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 and infinite speed

  1. Nov 2, 2005 #1
    If an object is located at the exact center of the universe, would time pass by infinitely fast for this object? An object at the exact center of the universe would not move at all. It would be independent of inflation. Using Einstein's time dilation equations (as I understand them) basically state that the faster an object moves, the slower time passes for it. Therefore a competely stationary object should have the exact opposite properties. or no.....?
    Last edited: Nov 2, 2005
  2. jcsd
  3. Nov 2, 2005 #2


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Well, for starters, we don't think there's any center to the universe. That doesn't seem to be the primary misconception here, however. It is true that time dilation implies that a moving clock runs slower, but it is not true that a stationary object's clock runs infinitely fast.

    Imagine you have two observers, one that is stationary and one that is passing by at a speed, v, and that the stationary observer claps his hands twice. The increment of time between these claps, as measured by the moving observer, is

    [tex]\Delta t' = \gamma \Delta t = \frac{1}{\sqrt{1-\frac{v^2}{c^2}}}\Delta t[/tex]

    where [itex]\Delta t[/itex] refers to the corresponding increment of time on the stationary observer's clock and c is the speed of light. Since [itex]0 \leq v < c[/itex], this will always give [itex]\Delta t' \geq \Delta t[/itex]. The important thing is that this expression is relative, not absolute. It can compare the amount of time that passes from one clock to the next, but it does not give a sense of absolute time.

    So what, then, is a stationary object? In special relativity, the basic answer is that it's an object moving with the same velocity as your frame of reference. To find the interval of time on a clock that's stationary with respect to yourself, you just plug v=0 into the above equation. Not surprisingly, you just get

    [tex]\Delta t = \Delta t'[/tex]

    meaning that the stationary object's clock runs at the same rate as yours.

    However, when describing gravity (and thus, the evolution of the universe), we need to use general relativity. In this theory, the passage of time between two observers depends not just on their relative velocity, but also on the curvature of spacetime. Thus, the above expression for time dilation will not be sufficient to describe the passage of time for a "stationary" observer.
  4. Nov 3, 2005 #3


    User Avatar
    Science Advisor
    Gold Member

    When you learn the mathematical foundations, and how Einstein derived relativity, these kind of questions quickly disappear. I don't at all wish to discourage you from exploring this. Learning how Einstein came to his conclusions is the most enlightening thing you can achieve in all of physics. What Einstein did, which was brilliantly done, was take the Maxwell equations to their logical limit.
  5. Nov 3, 2005 #4
    Am I right in saying a particle traveling at the speed of light is essentially 'timeless' and as far as its concerned it is stationary and the universe rushes past it at the speed of light.
  6. Nov 3, 2005 #5


    User Avatar
    Science Advisor
    Gold Member

    You are on the right lines but may I make three comments?

    If the particle has 'rest' mass then it cannot travel at the speed of light.

    The space-time interval, the proper time, does become zero as v -> c, but this applies only to (rest) massless particles such as photons.

    All observers think they are essentially stationary, we are at rest in our own frame of reference, and the universe rushes past. Have you ever looked out of a train window?

    I hope this helps.

  7. Nov 3, 2005 #6


    User Avatar
    Science Advisor
    Gold Member

    Wade your way through the Einstein equations and marvel at his brilliance.
  8. Nov 3, 2005 #7
    I am not sure that I understand how there is not a center of the universe. The universe started as a singularity that is expanding at an ever increasing rate from this point. Now this expansion is stronger than gravity's effect on the curvature of space, otherwise the universe would have already had the "big crunch." Therefore, it stands to reason that there is in fact a point in space that is not expanding.

    Chronos, I will look at the history of how Eistein developed his theories.
  9. Nov 3, 2005 #8


    User Avatar

    Staff: Mentor

    The big bang happened everywhere at once. Consider the surface of an expanding balloon. Where is the center of the surface?

    Universe looks (in 3d) exactly how the surface of the balloon looks (in 2d): every observer, at any point in space can look around and see everything moving away from him at a rate that depends only on distance (not angular separation from some center point). Every point looks like the center of the expansion.
  10. Nov 4, 2005 #9
    I need not wade through the equations of Einstein to marvel at his brilliance. I would have loved to have been a physicist, but alas, I just don't have the brains, so I went in to politics. His idea of gravity still amazes me. Such curiosity for truth and the ability to visulise something beond Newton has me perplexed. Relativity always has me thinking of what it is trying to say of the future. I share that curiosoty with him, I ask these quesions every day.
  11. Nov 8, 2005 #10


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Welcome to Physics Forums, bort. :smile:

    The universe is not expanding out FROM the point/singularity, it IS the point/singularity...and now the space contained within that singularity has expanded apart from itself.

    The real head-scratcher is trying to understand expansion without expansion into something, but such are the limits of our understanding based on our everyday experiences.

