does light have mass

Factual Questions
1

A friend told me light has mass. I told him I don’t think so and gave my friend this example.

If you put a 5 lb. weight on a scale in the dark it will weigh 5 lbs., if you put a 5 lb. weight on a scale where light is present it still only weighs 5 lb., proving that illumination does not have any mass to it.

He is absolute certain that I’m wrong. I remember having a similar conversation with another friend, where he explained that light does not have mass, and light does not fall into any of the categories (gas, solid, or liquid). I had had a few drinks and could not adequately recall the conversation to prove that light does not have mass.

So my question is what is light considered to be and does it have mass.

2

i think in order for soething to travel at the speed of light (ie. light) it must have a resistance of zero, therefore it weighs nothing, other wise if it had mass it would have resistance and not travel at the speed of light.
im so bad a describing things

3

Yeah, but if you had a more sensitive scale, you would notice a difference. Light does have mass, due to relativistic mass-energy conversion (E = mc[sup]2[/sup]), although it may have zero mass when at rest. But that’s different.

4

Light has mass. If a photon has energy E, then its mass is E/c^2. Look familiar? You need a pretty sensitive scale to weigh it, however.

5

It it to my understanding that light is made up of photons, and photons have a very slight mass. Also, light is affected by gravity, which suggests it does have mass.

6

light always travels the speed of light. It’s just that sometimes the speed of light is slower than other times.

Light travels faster through a vacuum than through the atmosphere, but it always travels the speed of light.

7

According to my physics teacher, light always travels at one velocity, (roughly) 3 x 10[sup]8[/sup] meters/second. However, when its traveling through a non-vacuum, it has to be absorbed and re-emited by the atoms or molecules it hits, which causes it to appear to move somewhat slower.

8

Does light therefore “bend” the space through which it passes?

9

I think so, although I’m not 100% sure. An actual physicist will be along shortly, I hope.

10

Heh… a while ago, I came across a tidbit of information that said that sunlight weighs two pounds per square mile… I’ll have to go back and find out where I heard that.

11

Oh dear. I’m afraid Ring is about to show up and come down on this thread with the obligatory “LIGHT HAS NO MASS AT ALL AND THE REST WHO SAY THAT IT DOES ARE SPEAKING NONSENSE!”

Well, that’s actually true. Technically speaking light has no mass.

But what does that mean in a practical sense? If I put a box with a photon inside of it is lined with (impossible) mirrors that are 100% reflective, will the system’s mass be larger or smaller than a plain empty box?

Well, it depends on how you look at it. In one way, the system actually has a higher mass! This is one place where the fundamentals of E/C^2 mass come into play for light.

Light, with zero rest mass, cannot bend space as it travels. It can only do that if it is a reference frame that allows it to do so and it must be in an interactive system (sort of like my photon in a box). This may seem strange at first, but think of it this way… light’s energy is all spent up in propagation (at the topspeed of the universe), it spends no energy on twisting spacetime. The energy of a thing with non-zero restmass will spend some of its energy on the twisting of spacetime. That’s one way of looking at what makes the thing have a nonzero-restmass.

We were talking in another thread about dimesions. One way to look at Energy (or mass energy) is in terms of dimension. Mass gets treated like time, as it has one degree of freedom and momentum gets treated like space as it is three degrees of freedom. The metric E^2-p^2 is therefore an invariant quantity for all formulations of “stuff”.

Light in and of itself has no restmass. This means that E^2-p^2 is zero. A direct consequence of this is that light does not warp space as it travels through it. It travels through space on geodescics, always at the speed of light, and it never slows down. When we put it in another medium, the speed of light appears to change, but as Qwerty rightly pointed out, that is due to interactions the light has with other constituents of the system.

That does not mean that a system that has mass cannot be converted into light. It just has to be converted into light in such a way the the metric measurement of the fourvector is conserved (E^2-p^2). This means that in certain ways of looking at systems of light particles (photons) you can have nonzero restmass. This is somewhat similar to the photon in a box analogy I opened with.

