# Double slit photon deflection

1. Mar 11, 2009

### Pjpic

What is the force that acts on a photon that causes its path to deflect when the second slit of the double slit experiment is opened?

Assuming that my understanding of the experiment is in the ball park. With one slit open the photon lands in one place but with two slits open it lands in different spot (one that helps create the interference pattern).

2. Mar 11, 2009

### malawi_glenn

there is no force, it is a wave-phenomena.

even with one slit you'll have a diffraction pattern...

3. Mar 11, 2009

### Pjpic

Wouldn't a photon land in one place if one slit is open but in another place if both slits are open? If so, why isn't it a force that causes the position to change?

4. Mar 11, 2009

### malawi_glenn

5. Mar 12, 2009

### brone

Doesn't Quantum Electrodynamics (QED) offer another explanation? The path of a photon is determined by the sum of the probability amplitude vectors of all possible paths.
Can it thus be stated that the photon's behavior sometimes resembling waves is purely a coincidence?

6. Mar 12, 2009

### malawi_glenn

Now path integral formalism is not the same thing as QED, but the path integral formalism in Quantum Mechanics is a good way to look at diffraction of particles at slits. See Feynmans Lectures on Physics.

7. Mar 12, 2009

### lightarrow

A photon is not a tiny corpuscle which pass through the slit (or the slits). If it did it, there wouldn't be any interference pattern on the next screen.
Unless you are talking about a special *interpretation* of QM, that is , for ex., DeBroglie-Bohm interpretation. Then the particles are guided by a "quantum potential".

8. Mar 12, 2009

### Pjpic

9. Mar 12, 2009

### Pjpic

Does this mean (in a general sort of a way) that opening up the second slit gives the photons more options and so the pattern becomes different?

10. Mar 12, 2009

### malawi_glenn

pjpic, nothing acts on the photons, your last post is the best one so far - that the number of possible trajectories changes and thus different pattern.

The "QM-path" is the sum of all possible paths, with each path is weighted with exp(-i S /hbar) Where S is the classical Action. So when hbar is small (i.e when the Action involved are in the classical regime) one obtains the classical path.

http://en.wikipedia.org/wiki/Path_integral_formulation

11. Mar 13, 2009

### lightarrow

Did you read my post? Photons ARE NOT tiny corpuscles which pass through the slit(s). Even saying "hit the screen" is speculation.
Why do people always think about corpuscles just from the name "photons"?

12. Mar 13, 2009

### DrChinese

Why? Probably because they are localized when they are observed in this experiment. In addition, the quantum nature of light has been demonstrated: photons are discrete, even though they sometimes behave like waves. (i.e. There is no such thing as half a photon.)

13. Mar 13, 2009

### Naty1

I like that. But clearly it doesn't explain much if you begin by thinking of a single photon as a "ping pong" type affair since it's difficult to figure out how one photon can pass through two slits simultaneously!

Brian Greene has extensive discussion about what the double slit means, or doesn't, in FABRIC OF THE COSMOS...he utilizes four different explanations/interpretations around page 205 (Reality and Quantum Measurement Problems) and mentions there are MANY others....He does mention that decoherence, still under development for several decades, is often viewed as a likely ingredient in any viable and final solution.

So don't think any here have THE absolute final and unambiguous answer with which all physicsts will ready agree!!!!.

14. Mar 13, 2009

### lightarrow

Instead I think that books and physicists don't stress enough the fact we shouldn't imagine photons as little balls.

15. Mar 13, 2009

### DrChinese

Probably true! They are strange little critters.

16. Mar 13, 2009

### T.O.E Dream

the reason why we think of individual photons as like little balls is because that in nature waves are made of tiny particles like how water is made of tiny water molecules. so if we think of photons as waves we are forced to think that it's made of smaller particles. so where will it end.

17. Mar 14, 2009

### lightarrow

This could be an explanation for not very cultured people. Someone who, at least, studied at high school, knows that those tiny water molecules are made of atoms, which are made of fields and of other particles, and not of "tiny particles" only. Soon one asks him/her self if those tiny particles, in turn, are made of other fields and particles, and soon discovers it's so. Then the question is: who was "born" before, fields or particles?
Quantum Field Theory answers this question:
http://www.physnet.org/modules/pdf_modules/m246.pdf

<<3b. Quantum Field Theory's Description.
The principle features of the quantum field theory description of light may be stated thusly:
1. Electromagnetic field components, E and B , still obey Maxwell's equations. When crossed and sinusoidally fluctuating, they still form an electromagnetic wave traveling at speed c. However, the E and B values no longer give exact forces on charged particles,
hence no longer predict exact energy and momentum transfer rates to those particles.
2. An electromagnetic wave can only transfer energy and momentum to and from charged particles in increments fixed by the wave's frequency: $$\Delta E = h\nu; \Delta p = h\nu/c$$.
3. The actual times and places of these energy and momentum transfers cannot be predicted exactly, but the probabilities that they will occur in specified time and space intervals can be precisely calculated. These precise probabilities are linearly proportional to the wave's electromagnetic field intensities.
The first statement delineates wavelike properties of light; the second, particle-like properties.