photon

[charlie-d]

So maybe someone can help me with a science question. Photons travel at the speed of light. As long as they are in our frame of reference that seems to make sense, except that they have these strange properties that they are both waves and particles and produce interference patterns and don't decide where they want to be until observed and seem to be able to communicate with each other in impossible ways. Yet mathematically if we apply our formulas to a photon from it's own frame of reference the photon doesn't have a frame of reference and time and place are undefined. So is it just a coincidence that these strange properties of photons seem to be "time" related? That in our frame of reference the spin of one knows the spin of the other from the same event even if the spin is changed, and the photon seems to be wave probabilities at one point in our frame of reference time and particle actualities in another point in our frame of reference time? How could these strange properties not be related to the fact that a photon has no time in its own frame of reference? Isn't there a missing piece somewhere?

[ArmillarySphere]

Several strange things here...

1) Photons don't communicate with each other. In fact, they don't interact at all (unless you start talking about their fields overlapping, but that's a result of their reception, not the photons themselves)
2) Nor do they have spin. Are you thinking about electrons?
3) The same photon can't be observed both for wave and particle properties. As soon as you observe it, you have absorbed it, ending its existence. Depending on the circumstances, the atom that absorbed it may immediately release another one in response, but that's a different photon.

You are correct to say that the proper frame of a photon is a little strange, though. In that frame, the universe has length zero in the direction the photon would be travelling. If it could, since the universe is also going to have a lifespan of zero. It doesn't make much sense to describe events in that frame, which is why you usually don't bother to.

[Fredrik]

Several strange things here...

1) Photons don't communicate with each other. In fact, they don't interact at all (unless you start talking about their fields overlapping, but that's a result of their reception, not the photons themselves)
2) Nor do they have spin. Are you thinking about electrons?
3) The same photon can't be observed both for wave and particle properties. As soon as you observe it, you have absorbed it, ending its existence. Depending on the circumstances, the atom that absorbed it may immediately release another one in response, but that's a different photon.

You are correct to say that the proper frame of a photon is a little strange, though. In that frame, the universe has length zero in the direction the photon would be travelling. If it could, since the universe is also going to have a lifespan of zero. It doesn't make much sense to describe events in that frame, which is why you usually don't bother to.
There are some strange things here too. :)

1. Actually they do. Think of a Feynman diagram with four straight lines forming a square, and four squiggly photon lines going out from the corners. The straight lines represent virtual electrons and positrons.

2. They have spin 1.

3. Since photons are identical, it doesn't quite make sense to say that it's a different photon. (It wouldn't make any more sense to say it's the same photon. It's just a photon in a specific quantum state, and we can't say more than that).

4. It's not just that it doesn't make much sense to describe events in that frame. There is no such frame. A frame must assign four numbers (coordinates in time and space) to each event, and this "frame" can't.

[charlie-d]

There are some strange things here too. :)

1. Actually they do. Think of a Feynman diagram with four straight lines forming a square, and four squiggly photon lines going out from the corners. The straight lines represent virtual electrons and positrons.

2. They have spin 1.

3. Since photons are identical, it doesn't quite make sense to say that it's a different photon. (It wouldn't make any more sense to say it's the same photon. It's just a photon in a specific quantum state, and we can't say more than that).

4. It's not just that it doesn't make much sense to describe events in that frame. There is no such frame. A frame must assign four numbers (coordinates in time and space) to each event, and this "frame" can't.


Do electrons travel at the speed of light?

[charlie-d]

There are some strange things here too. :)
4. It's not just that it doesn't make much sense to describe events in that frame. There is no such frame. A frame must assign four numbers (coordinates in time and space) to each event, and this "frame" can't.

Do you have an "interpretation" for what this means. I have a friend whose daughter is a physicist. When she was starting in school she would have discussions with him about what things like this "meant", but now she has graduated and doesn't do that any more.

[charlie-d]

Do you have an "interpretation" for what this means. I have a friend whose daughter is a physicist. When she was starting in school she would have discussions with him about what things like this "meant", but now she has graduated and doesn't do that any more.

All the forces in nature are not in space and time (from their own point of view) and light is a sort of half-thing. Starts sounding absolutely biblical.

[sol invictus]

4. It's not just that it doesn't make much sense to describe events in that frame. There is no such frame. A frame must assign four numbers (coordinates in time and space) to each event, and this "frame" can't.

Hang on - if I'm understanding what you're saying, that's not quite right. There's no problem going to a lightlike frame (in fact it's a very common and useful thing to do). The appropriate coordinates are called null coordinates.

