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Some strange questions about light

  1. Sep 17, 2006 #1
    1. Does light(photons) have 0 mass or infinite?(b/c it its mass would be zero, it would take no energy to slow it down, meaning, it wouldn't be traveling, unless it has infinite mass which means that it can't be slowed down even a bit but that contradicts the theory that it does have 0 mass)

    2. If you turn your torch on in a complete locked up dark place, and then turn it off, why isn't it always bright? I mean since light doesn't go through walls and it doesn't accelerate, shouldn't it be enough to give you brightness forever?

    3. I heard somewhere that photons travel twice the speed of light b/c all the other matter recieve info such as they are suppose to attract to each other or repel at the speed of light so photons have to travel twice the speed of light for them to know that they are traveling at teh speed of light. Well, doesn't it mean that they have negative mass?
  2. jcsd
  3. Sep 17, 2006 #2


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    It depends on how you define "mass" in relativity. Most physicists use what is also called "invariant mass", which is zero for a photon. It's also often called "rest mass" which is a confusing name when applied to a photon because a photon must always travel at speed = c. Some people like to define a relativistic mass for the photon using [itex]E = hf = mc^2[/itex] which gives [itex]m = hf/c^2[/itex].

    You should be very cautious about extrapolating your knowledge of classical physics to relativistic physics, and to photons in particular, because they do not behave at all classically. In particular, you can't speed up or slow down a photon, or increase or decrease its energy, by "pushing" on it. It's created with a certain amount of energy, and it keeps that energy until it's completely absorbed somewhere, so long as you continue to observe the photon from the same inertial reference frame (that is, if the observer does not himself accelerate).

    The light gets absorbed by the walls of the container. The only way to prevent this is to make the walls out of perfectly reflecting mirrors, which don't exist as far as anyone knows.

    Where did you read this? It sounds like the sort of thing I'd expect to find on a crackpot Web site somewhere.
  4. Sep 18, 2006 #3
    jtbell's explanation of photon mass is right, but there is another way to view it. If the rest mass of something is zero, you would need gamma to be infinity in order to give it relativistic mass. If gamma is infinite (i.e. v=c), relativistic mass would be some number (could be any number really its 0/0 but there might be some limit). Thats just how I imagine it, I dont think most people would agree but I dont see any real problems with it.
    this is just wrong. I think your talking about gravitons. If photons even emit gravitons, they would go at c relative to the photons.
    also, it wouldnt be negative mass it would be imaginary mass.
    Last edited: Sep 18, 2006
  5. Sep 18, 2006 #4


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    That's not true either, gravitons would always travel at c like photons. The phrase "relative to the photons" doesn't really mean anything in relativity, since something traveling at c cannot have its own valid inertial rest frame.
  6. Sep 18, 2006 #5
    1. You guys say that light can't be accelerated, well, when it bends due to gravity, isn't it accelerating? Or are you guys talking about tangential velocity?

    2. Wait so do gravitons travel as fast as light? Are there any other particles that travel at the speed of light?

    3. Jtbell gave a formula for relativistic mass, I thought it was constant. And I didn't understand the explanation from Michael 879 to the question "is the mass of light 0 or infinity?"

    Here is the understanding I have built, since any massful object can't travel at the speed of light, light's rest mass must be 0. if that is so, then I don't think stopping light would take any force.(obviously this contradicts the thoery, which says that stopping light takes infiniteous force). I am confused.
  7. Sep 18, 2006 #6


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    You can't stop light. The amount of force it might take is meaningless. Best you can do is absorb it into something, such as an electron orbital.
  8. Sep 18, 2006 #7
    Wait, I used to know this, this might seem like a random question but it isn't, the faster I travel, the heavier I get, right?
  9. Sep 18, 2006 #8


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    Sort of, but only from the point of view of someone who's moving at high speed relative to you and who's trying to push on you to accelerate you to an even higher speed relative to themselves. From your point of view it'd be them who gained mass (ie, became harder to accelerate), since in your own frame you're at rest while they're the one moving.
  10. Sep 18, 2006 #9


