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Who's going to age more?

  1. Apr 1, 2006 #1

    I've just got the following question for you guys...

    Say that Bob is travelling away from the Earth at 0.9999c in a glass spaceship. Phil, who is on Earth and can see Bob’s spaceship, notices that Bob’s clock is running slow. That makes sense. HOWEVER, since there is no ideal frame of reference and no frame is better than another, could one not equally argue that Bob is the one who’s stationary and that Phil (and the earth) is moving away from Bob at 0.9999c. Bob would therefore observe that Phil’s clock is running slow, right?

    So, who’s going to age more, Bob or Phil??

    Also, let’s consider the twin paradox.

    A stays on Earth (‘stationary’), B leaves at 0.9999c and returns to find that A is much older.

    Could one also not argue the converse – that B is stationary and that the earth (with A on it) leaves at 0.9999c and returns, leaving A (and everyone on earth) younger than B.

    How does one reconcile this apparent contradiction?
  2. jcsd
  3. Apr 2, 2006 #2


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    The frame that undergoes acceleration is considered the moving frame.
  4. Apr 2, 2006 #3


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    That is exactly where what the paradox is about. The crux is that the point of view of the observers are not equivalent.

    You are right when saying that, as Bob is flying away, Phil observes Bob's clocks run slow and that Phil observes Bob's clocks running slow. There's no contradiction here. To compare the rate at which Bob's clocks are going, Phil uses clocks in his system and concludes that Phil's clocks are running slow. From Phil's point of view this same analysis looks wrong, since Bob is using clocks which are not synchronized (they are not in Bob's frame). You can make this all quantitative using Lorentz transformations.

    But in the twin paradox, Bob returns home in his spaceship so he undergoes an acceleration, while Phil does not (neglecting acceleration of the Earth). So Bob is changing from one inertial frame to another when he turns around which causes him to 'gain' time wrt Phil. I.e. his perception of 'the time at Phil' changes when he jumps to a different inertial frame.
    Last edited: Apr 2, 2006
  5. Apr 2, 2006 #4
    The hardest thing in special relativity is to accept that time is not absolute. It is against our common sense.
    By asking who is aging more you assume that there is an absolute time scale by which we can compare Bob's and Phil's aging. But according to special relativity this is not the case. Each reference frame has its own time scale by which things are measured.
    If two bodies are moving away from each other in open space, can you ask who is moving faster? It depends, of course, on the reference frame from which their velocities are measured.
    In Bob's frame Phill is aging slower and in Phill's frame Bob is aging slower. Since they end up in Phill's frame things will be then as they were measured from there.

    You cam read more about it here:

    http://www.polarhome.com:763/~rafimoor/english/SRE.htm#Time Dilation

    And here

    http://www.polarhome.com:763/~rafimoor/english/SRE.htm#The Twins Paradox
    Last edited by a moderator: Apr 22, 2017
  6. Apr 2, 2006 #5
    Thinking about his lead me to question a colleague of mine as to how you could determine how fast you were going absolutely rather than relatively, I remember being less than shocked at the idea that you can't do that at all at the time, to determine absolute speed would require a zero point or motionless point and there's no real way to know this point, a sort of centre of the universe the point of creation, for all we know the whole universe is moving at light speed and the accelerating expansion of the universe is not measured from 0 but C as a speed, light speed could in fact be 2C? The misconception about realtivity that provoked the twins paradox is interesting and until you figure out that time is not a fixed value under acceleration you can't really grasp it. What's more interesting to me though is the lack of a zero point, and the lack of an absolutely positively provable absolute light speed. As with most things in physics there's really not alot you can pin an absolute on :smile:
  7. Apr 3, 2006 #6


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    But local light speed is absolutely constant.
  8. Apr 3, 2006 #7
    Actually some people believe that light speed is slower than it was at the start of the universe and that it is slowing down, so constant yes but not in terms of the life time of the universe, the point is only relative light speed is constant. Prove to me that what we see as light speed is in reality lights speed and I'll have to conceed you are right, mind you if you can do that your probably going to get some sort of Nobel prize so good luck to you :smile:

    Is the twins paradox still a paradox, I wonder? Riddle me that?
    Last edited: Apr 3, 2006
  9. Apr 3, 2006 #8


