# Two-slit experiment and The Truth

1. Apr 12, 2010

### GreatBigBore

Two-slit experiment and "The Truth"

I've heard the two-slit experiment described over and over, but there's still something about it that seems unclear to me.

When experimenters fire the first, single particle toward the screen, they don't get an interference pattern, do they? The interference pattern shows up only after lots of particles are fired, right? So the interference pattern doesn't indicate that a single particle is a wave that interferes with itself. It just shows that the behavior of large numbers of particles can be described with a wave function.

It seems that saying that a single particle goes through both slits and interferes with itself is about the same as saying that a coin flipped once can be both heads and tails, rather than saying that repeated coin-flip results can be described with a wave function.

If I'm making any sense at all, could someone please shed some light on my confusion? I really just want to know if we really think that a single particle behaves like a wave that can interfere with itself, and maybe a link to a site that makes it clear to interested laypeople.

2. Apr 12, 2010

Staff Emeritus
Re: Two-slit experiment and "The Truth"

You get an interference pattern if you fire particles one at a time, with arbitrarily long spacings between them. There's a nice picture of this in French and Taylor's textbook.

3. Apr 12, 2010

### Demystifier

Re: Two-slit experiment and "The Truth"

But if a single particle does not interfere with itself, how would you explain the fact that a single particle never appears at some places (at which the supposed interference is completely destructive)?

4. Apr 12, 2010

### faen

Re: Two-slit experiment and "The Truth"

The thing is that the "particle" is not a particle. There simply is no particle somewhere within the wave. Our perception of the particle is just an illusion. Its just waves. And when you measure the wave, it can become defined over a smaller position area, or defined over a smaller velocity area. Each wave is a superposition of other waves, since each state is a linear combination of the solution to any schr equation. So perhaps one can say that those waves in the superposition interfere with eachother..?

The perception about the particle or particle characteristics comes from that the waves react with eachother by forces. But that means that they just exchange energy somehow, not that a particle exist.

5. Apr 12, 2010

### Zarqon

Re: Two-slit experiment and "The Truth"

Not sure what you exactly mean by "fire lots of particles", but maybe you are helped by the following:

You can build up statistics to the see the interference pattern in many ways. You can take a single slit and fire a million (or how many you want) particles at it. However, you can equally well build up a million different slits (which may be separated by a large distance), and fire only a single particle through each one. When summing up the events, both experiments will yield the same interference pattern, and it should be fairly clear that the second experiment indicate that it is indeed necessary to describe every single particle in a way that contains the interference effect. Or in other words: The particle does act like it interfers with itself.

6. Apr 12, 2010

### zonde

Re: Two-slit experiment and "The Truth"

If it's not single particle that interacts with itself (or rather two superpositions of single particle) then it should be that particles within ensemble should interact (even when there is considerable delay between them). I suppose that this seems so counterintuitive that almost no one is considering that.

7. Apr 12, 2010

### Sillyboy

Re: Two-slit experiment and "The Truth"

For just a single particle, its state also behaves as a wave. That is to say, a single particle also is a wave. just emsamble of particles behave stasitically. Maybe we could comprehend this this way. for a coin, |state> = 0.5|up>+0.5|down>. But once the measurement happens, its states either goes into |up> or |down>, a large number of coins at the same sate can work out the probability or wave hehaviors!
If there is some wrong in my statement, please point them out. We can talk about them!

8. Apr 12, 2010

Re: Two-slit experiment and "The Truth"

No, it’s obviously impossible to get any 'pattern' with just a single dot...
Yes.
Wrong. In QM there is this 'weird' (for none scientist) http://en.wikipedia.org/wiki/Particle-wave_duality" [Broken], meaning an electron can be regarded as both a wave and particle – simultaneously. When you perform any measurement on the electron, the wave–particle duality is lost.

When a single electron is fired towards the http://en.wikipedia.org/wiki/Double-slit_experiment" [Broken], it passes as a wave, thus creating an interference pattern on the way out, towards the measuring screen.

If someone is 'sneaky', and try to measure which slit the electron actually passes; the wave–particle duality (and thus the interference pattern) is lost!

The position where the electron is measured on the screen is (mostly) random, with one very important exception – the interference (of the two waves coming out of the double-slit) increases the possibility for the electron to hit the screen where the waves are in phase (constructive/destructive interference).

