Thread: Object v Force

  1. #1

    Object v Force

    What happens when a photon hits something? Moving at the speed of light you'd think some damage would happen yet photons surround us and arent doing any thing I would expect from the speed of light with Newton's third law and all. Answers?

    yes, I know this is in offtopic ->fun stuff. some ppl find physics questions fun
    Isnt 10% of infinite still infinite?

  2. #2
    photons have no mass.

    question answered

  3. #3
    Quote Originally Posted by Freezymcgee View Post
    photons have no mass.

    question answered
    then how does gravity effect them? they are called black holes for a reason
    Last edited by ambigiouslynamed; 2012-02-12 at 04:40 AM.
    Isnt 10% of infinite still infinite?

  4. #4
    Photons hit you all the time - billions of times a second. Some bounce off, and some get absorbed. Either way, they transfer energy to you. But they energy they carry is miniscule, so there's generally no damage. And when there is damage, it's really small - fading pictures, sunburn, that sort of thing. If you get energetic enough photons, and enough of them, you can do real damage - think laser cutters, x-rays, frying ants with magnifying glasses, that sort of thing.

    For the mass question, photons have no rest mass. Therefore, all photons only exist in motion, travelling at the speed of light. But they do carry energy, which means via Einstein that they have mass.

    Finally, to really blow your mind: Photons are the reason you can sit in your chair or stand on your floor. They're the only thing keeping you from falling to the center of the earth. See, it's electrical repulsion that makes objects solid - the electrons in the atoms of chair repel the electrons in the atoms of your skin, and that keeps you from pushing through the chair. The photon is the particle that carries electromagnetic force - the repulsion between two electrons is actually a vast stream of (virtual) photons that travel between the two electrons pushing them apart.

  5. #5
    Quote Originally Posted by ambigiouslynamed View Post
    What happens when a photon hits something? Moving at the speed of light you'd think some damage would happen yet photons surround us and arent doing anything?
    Tell people, who has been vaporised by gamma-rays of nuclear expoision, that photons did nothing to them or put hand in metall-cutting industry laser and see what happens.
    Quote Originally Posted by ambigiouslynamed View Post
    then how does gravity effect them? they are called black holes for a reason
    Gravity affects not only mass but energy and momentum density and flux. Photons have energy and momentum.

  6. #6
    Dreadlord Cusco's Avatar
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    I, too, have been wondering about the contradictory question how black holes can affect photons despite our theory that photons are massless, but I think that the answer isn't being fairly explained by saying "the gravitational force is so immense that they pull photons in anyway". One definition of time is the distance between events in the fourt axis of space-time and since black holes infinitely bends space-time, distance between events is infinite and that is what makes it affect light as well. Light cannot travel past black holes since traveling is a series of events and the space between those events is infinitely bent. This makes it irrelevant that photons have no mass.

    Please correct me if I'm wrong; this is all abstract and it's 6 am. Good night.
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  7. #7
    Quote Originally Posted by braeldiil View Post
    Photons hit you all the time - billions of times a second. Some bounce off, and some get absorbed. Either way, they transfer energy to you. But they energy they carry is miniscule, so there's generally no damage. And when there is damage, it's really small - fading pictures, sunburn, that sort of thing. If you get energetic enough photons, and enough of them, you can do real damage - think laser cutters, x-rays, frying ants with magnifying glasses, that sort of thing.

    For the mass question, photons have no rest mass. Therefore, all photons only exist in motion, travelling at the speed of light. But they do carry energy, which means via Einstein that they have mass.

    Finally, to really blow your mind: Photons are the reason you can sit in your chair or stand on your floor. They're the only thing keeping you from falling to the center of the earth. See, it's electrical repulsion that makes objects solid - the electrons in the atoms of chair repel the electrons in the atoms of your skin, and that keeps you from pushing through the chair. The photon is the particle that carries electromagnetic force - the repulsion between two electrons is actually a vast stream of (virtual) photons that travel between the two electrons pushing them apart.
    do you know how much energy a photon carries? traveling at the speed of light you would think its a lot but according to you its not...
    Isnt 10% of infinite still infinite?

  8. #8
    Quote Originally Posted by ambigiouslynamed View Post
    do you know how much energy a photon carries? traveling at the speed of light you would think its a lot but according to you its not...
    Light is a low-density EM wave. It's relatively low-energy. The collisions are inelastic (or elastic, i forget which one is the "energy transfer minimized" one), so there's little effect. Look into it a little and you'll find more details, it's fairly straightforward.

    Compare microwaves or X-rays to visible light, for example. It's very energetic, and it does in fact transfer a lot of energy on impact with your molecules, so much so that it will break pieces right off your DNA.

    Visible light is just fairly low-energy.

  9. #9
    One interesting note is that when photons hit an object and bounce off, they do actually exert a force on the object in the same way as a ball bouncing off a wall would.

    The energy gained or lost by the object is reflected in the photon by a change in wavelength, for example a photon hitting a receding object would bounce off with a lower energy wavelength as the receding object accelerates slightly.

    This is how solar sails work.

  10. #10
    Around 0.0000000000000000000001 Joule for visible light.

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