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Do we also travel at a fixed velocity?

  1. Nov 22, 2008 #1
    Since, light is always travelling at a constant velocity with respect to everything else, does it also mean that everything else is also travelling at a constant velocity with light?

    As in, since light is always travelling at C irrespective of our velocity, then does it not also mean that we are also travelling at a constant fixed velocity with respect to light? Is our velocity then taken as 0? If that is the case what does velocity actually mean?

    With respect to one photon, what would be the velocity of another photon that is travelling towards it?

    Sorry about these basic questions, I am just a fan pf physics and don't really know anything much about it in detail.. :biggrin:
     
  2. jcsd
  3. Nov 22, 2008 #2

    JesseM

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    When physicists talk about the speed of one object "with respect to" another, they usually mean the speed of the first object in the inertial rest frame of the second ('rest frame' is a coordinate system where the object is at rest, and 'inertial' means the coordinate system is moving at constant velocity, not accelerating), but light doesn't have its own inertial rest frame in relativity--see this post for a little more info.
     
  4. Nov 22, 2008 #3

    DaveC426913

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    Yes, and in this is the answer to your question. You cannot pretend to "stand next to a photon" and measure the speeds of other objects. It is meaningless.
     
  5. Nov 22, 2008 #4

    russ_watters

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    Actually, I think the main question was asking it from the other direction. That makes this the answer:
    In other words, the OP asked the speed of "everything else" -- but since the speed of light is measured from our rest frame (and not the other way around), the answer [to a slightly different question maybe] is zero.
     
  6. Nov 22, 2008 #5

    A.T.

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    If you consider traveling trough spacetime, instead of space only, then everything is traveling at a constant velocity c, just in different directions:
    http://www.adamtoons.de/physics/relativity.swf
     
  7. Nov 22, 2008 #6

    JesseM

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    The statement that everything moves at c spacetime is only true if you adopt a weird mathematical definition of "speed through spacetime" (which isn't a term used by most physicists), see my post #3 on this thread.
     
  8. Nov 22, 2008 #7

    A.T.

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    I don't find it that weird. It a consequent extension of the classical "speed through space" by the adding the time dimension.
     
  9. Nov 22, 2008 #8

    JesseM

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    But nothing actually "moves" in a spacetime diagram, spacetime is just a fixed 4-dimensional structure with worldlines embedded in it, so this terminology tends to be confusing to people who haven't learned to take this geometric perspective on spacetime. What you're really doing when you talk about "speed through spacetime" is taking the spacetime interval between two events on the object's worldline and dividing by the proper time experienced by the object between those events, so it's really quite trivial that this will equal c since the spacetime interval is essentially defined as c * proper time (with the c there just to ensure that the spacetime interval has units of distance rather than time).
     
  10. Nov 22, 2008 #9

    A.T.

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    Yes it does! Just press "play" here and see it move. ;-)
    The picture of objects moving along their worldline doesn't seem confusing to me. It is a way to visualize the meaning of spacetime diagrams.
    The nice thing about these space-propertime diagrams is that you directly see all the quantities (space, coordinate time. proper time) as geometrical lengths. And the fact that everything "moves" with c trough this diagram is a nice simple rule.
     
  11. Nov 22, 2008 #10

    DaveC426913

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    But they don't move. In these diagrams time is fixed and unmoving.

    To set anything moving is to introduce an additional component that doesn't exist.
     
  12. Nov 22, 2008 #11

    JesseM

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    But what "meaning" is that? If you adjusted the animation so that objects traveled twice as fast along their worldline, or twice as slow, these types of "changes" would not seem to have any physical meaning, since the worldline itself would be unchanged. And to make sense of anything moving through spacetime, it seems you must implicitly be imagining a second time dimension, no?
     
  13. Nov 22, 2008 #12
    Either these guys are answering the wrong question, or I am.
    Velocities don't add like you would expect in relativity.
    30000 km/s combined with 30000 km/s does not equal 60000 km/s.
     
  14. Nov 22, 2008 #13
    Thank you for your replies,

    So, the way I understand what you people have said so far is this:

    1) All velocity is judged from an initial rest frame.

    2) Light has no initial rest frame, so judging velocity from its perspective is meaningless.

    Okay... this is just like another one of those, rules apply to everything, except electromagnetic ray things.

    Only EMRs can travel at c. Only EMRs can have no mass yet they can have energy and be affected by gravity. Only EMR's can have no initial rest frame.

    Is there any explanation of mass given by SR or GR that explains that light does not possess it yet it can affect things that do possess mass?
     
  15. Nov 23, 2008 #14

    Vanadium 50

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    Virtually none of what you have just posted is correct. You might want to reread the rest of the thread.
     
  16. Nov 23, 2008 #15

    LURCH

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    No no; not "initial," "inertial"! Meaning "not accelerating," from the root word "inertia."
     
  17. Nov 23, 2008 #16

    A.T.

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    Yes of course. The whole point of a time axis is to visualize movement in a static diagram. The animation is just meant to help to understand the relationship between the diagram and real world observation. Therefore the animated parameter is the coordinate time expierenced by the observer.
    In the case of a space-propertime diagram this "additional component" or "second time dimension" is the coordinate time measured by the observer. It determines how far the objects "move" along their worldlines. And since they all "move" the same "distance" in a given period of coordinate time, you could say: Everything moves at c trough space-propertime in regards to coordinate time.
     
    Last edited: Nov 23, 2008
  18. Nov 23, 2008 #17
    Technically precise or not I have found Brian Greene's explanation a very simple geometric way to think about relativity. It's real easy to visualize how motion in space "diverts" motion in time and linear acceleration becomes curved and rotational acceleration spiral (corkscrew) shaped.
     
  19. Nov 23, 2008 #18

    JesseM

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    But the fact that linear acceleration gives a curved worldline and rotational acceleration gives a corkscrew is just a fact about the shape of the worldlines, it has nothing to do with any notion of "moving" along the worldlines at speed c.
     
  20. Nov 24, 2008 #19

    A.T.

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    The notion of "moving" trough spacetime is only a visualization, just like the idea of spacetime itself. It is accessible to beginners, because it naturally extends the idea of spatial motion:

    space displacement during a period of observers time

    by adding a temporal component (proper time period) to the displacement vector, making it:

    spacetime displacement during a period of observers time
     
  21. Nov 24, 2008 #20

    JesseM

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    Mere spatial displacement doesn't specify whose time-coordinate you're using (whether the observer's or someone else's), it just involves the spatial distance between two different events on the object's worldline. Spatial velocity depends on a choice of time-coordinate, but it's not defined as space displacement divided by a period of observers time, it's defined as space displacement divided by a period of coordinate time in the same coordinate system you're using to measure space displacement. There isn't any obvious intuitive reason why it is more "natural" to define an object's "speed through spacetime" in terms of the object's own time rather than in terms of the time-coordinate of some outside observer's rest frame. And if you do choose to define "speed through spacetime" in terms of spacetime displacement divided by the object's own proper time, then as I said, my biggest problem with this is that the statement "everything moves at c through spacetime" is often presented as a significant physical insight when in fact it's basically a tautology given that the spacetime interval which measures "distance" in spacetime can be defined as proper time * c (with the only purpose of the c being to give the spacetime interval units of distance rather than time, c being the only physical constant that has units of distance/time). There's no reason we couldn't define the metric in terms of proper time itself rather than proper time * c, it's just a convention which I suppose owes to the fact that people find it more natural to think of the metric in terms of spatial distances than temporal distances.
     
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