# Thread: Concept of a Photon

1. Some simple Photon questions:

Does a photon have mass?

Can two photons collide and if so what happens if they do?

If a photon is light which moves at the speed of light then can a photon slow down? Can it stop? If it stops is it still a photon?

If a photon has mass then could I theoretically fill a bucket with photons? If not why not?  2.

3. 1. No, a photon has zero mass. Someone else may wish to go out to 10 or 15 decimal places to challenge this statement on the basis of uncertainty, but it is understood a photon does not have a rest mass.

2. It may be theoretically possible. In practice, we assume collisions do not take place.

3. A photon always travels at c, the speed of light in a vacuum. We know that light slows down when it enters any medium such as glass or air. This does not mean that the photons in the light have changed their velocity.

4. No, you cannot fill a bucket with photons. They must travel at c or they cease to exist.

You are on the verge of making that extremely common mistake, the one that all novices insist on making -- that of treating a photon like a classical particle governed by the laws of classical physics. You will want to imagine a photon as just another marble, that light is a stream of ball bearings. Sigh. It doesn't work. Photons must be treated by the laws of quantum physics or else you come up with screwy, meaningless conclusions.  4. Thanks SteveF,

I know some of my questions were geared toward the silly but I had thought of no better metaphors. When one sees terms like: "no resting mass" and "particle" then one might think there is a tiny dot with moving mass just as you alluded to with the ball bearing analogy.

Thank you.  5. If you truly wish to understand light you must study light, not photons. Treat light as a wave. There are no photons. This is the classical view of light and the mathematics is not at all difficult. High school algebra is all you need.

When you have the classical viewpoint snockered, then you can move on to photons and the dual nature of light. Here is a brief introductory article worth reading:

http://library.thinkquest.org/28383/...mla/2_10a.html

Too many beginners wish to rush past the classical treatment and start by inquiring about photons, treating them as classical objects. You know, dots with mass that can change velocity, bounce around inside a chunk of matter, and so on. They develop all sorts of weird ideas about light.

Try to explain polarization of light using photons if you will. It's a snap if you think of light as a wave, but you need heavy-duty quantum treatment when you need to work only in terms of photons. Good luck, my friend.
Â*  6. We can look at light not being made up of photons but as waves. But we look at other particles as particles. Maybe this is because they have mass, but we should really look at all subatomic particles as both particles and waves. Because that is how they behave. Even photons show some particle-like behaviour.  7. Originally Posted by oversteve
Some simple Photon questions:

Does a photon have mass?
Inertial mass (aka relativistic mass), yes. Proper mass (aka rest mass), no.

If a photon is light which moves at the speed of light then can a photon slow down? Can it stop? If it stops is it still a photon?
If a photon exists then, as observed from a locally inertial frame, it moves at c. If it stops then it no longer exists and is no longer a photon (which disapears from the universe).
If a photon has mass then could I theoretically fill a bucket with photons? If not why not?
A box of photons can exist regardless of how you define the term "photon." This is then a matter of fact rather than definition.

Best wishes

Pete  8. Originally Posted by SteveF
4. No, you cannot fill a bucket with photons. They must travel at c or they cease to exist.
I have heard and read many things in my day, but that is not one of them. Who says they "cease to exist"? Would you mind explaining?  9. It means that any attempt to slow down a photon is either futile or results in the photon being captured by matter. Even if the matter subsequently re-emits a photon the original particle has ceased to exist.

Is someone here actually claiming you can have a bucket full of photons? Keep the same photons for an hour or two? That a photon can exist at a velocity less than c? You explain.  10. Originally Posted by SteveF
It means that any attempt to slow down a photon is either futile or results in the photon being captured by matter. Even if the matter subsequently re-emits a photon the original particle has ceased to exist.
That told me nothing. That, fundamentally, doesn't mean it wont exist. You could explain yourself a bit better, if you please.

Specifically if this experiment was successful in a vacuum. There would be no matter to capture it.

Lastly, I know what it would mean. That doesn't answer my question.  11. beh, what a sea of confusion.
well done, i salute you.

photons are electromagnetic waves.
a wave is energy.
ienergy can be converted into mass, and vice versa.

so a photon cannot have mass, it can be converted into mass, in which it cease to exist as a photon(energy)

if 2 photons collided you'd have a higher wavelength of a photon.  12. I usually don't approve of vomiting excerpts from other places, but...

