The Nature of Photons: Do They Truly Travel Between Points A and B?

[jnorman]

Do photons actually "travel"?

since, at the speed of light, time does not exist and there is effectively no distance between any 2 objects in the universe, and since QM demonstrates that photons "take all possible paths" (ie, they are essentially everywhere between the time they are emitted and the time they are absorbed), do photons actually travel from point A to point B, or is that just a perception based on our particular perspective/reference frame?

 

[Dale]

See the FAQ:
https://www.physicsforums.com/showthread.php?p=3103865#post3103865

 

[Andrew Mason]

since, at the speed of light, time does not exist and there is effectively no distance between any 2 objects in the universe, and since QM demonstrates that photons "take all possible paths" (ie, they are essentially everywhere between the time they are emitted and the time they are absorbed), do photons actually travel from point A to point B, or is that just a perception based on our particular perspective/reference frame?

As you correctly point out, our concept of distance (and time) are rooted in our concept of mass or inertia and a reference frame. Distance and time have meaning only with respect to an inertial reference frame.

Photons do travel with respect to inertial reference frames ie. they do actually travel from point A to point B in the sense that they transport inertia/energy from one location to another as measured in a particular inertial reference frame.

As mentioned in the post that DaleSpam has directed you to, a photon can have no inertial frame of reference associated with it. Put another way, distance and time are not experienced by a photon so any kind of distance/time reference frame associated with a photon is meaningless. So, if you are asking whether photons actually travel in their own frame of reference, the question is rather meaningless. Still, it is a very good question.

AM

 

[Rap]

since, at the speed of light, time does not exist and there is effectively no distance between any 2 objects in the universe, and since QM demonstrates that photons "take all possible paths" (ie, they are essentially everywhere between the time they are emitted and the time they are absorbed), do photons actually travel from point A to point B, or is that just a perception based on our particular perspective/reference frame?

Its a perception based on our reference frame. Instead of talking about photons, where the math explodes, let's talk about a particle traveling closer and closer to the speed of light with respect to us. That particle does actually travel from point A to point B. The distance that it travels in its own frame is gets smaller and smaller compared to what we see as the distance from A to B, tending towards zero in the limit of v=c (v is the particle's velocity). The time that it takes to get from point A to point B gets smaller and smaller compared to the time we see it to be taking, tending towards zero in the limit of v=c. So yes, you might be able to argue that the photon actually travels from A to B in zero time, covering zero distance, in its own reference frame. The idea that the photon takes all possible paths is valid, but for most of those paths, if we are thinking of the photon as a particle (or wave packet), the quantum wave interferes with the other quantum waves destructively so it will behave like a single particle.

 

[Joshua.B]

wait...so do photons travel at the speed of light, because i thought that it was impossible for anything to travel at the speed of light

 

[jtbell]

wait...so do photons travel at the speed of light,

Yes, because they are light.

i thought that it was impossible for anything to travel at the speed of light

Nothing with a non-zero invariant mass (sometimes called "rest mass" which isn't a very appropriate name in connection with a photon which can never be at rest) can travel at speed c. A particle with zero invariant rest mass (such as a photon) must travel at speed c, in any inertial reference frame.

 

[Oldfart]

There's something maybe you guys can clear up for me. So sit back with a single malt and consider, if you will...

...the well established theory (or, law?) that a photon can take any path betweem being emitted at A and being absorbed at B. Two questions about this:

1. If the photon takes a path longer than a straight line path between A and B, would it not have to move faster than c to cover the longer distance at the same time as its fellows? Or is it OK if it gets there late?

2. Is there a limit to the path length? Like, if I shine a laser pulse from one side of my desk to the other side of my desk, where it is absorbed by my cat, is it possible that some photon on its way to the cat looped around the Orion nebula?

Thanks for your help! OF

 

[Oldfart]

There's something maybe you guys can clear up for me. So sit back with a single malt and consider, if you will...

...the well established theory (or, law?) that a photon can take any path betweem being emitted at A and being absorbed at B. Two questions about this:

1. If the photon takes a path longer than a straight line path between A and B, would it not have to move faster than c to cover the longer distance at the same time as its fellows? Or is it OK if it gets there late?

2. Is there a limit to the path length? Like, if I shine a laser pulse from one side of my desk to the other side of my desk, where it is absorbed by my cat, is it possible that some photon on its way to the cat looped around the Orion nebula?

Thanks for your help! OF

Thought I might give this a bump, may be a stupid question, but I'm really curious...

