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What happens biologically during time dilation?

  1. Mar 6, 2010 #1
    If someone is traveling close to the speed of light, relative to your reference frame, then the time observed on their clocks slows. But what does that mean when it comes to aging? Do their cells that make up their body replicate more slowly? Is there any type of disagreement in the amount of cells replicated in their body between your reference frame and theirs?
  2. jcsd
  3. Mar 6, 2010 #2


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    You are thinking about it wrong.

    In the frame of reference of the dude moving near c, there is no time distortion effect. There's no experiment he can perform upon himself or anywhere inside his spaceship that will show he's moving near c.

    In fact, there is no way you can show that it is not you who is moving. There is no objective frame wherein one of you is not moving. It's all relative (that's why it's called relativity). The dude's frame of reference is exactly as valid as yours - he is stationary, and it is you who are travelling at a near c.

    In conclusion, to ask what is happening to his cells is to ask what is happening to your cells.
  4. Mar 7, 2010 #3
    Well on average it takes a cell to completely replicate anywhere between 12-24 hours. So we’ll use 24 hours for this example, or 86400 seconds.

    So let’s use an example where someone is traveling .8c relative to your reference frame, which we’ll assume is an inertial reference frame.

    Event 1 we will call the beginning of replication for a cell, and event 2 will be the completion of replication. So in the observing reference frame

    t2 – t1 = 86400 seconds.

    But the time elapsed observed from your frame for the person traveling .8c is given by

    t’2 – t’1 = (t2 – (vx2/c^2))(gamma) – (t1 – (vx1/c^2)(gamma)

    We’ll say at t1 the x coordinate will equal 0 and with simple calculations knowing the person is traveling .8c we know that the x coordinate at t2 is 2.07x10^13m

    (86400 – ((.8*2.07E13)/2.99E8)(1/.6)) – 0 = 51936.31s

    Which only allows about 3/5 of the replication of the same type of cell in the reference frame of the observer. Therefore, for every 5 cells replicated on your body, you only observe 3 replicated on the person traveling .8c with respect to you.
    Am I right about this?
  5. Mar 7, 2010 #4


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    You're still on the wrong track.

    Let me ask you this:

    If you are watching an old film (you know the ones where the camera is cranked by hand, so everyone is running around faster than normal?) would you check to see whether the cars are fast by a factor of X and the people are walking fast by a factor of Y and the clocks are fast by a factor of Z and then check to see if there's a discrepancy between X and Y and Z?

    No, if the film is moving fast, then everything, everything is moving fast exactly the same. They're not moving fast individually; you can say that the entire-world-that-is-the-the-film as a single unit is moving fast.

    Furthermore, if the people in the film discussed it amongst themselves (if people-on-cellulose could somehow be alive), they would not see any problem with time. The fact that you are observing them at the wrong speed does not meant here's anything wrong with their world.


    It is not the individual components in the dude's spaceship that are slowed down. It is all of time in his frame of reference that is slowed down. And even that is only with respect to you.

    There is no absolute time against which both you and the dude can have events measured. The passage of time is a property of the frame of reference you or he are in.
    Last edited: Mar 7, 2010
  6. Mar 7, 2010 #5
    I understand that in the moving person’s reference frame (that is, moving relative to another frame) everything remains the same, as their frame in an inertial reference frame. Just like what is observed in the other person’s inertial reference frame. I’m talking about their observations relative to each other.

    There is less of a difference in time between events one and two for someone traveling an appreciable fraction of the speed of light relative to an inertial frame and IN the inertial reference frame that is observing. Is this not right?

    I actually think I got my own logic wrong in the last post. If event 1 indicated the beginning of a cell replicating and time 2 indicated the ending of it. Then the cells would replicate faster for the person traveling with respect to the observer, as it took 3/5 the time to finish replicating as in the observer’s frame, and this is of course IN the observer’s reference frame. NOT in the frame of the one traveling. For the one traveling the effects should be reciprocal. However, I know your still arguing against that claim in general.

    Are you claiming that the rate of cell replication is modified to compensate for time dilation, once again relative to the observer?
  7. Mar 7, 2010 #6

    Doc Al

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    Sure. Every process in the moving frame slows down according to the observing frame. (Of course, in the moving frame itself everything operates as usual. In fact, they see your clocks and bodily processes running slowly.)
    Not sure what you mean by "disagreement". Say that the rate of cell reproduction is 5 cells per hour. So, in one of your hours(note), 5 cells replicate on your body and only 3 cells replicate in the moving frame body according to you. Everyone agrees on this.

    What's more, the situation is completely symmetric (assuming the moving frame doesn't turn around and come back). The moving frame says the same thing about your cells. How can that be? To understand how they both can say these things requires you to understand not just time dilation, but the relativity of simultaneity. Note that the two events, the start of the first cell replication and the completion of the third in the moving frame, take place at different locations according to you, thus different 'clocks' must be used to record the time that they occur. The moving frame will see your clocks as being out of synch.
    Sure. But see my comments above.

    (Note: One of your hours as reckoned by you. The moving frame will see the time on your clocks change by a much smaller amount during the time that three cells have replicated. The relativity of simultaneity strikes again.)
    Last edited: Mar 7, 2010
  8. Mar 7, 2010 #7


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    No.This is wrong.

    The duration of a second in your frame of reference is one second. The duration of a second in the traveller's frame of reference is one second. Period.

    You are mistakenly applying some sort of "universal" time passage to both participants.

