A Universe with no Twin Paradox

Thecla
People are intrigued by Einstein's Special Theory of Relativity. They are fascinated by the Twin Paradox and the slowing down of moving clocks.
However it is often forgotten that Einstein's SR, in spite of being weird to the day to day speeds we are accustomed to, saved Physics from something worse.
In other words if you think SR is strange, imagine a universe without slowing down of moving clocks:
Suppose a pair of twins 20 years old. One travels at near the speed of light for 60 Earth years on a round trip. Upon return they are both 80 Earth years old-balding, wrinkles, fewer teeth.
In 1870 physicists would say so what; time travels at the same rate for everybody.
However,if moving clocks did not slow down, if both twin brothers ended up 80 years old, what would the great disaster that would occur to physics?

Mentor
However,if moving clocks did not slow down, if both twin brothers ended up 80 years old, what would the great disaster that would occur to physics?

If things were not as they are, they would be different.

I don't understand your question. Our theories of physics are based on what we actually observe. If we had observed different things, we would have different theories. So what?

harrylin
[..] it is often forgotten that Einstein's SR, in spite of being weird to the day to day speeds we are accustomed to, saved Physics from something worse.[..] if moving clocks did not slow down, if both twin brothers ended up 80 years old, what would the great disaster that would occur to physics?
Is this a quiz? Sorry I don't guess what disaster you imagine might happen if nature worked according to classical physics! And SR is not weird if you have a model of how it works.

Homework Helper
Okay, what "disaster" might happen? It would be possible for massive object to move faster than light, there would be no time or space dilation and there would be an absolute velocity for each object, determinable by electro-magnetic experiments such as the Michaelson-Morley experiment. I see no "disaster" yet.

Mentor
However,if moving clocks did not slow down, if both twin brothers ended up 80 years old, what would the great disaster that would occur to physics?
Moving clocks do run slow relative to clocks that aren't moving; that effect has been observed and measured many times. However AFAIK no one has actually tried it with twin brothers over a period of 80 years, so I'm going to take the liberty of reformulating your question to be:
if both twin brothers ended up 80 years old, what would the great disaster that would occur to physics?

Well, we would need a new theory, one that predicts everything relativity predicts and has been confirmed by experiment but that DOES NOT predict the differential aging of two twins over a period of eighty years. I wouldn't call that a "disaster", but it would make physics (even more) interesting and exciting for a lot of people for a long time.

I would also bet very long odds against such a thing happening. It is hard to imagine a theory that correctly predicts the time dilations and other relativistic effects that have already been experimentally confirmed, yet that does not predict something like the twin paradox (different proper time elapsed on different timelike paths between two events).

Thecla
I did not mean for this to be a quiz. The type of answer I had in mind:
1) Effects would precede causes
2) Ability to time travel and change the past.
Both of these would be a disaster to logic and physics.
3)Abilty in a closed lab to do an experiment that would determine the velocity of that lab.

Mentor
1) Effects would precede causes
2) Ability to time travel and change the past.
Both of these would be a disaster to logic and physics.
3)Abilty in a closed lab to do an experiment that would determine the velocity of that lab.

Yes, but under what hypothetical scenario, exactly, would these "disasters" happen? The fact is that we don't observe any of these phenomena in the real world, so our physical theories don't include them. If we *did* observe such phenomena, we would have different physical theories. So what?

harrylin
I did not mean for this to be a quiz. The type of answer I had in mind:
1) Effects would precede causes
2) Ability to time travel and change the past.
Both of these would be a disaster to logic and physics.
3)Abilty in a closed lab to do an experiment that would determine the velocity of that lab.
Ehm no, if 3) then not 1) and neither 2). As HallsofIvy and I indicated, one simply gets a situation as was expected until the end of the 19th century. No disaster at all.

georgir
Then light would necessarily have different speeds for different observers.
But the details depend on how light behaved in your hypothetical situation.
Light could have behaved like particles, i.e. having speed c only relative to the emitter (and any re-emitting particles in a medium). Then neither 1, 2 or 3 would be true.
Or it could have behaved like a wave in some absolute reference frame, having speed c only in that absolute frame. Then only 3 would be true.
Or it could have behaved like a wave, in some locally-dragged aether, and so changed its speed depending on the area it was in. Then something like 3 would be true but in a non-absolute and much more confusing sense... but it would have allowed us to measure that aether drag effect in some way.

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ImaLooser
Okay, what "disaster" might happen?

If the speed of light is infinite, and the Universe is infinite, then we all go Olber's. Would the Earth absorb an infinite amount of energy. I dunno.