You can imagine making a coordinate system out of lots of pulsed laser beams, so that near any point there's always three pulses intersecting. That specifies the spacetime point - the pulses are numbered, and each beam is directed in a certain direction.

Do electrons travel at the speed of light?

No. Massive particles travel at strictly less than the speed of light, and electrons have a mass.

I didn't understand your original question, but it sounded as though part of it had to do with entanglement. It might help to realize that electrons can be entangled in much the same way as photons - so that has nothing to do with traveling at the speed of light.

[charlie-d]

Hang on - if I'm understanding what you're saying, that's not quite right. There's no problem going to a lightlike frame (in fact it's a very common and useful thing to do). The appropriate coordinates are called null coordinates.

You can imagine making a coordinate system out of lots of pulsed laser beams, so that near any point there's always three pulses intersecting. That specifies the spacetime point - the pulses are numbered, and each beam is directed in a certain direction.



No. Massive particles travel at strictly less than the speed of light, and electrons have a mass.

I didn't understand your original question, but it sounded as though part of it had to do with entanglement. It might help to realize that electrons can be entangled in much the same way as photons - so that has nothing to do with traveling at the speed of light.

Thanks. I am getting the giste. Hard for us scientifically challenged folk.

So we know gravity travels at the speed of light, right? If the sun disappeared the earth would keep rotating for the amount of time it takes light to get from the sun to the earth. (I've been told). So does that mean that all the forces (weak, strong, etc) travel at the speed of light?

[Cuddles]

Thanks. I am getting the giste. Hard for us scientifically challenged folk.

So we know gravity travels at the speed of light, right? If the sun disappeared the earth would keep rotating for the amount of time it takes light to get from the sun to the earth. (I've been told). So does that mean that all the forces (weak, strong, etc) travel at the speed of light?

No. Photons are massless, so they travel at the speed of light. Gravitons have not been observed, but if they exist they must also be massless since gravity has an infinite range, and will therefore also travel at the speed of light. The force carriers for the strong and weak forces are both massive particles, and so they can neither have infinite range or travel at the speed of light.

[sol invictus]

So we know gravity travels at the speed of light, right? If the sun disappeared the earth would keep rotating for the amount of time it takes light to get from the sun to the earth. (I've been told). So does that mean that all the forces (weak, strong, etc) travel at the speed of light?

No. Photons are massless, so they travel at the speed of light. Gravitons have not been observed, but if they exist they must also be massless since gravity has an infinite range, and will therefore also travel at the speed of light. The force carriers for the strong and weak forces are both massive particles, and so they can neither have infinite range or travel at the speed of light.

Cuddles is correct that the weak force is mediated by massive particles, but the strong force is mediated by particles called gluons which are in fact massless. However it doesn't make much sense to ask at what speed they propagate because of a phenomenon called confinement. Basically you'd never be able be able to measure the strong force on distances large enough to determine how fast it propagated - it only acts on subatomic length scales.

As for gravity, the existence of gravitons is not relevant to this. Classical gravity waves (which have been experimentally confirmed indirectly, and almost certainly exist) propagate at the speed of light.

Think of it like this: suppose any event takes place at a certain place and time (a bomb goes off, someone grabs a lead weight and moves it back and forth, or whatever). The first sign of that event, no matter what it is, will arrive at a distant point at precisely the same time the light from it does. Later more stuff might arrive too, but all of it is confined within what's called the future light cone, or cone of influence, of that event (which is the solid expanding sphere defined by a pulse of light emitted in all directions at the moment the event took place).

[robinson]

Photons are considered to have no rest mass.

[sol invictus]

Photons are considered to have no rest mass.

No, they are known to have no rest mass. The limits on the mass of the photon are incredibly good.

[robinson]

According to current theory, photons have no rest mass. They do have relativistic mass.

[Yllanes]

Hang on - if I'm understanding what you're saying, that's not quite right. There's no problem going to a lightlike frame (in fact it's a very common and useful thing to do). The appropriate coordinates are called null coordinates.

I think he just means you can't do a boost to a reference system with v = c, which would be something like the rest frame of a photon.

[sol invictus]

I think he just means you can't do a boost to a reference system with v = c, which would be something like the rest frame of a photon.

I guess there are two different questions. One is whether it's possible to describe spacetime in coordinates in which photons can be at fixed coordinate position in three of the coordinates (just like a particle at rest). The answer to that is yes - those are called null coordinates.

Another question is whether it makes sense to take a limit of infinite velocity and describe spacetime from the point of view of someone moving at that speed. The answer to that is again yes, under some circumstances at least. Look up Penrose limit and infinite momentum frame.

