What happens when light stops?

Discussion in 'Physics & Math' started by Xelios, Dec 1, 2001.

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  1. Xelios We're setting you adrift idiot Registered Senior Member

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    I got to thinking today, if light has no rest mass (therefor allowing it to travel at light speed) what happens in the split second after it has struck a reflector of some kind and come to a complete stop while reversing directions? Why doesn't the light simply disappear? I mean, using common sense one would think if an object has zero rest mass, and it came to rest it would cease to exist. Anyway, thanks for your help.
     
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  3. James R Just this guy, you know? Staff Member

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    It disappears!

    Light does disappear when it stops. It stops when it is absorbed by something. To be reflected, a photon must be absorbed by the reflector, which then emits a new photon in the opposite direction.
     
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  5. Rick Valued Senior Member

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    hi,
    let us say i take a pulse of photons in a big container.and i have supernatural powers to see distinctly these particles,say these are 30 in no.now i deaccelerate these,without hitting or by anything,but with some medium,then hypotheically what would happen if they stop,in the middle of the container.(disregard HOW will i stop them)what would i see?would i see them vanish?

    if photons are included in tachyons category then they should never stop.they should continue to move with "c" speed.
     
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  7. Mr. G reality.sys Valued Senior Member

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    It gets absorbed by an electron.

    The electron later can emit another photon, the direction of emission being random and only sometimes opposite the direction of the first photon.

    Reflection of photons by a lattice of atoms (an aluminized mirror, for instance) is emission of new photons by electrons which have previously absorbed photons.
     
  8. Stryder Keeper of "good" ideas. Valued Senior Member

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    I would say that Photon's react similar to electrons or Neutrons when they are singled out, they have a period of time that they exist before their energy disperses into nothing by Zero-point energy.

    Since a Photon is lights mass, I would guess it's existance at rest would be such a short period that were wouldn't percieve it existing at rest before it disperses to Zero-point.
     
  9. Rick Valued Senior Member

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    are you conveying that it does exist at rest,but due to our techno-inability we just cant freeze a shot of it?

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  10. James R Just this guy, you know? Staff Member

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    The problem with this is that <i>how</i> you stop them will be very important to the answer. What medium are you talking about?

    In fact, you can't decelerate a photon. You can only absorb it.

    Of course, if you have supernatural powers, all bets are off and you can do what you like. Since you're not constrained by the lws of nature, you're free to make up any explanation which makes you happy. It's your fantasy.

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  11. Mr. G reality.sys Valued Senior Member

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    ...how you stop them will be very important...

    There is only one way to stop a photon -- electron absorbtion.

    The extra-electron 'medium' is irrelevent (unless you're talking statistics: the chance a photon will be absorbed).

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    Last edited: Dec 2, 2001
  12. Stryder Keeper of "good" ideas. Valued Senior Member

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    I found a post that explains MIT freezing a Photon within an Sodium Atom
    http://csep10.phys.utk.edu/newsgroups/daunt152/messages/66.html

    It basically means that the energy can be frozen, but only when it's within a Stiffened electromagnetic field.
    I've mentioned before my thoughts on Zero-point energy being the universes natural background level of electromagnetics, that's why originally I said it disperses and my mentioning for a photon dispersing too quickly was only if it had been frozen in space (only with the stiffened Zero-point to hold it).

    Of course you could say that freezing a photon in a Sodium atom is just only Freezing it's photoelectric effect.
     
    Last edited: Dec 2, 2001
  13. Mr. G reality.sys Valued Senior Member

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    <<...freezing a Photon within an Sodium Atom ...>>

    Consider that the photon (apparently frozen in place within the sodium atom) and the sodium atom both where observed in a laboratory attached to rotating Earth, a planet revolving around the sun, a star revolving around the center of the Milkyway galaxy, a galaxy moving around the center of gravity of the Local Group of galaxies, a group of galaxies moving around the center of gravity of the Virgo Group of galaxies, a group of galactic groups moving...

    So, was the photon actually at rest or did it just appear to be at rest with respect to the also moving sodium atom, like resident astronauts and cosmonauts seem to appear at rest with respect to the moving International Space Station?
     
  14. Xelios We're setting you adrift idiot Registered Senior Member

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    What if there are no electrons?

    So suppose I had a container containing absolutly nothing inside (aside from the virtual particles and such). If I injected hydrogen ions (H+), which have no electrons, (or even just a mass of neutrons), then "shot" them with photons, what would happen to the photons as they hit the atoms? I'm guessing the don't just disappear into thin air, so are they absorbed by the protons? Or do they simply not interact with the atoms? Thanks again =)
     
  15. Rick Valued Senior Member

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    ==============================================
    the whole point was what would be my observation once i stop it, how is it going to look like at rest.my theory is that photons cant stop or deaccalerate,they continue to go on their speed and they can never stop,just like tachyons,unless they get hit by something or get absorbed etc.otherwise that could lead to violation of energy,since photons cant exist at rest,so they cant come to rest,any corrections,or inputs to my understandings are welcome.
     
