age slower and one faster. However how would they interact, would one be acting super fast looking like a VHS stuck in fast forward, and the other really slow with a deep voice? I'm sorry if this seems basic to everyone else, but I'm struggling to get my head around it.

Could someone please clarify

Samuel Baulch

P.S Please don't answer if you are just guessing, thanks

Ken G
2009-May-22, 06:40 AM
My question has to do with the twin paradox, I understand that information would stop this from being possible because it can't travel faster than light, but lets say for one moment it could. If both twins had a video conference going while one was traveling, my question is. What would the other see?The question is very profound, but impossible to answer, because you are basically asking "what would happen if the way things worked was different from the way things work". If you are creating a different universe to ask your question, then the answer is however you create that universe. If you are asking about this one, then it is simply not possible to suspend the rules that say any such "video conference" cannot propagate information faster than light. You see, it's not just a technicality, it is central to the whole way that things work, and central to the limitations in how your question can be answered. The speed of light is not a problematic limitation we can imagine (even hypothetically) getting past, it is central to how space and time actually work.


I know one would seem to age slower and one faster. Even that is not necessarily true-- in the normal way that both twins conceptualize time for the other (the special relativity way), each twin thinks the other is the one that is aging slower, as long as both twins are moving at constant speed (i.e., both are separating from each other at a constant rate, so they are both inertial observers). So they simply don't agree on how to match up their "nows", and that's why there's no absolute way to have that video conference-- and even that discrepant matching is just an artifact of special relativity, it's nothing uniquely real that the universe needs to respect or care about. You are imagining that the universe knows how to match up their "nows", and put them in the same instant in the video conference, but the universe does not apparently know how to do that, it has no interest in doing that and no need to do that-- this is the message of relativity, there is no absolute one-to-one matching between the "nows" of those two twins. Any such matching is quite arbitrary, all you can say is that if one turns around and comes back, that twin will have experienced a shorter time interval when they reunite. How you match up the various moments in that shorter time interval to the longer one of the other twin is completely 100% arbitrary-- the universe simply doesn't do it in a unique way, it is never necessary to do it to establish any of the measurables of that scenario.

alainprice
2009-May-22, 02:08 PM
I don't see a problem with the question.

The twins must travel at less than the speed of light, while the video conference signal is light.

So yes, each would see the 'twin' talking and moving in slow mo. It's not so much that each 'twin' is slower, but the video is basically transmitting at a lower frame rate.

If one of the twins turns around and comes back, the situation gets a little more complicated.

StupendousMan
2009-May-22, 02:45 PM
I wrote this up for a special relativity course I
taught earlier this year. It examines in depth the
case of twin brothers, one of whom stays home
on Earth, one of whom goes off on a round-trip
relativistic rocket trip.

http://spiff.rit.edu/classes/phys200/lectures/twin_2/twin_2.html

hhEb09'1
2009-May-22, 02:56 PM
I don't see a problem with the question.I would agree with KenG. The OP seems to be suggesting some sort of instantaneous video conferencing (otherwise, why the comment about faster than light?), and imagining how that would work.

alainprice
2009-May-22, 03:01 PM
Okay, I'll still help him out then.

What if we create a wormhole, enlarge the openings, and keep it stable? I've got one opening and my twin has the other. It's like a magic mirror video conference. Now what?

I think it's fine to say that all video conferences operate at light speed, so we'll have to use that as one of our assumptions.

thoth II
2009-May-22, 03:46 PM
If the video signal didn't travel at "c" speed of light, then Einstein's fundamental postulate would be violated. Then all the consequences that derive from this, like time-space transformation, would be invalid too. So to have the video signals faster than light, we would have a new universe with unknown laws. So to try to picture what the first twin would see if he had instantaneous information from the second twin, the laws of physics wouldn't tell us; so your guess would be as good as mine as to what he'd see.

fifelad55
2009-May-22, 04:55 PM
A further hypothetical question in the same vein.

I'm also still trying to get my head around much of what is discussed here.

Let's imagine that rather than expanding the universe is contracting.

The twins are travelling in opposite directions at 51% of the speed of light. Is there a rate of contraction that would allow the twins to communicate even though they are apparently moving away from each other faster than the speed of light?

My own guess is that the rate of contraction would have to be about 100 times plus greater than the current rate of expansion.

Alan

Ken G
2009-May-22, 05:27 PM
The twins are travelling in opposite directions at 51% of the speed of light. Is there a rate of contraction that would allow the twins to communicate even though they are apparently moving away from each other faster than the speed of light? Actually, no contraction at all is required for that-- the twins can still communicate even in a perfectly normal special-relativity (Minkowski) spacetime. That is because in the frame you are describing, the twins are each moving at a speed 51% of light, but light is moving at 100% that speed-- i.e., light is twice as fast as either twin and can overtake either at its leisure. (In the frame of either twin, the other twin is separating from them at a speed that is less than c, not 102% of c, because velocities just don't add that way in SR.)

If you really do want to ask what happens if the universe is contracting, though, you are now doing general relativity and it's a lot harder. It's very hard to make true statements without doing the full calculation! But generally speaking, I believe the "ants crawling at constant speed across a rubber contracting/expanding tablecloth" picture works fine to handle questions like that.

robross
2009-May-22, 08:33 PM
If the video signal didn't travel at "c" speed of light, then Einstein's fundamental postulate would be violated. Then all the consequences that derive from this, like time-space transformation, would be invalid too. So to have the video signals faster than light, we would have a new universe with unknown laws. So to try to picture what the first twin would see if he had instantaneous information from the second twin, the laws of physics wouldn't tell us; so your guess would be as good as mine as to what he'd see.

I can't find the link, but there's a wiki link talking about GR and the Twin Paradox and they explore what FTL light-communication would mean, if it were allowed, and they come to the conclusion (with tons of math symbols I must confess are over my head) that this situation would permit time travel, and would allow one of the senders to receive a signal from himself in the future before he sent it. It was pretty cool. :lol:

Rob

pzkpfw
2009-May-22, 08:46 PM
I can't find the link, but there's a wiki link talking about GR and the Twin Paradox and they explore what FTL light-communication would mean, if it were allowed, and they come to the conclusion (with tons of math symbols I must confess are over my head) that this situation would permit time travel, and would allow one of the senders to receive a signal from himself in the future before he sent it. It was pretty cool. :lol:

Rob

Long thread about that here: http://www.bautforum.com/against-mainstream/75007-instant-communications.html