From another forum:

http://www.sciencechatforum.com/viewtopic.php?f=84&t=26847

“A common explanation of this paradox is that the travelling twin experienced acceleration to slow down and reverse velocity. While it is clearly true that a single person must experience this acceleration, you can show that the acceleration is not crucial. What is crucial is that the travelling twin experienced time in two reference frames, while the homebody experienced time in one. We can demonstrate this by a modification of the problem. In the modification, there is still a homebody and a person travelling to a distant star. The modification is that there is a third person even farther away than the distant star. This person travels at the same speed as the original traveler, but in the opposite direction. The third person’s trajectory is timed so that both of them pass the distant star at the same time. As the two travelers pass, the Earthbound person reads the clock of the outbound traveler. He then adds the time he experiences travelling from the distant star to Earth to the duration experienced by the outbound person. The sum of these times is the transit time.

“Note that no acceleration occurs in this problem…just three people experiencing relative inertial motion.”

I understand what is being described here, but I don’t understand the significance. My problem is that I’ve long understood to be acceleration to be a crucial aspect of the relativity general theory as it applies to the twin paradox, and the differences between special relativity and general relativity, but I’ve never really understood why. So it doesn’t really conflict with any of me preconceived notions, even though I have those notions, to learn that acceleration is not actually a factor in the twin paradox.

I don’t have a specific question. I’m just hoping someone finds something interesting enough in the above quote to make an educational comment on it.

yes, acceleration is not necessary. a current issue of scientific american is all about time. it explains the twin paradox quite well. it is available online but by subscription only.

some scans of the article

Page 1

Page 2

Page 3

hopefully the little bits missing wont confuse you.

esselte said:

“A common explanation of this paradox is that the travelling twin experienced acceleration to slow down and reverse velocity. While it is clearly true that a single person must experience this acceleration, you can show that the acceleration is not crucial. What is crucial is that the travelling twin experienced time in two reference frames, while the homebody experienced time in one. We can demonstrate this by a modification of the problem. In the modification, there is still a homebody and a person travelling to a distant star. The modification is that there is a third person even farther away than the distant star. This person travels at the same speed as the original traveler, but in the opposite direction. The third person’s trajectory is timed so that both of them pass the distant star at the same time. As the two travelers pass, the Earthbound person reads the clock of the outbound traveler. He then adds the time he experiences travelling from the distant star to Earth to the duration experienced by the outbound person. The sum of these times is the transit time.

“Note that no acceleration occurs in this problem…just three people experiencing relative inertial motion.”

Yeah, ok. I assume that all 3 clocks start at zero at the beginning of the experiment. The inbound traveller’s clock will read less than the stay-at-home clock at the end of the journey. However, I disagree that “the travelling twin experienced time in two reference frames”, it’s just that the numbers on his clock result from combining measurements in two frames, so they don’t really represent the proper time of any of the trajectories in the experiment.

OTOH, if we modify the experiment slightly so that the actual clock carried by the outbound traveller is transferred to the inbound traveller, rather than just transferring the clock’s data, then the clock experiences extreme acceleration during the transfer and we’re back to the traditional version of the twin paradox.

esselte said:

I understand what is being described here, but I don’t understand the significance. My problem is that I’ve long understood to be acceleration to be a crucial aspect of the relativity general theory as it applies to the twin paradox, and the differences between special relativity and general relativity, but I’ve never really understood why. So it doesn’t really conflict with any of me preconceived notions, even though I have those notions, to learn that acceleration is not actually a factor in the twin paradox.

I don’t have a specific question. I’m just hoping someone finds something interesting enough in the above quote to make an educational comment on it.

FWIW, it is possible to handle acceleration in special relativity, as long as the spacetime “region” it’s happening in is flat. Of course, no region of spacetime is truly flat, but the GR corrections to SR are generally fairly small unless you’re in a powerful gravitational field or are trying to do calculations that cover a very large region of space or span of time.

PM 2Ring said:

Yeah, ok. I assume that all 3 clocks start at zero at the beginning of the experiment. The inbound traveller’s clock will read less than the stay-at-home clock at the end of the journey. However, I disagree that “the travelling twin experienced time in two reference frames”, it’s just that the numbers on his clock result from combining measurements in two frames, so they don’t really represent the proper time of any of the trajectories in the experiment.

OTOH, if we modify the experiment slightly so that the actual clock carried by the outbound traveller is transferred to the inbound traveller, rather than just transferring the clock’s data, then the clock experiences extreme acceleration during the transfer and we’re back to the traditional version of the twin paradox.

In the first case the numbers on his (the inbound traveller) clock result from the transfer of information from one moving object to another using the electromagnetic spectrum.

In the second case the numbers result from the physical transfer of the clock from one to another.

Presumably these two cases are equivalent to each other, somehow. Otherwise you could really screw with time by transferring both the information and the physical clock and getting a double wammy of temporal weirdness.

So does this imply that during the transfer of information, the information itself undergoes an acceleration in the same way that a physical clock flung from one starship to another does? If so, what is the force that is causing this acceleration of information? What is actually being accelerated?

PM 2Ring said:

FWIW, it is possible to handle acceleration in special relativity, as long as the spacetime “region” it’s happening in is flat. Of course, no region of spacetime is truly flat, but the GR corrections to SR are generally fairly small unless you’re in a powerful gravitational field or are trying to do calculations that cover a very large region of space or span of time.

> A common explanation of this paradox is that the travelling twin experienced acceleration to slow down and reverse velocity. While it is clearly true that a single person must experience this acceleration, you can show that the acceleration is not crucial. What is crucial is that the travelling twin experienced time in two reference frames

That is plain wrong. One cannot get from one inertial reference frame to another without experiencing acceleration, and therefore acceleration is necessary. Further, when interpreted in general relativity there is no need for the travelling twin to at any time to experience an inertial reference frame at all, so the travelling twin doesn’t need to experience time in two inertial reference frames.

mollwollfumble said:

> A common explanation of this paradox is that the travelling twin experienced acceleration to slow down and reverse velocity. While it is clearly true that a single person must experience this acceleration, you can show that the acceleration is not crucial. What is crucial is that the travelling twin experienced time in two reference frames

That is plain wrong. One cannot get from one inertial reference frame to another without experiencing acceleration, and therefore acceleration is necessary. Further, when interpreted in general relativity there is no need for the travelling twin to at any time to experience an inertial reference frame at all, so the travelling twin doesn’t need to experience time in two inertial reference frames.

I don’t think it is wrong. The experiment can be set up so that all the acceleration takes place before the timing commences, so acceleration isn’t necessary. The important thing in the experiment as described is that one clock is always in the same reference frame and in the other the clock or clocks are in different reference frames. Making it two inertial reference frames simplifies everything and shows that acceleration is not essential.

The experiment can be set up so that all the acceleration takes place before the timing commences…

this was my thinking too. plus what i read in th article i posted.