If photons are "massless" how are these things possible? (read details)?

Question

Accodring to the Theory of Special Relativity, light, passing by a body, travels in a path that is curved according to the gravity and the electromagnetic field of that body. The force generated by gravity is calculated using mass. Albeit, electromagnetic force would have the greater pull, but it is theorized that gravity has a pull on light. Furthermore, it is theorized that black holes, which generate extreme gravitational pulls, do not allow light to escape.

Photons, upon striking a special metalic surface, transfer their kinetic energy to electrons, which begin to flow. Kinetic energy is calculated with mass and velocity.

Photons are said to have a non-zero energy. According to Einstein, Energy equals the mass of an object multiplied by its velocity squared. If mass is zero, doesn't that also make energy zero?

I don't understand this concept. Can someone please clarify this to me in simple terms. I've read all the websites that talk about the definitions of mass.

Answer 1

Hi,

First think about space as a big sheet of saran wrap. Then imagine that mass distorts the sheet, causing wells, or dents in it. now imagine that the photons need to travel along the saran wrap (i.e. traveling through space) and it makes sense that they would seem to change directions with respect to gravity.

That is a gross oversimplification, but I think that the analogy is roughly correct.

Hope that helps,
Matt

Answer 2

The pull of gravity on light isn't just theory, its been measured on numerous occassions and the degree by which a light beam is deflected always agrees with Einsteins theory. Gravity acts on light because it does have a mass, what light doesn't have is a rest mass which is what people are usually referring to when they talk about mass.

You can simplify Einsteins equation down to E=M because the speed of light is a constant and so can be removed (but you get really awkward units for mass if you do, so C^2 is usually left in). But E=M really emphasises that mass and energy are the same thing so because a photon has energy it can also be expressed as having a mass.
Putting C^2 into the equation also emphasises that you get an awful lot of energy for just a little bit of mass or to turn it on its head there is a miniscule mass associated with a photon which is why gravity doesn't have much of a grip on it. So its only around intense sources of gravity that you see a significant effect i.e. a black hole.

Answer 3

Very simply put, a photon, like all quanta, is massless and carries no electric charge. Therefore, an electromagnetic field has no effect on the path of a beam of light.

I think your E=MC2 concern is easily explained by the fact that "M" in this equation is equal to the rest mass - this is important, since this equation states that energy and mass are equivalent. Light's mass is in the form of pure energy - and the energy is indeed equal to it's rest mass times it's speed squared.

Answer 4

Energy=Mass times speed of light squared

This equation tells you how much energy is in a given mass of matter. Energy (such as photons) are already energy and do not have mass.

Photons are quanta and behave both like waves and particles, even though they composed of energy; this is called the wave-particle duality. A different equation is needed to interpet momentum of photons: E = cp where p is the magnitude of the momentum. The formula for Kinetic energy of a photon is Ke=E/2. if a photon is reduced to Ke=0 then it ceases to exist.

Gravity does not "attract" photons...it bends space and they continue going in a 'straight' line. Black holes bend space so tightly that not even photons can get out.

Answer 5

All you need to know to answer all your questions is that energy of electromagnetic waves is proportional to the frequency, more specifically E = hf, h being the planck constant and f being the frequency. The idea of energy being proportional to mass is pretty much only used in mechanics. Physics is WAY more than just mechanics.

It is not just a theory that gravity bends light, there is evidence of light bending as it passes by stars and other massive objects.

Answer 6

Hmm, here's a start:

I believe part of your problem is thinking of light as an "either or" situation in that it either has mass and should act as if it has mass, or it doesn't and so it shouldn't. In fact, light is more of a "both" situation. Light acts and behaves as if it doesn't have mass and as if it does have mass depending upon the experiment. Light belongs to the realm of quantum physics where just about everything goes against common sense.

Hope that helps a little...

Answer 7

It took Einstein 12 years to develop a model of the photon that met the experimental data for both wave and particle behavior. It is this duality that you are struggling with. I recommend that you read:
http://en.wikipedia.org/wiki/Photon
in its entirety to try to grasp this concept. It contains both a historical as well as theoretical explanation of the photon.

J

Answer 8

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