# Does light experience time?

1. Jun 26, 2013

### 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?

2. Jun 26, 2013

### 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").

3. Jun 26, 2013

### Staff: Mentor

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.

4. Jun 26, 2013

### Staff: Mentor

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.

5. Jun 26, 2013

### naturale

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).

Last edited: Jun 26, 2013
6. Jun 26, 2013

### 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 )

Last edited: Jun 26, 2013
7. Jun 26, 2013

### Naty1

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'.

8. Jun 26, 2013

### 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?

9. Jun 26, 2013

### 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.

10. Jun 27, 2013

### 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.

11. Jun 27, 2013

### nitsuj

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)

Last edited: Jun 27, 2013
12. Jun 27, 2013

### Naty1

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

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

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.

Last edited: Jun 27, 2013
13. Jun 27, 2013

### Staff: Mentor

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

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.

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

14. Jun 27, 2013

### nitsuj

That explains it!, Thanks Naty1

15. Jun 27, 2013

### Staff: Mentor

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:

16. Jun 27, 2013

### tolove

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

17. Jun 27, 2013

### Staff: Mentor

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.

18. Jun 27, 2013

### wotanub

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.

19. Jun 27, 2013

### Phy_Man

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)

20. Jun 27, 2013

### nitsuj

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."

21. Jun 27, 2013

### naturale

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 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).

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.

Last edited: Jun 27, 2013
22. Jun 27, 2013

### wotanub

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.

23. Jun 27, 2013

### naturale

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.

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.

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

24. Jun 27, 2013

### Staff: Mentor

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

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.

25. Jun 27, 2013

### naturale

: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.

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.