Does a photon experience time when slowed down by gravity?

[MR GREY]

Does light experience time? If you were a beam of light would you experience time?

That's pretty much my question. From what I've heard light would not experience time because it's going at light speed but...

What happens when light slows down? Does it experience time then?

As gravity slows light down, light 'should' experience more and more time, right?

 

[naturale]

The internal clock of photons is frozen. According to the wave-particle duality, particles are clock with ticks of periods T = h / m c^2 (Compton time) determined by the particles' mass. As the photon has zero mass, it is a clock that takes an infinite time to do a tick, so it has no time.

However light defines the underlying casual structure to determine events in spacetime. It allows us to compare the ticks of clocks (e.g. the internal clocks of the particles or the atomic clocks) and determine the relative time of different objects that experience time (clocks with finite periodicity).

This is an intuitive way to answer your question. I hope this help (read R. Penrose "the cycles of time").

 

[PeterDonis]

Light doesn't really "experience" anything, since it isn't conscious, but I'll assume that by "experience" you really mean something objectively measurable, like the reading on a clock.

As far as "time" goes, any object, whether light or ordinary matter, can be described by a worldline, a curve in spacetime that describes its history--where it is in space at each instant of time. For an ordinary object, like you or me or a rock, this curve has nonzero length between any two distinct points (which are called "events"). This length is called the "proper time" experienced by the object between those two events.

Light, however, or anything that moves at the speed of light, has a worldline which has zero length between any two distinct points. (This is possible because the geometry of spacetime is different from the ordinary Euclidean geometry you're used to.) Sometimes this is described as light "experiencing zero time", but it would be more accurate to say the concept of "proper time" simply doesn't apply to light.

 

[PeterDonis]

The internal clock of photons is frozen. According to the wave-particle duality, particles are clock with ticks of periods T = h / m c^2 (Compton time) determined by the particles' mass. As the photon has zero mass, it is a clock that takes an infinite time to do a tick, so it has no time.

No, this is not correct. See my previous post. Also, your understanding of wave-particle duality is incorrect; photons have finite frequencies and wavelengths, even though they have zero rest mass.

 

[naturale]

No, this is not correct. See my previous post. Also, your understanding of wave-particle duality is incorrect; photons have finite frequencies and wavelengths, even though they have zero rest mass.

Yes, but if you are a photon, you travel at speed of light and your ticks are infinite, so you don't experience time, your clock is frozen (your "rest" energy is zero and your have zero frequency in your worldline, according to your answer).

My answer is absolutely correct.

 

[robphy]

Although a lightlike worldline (say, between events A and C, with C in the future of A) has zero square-interval so that no proper-time ticks off, if B is an event on that worldline [between A and C], there is still a causal ordering that A happens, then B happens, then C happens.

That is one piece of an old post of mine ( from an old thread "Photon's Perspective of Time" from 2006 )
https://www.physicsforums.com/showthread.php?p=899778#post899778

 

[Naty1]

What happens when light slows down? Does it experience time then?

As gravity slows light down, light 'should' experience more and more time, right?

Light always moves at c locally in a vacuum. It may APPEAR to slow down when observed from a distance because of spacetime curvature...but an observer right there will observe it at it's usual 'c'. And it may appear to slow down in a medium, but even there individual photons still move at 'c'.

 

[MR GREY]

I still need to think over the rest of the posts but just a quick question about Naty1's post.

Does light still move at c in a black hole?

 

[Khashishi]

Neutrinos were previously thought to be massless and travel at the speed of light. Since we have measured neutrino oscillation, this requires that neutrinos "experience time" and therefore move at less than the speed of light. This is why neutrinos are thought to have mass now, although the masses haven't been measured.

Light still moves at c in a black hole.

 

[sWozzAres]

Is it the case that when a photon "travels" from point A to point B, then due to length contraction, the distance from A to B effectively shrinks to zero. The photon then has no distance to travel, therefore it takes no time to get there.

Also, since a photon gets absorbed by an electron and then later emitted, does this imply that it will "experience" time while it is trapped inside the atom.

