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How can anyone know photons exist?

  1. Mar 28, 2012 #1
    these should be some very easily answered questions. how does anyone know photons exist? im guessing they can be "stopped" and studdied?
    and also, after reading about this relativity stuff for the last few days i feel like im getting an understanding, but please, entertain this scenario and try to give me a simple answer:
    ok, so ive heard the idea that if some left earth and travelled the universe at near light speed they would come back to an earth billions of years in the future.
    so just to clarify, if i was sitting far enough away from someone that i could watch them move at 90% c with a telescope. would they appear squished length-wise, and would they be moving in slow motion(blinking, heart-beat, muscle movement, ect.. not speed)?

    thanks, i really need this curiosity satisfyed or it will keep bugging me and finding its way into every damn train of thought i have!
  2. jcsd
  3. Mar 28, 2012 #2


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    There was indirect evidence from things like the Lamb shift, which requires a quantized electromagnetic field for its explanation.

    More "direct" evidence is summarized in these lectures by Alain Aspect:

    Last edited by a moderator: Sep 25, 2014
  4. Mar 28, 2012 #3


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    If you have some specific reason for doubting the existence of photons, it would help if you tell us what it is, so people can address that issue instead of wasting time shooting blindly in the dark.
  5. Mar 29, 2012 #4
    ok, dont worry about the photon thing, it seems to be beyond my grasp at the moment. but the scenario i mentioned?
  6. Mar 29, 2012 #5
    Scientists have slowed photons to below walking pace (not sure if actually stopped) in certain mediums. Relativity only requires that photons travel at the speed of light according to all observers when in a vacuum. However you are right that photons have a certain mystery about them. When a ball has a trajectory from A to B we can observer the ball at all points along its trajectory from the photons bouncing of it. For individual photons we can not observe them in transit, only when they arrive. This means we have to deduce their path rather than explicitly know their path. In certain experiments where photons are passed through half silvered mirrors so that they have a choice of two paths they even appear to travel both paths at once and form an interference pattern with themselves. Any attempt to detect which route was taken in the experiment destroys the interference pattern.

    You are right that the heart-beat etc. of the relativistic pilot would appear to be in slow motion. However, you should be aware that if the pilot was looking at you on the ground through his telescope, your heart-beat etc would also appear to be in slow motion from his point of view. Now what you would see from the length contraction point of view is a little more complex as you have to take light travel times into account. As a long object such as typical rocket approaches you it can appear longer than its proper length and after it has passed you it can appear shorter than its length contracted length. As it passes the point in its path that is closest to you, it will appear more or less as its length contracted length. I say "more or less" as there is an optical rotation effect (see Penrose Terell rotation) that makes the object appear rotated so that you can see the back of the object before it actually passes you. For a sphere, this rotation effect means you cannot see the length contracted sphere as squished shape. Imagine a ball that is squished into a M&M shape. You can rotate the M&M in such a way that its outline is a perfect circle so that superficially it appears that the M&M is actually a sphere. This optical cancellation of the appearance of length contraction only occurs perfectly for objects with rotational symmetry about an axis that is perpendicular to the plane that contains the path of the object and the light paths to the observer. Other than that you will be able to see the length contraction of objects at the point of closest approach. At any point, any object will be measured to be its length contracted length by noting where the front and back of the object are simultaneously using two synchronised clocks. You can also measure the length of a moving object using a single clock if you know its speed. All you have to do is note when the front of the object passes the clock and when the back of the object passes the clock. The difference between these two times multiplied by the speed gives the length of the object and this will be less than its rest length. However, you will require two synchronised clocks to determine the objects speed.
  7. Mar 29, 2012 #6


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  8. Mar 30, 2012 #7
    I don't get it.. How is this achieved? The scientists expanded space inside the cooled gas chamber, or is this some effect of bouncing the photons around the gas molecules?

    I can imagine, myself entering the box of pain and nothingness to only come out of it long dead and decomposed :) Probably a good way to make fuel or get rid of radiation:)))
  9. Mar 30, 2012 #8


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    The article explains it. They use interference to stop the light inside the gas.
  10. Mar 31, 2012 #9
    AM I right in thinking that the coupling beam is being absorbed and re-emitted thus driving the atoms between two energy states?

    How is it determined that it is the same set of photons of the test beam coming out that went in?
    Isn't it possible that they were absorbed as expected but were re-emitted after a delay?

    Possibly many times during transit and this might be the cause of the extended transit time?

    As to the motionless storage after turning off the driving laser; is it possible that it was not photons that were stored but energy states of electrons that then did not re-emit in the stable, energy poor, low temp environment until the gas was re-excited by the coupling laser??
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