Quote by croghan27 Somewhere between: "Acceleration affects the periods of oscillators." and "What velocity does the third clock have with respect to the earth clock? What velocity with respect to the ship clock?" I have become lost again. While velocity and acceleration are connected, they are not, in my philosophy, Horatio, the same thing. One involves achieving and is a change of velocity; while the other is a constant. One must accelerate to achieve a certain velocity, or conversely decelerate, but the latter is the result of the former. What is the connection here and which results in the ageing or less ageing of the twins? (At the risk of being a drag, there is another question in the background of this.)
The differential aging is the result of less elapsed time for the ship's twin, which is a function of velocity. Acceleration is relevant as the time derivative of velocity.

 Quote by Al68 The differential aging is the result of less elapsed time for the ship's twin, which is a function of velocity. Acceleration is relevant as the time derivative of velocity.
Thanks for that ... both terms seemed to be used interchangeably and I was being led astray.

The other question I have may be somewhat off the wall .... but as we sit on earth we are busy whurrling about in the motion that makes days, on top of that we are circulating about the sun, in the circuit that defines years. The sun is just one of the stars in a very mobile galaxy grandly twisting in 'space'. So we are moving in all direction at once when compared to just about any reference point.

What effect does all this motion have upon us relative to ...er....er.. relativity?

 Quote by croghan27 Thanks for that ... both terms seemed to be used interchangeably and I was being led astray. The other question I have may be somewhat off the wall .... but as we sit on earth we are busy whurrling about in the motion that makes days, on top of that we are circulating about the sun, in the circuit that defines years. The sun is just one of the stars in a very mobile galaxy grandly twisting in 'space'. So we are moving in all direction at once when compared to just about any reference point. What effect does all this motion have upon us relative to ...er....er.. relativity?
All motion is relative to a defined reference frame or "reference point", like you say, so the effect of our motion relative to a particular reference frame would depend on the reference frame chosen.

 Quote by Al68 If a clock is affected by acceleration, then it is simply not a valid clock in SR. This is the "clock hypothesis", that a clock's rate is unaffected by acceleration. Of course a real clock may be affected by acceleration, but the predictions of SR are not valid for such a clock. I think you must have misread my post. My point was that the traveler's clock is predicted to show a lower reading and the ship twin is predicted to age less for a common underlying reason: Less elapsed time passes.
I don't think this addressed the OP's concern: adwuk wrote, "What I can't get my head around is how in the twins paradox one twin has physically aged more than the other. How does travelling at the speed of light affect the chemical reactions involved in the ageing of (biological) cells?"

SR can't answer the question of the deep physical cause(s) for differential aging.

Relative position in gravitational field is also connected to differential aging.

Atoms, quartz clocks, humans, etc., all material objects are bounded, standing wave structures, ie., oscillators, of lesser or greater complexity.

One take on differential aging is that acceleration affects the periods of oscillators.

 Quote by ThomasT One take on differential aging is that acceleration affects the periods of oscillators.
While real clocks, that is periodic effects may well be affected mechanically by acceleration, the clock hypothesis assumes that for ideal clocks this is not the case. Differential ageing assumes the clock hypothesis and so definitely precludes the varying periodicity of clocks as a cause for the effect.

Differential aging is a logical consequence of clocks following different spacetime paths. This requires acceleration on the part of one or both of the clocks but the acceleration is not the direct cause of the effect. Remember we are using ideal clocks that satisfy the clock hypothesis.

Matheinste.

 Quote by ThomasT SR can't answer the question of the deep physical cause(s) for differential aging.
It's not supposed to. SR isn't biology. SR only answers the question of how much time elapses. Common sense says more elapsed time equals more aging.
 Relative position in gravitational field is also connected to differential aging. Atoms, quartz clocks, humans, etc., all material objects are bounded, standing wave structures, ie., oscillators, of lesser or greater complexity. One take on differential aging is that acceleration affects the periods of oscillators.
SR predicts what a hypothetical clock will read if it is not unaffected by acceleration.

Clocks which are unaffected by acceleration will show that the ship twin has less elapsed time.
 Thanks for the input(s). My concern is that in calculating in terms of instantaneous velocities (vis clock postulate), and visualizing in terms of paths in spacetime geometry, then maybe some important physical considerations get glossed over. We agree that relativity theory is not designed to provide an answer to the OP's question about deeper physical cause(s) of differential aging. A more fundamental (wave?) theory is required. The physical evidence does suggest that modifications of oscillatory periods happen during intervals of acceleration.

 Quote by ThomasT The physical evidence does suggest that modifications of oscillatory periods happen during intervals of acceleration.
What evidence are we talking about. I thought that the clock hypothesis had been confirmed to a high very high degree for atomic clocks.

Matheinste.

 Quote by matheinste What evidence are we talking about. I thought that the clock hypothesis had been confirmed to a high very high degree for atomic clocks. Matheinste.
The most compelling evidence is that you can feel when you're accelerating. It seems logical to assume that it's during these intervals that changes in oscillatory periods are occuring.

I'm not familiar with the experiments you're talking about. I'd be interested to see an experiment that shows that acceleration has no effect on clocks.

 Quote by ThomasT The most compelling evidence is that you can feel when you're accelerating. It seems logical to assume that it's during these intervals that changes in oscillatory periods are occuring. I'm not familiar with the experiments you're talking about. I'd be interested to see an experiment that shows that acceleration has no effect on clocks.
Look at the FAQ at the top of the forum entitled Experimental Basis for Special Relativity and follow the links. The hypothesis has been confirmed up to $$10^{18}g$$

Matheinste.

 Quote by Al68 All motion is relative to a defined reference frame or "reference point", like you say, so the effect of our motion relative to a particular reference frame would depend on the reference frame chosen.
I guess I am going to have to accept there is no such thing as a stationary object - it is always relative. Motion seems to be something that was/is and forever shall be present.

Now we are speaking of a innate characteristic of the universe - is there something that is responsible for this? CERN is busy spending billions to find a particle that may or may not hold the secret to mass - is there a chronological particle/wave/force?

 Quote by ThomasT The most compelling evidence is that you can feel when you're accelerating. It seems logical to assume that it's during these intervals that changes in oscillatory periods are occuring. I'm not familiar with the experiments you're talking about. I'd be interested to see an experiment that shows that acceleration has no effect on clocks.
Some clocks might very well be affected by acceleration, they might break completely, but those clocks are not valid in SR. SR predicts only what a clock will read if the clock keeps proper time regardless of acceleration.

 Quote by matheinste Look at the FAQ at the top of the forum entitled Experimental Basis for Special Relativity and follow the links. The hypothesis has been confirmed up to $$10^{18}g$$ Matheinste.
Thanks. Unfortunately, I checked out the link "bailey et al" from a couple of places and it doesn't lead to an article that I can read.
 Mentor Here is a link to the abstract: http://www.nature.com/nature/journal.../268301a0.html You can probably find it at a local library. Nature and Science are widely subscribed to. There also used to be a hyperphysics page describing it, but I couldn't connect to it today.

 Quote by Al68 Some clocks might very well be affected by acceleration, they might break completely, but those clocks are not valid in SR. SR predicts only what a clock will read if the clock keeps proper time regardless of acceleration.
I'm not sure what you're saying. If you change a real clock's velocity, then won't it, at the different velocity, keep different time? This has been experimentally confirmed, hasn't it?

So, assuming that a clock's tick rate is proportional to the speed at which the clock is moving, the question I'm interested in is: when tick rates change -- during what are called acceleration intervals -- then what are the mechanics of the change?

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 Quote by ThomasT Thanks. Unfortunately, I checked out the link "bailey et al" from a couple of places and it doesn't lead to an article that I can read.
Put simply, it works like this:

Put a clock at the end of a centrifuge. Spin up the centrifuge so that the clock is traveling in circle at a given speed while experiencing an acceleration. Compare the clock's rate with that expected just due to its velocity and see if it varies. (is the acceleration having an additional effect on the clock rate).

By varying the radius of the centrifuge and its rate of spin you can create situations where you have different accelerations but maintain the same speed for the clock or maintain the same acceleration for different speeds of the clock.

Th experiment has been done with high speed centrifuges and using samples of a radioisotope for the clock. To the accuracy already stated, it has been found that the measured decay rate of the sample is only determined by the speed at which it moves and that the acceleration has no effect.

 Quote by DaleSpam Here is a link to the abstract: http://www.nature.com/nature/journal.../268301a0.html You can probably find it at a local library. Nature and Science are widely subscribed to. There also used to be a hyperphysics page describing it, but I couldn't connect to it today.
Thanks. I'm in Fort Lauderdale. I'll try to get a copy within the next few days.