 

[nitsuj]

Also, since a photon gets absorbed by an electron and then later emitted, does this imply that it will "experience" time while it is trapped inside the atom.

Can't help but think the photon has literally been absorb, it is no longer a photon. Then the atom is "uncomfortable" and "poops" out a bit of energy...a photon. :tongue2:

That's a neat question. I wonder what changes about the atom while it has this "extra" energy. Does it have more mass? Or does the electron(s) just gain more momentum and it's "orbit" is not stable given the charge of the nucleus? (thinking that)

 

[Naty1]

Quote by sWozzAres

Also, since a photon gets absorbed by an electron and then later emitted, does this imply that it will "experience" time while it is trapped inside the atom.

It is not trapped as an identifiable entity...a photon always moves at 'c'...

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

When a photon is absorbed the electromagnetic field of the photon disappears and the state of the system that absorbs the photon changes. Energy, momentum, angular momentum, magnetic dipole moment and electric dipole moment are transported from the photon to the system. Because there are conservation laws, that have to be satisfied, the transition has to meet a series of constraints. This results in a series of selection rules. It is not possible to make any transition that lies within the energy or frequency range that is observed...Applications[edit]

Materials with broad absorption bands are ... in pigments, dyes and optical filters. Titanium dioxide, zinc oxide and chromophores are applied as UV absorbers and reflectors in sunscreen...

edit: Any photon subsequently emitted is a different photon..a 'new'particle...that might well reflect other energy changes that have taken place between absorption and emission.

 

[PeterDonis]

Yes, but if you are a photon, you travel at speed of light and your ticks are infinite

No, they aren't. If you think they are, show us the math. Where in the math are the infinite ticks?

your "rest" energy is zero

A photon can never be at rest, so the concept of rest energy (or rest mass) doesn't apply to it. The correct statement is that a photon has a 4-momentum with zero length.

your have zero frequency in your worldline

Wrong. As I said before, a photon has a finite frequency and wavelength.

 

[nitsuj]

That explains it!, Thanks Naty1

 

[PeterDonis]

Is it the case that when a photon "travels" from point A to point B, then due to length contraction, the distance from A to B effectively shrinks to zero. The photon then has no distance to travel, therefore it takes no time to get there.

No. The concept of length contraction comes from Lorentz transformations, and the Lorentz transformation is singular (i.e., mathematically invalid) for a relative velocity of c. So there is no way to define "distance relative to a photon".

Another way of putting this is that a photon has no "rest frame"; we have a forum FAQ on this:

https://www.physicsforums.com/showthread.php?t=511170

 

[tolove]

A photon can never be at rest, so the concept of rest energy (or rest mass) doesn't apply to it. The correct statement is that a photon has a 4-momentum with zero length.

Could you explain this concept in more detail, Peter? I'm unfamiliar with Four-momentum.

 

[PeterDonis]

Could you explain this concept in more detail, Peter? I'm unfamiliar with Four-momentum.

Try this Wikipedia page:

http://en.wikipedia.org/wiki/Four-momentum

The "length" I'm talking about is what this page calls the Minkowski norm; a photon's 4-momentum has a Minkowski norm of zero.

 

[wotanub]

That's a neat question. I wonder what changes about the atom while it has this "extra" energy. Does it have more mass? Or does the electron(s) just gain more momentum and it's "orbit" is not stable given the charge of the nucleus? (thinking that)

The electron that absorbs the energy is put into a different quantum state. This changes things like the probability density of where it could be observed.

 

[Phy_Man]

Could you explain this concept in more detail, Peter? I'm unfamiliar with Four-momentum.

4-momentum is what is called a 4-vector and every particle has one. The 4-momentum of a particle is defined as either the product of the particle's 4-velocity and the particles proper mass. It has the value P = (mc, p) where m is the particle's inertial mass. 4-velocity U is defined as U = dX/dT where T is proper time and dX is the spacetime displacement dX = (cdt, dx, dy, dz)

 

[nitsuj]

The electron that absorbs the energy is put into a different quantum state. This changes things like the probability density of where it could be observed.

Naty1 pull from wiki explained the change to the "system" very clearly


"Energy, momentum, angular momentum, magnetic dipole moment and electric dipole moment are transported from the photon to the system."

