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The crossing of two light waves, a point standing still ?

  1. Oct 10, 2011 #1
    A light wave has mass 0, so except his movement in one direction with lightspeed C it can't be moved in an other direction.

    So the crossing is always the same point (don't consider that space itself is also expanding according theory). Consider you can still track the light waves after a while.

    When the light source was there in that point, and not anymore after a while, the source was/is moving.

    In theory you could determine if an object is moving or not somewhere in space ?
  2. jcsd
  3. Oct 10, 2011 #2


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    Moving with respect to what? If you have the entire system floating along at 100km/s, you would observe exactly the same thing.
  4. Oct 10, 2011 #3


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    If you could track the progress of a light wave, then, yes, you could determine if an object is moving or not somewhere in space, in an absolute sense. But, you cannot tell where light is out in space or how fast it is going. You have to wait for a reflection of that light off an object to even know that it arrived at any particular location and then you can only tell that it got to that object sometime between when it left you and when the reflection arrived back at you.

    In Special Relativity, we state that the two halves of the round trip take an equal time and thus define the time that it arrived at any particular location. This is the basis of a Frame of Reference as defined by Einstein. But we can do this for any state of motion and they are all equally valid so they do not provide us with knowing an absolute motion in space.
  5. Oct 10, 2011 #4


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    If you just have a pair of light beams, you can't tell your state of motion. Both light beams will have a velocity of 'c' always.

    If you give each light beam a freqauency, then there will be some particular frame where both light beams have the same frequency. This frame is special only for that particlar setup, though - some other frequency assignment will define some other rest frame.

    You'll always be able to mark "equal intervals" along a light beam (by using an affine parameterization). If you pick some particular interval though an make it have physical significance by tying it to some parrticular electromagnetic wave, you can determine the motion of the source via the doppler shift if you also know the frequency of the light in the rest frame of the source.
  6. Oct 11, 2011 #5
    Thanks for all the answers, this helps a lot for reading the SRT.

    I understand what you all say.

    But in my view I can see space of points (standing still) in an absolute way and all other objects are moving. I know there is no reference, so it is always relative. It's only for my own view, it thinks easier.

    So I understand that reading the SRT (object A, B moves compared with A) one rest frame with A can be considered as a situation standing still (both ways of the reflecting light has same length/time) and the other rest frame with B has a speed V and so the ways/times of the reflecting light is different, and locations for equal moments are different.

    Finally this is the same situation (and results in same mathematics/formulas) considering both frames with moving objects in relation to each other (compared to a third frame with a standing still situation). So it's all relative.
  7. Oct 11, 2011 #6


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    Except that, for any volume of space you call "standing still", I can always say that entire volume of space is moving at 100km/s somewhere (and I would be right), and you have no way knowing or claiming it isn't. Thus you cannot label it as absolute.
  8. Oct 11, 2011 #7
    Yes I agree, but if light has the same qualities in other universa than I don't agree because light is unmoveable except in its own direction .. but except that an absolute point in a endless space has no meaning, .. but it helps me in my view for physics ... we all never will know the whole truth in our lives .. but like Einstein (and others) have opened a gate, maybe some day there will be another gate opened (probably first with mathematics) ...
  9. Oct 11, 2011 #8


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    I have no idea what "light is unmovable except in its own direction" could mean. Would you please explain that.
  10. Oct 11, 2011 #9
    I am just busy in reading Einstein so I say maybe things to quick.

    But I thought if the mass of light is 0, it has no qualities as things with a mass > 0, that's why the speed is always constant, it can't be changed (in vaccuum) in speed and direction. I thougt light is always going straight on, even in bending space (but when the bending space changes, your direction of light is changed).

    So I mean in the right circumstances (no bending etc.) light goes straight on in space and speed can't be changed (in space, vaccuum). So you could track (in theory) the same point in space (crossing two light waves from same source at the same moment in theory).

    So that's what I mean with light is unmoveable (in the right circumstances).

    That's why for me is it no problem in mind to see absolute points .. if one is trackable in such a way, than you have a reference point in space for some others (old points, because when space itself is expanding new points will coming) .. but is all in theory (in thought and no problem for ME) ...

    But I agree there are no absolute points, except in my mind :smile: .. during reading the SRT ... Einstein is also using lightspeed (light waves) in his theory, so if you think every light wave can be changed from direction (under circumstances we don't know all) than you get it very difficult to understand and follow the SRT .. (also under ideal circumstances light is reflecting in the same direction) ...
  11. Oct 22, 2011 #10
    Newton's bucket problem shows that we can't rule out absolute motion
  12. Oct 22, 2011 #11
    I am trying my best to follow what you are trying to say but I do not get it.

    How do imagine two light waves to cross if they come from the same source?
  13. Oct 22, 2011 #12


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    With a beam splitter.
  14. Oct 28, 2011 #13
    Ok imagine two light waves start at the same time from a light source and go into space.

    Would it be possible to track the point where the light waves were started ?

    If light is going straight on and you could track both waves somewhere in the universe, and if you had two very long rulers, lay them along the light waves and that crossing point would be the point where the light waves were started. If the light source is still in that point it was not moving at all in our universe. If light has the same behavior in other universa as well, such crossing point could be an absolute restpoint for objects with a mass > 0.
  15. Oct 28, 2011 #14


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    Is this scenario any different than the one where there are two observers approaching each other at a high speed and at the moment they coincide, one of them sets off a flash of light, and they both measure that they are each in the center of the expanding sphere of light? I don't think so. You need to give up on your quest for an absolute restpoint.
  16. Oct 28, 2011 #15
    Hi Gwh, but after a while they are not in the center anymore ? So with the rulers you detect nobody is there, so they are/were moving (or after).
  17. Oct 28, 2011 #16


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    No matter how much time has gone by and no matter how far apart they are, they both will think they are in the center of the expanding sphere of light. There is no way to dissuade each of them that they are. That is the correct answer no matter how strange or unbelievable it seems. Look up "expanding sphere of light" in google to find lots of discussion on the subject.
  18. Oct 29, 2011 #17
    Ghw, you are talking now what they experience, but in fact you could not do my experiment in thought because you could not track the light waves because their speeds.

    But yes since my topic you know where I learned the behavior of light (I should start a book about the SRT in a different way to save time for the readers with another profession, by first explaining the magic behavior of light), I can imagine that they see a sphere of light ...
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