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Twins paradox and ageing

  1. Nov 10, 2005 #1
    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.

    They have explained to me (countless times) all the examples and experiments that show that time passes more slowly for objects travelling at speed (atomic clocks on planes, flashing light at one second intervals, looking at a stationary clock while travelling at the speed of light).

    I have got my head around the fact that speed and time (I think) is relative (except the speed of light) to the frame of reference of the observer.

    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?

    I have tried to get my head around Einsteins publications, but are a bit over my head so you'll need to bring any explanation down to a low level!

    Any help would be appreciated before I finally fry my brain pondering this one!

  2. jcsd
  3. Nov 10, 2005 #2


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    I quess the simpliest thing I could tell you is to remember that all chenical reactions take place by electromagnetic interaction.
  4. Nov 10, 2005 #3


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    How would a clock be affected, but not biological processes?
  5. Nov 10, 2005 #4
    Even simpler – it doesn’t! Why would a chemical reaction change because your moving any more than juggling a ball on a jet flight would change. You don’t toss a ball up and have it slam into the back of the plane because you and the plane traveled away from it, the ball stays with you. Just like the traveling chemical reactions moving along with the Twin with his ‘official time wrist watch’. When a one hour chemical reaction is complete and reports to the twin, the clock on the wall also reports to the twin that one hour is up, and sure enough when the twin checks the ‘official time wrist watch’ they are right on time. I.E. Chemical reactions don’t change, time is always normal within a common reference frame!

    BUT to report this info back to the #2 Twin back home that’s a different issue. Lets relay the info to a station #1 twin is passing by – that station being in the SAME FRAME as #2 back home. Only now can he can see the time change on the station clock is already reading two hours. This information sent back to #2 will confirm traveler #1 is younger than #2. It will take awhile to get the info back there even sent by light speed radio, even longer for a reply, all the while getting father apart .

    But wait – after waiting a while the same kind of info is coming back ahead of schedule! Not a reply but twin #2, is relaying the same kind of info via a ship passing twin #2 at home using the same exact traveling frame as twin #1 including a perfectly synchronized clock. The news -- #2 is claiming to be the younger and can see #1 is aging faster!

    Only one way to see who is right, that’s to put them back together and resolve it once and for all by looking at both wrist watches at the same time in the same place. NOW is when you get to the point that makes all the difference.
    Who’s room do we go to??
    They are both perfectly happy AND STATIONARY in their own current room since separating.
    Once they are together to settle it the thing that makes THE a difference is where is the point of meeting in comparison to the point separating.

    IF it’s where twin #1 has been stationary all this time in his traveling frame his room is the departure point, with twin #2 moving away. So, for the meeting to occur the non-traveling twin #2 must now move and travel at twice the speed to catch up. Now even though #1 was traveling all the time since that frame has both the start and end from the relative view of that frame #1 wasn’t traveling at all so yes the twin #2 will be younger.

    But how about meeting back at earth – that means the traveler #1 must go back leaving his original “departure point/room” to continue on. And must also leave that point at Twice the original speed in order to approach home at the same speed a the original departure speed. Now when they do meet the start and end point will be the same only in the home ref frame and Twin #1 will be the younger.

    But what if they both decide to move to reconnect – how do we get the start and end point to be the same place in just one frame. Well there is always one place in some frame where any two events occur at that one location. In this case it’s in the frame that follows the behind #1 at 1/2 speed. (remember you can not get half by dividing by 2 in this case). This point will always be exactly half way between the twins. Thus this time when they compare their ages, they will be exactly the same. As if they had both left earth in different directions and returned.

    So for chemical reactions – it’s like tossing a ball in a moving car – simple.
    Understanding the twins,
    best make sure the physics teachers get you to understand all three examples above.
    Have them slow down a little when they start talking about “simultaneity” or you will get a head ache.

  6. Nov 11, 2005 #5
    Thanks for the answers guys - Though I have a headache now thanks RandallB! I suppose the answer I was looking for was the connection with electromagetism. Good point Russ, I guess I had accepted the clock theory as it has been experimentally tested and didn't stop to think why the clock runs more slowly. Being a biologist I more used to considering the 'concrete' - or maybe I'm a little thick!

    I've posed the same question to some of my students who are also studying physics. I’ll post any of the explanations that they come up with.
  7. Nov 11, 2005 #6
    No you aren't thick, the theory is very unintuitive. However, I think it would be very odd if biological processes were exempt from time dilation. It seems equally unlikely that we could experience time outside our biological constraints from an ivory tower. This basically means we could only experience time dilation by comparing clocks or watching other objects undergoing it differently from us.

    As an illustration, I sometimes imagine a small black hole appearing in my front room. On this occasion, instead of getting crushed to a paste and sucked in, I imagine that I can experience the time dilation alone, while outside the world goes on as normal. My time has slowed right down (or suppose - basically completely frozen) but because my brain is frozen I don't notice anything. Everything stops. When the black hole disappears two minutes later (measured from outside) I unfreeze. Everything restarts from where it got to before. I notice two things. My clocks are behind and the man I was watching run round the field outside my window has jumped into the future. I will probably never be able to measure the small change to my biological age.