 

[naturale]

No, they aren't. If you think they are, show us the math. Where in the math are the infinite ticks?

See discussion in How a particle tells time (Holger Mueller et al in *Science*).

The ticks of a particles of energy $E(p)$ and momentum $p$ has period $T(p) = \frac{h}{E(p)}$. At zero momentum $p\equiv 0$ you get the Compton time, that is the recurrence along its propertime $T(0) = \frac{h}{E(0)}= \frac{h}{M c^2}$. Light which has zero mass has infinite periodicity in the propertime (or worldline).

A photon can never be at rest, so the concept of rest energy (or rest mass) doesn't apply to it. The correct statement is that a photon has a 4-momentum with zero length.

A photon can have zero momentum. Since it is massless, at zero momentum it has zero energy $E = c p$, so it has a frozen clock (infinite Compton time).

Wrong. As I said before, a photon has a finite frequency and wavelength.

A photon is massless and can have zero frequency and zero wavenumber: its Compton time is frozen.

It is surprising how simple concepts are so misunderstood when expressed in a slightly different way with respect graduate's textbooks.

 

[wotanub]

A photon can have zero momentum. Since it is massless, at zero momentum it has zero energy, so it has a frozen clock (infinite Compton time).

A photon is massless and can have zero frequency and zero wavelength: its Compton time is frozen.

A zero momentum photon would be no photon at all.

If it has no frequency, that means the EM field isn't oscillating, meaning there is no photon.

λ = c/f. If frequency and wavelength are both zero, doesn't that mean the wave speed is indeterminate? Which isn't true.

Physics is the study of matter and energy. If it isn't matter, and it doesn't have energy, then it doesn't exist.

 

[naturale]

A zero momentum photon would be no photon at all.

If it has no frequency, that means the EM field isn't oscillating, meaning there is no photon.

In principle you can have a photon (or observe a photon in a reference frame) such that its momentum tends to zero, that is, its wavelength tends to infinity. If you try to go in the reference frame of the photon you see that the photon doesn't experience time, it has frozen clock. This was one of the keypoints in Einstein's derivation of relativity.

λ = c/f. If frequency and wavelength are both zero, doesn't that mean the wave speed is indeterminate? Which isn't true.

Sorry, here there was a typo. I meant that photons can have zero frequency and wavenumber, or that equivalently that the temporal period and wavelength tend to infinity.

Physics is the study of matter and energy. If it isn't matter, and it doesn't have energy, then it doesn't exist.

It is not my fault that photons have zero mass (rest energy)!

 

[Nugatory]

In principle you can have a photon (or observe a photon in a reference frame) such that its momentum tends to zero, that is, its wavelength tends to infinity.

"Tends to", yes. But that doesn't mean it gets there.

If you try to go in the reference frame of the photon you see that the photon doesn't experience time, it has frozen clock. This was one of the keypoints in Einstein's derivation of relativity.

No, that is not a key point of Einstein's derivation of relativity.

He started with two postulates (the principle of relativity and the invariance of the speed of light) and from these derived a set of transformations (the Lorentz transformations) which relate coordinates in a frame in which some object is at rest to coordinates in a frame in which something else is at rest - and these transformations disallow any frame in which a photon could be at rest.

 

[naturale]

"Tends to", yes. But that doesn't mean it gets there.

:yuck:
This discussion is becoming obtuse. Study the concept of limit. Neglecting possible physics at the Planck scale and assuming that electromagnetic gauge invariance is not broken in nature, the possible momentum for the electromagnetic field is from zero to infinity. I am not saying that photon can be at rest, they always travel at speed c, I am considering photon with zero momentum.

No, that is not a key point of Einstein's derivation of relativity.

He started with two postulates (the principle of relativity and the invariance of the speed of light) and from these derived a set of transformations (the Lorentz transformations) which relate coordinates in a frame in which some object is at rest to coordinates in a frame in which something else is at rest - and these transformations disallow any frame in which a photon could be at rest.

