Physics Help Forum What is the frequency of a single photon?

 Theoretical Physics Theoretical Physics Help Forum

Jan 19th 2015, 11:36 AM   #11

Join Date: Apr 2008
Location: On the dance floor, baby!
Posts: 2,558
 Originally Posted by Farsight An ascending photon doesn't get redshifted! Its frequency doesn't change one jot! When I lift you up to space I do work on you. You gain energy. So the photon appears to have lost energy. But it hasn't. There's some really interesting things lurking in general relativity. You tend not to bump into them, and they don't seem to get taught. It's only when you "do your own research" that you find out about them. A related matter is invariant mass. And as you may have guessed already: it varies! How's that for shiny?
I understand what you were saying about the photon wavelength. I was making a joke about it because some would see it blue-shifted and others would see it red-shifted.

Most Physicists work with the rest mass (what I believe you are talking about when you say "invariant") and use the momentum as a variable when talking about effects on the particle due it's speed.

As far as any little nuggets of information that you can only find out about after doing your own research, what you are speaking about is true any field of Physics. GR is nothing mysterious it just involves a bunch of concepts that people tend to find difficult because experience in the low speed, low mass limit is too comfortable to easily let go of. Many fields of Physics have similar issues.

-Dan
__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.

Jan 19th 2015, 12:09 PM   #12
Senior Member

Join Date: May 2014
Location: Poole, UK
Posts: 132
 Originally Posted by StarsBaby I'm guessing that Farsight is suggesting that there is one absolute system of coordinates in which to consider the physical events
No, I root for relativity. It's just conservation of energy. Go back to SR. Imagine you're in space. There's a stream of identical photons coming at you. You measure their frequency. Then you fire up your gedanken rocket and head towards them, then turn the rocket off and measure their frequency again. You now measure a higher frequency. But those photons didn't change. You did. It's the same for GR where a stream of photons is moving straight up and you measure the frequency, then descend and measure it again.

 Originally Posted by topsquark I understand what you were saying about the photon wavelength. I was making a joke about it because some would see it blue-shifted and others would see it red-shifted.
Oh. OK, sorry, I missed that.

 Originally Posted by topsquark Most Physicists work with the rest mass (what I believe you are talking about when you say "invariant") and use the momentum as a variable when talking about effects on the particle due it's speed.
The rest mass varies with gravitational potential. Yes. Wash my mouth out with soap, Scout's honour, cross my heart and hope to die, it varies. This is why we have a mass deficit. Binding energy is said to be negative energy, but in truth what we've got is less positive energy.

 Originally Posted by topsquark As far as any little nuggets of information that you can only find out about after doing your own research, what you are speaking about is true any field of Physics. GR is nothing mysterious it just involves a bunch of concepts that people tend to find difficult because experience in the low speed, low mass limit is too comfortable to easily let go of. Many fields of Physics have similar issues.
You sound unimpressed. Maybe you need another shiny little nugget. Here: you know if you shoot a cannonball straight up in the air, it slows down? Well if you do the same with a photon, it speeds up!

Jan 19th 2015, 12:24 PM   #13
Junior Member

Join Date: Oct 2014
Posts: 13
 Originally Posted by Farsight No, I root for relativity. It's just conservation of energy. Go back to SR. Imagine you're in space. There's a stream of identical photons coming at you. You measure their frequency. Then you fire up your gedanken rocket and head towards them, then turn the rocket off and measure their frequency again. You now measure a higher frequency. But those photons didn't change. You did. It's the same for GR where a stream of photons is moving straight up and you measure the frequency, then descend and measure it again.
Your idea is fine for SR, but not for GR. In the SR case, one is considering the photons in two separate systems of coordinates. There is no change in system of coordinates associated with the change in frequency associated with GR.

 Jan 19th 2015, 12:39 PM #14 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 It isn't my idea, and it is fine for GR. There is a change of coordinate system. When you descend your clocks are running slower, so you measure the selfsame photon frequency to be higher. Alternatively, think this through: you're out in space. You have a 511keV photon in your hand. You throw it into a black hole. What's the black hole mass increase?
Jan 19th 2015, 04:28 PM   #15

Join Date: Apr 2008
Location: On the dance floor, baby!
Posts: 2,558
 Originally Posted by Farsight No, I root for relativity. It's just conservation of energy. Go back to SR. Imagine you're in space. There's a stream of identical photons coming at you. You measure their frequency. Then you fire up your gedanken rocket and head towards them, then turn the rocket off and measure their frequency again. You now measure a higher frequency. But those photons didn't change. You did. It's the same for GR where a stream of photons is moving straight up and you measure the frequency, then descend and measure it again.
This is the second time today I've had to admit that I haven't looked over this problem so I can't put a concrete argument on the table. But my gut tells me this: The total energy of a particle is the time component of the momentum 4-vector. Hence in different reference frames we will observe the particle to contain different energies in different frames. Total energy is not "conserved" in this sense. Remember it is p_(mu) (dot) p^(mu) that is constant, not the momentum or energy. (Don't panic folks. In the low speed limit the energy and momentum are still conserved separately.)

Please don't quote me on that one. I haven't worked out the explicit details yet, but it seems sound to me.

