Why do people keep saying photons are timeless?

[weezy]

If you search for "does a photon experience time", almost every other link says that they travel at the speed of light and so STR tells us that its clock doesn't tick at all. However why do they use the arguments for special relativity which was developed for massive particles moving close to the speed of light and extrapolate them to at speed of light? As far as I remember the time dilation formula was devised by Einstein on analyzing the light clock which always moved less than the speed of light. If the clock now runs at velocity 'c' then the pulse of light bouncing off one mirror completely misses out the second mirror during the instantaneous acceleration of the light clock apparatus from subluminal to luminal speed and our clock thus 'breaks'. Instead if we have a light clock which always moved at the speed of light, then a massless observer moving along the light clock, were he to shoot a pulse of light towards the bottom mirror would just rebound fine and he would experience normal time if he chooses to ignore the rest of the universe outside his frame. This last bit is given light still holds the same meaning as it does at subluminal observer speeds.

I don't know if my reasoning is sound but I will say that claiming the photons don't experience time is just speculation and not a fact. We don't know about if relativity applies at speeds of light

 

[Orodruin]

If you search for "does a photon experience time", almost every other link says that they travel at the speed of light and so STR tells us that its clock doesn't tick at all.

There is no clock that can move at the speed of light. Photons do not have a rest frame.

This colloquial statement (generally in popular science) stems from the length of the world-line (which is the proper time for time-like world lines) is zero for light-like world lines.

As far as I remember the time dilation formula was devised by Einstein on analyzing the light clock which always moved less than the speed of light.

The light-clock is an argument for popular science and introductory relativity. The time dilation formula can be devised without reference to an actual constructed clock from the first principles behind SR.

Instead if we have a light clock which always moved at the speed of light, then a massless observer moving along the light clock, were he to shoot a pulse of light towards the bottom mirror would just rebound fine and he would experience normal time if he chooses to ignore the rest of the universe outside his frame.

Some very common misconceptions. Photons do not have rest frames and there is no such thing as "outside a frame". In SR, everything exists in all frames. Different frames are just different tools for describing the same physical situation. If something happens in one frame, it must happen in all frames.

 

[Ibix]

We don't know about if relativity applies at speeds of light

Special relativity applies to everything, in the absence of gravity.

However, that the speed of light is constant in all inertial reference frames is one of the principles of relativity. So a rest frame for light - where it is stationary - is a contradiction in terms. So trying to describe what a photon does or does not experience is impossible - there is no way to "put yourself in its shoes".

That doesn't seem to stop people trying.

 

[I like Serena]

Hmm... actually photons would only travel at the speed of light in vacuum - and true vacuum does not exist - so they can't.
So in practice photons would always go a bit slower, since they're always in a medium.
I wonder... doesn't that mean that the clock does tick for them after all?

 

[Biker]

Hmm... actually photons would only travel at the speed of light in vacuum - and true vacuum does not exist - so they can't.
So in practice photons always go a bit slower, since they're always in a medium.
I wonder... doesn't that mean that the clock does tick for them after all?

I don't think that is true. Light is always moving at c but the light wave is slower in mediums. There are a lot of explanations such as absorption and emitting but that is wrong.

The only one that is true is something about group velocity.

 

[SiennaTheGr8]

Hmm... actually photons would only travel at the speed of light in vacuum - and true vacuum does not exist - so they can't.
So in practice photons always go a bit slower, since they're always in a medium.
I wonder... doesn't that mean that the clock does tick for them after all?

I don't think it's true that a photon ever moves slower than $c$.

I'm no expert(!), but my understanding is that the QM explanation of light propagating through a medium involves not slower-than-$c$ photons but rather "quasiparticles" (polaritons?) that are something like a superposition of photons and the excited particles in the medium (excited because they're interacting with the photons). It's these quasiparticles that you might say travel slower than $c$.

Perhaps someone who knows better could correct me or improve my phrasing.

(I believe that a common incorrect explanation is the delay between an atom's absorption and emission of a photon—i.e., that the photons travel at $c$ from atom to atom but are delayed every time they're absorbed.)

 

[DrStupid]

So in practice photons would always go a bit slower, since they're always in a medium.
I wonder... doesn't that mean that the clock does tick for them after all?

