Photons and mirrors: questions

[Mason, Larry K]

As I understand it, photons are discrete packets of energy which can be emitted by electrons. The energy of the photon must be in one of a set of specific amounts and the photon also has momentum. Photons travel at the speed of light. I would like to better understand that "travel" part.

Since the photon must always move at the speed of light (given the medium), the photon experiences no passage of time. So from the point of view of the photon, there is no time between when it is emitted by atom "S" (source) and when it is absorbed by atom "A". An outside observer may experience any period of time between the two events (events from the point of view of the observer) but from the photon's point of view there is only a single occurrence, that of being associated with a different atom. The photon is only a change in the energy status of two atoms in some respects from the photon's point of view.

So from the point of view of the photon, there is no "path" since there is no "time" at which it is only part way from atom "S" to atom "A". The photon's "clock" does not "tick" during the energy transfer. It's more like "everything is lined up right so the transaction can take place" and the transaction is "instant."

From the point of view of the outside observer the arrangement of physical circumstances between the two atoms appears to influence the photon. But since the observer can "observe" a photon only by having it absorbed by some instrument (some atom which is *not* atom "A"), the observer cannot actually watch (measure, detect) the photon itself between atom "S" and atom "A".

If the above description is correct (and I am not at all confident that it is correct), then the use of a mirror brings questions to mind.

Exactly what happens to a photon when it strikes a mirror (either fully or half silvered). We talk about it as if the photon bounced. But it seems to me that such a physical analogy is not an accurate description of what is happening. Does the photon get absorbed by some atom and re-emitted with a directional component? Does the set of atoms at the surface of the mirror provide some field which interacts with the photon (and the photon's momentum must come into play here)? Just what is going on during reflection? Is the photon leaving a mirror the same photon as the photon striking the mirror?

PS: I realize that there are many instances of the photon and I hope that what I am asking makes sense given that situation. David gives a description of what the absorbing atom experiences in its many instances but does not describe the photon's experience that I could find.

[Rami Rustom]

On Mon, May 20, 2013 at 8:27 AM, Mason, Larry K <ma...@email.unc.edu> wrote:
> As I understand it, photons are discrete packets of energy which can be emitted by electrons. The energy of the photon must be in one of a set of specific amounts and the photon also has momentum. Photons travel at the speed of light. I would like to better understand that "travel" part.

Well maybe think about the photon as the electromagnetic wave.

When an electric charge vibrates, that causes a chain reaction to take
place, a wave of changing electric field and magnetic field, and the
wave never ends until it is absorbed.

This happens because a moving electric charges induces a magnetic
field, and similarly a changing magnetic field induces an electric
field.

> Since the photon must always move at the speed of light (given the medium), the photon experiences no passage of time.

What does "experiences" mean? How can a photon experience anything?

And I don't see how *not experiencing the passage of time* follows
from *photons move at speed of light*.

> So from the point of view of the photon, there is no time between when it is emitted by atom "S" (source) and when it is absorbed by atom "A".

How so? It takes 8 minutes for a photon to travel the distance from
the sun to earth.

> An outside observer may experience any period of time between the two events (events from the point of view of the observer) but from the photon's point of view there is only a single occurrence, that of being associated with a different atom.
>
> The photon is only a change in the energy status of two atoms in some respects from the photon's point of view.

That's nice for math purposes. But I don't see how that's relevant to
what you're saying.

> So from the point of view of the photon, there is no "path" since there is no "time" at which it is only part way from atom "S" to atom "A". The photon's "clock" does not "tick" during the energy transfer.

Why does the photon have a clock?

> It's more like "everything is lined up right so the transaction can take place" and the transaction is "instant."

I don't see how its like that.

> From the point of view of the outside observer the arrangement of physical circumstances between the two atoms appears to influence the photon. But since the observer can "observe" a photon only by having it absorbed by some instrument (some atom which is *not* atom "A"), the observer cannot actually watch (measure, detect) the photon itself between atom "S" and atom "A".

Why does that matter?

> If the above description is correct (and I am not at all confident that it is correct), then the use of a mirror brings questions to mind.
>
> Exactly what happens to a photon when it strikes a mirror (either fully or half silvered). We talk about it as if the photon bounced.

I think it gets absorbed, and another one is created/emitted.

> But it seems to me that such a physical analogy is not an accurate description of what is happening. Does the photon get absorbed by some atom and re-emitted with a directional component?

I think so.

> Does the set of atoms at the surface of the mirror provide some field which interacts with the photon (and the photon's momentum must come into play here)?