    But observations of distant galaxies does not show a motion away from any central point.
  12. Nov 8, 2005 #11


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Or think of yourself on the surface of the Earth (2 dimensional surface). Where is the center of the surface of the Earth? There isn't one (although people from New York City may disagree :wink: ). If the surface of the Earth were suddenly twice the size, there's still no apparent center of the expansion. Of course, when you add the third dimension, the center becomes clear...but there is no 4th dimension of space (at least not according to the cosmological model from Relativity).
  13. Nov 8, 2005 #12


    User Avatar

    Staff: Mentor

    Being from Philly, I don't give a damn what people from New York think! :tongue:
  14. Nov 8, 2005 #13
    An Object with no motion absolute is timeless, It is impossible to have absolutely no motion so the word Timeless is non-existant.
  15. Nov 8, 2005 #14
    What? too advanced.

    What do you mean with " no motion absolute is timeless? " I think your argument could have any case if you are talking about something like third law of thermodynamics (that would account for saying that no object can reach the absolte zero, or viewed different, that a object that reaches the absolute zero converts in another kind of energy) ; I don't see what is the point with it.
    Last edited: Nov 8, 2005
  16. Nov 8, 2005 #15
    It would only make sence, Nature abhores a vacuum, Why would there be a limited Universe size in an infinite void, If Nature abhores a vacuum then there should be an infinite Universe to fill its emptiness.

    I do believe more in the infinite Universe theory more so than a size limit on the Universe.

    Everyone thought the Earth was the center of the Universe at one time and now it seems that some think the Universe is the center of it all now, in my humble opinion from a common sence point of view is that there is no real center, The Universe may be just a super sized Galaxy among many others super far away and can not be detected as of yet.:smile:
  17. Nov 8, 2005 #16
    I was referring to (Relative aging), Some Bacteria that have been trapped in the Arctic Ice Cap that were millions of years old were brought back to life being millions of years old, Their systems were suspended to a slower state, The Bacteria experienced little time, But they were not timeless, Just slowed.
  18. Nov 9, 2005 #17
    Are you talking about biology or the prevention of decompsition by the ice in the bacteria? That doesn't have any sense to me, cause even then, this bacterias experienced the same time rate as you do over this millions of years. "Experienced" as a physical term, not the ilusion of less aging that women try to achieve with creams - well, some men too - or when you see a decrepit man of 70 in US compared with a guy of 110 in an asian island that feels totally comfortable with his body. You can''t give a physical argument based on biologiczal conditions.
  19. Nov 9, 2005 #18

    That has nothing to do with time dilation. Cryogenic suspended animation is a completely different thing.
  20. Nov 9, 2005 #19
    How can a suspended Bacteria have Experience of time?

    Besides, If the Bacteria could see everything moving around them while in a suspended state then they would see everything moving very fast to there cognitive abilty, Their thinking would be slowed just as their body.

    Besides all this lets get back to the topic of Time Dilation.

    If I was traveling at sub light speed and passed by a planet containing a person and my space ship came very close to this person, I would see this person as moving incredibly slow, (To Me) this person would appear to be experiencing less time. It would be the opposite if the person saw me, this person would see me as experiencing a faster rate of time.

    It would still be this way even if travelling at light speed was possible for which it is not.:smile:
  21. Nov 9, 2005 #20


    User Avatar
    Science Advisor

    Whether they experience anything isn't really the point, the point is just that all natural clocks like rates of radioactive decay, the speed with which various chemical reactions happen, the rate that different atoms oscillate, etc. all happen at the same rate in the frozen bacteria that they would in a live one. This is unlike genuine time dilation, where all these physical clocks would genuinely seem to slow down, not just gross macroscopic features like metabolism rates or rates of information-processing in the brain.
  22. Nov 9, 2005 #21

    No. Time dilation, and the fact that travelling at the speed of light is impossible are closely related. They are not seperable in Special Relativity. The postulates that lead to time dilation also lead to the impossibility of faster than light travel. They cannot be seperated.
  23. Nov 9, 2005 #22
    Compared to the Bacteria, We would experience more of a contraction of time then the Bacteria would because the relativity of time cognitively for the Bacteria would be slower, we would see the Bacteria as experiencing a slowed rate of Relativity from our perspective because it would appear to be suspended.
  24. Nov 9, 2005 #23
    Doesn't SR say that a particle travelling at the speed of light has used all its energy to do so and thus has no energy to travel through time. For instance I am at this moment in motion with the earth and the galaxy going through space and the rest of the energy is used up travelling through time.

    Turning this on its head, could it be said that there is no such thing as a motionless object due to the fact that all its energy would be used to travel through time. This begs the question, if we just assume for fun that an object could become motionless; at what speed would it travel through time, infinite speed?
  25. Nov 9, 2005 #24

    "Thats not right. Its not even wrong." ~ Dirac.

    Cryogenic suspended animation has NOTHING to do with special relativity. Special relativity is about the laws of physics in inertial frames that have relative motion.
  26. Nov 9, 2005 #25
    No. SR does state that it would take inifinite energy to accelerate to the speed of light, and time would slow down to a standstill, but there is no "energy to travel through time".

    Kinda and no.

    In relativity there is no such thing as a motionless object, because motion only has meaning relative to another object. Two objects can be comoving (moving with the same velocity, or zero velocity relative to each other) but that is as close as it gets.
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook