Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Time dilation

  1. Nov 11, 2003 #1
    imagine two people in deep space, with no frame of reference but each other. if one of them is speeding along at near lightspeed, he might just as well assume he is stationary and the other guy is moving.

    how can we get around this problem, without something like the ether??

    secondly, does light speed up when traveling towards a mass, as it slows down when traveling away from it?
    or am i just misunderstanding something? i am not too clear about relativity, so please keep explanations simple...

  2. jcsd
  3. Nov 11, 2003 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    What problem? What's wrong with each person assuming he is at rest and the other is moving relative to him?
    The speed of light, locally, is always c. What this means is that if you arrange for light to travel through some experimental apparatus at rest with respect to you, the apparatus will always indicate light is travelling at c. Only a distant observer will see light slowing down as it moves more deeply into a gravitational field -- but he can't measure that speed directly, only indirectly. To an observer in that field, the same light will appear to be moving at c. This is all caused by what is called "gravitational time dilation."

    - Warren
  4. Nov 11, 2003 #3
    Re: Re: time dilation

    the problem is, that i do not see how time can be slowed down for both at the same time (no pun intended); time dilation should only occur for the one moving at near c, but if there is no reference frame, who experiences time dilation???

    do both guys see the other guy moving in slow-motion?
    if they do, i need to seriously refine my concept of time...
  5. Nov 11, 2003 #4


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Re: time dilation

    You probably do, but so did everyone when Relativity came to the fore.
  6. Nov 11, 2003 #5


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Re: Re: time dilation

    You're still hanging onto the Newtonian definition of 'time.' In Newton's day, it was believed that time was independent of space -- it was an absolute quantity. If time were to slow down, then it slowed down from some "normal" rate experienced by the rest of the universe.

    This, however, is not correct. It turns out that time is a uniquely local experience -- the time elapsed on your wristwatch does not have to agree with the time elapsed on anyone else's. Furthermore, it is not correct to say that time has slowed down for someone -- all that you can say is that one person, when viewing another person, will measure that other person's time as running slowly.

    The key word of the previous sentence is measure. Physics can tell you nothing about reality beyond the measurements that an observer will make on some system. For two people passing one another at high relative velocity (call them Captain A and Captain B), both will measure the others' time as running slowly. Remember that time is a uniquely local phenomenon, and there is absolutely no reason why Captain A's wristwatch on one spaceship should have anything to do with the wristwatch worn by Captain B on another spaceship. It would be very non-symmetrical, and very confusing, for one observer to measure the other's clock running slow, while the other measures the first's as running fast. Instead, both Captains measure the others' time as running slowly, as compared to their own local clocks.
    It is critical to realize that time dilation is not experienced -- it is observed. Captain A can watch Captain B, and measure B's clock as running slow. He will never measure his own clock as running slowly, however -- and this makes complete sense: how could he? At the same time, Captain B thinks his clock is running just fine, and A's clock is running slowly.
    Yes indeed!

    - Warren
  7. Nov 16, 2003 #6
    I read on howstuffworks.com that in relativity there is time dialation and another dialation where the object shinks in the direction it is going. is this correct or not(if its not correct it will probably make no sence to you)
  8. Nov 16, 2003 #7
    The latter is called "length contraction". (Dilation means getting bigger.)
  9. Nov 17, 2003 #8
    so how do you explain length contraction??

    i can sort of picture time dilation, due to the constancy (is that a word?) of c, but i can't see how this could influence the size of an object...

    oh, and what about the mass increase?? would you "experience" an increase in mass, or is it like time dilation and can only be observed by someone else?

    is there any way of explaining mass dilation (other than just quoting E=mc^2, i mean).

    this is really doing my head in. it's GREAT!! :wink:
  10. Nov 17, 2003 #9


    User Avatar
    Science Advisor

    That's exactly right. Your mass increase is related to your velocity, and your velocity is entirely relative. If you travel by at .9c (from my frame of reference), your mass is increased. But to someone travelling alongside you, your velocity is 0, so your mass is the same as it would be if you are at rest ("rest mass" it's called) because to him, you are. Since you are never in motion relative to yourself, you will always measure your own mass to be equall to your rest mass.

    Also, if you can grasp time dilation, even vaguely, you are already most of the way to getting length contraction, as they are like two ways of looking at the same thing. The number of seconds it takes light to get from point A to point B changes, but the speed of light doesn't. So we can say the seconds have gotten longer (time dilation) or the distance has gotten shorter (length contraction), whichever is more convenient for the solving the particular equation at hand.
  11. Nov 17, 2003 #10


    User Avatar
    Science Advisor
    Gold Member

    Think of it this way, imagine your on a spaceship of a certain length travelling past a planet at a speed of 0.9c, you turn on a light at the back of the spaceship and you measure how long (tspaceship) it takes to travel the length of your spaceship (xspaceship) because the speed of light is always constant xspaceship/tspaceship must equal c . Now imagine someone on the planet making the same measurement and gets xplanet and tplanet. Now we know because of time dialtion that tplanet can't the same as tspaceship yet xplanet/tplanet must also equal c, so th conclusion is the person on the planet must of measured a different value for the length of the spaceship.
    Last edited: Nov 17, 2003
  12. Nov 17, 2003 #11


    User Avatar
    Science Advisor
    Gold Member

    I should add what this shows is that the different contractions/dialtions can be derived from each other (infact historically time dialtion was derived from lenght contraction)
  13. Nov 17, 2003 #12


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    here you are talking about space time and almost in the same breath quoting rhymed verse by an outstanding poet. I'm going to try to recall a Jorge Borges sonnet about spacetime (the 4D universe as a crystaline memory containing all the past and future---the world line of each entity that ever has existed or will exist.) Borges did not, IMO, believe in anything like God he just used the idea. The translator is Richard Wilbur, who managed to reproduce the rhymed form of the original in his translation.

