Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Does light have an infinite lifetime?

bigthink.com

Post reply on HN
Print view View original

91–100 of 106 posts

Re: Does light have an infinite lifetime?

#91

Post by fnordpiglet »

_factor wrote: post #83
doodlebugging wrote: post #37

Where do the photons that hit my retina go when I finish with them? If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes? If photons are infinite then harvesting light would…

What makes you so sure the photon didn’t originate from your eye and finish where you saw them?

I assume you’re referring to Feynmans demonstration that since photons are neutral with no antiparticle in QED a photon going forward in time and backwards in time are the same photon. But from our frame of reference the photon always originates at the origin and hits our eye. From the photons point of reference it’s not meaningful tho to say they go backwards or forwards because time actually doesn’t elapse for them.

Re: Does light have an infinite lifetime?

#92

Post by fnordpiglet »

dataflow wrote: post #70
henry2023 wrote: post #67

From the perspective of a photon, there is no such thing as time. It's emitted, and might exist for hundreds of trillions of years, but for the photon, there's zero time elapsed between when it's emitted and when it's absorbed again. It doesn't experience distance either.

Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren'…

This is because a photon has no mass and always travels at the speed of light in all reference frames. A photon never observes time because the effects of time dilation at the speed of light is infinite and due to relativity they don’t experience the events between their emission and absorption as they don’t decelerate when they’re absorbed. In a very real sense from the photons perspective they never exist and don’t experience a temporal dimension. However from a frame of reference less than the speed of light - which all objects of mass experience - we can observe their creation, propagation, and absorption.

Reality is stranger than fiction!!

Re: Does light have an infinite lifetime?

#93

Post by fnordpiglet »

Leftium wrote: post #87
dataflow wrote: post #70

Earlier quoted context omitted.

Something about this doesn't make sense. This means that, from its own perspective, every photon is emitted at time T at location x, then absorbed at time T at location y, meaning there is no time "in between" those... yet somehow, at a distinct point in between those events, an object can get in the way and block the photon from reaching point y. Meaning you have two points in time that have the same value but aren'…

If something blocks the photon, that was "agreed" upon before the photon was created. A photon travels between an electron that emits it and an electron that absorbs it. These two electrons must agree on the photon transfer beforehand. This has some strange implications: - When you see a star at night, it's because photons from the distant star hit your eyes. - These photons started traveling billions of years ago, w…

While I think you can conceive of it this way I don’t think it’s precisely right. Photons don’t experience time at all and never exist from their own perspective as they have no mass and always travel at the speed of light in all reference frames. However causality at speeds less than the speed of light is a real thing and due to special relativity we can interact with photons with a different simultaneity. This is partially because they can’t experience the events of the universe going less than the speed of light due to the fact they absorb rather than decelerate.

This doesn’t mean there’s some cross time agreement between particles mediated by the photon because the reference frame of those particles are different than the photons and are necessarily slower than the speed of light.

Re: Does light have an infinite lifetime?

#94

Post by lolc »

Kranar wrote: post #59
lolc wrote: post #56

Earlier quoted context omitted.

My understanding is that photons don't have a life the way we do. They move at the speed of light and thus time does not advance for them. They cannot change between emission and absorption, no matter the distance. Always bends my mind to think about it.

Almost everything you said is correct. Photons do not have a reference frame, so time does not advance because for a photon there is no time coordinate system in the first place. It's not simply that photons don't experience time, it's that time and space don't exist for photons. >They cannot change between emission and absorption, no matter the distance. From the point of view of a photon, neither time or space exis…

Not to quibble about definitions of what "change" means, but I thought the red shift depended entirely on the relative speed of the emitting and receiving body? A laser beam for example could not be analyzed for its red shift, unless we knew the original frequency, because there would be no spectral absorption pattern to determine the shift. So we cannot tell how far laser light traveled before it reached our sensor.

Re: Does light have an infinite lifetime?

#95

Post by out_of_protocol »

dylan604 wrote: post #41
adonovan wrote: post #38

Earlier quoted context omitted.

Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted. He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)

Electrons? We were just told that there is only one single electron in yesterday's posts. It's almost like they don't know what an electron is

> only one single electron

> an electron

THE electron? /s

Re: Does light have an infinite lifetime?

#96

Post by credit_guy »

adonovan wrote: post #38
doodlebugging wrote: post #37

Where do the photons that hit my retina go when I finish with them? If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes? If photons are infinite then harvesting light would…

Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted. He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)

Does Feynman say that? If an electron absorbs a photon, why would it create another one of the same frequency, and with polarization perfectly rotated by 90°?

Re: Does light have an infinite lifetime?

#98

Post by wholinator2 »

fnordpiglet wrote: post #90
doodlebugging wrote: post #37

Where do the photons that hit my retina go when I finish with them? If they have infinite lifetimes then does each one carry a memory of it's creation event? When I burn a log in my fireplace and a cinder flares and pops creating a spark, does the photon exist after the spark energy flares out and the cinder is no longer illuminated enough to be detected by my eyes? If photons are infinite then harvesting light would…

Photons don’t experience time, from their relativistic frame they are created and destroyed instantly. This is because they are massless and therefore always travel at the speed of light. Due to relativity they experience a different simultaneity - in our reference frame we can observe their emission and absorption as energy and existence and measure them etc but, even given they can’t experience anything anyways, ti…

Right, this would be true if it were possible for a photon to"experience", but at i understand it we currently believe it's not.

Re: Does light have an infinite lifetime?

#99

Post by candiodari »

credit_guy wrote: post #96
adonovan wrote: post #38

Earlier quoted context omitted.

Feynmann is very clear that the photons bouncing off a mirror are not the same ones that hit it. They are absorbed by electric oscillators (free electrons in the material) and destroyed; then new ones are created and emitted. He's also careful enough to point out that electrons don't really have an identity that would allow you to meaningfully define "not the same". :)

Does Feynman say that? If an electron absorbs a photon, why would it create another one of the same frequency, and with polarization perfectly rotated by 90°?

Because of the layout of the electron orbitals.

Re: Does light have an infinite lifetime?

#100

Post by Kranar »

lolc wrote: post #94
Kranar wrote: post #59

Earlier quoted context omitted.

Almost everything you said is correct. Photons do not have a reference frame, so time does not advance because for a photon there is no time coordinate system in the first place. It's not simply that photons don't experience time, it's that time and space don't exist for photons. >They cannot change between emission and absorption, no matter the distance. From the point of view of a photon, neither time or space exis…

Not to quibble about definitions of what "change" means, but I thought the red shift depended entirely on the relative speed of the emitting and receiving body? A laser beam for example could not be analyzed for its red shift, unless we knew the original frequency, because there would be no spectral absorption pattern to determine the shift. So we cannot tell how far laser light traveled before it reached our sensor.

Not a quibble at all. The redshift I'm referring to is the kind of redshift due to gravity, as opposed what you're describing which is redshift due to the Doppler effect.
Post reply on HN