Practical photons are irrelevant for clocks. In the second postulate Einstein refers to plane light waves in vacuum. And it is not even neccecary to refer to light at all. It is sufficient to say that there is an invariant speed. This speed can be determined experimentally and it appears to be equal to the speed of light in vacuum.

 

[stoomart]

I don't know if my reasoning is sound but I will say that claiming the photons don't experience time is just speculation and not a fact.

I imagine one needs to seriously study relativity before claiming it is speculation. My understanding of photons being timeless comes from posts like the one from @Orodruin, and researching the issue myself. Per Wikipedia's Spacetime article:

Spacetime intervals are zero when $x=\pm ct$. In other words, the spacetime interval between two events on the world line of something moving at the speed of light is zero. Such an interval is termed lightlike or null. A photon arriving in our eye from a distant star will not have aged, despite having (from our perspective) spent years in its passage.

• Time by itself and length by itself are not invariants, since observers in relative motion will disagree on the time between events or the distance between events.
• On the other hand, observers in relative motion will agree on the measure of a particular combination of distance and time called the "spacetime interval."
• Spacetime intervals can be positive, negative or zero. Particles moving at the speed of light have zero spacetime intervals and do not age.
• Spacetime diagrams are typically drawn with only a single space and a single time coordinate. The time axis is scaled by $c$ so that the space and time coordinates have the same units (meters).

 

[GrayGhost]

If you search for "does a photon experience time", almost every other link says that they travel at the speed of light and so STR tells us that its clock doesn't tick at all. However why do they use the arguments for special relativity which was developed for massive particles moving close to the speed of light and extrapolate them to at speed of light?

Hello weezy, nice to make your acquaintance.

The reason they do that, is because there is no other option to predict a speed c POV. The photon cannot be held in a state of rest, and thus has no inertial POV (even though it travels at a steady speed). We only know what material POVs experience. We cannot know what a photon's experience is. So we ask the "what if" ... "what if" a material POV could attain a relative c, what would its experience be? Or, "what if" a particle of rest mass does exist that always travels at c (never seen to date)? All that we can do, is to explore what the mathematics suggests said experience should be. We consider an SR thought experiment, for higher and higher speeds, to determine the trend in the relativistic effects, then we extrapolate kinematically what the experience should be in the limit as v attains c.

In the initial instant an inertial wonder-POV attains a relative c (call it a wonder clock), it hands then reach a zero tick-rate per normal inertial cosmic POVs. Let's say the clock's hands then read 2:30. Observers record the clock to move at c thru 3-space over duration, the clock's hands never moving always reading 2:30 as it travels. So per normal inertial observers, the speed-c clock should not experience the passage of time as it goes, since its hands never move while at c. But wrt any finite traversal distance considered, and per the wonder clock itself, it is everywhere AT ONCE along its propagation path for the portion of its path while its hands read 2:30. Existing at multiple contiguous locations AT ONCE is to traverse a finite distance in zero duration, ie an infinite velocity. AT ONCE, requires no passage of time. This is why it is said that the hands of a clock carried by a photon never move, or that a photon does not experience the passage of time.

Yet, photons do not possesses a material POV since they cannot be held in a state of rest. As such, it is said that the extrapolated experience of a speed c wonder clock cannot be known to correctly represent the actual experience of a photon, assuming it could have such a thing. There are other obstacles as well, physically speaking. For example, to attain a relative speed c wrt a single cosmic body, you must also attain a relative c wrt every other cosmic body. It must all happen AT ONCE per you. You do not attain a speed c wrt this guy, then later wrt that guy, and so on. As such, and logically speaking (and besides the fact that all heavenly material entities would be infinite in mass per you), it seems quite impossible to attain a relative speed c. A relative speed c plays the role of an infinite velocity (per SR) ... and it's most difficult to imagine infinities in nature.GrayGhost

 

[Orodruin]

My understanding of photons being timeless comes from posts like the one from @Orodruin

I would certainly hope not, because that was not my intended meaning at all. It was meant as an explanation of why people might say things like that. Also, Wikipedia is generally not considered a reliable source.

 

[Ibix]

So we ask the "what if" ... "what if" a material POV could attain a relative c, what would its experience be? Or, "what if" a particle of rest mass does exist that always travels at c (never seen to date)?

