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What happens to a laser pulse that is beamed through a gravity well?

  1. May 29, 2010 #1

    TCS

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    I believe that the path of the photons will bend towards the center of mass, but I wonder what happens to the wavelength and the pulse shape while the photons are in the gravity well.

    The photons don't undergo any real acceleration, since they are in free fall, so their wavelengths should stay the same but they traveling through shrunken space and slowed down time.

    I wonder if it affects the pulse like an inductor.
     
    Last edited: May 29, 2010
  2. jcsd
  3. May 29, 2010 #2
    The fact that the light is in the form of a laser pulse does not change how it will behave in relation to gravity.
     
  4. May 29, 2010 #3

    TCS

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    Why does a spherical wave behave exactly the the same as a colimated spacially and temporally restricted wave.
     
  5. May 30, 2010 #4

    Nope, the wavelkength does not "stay the same". Google "Pound-Rebka experiment".
     
  6. May 31, 2010 #5

    TCS

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    Light that is emitted in a gravity well will red shift when it comes out of the well. However, light emitted outside a gravity well has the same frequency after it leaves the welll than when it went in.

    I got a little mixed up and I was thinking that the red/blue shift of photons related to whether a particle would have to undergo real accleleration to fllow the same path as the photon because I was trying to understand why the oscillation of a clock speeds up but the oscillation of light slows down. However, I think the correct correlation is that the frequency of the light will change whenever the velocity of a particle would change.

    Anyway, it seems that the light will have a red/blue as long as the path of the light is along the gradient of mass/energy density. also, Also, motition perpendicular to the gradient causes the direction of the light to shift in the direction of the gradident. Accordingly, the wavelngth of light will decrease until it reaches its lowest point in the well and then increase until it leaves. This makes sense to me since the contraction of space causes more energy to be contained within a smaller space.

    However, I still don't know if a gravity well will cause a change of pulse shape.
     
  7. May 31, 2010 #6
    It doesn't, anymore than a flash from a lamp would. Collimated or not, only the wavelength changes as starthaus has said. The spread is going to increase based on the distance it travels, and as gravity is geometry, that is just another aspect of the pulse's travel.
     
  8. May 31, 2010 #7

    TCS

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    It's postition changes not just its wavelength. It's direction varies as it trravels through the well and its positioon on the other side of the well is shifted not unlike the positiion of a photon that has passed through an acuarium.
     
    Last edited: May 31, 2010
  9. May 31, 2010 #8
    Yes, but unless there is shearing it will emerge as it entered. If you fired a pulse from A -> B over distance Y, and a pulse from C -> D over the same distance Y, but through a gravity well, the distance pulse C-D covers will be greater, and it will have diffused more than A-B. Beyond that, there is net result once you reach B, and D respectively. The result is that C-D will have traveled "more" than A-B because of its trip through a gravity well.
     
  10. May 31, 2010 #9

    TCS

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    If you send a dc current through a metal slinky but you compress the slinky in the middle of the path, the current will be the same out as in even though the energy density and magnetic field vary along length the slinky. However, if send a pulse through the slinky, the pulse shape will shift back and flatten.


    It seems like there should be a similar situation on the waves traveling through the gravity well, where the charging up of the energy density causes a delay in the pulse.
     
  11. May 31, 2010 #10
    The frequency must remain constant to an outside observer, but due to gravitational time dilation to an inside observer the frequency will look faster.

    Think of it as pulses, if I am outside and shoots one pulse every second, and inside there is 50% relative time dilation a person inside will see 2 pulses per second. In the same scenario since light is just a series of pulses they would also double in frequency. But to an observer at the other side of the gravity well the pulses would look just like normal again.

    This should answer your original question, the photon composition don't change at all since it is just an issue about perception.
     
  12. May 31, 2010 #11

    TCS

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    You are always going to see the light as the same frequency because it has to reflect out for you to see it. However, from your reference frame, the light has a higher frequency while it is in the gravity well because of space dilation.
     
    Last edited: May 31, 2010
  13. Jun 3, 2010 #12
    On the surface of earth I am measuring Light with a clock that has a period dilated from the clock you have in orbit, all things else being equal, I will always count more cycles per second than you.
     
  14. Jun 3, 2010 #13

    TCS

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    My question was meant to be about what happens to the light from the perspective of the external frame not the internal frame.

    If you put a ruler in a gravity well, it will shrink to someone in the external frame. If you put light in the same well, its wavelength will likewise shrink.
     
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