    I'm sure this illustration isn't water-tight, it helps me sometimes.
    Last edited: Nov 11, 2005
  8. Dec 24, 2005 #7
    Thanks for all your help. It is becoming clearer in my mind, just the act of 'discussing' it is helping. Cheers!
  9. Dec 21, 2009 #8
    If it is all relative - who is to say that one twin is going faster than the other - they both are traveling at the same speed relative to each other. Why would one age and not the other, or conversely, what decides which clock goes faster: they both are traveling the same speed relatively.
  10. Dec 21, 2009 #9


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    Because one of them accelerated and the other didn't. That breaks the symmetry of relativity.
  11. Dec 21, 2009 #10
    Does the fact that inertia of the travelers as seen by the stay behind folks increases by a factor of gamma [(1/sqrt(1-(v/c)^2))] influence our thinking in any way? After all if F=ma and we increase m does not a decrease?
  12. Dec 21, 2009 #11


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    Well, yes. That's one of the ways of rationalizing why the ship can never reach c. As its velocity approaches c, so does its mass climb without limit, so does its acceleration approach zero.
  13. Dec 21, 2009 #12
    I was thinking about the biological aging processes that are going on in the ship. The forces are not effected by the velocity, but the inertia is, so the rates of chemical reactions are slowed.(?)
  14. Dec 21, 2009 #13
    Yet at the time they pass the imagined railroad station the acceleration stage is over (Although speaking of past/present/future in this instance is fraught with danger) .... I am sure you are correct - and the confusion lies with me ... is there a explanation of this you could direct me to?

    I have this website: http://www.einstein-online.info/en/spotlights/sr/index.html [Broken]

    Hope I am not disrupting a discussion that has gone beyond my simple knowledge.
    Last edited by a moderator: May 4, 2017
  15. Dec 21, 2009 #14
    All clocks are time dilated by relative motion. Biological processes could be used as crude clocks. For example the time it takes a yeast culture to increase its population one thousand times or the time it takes a young seedling to double in height, could be used as a form of clock.
  16. Dec 21, 2009 #15
    So I understand, kev, but thank you. My question is what mechanism governs which biological process is the one to be affected. (Even if I suspect that is not the word.) I thought it was related to speed - as in how close it approaches that of light, but now I find that it is a previous happening - the acceleration, not the % of the SOL, is the culprit here.

    Is it possible I am not understanding acceleration - I took it to mean the achieving of a difference in speed between what it was when the twin was stand with the other twin and what was eventually arrived at. Once there, the 'acceleration' ceases, and a constant rate of movement is in place.

    Is it that 'acceleration' imparts a potential, an inertia to one that is not given to the other?
  17. Dec 21, 2009 #16
    In other threads Dr Greg and I have shown it is possible to set up a version of the twin's paradox that elliminates the acceleration as a cause of differential ageing. Basically both twins accelerate away from the Earth with identical acceleration. They cruise at constant velocity for a while and then twin 1 turns around and cruises back to the Earth where he comes to stop. Twin 2 continues to cruise for a while and then using an identical acceleration pattern to twin 1, he turns around and cruises back and then stops. When they reunite, twin 2 has aged less than twin 1 despite the fact they have both experienced identical acceleration patterns. So you see it is not simply differences in acceleration that causes differential ageing. To be more technical it the path lengths through spacetime that causes the differential. The twin that takes the longest path through spacetime experiences the least proper time. Another analogy I once read is wind chill factor. If you are running into the wind, the chill factor might be greater than the chill factor when you are standing still. In order to go from standing to running you have to accelerate, but it is not the acceleration per se that causes wind chill.
  18. Dec 21, 2009 #17
    Like any good explanation, kev, that opens all manner of possibilities for further inquirery. I fear they would stray beyond the parameters of this thread, which was indeed begun by a biology teacher - so thank you for that.
  19. Dec 21, 2009 #18


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    This leads me to believe you are still thinking there's some sort of discrete effect on particualr aspects of the moving frame.

    It affects everything. There is no experiment you can do within your frame of reference - even in principle - that might show a discrepancy in the passage of time between one mechanism and another. You might as well look at it as time itself has been dilated.
  20. Dec 21, 2009 #19
    In space/time every thing is always traveling at the speed of light. That is

    (vt)^2 + (ct)^2 = c^2

    It is only a question of how much of the traveling is done in the spacial direction and how much is done in the time direction.
    Last edited: Dec 21, 2009
  21. Dec 21, 2009 #20
    Kev, this all seems very Machian to me. There seems to be something special about the stay behind folks they age the most. How? Given that velocity is "relative". How do account for the asymmetry? The folks left behind are moving at exactly the same speed as the folks in the ship (when seen from the ship) (I say speed not velocity, the velocities are equal and opposite).
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