Lifetime of a Photon: Electron-Positron Annihilation & Speed of Light

[vivitar02]

If electron-positron annihilation creates a photon, with "mass-energy", how long will the photon live for? It will travel at the speed of light until when?

 

[mathman]

Photons always travel at the speed of light. The will continue indefinitely until they interact with something.

 

[Sojourner01]

Photons, as far as we know, don't decay to anything. Several fundamental quanta are thought to be the same, in particular, electrons and protons. There's an experiment looking for proton decay which would have expected to find one by now if it ever happens, and they haven't. All the others are thought to be even less likely. The photon is included.

 

[JesseC]

The clock of an object traveling near the speed of light slows down relative to our own clock. Thus particles traveling at high speeds appear to decay more slowly (at least the half life lengthened) from our point of view.

Closer things get to the speed of light, longer their half lives. At the speed of light, the half life would become infinite... so photons cannot decay in the sense of nuclei or particles.

 

[JesseC]

Photons, as far as we know, don't decay to anything. Several fundamental quanta are thought to be the same, in particular, electrons, protons and neutrons. There's an experiment looking for proton decay which would have expected to find one by now if it ever happens, and they haven't. All the others are thought to be even less likely. The photon is included.

Free neutrons do decay, with a half life of about 13 minutes.

 

[Sojourner01]

Free neutrons do decay, with a half life of about 13 minutes.

Duh, yes, you're absolutely correct. I knew that as well. Post edited.

 

[Dr Lots-o'watts]

If electron-positron annihilation creates a photon, with "mass-energy", how long will the photon live for? It will travel at the speed of light until when?

Actually, annihilation creates at least two photons, not a single one. The process has to obey conservation of momentum.

I'm being a wise guy because I use to forget about this, but now I can't so I share my curse.

 

[mrspeedybob]

From a cosmological standpoint it's easy to see that if photons do decay it takes a loooooong time. We're still seeing cosmic background radiation from 14 billion years ago. In another sense though they decay constantly. The expansion of space causes photons to redshift / decay to lower energies.

 

[Phrak]

I think this thread needs some rewinding.

Since when do photons propagate at the velocity c? QuantumElectroDynamics makes no such assumption, and off shell photons have a range as a function of their velocity.

 

[DrChinese]

Looking at the existence of a photon: can a free photon truly exist? That is, a photon which is just flying through free space awaiting something to interact with? I am thinking of a photon created long ago, and traveling through space today. If the photon was formally an excitation of a field - and not an independent entity itself - then this wouldn't happen... would it? There would need to be a start point and an end point. The end point being some spot in the distant future in which there is an interaction.

 

[inflector]

Since when do photons propagate at the velocity c? QuantumElectroDynamics makes no such assumption, and off shell photons have a range as a function of their velocity.

Do you have some references for this assertion? Specifically that:

• Photons don't travel at c
• Photons have variable velocity
• Photons have a range determined by that velocity

 

[vivitar02]

From a cosmological standpoint it's easy to see that if photons do decay it takes a loooooong time. We're still seeing cosmic background radiation from 14 billion years ago. In another sense though they decay constantly. The expansion of space causes photons to redshift / decay to lower energies.

1. They decay to lower energies, but remain as photons ?

2. As it propogates through matter (mass?), is the photon's energy changing ?

3. Sorry, when I say mass, is that the same as referring to massive particles, i.e particles with
a non-zero rest mass?

Sorry, I'm fascinated by the whole concept of massless particles, mass-energy etc.

Thanks

 

[TheTechNoir]

1. They decay to lower energies, but remain as photons ?

To my understanding the decay to lower energies is the red shift as mentioned, or specifically the change in the wavelength. That is why the microwave background radiation which I believe was once actually visible light has left the visible light spectrum and is now only visible in microwaves because of the amount of energy change aka amount of redshift/stretching of the wavelength.

 

[mrspeedybob]

To my understanding the decay to lower energies is the red shift as mentioned, or specifically the change in the wavelength. That is why the microwave background radiation which I believe was once actually visible light has left the visible light spectrum and is now only visible in microwaves because of the amount of energy change aka amount of redshift/stretching of the wavelength.