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    In special relativity, the idea that light always travels at c is only true in inertial coordinate systems, which means the coordinate systems of non-accelerating observers; in non-inertial coordinate systems light can travel at other speeds. But special relativity only deals with flat spacetime, while in general relativity gravity is understood as curved spacetime. In curved spacetime there's no such thing as an "inertial coordinate system" to cover the whole spacetime, but you can still talk about an inertial coordinate system in an infinitesimally small neighborhood of spacetime...you can think of this in terms of the idea that if you zoomed in very closely on a curved surface, like an ant's-eye view of a beach ball, the surface will be pretty close to a flat plane in that small zoomed-in region. So if you use a "locally inertial" coordinate system in curved spacetime, light is still always traveling in a straight line at c, even though a larger view of its path will appear curved (in the larger view, its path is something called a 'geodesic', which is basically the closest approximation to a straight line on a curved surface, like a great circle on a sphere--on a curved 2D surface the geodesic path would be the shortest path between two points on that surface, just like a straight line is the shortest path between points in a plane, although in curved spacetime a geodesic is usually the path through spacetime with the largest value of the proper time).
    Any massless particle...the only other one I know of is the gluon, which carries the "strong force" that holds the nucleus of the atom together, just like photons carry the electromagnetic force.
    Usually in relativity physicists just talk about the "rest mass", which is constant; but there's a separate concept called "relativistic mass", which is gamma*rest mass, where "gamma" is given by the formula [tex]1/\sqrt{1 - v^2/c^2}[/tex]. If v=c, then gamma is 1/0; but if the rest mass is also 0, as in the case of a photon, then this formula tells you the relativistic mass would be 0/0, an undefined quantity, which I think is what Michael879 was talking about. To find the actual relativistic mass of the photon, you have to use a formula from quantum mechanics relating momentum and wavelength.
    You're correct about the first part, but as for the second part, keep in mind that the equation F=ma is specific to Newtonian physics, it won't work in relativity...I'm not sure if the concept of "force" is even used in relativity, or what formulas would apply to relativistic forces.
    Last edited: Sep 18, 2006
  11. Sep 18, 2006 #10
    Well the concept of 3-force is superseded by the concept of 4-force:

    [tex]f_{\mu} = \frac{d}{d\tau}p_{\mu}[/tex]

    where [itex]p_{\mu}[/itex] is the four momentum. Four-force is useful in describing electromagnetic interactions.
  12. Sep 18, 2006 #11
    ok whether or not c is an inertial frame of reference you can talk about the properties a reference frame approaches as it goes to v=c. anything traveling at c appears to travel at c in all inertial frames. gravitons are thought to travel at c so that even in a photons reference frame, it would appear to go at c.
    any massless particle must travel at c (the only speed it can have relativistic mass at).
    rest mass of an object cannot be changed by changing its velocity. I wouldnt call it constant. Relativistic mass is rest mass * gamma. light's rest mass is 0. Its relativistic mass would be mr = hf/c^2. Even though light has no rest mass it still acts as though it had mass (i.e. gravity, momentum) because it is going at c.
  13. Sep 18, 2006 #12
    Since gravity only attracts mass, why would it attract light? Unless light has infinite mass but that would only mean that mass accelerates as velocity does(I remember messing up with the mass, lenght, and time as velocity increases last year in intro to relativty.) I forget which one increases though. However, even if it did increase( which you guys say it didn't) whatevertimes0 will still be zero. Then why does gravity have an effect on light?

    Also, I know that relativistic mass is current, rest mass is w/ no velocity, but then what is gamma mass? Also, the formula explain by Jesse, is that b/c since light is changing direction, its changing its velocity too? B/c even in relativistic world, why would light change speed?
  14. Sep 18, 2006 #13


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    Sigh. It gets old pointing out that Einstein's gravity tensor is affected by mass, and momentum, and energy! Light does have momentum, even classical wave theory light, so it gravitates!
  15. Sep 18, 2006 #14
    sorryyyyyyyyyy, I didn't know that. What about my other questions?
    1. What is gamma mass exactly?(not rest, but specifically)
    2. So the formula mr = hf/c^2 for light is b/c it bends?(b/c I still dont think its tangentical velocity would change)
  16. Sep 18, 2006 #15


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    As I explained, light never locally changes speed or direction in relativity. In curved spacetime, I don't know if it really makes sense to say light is "changing directions", since it's impossible to have a truly straight line on a curved surface--try drawing one on a globe, for example. The closest approximation to a straight line is a geodesic, as I explained, and light does follow a geodesic path through curved spacetime.

