Twins paradox and ageing

Al68

No they don't. They all agree that 20 years elapsed in earth's path and 5 years elapsed in the ship's path.and they and all other observers actually counted 5 years elapsed wrt the ship's system. You only listed three of the four relevant facts.Not really, since the exact same change in the period of an oscillator is observed when the observer changes velocity instead of the oscillator. The change in the period of the oscillator occurs because of a change in relative velocity between oscillator and reference frame, whether or not the oscillator accelerates.It may be more intuitive, but it's conceptually wrong. Each twin ages 1 year per year of elapsed time. Again, it's time itself that "flows" differently in different frames, not a change in the operation of various devices used to measure it.

Conceptually, it's like saying that my bathroom scales read higher because I ate too much ice cream and cake last week. The scales read higher because I'm heavier, not because of any difference in the scales' operation. This is true of any measuring device: different results do not imply a difference in the measuring device itself.

ThomasT

Sorry for the delay in replying.

We're setting aside the spacetime interpretation for the moment.
Right. The travelling clock actually recorded fewer ticks than the earthbound clock.
No, the physical fact is that only one clock ran slow -- this is the point of including the visual tracking of the earth-sun system.
No, the change in the period of an oscillator is due to the acceleration of that oscillator. What you're saying applies to situations where we don't have sufficient info regarding the acceleration histories (or we simply disregard it) of the two clocks being compared. But in the usual earthbound-travelling twin scenario, we do.

edpell

We have to be careful on the word here. You can not get velocity without acceleration.

Is it the velocity that alters that state of the clock or is it the acceleration that alters the state of the clock? What experiment can we do to measure which is responsible? Two things happen and a third is found to be correlated. If you can never separate the two things how will you ever be able to call one the cause and the other not the cause?

ThomasT

Time itself ... flows differently??

The traveller, and all other observers, watched the earth-sun system rotate 20 times during the trip. Yet the traveller and his clock only ticked off 5 years.

We agree that difference in tick rate is due to difference in velocity, don't we? It seems clear to me from experiments that it isn't intervals of constant, uniform velocity that exhibit tick rate changes, but intervals of acceleration.

In this case they do.

croghan27

This may have been gone over before ... but kindly humour me.

If there was a clock on the travelling ship and a similar clock at the stationary take off location - say they are old grandfather clocks .. with swinging pendulums as a driving force. (the sort of things that Galileo worked with). Let us suppose there is a way to track the swing of the pendulum in the ship, but at the point of origin.

Would the swing of the pendulum on the ship match that of the clock at the stationary place? OR If it moved one centimetre in its' swing would it take less time than for the stationary clock's pendulum to move the same distance?

I guess this is a classical physics question being applied to a quantum physics situation.

Oh - by the way: look for the Bengals to take less time to move the football over the Jets goal line and do it more often than the Jets can do the reverse this afternoon.

edpell

NO!

This case seems to have gamma of 4. So the traveler only saw 5 rotations of the earth around the sun when he/she looked in their telescope. Due to the fact that the light from rotations 6-20 has not yet reached the traveler. The traveler will have to keep looking for another 15 years (after arrival) to see the light from rotations 6-20.

Janus

No, the traveler watched the Earth-Sun make .9 rotations during the trip. (Relativistic Doppler effect). From his reckoning, figuring in his relative velocity with respect to the Solar system, The Earth-Sun has made 0.635 rotations. He now stops at the destination (assume a short enough time of deceleration that he is, in all intents and purposes, stops dead. He is seeing the same light as he was before he decelerated, but his reckoning of how much time has passed on Earth changes. He is now 19.365 ly from Earth, which means that the information is 19.365 years old. Therefore, by his reckoning the Earth has aged 20 yrs since he left. Let's try to explain this by way of analogy:

You have two men(A&B) walking in the Same direction at the same speed on a featureless plain.
If you consider the direction they are walking as "time", this represents our twins At the starting point, at rest we each other. They each progress through time at the same rate and age at the same rate.

Now one man(B) turns and starts to walk in a new direction. As each walks in his own direction, they get further apart, or as each progresses through time, the distance increases between them. This represents one of the twins while traveling away from his other twin.

Now consider the perspective of man A. As he walks, B falls further behind with respect to the direction that A is walking. This represents B progressing through time more slowly or aging less than A.

But now consider man B. By his perspective, it is A that it falling behind. and he is just as entitled to claim that the direction that he is walking is the direction of time progression, and that it is A that is making slower progress/aging slower. This is the whole point behind the principle of Relativity. Each inertial frame judges other with respect to itself and there is no absolute reference. Each judges time progression as progress in the direction he is walking.

Now consider what happens when Man B turns to walk in the same direction as A again. This represents the traveling twin reaching his destination and stopping. The distance between them no longer changes, and they are again "at rest" with respect to each other.

From A's perspective, this just means that B stops losing ground, and starts to age at the same rate. He doesn't make up lost ground, however, and his total progression through time remains less. He remains younger.

From B's perspective, as he turns, A's position with respect to Him changes. He goes from being behind to being in front. (Stand in the middle of the room with an object to one side and slightly behind you. Now turn 45° in that direction. The object, from your perspective moves from behind you to in front of you.)