    One thing does not exist: Oblivion.
    God saves the metal and he saves the dross,
    and his prophetic memory guards from loss
    the moons to come and those of evenings gone.
    Everything is: the shadows in the glass,
    which, in between the days two twilights, you
    have scattered by the thousands, or shall strew
    henceforward in the mirrors that you pass.
    And everything is part of that diverse
    crystalline memory, the Universe.
    Whoever through its endless mazes wanders,
    hears door-on-door click shut behind his stride,
    and only at the sunset's farther side
    shall view at last the Archetypes and Splendors.
  14. Nov 25, 2003 #13
    How can clocks be simultaneously slower than each other?

    Personally, I think it's impossible. The example that's often given is that of two people standing a distance apart from each other. If you have two people that are the exact same size and they are separated by a distance, they both appear smaller to the other person just like each person measures the other persons' time as moving more slowly than their own. But the problem is that they aren't smaller than each other, obviously that's impossible. They only appear smaller than each other. How can I have a clock that moves more slowly than your clock and at the same time your clock moves more slowly than my clock? This is the same problem with the people, the people are still the same size they only appear smaller. This could be illustrated with our clocks: My clock reads 3:00, but when you look at it it reads 2:50. Your clock reads 3:00, but when I look at it it reads 2:50. The clocks only appear to be slower than each other. In reality they are not, that's impossible.
  15. Nov 25, 2003 #14


    User Avatar

    And I guess muons only appear to last lthousands of times longer than when they are stationary, and so when we repeatedly measure them when they shouldn't exist we are just having illusions? Really.

    Perhaps you should work on WHY the time dilation result was found - it is a logical neccessity of the postulates of SR.
  16. Nov 28, 2003 #15
    Time slows for an object that accelerates and/or decelerates. This is the answer to, say, the twin paradox. Why does the traveling twin stay young? Becouse he leaves, turns back and stops at home while the other twin stays at earth all the time.
  17. Nov 28, 2003 #16


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm sorry, but it sounds like you've got some major misconceptions about how relativity works. It sounds to me like you don't exactly know what the twin paradox is either.

    - Warren
  18. Nov 28, 2003 #17


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: How can clocks be simultaneously slower than each other?

    The problem is that you are trying to treat time as an absolute measure, when it isn't.

    To use the people example, When you say the one person "appears" smaller, you mean that their angular size is smaller. So in this example, angular size relates to time. Now when you say that they aren't "really" smaller, you are talking linear size, a different type of measurement. This measurement relates to the spacetime interval.

    Thus angular size is "distance variant" while linear size is distance "invarient"


    Time is "Lorentz varient"
    While the spacetime interval is "Lorentz invarient".

    Time measurement behaves like angular size measurement, not linear measurement.

    When you say that it is impossible for the clocks to each be slower than the other, you are trying to treat time like linear measure(As we used to believe it behaved, and how it appears to behave under everyday conditions)

    But is not impossible if time behaves like angular measure, and this indeed is how we now know it really behaves.
    Last edited: Nov 28, 2003
  19. Nov 28, 2003 #18
    For example if the sun explodes, light would be given off from this explosion. What if you could follow this light that is flying away from the sun at any speed you like. If you were to follow it at slighty below the speed of light you would see the suns light moving slowly through or ahead of you. If you were to follow it at above the speed of light you would see the sun's light that is in front of you appear to flow in reverse since you are catching upto and passing light that is ahead of you. This is where "time travel occurs." But it is not really time travel, it is more like placing yourself into a position in space more quickly than the speed of light creating the illusion of going back into time. Eventually the light will stop and you will have nothing more to view leaving the possibility of traveling into the past or the future impossible. But what happens if you travel with the sun's light at the speed of light? What i believe you would see then is the mechanics of light itself and how it moves and functions in space.
  20. Dec 7, 2003 #19
    Can we regard time dilation as a dual phenomenon? Since the speed of light is always the same regardless of your velocity, the faster you go though space, the more slowly you must move in time. Time dilation is not merely the stretching of time but the stretching of a slowed-down time. Thus two things seem to be happening here. Can we decouple the concepts of time dilation and time slowing? Can we conceive of a time that slows down but is not dilated?

    Let's say the constancy of the speed of light causes a rocket moving at .5c to slow down in time by one-half. The time is slowed but not dilated. After one year the pilot of the rocket returns to Earth but does not discover that Earth has aged two years. Instead, he's back one year in everyone else's past, on a different time-line.

    Of course, this is not what would really happen. In reality, the pilot would return to find that two years had passed on Earth, that he was now one year younger relative to everyone else but very much in their present.

    While c causes time to slow down for objects in motion, what is it that causes to time to dilate? It seems the only answer is another constant-- an absolute present which must be maintained regardless of how far time is slowed down. Just as time must vary in order to maintain the constancy of the speed of light, aging must vary in order to maintain the constancy of the present. As you slow down in time, you simply age more slowly as your time is stretched out to keep you in the present.

    Does this make sense? Is there some obvious error in my analysis I'm not aware of?
  21. Dec 7, 2003 #20
    "Time dilation" is just a fancy way of saying "time slowing".
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Time dilation
  1. Time dilation (Replies: 31)

  2. Time dilation (Replies: 21)

  3. Time dilation argument (Replies: 31)