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Relativity and the question of age

  1. Jul 9, 2013 #1
    Suppose our galaxy, say A, is moving at near the speed of light with respect to another galaxy, say B. (which I think it indeed is doing ).

    My age is say 80 years in my position (at earth, in the solar system, in the galaxy).

    1. Persons in the 'another' galaxy are doing experiments regarding my age. they see me grow day by day but in their galaxy generations pass before I die. It's because as compared to them my time has slowed down extremely.

    Now, my galaxy may be at rest as compared to some yet another galaxy, say C. There people will find my age to be just 80 years only.

    So, what is my correct age/ Can someone measure it be less than 80 years? How?

    2. When my 'time' slows down for galaxy B, do the motions of electrons around the atoms that constitute my body also slow down? How is matter or life possible in such a case. Won't the electrons fall inside the nucleus (relative to the person who is seeing me from galaxy B).

    How can we be so sure that if the motion of the cells of our body, the chemical reactions taking place in them, and all the processes of our body like heartbeat etc. slow down (even if relative to another), we will continue to live. Won't the person who is viewing us from galaxy B see us dead and even .disintegrated
  2. jcsd
  3. Jul 9, 2013 #2


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    What happens to you cannot be affected by the fact that you are being watched by these extra-galactic observed. Your constituent electrons will behave exactly as they should.

    Your age is always what the clocks and calendars say in your local frame, and all observers will agree on that.
  4. Jul 9, 2013 #3
    That's an excellent question, Naveen, and very legitimate. What you are making, though, is a classic "category mistake" by assuming that only the motions of the electrons in atoms are going to be affected by this time dilation, while everything else behaves normally. What actually happens is that ALL of the physical and physiological systems are slowed down proportionally to one another so the global systemic operation is unchanged. I'll admit I'm not sure if we are to take these slowed down physiological processes as really occurring and functioning in a slowed down time, or that they are really occurring the "normal" time frame for you (A), and the slowing down of those physiological processes is simply an optical illusion given to those in galaxy B. Perhaps a relativist in this forum can enlighten us.

    Whatever the case, though, the reason that these processes are allowed to continue and DO continue, is that everything is functioning normally in your rest frame, regardless of what's going on in B's rest frame. B is just witnessing the normality of the physiological processes in your frame.
  5. Jul 9, 2013 #4
    Also, our galaxy is not, in fact, moving near the speed of light with respect to any galaxies. You may be thinking of the expansion of space between our galaxy and distant ones, but that is different, and doesn't involve time dilation.
  6. Jul 9, 2013 #5
    Mentz already answered this in the prior post. Local clocks, for example, tick at a steady even pace.

    What is 'real' locally is not necessarily what is 'real' from a distant frame of reference. It is not an 'optical illusion'.....bring different clocks together in the twin paradox and sure enough they have ticked off comparatively different times...but each maintains it's proper time in it's local frame.

    A simple example: You view a friend alongside a distant house.....gee they look so small....but so do you from their perspective.....what is the 'real' size of each of you? Which view is 'real'??
    And in this example, there is not even any time nor relative motion...just as simplistic an example as I can suggest.
  7. Jul 9, 2013 #6
    Yeah, and this continues to be the problem with the twin paradox that drives lesser mortals crazy. Your statement is even paradoxical, even though you're asserting it to be otherwise.

    Well, if my twin shows up after his "trip" younger than me than the fact that his physiological processes were slowed relative to mine is real. But they weren't slowed down for him, right? So how do you reconcile that? At what rate were his physiological processes actually occurring at? To say it depends on your frame of reference doesn't add up for me. It seems as if you're having it both ways.

    So then the OP's concern comes back into frame (no pun intended). Are physiological processes scale invariant? Can we take the rate of the reactions of cellular respiration, cut them in half proportionally across the board, and everything still works as normal? Perhaps... Any cryogenecists wanna chime in?
  8. Jul 9, 2013 #7


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    The twin paradox doesn't come from frames of reference, it comes from the path that each twin takes through spacetime.

    I and my twin shake hands, get into our cars, set the odometers of our cars to zero, then drive off at a constant speed (as indicated by our speedometers) of 100 km/hr on different routes. Some hours later we meet in a distant city (my twin arrives there before me and waits for me to show up), swear under oath that were driving at 100 km/hr the whole time between our departure and our arrival at the destination... Yet the odometer of my car reads a higher value than the odometer of my twin's car. There's no great mystery here, I just took a longer route through space than my twin did.

    The twin paradox is the same thing except with spacetime instead of space, so not only do we record a different distance traveled, we also record a different time elapsed.
  9. Jul 10, 2013 #8
    Why does the house at a distance appear small?

    A person is standing near a house. Light bounces off the house and enters his eyes with all the information about the house and he sees the particular 'size' of the house.