[Fredrik]

Hang on - if I'm understanding what you're saying, that's not quite right. There's no problem going to a lightlike frame (in fact it's a very common and useful thing to do). The appropriate coordinates are called null coordinates.

I had completely forgotten about null coordinates. I googled and found this explanation in a post by "pervect" at physicsforums.com:


In an abstract sense, one can construct coordinate systems in which the trajectory of a photon is stationary.

These are called "null coordinates" and the transformation rules are quite simple. Using geometrized units in which c=1, the transformations to null coordinates are just:

u = x-t
v = x+t

These coordinates are perfectly valid mathematically, and are even used in General Relativity. They describe a coordinate system in whch a photon moving along the x axis is represented by a single number (u, or v, depending on which way the photon is going).

Combined with standard 'y' and 'z' coordinates, one can construct a complete 3-d coordinate system out of these null coordinates.

This abstract mathematical description is probably as close as one can come to ascribing a "point of view" to a photon. Note that this coordinate system does not have any such thing as a "time" coordinate - instead, one has two null coordinates (one for photons moving in the +x direction, another for photons moving in the -x direction), and two spatial coordinates.
(That guy always makes very high quality posts in the relativity forum). OK, I understand now that it makes some sense to think of these coordinates as the photon's frame, since a photon is stationary in these coordinates.

What I had in mind when I said what I said is that it doesn't make sense to let v go to 1 in the usual Lorentz transformation:

\scriptsize\begin{pmatrix}t' \\ x'\end{pmatrix}=\gamma\begin{pmatrix}1 & -v\\ -v & 1\end{pmatrix}\begin{pmatrix}t \\ x\end{pmatrix}

I thought that's what "the photon's frame" meant.

[sol invictus]

One thing to bear in mind is that it's not that the trajectory of a photon is stationary in both null coordinates - it is stationary in one, but not the other. But that's no different than a massive particle at rest, which is at fixed spatial position but "moving" in time, so it's a good analogy to a rest frame.

As for the limit of infinite boost, it does make sense sometimes and has been used for many purposes over the years. It is subtle, though.

[Fredrik]

charlie-d, you might find this page interesting: http://math.ucr.edu/home/baez/physics/Relativity/SpeedOfLight/headlights.html.

[Fredrik]

As for the limit of infinite boost, it does make sense sometimes and has been used for many purposes over the years. It is subtle, though.
I didn't know about that, but I don't mind learning something new. :)

[Yllanes]

I guess there are two different questions. One is whether it's possible to describe spacetime in coordinates in which photons can be at fixed coordinate position in three of the coordinates (just like a particle at rest). The answer to that is yes - those are called null coordinates.

Another question is whether it makes sense to take a limit of infinite velocity and describe spacetime from the point of view of someone moving at that speed. The answer to that is again yes, under some circumstances at least. Look up Penrose limit and infinite momentum frame.

Of course there are subtle ways to handle these sorts of things. But I think things like the infinite boost limit are out of place in this thread. I think the audience calls for an elementary exposition where the naive way of defining a rest frame for a photon (a boost with v = c) doesn't work.

[Ziggurat]

According to current theory, photons have no rest mass. They do have relativistic mass.

Relativistic mass is redundant with energy. Rest mass (aka invariant mass) is not. Pretty much nobody uses relativistic mass anymore because it's useless, and "mass" without any qualifier always means invariant mass.

[NobbyNobbs]

As for gravity, the existence of gravitons is not relevant to this. Classical gravity waves (which have been experimentally confirmed indirectly, and almost certainly exist) propagate at the speed of light.


Ok, to expand on this, I have a question of my own that's been bugging me.

We've established that, should the sun disappear, the earth would continue to revolve for another 8 minutes. That is, gravity moves at the speed of light.

The escape velocity of a black hole is greater than the speed of light. Yet, gravity "escapes" a black hole; that is, something in the vicinity of the black hole is affected by the gravity of the hole and is pulled in. This couldn't happen unless the gravity waves were able to escape.

Does this mean that gravity waves move *faster* than light? If not, then how can a black hole affect its surroundings?

Thanks.

[Ziggurat]

The escape velocity of a black hole is greater than the speed of light. Yet, gravity "escapes" a black hole; that is, something in the vicinity of the black hole is affected by the gravity of the hole and is pulled in. This couldn't happen unless the gravity waves were able to escape.

There's a distinction between the field itself and changes in the field (which are what gravity waves are). The field itself exists because the mass is there. Gravity waves don't need to come from inside the event horizon, and if the black hole isn't accelerating, there won't be any gravity waves at all. For the same reason, black holes can also have electric fields if they have a net charge.