    Einstein's theory of relativity postulates that his frame of reference is exactly as valid as yours. i.e. you here on planet Earth have no special claim to any universal passage of time. From the frame of reference of the spaceship, it is stationary and you on Earth are moving away at near c.
  9. Mar 7, 2010 #8
    check out the similar discussion in this thread: https://www.physicsforums.com/showthread.php?t=382591
    Apparently some people think that time dilation is a real slowing down, whereas it is really only an effect of observation between two different inertial frames.
    In Special Relativity, all inertial frames are equal; none can be considered an absolute reference for any other.
  10. Mar 7, 2010 #9
    Including Einstein, the majority of physics graduates, postgrads and PhD's, and the guys who ran this experiment
    Last edited by a moderator: Sep 25, 2014
  11. Mar 7, 2010 #10


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    Ron, you need to stop making false claims about SR. It's OK to be confused and to ask questions about the things you don't understand, but it's not OK to repeat the same mistakes over and over after they've been pointed out to you.

    No one is saying that time is slowing down in an absolute sense, or that there's a preferred frame. When A's clock is slow in B's rest frame, B's clock is slow in A's rest frame as well. Both are correct to say that "the other clock is slow". That's not a statement about something absolute. What the statement really means is that the coordinate system that's naturally associated with B's world line is assigning time coordinates to the events where A's clock is present that are higher than the numbers that are displayed by A's clock at those events.

    These are not controversial claims. Everyone who understands SR agrees that this is what SR is saying.
  12. Mar 7, 2010 #11


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    I agree. Time dilation is an observer dependent effect, differential ageing is not.

    (Fredrik, did you misread this post, perhaps ? It doesn't seem controversial.)
  13. Mar 7, 2010 #12
    actually that is what several posters are claiming.
  14. Mar 7, 2010 #13


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    Name names. Who is claiming this ?
  15. Mar 7, 2010 #14
    Since you asked, check out 'atyy' and 'sylas' in the thread I linked earlier, and apparently 'TcheQ' above. Not sure where 'stevmg' stands, seems to be leaning to the other side as well.
    I think the difference between me and Fredrik is semantic, where I say clock B appears to observer A to be ticking slower, Fredrik says that clock B is ticking more slowly in A's inertial frame. as long as he doesn't believe clock B actually slows down in its own ('B') inertial frame, then we are on the same page. that's how I read his postings.
  16. Mar 7, 2010 #15
    another one early on. 'Ich' typed:
    "This is not about the aging process, and not abhout different conditions the twins have been living in. For one twin, 10 years passed, for the other 5. With all consequences."
  17. Mar 7, 2010 #16


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    There is a problem here over the use of language. When people try to answer questions such as "Is time dilation real?" or other variants, different people interpret the words in different ways. There's no unambiguous definition of "real" that everyone agrees with.

    If two observers begin together, then separate, then come back together again, then there is no doubt, experimentally, whether the two observers have elapsed the same time or not. You just look at the two clocks , which are both co-located, before and after, and read off the values. In this case we are not making any instantaneous comparison of clock rates, we are comparing the two average clock rates over the whole journey.

    On the other hand, if you want to compare two clocks which are separated by a distance, there is no unambiguous way of doing this. To make an instantaneous comparison you need to read both clocks simultaneously, and in relativity different observers disagree over what "simultaneous" means, with no one definition taking priority over any other. Thus whether clock A is instantaneously ticking faster, slower or at the same rate as clock B depends entirely on who is making the comparison. There is no absolute answer to that question.

    In the case of the twins' paradox, everyone agrees that the travelling twin (who is not inertial for at least part of the journey) ages less than the stay-at-home inertial twin averaged over the whole journey, but observers disagree over the instantaneous rates at any point during the journey.
  18. Mar 7, 2010 #17
    there's another one.
    I think y'all need to hash this over and decide which version of Special Relativity you believe in - the one where all inertial frames are equal, or the one where some inertial frames are more equal than others.
    Either way it's been an interesting weekend. Toodles.
  19. Mar 7, 2010 #18

    Doc Al

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    You seemed to have missed the point that one of the twins accelerates and thus does not remain in a single inertial frame. Basic stuff.
  20. Mar 7, 2010 #19


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    I think you might be conflating two phenomena

    1. Time dilation : where two IRFs see one and others clocks running slower. This depends on the relative speed and seems paradoxical ( but isn't ).

    2. Differential ageing : this is not dependent on relative velocity at one instant, but the proper length of the journeys before the clocks are brought together and compared. All IRFs agree on this.

    This is what DrGreg and earlier posts are saying.
  21. Mar 7, 2010 #20


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    Ron, did you look at the numerical example I gave back in post 63 of the other thread? I showed that in an example where one twin (Stella) travels away from another twin (Terence) and then turns around and returns, you can analyze it from the perspective of two different frame, both of which are considered "equal" and which disagree about the rates the two twins were aging during each phase of the trip, yet both frames end up agreeing on the total amount that each twin has aged at the point when they reunite, with the twin who turned around having aged less in total.

    As Mentz114 said, you have to differentiate between the rate that each twin is aging (or that their clock is ticking) at a particular moment or during a particular phase of the trip, which different inertial frames can disagree on, and the total time elapsed for each twin during the course of the entire trip from start to finish, which all frames actually agree on despite disagreeing about the rates at different moments. It turns out that in SR all frames always agree in their predictions about local events that happen at a single point in space and time, like two twins meeting and comparing their ages at a single location and a single moment, even though they can disagree about other things like which of two twins is aging faster at a particular moment, or even things like which of two twins that are far apart celebrates their 40th birthday first (this is the relativity of simultaneity, which says that when it comes to events at different locations in space, different frames can sometimes disagree on which happened earlier and which happened later...so different frames can disagree about which twin is older when they're far apart, just not when they actually meet at a single location).
    Last edited: Mar 7, 2010
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