    Now, he retreats 500 meters back. Light is still reaching his eyes with all the information about the house. The distance that light has travelled is miniscule (500m) as compares to its speed (300000 km/s). The intensity of the light also has not diminished.

    So, the person should be getting the same information about the house and its size should also be same from that small distance.

    What actually is happening here?

    Just a bit of 'useless' thinking on my part.
    Last edited: Jul 10, 2013
  10. Jul 10, 2013 #9
    In your example, speed is same where as time and distance are different. This is understandable.

    But in the twin paradox case, not only is speed different but time, distance( length contraction), mass (increase in mass), KE, age etc. are different. This is a completely bizarre scenario. Everything is different.
  11. Jul 10, 2013 #10
    The intensity of the light has diminished, it falls off as the square of the distance of the person from the house.

    No it shouldn't. The person is farther away. The house is going to appear smaller. This is an issue of optics and perspective, it has nothing to do with relativity and the twin paradox.

    See above.
  12. Jul 10, 2013 #11


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    It gets a lot less bizarre if you draw a space-time diagram showing the two twin's paths through space-time and then calculate the length of each path.
  13. Jul 10, 2013 #12
    OK, I agree that only maths can help me out here. No common sensical picture is going to help.

    But how would the twin who is on earth reconcile to what is happening on the space ship that his twin is in.

    He wouldn't be able to recognize his twin's face/body due to length contraction. There is a billiards table in the space ship, it would appear squeezed or warped. Everything in the space ship would be beyond recognition/warped.

    How would he understand that extremely low 'heart beat' on earth is a sign of 'extreme weakness' and even death, but his twin is hale and hearty with such a slowed down heartbeat or lung function or the like. How will he understand that everything is happening in 'slow motion' in the space ship?

    Then, if we tell him that laws of physics are same for you as well as for your twin/spaceship, how is he going to understand/compute/theorize all this.

    It is assumed that the space ship is moving near the speed of light in uniform motion with respect to earth, the home of first twin.
  14. Jul 10, 2013 #13


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    If the distant twin sends a video recording of himself that lasts an hour on his clock ( ie a certain number of frames) then the receiving twin, using the correct video protocol will see everything is normal. The video would last one hour on his clock also.
  15. Jul 10, 2013 #14
    In relativity it is not paradoxical; in Newtonian physics it sure is considered 'paradoxical'.

    It appears as such because none of us are used to observing such phenomena and we are not taught about the variable nature of space and time. [like velocities not directly adding at relativistic speeds]
    It is time and space that are dynamic variables, not immutable constants, and they sure do NOT appear to be that way in our everyday experience. THAT is the root cause of 'paradoxical' here.[ We also tend to 'think' things are deterministic when in fact they are probabilistic [as in quantum mechanics]...so nature again confounds us!! see my signature....Nature IS bizzare!!]

    Well if you are going to take my simplistic example too far, it flops!! But it is simply a lense [focus] type effect....nothing much really to do with relativity....

    This of course is a better way to explain some things that I did, but it doesn't seem to satisfy. Taking this one step further, geodesic paths [worldines], are the shortest paths between any two events, with distance being defined in terms of spacetime intervals:

    yes....and a precise way to think about this is that when one twin 'powers up' and rides 'faster in space' [as Nugatory describes]...they take a LONGER spacetime route. Why is this: the distance metric contains opposite signs for time and space, if space is bigger, time is smaller. [Of course none of this really explains WHY it happens; we know it follows such math from observational confirmation of relativity. ]
  16. Jul 10, 2013 #15
    Well you never hear anything theorized that way. Let's try it now.

    Twins are both in the same spaceship, having a rope pulling contest. The pulling directions are perpendicular, so that one flattened twin is pulling a contracted rope, while the other twin, who has a small width, is pulling a thin band that is not contracted lengthwise.

    (there's a pulley which makes the different pulling directions possible)

    So we may guess that the flattened twin is able to pull with quite large force, but just a short distance, while the other twin is weak, but has longer hands which make a longer pulling distance possible.
  17. Jul 15, 2013 #16
    This is a great time to learn about invariant measurements.

    one of the most fundamental invariant measurements is the order of events. Every observer must agree on the order of events. This is called an invariant causal system/structure.

    This is represented by the spacetime interval. This is more Physically fundamental then time/length.

    One of the consequences of every observer measuring the same order of events is differential aging. its strange at first to think that your age is not as physically fundamental as causal structure, but age/aging not as important as the sanity of the universe :tongue2:

    Makes me wonder if aging could be is a weak emergence

    Mass is also invariant.