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    I encountered a problem parsing this part of your post:
    Please rephrase.
  10. Apr 3, 2006 #9
    Essentially the only way to know how fast you are absolutuely travelling is to find a motionless point in the universe, if you want say the origin point of the big bang or a centre of the universe or whatever, without that specific motionless point you cannot guess at what speed you are travelling because all other points in space may be travelling in the same direction and speed as you so you have no way of telling wether you are motionless. The light thing is a red herring. I was merely saying that thinking about the twins paradox lead me to ask how we know that in fact we are travelling at a given speed if we don't first know our actual absolute speed.

    As I sit here now I can see two people sitting near me, they appear to be motionless, sat as they are, not really moving from their place, tell me how fast they are travelling? It's the same kind of idea but scaled up to the size of the universe. I'm explaining this really badly. It was an idle discussion at work, that lead to an interesting idea, but I'm not sure I can convey the point without diagrams.
  11. Apr 3, 2006 #10
    One thing we CAN measure our speed relative to is the speed of light. Because we (i.e. all massive particles) agree on that.
  12. Apr 3, 2006 #11


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    That may or may not be true (it is being researched), but really isn't relevant here because this thread is about light speed being constant regardless of the motion of an object (not its age in the universe).
    I don't understand what you mean - there aren't any more Nobel prizes to give for that - they were all given out long ago.
    In science, certain phenomena are called paradoxes only because they challenge our preconceptions. There is no paradox in Relativity regarding this phenomena - the paradox is in the minds of people operating on the incorrect preconceptions.
  13. Apr 3, 2006 #12


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    And since neither of those absolute rest points exists, there is no absolute speed to measure. edit: actually, its probably the other way around - there is no absolute speed, therefore there is no absolute rest point.
    No. Relativity - even Galileo's relativity - states that the concept of speed is something that is measured between two observers - any two observers. So the concept of absolute rest is irrelevant to a discussion of speed. If speed were dependent on an absolute rest frame, you wouldn't be able to play ping pong on a train (or on a rotating and revolving earth, for that matter).
    See above, but to state it again: absolute speed isn't required and never has been for such a discussion - even if it existed, which we now know it doesn't.
    You just answered your own question: to you, they are motionless.
    I think you are conveying your point well enough to understand your idea - but your idea is wrong.
    Last edited: Apr 3, 2006
  14. Apr 3, 2006 #13


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    Well, sorta - but since we always measure the speed of light to be C, we simply always measure ourselves to be motionless wrt it.
  15. Apr 3, 2006 #14
    Yes, that is quite correct. I was just trying to draw attention to the fact that in some of the discussion above people were implicitly assuming that velocities transform as scalars between frames.
  16. Apr 3, 2006 #15
    There may well be a large volume of locations in the universe where clocks run at a maximum speed. When these clocks are moved into G fields they run slower than clocks that remain in a location that is not subjected to a G potential - its also possible that velocities can be similarly related and identified with motion wrt to locations where local clocks run at at a maximum rate. In other words, it is possible to have a place or places associated with a universal frame where time advances at a maximum rate and still have a completely consistent SR based upon the LT that predict the clock rate in moving systems relative to one another.
  17. Apr 3, 2006 #16


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    and some people belive that the universe, earth, and all life on it was created about 6010 years ago.

    the speed of light, measured as a dimensionful number, is a human construct. by adjusting our definitions of units, we can make it equal any positive number we want.

    if a dimensionless universal "constant" (such as the fine-structure constant) changed, that would have physical significance. a variable c, or variable G, or variable [itex] \hbar [/itex] theory cannot be measured and is "operationally meaningless" (Michael Duff's term for it).