Last edited by a moderator: May 4, 2017
9. Apr 12, 2010

### LostConjugate

Re: Two-slit experiment and "The Truth"

The confusion is in the fact that if the electron is a wave packet how is it that after detection the rest of the wave(s) suddenly no longer exist. It is as if though we held the detector in place for an infinite amount of time until it absorbed the entire wave across all space.

So the question becomes more about time, and our perception of time.

Perhaps the individual wave functions of different momentum are traveling at an infinite relative speed to the detector so that when we place a detector all the waves are absorbed instantly. Since only the group velocity can transfer energy this would obey the laws of special relativity. Just a thought.

10. Apr 12, 2010

Re: Two-slit experiment and "The Truth"

Just another thought:

Would it be possible to use 'entangled' electrons (as entangled photons in EPR) to make a "sneak preview" on which slit the entangled electron passes – thus knowing that the partner is passing the opposite – without breaking the wave-particle duality...??

11. Apr 12, 2010

### Antiphon

Just a point of clarification: different photons cannot interfere with one one another. Any given photon can only interfere with itself.

If you only shoot one photon in the whole experiment it will create an intereference with itself. Not a pattern because (as noted) one dot doesn't make a pattern. But it will never land in the node of what will become an interfetence pattern should you ever decide to shoot additional photons. (photons interfering with one another would result in a non-linear theory.)

12. Apr 12, 2010

### LostConjugate

Re: Two-slit experiment and "The Truth"

Not possible, you can't send information via entanglement. For one it would violate special relativity, though there is a QM explanation as well.

13. Apr 12, 2010

### LostConjugate

Re: Two-slit experiment and "The Truth"

If this is the case why did the scientists even experiment with only sending one photon at a time through the slits. The experiment of sending large amounts of photons at once would be just as convincing if they could not interfere with each other.

14. Apr 12, 2010

### ThomasT

Re: Two-slit experiment and "The Truth"

Or that an (emitted) expanding wavefront passes through both slits (thus transmitting two expanding, interfering wavefronts), but only produces a single pointlike detection.

Maybe the passage through the slits produces two slightly different wavefronts which subsequently interfere with each other leaving only a small area with enough energy to produce a detection. Or maybe not.

Anyway, it's (whatever it is) only a particle on detection, otherwise it's treated as a wave. So, if you have two-slits open, then you (eventually) get an interference pattern, and with just one slit open you get a diffraction pattern.

The truth of the two-slit experiment is still a mystery.

15. Apr 12, 2010

Re: Two-slit experiment and "The Truth"

The actual 'decision' to go left or right is of course 100% random, as in 'normal' double-slit, analogous to EPR spin up/down.

And EPR is not regarded as a violation of information FTL...?

Last edited: Apr 13, 2010
16. Apr 12, 2010

### LostConjugate

Re: Two-slit experiment and "The Truth"

I am not too familiar with entanglement or EPR entanglement, though I read somewhere that no information can be found until both entangled particles are measured. How would the spin or the paramagnetic resonance of the electron tell you about which slit it went through?

17. Apr 12, 2010

Re: Two-slit experiment and "The Truth"

No real information can be transmitted in EPR, the outcome 100% random (up/down), but you don’t need to measure both particles – if P1 is measured 'spin up', then P2 must be 'spin down', even if P2 is on the other side of the universe.
Good question, I have absolutely no idea, that’s why I asked!

Maybe it’s impossible… when thinking more about it, a measurement on an entangled twin breaks the superposition by measurement, and the other twin is now 'forced' to 'choose' it’s opposite state instantly = measurement = breaking the wave-particle duality... maybe...

18. Apr 12, 2010

### jeblack3

Re: Two-slit experiment and "The Truth"

When two particles are entangled so that they both go through the left slit or both go through the right slit, then neither particle by itself creates an interference pattern. You only get an interference pattern when you compare the two particles; select only the events where the first particle lands at a given place, and the places the second particle is detected form an interference pattern. Thus if you measure which slit one particle goes through, you cannot detect any interference pattern. Experiments of this type are called "quantum eraser" experiments.

19. Apr 12, 2010

### RUTA

Re: Two-slit experiment and "The Truth"

The short answer is "no." See "Time and the Quantum: Erasing the Past and Impacting the Future," Y. Aharonov & M.S. Zubairy, Science 307, 11 Feb 2005, 875-879.

20. Apr 12, 2010

### LostConjugate

Re: Two-slit experiment and "The Truth"

So what is to stop you from configuring a series of entangled particles of spin up and spin down, then flip the spin at one location of specific particles to send a message to another location. I read about this and why it was impossible many years ago.