 

[Andrew Mason]

There's something maybe you guys can clear up for me. So sit back with a single malt and consider, if you will...

...the well established theory (or, law?) that a photon can take any path betweem being emitted at A and being absorbed at B. Two questions about this:

1. If the photon takes a path longer than a straight line path between A and B, would it not have to move faster than c to cover the longer distance at the same time as its fellows? Or is it OK if it gets there late?

I am not an expert on quantum physics by any means but I think the answer to that is that the photon can only move at one speed, c. The wave function amplitudes for all possible paths are added vectorally and cancel each other out (sum to zero) except for the amplitudes for paths very, very close to a straight line. The probability that a photon will take some path that differs from a straight line between two points is the square of the resultant amplitude for such paths (the sum of all amplitudes for non-straight line paths), which is the square of something very very close to 0. So the probability that it deviates much from a straight line is virtually 0.

2. Is there a limit to the path length? Like, if I shine a laser pulse from one side of my desk to the other side of my desk, where it is absorbed by my cat, is it possible that some photon on its way to the cat looped around the Orion nebula?

That is not possible because if the light is received we know it did not go to Orion because it would take hundreds of years to make that journey. The probability amplitudes for anything but a straight line sum to virtually 0. The probability that it deviated a degree from a straight line is extremely small let alone to the Orion constellation - like the probability that all of the molecules in room will suddenly all go in one direction. It never happens.

AM

 

[Phrak]

There's something maybe you guys can clear up for me. So sit back with a single malt and consider, if you will...

...the well established theory (or, law?) that a photon can take any path betweem being emitted at A and being absorbed at B. Two questions about this:

Not any path. All paths whether spacelike or timelike. This means that the world lines are both faster and slower than c. Second, throw out the notion of a 'photon' and call it a phasor, or something. All these phase carrying entities add together according to phase, so you don't have some photon-billiard-ball flying from point to point, but all these phasors going on all at once. Next, presume the 'phasor' is not a physical field but a construction useful in obtaining probability densities. Then call this construction a theory of light.

When it's all calculated properly the faster and slow phasors cancel each other out for the most part, and the remainder looks like light properly bouncing off of surfaces at an angle about equal to the angle of incidence, and propagating at about c. It's called quantum electrodynamics.

 

[Oldfart]

Many thanks for your replies, gentlemen!

 

[sophiecentaur]

You could say that a photon can take any number of paths from where it starts. You only know what path it has taken when you actually detect it arriving somewhere. So the tense of your question may need adjustment
"Did photons actually travel?" may be better. And , as it (the quantum of energy, at least) started somewhere and ended, later, somewhere then that is the definition of 'travel'. In between, though, what can you say? Along with Schrodinger's cat, you just can't describe what it's up to.

 

[Oldfart]

In between, though, what can you say? Along with Schrodinger's cat, you just can't describe what it's up to.

Can't you say that it has followed a spacetime geodesic? I'm a little confused about book illustrations showing photons sort of wandering about between A and B in their various possible paths, and then reading (my interpretation) that photons must follow spacetime geodesics. Duuh...

Thanks!

 

[epenguin]

As you correctly point out, our concept of distance (and time) are rooted in our concept of mass or inertia and a reference frame. Distance and time have meaning only with respect to an inertial reference frame.

Photons do travel with respect to inertial reference frames ie. they do actually travel from point A to point B in the sense that they transport inertia/energy from one location to another as measured in a particular inertial reference frame.

As mentioned in the post that DaleSpam has directed you to, a photon can have no inertial frame of reference associated with it. Put another way, distance and time are not experienced by a photon so any kind of distance/time reference frame associated with a photon is meaningless. So, if you are asking whether photons actually travel in their own frame of reference, the question is rather meaningless. Still, it is a very good question.

AM

[PLAIN]http://www.pushupstairs.com/images/emoticon/extra3/thinking.gif H'mm a photon has no experiences between birth and absorption (will this do?:shy:) but a then photon does know at least that it came from a different time and/or place than another photon independent of anyone else's reference frame since it can interfere with it - is this a real point?

 

[Andrew Mason]

[PLAIN]http://www.pushupstairs.com/images/emoticon/extra3/thinking.gif H'mm a photon has no experiences between birth and absorption (will this do?:shy:) but a then photon does know at least that it came from a different time and/or place than another photon independent of anyone else's reference frame since it can interfere with it - is this a real point?

Well, it stops being a photon when it interacts with matter (and does not become a photon until it stops interacting with matter). So it is difficult to give any meaning to the question whether a photon would "know" about where it came from or where it ended up.

AM