Exactly. One of the properties of empty space is that it expands for some reason I know not. The expansion is so slight that on terrestrial scales it is not detectable. On a cosmic scale it means that everything is moving away from everything else. That means that very distant galaxies are moving away at great speed, therefore, the light we see from these galaxies is stretched out to a longer wavelength. Another way to view this is that if you have a wave of 600 nm light, as that light travels 600 nm, 600 nm gets just a tad longer so the next wavelength may be 600.000000000000000000000000000000000000000000000000000000000000000000001 nm. Repeat this process for a few billion years and it turns into infra-red, then microwave, then radio, etc...

 

[negru]

I think this thread needs some rewinding.

Since when do photons propagate at the velocity c? QuantumElectroDynamics makes no such assumption, and off shell photons have a range as a function of their velocity.

Any "off-shell" (otherwise known as virtual) photon you'd measure becomes on-shell, hence must move with speed c, as all massless particles. Quantum electrodynamics=quantum mechanics+special relativity, so it'd better respect special relativity.

 

[FREEDOM2]

Any "off-shell" (otherwise known as virtual) photon you'd measure becomes on-shell, hence must move with speed c, as all massless particles. Quantum electrodynamics=quantum mechanics+special relativity, so it'd better respect special relativity.

Can you explain the concept of off shell and on shell photons to me?... I don't quite understand them fully.

 

[Phrak]

Any "off-shell" (otherwise known as virtual) photon you'd measure becomes on-shell, hence must move with speed c, as all massless particles. Quantum electrodynamics=quantum mechanics+special relativity, so it'd better respect special relativity.

Not at all. Measurement or detection has nothing to do with time evolution of fields.

 

[Phrak]

Do you have some references for this assertion? Specifically that:

• Photons don't travel at c
• Photons have variable velocity
• Photons have a range determined by that velocity

Feynman has a short popular book QED you might like. I suppose you could look up QED in the wikipedia.

 

[negru]

Not at all. Measurement or detection has nothing to do with time evolution of fields.

Are you serious? Do you know what an S-matrix is? You can't detect an off-shell particle, pretty much by definition.

 

[inflector]

I suppose you could look up QED in the wikipedia.

I'm somewhat familiar with QED and just reread the Wiki page:

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

I don't follow all of it, but I didn't see anything about velocity of photons being variable. Is this somehow implied for virtual photons somewhere in the math?

 

[negru]

Yes he's referring to virtual photons (probably). Pretty much due to the uncertainty principle, it's possible for virtual particles to temporarily violate conservation of energy, and not obey the classical equations of motion (this is what off-shell means). However, as soon as you measure a virtual particle it has to become on shell. It follows from the LSZ reduction formula

 

[Kothar]

A proton hits a molecule or an atom and because of the collision it gives energy to the collided object. And as I know the proton reflects from the surface.
In this case;
1- does the proton have the same energy before the collision
2- if it is absorbed by the collided object since mass turns into energy does that mean that the proton turns into mass.

 

[tijaska]

Talking of red-shifted photons, does anyone know if virtual photons also get red-shifted when they climb out of a gravity well? When a virtual photon encounters an electrostatic charge, it transfers momentum to it, either attractive or repulsive. When a "real" photon is red-shifted its momentum is reduced, so I'm guessing that the same would be true for a virtual photon as well.

 

[mrspeedybob]

Looking at the existence of a photon: can a free photon truly exist? That is, a photon which is just flying through free space awaiting something to interact with? I am thinking of a photon created long ago, and traveling through space today. If the photon was formally an excitation of a field - and not an independent entity itself - then this wouldn't happen... would it? There would need to be a start point and an end point. The end point being some spot in the distant future in which there is an interaction.

If I understand you correctly you are postulating that a photon could not exist without definite beginning and end points. This would mean that if a hot body were to exist in a hypothetical, otherwise empty universe it would never cool off because there would be no other matter in the universe for the energy to be radiated to. In our universe it means that If a hot body were to be located in a part of the universe so devoid of matter that the nearest object was receding at the speed of light, that body could not radiate.