  16. Stryder Keeper of "good" ideas. Valued Senior Member

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    Xelios

    As far as I know about taking a preportion of Quanta from an Atom (Electrons, Protons or Neutrons) They have a period of life span before their energy disperses.

    (This is similar to how radiation occurs with high electron atoms, but it's the very quanta that disperses energy and since it isn't apart of a orbital structure it doesn't replenish it's energy through kinetic energy or friction)

    So for you to shoot one with a photon you would have to be pretty quick.

    You might be able to find more relevant information at:

    http://www.cern.ch

    Since they have been conducting these sorts of experiments and discussing quantum theory for years.
     
  17. Avatar smoking revolver Valued Senior Member

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    light has mass

    I would say tht light[photons] has mass. It is proved that the light of the stars is curved(contorted)i.e.affected, by suns gravity force.
    In time of solar eclipse stars by the sun appear in different place(of course stars don't change their position, so during the eclipse we can see the true location of stars, of course if their light isn't affected by other big mass objects)
     
  18. Mr. G reality.sys Valued Senior Member

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    When the light of a more distant star passing near a massive object in space appears to curve it is not because the photons of light are responding to the nearby mass as if they themselves have mass. The photons are moving in straight-line paths. It is spacetime through which they are moving that is curved by the massive object nearby.

    It is analogous to a spoon in a glass of water. It appears bent but is not physically bent.

    The photon is moving straight. The mass is curving spacetime. The curvature of spacetime makes the photon's straight-line behavior appear to the outside observer to be curved as if in response to the mass, but the photon only knows it is moving straight.

    More Einstein, less Newton.
     
    Last edited: Dec 2, 2001
  19. Rick Valued Senior Member

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    As my understanding goes, when two balls are thrown one with a high velocity and other with less,both will have same trajectory in space-time coordinates,c-t coordinates.this means that light has curved path(ALWAYS)in c-t coordinates,space-time coordinates,its just that it is too fast to see.

    bye!
     
  20. Avatar smoking revolver Valued Senior Member

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    got that

    How could I forget that!!!!!!!!!!!!!! I read about spacetime curving in "Short History About Time". Thanx for reminding me tht

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  21. Chagur .Seeker. Registered Senior Member

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    Gee, Mr. G ...

    Re. "There is only one way to stop a photon -- electron absorbtion."

    1999

    "Recently, the Hau group succeeded in reducing the light speed to 17 m/s (the speed of a racing bicycle) by optically inducing a quantum interference in a Bose-Einstein condensate."

    <a href=http://www.physics.harvard.edu/fac_staff/hau.html><font color=red>1999 Article</font></a>

    2001

    "The sample becomes opaque once more, and the light pulse cannot emerge."

    <a href=http://www.nature.com/nature/fow/010125.html><font color=red>2001 Article</font></a>

    My, doesn't time fly (and not "in a sodium atom")?

    Take care.
     
  22. James R Just this guy, you know? Staff Member

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    Xelios:

    An H+ ion is a proton. I suspect that an isolated proton would not normally absorb photons, unless they were of very high energy.

    zion:

    <i>As my understanding goes, when two balls are thrown one with a high velocity and other with less,both will have same trajectory in space-time coordinates,c-t coordinates.this means that light has curved path(ALWAYS)in c-t coordinates,space-time coordinates,its just that it is too fast to see.</i>

    IF two balls have different speeds, their spacetime trajectories (called <i>worldlines</i>, by the way) will be different.

    What are c-t coordinates? You're graphing what against what? Distance against time, perhaps? In that case, in Euclidean spacetime light will always have a straight line worldline.

    Chagur:

    The "slow light" experiments involve atoms absorbing and re-emitting photons (i.e. electron absorption). A specially prepared medium was used in those experiments. A recent experiment actually <i>stored</i> a light pulse in a medium for some time, before re-releasing it. The energy and information about the pulse was stored in the quantum ensemble of atoms in that experiment.
     
  23. Crisp Gone 4ever Registered Senior Member

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    Hi all,

    I agree with James R here, the only way to actually stop a photon is by absorption, leading to the destruction of the photon.

    However, if - very hypothetically speaking, with some magic wink-wank - you could get a photon to stop, you wouldn't see anything. In order for you to see or detect an object, it has to emit ... light or photons. I still have to hear about photons emitting photons in order to be detected.

    Bye!

    Crisp
     
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