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I LIGO Discovery - A question about space-time properties

  1. Feb 18, 2016 #1
    A question from a physics laymen to those more advanced: if eLIGO detects gravitational waves by the difference to the combined laser wavelengths (a difference to the destructive interference pattern following curvature of space-time in each individual pathway), how is it that the lasers themselves are not exposed to a stretching of time, rather than spatial lengthening alone, since EM waves are, too, subject to gravitational waves? Would this not cause the pattern to remain the same if there really is a gravitational wave? Thank you for your time.
     
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  3. Feb 18, 2016 #2
    A very good question. I was thinking the same thing but am having trouble formulating good search keywords to possibly find relevant discussions on the internet.

    If gravitational waves / ripples are a temporary cyclical expansion and contraction of space observed in a relatively local region, wouldn't all phenomena in that region be equally affected? It's not the physical lengths of the 4 km laser paths that are measurably changing but rather their encompassing space density.

    You just can't measure variations if you are wholly enveloped in a frame-of-reference changing environment.

    At least that's how I understand gravitational waves. Therefore I'm having trouble understanding how they can detect anything at all.
     
  4. Feb 18, 2016 #3

    phyzguy

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  5. Feb 18, 2016 #4
    The paths along which the light waves travel do experience the stretching and contracting, and if c weren't constant for all inertial reference frames, the light itself would experience this, too, and we'd never detect anything. But c is constant, whether in the presence or absence of g-waves. G-waves can't cause c to speed up or slow down but they can cause the paths that light follows to stretch and contract. Does that help? I honestly can't tell if that answers your question. My bad :)
     
    Last edited: Feb 18, 2016
  6. Feb 18, 2016 #5
    If you don't read the link (thank you phyzguy), it was the time difference that caused them to see the split beam go out of sync (they used the light beams as a clock and the stretching or compression of space by the gravity wave caused the arrival times to go out of sync).
     
  7. Feb 18, 2016 #6
    So in the elongated or compressed pathways, do the subatomic particles out of which they are constructed undergo growth and shrinkage, along with their inter-particle distances? Or at least along one axis? The electron orbits become elongated? It doesn't make sense to me. If light speed is constant independent of matter-affected G waves, then light just isn't part of space-time at all. It's "outside" of it?

    I suppose if light were affected by space expansion we wouldn't have red-shift. But we do.
     
  8. Feb 19, 2016 #7
    That can be a tricky question without more contexts but I’ll take a stab at it. If the space-time dynamics for a place in space uniformly change then the particle is defined by the new dynamics and hasn’t changed in shape or size according to those dynamics.

    Light is very much a part of space-time, it’s a constant for the space-time it travels. In other words, if you observe light from a position close to the surface of a black hole, relative to how you observe distance and time it’s still c, if another observer is a great distance from your position (and the gravitational field) to them your time is slowed and they will see time and distance differently than you, yet c will remain constant to their view of time and distance.
     
  9. Feb 20, 2016 #8
    Thank you for all your replies. I have read the articles you provided that explain the light as more of a clock, rather than ruler; and that red and blue shift effects following a gravitational wave cause the arrival times to differ. But I was more interested in the effect gravitational waves have on time dilation and compression and their influence on the readings: if time in the reference of the light is affected along with its space, would that not have also an influence on the readings of their arrival time from our reference point? Specifically, if one arm is stretched in space but also in time, would that not mean that while it appears longer to us, in the reference of the stretched arm, time sped up relative to us, and upon return would match the other arm's arrival time, in which the distance was compressed, while time slowed down - thus, both equaling out at the measurement stage? Thank you.
     
  10. Feb 20, 2016 #9

    phyzguy

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    Gravitational waves are usually understood by approximating the metric as a flat space Minkowski metric (usually called η) plus a small perturbation (usually called h), then inserting this into the Einstein Field Equations and solving the resulting linearized equations. When this is done (see for example this Wikipedia article), the perturbation h contains only changes to the space part of the metric, so there are no changes to the time components. So we can interpret the passing of the GW as periodically stretching and compressing space while time passes unchanged.
     
  11. Feb 21, 2016 #10
    I see, thank you for your insights. If time relativity is not a factor in that setting, then the calculations make sense to me now.
     
  12. Feb 21, 2016 #11

    mfb

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    Electrons are bound so tightly that gravitational waves cannot change the size of atoms in any relevant way. The effect is not completely zero but tens of orders of magnitude too weak to be of any relevance.
     
  13. Feb 21, 2016 #12

    phyzguy

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    You're welcome. Glad I could help.
     
  14. Feb 23, 2016 #13

    ewq

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    "So we can interpret the passing of the GW as periodically stretching and compressing space while time passes unchanged."

    So "deformation" of spacetime is really just the deformation in space while time is unaffected? It affects matter but not time? Which would imply that you cannot detect any changes in length as your ruler (whatever it is) expands or contracts along with spacetime but you can detect the difference in time?

    If so this would invalidate the constancy of the speed of light as now you would measure that light takes more / less time to travel the same distance?
     
  15. Feb 23, 2016 #14

    mfb

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    You cannot use a regular ruler. You can use light as ruler. A very, very stiff ruler would work as well in theory.
    The distance does change, the speed of the light does not.
     
  16. Feb 23, 2016 #15

    ewq

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    Doesnt make any difference what the ruler is, a stick or a laser it follows the deformation of spacetime. The point is you are unable to measure the change in length due to spacetime distortion.

    So say your spacetime is undistorted and your arm of interferometer is 300000 km long. You measure that light takes 1s to travel that path.

    Then gravity wave pases and stretches your 300000km, you canot notice it because of above and you still measure 300000km.
    But because light travels at the same speed as in the first measurement you find it takes more timethan 1s to travel your measured 300000. Calculate the speed of light and its different from c now?

    something is not right here
     
  17. Feb 23, 2016 #16

    mfb

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    Your assumption that the length does not change is wrong. And you do measure the difference.
     
  18. Feb 23, 2016 #17

    ewq

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    How do you measure a difference in length? Ligo is set up only to measure interference pattern due to time delay of the signal?
     
  19. Feb 23, 2016 #18

    phyzguy

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    Instead you should say, " But because light travels at the same speed as in the first measurement you find it takes more time than 1s to travel the stretched distance. Knowing that the speed of light is constant, you conclude that the distance between the ends of your interferometer has increased."

    This is how LIGO works. As the gravitational wave stretches and compresses the arms of the interferometer, the wave crests of the laser light (which travel at a constant speed) arrive slightly sooner or slightly later, shifting the interference signal.
     
  20. Feb 23, 2016 #19

    mfb

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    You answered your own question. The time delay comes from the changed distance.
     
  21. Feb 23, 2016 #20

    ewq

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    In response to physguy:

    In that case time ticks differently in "stretched" and "unstretched" reference frame which contradicts what you said in previous post.
    You just cant have two space and time coordinate systems where c and time is constant and lenghts are different.

    And second, how can the constancy of c be taken as the very premise of the experiment? How do we know we would measure same speed of light in "deformed" spacetime as measured from "undeformed" reference frame? Has any experiment ever tested this? Or have we just blindly taken Einsteins postulate for inertial reference frames?
     
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