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

How Is The Relativity Of Simultaneity Consistent With Physics And Relativity

  1. Jun 19, 2011 #1
    Hi.
    The concept of relativity is that the laws of physics are the same in all inertial frames of reference.
    As a result of relativity we have the relativity of simultaneity, which says that two events that are simultaneous in one inertial frame are not simultaneous in other frames.

    Now consider we have a piece of wood and we put on a fire in exactly its middle, then according to the laws of physics (if the conditions are equal on both sides of the piece of wood) then the fire will arrive at the ends of the piece of wood simultaneously.
    Or consider if we stand in the middle of a train and we trow two balls to the two ends of the train car with the same force, then according to physics the balls will arrive at the ends of the train simultaneously.

    This is a result of the laws of physics and therefore according to relativity should be true regardless of the frame the situation is being observed.
    However the relativity of simultaneity says that simultaneous events in one frame are not to be simultaneous according to another frame, so how is the situation to be explained?

    (Note that every frame will agree on the middle point of objects since both halves are equaly contracted on frames that the object is in motion)
     
  2. jcsd
  3. Jun 19, 2011 #2

    bcrowell

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Hi, hprog,

    In the two-balls version, there is nothing in the laws of physics that says that the balls have to have equal velocities. They just happen to have equal velocities in one frame.

    In the fire version, there is nothing in the laws of physics that says that fire has to spread at equal speeds in both directions along a piece of wood that is moving with a certain velocity. The symmetry of the laws of physics only implies that the fire has to spread at equal speeds in both directions in a frame where nothing else breaks the symmetry, i.e., in a frame where the wood is at rest.

    -Ben
     
  4. Jun 19, 2011 #3

    DaveC426913

    User Avatar
    Gold Member

    Wait wait.

    Relatviity does not say that cannot be the same; it simply says they may not be the same.

    Nothing in your scenarios would cause any frame to see those events as asimultanoeus, so both frames might see them the same (excepting some things you might do to change them).

    What exactly is the problem?
     
  5. Jun 19, 2011 #4
    What situation??

    oops, I just realized DaveC asked that...

    So let me ask it this way: Are you asking if something would be non simultaneous from another inertial frame? from an accelerating frame?? Are your objects in an inertial frame or accelerating frame...say even rotating???
     
  6. Jun 19, 2011 #5

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Did you perhaps mean "every" when you wrote "any"? The two events "fire reaches the left end of the piece" and "fire reaches the right end of the piece" aren't simultaneous to an observer with a non-zero (relative to the piece of wood) velocity component in the left-to-right direction.

    The fire reaches both ends simultaneously in one frame, but not in another. So he has found something that's different in two frames, and is asking "hey wait a minute, wasn't everything supposed to be the same in both frames?"

    A part of the answer is that the idea isn't that everything is the same. That wouldn't make much sense actually, because it would mean that if you get up from your chair and start walking towards the door, your speed relative to the chair would still be 0.

    So what is the same? First of all, we need to understand that "the principle of relativity" is a rather loosely stated idea, so we shouldn't expect a complete answer to follow from it. The complete answer is contained in the theory that these ideas helped us find (i.e. special relativity). There are however lots of little details in the theory that could be interpreted as aspects of the principle of the relativity. For example, if A and B are two clocks, and the rate of change of B's clock in the frame comoving with A is slow by a factor of 25%, then the rate of change of A's clock in the frame comoving with B must also be slow by a factor of 25%. So I don't think it's possible to write down a complete answer.

    One of the most important things that can be described as an aspect of the principle of relativity (the main thing really) is that equations that describe how properties of particles and fields change with time, can be stated in a coordinate-independent way. There's a slightly different version of each such equation associated with each inertial frame, but they will look more or less the same. (If a term that appears in the equation associated with one frame doesn't appear in the equation associated with another, it's just because its value is non-zero in the first frame and zero in the other). The coordinate-independent equations have coordinate-independent solutions, but there's a coordinate-dependent solution associated with each inertial frame. Example: The coordinate-dependent versions of Maxwell's equation associated with different inertial frames look the same, but the coordinate-dependent values of the E and B fields may be different in different frames.
     
    Last edited: Jun 19, 2011
  7. Jun 19, 2011 #6

    DaveC426913

    User Avatar
    Gold Member

    Ok. Sure.

    And here I thought he was asking: "hey wait a minute, wasn't everythning supposed to be different in each frame?"

    See here:
     
  8. Jun 19, 2011 #7

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    He's arguing that the principle of relativity seems to say that everything is the same, and that relativity of simultaneity seems to contradict this by saying that this one thing isn't the same.
     
  9. Jun 19, 2011 #8

    DaveC426913

    User Avatar
    Gold Member

    Hm. OK.

