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

Is there a universal frame of reference for which to measure speed with? Consider

  1. Jun 13, 2010 #1
    Sorry if this question seems more philosophical than scientific. It's my first post in this forum. I love learning about the way the universe works and I always have lots of questions :-)

    Alright, so to my point. It seems like the theory of relativity says that the faster you move (plus how much gravity you are experiencing) determines the rate at which time passes (relative to an observer).

    For a moment, let's imagine a region in space where gravity is so small, that it can be ignored, and that you have lots of space to travel in without worrying about running into any other gravitational field excerpted by any other object.

    If we keep in mind that the theoretical maximum speed that anything can move at is the speed of light, then the slowest speed is "absolute rest". Yeah yeah, "there is no absolute rest" (https://www.physicsforums.com/showthread.php?t=9806) you say because you can only measure rest as compared to other observers...

    But I see a double standard here... Why can't you have absolute rest when you can have an absolute speed limit (speed of light) notwithstanding a second observer?

    In the setting above (no gravity, nothing else to compare speed with), there is still a limit of how fast you can move (plug your preferred imaginary ship that can accelerate to 0.9999 c here); your mass becomes infinite and you just can't push through the magical 300,000 km/s speed limit. Even if there is nothing else around to compare your speed with, you will experience this limit. So, what is telling you that you can't go any faster than c? What is making it so that you can't travel that fast when nothing else is around?

    Seems like there must be an overarching (universal) frame of reference that physics follow to limit anything to move at less than the speed of light; therefore it follows that there must be absolute rest as measured against that same overarching frame reference that exists even when there is nothing to "be relative" with.

    Thoughts?
     
  2. jcsd
  3. Jun 13, 2010 #2

    phyzguy

    User Avatar
    Science Advisor

    Re: Is there a universal frame of reference for which to measure speed with? Consider

    Interesting argument, but that's not the way the universe works. The facts are, and these are amply verified by experiment:

    (1) There is no universal reference frame.
    (2) All reference frames see c as an absolute maximum speed.

    It's not at all obvious that this is the case, but reasoning from these two postulates leads to special relativity, and every experiment that has ever been done has shown that this is the way things are. Perhaps it is a double standard, but to quote Richard Feynman, "This is the way the universe is, if you don't like it, move somewhere else!"
     
  4. Jun 13, 2010 #3
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    Hey Yupper,

    Well, there is no rest in our universe since its not possible to stop the time. Even if you find a means to do so, that moment you are no longer a part of space-time continuum.

    There are 2 *impossible* ways to do so,

    1- Get into a singularity, results are unknown and open to speculation.
    2- Achieve speed of light, which is quite a contradiction in itself from the point of view you explain with your question.

    Don't know about 1, but 2 tells me "absolute rest" and V = c is the same thing.
     
  5. Jun 13, 2010 #4
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    There's no double standard, the speed limit of c is also relative. It's a limit to the speed of an object relative to an inertial reference frame.

    And there may very well be some "overarching" reference frame, but if the speed limit of objects relative to that frame is c, and light travels at c relative to that frame, then it's exactly equivalent to any other inertial reference frame.

    When physicists say there is no "preferred" reference frame, they only mean that the standard laws of physics are valid in any inertial reference frame, and the speed limit of c is a law of physics.
     
  6. Jun 13, 2010 #5
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    I am trying to follow you but it seems that you think you can know a speed without having anything to compare yourself to? In one frame of reference you could be stopped and at the same time in another you could be moving at .999c

    Or look at it like this if you are moving away from earth at .9c, that doesn't stop something else moving away from you at .9c in that same direction.

    If the speed of light is the limit then the "absolute rest" is like right now I am at absolute rest when compared to my computer.
     
  7. Jun 14, 2010 #6

    atyy

    User Avatar
    Science Advisor

    Re: Is there a universal frame of reference for which to measure speed with? Consider


    Yes, there is at least one overarching frame - such a frame is called an "inertial frame". The reason that such an overarching frame is not considered "absolute" is that more than one inertial frame exists. So rather than there being a preferred frame, there is a class of preferred frames.
     
  8. Jun 14, 2010 #7
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    Hi, thanks everyone for your answers! Glad to have found this forum!


    This is all very interesting. I need to do some reading on the "http://en.wikipedia.org/wiki/Inertial_frame_of_reference" [Broken]" concept.

    So, I guess it's not possible to tell where in the 0 to c speed some matter is traveling at? For example, take the popular description of a person out for a jog. That person is jogging on the surface of the earth, which is rotating in it's own axis, which is in turn orbiting around the sun, which all of it is orbiting around the center of the galaxy, which is in turn moving away from other galaxies... How fast are we moving then? Given that we are certainly moving, we must be moving at some fraction of c. It follows that time here must be slower (relatively speaking) than time somewhere else where not as much motion is happening... If so, there must be at least one place where time is the "slowest" of all.

