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Does light have an infinite lifetime?

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Re: Does light have an infinite lifetime?

#21

Post by pixl97 »

amluto wrote: post #13
colechristensen wrote: post #3

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

I think this is sort of besides the point. If you build a box, paint the walls black, and put a flashlight in the box, then the photons coming from the flashlight are shorter lived than if you shine the flashlight into the sky on a cloudless day or night. Not shorter lived from their own perspective — shorter lived from an outside observer’s perspective. Sure, one could quibble about the choice of observer, but you w…

Why is the speed of causality beside the point?

lets take two magical particles that have clocks on it. One is a photon and the other is a neutrino. I send these off towards you in a perfect vacuum. When you receive these particles the clock on the photon will be 0. It will be be the exact same photon that left my emitter, it will not have changed in any way as it did not interact with anything along the way. And as long as you are not moving relative to me, you'll perceive the photon as the same color/wavelength I emitted it at.

Meanwhile that neutrino will arrive billions of a second later (well depending on our distance) and will have oscallated at least trillions of times if not far more. The clock on the neutrino will have ticked the difference between the photon arrival to the neutrino arrival.

Don't apply classical behavior to light-like objects. They play be different sets of rules.

Re: Does light have an infinite lifetime?

#23

Post by ars »

cwillu wrote: post #17
gwern wrote: post #2

Interesting. I had no idea that the universe would eventually be basically just photons. I wonder if that is why Stephen Baxter had his 'photino birds' as the final form of life in his Xeeleverse?

I believe proton decay is still considered hypothetical, and the electron is pretty confidently believed to be stable; https://profmattstrassler.com/articles-and-posts/particle-ph... is a good overview.

If protons do decay, their decay product would include a positron that would in turn annihilate with the electron. Leaving a universe with just photons and neutrinos.

If neutrinos are their own antiparticle then they could react with each other and produce ??? Not sure, because you would have to find out of the lepton number is conserved.

Re: Does light have an infinite lifetime?

#24

Post by Modified3019 »

ryandamm wrote: post #12
colechristensen wrote: post #3

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

Hoping a physicist can correct me here, but... I believe index of refraction is a function of the photon being absorbed and reemitted by the electrons in the dielectric material, so it's no longer correct to think of a photon moving at a fraction of the speed of light inside the material, it's more like a churning series of them being created (always moving at c) but constantly being absorbed and canceling each other…

For those curious about what the above poster is talking about, here’s a well done video explaining the topic of the apparent changes of light’s “speed” through materials

3Blue1Brown - But why would light "slow down"? | Optics puzzles 3: https://m.youtube.com/watch?v=KTzGBJPuJwM

Re: Does light have an infinite lifetime?

#25

Post by TheOtherHobbes »

colechristensen wrote: post #3

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

I wish people would stop repeating this.

If light is emitted and absorbed at the same instant, how does it know when/where in spacetime it's been emitted and absorbed?

You cannot apply SR to light in this naive way. SR works just fine for anything with mass, but without a theory of quantum gravity no one has the first clue how massless particles operate in spacetime, or what they do or don't "experience."

Re: Does light have an infinite lifetime?

#26

Post by akira2501 »

adrian_b wrote: post #19
colechristensen wrote: post #3

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

From the point of view of any observer, a photon has a definite lifetime, between the moments of its emission and its absorption. The Lorentz transformations are defined only between reference systems where the relative speed between them is less than the speed of light. It is not possible to attach a reference system to a photon or to any other particle that moves with the speed of light, because there are no conver…

If it's travelling at c then isn't the length contraction infinite? Or is that dependent on Lorentz transformations as well?

Re: Does light have an infinite lifetime?

#28

Post by adrian_b »

akira2501 wrote: post #26
adrian_b wrote: post #19

Earlier quoted context omitted.

From the point of view of any observer, a photon has a definite lifetime, between the moments of its emission and its absorption. The Lorentz transformations are defined only between reference systems where the relative speed between them is less than the speed of light. It is not possible to attach a reference system to a photon or to any other particle that moves with the speed of light, because there are no conver…

If it's travelling at c then isn't the length contraction infinite? Or is that dependent on Lorentz transformations as well?

Length contraction and time dilation are words that describe changes that are the consequence of a Lorentz transformation.

Like I have said, the formulae of a Lorentz transformation are defined only when the relative velocity between the two systems is less than the speed of light.

Attempting to pass to a limit when the relative speed approaches the speed of light does not produce any useful result, because at the limit you no longer obtain a reference system, so you no longer get a transformation between reference systems.

Without a reference system, there is no meaning for the concepts of distance and time.

Any reference system for the 4-dimensional space-time must be attached to normal matter made of leptons and quarks, it cannot be attached to photons. In any reference system for the 4-dimensional space-time, the photons are particles that move with equal speeds in space and in time, while the normal matter moves faster in time than in space. The notion of proper time (i.e. the time measured for an object that moves only in time, without moving in space) is not defined for photons, because they always also move in space, not only in time.

This should be obvious from the rule introduced by Einstein that the speed of light is the same in all possible reference systems, from which the Lorentz transformations can be deduced. If a reference system were attached to a photon, in that reference system the speed of light could not have the same value as in the normal reference systems, so within Einstein's theory such a reference system cannot exist.

Re: Does light have an infinite lifetime?

#29

Post by akomtu »

Isn't it a meaningless question? Photons aren't things, but loosely localized areas of motion energy that's temporarily assumed the shape of a photon. Upon collision with other similar waves, it may change shape and become another partickey like an electron. But the light itself is a motion itself, which is a pure abstraction. At the end of the universe, photons probably will keep spreading over larger and larger areas, slowly turning into a uniform sea of that pure motion.

Re: Does light have an infinite lifetime?

#30

Post by bequanna »

colechristensen wrote: post #3

Huh? Light has zero lifetime, along with all massless particles. When you travel at c your clock doesn’t tick. From the viewpoint of a photon it is emitted and absorbed at the same instant. You cannot decay if you don’t experience time. I’m not sure exactly how index of refraction works with this.

Right, but doesn’t light travel at less than c in some situations (passing through glass, etc)? Would we say they experience “time” in those cases?
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