Something with mass traveling at c is another contradiction in terms (it must have a null energy-momentum vector because that's what traveling at c means - but mass is the modulus of the energy-momentum vector, and the modulus of a null vector is zero). So what this quote boils down to is "we try to imagine the logical consequences of contradicting ourselves". That's why the outcome is inevitably nonsense.

 

[puzzled fish]

This is going to be my last post in PhysicsForums as I have decided to go.

One must be very careful when dealing with infinities. Does infinity minus infinity when both x and t are infinite in spacetime interval, tend to zero? Or does 0 multiple infinite times ( for our little trivial measurements of our theories) tend to zero?

 

[weezy]

This is going to be my last post in PhysicsForums as I have decided to go.

One must be very careful when dealing with infinities. Does infinity minus infinity when both x and t are infinite in spacetime interval, tend to zero? Or does 0 multiple infinite times ( for our little trivial measurements of our theories) tend to zero?

I can say for sure that an infinite sum of 0s will always be 0. Not sure about the rest.

 

[weezy]

I don't think that is true. Light is always moving at c but the light wave is slower in mediums. There are a lot of explanations such as absorption and emitting but that is wrong.

The only one that is true is something about group velocity.

Why is the absorption emitting explanation wrong? How do you explain refraction at photon level then?

 

[Biker]

Why is the absorption emitting explanation wrong? How do you explain refraction at photon level then?

Lets assume that the situation is true, We know the atoms absorb specific wavelengths, right? so the refractive index of these wavelengths should change drastically because they will get absorbed a lot which means delayed but that doesn't happen. Yes the refractive index depend on the wavelength but not as discrete but smoothly. Also specific wavelength shouldn't be absorbed so I guess no change for the speed of light?

2nd thing, Absorbing and re-emitting is just a random process. It takes different amount of time. You could say that it kinda averages out but I don't think that happens( I need more insight on that). Whenever you measure the time it takes light to pass it is always constant.

 

[DrStupid]

Why is the absorption emitting explanation wrong?

As Biker already mentioned, repeatedly absorption and emission would result in a random walk and that doesn't fit to experimental observations.

How do you explain refraction at photon level then?

By describing the photon as an elecromagnetic wave. The speed of electromagnetic waves depends on the permittivity and permeability of the environment. And these properties change in presence of matter.

 

[SiennaTheGr8]

https://physics.stackexchange.com/q...fter-coming-out-of-a-glass-slab/153907#153907

 

[I like Serena]

Wiki explains that an electromagnetic pulse has a front speed, a phase speed, and a group speed (link).
And the front speed (under certain assumptions) is always equal to c, the speed of light in vacuum.
The phase speed and group speed deviate because the electromagnetic wave interacts with the particles in the medium.

To be honest, I still don't quite get this, since it seems to suggest that whether we shoot a laser through a vacuum or through a medium, that the first effect of the waves arrive on the other side at the same time.

 

[vanhees71]

Well, don't read only Wikipedia but also some good physics book ;-). For the classical dispersion theory, have a look in

A. Sommerfeld, Lectures on theoretical physics, vol. 4 (Optics)

Sommerfeld and Brillouin wrote two very famous papers to clarify the apparent problem with the special theory of relativity and dispersion in the realm of anomalous dispersion, where the phase as well as the group velocity formally get $>c$. The true answer is that, in this realm the group velocity looses its usual physical meaning, because the corresponding approximate solution of the involved Fourier integrals, using the saddle-point method, is not applicable in this region. Of course, the full solution is always causal in accordance with special relativity. Last but not least it's one of the most beautiful applications of complex-function theory (theorem of residues) ever . You find it all in Sommerfeld's textbook.

I'm not aware, whether the original articles by Sommerfeld and Brillouin have ever been translated to English (in any case they should have been!). Here are the German references:

A. Sommerfeld, Über die Fortpflanzung des Lichtes in dispergierenden Medien, Ann. Phys. (Leipzig), 349 (1914), p. 177–202.
http://dx.doi.org/10.1002/andp.19143491002

L. Brillouin, Über die Fortpflanzung des Lichtes in dispergierenden Medien, Ann. Phys. (Leipzig), 349 (1914), p. 203.
http://dx.doi.org/10.1002/andp.19143491003

 

[haushofer]

Ask for an operational definition of how much time elapsed for a photon. That will shut them up. :P

 

[stoomart]

I would certainly hope not, because that was not my intended meaning at all. It was meant as an explanation of why people might say things like that. Also, Wikipedia is generally not considered a reliable source.