To derive special relativity and its postulate Einstein thought to the problem of the time experienced by light, realizing that it doesn't experience time. Lorentz formulated his transformations several years before Einstein.

 

[PeterDonis]

See discussion in How a particle tells time (Holger Mueller et al in *Science*).

This is just an experiment to precisely measure the Compton frequency of a massive particle. Photons do not have a Compton frequency; the whole point of the Compton frequency is that it is the frequency of a photon whose energy is equal to the rest energy of the massive particle.

Light which has zero mass has infinite periodicity in the propertime (or worldline).

No, it doesn't, because the concept of Compton frequency does not apply to a photon. A photon's frequency and wavelength are related to its energy by the Planck formula: $E = h \nu = h c / \lambda$.

A photon can have zero momentum.

I didn't say zero momentum, I said zero 4-momentum. They are different things. A photon with zero ordinary momentum is not possible, since a photon cannot be at rest.

A photon is massless and can have zero frequency and zero wavenumber: its Compton time is frozen.

Incorrect (except for the "massless"). See above.

It is surprising how simple concepts are so misunderstood when expressed in a slightly different way with respect graduate's textbooks.

It is surprising how people can think they understand simple concepts when they actually don't. You really need to review basic relativity and quantum mechanics before making further pronouncements.

 

[PeterDonis]

If you try to go in the reference frame of the photon you see that the photon doesn't experience time, it has frozen clock.

No, you find that the concept of "reference frame of the photon" doesn't make sense. We have a forum FAQ on this:

https://www.physicsforums.com/showthread.php?t=511170

This was one of the keypoints in Einstein's derivation of relativity.

Do you have an actual reference for this claim? It looks to me like you are misinterpreting something.

 

[PeterDonis]

the possible momentum for the electromagnetic field is from zero to infinity.

Not including the endpoints. A momentum of infinity makes no physical sense, and a photon can't have zero momentum because it can't be at rest.

I am not saying that photon can be at rest, they always travel at speed c, I am considering photon with zero momentum.

Which is impossible if a photon can't be at rest.

 

[DrGreg]

...realizing that it doesn't experience time.

That doesn't mean "experiences a time equal to zero". It means the concept of time relative to a photon doesn't make sense, it's undefined.

 

[Nugatory]

Lorentz formulated his transformations several years before Einstein.

Indeed he did - that's why we call them the Lorentz transformations. Einstein didn't discover them, Einstein discovered that they could be derived from his two postulates.

And when I read Einstein's 1905 paper I'm not finding a lot of support for your clam that "If you try to go in the reference frame of the photon you see that the photon doesn't experience time, it has frozen clock. This was one of the keypoints in Einstein's derivation of relativity".

 

[Simon Bridge]

[zero momentum] is impossible if a photon can't be at rest.

... it is more proper to consider the long wavelength limit isn't it?

Similarly, though the reference frame of a photon makes no sense, we can ask what things look like at ultra-relativistic speeds.

 

[Phy_Man]

That doesn't mean "experiences a time equal to zero". It means the concept of time relative to a photon doesn't make sense, it's undefined.

Myself, I question what "a photon experiences time" means.

 

[PeterDonis]

... it is more proper to consider the long wavelength limit isn't it?

A long finite wavelength is fine. An infinite wavelength is not.

Similarly, though the reference frame of a photon makes no sense, we can ask what things look like at ultra-relativistic speeds.

Sure, but that's not the same thing.

 

[PeterDonis]

The 4-momentum of a particle is defined as either the product of the particle's 4-velocity and the particles proper mass.

This is only true for particles with nonzero rest mass. It doesn't work for photons, but photons still have a perfectly well-defined 4-momentum.

 

[Simon Bridge]

A long finite wavelength is fine. An infinite wavelength is not.

Is there a limit to how long the wavelength can be?
Of course an infinite wavelength is a horizontal line ... you don't have a wave (and other issues).

It can be sensible to talk about a limit to a value even though the value itself is not attainable and does not make sense in the context.

Sure, but that's not the same thing.

And I'm not saying that - it's just a way forward. Now people get to say whether this is the sort of thing intended and will remember to be more careful with language in future. It's a common-enough mistake.