    Anyway, the formula I explained for relativistic mass has nothing to do with curved spacetime specifically, it is used in special relativity where there is no gravity and spacetime is flat. "Relativistic mass" basically tells you the difficulty of accelerating an object along its direction of motion--the faster it goes, the more energy it takes to accelerate it by a given amount, so it's as if the object had gained mass (you know that in Newtonian physics the difficulty in accelerating an object is proportional to its inertial mass). Of course in the object's own rest frame, its inertial mass is unchanged, the difficulty of accelerating it in that frame is just proportional to the rest mass.
  17. Sep 18, 2006 #16


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    See above, it basically is the apparent inertial mass, which tells you the difficulty in accelerating the mass in its direction of motion from the perspective of your reference frame.
    That formula comes from the quantum equation for the energy of a photon, E = hf, and then you take the relativistic equation E = mr*c^2 and substitute for E, giving mr*c^2 = hf, or mr = hf/c^2.
  18. Sep 18, 2006 #17
    aah, I get it, so what you are saying is that since it gives undefined, stopping infinite mass take infinite energy and same goes for something that doesnt have mass at all b/c then it takes the speed of something so fast that time stops for it at that speed.(basically reaches the limit)

    So this is the reason that when an object is dropping to a planet, it doesn't just accelerate to the speed of light b/c the faster it gets, the slower the acceleration gets, b/c hte more energy it needs to accelerate it.

    Another question I asked earlier but forgot to clear my doubts. When you shed a light in a completely dark room, the only way, that certain amount of light can stay there is if you have a perfect mirror. But what I was wondering is what actually causes light to reflect from mirror, and absorb in the wall. I mean for light, what is the the diff. b/w those two?
    Also, if a wall absorbs the photons, what does it do to the electrons inside atoms, since they are about the same size? I mean if light strikes the electrons, it sure speeds them up, then something similar to what happens in fission occurs. If that happens, then why don't we have a blast?
    Last edited: Sep 18, 2006
  19. Sep 18, 2006 #18
    gamma mass is gamma * rest mass. gamma is 1/sqrt(1-v^2/c^2). Ive never heard gamma mass before maybe you misread one of the posts. Its called relativistic mass.
    it gives undefined as in 0/0 (which isnt rly undefined its a set of all numbers) not 1/0. the relativistic mass of light isnt infinite.
    even in classical mechanics an object would require a blackhole to accelerate to c. If you take an object from "infinity" and a planet with escape velocity v, the object will have velocity v by the time it reaches from planet. Black holes are the only things with escape velocity v >= c.
    huh? I think your slightly confused. Fission is caused by something splitting the nucleus of an atom (in the case of uranium its a neutron being shot into the nucleus). When light hits an electron it can do 3 thing, be absorbed, be absorbed and reemitted at the same frequency, or be absorbed and reemitted at a lower frequency (not sure if the last one is true). What happens depends on the energy level of the electron or proton the photon hits. Mirrors just reemit the same frequency they absorb in the visible range. Im guessing this is just because metals contain electrons with energy levels that match up with visible light well. Never actually learned that part though, just kinda making it up.
  20. Sep 18, 2006 #19
    Exactly, this is what I am taklig about, you shoot some electrons, those electrons shoot other electrons and the chain reaction starts. Well, since photons are as big as electrons(close enough), wouldn't hte same thing happen? btw, how can an electron absorb photon? And I was looking for a definate answer for the one w/ the mirror and wall for why do mirrors reflect light and wall doesn't.
  21. Sep 18, 2006 #20
    mass can become energy. When the photon hits the electron it elevates the electron to a higher energy level (gives it more KE). It takes a certain energy photon to actually knock electrons out of an atom. Even if it was the right energy, I dont think it would not cause a chain reaction. Also, having a bunch of electrons moving around a box wouldnt cause a fission reaction.
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