# Journey of a Photon in the Double Slit Experiment

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1. Jul 22, 2015

### JoshuaMandlazi

The results of the double slit experiment lead to the conclusion that a photon travels as a wave. Question 1: Is it possible to track the journey of the photon? It seems to me (correct me if I'm wrong) that from the moment we release the photon till contact with the detector we don't know what happens. Question 2: Isn't the wave then a calculation based on all the possibilities which tries to explain the probabilities of where the photon could be? It seems to me that we don't know how a photon travels, however that we have figured that a wave best describes where the photon most likely could be. Please help, thanks

2. Jul 22, 2015

### DrChinese

Welcome to PhysicsForums, JoshuaMandlazi!

If you track the path of the photon, then you essentially constrain it to a single path. That changes its behavior as it moves to the detector. There can be no interference.

When it goes through the double slit unobserved, there is interference between possible paths and it behaves wave-like. So yes, that is the calculation to explain the probability.

What exactly are you asking past this?

3. Jul 22, 2015

### JoshuaMandlazi

Thank you DrChinese. Looking at matter in normal everyday life it is hard to believe that the building blocks or subatomic particles move in a different way following different sets of rules. Does the act of tracking the photon actually alter its path? Thus as you put it constrains it to one path? Or does the act of tracking allow you to find which of the multiple possible paths it took? Does the act of measuring alter the state of a subatomic particle?

4. Jul 22, 2015

### jfizzix

The only way we know if a photon is in the setup to begin with is if it interacts with something we can measure, like a photon-detector. Whatever we can deduce about the behavior of the photon comes exclusively from whatever measurements we can make.

So yes, we don't necessarily know what the photon is doing before it hits a detector. What we can do is make up theories that seem to give good predictions no matter what sort of measurement we do. In that case, the best we can say is that before it hits the detector, it is exceedingly likely that it behaves as quantum mechanics predicts it will.

If it makes no sense to talk about the single trajectory of a single photon, but the quantum state of the photon seems to be a consistent and accurate description of what we're likely to see, we might be tempted to shift what we base our models of reality on.

That'd be a bit of a stretch, though.
At best, we simply don't know enough yet.

5. Jul 22, 2015

### stedwards

The only theoretical structure I am aware of, which has a photon propagating as a particle, is within Bohmian mechanics where the particle is guided by 'pilot waves'.

6. Jul 23, 2015

### Staff: Mentor

It doesn't lead to that conclusion.

The problem is beginner texts and popularisations to not give a fully quantum analysis of it. To fix that please have a look at the following:
http://arxiv.org/ftp/quant-ph/papers/0703/0703126.pdf

Thanks
Bill

7. Jul 23, 2015

### Staff: Mentor

In fact they do.

But you need to understand exactly what QM says which can be obscured by popularisations. Here is a much better way to view QM:
http://www.scottaaronson.com/democritus/lec9.html

Thanks
Bill

8. Jul 23, 2015

### DrChinese

It's tough to "make sense of", but consider this:

1. The building blocks - quantum particles/systems - follow a single set of rules (called Quantum Mechanics). Their classical behavior (what you see everyday) follows from that, not the other way around. You don't see the quantum behavior directly! If you could, it would seem normal to you. What you see is essentially an averaging of the quantum behavior.

2. You probably are aware there is something called the Heisenberg Uncertainty Principle (HUP). You need to have a basic understanding of this - which is a consequence of Quantum Mechanics. The HUP makes it difficult to answer your questions as you have asked them. So here are a few comments:

a. A particle in an unknown (indeterminate) state WILL be altered by a measurement or observation. For example, an x-spin measurement will place the particle in either a + state or a - state.

b. Once the state is known from a. above, generally a subsequent measurement (x-spin in our example) will reveal the same value (there is NO change).

c. Quantum particles ALWAYS have indeterminacy in some or all of their quantum properties. Measurements can yield information about some quantum properties, but that information is limited in accordance with the HUP. Indeterminacy means: the property does not possess a well-defined value.

d. The relative number of available paths for a particle determines the likelihood of a particle result. The paths do interfere as if each is actually occurring. Yet the final observation always reveals a single outcome from available outcomes. Measuring position repeatedly and often therefore reduces the available paths and eliminates most of the interference. That yields a path that looks (to us) as if it agrees with a classical particle path. But that is a very special case, something seen in a laboratory and is rare elsewhere. The particles in your body do not behave classically and do not have a specific position, momentum, etc. because those attributes are not being observed.

9. Jul 23, 2015

### alghe

10. Jul 23, 2015

### DrChinese

Entangled properties are in a superposition and therefore indeterminate individually. So an observation (say spin) on that will place the property in a specific state. Subsequent observations will be consistent with that.

11. Jul 24, 2015

### qopdaniela

You may want to take a look at this
http://materias.df.uba.ar/labo5Aa2012c2/files/2012/10/Weak-measurement.pdf [Broken]

Last edited by a moderator: May 7, 2017
12. Jul 24, 2015

### Derek Potter

A great deal of the wierdness of QM is due to the fact that nature does not restrict herself to particular "possibilities" but allows intermediate states. Think of Schrodinger's Cat (with its environment of course)! But then, which is wierder - that nature should prefer certain states over others or that she couldn't care less?

13. Jul 24, 2015

### alghe

What do you think of this representation: ?