I'm not familiar with relativity, but I thought the age only depends on how fast they move. What does it have to do with acceleration and deceleration?[You asking this question implies familiarity with relativity (at least to some extent).](In the absence of gravity) the relative time depends only on the relative velocity between reference frames.I refered to the inexistence of acelaration in the inertial frame of Jack to make the point that whatever acelarates wrt it,will be bounded to have a nonzero speed (in this inertial frame).Therefore, this object, if it ever returns to this frame, will be younger than Jack.Note:I have written some wrong stuff in my earlier posts.Aaron is right (as usual!)Will not edit those, so that future readers can understand the discussion.

In addition to AlexandreC's excellent reply, I will add a more simple-minded answer.We all know that if an astronaut takes off from earth and flies near the speed of light, he will be younger than his twin when he returns. But if all motion is relative, can't his twin on earth pretend that the astronaut stayed still and the earth blasted off and came back?The simplified answer is that only one party feels the acceleration and deceleration, that is the person who ends up younger.

QuoteOriginally posted by: AaronIn addition to AlexandreC's excellent reply, I will add a more simple-minded answer.We all know that if an astronaut takes off from earth and flies near the speed of light, he will be younger than his twin when he returns. But if all motion is relative, can't his twin on earth pretend that the astronaut stayed still and the earth blasted off and came back?The simplified answer is that only one party feels the acceleration and deceleration, that is the person who ends up younger.If the astronaut were to take off from earth carrying a mechanical watch (manufactured and calibrated at earth) ... will he find that once he has reached near speed of light, the mechanical watch is giving a wrong time. (probably needing a change in the gear ratio which controls the motion of the hands of the watch).1. Suppose the astronaut chooses to measure his age with the same watch, without any corrections (even though it is faulty) .... returns to earth .... will he find that his age as per the faulty watch is the same as the age of his twin on earth ?2. Will the faulty watch during the journey, correct itself automatically .... while the astronaut decelerates and lands on earth?3. Suppose, the astronaut took the mechanical watch from earth, notes the time at which the flight started .... but forgets to take a look at it, during the journey. As soon as he lands back on earth, he wishes to know what the time spent on the journey was -- pulls out the faithful watch ... finds it's still working fine. Notes the time of the journey !!!!! -- will we still see a "twin paradox" ?

The simplified answer is that only one party feels the acceleration and deceleration, that is the person who ends up younger.Actually it is possible to formulate the twin paradox without involving any acceleration. Suppose that we have two astronauts A, and B, in separate rockets and both moving at (different) constant velocities. Consider the inertial frame in which A is at rest, suppose that in this frame B is moving at velocity v. Now suppose that B passes A and at the instant he passes, A and B synchronize clocks. Then at some subsequent time, B passes a third astronaut, C travelling in the opposite direction with velocity -v. At the instant that B passes C, C synchronizes her clock with B's. Then C continues until she meets A. At that point A and C compare clocks. They will find that their clocks show exactly the same discrepancy as would occur in the conventional formulation of the twin paradox. However, in the current formulation, it's all done without any acceleration.

bhutes,The fact that Jill and Jack watches measure the time differently,does not mean that there is a faulty watch. RealIllusion,Aaron explicitely mentioned "simplified answer"- no one said acceleration (or gravity) were necessary to experience "faulty watches".

QuoteOriginally posted by: alexandreCThe fact that Jill and Jack watches measure the time differently,does not mean that there is a faulty watch. If you give a watch, manufactured and calibrated on earth to guys who travel at near the speed of light ... those guys will call the watch "faulty", even as they continue to move at near speed of light, holding the earth-manufactured watch in their own hands. Is that a fair statement to make? (Assume the near light speed traveling guys made a stopover to pick the watch from earth)Then, if those same guys were to slow down and land on our earth again, to their astonishment, they will find that the "faulty" earth-made watch corrected itself automatically. Now, is that again a fair statement?Though it's important to note, I am having a differently designed watch in mind -- first idea I get is, something using the oscillations of a quartz crystal. (Second thought was using a cesium-133 based atomic clock ... but scared to think that it may give a different value of a second, if the entire atomic clock apparatus were move to near speeds of light -- just incase someone needs to recall cesium-133 is used to define the IUPAC unit of time i.e. second).Will the oscillations of a quatz crystal be any different than earth, if we were to move the crystal near speeds of light?I just want to measure time, without a pendulum !!!In other words, if I take 2 clocks with me, 1 based on pendulum and the other based on quartz crystal and blast off from earth .... will the two watches agree with each other? (They did agree with each other, when I was on earth).

yes, a quartz watch, or a pendulum watch, or a sand clock, or any physical system capable of measuring time will give the same output, independently of the inertial frame.thats a consequence of the second principle of relativity - Any 2 inertial frames are equivelent.There is no such thing as a priveleged reference frame.