After B completes his turn, He finds that A is now ahead of him in time, and progressing at the same speed. His has made more progression through time. His has aged more and is older.

Both men agree in the final result, but have different views of how that result came to be. And this is the important part: Each man's view of what happened is just as valid at the others.

So when you ask what causes one twin to be older than the other at the end of the trip, the answer is: It depends on which twin you are.

Relativity makes us rethink how we measure time. To use the direction analogy again: Before Relativity, we could think of time as the direction North. No matter who you asked and what relative directions they where facing, they all agreed on what direction North was. Ask them to point North, and they all point the same direction. It is, in a sense an absolute direction.

Relativity tells us however that time is like the the direction Left. Ask a number of people to point left, and they will all point in different directions depending on the relative directions they are facing. There is no absolute "left". Left is determined by the individual and moves with him. And in Relativity, time measurement is determined by what frame you measure it from.

vela

The difference in time between the two frame has nothing to do with the light travel time. SR says that even after you take into account those effects, the time elapsed between two events is different in the two frames. Also, in the twin paradox problem, the traveller's twin aged twenty years, so the traveller would see the Earth going around the Sun twenty times during the trip. You're thinking of a different situation.

Wikipedia has a good explanation of the twin paradox from the traveling twin's point of view.

http://en.wikipedia.org/wiki/Twin_pa...ial_relativity

DaleSpam

If two surveyors were to measure the distance along routes from New York to Miami, and one surveyor's route went through Washington DC and the other surveyor's route went through Los Angeles, would you say "the physical fact is that only one surveyor's chain was shrunk" or would you say "the physical fact is that only one surveyor's route was longer"? In other words, would you attribute the difference in measurement to a difference in the thing being measured or to a difference in the thing doing the measuring?

DaleSpam

Actually, it is neither velocity nor acceleration (both vector quantities), it is speed (a scalar quantity). Although you cannot have acceleration without a change in velocity you can have acceleration without a change in speed. This experiment has been done with accelerations on the order of 10¹⁸ g, and the results are that the time dilation is not a function of the acceleration.

GRDixon

[QUOTE=adw73uk;820926]Hi, I'm a Biology teacher, constantly getting into discussions with the physics teachers at my school regarding the effect of travelling at speed on ageing.

This isn't exactly chemistry, but the following fact may provide insight. Consider a macroscopic "Bohr Atom," consisting of a resting, massive, positively charged central body orbited at constant speed in a circle by an equal magnitude negatively charged particle. For a given satellite speed, mass and orbital radius, one can readily calculate what the common magnitude charge must be. This system can be viewed from an alternate inertial frame, relative to which the central body moves with a constant velocity. When the electron's motion is computed relative to this second frame (using the laws of Maxwell, Lorentz and Newton) then the motion is a quasi-cycloid that cuts "above" and "below" the central body at distance R, but "in front of" and "behind" the central body at distance R/gamma. The computed motion also gives the result that the cycloid cycle time is (T)(gamma), where T=(2)(pi)(r)/(v). The interesting thing is that this is true no matter what inertial frame we initially assume the "atom" to be "at rest" in. Indeed, thanks to the way the clocks in the inertial frames are synchronized, each frame measures a moving "atom" to be length contracted. etc.! A more detailed account can be viewed at www.maxwellsociety.net/LovingLorentz.html

Evolver

But doesn't that defeat the point of relativity? When you say one twin accelerated away, that's only relative to the other twin... but the exact opposite is true if you take the relative experience of the second twin. Relatively speaking, it's always the other twin that accelerated away.

jtbell

Accdeleration is not relative. One twin feels a kick in the seat of his pants as his spaceship engines fire, the other twin does not. The first twin knows, without having to look outside his spaceship at all, that he is not at rest (or moving at constant velocity) in an inertial reference frame.

Evolver

To say acceleration is not relative means you have already decided that one frame of reference is more absolute than the other. Though one twin experiences the other twin AND the universe accelerating away from him (hence the kick in the pants), while the other twin only experiences the first twin accelerating away... both experience the opposite twin accelerating away from them.

Also, being at rest is completely relative as well. Would you consider yourself at rest when typing your response to this on your computer? Because in reality you are rocketing through space on the Earth, and the Earth is swirling about the Milky Way, and who knows what else motions the entire universe is actually performing. But, in your current reference frame you are at rest... Everything is relative.

DaleSpam

Hi Evolver, did you miss this part of jtbell's response:Nothing has to be decided in advance or with reference to the rest of the universe. An onboard accelerometer will suffice.

Evolver

I did not miss it, in fact I commented on it directly (please read above.) "The kick in the pants" you refer to is only unique to twin #1 because the entire universe around him is accelerating away from him (including twin #2). That is the only unique difference between twin #1 and twin #2, and that is what the on board meters are registering. Twin #2, nonetheless, still experiences twin #1 accelerating away from him... if this were not true twin #1 would never leave twin #2's reference frame.

Mentz114

That's a lot of rot. It's been explained to you that only one twin in this scenario accelerates so it is not symmetrical as you insist.

JTBell and DaleSpam are far too polite to tell you - but you're being really stupid and arrogant.