aren't photons "massless"?

I think photons have mass, but even if they didn't they have other properties that differ from Neutrinos.

Can you link to a page which supports that? From what i can tell, they have something conceptual called a 'rest mass'... but i'm unsure if a wavicle traveling at (or near) lightspeed can be convinced to come to a rest very easily (or for very long).

Anyway, it seems many quantum schools of thought need parallel universes and other abstract models to "explain" some of this stuff. And until string theory solves the mystery of gravity, then we're all just guessing. ;)

[think i hear crarko biting his tongue. :) ]

Maybe they don't have mass. I really don't know. But how would a solar sail work if they don't?

Black piece of paper brings a photon to a halt pretty quick and converts it into something else. Acceleration is something like 10^24 gravities - good thing it's got no mass.

Any star gives off *plenty* of pretty much any particle you can think of.

Read the rest of the article. Especially the "Mechanics" bit.

Photons have no rest mass.

If you say so. ;)

Good thing black paper lets photons rest... they must be really tired from moving so fast all of [?] the time. ;)

Read a strange thing about photons recently. If a photon leaves a galaxy a million light years away, then it will reach us in a million years. But from the photon's perspective, it arrived here instantly. Does this mean that all photons are everywhere at the same time?

Not really. It's one of the problems of trying to apply non-relativistic reference frames to relativistic speeds. Time slows down for the reference frame that is moving at relativistic speeds (near the speed of light), but it's still constrained within the time of external frames.

Imagine you and a friend are a great distance away from each other holding flashlights. You turn yours on and then off. When your friend sees yours go off, they turn theirs on and then back off. From the perspective of the light from each flashlight the journey is instantaneous, but clearly the light from the two sources was not travelling simultaneously.

That is the idea currently.

It stems from the famous double-slit experiment. The one that creates the interference patterns. Those patterns are easily explained but somebody had the bright idea to fire off one(1) photon at a time. This still produces the interference pattern. That's only possible if the single photon moves through both slits at the same time. However, if you start counting the photons that pass through one of the slits the interference pattern disappears. The conclusion is that the photon is everywhere at the same time unless you look at it.

http://en.wikipedia.org/wiki/Double-...dual_particles

http://en.wikipedia.org/wiki/Photon

From Wikipedia: In quantum mechanics, the Heisenberg uncertainty principle states a fundamental limit on the accuracy with which certain pairs of physical properties of a particle, such as position and momentum, cannot be simultaneously known. In other words, the more precisely one property is measured, the less precisely the other can be controlled, determined, or known.

So then, have we actually brought a photon to "rest" and measured its mass, :) or is this merely all conceptual? (as i initially stated)


What is the mass of a photon?

We use terms like 'mass' and 'gravity' every single day, but that doesn't mean we have full understanding of every aspect of their nature. We use math and models to represent our (human) perception of such phenomena. So, eventually, we all wind up on this page...

Interpretations of Quantum Mechanics

Even though the word photon doen not appear on the that page, it gives us an idea of how much of this is still open to interpretation.

My reading of that section conveys that it's not that "the photon is everywhere at once unless you look at it", but that "light (and electrons, etc.) can behave as either particles or waves, but not both at the same".

Maybe the meaning of "everywhere at the same time" could use some clarification. As used by kaptagat, SirDice, and myself; do we all mean the same thing?

Yes, that's correct. With the single photon double slit experiment the only way an interference pattern would show is when a single photon interferes with itself. And the only way it could do that is if it would go though both slits at the same time. Hence the conclusion the photon is everywhere at the same time.

Basic particle physics questions
 

Uhm...I dont understand the question. In...before hitting is impossible in all cases.

Though, come to think of it, if the particle gives the electron cloud the necessary amount of energy to trigger K-capture which usually gives out energy, and doesnt require it, the particle would not be in the core, but rather an neutron being changed into a proton.

OK, I think it's the definition of "everywhere". It certainly doesn't mean that a light that I shine in front of me is going to throw photons behind me, unless they "bounce" there. A single photon may appear to go through both slits, but that doesn't mean it's literally everywhere. Anyway, that was my disagreement with the wording.

As for the conclusion, I don't think it's entirely accurate to state that the photon actually went through both slits, just that the photon as a wave did so. Or, as the article states: It's a bit like the wave-like features of the photon go through both slits, but the particle features go through one slite, as determined by the interaction of the wave with the slits.