Another way of thinking about this is to consider the fact that when the black hole is first collapsing, the information about the soon-to-be black hole's mass is already out there in the form of its field far from the object. When the collapse happens, that information doesn't disappear, and it need not travel through the event horizon because it already exists outside it.

[RecoveringYuppy]

Gravity waves and gravity are two different things. Gravity waves are a propagation of the change of the gravitational field and they don't escape a black hole (another way of saying we can't detect any motion of the mass inside the BH).

[sol invictus]

Does this mean that gravity waves move *faster* than light? If not, then how can a black hole affect its surroundings?


To add something to the two posts above, here's a very interesting and relevant fact about black holes: they have a gravitational field which is absolutely identical to the field of a star of the same mass (until you get very close). So the only thing you can determine from that field is the mass, and actually the angular momentum as well if the star/black hole is rotating. Other than that, the only other characteristic they can possess is an electric field due to a net charge. So all black holes are characterized by three quantities: mass, charge, and angular momentum, and their fields are identical to the fields of any other spherical object with the same mass, charge, and angular momentum.

When you put that fact together with the existence of Hawking radiation, you end up with an information paradox. The resolution is that quantum mechanically, black holes differ from each other, but that's a long and convoluted story. The short version is that your intuition is almost right - all black holes look almost exactly the same, because nothing can escape from them to differentiate between them.

[RecoveringYuppy]

I'm trying to increase my knowledge of physics by attempting to make the same post as Ziggurat at the same time. Almost have the timing down.

[ArmillarySphere]

I see that I have some reading to do before I comment on quantum mechanics again. It's been well over 10 years since I studied it, and I never moved beyond the basics.

I was thinking of the null coordinate system, but I didn't realise that the equations had two solutions. Go figure.

[Olowkow]

I'm trying to increase my knowledge of physics by attempting to make the same post as Ziggurat at the same time. Almost have the timing down.

I now know as much as Sol and Zig...I just know it now..and they knew it then.

I'm working on post-posting their posts.:)

[charlie-d]

I now know as much as Sol and Zig...I just know it now..and they knew it then.

I'm working on post-posting their posts.:)

Thanks everyone. That was more than helpful. I read through the posts, but I can't say I understand them all. I have to reread the null coordinates and see if I can understand better. I was of the impression that the frame of reference was more like "undefined" for a photon point of view. That the math stopped working.

[Ziggurat]

I was of the impression that the frame of reference was more like "undefined" for a photon point of view. That the math stopped working.

For ordinary (non-null) coordinates, you can get from one reference frame to another using Lorenz transformations, where there's a term in the transform for the velocity. If you plug in c for v to those transforms, you do indeed get something that stops working, so your impression isn't without basis. Null coordinates really are different from ordinary coordinates, and the transform to get to them isn't a Lorenz transform.

[charlie-d]

Think of it like this: suppose any event takes place at a certain place and time (a bomb goes off, someone grabs a lead weight and moves it back and forth, or whatever). The first sign of that event, no matter what it is, will arrive at a distant point at precisely the same time the light from it does. Later more stuff might arrive too, but all of it is confined within what's called the future light cone, or cone of influence, of that event (which is the solid expanding sphere defined by a pulse of light emitted in all directions at the moment the event took place).

So I guess the question is what is the "cone of influence" from a photon point of view. Does a photon have all information that ever was, or does it have no information, or is the question ridiculous?

[Cuddles]

The escape velocity of a black hole is greater than the speed of light. Yet, gravity "escapes" a black hole; that is, something in the vicinity of the black hole is affected by the gravity of the hole and is pulled in. This couldn't happen unless the gravity waves were able to escape.

Does this mean that gravity waves move *faster* than light? If not, then how can a black hole affect its surroundings?

Thanks.

To add to what everyone else has already said, Gauss' law is very helpful when thinking about this sort of thing. One of the important consequences of Gauss' law is that the electric field outside a spherically symmetric body does not depend on the radius of the body. An equivalent formulation applies to gravity as well. So, for example, it makes no difference if the Sun is the size it actually is, compressed down to a football or only just smaller than the Earth's orbit, the Earth would still fell exactly the same force and orbit in exactly the way.

When this is applied to black holes, you can see that it doesn't make any difference if the mass is concentrated at a point in the centre, inside the event horizon, or spread around a sphere outside the event horizon, the graviational field seen by anything outside the black hole will be exactly the same.

[sol invictus]

So I guess the question is what is the "cone of influence" from a photon point of view. Does a photon have all information that ever was, or does it have no information, or is the question ridiculous?

Photons can be created and absorbed just like any other particles. They carry (a little) information about the event that produced them - nothing more.