    Maybe I just need to do more reading on the inertial frame concept to avoid falling in these thought rabbit holes :-)

    Ha, after I posted my question I realized what you point out here; how do you know how fast you are going, if you have nothing else to compare it with right? One thought I had after that was that, indeed you would not know how fast you may be going, but you can safely assume that as long as you feel acceleration, you have not yet achieved light speed. (Of course this is an imaginary ship that would have the necessary thrust to get as close to c as possible, and you are traveling in a straight line; yeah lots of footprint on this imaginary experiment!).

    Wouldn't time "speed up" (relative to an observer) the slowest you move (and the least amount of gravity you experience)?
     
    Last edited by a moderator: May 4, 2017
  9. Jun 14, 2010 #8

    Janus

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Re: Is there a universal frame of reference for which to measure speed with? Consider


    You still seem to be hung up on the idea of there being a "preferred" frame of absolute rest against with motion can be measured by. This isn't true. Imagine that you have two astronauts floating in space with nothing else around. They are moving relative to each other. There is no way to say which is moving "faster or slower" than the other. Even if they compare their clocks. Each will note that since the other astronaut is moving realtive to himself, it is the other astronaut's clock which is running slow.

    If you add a third observer, so that his relative velocity to both astronauts is the same, he will note that both of the other astronaut's clocks run at the same rate(though slower than his own).
     
    Last edited by a moderator: May 4, 2017
  10. Jun 14, 2010 #9
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    It really does. What i am trying to say is, considering x,y,z coordinate system it is possible to talk about absolute rest. But when you are talking about x,y,z,t coordinate system it is not possible since t changes according to your relative speed.

    What i am saying is, stopping time means moving t out of the equation. Thus you get back to x,y,z thus achieve absolute rest.

    I most of the time don't understand what i am talking about when i write it, so well i am open to suggestions =)

    Also need to note that, when you are no longer part of x,y,z,t universe, the x,y,z universe has noting to do with the one you left. =)
     
  11. Jun 14, 2010 #10
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    How about you look at this, you can accelerate as long as you want, lets say you had a space ship that could accelerate at 1g. You could accelerate at 1g for 100 billion years from your own point of view.
     
  12. Jul 1, 2010 #11
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    "Is there a universal frame of reference for which to measure speed with?"

    Contrary to every reply saying there is no universal reference frame, I think the answer is yes there is: the cosmic background radiation. Would'nt it be blueshifted in the direction of motion and redshifted in the opposite direction? My astronomy professor answered yes.
     
  13. Jul 1, 2010 #12
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    This is a logical concept and in fact might possibly be true.
    But even if absolutely correct and there is a set inertial frames in the universe actually at rest wrt light, this does not change the fact that within that universe we have no possible means to determine which of the various frames those are.

    The majority of the responces have been based on and addressed this inability to determine a state of rest and are all correct and relevant in that context.

    I certainly agree with all in that context but dont agree with many who assert that there is no possible rest frame even as a hypothetical concept. That reject your idea as either a valid logical construct or as a possible reality.

    It is the same question regarding inertial motion itself.
    The inescapable empirical reality is that we have no means to detect inertial motion from within a frame let alone quantify that state of motion in any but relative terms.

    Based on this many have made the logical inference that inertial motion itself is not merely unquantifiable but fundamentaly unreal. That it has no physical implications or consequences whatever. That all frames are traveling at the same velocity relative to light and the apparent differences have no meaning.

    Others have logically arrived at the alternative view that actually differing velocities relative to light are found in differing frames but that the "physics of the universe conspires to veil that from our perception or measurement." (A rough paraphrase of Poincare I believe??? ) That the real physical effects resulting from either acceleration or inertial motion make it impossible to either measure the frames velocity relative to light or perceive those effects themselves within the frame.

    It is evident that neither empirical tests nor logic can prove either viewpoint or conversely disprove the other so IMHO it is just personal preference.
    I myself favor The "Conspiracy Theorem"
     
  14. Jul 1, 2010 #13

    jtbell

    User Avatar

    Staff: Mentor

    Re: Is there a universal frame of reference for which to measure speed with? Consider

    But what makes this reference frame special besides the fact that in it, the CBR is neither redshifted nor blueshifted?

    In relativity, when we talk about a hypothetical "preferred" or "universal" reference frame, we mean a frame in which the laws of physics take on a simple form. That is, the mathematical relationships among physical quantities, not simply the values of those quantities; things like Newton's laws of motion (or rather their relativistic versions), Maxwell's equations for electromagnetism, etc.
     
  15. Mar 17, 2011 #14
    Re: Is there a universal frame of reference for which to measure speed with? Consider

    I think the best way of considering this is to ask the right question. For me it was how would you determine a body was actually at rest. Then twins paradox, triplets paradox made it hit me like a thunderbolt. There are no provably fixed points in space, hence no frame is privileged there are only frames of relative motion.

    This did of course come in conjunction with most of the things people have already mentioned or it would be a leap of genius. :smile:

    Another bolt was that time for c was undefined, but if we look at it as 0 despite that having no real meaning then the whole shooting match natural falls out of the equations.

    Maths and intuition then meet at a point. And an understanding was born.

    It's not all that easy to understand this, but I am thankful that I had a well qualified physics PhD to bounce off while learning about this as no doubt now you have. :D
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: Is there a universal frame of reference for which to measure speed with? Consider
Loading...