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Does light have an infinite lifetime?

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Re: Does light have an infinite lifetime?

#11

Post by Nevermark »

supportengineer wrote: post #6
Nevermark wrote: post #4

Earlier quoted context omitted.

> You cannot decay if you don’t experience time. Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either. The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events. Given that interpretation, it might b…

Would that seem like a fold in spacetime to the photon?

I can't top the sibling comment about a summer breeze! But it is an interesting question.

Not only does the photon not experience any delay between its two end points, but it experiences its path between them as a simple shortest-distance straight line segment, even if the same path looks like a curve through gravitationally warped space-time to us.

The photon does experience a form of distance, i.e. the number of wave lengths between its ends. But just the number of cycles, not the actual wave lengths which we would see varying as we experienced dark energy and space stretching the photon's wavelength from our viewpoint.

So a photon "experiences" two spacially separated ends, and a number of wave cycles between them, and that's it? Perhaps.

Re: Does light have an infinite lifetime?

#12

Post by ryandamm »

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 out.

I also seem to recall that the speed of light below c is actually the group velocity, and each individual photon still would move at c. I'm also not entirely sure if photons can be said to exist except at creation and absorption; isn't a photon a phenomenon best described by particle interactions, and moving through free space it's more correctly described as a field? Genuine question, though I somehow doubt I'd understand any good elaboration.

IANAPhysicist, though. I just play with light recreationally.

Re: Does light have an infinite lifetime?

#13

Post by amluto »

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 would he hard-pressed to put an observer in the box who thinks the photons last very long.

Re: Does light have an infinite lifetime?

#15

Post by amelius »

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.

But if your clock doesn't tick, then infinite lifetime means the same as zero lifetime.

From the point of view of the photon, time doesn't even exist. So it is pointless to ask the question from the point of view of the photon.

Re: Does light have an infinite lifetime?

#16

Post by codyd51 »

barfbagginus wrote: post #10
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?

The photino birds want the universe to be populated with white dwarfs, because they can feed off their gravity wells without the risk of deadly supernovas and black holes, which can kill them. After the xelee and humans leave, the universe becomes a cold place dominated by photino birds living in the cold pinpricks of white dwarfs. Eventually matter evaporates into photons, and the photino birds die. However, it turn…

One potential hiccup with your one-photino-bird-universe theory (which is quite fun!): I believe I remember a scene in which the ‘birth’ of a photino bird was described. If I remember correctly, it was indeed described as a clone of its parent.

Re: Does light have an infinite lifetime?

#17

Post by cwillu »

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.

Re: Does light have an infinite lifetime?

#18

Post by pixl97 »

Nevermark wrote: post #4
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.

> You cannot decay if you don’t experience time. Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either. The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events. Given that interpretation, it might b…

>But if photons couldn't decay due to not experiencing time, they couldn't do anything else either.

I mean we are jumping way out of the classical behavior that objects like you and I exist in. To the photon itself is a timeless object. It 'moves' in a null geodesic where t=0. Attempting to apply any classical behavior that occurs in time-like objects just isn't going to work when applying them to massless light-like objects.

Re: Does light have an infinite lifetime?

#19

Post by adrian_b »

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 conversion rules between the coordinates in such a system and those in a normal reference system.

Therefore it is not correct to say that the lifetime of a photon in a reference system attached to it is zero or infinite or it has any other value.

This lifetime is just undefined, while the lifetimes in any other reference systems are well defined.

The photon does not decay in the absence of interactions with other particles because that would violate several conservation laws. However, when the photons have energies that are high enough, the interaction between themselves can generate other particles, in particle-antiparticle pairs, in order to satisfy all conservation laws.

Re: Does light have an infinite lifetime?

#20

Post by nyrikki »

Nevermark wrote: post #4
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.

> You cannot decay if you don’t experience time. Interesting take! But if photons couldn't decay due to not experiencing time, they couldn't do anything else either. The reality is that a photons creation and destruction are not prohibited, but simply "experienced" as two events at different locations at the same time, with the photon being the "thing" that connects those events. Given that interpretation, it might b…

From a photons perspective, emition and absorption are simultaneous.

While from our perspective it is a form of causal connection, that is mearly due to the frame of reference.

While we can infer the connection between each, it is possibly better to consider the speed of light as the speed of causality.

But as there are no privileged reference frames under GR the choice is yours.

But from the photons perspective, it doesn't experience time at all so it can't be a barrier.

But don't confuse the map for the territory. GR is a model, not the system itself.

The fact that almost every test we can figure out has only confirmed it doesn't change that.

Under the 'all models are wrong but some are useful' idea, in GR photons not experiencing time is important to that model.

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