 Originally Posted by Farsight The rest mass varies with gravitational potential.
Owie! Owie! Mommy! My head hurts!

Rest mass is just that. If you measure the mass of a particle when you are in the same frame as the particle (ie. the frame where the particle is stationary) you will measure the mass to be its rest mass. That is the definition of the rest mass of a particle. The rest mass is constant.

Physics used to use the equation E^2 = (pc)^2 + (mc^2)^2 = Mc^2 to talk about a mass M that varies with the speed. Few do this now-a-days and say that m is the rest mass and it is p that changes.

 Originally Posted by Farsight Maybe you need another shiny little nugget. Here: you know if you shoot a cannonball straight up in the air, it slows down? Well if you do the same with a photon, it speeds up!
Owie! Owie! Mommy make it stop!

Are you for real? I'm going to use not just soap, but lye soap, and follow up with 50 lashings with a wet textbook.

The speed of light is the same constant in any reference frame. That's one of the founding laws of SR and GR. You said before that you were rooting for Relativity. Did you just change your mind?

-Dan
__________________
Do not meddle in the affairs of dragons for you are crunchy and taste good with ketchup.

See the forum rules here.

Jan 20th 2015, 04:13 AM   #16
Senior Member

Join Date: May 2014
Location: Poole, UK
Posts: 132
I tell you topsquark, energy is conserved. It's the one thing we can neither create nor destroy. You observe the particle to contain different energies in different frames, but the particle didn't change just because you changed your state of motion. Instead you changed.

 Originally Posted by topsquark Rest mass is just that. If you measure the mass of a particle when you are in the same frame as the particle (ie. the frame where the particle is stationary) you will measure the mass to be its rest mass. That is the definition of the rest mass of a particle. The rest mass is constant.
Let's say you measure the rest mass of a particle at some altitude x, then you measure the same rest mass at altitude y. You naturally conclude that the particle's rest mass hasn't changed. But it has. That rest mass is a measure of the particle's energy-content. And that energy-content has reduced. Gravity is not a force in the Newtonian sense. It converts potential energy into kinetic energy, and when the latter is radiated away into space, you're left with a mass deficit. But this applies to both you and the particle. You both change, so you think the particle doesn't.

 Originally Posted by topsquark Are you for real? I'm going to use not just soap, but lye soap, and follow up with 50 lashings with a wet textbook.
Yes I'm for real Dan. So is the mass deficit.

 Originally Posted by topsquark The speed of light is the same constant in any reference frame. That's one of the founding laws of SR and GR. You said before that you were rooting for Relativity. Did you just change your mind?
No. I root for relativity. Take a look at this. Second paragraph:

Attached Thumbnails

Last edited by Farsight; Jan 20th 2015 at 04:21 AM.

 Jan 20th 2015, 05:23 AM #17 Junior Member   Join Date: Oct 2014 Posts: 13 That's not physics help. I think I've found the source of the problem here: http://rationalwiki.org/wiki/List_of_Internet_kooks
 Jan 20th 2015, 07:30 AM #18 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 The anonymous person who put up that malicious entry has an IP address in Dublin. I'm not some "internet kook". I'm a guy who loves physics, and who cares about physics.
Jan 20th 2015, 07:57 AM   #19
Junior Member

Join Date: Oct 2014
Posts: 13
 Originally Posted by Farsight The anonymous person who put up that malicious entry has an IP address in Dublin. I'm not some "internet kook". I'm a guy who loves physics, and who cares about physics.
You may love physics, but unfortunately you don't seem to be helping people to understand physics. You are, in your own words, "relatively weak" at mathematics and "an amateur" at physics. Rather than provide help here, you have presented your own, peculiar, interpretation of physics (to be charitable).

 Jan 20th 2015, 08:58 AM #20 Senior Member   Join Date: May 2014 Location: Poole, UK Posts: 132 I have provided help. Check my posts. But it is a little quiet, and it's all too easy to get into rather deep discussions with the likes of topsquark. Note that there's nothing "peculiar" about quoting Einstein or giving some other robust reference to clarify a point. Having checked your posts, it's clear that you haven't provided help. It's clear that you're only here to cause trouble. I suspect we've met before, and/or that you added me to the rationalwiki list of "internet kooks", and/or you're now stalking me on the internet and linking to that list. I'm John Duffield from Poole in the UK, my current IP address is 86.128.49.84, and I'm pleased to make your acquaintance. And you are? topsquark: feel free to "clean up" this unseemly conversation. Whilst I'd like to ignore an attempt to discredit me, I feel I need to say something to counter the ad-hominems.

 Tags frequency, photon, single

 Thread Tools Display Modes Linear Mode

 Similar Physics Forum Discussions Thread Thread Starter Forum Replies Last Post backtophysics Quantum Physics 0 May 9th 2012 07:22 AM Austic Advanced Electricity and Magnetism 2 Mar 17th 2011 02:39 PM labview1958 Light and Optics 4 Sep 24th 2010 07:23 AM Lil_Aziz1 Light and Optics 3 Sep 3rd 2010 11:51 PM yzc717 Waves and Sound 0 Feb 18th 2009 12:45 AM