Yes. Electromagnetic waves interact with electric and magnetic fields
(since they are themselves electric and magnetic fields).

> Just what is going on during reflection? Is the photon leaving a mirror the same photon as the photon striking the mirror?

No.

> PS: I realize that there are many instances of the photon and I hope that what I am asking makes sense given that situation. David gives a description of what the absorbing atom experiences in its many instances but does not describe the photon's experience that I could find.

which page?

-- Rami Rustom
http://ramirustom.blogspot.com

[Alan Forrester]

On 20 May 2013, at 14:27, "Mason, Larry K" <ma...@email.unc.edu> wrote:

> As I understand it, photons are discrete packets of energy which can be emitted by electrons. The energy of the photon must be in one of a set of specific amounts and the photon also has momentum. Photons travel at the speed of light. I would like to better understand that "travel" part.
>
> Since the photon must always move at the speed of light (given the medium), the photon experiences no passage of time. So from the point of view of the photon, there is no time between when it is emitted by atom "S" (source) and when it is absorbed by atom "A". An outside observer may experience any period of time between the two events (events from the point of view of the observer) but from the photon's point of view there is only a single occurrence, that of being associated with a different atom. The photon is only a change in the energy status of two atoms in some respects from the photon's point of view.

The photon isn't a person. It doesn't think or perceive or have a point of view.

> So from the point of view of the photon, there is no "path" since there is no "time" at which it is only part way from atom "S" to atom "A". The photon's "clock" does not "tick" during the energy transfer. It's more like "everything is lined up right so the transaction can take place" and the transaction is "instant."

It is true that the proper time along a photon's world line is zero, but what you said about the photon experiencing stuff doesn't follow.

> From the point of view of the outside observer the arrangement of physical circumstances between the two atoms appears to influence the photon. But since the observer can "observe" a photon only by having it absorbed by some instrument (some atom which is *not* atom "A"), the observer cannot actually watch (measure, detect) the photon itself between atom "S" and atom "A".

That's a very misleading statement:

http://lib.semi.ac.cn:8080/tsh/dzzy/wsqk/Nature/nature400-239.pdf

> If the above description is correct (and I am not at all confident that it is correct), then the use of a mirror brings questions to mind.
>
> Exactly what happens to a photon when it strikes a mirror (either fully or half silvered). We talk about it as if the photon bounced. But it seems to me that such a physical analogy is not an accurate description of what is happening. Does the photon get absorbed by some atom and re-emitted with a directional component? Does the set of atoms at the surface of the mirror provide some field which interacts with the photon (and the photon's momentum must come into play here)? Just what is going on during reflection? Is the photon leaving a mirror the same photon as the photon striking the mirror?

Exactly what is going on depends on the type of mirror. It always involves some interaction between the photon and the field of the atoms in the mirror. The photon that leaves the mirror instantiates the same information as the photon that went in. Whether it is reasonable to say that it is the same photon depends on the exact details of the experiment in which the mirror is being used.

Alan

[David Deutsch]

On 24 May 2013, at 09:02, Alan Forrester <alanmichae...@googlemail.com> wrote:

> On 20 May 2013, at 14:27, "Mason, Larry K" <ma...@email.unc.edu> wrote:
>
>> As I understand it, photons are discrete packets of energy which can be emitted by electrons. The energy of the photon must be in one of a set of specific amounts and the photon also has momentum. Photons travel at the speed of light. I would like to better understand that "travel" part.
>>
>> Since the photon must always move at the speed of light (given the medium), the photon experiences no passage of time. So from the point of view of the photon, there is no time between when it is emitted by atom "S" (source) and when it is absorbed by atom "A". An outside observer may experience any period of time between the two events (events from the point of view of the observer)

BTW an observer isn't normally present at both events. Only in the exotic situation of the photon having orbited a massive object can this happen.

>> but from the photon's point of view there is only a single occurrence,

I agree with Alan (below) that the photon doesn't have a point of view. But it's not true that the photon undergoes no occurrences during its path. It exists along a line in spacetime. The line has zero length (proper time), but the electromagnetic field that constitutes that photon does not take the same value all along the line. On the contrary, it oscillates. Typically, an enormous number of times. And those oscillations are real: what the photon does when it finally strikes the target may depend critically on the exact phase of the oscillation that it has reached during that zero proper time.

>> that of being associated with a different atom. The photon is only a change in the energy status of two atoms in some respects from the photon's point of view.
>
> The photon isn't a person. It doesn't think or perceive or have a point of view.

-- David Deutsch