# B Heisenberg's uncertainty principle and photons

1. Aug 25, 2016

### Comscistudent

Hi everyone,

Apologies if this is common knowledge or a silly question, I'm just coming back to physics and I've been looking through the double slit experiments ( both double slit and delayed choice quantum eraser ) and it got me thinking about the uncertainty principle.

With a photon of light in either version if we know through which slit the photon passes does that not mean that the momentum at that point must be unknown?

Since a photon is a massless particle ( I think ) that implies that the speed of the photon must not be exactly C. Since C is the absolute limit the photon at that point must have the ability to be moving slower than C. Is that even possible? The opposite is also true, that the mass could have changed but if the photon has mass it again cannot be travelling at C.

I assume I've missed something basic here so please correct me.

2. Aug 25, 2016

### Staff: Mentor

Last edited: Aug 25, 2016
3. Aug 25, 2016

### Comscistudent

That helps a lot thanks!

4. Aug 25, 2016

### Jilang

It's not an uncertainty in the speed, but an uncertainty in the direction of travel.

5. Aug 25, 2016

### Sophrosyne

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Although photon "particles" do not have a mass, they do have what's called a "rest mass"- that means if you take the energy of a photon and translate it to what it WOULD be if it had mass, you would come up with a number (remember Einsteins equation relating mass and energy). This number actually does have "mass-like", effects, because photons actually have a momentum which you can then calculate through the famous p=mv equation. If you shoot out enough photons in one direction, you could even accelerate a rocket into space or knock a person down. Granted, it would take a lot of photons to do that, but theoretically it would be possible.

Last edited: Aug 25, 2016
6. Aug 25, 2016

### Staff: Mentor

Photons are massless. And the concept of "rest mass," which applied to massive particles, is no longer employed (you will find many threads on the subject on PF).

7. Aug 26, 2016

### vanhees71

"Rest mass" is very ridiculous for photons. Even if you use a (totally wrong) picture of a classical massless particle (which in fact doesn't exist in nature) it's very clear that there is no inertial reference frame, where it can be at rest. That's why the idea of a "relativistic mass" is abandoned in the modern treatment (if you call a paper of 1907 "modern"), and you only use invariant mass (a Lorentz scalar) as a concept to describe mass. What has been called "relativistic mass" is just relativistic energy (divided by $c^2$) which is the temporal component of the Minkowski energy-momentum four-vector.