Basic particle physics questions

kaptagat · 11-01-2011, 11:11 AM

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

SirDice · 11-01-2011, 11:49 AM

Quote:

Originally Posted by kaptagat


	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?

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.

chabig · 11-01-2011, 03:10 PM

Quote:

Originally Posted by SirDice


	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.

chabig · 11-01-2011, 03:14 PM

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

acme.mail.order · 11-01-2011, 05:37 PM

Quote:

Originally Posted by SirDice


	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.

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?

Quote:

Originally Posted by kaptagat


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

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

NovaScotian · 11-01-2011, 07:08 PM

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.

SirDice · 11-02-2011, 02:20 AM

Quote:

Originally Posted by chabig


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

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

SirDice · 11-02-2011, 02:24 AM

Quote:

Originally Posted by acme.mail.order


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

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.

Quote:


	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?

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

acme.mail.order · 11-02-2011, 02:28 AM

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

acme.mail.order · 11-02-2011, 02:39 AM

Quote:

Originally Posted by SirDice


	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.

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.

SirDice · 11-02-2011, 02:39 AM

Quote:

Originally Posted by acme.mail.order


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

http://en.wikipedia.org/wiki/Reflect...s%29#Mechanism

SirDice · 11-02-2011, 02:41 AM

Quote:

Originally Posted by acme.mail.order


	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.

http://en.wikipedia.org/wiki/Huygens-Fresnel_principle

acme.mail.order · 11-02-2011, 02:47 AM

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

SirDice · 11-02-2011, 02:51 AM

Quote:

Originally Posted by acme.mail.order


	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.

acme.mail.order · 11-02-2011, 02:53 AM

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

kaptagat · 11-02-2011, 03:40 AM

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

SirDice · 11-02-2011, 04:06 AM

Quote:

Originally Posted by acme.mail.order


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

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

SirDice · 11-02-2011, 04:09 AM

Quote:

Originally Posted by kaptagat


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

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

acme.mail.order · 11-02-2011, 07:21 AM

Quote:

Originally Posted by SirDice


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

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

chabig · 11-02-2011, 04:05 PM

Quote:

Originally Posted by SirDice


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

That's not the same as reflection:

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

11-01-2011, 11:11 AM	#1
kaptagat Major Leaguer Join Date: Jul 2003 Posts: 329	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

11-01-2011, 07:08 PM	#6
NovaScotian Hall of Famer Join Date: Oct 2002 Location: Halifax, Canada Posts: 4,946	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. __________________ 17" MBP, OS X 10.8.3; 27" iMac, OS X 10.8.3

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11-01-2011, 03:14 PM	#4
chabig Hall of Famer Join Date: Jan 2002 Posts: 2,934	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

11-02-2011, 02:28 AM	#9
acme.mail.order League Commissioner Join Date: Sep 2003 Location: Tokyo Posts: 6,046	Most materials that are commonly considered photoelectric are usually not also considered reflective.

11-02-2011, 02:47 AM	#13
acme.mail.order League Commissioner Join Date: Sep 2003 Location: Tokyo Posts: 6,046	Largely concerned with the wave principle. OP seemed more concerned with the particle end of things.

11-02-2011, 02:53 AM	#15
acme.mail.order League Commissioner Join Date: Sep 2003 Location: Tokyo Posts: 6,046	Eventually. But you can't use one to explain the other.

11-02-2011, 03:40 AM	#16
kaptagat Major Leaguer Join Date: Jul 2003 Posts: 329	OK thanks for the replies about bouncing photons but why don't they go straight through things like neutrinos?