In short, does light have mass? no. Can we get light to behave like we would expect a massive particle to? sort of. As with most things, it’s all in the way you look at things. Physicists, to avoid this utter confusion are abundantly clear. Light does not have mass, and a photon observed in isolation does not bend spacetime. It merely travels through spacetime on a geodescic.

12

Let’s say there’s a beam of light shining in the +x direction. If you move in the +x direction the energy density of the beam can be as high as you wish it to be (blueshift). If you move in the opposite direction the energy density can be as small as you wish (redshift).

So given that an object that has an energy density will curve spacetime this means that curvature can’t be an invariant.

Does light have mass? Certainly it doesn’t have a rest mass. But can an effective non-invariant gravitational mass be ascribed to it? Scientists differ on this question.
Here’s what Alan Guth says:

We are perhaps not used to thinking of electromagnetic radiation as having mass, but it is well-known that radiation has an energy density. If the energy density is denoted by u, the special relativity implies that the electromagnetic radiation has a mass density

rho = u/c[sup]2[/sup]

Rindler also says that its ok to write m = E/c[sup]2[/sup]

http://www.geocities.com/physics_world/rel_mass.htm

So does light have a non-ivariant mass? I’ll be damned if I know.

13

Light does have mass, but not very much. That’s why it’s light. Otherwise it’d be heavy.

Sorry. Couldn’t resist.

14

I think I saw a program on Discover or TLC saying that Earth absorbs 14 pounds of photons a second.

15

I did some research myself and here is what I found

http://infoplease.com/ce6/sci/A0859284.html

Modern Theory of the Nature of Light

With the acceptance of the electromagnetic theory of light, only two general problems remained. One of these was that of the luminiferous ether, a hypothetical medium suggested as the carrier of light waves, just as air or water carries sound waves. The ether was assumed to have some very unusual properties, e.g., being massless but having high elasticity. A number of experiments performed to give evidence of the ether, most notably by A. A. Michelson in 1881 and by Michelson and E. W. Morley in 1887, failed to support the ether hypothesis. With the publication of the special theory of relativity in 1905 by Albert Einstein, the ether was shown to be unnecessary to the electromagnetic theory.

The second main problem, and the more serious of the two, was the explanation of various phenomena, such as the photoelectric effect, that involved the interaction of light with matter. Again the solution to the problem was proposed by Einstein, also in 1905. Einstein extended the quantum theory of thermal radiation proposed by Max Planck in 1900 to cover not only vibrations of the source of radiation but also vibrations of the radiation itself. He thus suggested that light, and other forms of electromagnetic radiation as well, travel as tiny bundles of energy called light quanta, or photons. The energy of each photon is directly proportional to its frequency.

With the development of the quantum theory of atomic and molecular structure by Niels Bohr and others, it became apparent that light and other forms of electromagnetic radiation are emitted and absorbed in connection with energy transitions of the particles of the substance radiating or absorbing the light. In these processes, the quantum, or particle, nature of light is more important than its wave nature. When the transmission of light is under consideration, however, the wave nature dominates over the particle nature. In 1924, Louis de Broglie showed that an analogous picture holds for particle behavior, with moving particles having certain wavelike properties that govern their motion, so that there exists a complementarity between particles and waves known as particle-wave duality (see also complementarity principle). The quantum theory of light has successfully explained all aspects of the behavior of light.

http://infoplease.com/ce6/sci/A0838876.html

photon , the particle composing light and other forms of electromagnetic radiation, sometimes called light quantum. The photon has no charge and no mass.

16

The photon is believed to have no rest mass. But it does have energy while in motion, and that’s equivalent to mass under relativity.

17

The photon is believed to have no rest mass. But it does have energy while in motion, and that’s equivalent to mass under relativity.

18

What confuses many is that light, while it has no mass, nonetheless has momentum and can exert force. Force is measured in pounds and is often confused with mass.

19

check out this thread from just a few weeks ago:

what is the effect of gravity on light ? and other light [heavy] questions…

the mass question as well as the “Does light therefore “bend” the space through which it passes?” question, among others, was answered.

p.s. i’ve been trying to reply to this thread since it was posted, but the boards were too slow to allow me to do a search… finally managed it…