Photons in free space act almost exclusively as waves. Therefore, when they cross paths they merely set up an interference pattern for the very brief time of their interaction. No energy is exchanged and the quantum state of each photon is unchanged after they pass each other.

This interference pattern is akin to ripples on water that approach each other, form an interference pattern of peaks and troughs and then continue on their way.

If matter is present where the photons cross, non-linear effects caused by accelerated electric charges may allow the photons to interact. This interaction could be considered a collision of sorts, resulting in exchange of energies with many possible outcomes.

One such outcome is called frequency doubling, where two photons are combined to form one photon at twice the frequency.

-Scott Wilber, President, ComScire - Quantum World Corporation  13. Originally Posted by Jeremyhfht Originally Posted by SteveF
It means that any attempt to slow down a photon is either futile or results in the photon being captured by matter. Even if the matter subsequently re-emits a photon the original particle has ceased to exist.
That told me nothing. That, fundamentally, doesn't mean it wont exist. You could explain yourself a bit better, if you please.

Specifically if this experiment was successful in a vacuum. There would be no matter to capture it.

Lastly, I know what it would mean. That doesn't answer my question.
The usual way of explaining this is to say that a photon is not a valid rest frame (where it would have zero velocity) as according to special relativity a photon always travels at c, so it would be at rest and travelling at c in its own rest frame - a contradiction.  14. yes, a photon does have mass and it can be calculated using de broglie's equation which relates the 2 equations: E=hf and E=mc^2, where h is planck's constant and f is frequency.

giving you hf = mc^2, so the mass of a photon is (hf)/(c^2), giving you an almost negligible mass, which is why the dual nature has been discovered only recently (beginning of 20th century)

the way light acts as a particle is that it is quantized, as was discovered by MAx Planck. what this means is that light is not continuous, it has distinct and separate packets (called photons, that is the definition of photon), and you cannot have quantities of energy that are less than these packets. therefore, a photon is the smallest quantity of light of that particular frequency, and it's energy can be calculated by E=hf

sorry if its a little out of order and confusing, i am sorta inarticulate  15. Originally Posted by ZebraFiesta
yes, a photon does have mass and it can be calculated using de broglie's equation which relates the 2 equations: E=hf and E=mc^2, where h is planck's constant and f is frequency.

giving you hf = mc^2, so the mass of a photon is (hf)/(c^2), giving you an almost negligible mass, which is why the dual nature has been discovered only recently (beginning of 20th century)

the way light acts as a particle is that it is quantized, as was discovered by MAx Planck. what this means is that light is not continuous, it has distinct and separate packets (called photons, that is the definition of photon), and you cannot have quantities of energy that are less than these packets. therefore, a photon is the smallest quantity of light of that particular frequency, and it's energy can be calculated by E=hf

sorry if its a little out of order and confusing, i am sorta inarticulate
Its better to say a photon has momentum given by hf/c as talking about the rest mass of a photon is meaningless.  16. Its better to say a photon has momentum given by hf/c as talking about the rest mass of a photon is meaningless.
That is the reason I use the term proper mass instead of rest mass. In fact these are two terms which refer to two different things in general. In the most general case gamma has the vale dt/dtau which is

gamma = 1/sqrt(1 + 2Phi/c^2 - beta^2)

Thus

m = u/sqrt(1 + 2Phi/c^2 - beta^2)

where u is the particle's proper mass. The rest mass is given by

m_0 = u/sqrt(1 + 2Phi/c^2)

It is this mass that Einstein speaks about in his text "The Meaning of Relativity" in the section on Mach's principle.

Pete  17. Originally Posted by river_rat Originally Posted by ZebraFiesta
yes, a photon does have mass and it can be calculated using de broglie's equation which relates the 2 equations: E=hf and E=mc^2, where h is planck's constant and f is frequency.

giving you hf = mc^2, so the mass of a photon is (hf)/(c^2), giving you an almost negligible mass, which is why the dual nature has been discovered only recently (beginning of 20th century)

the way light acts as a particle is that it is quantized, as was discovered by MAx Planck. what this means is that light is not continuous, it has distinct and separate packets (called photons, that is the definition of photon), and you cannot have quantities of energy that are less than these packets. therefore, a photon is the smallest quantity of light of that particular frequency, and it's energy can be calculated by E=hf

sorry if its a little out of order and confusing, i am sorta inarticulate
Its better to say a photon has momentum given by hf/c as talking about the rest mass of a photon is meaningless.
true, my mistake. however, a photon cannot be at rest, since it is traveling at the speed of light  Bookmarks
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