    Also, once you learn more about the temporal/spatial dimensions you will see what a simple easy to understand shockingly accurate analogy the shorter path driven, earlier arrival is.
    Last edited: Jul 16, 2013
  18. Jul 15, 2013 #17


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    So both twins are at rest relative to each other, the pulley, and the spaceship? And, in the frame in which all of them are at rest, both twins' ropes are the same length?

    If my assumptions above are correct, then neither twin will have any mechanical advantage. The fact that in a frame in which the spaceship/pulley/twins are moving, one twin's rope appears shorter, has no effect on the actual mechanics of the rope pulling.
  19. Jul 17, 2013 #18

  20. Jul 17, 2013 #19


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    Every observer must agree on the order of events??? It seems to me that this could only be true if there were only one frame to consider--unless I'm totally misunderstanding you. Can you please explain?

    And can you point to a definition of "invariant causal system/structure"? I never heard of this.
  21. Jul 17, 2013 #20
    Ill get a decent reply up, but wiki causal system. essentially the future doesn't effect the past.

    A very simple way to picture this is despite differential aging...the incremental nature of aging is always there. Going faster then c breaks this incremental nature. the continuum always "goes" 1-2-3-4-5 and we all agree on that ordering. it never goes 1-3-2-5-4...if that's the case you have gone too fast and ruined the universe for everyone else :tongue2:

    Doppler is the most clear example I can think of at this moment, another fun one is if everyone is observing you, we all will agree on the order of the physical occurrences as they happen to you, no matter the comparative motion. however that does mean the observers will (generally) not agree on the simultaneity of events where there is a spacelike separation. I.e. the ordering of your's and mine birthdays. Since we don't celebrate them together :cry: there is a spcelike separation between those occurrences, so in some frame it could be I turn older then you, and in another frame you turn older then me.

    I really like this way of thinking of it because it precedes the measurements of length/time between the physical occurrences. If someone agrees that we all observe the same order of events/happenings/physical occurrences, it should follow that measures of time/length would be different depending on the comparative motion/speed.
    Last edited: Jul 17, 2013
  22. Jul 17, 2013 #21


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    That is not correct. What is correct:
  23. Jul 17, 2013 #22


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    Your age at a given event along your worldline is the proper time along your worldline from your birth to that event. It is frame invariant, so all reference frames agree on its value.

    It helps to think geometrically. If you have a curve in a plane and two points on that curve, then the length of the curve from one point to the other is independent of the coordinate system that you use. Age is the "length" of a massive particle's line in spacetime.
  24. Jul 17, 2013 #23
    Actually that's more specific. The correctness depends on "event". Here it is a physical occurrence, an interaction between to previously separated objects (objects = not spacetime).
    Last edited: Jul 17, 2013
  25. Jul 17, 2013 #24


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    What you said is only true for two events that lie in the same light cone. To see this, if ##t_{1} < t_{2}## in one frame but ##t'_{1} \geq t'_{2}## in another frame, then ##\Delta t'_{12} = \gamma(\Delta t_{12} - v\Delta x_{12})\leq 0##. Now if ##\Delta t_{12}\leq 0## as well then this would be perfectly possible but we know that ##\Delta t_{12} > 0## so ##\left | \Delta t_{12} \right | \leq v\left | \Delta x_{12} \right |< \left | \Delta x_{12} \right |## thus ##-\left | \Delta t_{12} \right |^{2} + \left | \Delta x_{12} \right |^{2} > 0## which is a contradiction if both events lie in the same light cone.

    An event that is outside the lightcone of a given event need not obey this. Think of the train and lightning thought experiment: the two events occur simultaneously in one frame but occur at different times in another so the order of the events is not preserved under Lorentz transformations for space-like separated events.
  26. Jul 17, 2013 #25
    And what is the physical significance of RoS? non-physical need not obey physical, literally that simple. The lighting strikes are physically independent. So it doesn't matter if me on the train sees both happen at the same time and you on the platform sees them occurring separately, or vice-versa. All that needs to be done is change the angle of perspective through 4D to change that type of spacelike ordering, clearly you or me moving has no impact on the individual lighting strikes.

    For ordering, every observer agrees I am in the centre of the train, right in the middle of the "event" that's the front of the train & the "event" that's the rear of the train. There is no (inertial) physical angle of approach through spacetime that can change the fact I am in the middle of the train. The are many angles of approach that can change the simultaneity of the what happens at the front of the train compared to the back of the train, that perspective ignores the causal structure of the whole system (lightning, me , you, train) and focuses just on our comparative coordinating of the spacelike separated lighting strikes.

    the causal presentation of c "ignores" what's physically meaningless...i.e. the comparative measures of length/time. What is physically meaningful is the order of the physical occurrence. That's all that matters for comparative observations, not how long it took and the length of the distance.
    Last edited: Jul 17, 2013
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