Basic particle physics questions
 

Hello Everyone

Can someone explain why neutrinos (very small and with no electrical charge) can go through everything while photons (also very small with no electrical charge) bounce off everything enabling us to see things.

Secondly, how to photons bounce off things at the same speed they bumped into them?

Thanks

Photons aren't 'bounced off' they are absorbed causing electrons to go into a higher state. When those electrons drop back to their stable shell they give off a photon.

I don't think so. It's well known that light reflects off things.

I doubt any of us are experts in this subject, so let's go to the web:

http://www.physicsforums.com/showthread.php?t=309082

http://www.natscience.com/Uwe/Forum....and-a-neutrino

And of course they do this at the exact same frequency and angle as the incoming photon?

Now that I think more about it, how does a stream of subatomic particles reflect cleanly off of a surface that is "smooth" only at a scale many orders of magnitude larger than itself?


That part's easy. Plug something with zero mass into a conservation of momentum equation.

These questions are pushing the boundary between particle physics and quantum mechanics. In the latter, the particle is considered as a wave so a clean reflection is a wave question -- Same question -- how does an ocean wave reflect off a rocky wall? By constructive interference. Having studied this on the way to an engineering degree about 50 years ago, I have nothing more to say lest I make an ass of myself.

Yes. The amount of energy needed to knock an electron to a higher orbit is the same amount of energy it gives off when it falls back to it's stable orbit.

http://en.wikipedia.org/wiki/Geiger-Marsden_experiment

Most materials that are commonly considered photoelectric are usually not also considered reflective.

I'll take that at face value. How are you planning to account (at the quantum level) for the angle of incidence being equal to the angle of reflection at the macroscopic level. If you toss a particle into the whirling mass of the electron cloud something's (probably) going to come back out. Somewhere. Maybe. But only when you're not looking.

Largely concerned with the wave principle. OP seemed more concerned with the particle end of things.

A photon is both a wave and a particle, so you have to take both into account.

Eventually. But you can't use one to explain the other.

OK thanks for the replies about bouncing photons but why don't they go straight through things like neutrinos?

If I could explain why this duality exists I would have received a Nobel prize by now ;)

http://en.wikipedia.org/wiki/Wave%E2...rticle_duality

Photons interact with atoms. Neutrinos also interact just on a very, very small scale. The weak nuclear force of a neutrino is several orders of magnitude smaller then the electomagnetic force of a photon.

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

Didn't ask to have the duality explained (it may never be explained), just said you can't use one to explain the other.

That's not the same as reflection:

http://en.wikipedia.org/wiki/Reflect...ction_of_light

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.

One of the problems with quantum physics is that we are at the edge of useful analogy.
Most of the concepts that we are familiar with break down, so trying to say "a photon is like a wave" (sometimes) is uncomfortable at best. Even ideas like time and mass start to fall to bits in terms that we find it comfortable to relate to.

Remember that evidence that the photon (as a wave) transited both slits is that the appropriate interference pattern shows. Think about how that works for a particle.

I have to admit it's been 20 years since I last took atomic and nuclear physics classes. Can't exactly remember where I picked it up.

This seems to explain it relatively simple but it also includes some links to more detailed descriptions.

http://www.newton.dep.anl.gov/askasc...5/phy05010.htm

This one's related to the single photon double slit experiment: Google cache

There is no way a high school science project using a child-safe laser, a piece of Kodak film and some glass filters is going to come anywhere close to a single-photon event. There will be stray photons bouncing around the room from some source or another.

I would also strongly doubt the accuracy of $erious $cience claiming a single-photon event. Prove that there were no other particles* involved. Explain your proof to Werner Heisenberg. If he agrees, I'm convinced.

* other photons, related bosons, quarks, leptons, hadrons, neutrinos (good luck with that one) or any other subatomic flotsam that could jigger the results.

He's been dead for almost 50 years.

I know.

OK, but the rest of the scientific community is satisifed by multiple, independently replicated peer reviewed studies, experiments, and research papers. Heisenberg himself could disagree with the result and it still be accepted as valid. Einstein, for example, didn't like the implications of Quantum Mechanics, but he still worked on research and experiments that would ultimately support it.


benwiggy's earlier comment is perfect here.

Photons / electrons / etc. at this level of observation exhibit particle and wave characteristics. This may lend itself to describing these wave characteristics as the particle being in multiple places at the same time, but that's at best a great simplification.

A quote attributed to Feynman sums this up pretty well, "If you think you understand quantum mechanics, you don't understand quantum mechanics." Or, "'It is safe to say that nobody understands quantum mechanics."

This is not uncommon in Physics. As a Mechanical Engineer, I studied Fluid Mechanics and Heat Transfer. One of the concepts there for explaining the growth of boundary layers on solid surfaces immersed in flowing fluids was Ludwig Prandl's boundary layer theory based on a concept called mixing length. Works beautifully as a predictive tool. Unfortunately, with the advent of modern computational fluid mechanics and much finer measurements than Prandl could make, it's been proven wrong. Still works, though.