How Do Photons Relate to Quantum Mechanics and the Uncertainty Principle?

[jack47]

I was just thinking how photons fit into QM, I've heard people speak of "photon wavefunctions".

In particular I am wondering whether its bound by the Uncertainty priciple, I guess it can't be... since we know its exact velocity (i.e delta(x) = 0, at least in a 1-d case =P) how could we know its position?

 

[selfAdjoint]

Exactly. Where along a beam of light, is an individual photon? You don't know; you can't know; it's uncertain.

The photon is a full-fledged quantum particle, uncertainty and all.

 

[Goalie_Ca]

If we don't know its position then what is it we observe? Naturally it takes a certain amount of time for some light to travel from point a to point b. Can't we place a limit on its path or general area (somewhere between a and b, thus finite area).

Does this help to explain the young's double slit experiment at all?

Sorry if these questions sound silly. I'm just a 2nd year electrical/computer engineering student with lots of questions and few answers :D

 

[techwonder]

Actually, the speed of a photon is not THAT well defined in QM as it is SR and GR. In QED photons have a finite possibility to go slower or faster than c - on short distances.

 

[hellfire]

I thought a causality condition is imposed in QFT, which makes FTL propagation impossible for real photons.

 

[swansont]

I was just thinking how photons fit into QM, I've heard people speak of "photon wavefunctions".

In particular I am wondering whether its bound by the Uncertainty priciple, I guess it can't be... since we know its exact velocity (i.e delta(x) = 0, at least in a 1-d case =P) how could we know its position?

The uncertainty principle contains a momentum term, not a velocity term. A photon's momentum is E/c.

 

[TeV]

If we don't know its position then what is it we observe?

Interactions.

 

[turin]

If you do the double slit experiment, for instance, with just one photon at a time (that is, an extremely low intensity for which you would notice the time between the appearance of spots), then do the spots show up on the screen with a uniform frequency, or do they show up erratically?

 

[Magg$]

What are you talking about!

Photons have quantum behavior, yes... But it is definitely impossible to talk about the position of a photon, as its movement, due it behaving like quantum, is completley random!

 

[Dina-Moe Hum]

If you do the double slit experiment, for instance, with just one photon at a time (that is, an extremely low intensity for which you would notice the time between the appearance of spots), then do the spots show up on the screen with a uniform frequency, or do they show up erratically?

They will form interference pattern anyway, unless you try to locate these photons during their journey to the target. As long as you don't interfere with the process they "pass through both slits" and they interfere with themselves

 

[turin]

They will form interference pattern anyway, unless you try to locate these photons during their journey to the target. As long as you don't interfere with the process they "pass through both slits" and they interfere with themselves

Sorry, perhaps I was unclear. The (reworded) question is:

Do they have a reqular occurance (let's say one blip every second, give or take only a few milliseconds), or do they occur erratically (like one a few milliseconds after the previous, and then maybe two or three seconds until the next one, and then half a second until the next one, and then one second, and then a hundred milliseconds, etc.. All of these dead times may average out to one second, but there is a high degree of uncertainty that the next blip will occur within some window of time)?

I'm asking about a temporal measure, not a spatial measure.

I have never done the double slit experiment with such a low intensity or sensitive detector. I would like to here from someone who has. Judging from the silence, I suppose I'll have to go look up a paper or something.

 

[chroot]

Photons have quantum behavior, yes... But it is definitely impossible to talk about the position of a photon, as its movement, due it behaving like quantum, is completley random!

No, it's not random. Photons move according to well-understood and very predictable, though probabilistic, rules.

- Warren

 

[Chronos]

a single photon can pass through both slits at the same time. even more mysterious is that a single electron can too!

 

[jtolliver]

swansont was right, it is impossible to know the position and momentum(not velocity) at the same time. In classical physics this can be simplified by the formula p=mv to say that you can't know the position and velocity at the same time. For the photon you can't use the formula p=mv(which is of course not lorentz invariant). However you can say that it is impossible to know the photons position and wavelength at the same time.

 

[Tyger]

Don't forget that the two transverse degrees of freedom and the longitudinal degree of freedom are independent and uncertainty applies to all of them.

 

[Imparcticle]

Actually, the speed of a photon is not THAT well defined in QM as it is SR and GR. In QED photons have a finite possibility to go slower or faster than c - on short distances.

If a photon did go slower than c, then what would happen to it?

Exactly. Where along a beam of light, is an individual photon? You don't know; you can't know; it's uncertain.

The photon is a full-fledged quantum particle, uncertainty and all.

Have you heard of a light clock? A single photon is reflected and directed on various paths and the time it takes for it to finish the path is equal to one second, I believe. Isn't its position then able to be determined when it reaches the end of its path at the end of one second?

 

[selfAdjoint]

If a photon did go slower than c, then what would happen to it?



Have you heard of a light clock? A single photon is reflected and directed on various paths and the time it takes for it to finish the path is equal to one second, I believe. Isn't its position then able to be determined when it reaches the end of its path at the end of one second?

The light clock in relativity doesn't refer to photons at all (though some presentations of it may unwisely use photon language). A simple classical beam will do the job. Think of it as a wave front rather than a photon.