A third thought !!! -- how do the biological processes in the human body measure time: do they respect the "pendulum" clock?The heart: Oh! it's "80 years" now that I have been pumping blood ... time to switch off !!!Can I base a clock on my pulse rate!Will the heart keep adjusting its pulse rate under varying "pendulum clock regimes" at different velocities?Case 1: If the heart keeps adjusting, I will have had lower no. of heartbeats during my space travel, than my twin on earth.Case 2: If "the heart doesn't adjust" ... my pulse rate will vary at different velocities. This would mean my biological clock is as bad as earth ... I return to earth in "40 years as per my pendulum clock" .... and the heart will already be saying "time to switch off" !!!

yes, biological clocks measure the same time.(Biological clocks are nothing but complicated physical clocks.)

I get to agree with you entirely.I guess, I managed to improve my understanding on this a good deal. Thanks.

Bhutes,The answer to your biological clock question is neither that simple nor easy to conclude. How human body react or function at those speeds will stay as a question until we reach at those speeds and do research on humans. One truth we know so far is that the astronauts who stay long enough in space (to put simply) have muscle weakening problems because they don't use them as much in a no-gravity environment. This is good from one point that you don't need to spend too much energy in some parts of your organs so your body doesn't get tired relatively fast. (It's like a marathon; if you run at a steady pace you will run longer than who runs fasts and slows down to rest and tires to run as fast as he can again). However that may not be good for some other organs those are having hard time to function properly in that environment. So there is a fine line between the benefits and hurdles in this situation and no one has enough research data to conclude that one will outperform other and live longer/shorter or age slower/faster. Also there are lot of other psychological issued that needs to be addressed. Some genetic concerns for adaptation and many more. To make the long story short nobody knows in reality for sure which one of the twins will age faster. IMHO since it's almost impossible to have same environment when going different directions in the universe, twins will age differently but I can't say which one will be faster or slower, that will depend on their reactions to the different environments. One more constrain I like to add to twin paradox problem is not to allow genetic manipulation for aging in twins. If we do, we already know the answer from the start. Our scientific understanding of aging process is very good and many research results have proved that it can be slowed down very significantly in many living organisms (ex. doubling the life span of a mouse). That is good enough to assume that we might have good chances of being successful in slowing down human aging. Many scientists even think that we are not that far from stopping human aging.

quantumar -the points you are mentioning(related to hostile enverionments and its' influence in aging)are irrelevant for what we have been discussing;the dilation of time due to the relativity principle.If you make a space shutle that reproduces the enviorement of earth (gravity can be replaced by acceleration),any biological watch will produce the same result as a physical one.

My answer to Bhutes's question is totally from biological point of view, it may be irrelevant from a physics approach to expansion of time which you mentioned we are discussing here but when it comes to applying those physics rules on human beings we are not just talking about the dilation of time due to the relativity principle. We are also talking how human beings are effected by it, that complicates the situation.If we were talking about just physics here I would totally agree with you. When you try to explain aging of a living organism with respect to applying physics laws to time and speed, that will be incomplete therefore misleading. Biological clock of aging does not tick the same way as other physical clocks. It accelerates, decelerates, even sometimes stops or jumps depending on many things. Thus thinking that same time passage for twins will produce the same aging results is not correct no matter what the physics laws say about physical time. There is biological aspect to it regardless of how time passes. Environment (doesn’t have to be hostile) is not just about gravity I gave that as an example. To make it more clear, assume twins are in the same ship, going the same direction, eating and doing exactly the same things the same way but one of them walks faster than other one and walks an average of 100 steps more a day, and both spend quiet some time in the ship, at the end of the journey if you look at the circulation levels of Catecholamines, they will be more likely different thus their relative ages. Age in humans is relative not absolute. A person who is 40 years old might be younger compare to a person who is 32 years old but older compare to another person who is 35.This is just an example, you can create unlimited number of scenarios that not everything will be same so their aging process.Aging is complicated matter that’s why most of the research in this area go to source of it ; genetics, trying to slow and stop the process.My only purpose here is to give a realistic biological point of view to Bhutes question, not to argue physical laws about it. I still think we don't know which one will age faster for sure since there are so many different unknowns.If I am wrong please correct me, our definition of aging is different this is why I think we have different point of views. I see age as less dependent on time and more on biological reasons, you see it as depended more on time for this example.

Nice post quantumar.sorry if I phrased my last post too strongly- no intention to offend or anything, obviously!