    "[An] important lesson we learn from the way that pure numbers like α define the world is what it really means for worlds to be different. The pure number we call the fine structure constant and denote by α is a combination of the electron charge, e, the speed of light, c, and Planck's constant, h. At first we might be tempted to think that a world in which the speed of light was slower would be a different world. But this would be a mistake. If c, h, and e were all changed so that the values they have in metric (or any other) units were different when we looked them up in our tables of physical constants, but the value of α remained the same, this new world would be observationally indistinguishable from our world. The only thing that counts in the definition of worlds are the values of the dimensionless constants of Nature. If all masses were doubled in value [ including the Planck mass mP ] you cannot tell because all the pure numbers defined by the ratios of any pair of masses are unchanged." (From John Barrow (2002), The Constants of Nature; From Alpha to Omega - The Numbers that Encode the Deepest Secrets of the Universe.)

    also see http://en.wikipedia.org/wiki/Planck_units#Planck_units_and_the_invariant_scaling_of_nature .

    what? prove a tautology?

    so proving something to you is what gets a Nobel?
  18. Apr 4, 2006 #17
    Oh believe me I'm not saying it is of huge releveance to his conversation, it's just an idea that gets spawned when you start talking about speed. I think my colleague ended up saying well absolutely there is no way to define your absolute speed only as you say a relative one. Sorry for derailing the discussion, it was an idle discussion and an idle thought, something that I found of interest

    Actually not all people are satisfied with the idea that the twins paradox is not a paradox merely a misprehension, some people still argue that it's not compatible with general relativity, I'm not one of them but my colleague is, he knows more than me though as he has a doctorate in cosmology, that doesn't mean he's right of course or wrong for that matter, but call me old fashioned I do tend to go with the core beliefs when a lack of proof is forthcoming. He also suspects that there is an ether and this field gravity thing that produces drag on an object is just a clever way of avoiding using the word ether and that big bang is not the most viable theory out there, who am I though to argue with that :wink:
    Last edited: Apr 4, 2006
  19. Apr 4, 2006 #18
    Bold by me
    Last edited: Apr 4, 2006
  20. Apr 4, 2006 #19


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    He doesn't sound like he knows any more than you. Undergraduate physics students know better than this.

    I'd hate to think that someone with a degree from an accredited institution would believe sucn nonsense.

    Say - is this another april fools joke?
  21. Apr 4, 2006 #20
    No April Fools jokes are by tradition only suposed to be carried out on April Fools day and even then only in the AM, according to tradition. I'm not saying he believes in ether but he has problems with the idea of an infinite action over distance and a problem with the existence of gravitons.

    As for the paradox thing it's to do with a triplets paradox idea he came up with that rubishes the solutions that are proposed to the twins paradox, more than that I could not say, apparently he wrote a paper on it 30 years ago, which was sumarrily dismissed without his lecturer even reading it, he could explain it far better than me, I have no real idea without getting hold of the paper, it's probably beyond my understanding anyway and has something to do with aplication of special relativity to general relativity.

    With ether I think he was simply pointing out that if a gravitational field is some sort of drag effect then maybe we are looking for a graviton that doesn't exist and that the ether idea that Einstein succeeded in burying could be reinvented, not that he believed it necessarily.

    I think his ideas work around a sort of sea idea of reality with waves on the surface that we see and things under the surface we don't and emergent properties. The whole idea of virtual particles being real just not visible untill they break the surface, hey he's toyed with loads of ideas, I don't think he believes them but, maybe he's one of those unusual theoretical physisist guys that come up with preposterous stuff like strings or branes :wink: At least he hates the whole idea of creating extra dimensions for consistency reasons. He though Heim Theory was a bit of a joke. Not everyone looks into mainstream areas especially when you get to his level, I supose science would be terribly dull if they did.

    Anyway interesting as it maybe to your incredulous establishment guys, I'm really gonna have to apologise for threadjacking this whole thing. if someone could answer the OP's OP it would be nice, I could always create another thread on my wacky colleagues way out ideas if you'd like?

    EDIT: Actually on talking to him again he said it's not that he disagrees with special and general relativity, and that length contraction and time dilation explain the twins paradox exactly, well it is he has some ideas in a paradox that show that it may well be related to a relative speed but not to an absolute one, and so since no absolute speed is known the whole time dilation and length contraction thing are throw into some sort of doubt, I'm unsure of specifics, because he didn't have time to explain it but it's tied up somewhere with 3 twins non absolute speed and something else, maybe I should ask him for a copy and put it up in the skeptecism and debunking or new theory thread? He does say he's still working on ideas he proposed back then though, who knows what exactly he's referring to?
    Last edited: Apr 4, 2006
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