# I Twin paradox not including accelerations, it is wrong where?

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1. Oct 2, 2017

2. Oct 2, 2017

### sweet springs

I think that gravity is more appropriate term in explanation of twin-paradox than acceleration. An egocentric traveler twin who believes he never move and keep still, would interpret that time dilation is due to a sudden appearance of gravitation field in all over the space including the Earth though he cannot explain why it appears.

3. Oct 2, 2017

### Staff: Mentor

The best explanation, as has already been said in this thread, is the different lengths of the two paths through spacetime.

Which is a good reason not to say that gravity is a "more appropriate" explanation.

4. Oct 2, 2017

### sweet springs

Yea, the best explanation, right.
I would like to just add transfer of inertial frames is more appropriate (not the best) word than acceleration in twin-paradox explanation. When a massive body keep still during its changing the frame of reference, acceleration or force always work on it and vice versa. These are two equivalent sayings. But for information transfer as in post #7 the former applies, the latter does not.

5. Oct 3, 2017

### Staff: Mentor

"Transfer of inertial frames" is vague. Also it's not physical--"inertial frames" is a term for an abstract model made by humans, not a physical happening. Also it's unnecessary: you can just say "path through spacetime" and not have to worry about frames at all.

I think I understand what you're trying to say, but this is not a good way to say it. A better way would be to say that if an object has nonzero proper acceleration, any frame in which it is at rest must be a non-inertial frame; it can't be at rest in an inertial frame for more than an instant. That puts the physical observable (proper acceleration) before the abstract model (a frame).

See above.

6. Oct 3, 2017

### f todd baker

The twin paradox absolutely does not need any mention of acceleration. And it is very easy to understand the asymmetry to boot. The traveling twin sees the distance to the destination to be length contracted, the earth-bound twin does not.

7. Oct 3, 2017

### Staff: Mentor

You are right that the acceleration is not required to explain the twin paradox, but mistaken about the role of length contraction. If we use coordinates in which the travelling twin is at rest on the outbound leg, the earthbound twin will find that leg is shorter than the travelling twin; and likewise for the return leg when we use coordinates in which the traveller is at rest on the return leg.

8. Nov 14, 2017

### f todd baker

If the traveler is taken to be at rest, there is no length contraction because the earth-bound twin and the destination are in the same frame.

9. Nov 14, 2017

### jbriggs444

There is length contraction because the frame in which the earth bound twin and the destination are at rest is one you have chosen to consider as being in motion.

10. Nov 14, 2017

### f todd baker

Sorry, I am not getting your point. Regardless of which frame you choose to be at rest, the traveling twin sees the distance from earth to destination to be contracted and the earth bound twin does not see that distance contracted. What we have here is a failure to communicate!

11. Nov 14, 2017

### jbriggs444

Ummm, is that not "length contraction"?

12. Nov 14, 2017

### Mister T

When you choose to observe things from the traveling twin's perspective, you choose the frame in which he is at rest.

Likewise when you choose to observe things from the staying twin's perspective, you choose the frame in which he is at rest.

Thus you cannot claim to to be making observations without regard to which frame you've chosen.

13. Nov 15, 2017

### 1977ub

In the Hitchcock version, during the outgoing trip, it is suspensefully anybody's guess which twin will turn out to be the "outgoing" one, and whose frame-dependent length contraction will appear to be relevant at the end of the scenario.