I understand, unfortunately I neglected to put "timeless" in quotes.

 

[Mister T]

Instead if we have a light clock which always moved at the speed of light,

You can't have that as it would violate the postulates upon which the light clock derivation is based. Time dilation approaches infinity as speed approaches $c$. Saying it is infinite at speed $c$ is wrong. It's wrong physically because time-measuring devices can't move at speed $c$ and it's wrong mathematically because a function approaching infinity means the value of the function increases beyond all bounds; it's nonsense to say the value of the function is infinite.

 

[Mister T]

Hmm... actually photons would only travel at the speed of light in vacuum - and true vacuum does not exist - so they can't.

By that logic the speed of light in a vacuum doesn't exist, either, so your statement is self-contradictory.

The fact that there is no such a thing as a pure vacuum does not prevent us from measuring and knowing the speed of light in a vacuum. There's no such thing as an exact value for the measurement of a circle's area, but that doesn't stop us from knowing its area is exactly $\pi r^2$.

 

[I like Serena]

Well, don't read only Wikipedia but also some good physics book ;-).

Erm... I wasn't planning on a dissertation on how light behaves in a medium...
If wiki is wrong, please enlighten us. I don't mind updating the wiki article as soon as I know what's wrong with it...

Ask for an operational definition of how much time elapsed for a photon. That will shut them up. :P

Erm... who are you trying to shut up?

 

[I like Serena]

By that logic the speed of light in a vacuum doesn't exist, either, so your statement is self-contradictory.

The fact that there is no such a thing as a pure vacuum does not prevent us from measuring and knowing the speed of light in a vacuum. There's no such thing as an exact value for the measurement of a circle's area, but that doesn't stop us from knowing its area is exactly $\pi r^2$.

The speed of light in vacuum is a limit. The way to measure it, is to measure it in thinner and thinner mediums, and extrapolate it into vacuum.
In other words, it's not real - it's an extrapolation to get to a nice constant.

 

[stoomart]

The speed of light in vacuum is a limit. The way to measure it, is to measure it in thinner and thinner mediums, and extrapolate it into vacuum.
In other words, it's not real - it's an extrapolation to get to a nice constant.

A bunch of smart people beat this topic into the ground for me in the recent thread "Light and its ideal speed".https://www.physicsforums.com/threads/light-and-its-ideal-speed.920769/#post-5808314

 

[vanhees71]

Erm... I wasn't planning on a dissertation on how light behaves in a medium...
If wiki is wrong, please enlighten us. I don't mind updating the wiki article as soon as I know what's wrong with it...

Ok, I admit that I wasn't able to write my long planned Insights article about photons (lack of time), but it should be clear that I cannot write a textbook on QED (including many-body QED) in the form of forum postings or Insights articles. Also I think one should first understand classical electrodynamics in its right form, i.e., as a relativistic field theory. That's why I gave hints, where to find the physics in good textbooks and original research articles (admittedly over 100 years old, but obviously the people haven't sufficiently taken care to read these articles in the last 100 years, so it doesn't hurt to quote them).

 

[spacecadet11]

Wiki explains that an electromagnetic pulse has a front speed, a phase speed, and a group speed (link).
And the front speed (under certain assumptions) is always equal to c, the speed of light in vacuum.
The phase speed and group speed deviate because the electromagnetic wave interacts with the particles in the medium.

To be honest, I still don't quite get this, since it seems to suggest that whether we shoot a laser through a vacuum or through a medium, that the first effect of the waves arrive on the other side at the same time.

Hmm...Light in a vacuum can move 'without' being hindered..it accomplishes it's task by getting to the other side of the medium in a set time..which would be it's velocity.
I light wave traveling in a medium..is being hindered in it's travels..which means it will take 'longer' to accomplish the task of getting to the other side of the medium. That's considered a 'slower speed'.
I believe I read once that nature 'conspires' such though..that it will get to the other side of the medium in the 'same set time'. If it doesn't..you will simply not 'see' the light...the medium is opaque.