Given that we exist in a part of the universe where, if a free photon could exist, it would have a certain probability of eventually interacting and a certain probability of never interacting, and that probability spread will shift towered never interacting as the universe expands and our visible portion of it gets less densely populated. It should be possible to construct an experiment that would compare the radiation rate of two hot bodies at different times and determine if there was a difference due to the expansion of the universe. If there is a difference then free photons cannot exist. If there isn't then either free photons can exist OR we live in a universe that will eventually stop expanding giving every photon an opportunity to interact and therefore exist.

Has anybody done an experiment like this?

 

[Naty1]

Phrak's comment likely has nothing to do with the OP's intent ...only he/she knows for sure...but is interesting...

For example, cosmic microwave background radiation (photons or quanta of electromagnetic radiation) has been around for 99%+ the age of the universe and will be present until the end...such photons don't decay as far as is known today...and electromagnetic radiation extends asymtotically to infinity as 1/r²...unless the universe 'ends' ...

You can read about virtual photons with mass and limited range here:

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

and also follow the link in the article for definitons of on shell and off shell...

The descriptions of such static force fields via virtual particles seems rather artifical...maybe a result of our imprecise mathematical models?...or things may be even stranger than we suspect! Anyway, it's all theoretical because we can't measure virtual anything.

 

[Naty1]

This would mean that if a hot body were to exist in a hypothetical, otherwise empty universe it would never cool off because there would be no other matter in the universe for the energy to be radiated to.

not so...Our sun cools...radiates away it's energy, via electromagnetic radiation... whether or not the Earth is nearby to be warmed by that radiation...the only criteria for cooling is that the sun be hotter than it's (empty, vacuum) surroundings...an example would be black hole (Hawking) radiation which does not currently take place because black holes are colder than empty space as measured by the 2.7 degree CMBR...

 

[Naty1]

I never thought of it this way before: virtual photons have no life while "real" photons have unending lives...

 

[Orionabc]

Looking at the existence of a photon: can a free photon truly exist? That is, a photon which is just flying through free space awaiting something to interact with? I am thinking of a photon created long ago, and traveling through space today. If the photon was formally an excitation of a field - and not an independent entity itself - then this wouldn't happen... would it? There would need to be a start point and an end point. The end point being some spot in the distant future in which there is an interaction.

I wonder exactly that too. It makes sense to talk about the point at which a photon originates and the point at which it is detected. Anything in between has to be inferred and in order to infer it we assign a plethora of physical properties to it without any way of measuring them. We dogmatically pronounce...in order for a proton to originate at point A and register its existence at point B (notice I am avoiding the words create and annihilate here) it must persist in space and time between those events. Our evidence is our own experience of what it means personally to exist. In actuality what we have is a high degree of correlation between two events.

 

[DrChinese]

If I understand you correctly you are postulating that a photon could not exist without definite beginning and end points. This would mean that if a hot body were to exist in a hypothetical, otherwise empty universe it would never cool off because there would be no other matter in the universe for the energy to be radiated to. In our universe it means that If a hot body were to be located in a part of the universe so devoid of matter that the nearest object was receding at the speed of light, that body could not radiate.

Given that we exist in a part of the universe where, if a free photon could exist, it would have a certain probability of eventually interacting and a certain probability of never interacting, and that probability spread will shift towered never interacting as the universe expands and our visible portion of it gets less densely populated. It should be possible to construct an experiment that would compare the radiation rate of two hot bodies at different times and determine if there was a difference due to the expansion of the universe. If there is a difference then free photons cannot exist. If there isn't then either free photons can exist OR we live in a universe that will eventually stop expanding giving every photon an opportunity to interact and therefore exist.

Has anybody done an experiment like this?

Yes, this is pretty much my question. If there can be free photons then presumably there is some amount of light traveling along that will never interact with anything. I think of this as a halo of unseen light. But if that were true, then all of the other paths that light could take would not exist as there could be no contingency. On the other hand, once all of the possible interactions (with matter in the vicinity) come to nothing: does that constitute an observation of sorts?