Photons lack mass, how can they have a half life?

Question

While reading an article this morning concerning the recent and long awaited discovery of the Higgs Boson particle, a statement about massless particles and the idea that from their perspective time does not exist piqued my curiosity.

"Massless particles are doomed by Einstein's theory of relativity to travel at the speed of light. That means, for them, that the past, the present and the future are the same thing."

So my question is this:
If photons lack mass, and particles devoid of mass do not experience time, and decay requires time, how can photons have a half life?

Answer 1

You have good responses so far. Photons have no half life. We have photons coming to us that are 13 billion years old, almost as old as we believe the Universe to be. And they are coming to us in numbers that show no "half life decay".



Additionally, for a photon to decay, it would face conservation laws. So it would have to conserve momentum, spin, and charge, in which three or more particles would have to be involved. The only way we have seen this work in this Universe is interaction with a charge... electron + photon -> fast moving electron, or electron -> photon + recoiling electron. There are photon-virtual photon interactions, but no interactions of a photon simply disappearing, taking its momentum and spin out of the Universe.



One thing I'd like to point out, is that the equations that apply to massive particles *cannot* be applied to light. We cannot say what "time sense" photons might have. We do know they do proportionally react to the space through which they pass, and the apparatus that might flip their polarization. It is hard to do "proportional" and do "in no time at all".

Answer 2

The physicsforum site debates the issue.



Opinion: Photons do not have a half life. How do you observe a group of photons to observe and document a decrease in number of photons present for 6 (?) + billion years.



Are we to conclude that the number of photons arriving from the edge of the observable universe is less than the number of photons emitted from the source? I think not



Edit: A random thought. Photons do lose energy to gravity and if they lose all their energy they disappear. This is normally called "Red-Shift." In the case of Black Holes, we can view the photons not escaping as the photons lose all their energy to the gravity well of the warp in space-time. I would not call that a half life though. The photons do not decay to something else, they just lose all their energy and disappear.

Answer 3

AWolf on the blog is mistaken. Photons do not have a half life.

Evidence: Hubble telescope can see visual light 13.75 Billion years away (meaning into the past, thanks to the speed of light).



Photons (waves) do stretch out, red shift, over distance/time, so they are feet long when they get here from there and are deeply into the infrared end of the spectrum. I don't understand that part yet, but it happens.



What I want to know is this: Physicists claim this universe began with the Big Bang, a point of singularity. And that the universe is slightly less than 14 Billion years old. So when we look 13.75 years into the past, why do we see an ever-expanding universe instead of a much smaller infantile universe? The farther we look the bigger it is, when science claims it should be smaller in the past.



The experts are talking out their a_s and have to invent things we can't detect like dark matter and energy to make their math work. THUS SAYETH the high School graduate! ;)



You might be interested in a better unified field theory. If the author is right, we can throw an anchor into the heart of Creation and pull out infinite electrical energy... Meaning we can build Starship Enterprise and no longer have to burn oil.

(Well, except we haven't invented a warp drive yet... ;)

http://gravitydrivenuniverse.org/

http://www.fhu.com/books/finding_god/

http://www.fhu.com/books/finding_god/findinggod_review.html

Answer 4

Free photons don't decay; they effectively have an infinite half-life. They are emitted and are then stable (up to the time they get absorbed).



The 2nd link you give does not refer to photon half-life. Maybe you are confusing proton and photon? Protons do have mass and don't travel at the speed off light.

Answer 5

Who says photons have a half life? Ok... somebody mentions it in a forum entry. That's not a scientific citation. It's just someone who doesn't know what they're talking about asking a question.

Answer 6

They are of a mass yet unmeasurable by our tech.

The ability to calculate half life requires the measure of mass degradation. So it's incalculable in the past or present. In the future an accurate measure may be found through an enlarged triangulation of observation.

LED's do burn out eventually.

Answer 7

That is nonsense. The reference you give (AWolf) is by an idiot with no knowledge of physics.



And go learn what "half life" means and how it comes about and you'll realize that your question is senseless.



You might as well have asked :" What religion are school buses?" Grammatically correct, but meaningless.

Answer 8

Photons do not have a half life. They can be absorbed or emitted, but free photons are stable and do not decay.