    To which my response would be:

    Relativity does not say "everything is the same", it says "everyone follows the same rules , even though that might (and often does) lead them to completely different results. And everyone who does observe the rules has an equally valid viewpoint."
     
  10. Jun 19, 2011 #9
    OK thanks all of you for your effort, but I do not understand your answers.
    Forget a moment about the relativity of simultaneity, if fire spreads out in two directions with every thing being equal is there any reason for the fire going in one side faster than the other, or the ball flying in one direction faster than the other? this is physics and we need to have an answer and rule why this should happen.
    Here is another such experiment, suppose we have a pool and we pure water directly in the middle of the pool and the water is spreading out to both sides, is there a reason why the water should arrive to one end of the pool before the other? farther more if the pool is an exact rectangle the water will arrive to the left and right sides (perpendicular to the direction of motion) simultaneously so why is this not happening in the direction of motion (and the same can be said for the ball and fire in which going sideways will yield simultaneous results).
     
  11. Jun 19, 2011 #10

    russ_watters

    User Avatar

    Staff: Mentor

    The fire can be moving at different speeds with respect to different observers and be different distances from different observers. This doesn't even have anything to do with Einstein - Newton and Galileo would be fine with this as well.
     
  12. Jun 19, 2011 #11

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    There are many different ways to obtain this result. You could e.g. get it from the relativistic formula for addition of velocities. I think the best way to see it is by drawing a spacetime diagram, but you would have to work on understanding relativity of simultaneity first.

    But in Galilean spacetime, a boost to a different inertial frame would change the velocities of the two "fronts" of the fire by the same amount, [strike]so that the two speeds would be the same in the new frame too[/strike] so that the two speeds in the new frame would differ from their values in the old frame by the same amount.

    Edit: I corrected the last part after russ watters pointed out that I messed it up.
     
    Last edited: Jun 19, 2011
  13. Jun 19, 2011 #12
    You are using the velocity addition formula which is based on the relativity of simultaneity, so it can not be considered a solution rather it is part of the problem itself.
    Again my question every thing being equal why should things on one side be different than the other, and why should it be (1) only for different frames, (2) and only in the direction of motion.
     
  14. Jun 19, 2011 #13
    No, in physics if every thing is being equal than we should get equal results.
    In fact you have not disagreed that in the same frame every thing will be equal and that even in different frames it will be equal for the perpendicular direction, although the relativity of simultaneity does not require it...
    This is clearly because of equal things will yield equal results, and the relativity of simultaneity cannot change this without breaking general physics, unless you come up with a better answer.
     
  15. Jun 19, 2011 #14

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    I'm not going to spend a lot of time on finding a solution that doesn't require you to understand relativity of simultaneity first. Maybe there is one, maybe there isn't. What's important here is what SR says about this situation. The Lorentz transformation is a part of the definition of SR, and the addition of velocities formula follow easily from that, so "what SR says" is precisely what you get from that formula.
     
  16. Jun 19, 2011 #15

    russ_watters

    User Avatar

    Staff: Mentor

    No, one might go up while the other goes down, say, if you're in a frame traveling with one flame front that one is motionless while the other is moving at twice the speed it moves wrt to the object that's burning.
     
  17. Jun 19, 2011 #16

    russ_watters

    User Avatar

    Staff: Mentor

    No, that's not what relativity means at all. Relativity says they obey the same laws of nature, not that you get the same results when measuring the same phenomena from different reference frames.
     
  18. Jun 19, 2011 #17

    Fredrik

    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    Yes, I messed up what I was trying to say. :smile:
     
  19. Jun 19, 2011 #18

    atyy

    User Avatar
    Science Advisor

    Quick guess: It's not sufficient to apply equal force. Equal force must be applied for equal time. While the force is being applied, the balls will move apart, so they are not released from exactly the same point.
     
  20. Jun 21, 2011 #19

    atyy

    User Avatar
    Science Advisor

    OK, I give up. It's a mess. And they don't even have to learn about relativistic forces at http://ocw.mit.edu/courses/physics/8-033-relativity-fall-2006/lecture-notes/lecture11_dyn2.pdf" [Broken] :biggrin: I'll stick with relativity of simultaneity.
     
    Last edited by a moderator: May 5, 2017
  21. Jun 22, 2011 #20

    Dale

    Staff: Mentor

    All three scenarios fire/balls/water are governed by the same thing: relativistic velocity addition. If you know the velocity in one frame you use relativistic velocity addition to get the velocity in another frame. This ensures that if they arrive simultaneously in one frame then they do not in any other frame.
     
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook




Similar Discussions: How Is The Relativity Of Simultaneity Consistent With Physics And Relativity
Loading...