[BegeMoT]

If a tree falls in a forest and no one is around to hear it, does it make a sound?

[intisgunkas]

Is this similar to "spooky action at a distance" that Einstein describes? I'm not a physicist or anything and I can't watch the video on my work computer.

Einstein had written a paper in the 1950s, along with a couple of other top physicists, who pointed out that quantum mechanics "allowed" a cause to have an effect elsewhere instantaneously (seemingly violating relativity, for one). He called it "spooky action at a distance" to describe his feelings on the subject, i.e. that quantum mechanics was flawed. These days it's generally known as quantum entanglement; it was verified to be true by John Bell (theoretically) and a multiple of experiments.

It doesn't make sense. if electron A travels through hole A and leaves A mark on the board when it is being observed (same for B), BUT when it is not observed, it does NOT leave A (or B) mark on the board...

The electron always gets "observed" - it's a question of where and how it gets observed that matters. If one doesn't use any detectors at the slits and simply lets the electron strike a distant screen after the slits, then the observation takes place at the screen (and the electrons exhibit wave behaviour). Place detectors at the slits, to count the electrons as they go through, and one now observes them there, causing them to exhibit particle behaviour.

Strictly speaking, it should be noted the process of observation always ultimately results in particle behaviour - to observe the electrons at the screen, they must collide with the atoms of it; to observe them at the slits, they must emit electromagnetic waves or interact with a magnetic field. Either way, that's particle behaviour. What the double slit experiment is showing is that, in the space between the emission of the electron and its final observation, it can exhibit both particle and wave behaviour - hence, it can show a diffraction pattern on the screen.

[intisgunkas]

There's one thing I'm still not sure about though... does it stop behaving like a wave if you put your eyes near the slots and look across the electron beam?

Bit of a moot question considering that you can't see electrons but if we assumed that one could do this, then you'd still get an interference pattern - just a very poor one. Step too close to the slits and you wouldn't see the pattern at all, even though it's still present.

[loyalgagora]

Bit of a moot question considering that you can't see electrons but if we assumed that one could do this, then you'd still get an interference pattern - just a very poor one. Step too close to the slits and you wouldn't see the pattern at all, even though it's still present.

I see, so the better the detector at the slits the less interference pattern you will get? Even if you had a detector a mile away that can detect every electron passing through there will still be no interference?

Another question is, after an electron is detected does it forever behave like a particle or can it keep on behaving like a wave? E.g. if we had 2 sets of double slits, the first with a detector before it, and the second without, will we get an interference pattern?

[Kamendoriks]

So can anyone shed some light on this?

There must be information passed from the observer to the electron in order for the electron to 'know' that is has been observed and thus alter it's behaviour. What form does this information take? Is it another particle / wave?

[intisgunkas]

I see, so the better the detector at the slits the less interference pattern you will get? Even if you had a detector a mile away that can detect every electron passing through there will still be no interference?

If the electrons are interacted with at the slits, then no diffraction pattern forms at all. It's been a while since I've read the paper on the Japanese research into this, so I can't remember any other particular details.

There must be information passed from the observer to the electron in order for the electron to 'know' that is has been observed and thus alter it's behaviour. What form does this information take? Is it another particle / wave?

The electron doesn't alter at all - it remains exactly what it is at all times, a quantum entity. It is how we choose to observe the entity that determines what behaviour we imply: leave electrons to pass through the double slits and reach the screen? Then they form a diffraction pattern on the screen, which to us is wave behaviour. Stick a detector right by a slit, so we can count individual electrons as they pass through the slit? Then we interact with them in terms of charged particle behaviour and thus we think of it being a particle.

However, to answer your question, all interactions and observations with charged particles are done via photons (the quantisation, i.e. "particle", of electromagnetic waves).

[Kamendoriks]

However, to answer your question, all interactions and observations with charged particles are done via photons (the quantisation, i.e. "particle", of electromagnetic waves).

So are those photons present whether or not the electron is being observed, or are they somehow bought into being by the act of observing?

[intisgunkas]

Well, accelerating charged particles emit photons but non-accelerating charged particles will generate a magnetic field though which can be detected via Hall Effect probes or something similar; the field and the detector though interact through the exchange of photons.

[PheliarearY]

This is the first time I've heard of this experiment, but after looking at it, this is how I have come to understand it:

When an electron/quantum particle is fired, it travels as a wave of probabilities - i.e. a wave where the particle has a probability of being anywhere on the wavefront, with the most probability being in the direction of travel and the probability decreasing as you deviate from the direction of travel. When this wave hits the double slits, it will form an interference pattern of probabilities, and when this pattern hits the screen, an actual position for the electron is determined according to the probability patterns. This explains the interference pattern observed on the screen.

The important thing to understand is, the actual position of the electron is ONLY determined (calculated there and then according to the probability function of the wave) when it is observed (this can be when it hits the screen, or when it is monitored by a detector) before it is observed, it remains simply as a wave of probabilities. After it is observed, the wave disappears and the particle materialises. From then on, the particle will will travel to its destination knowing exactly where it is, hence behaving like a particle. This explains why observing the wave/particle before it hits the screen will remove the interference pattern.

Does this make any sense to anyone?

I give this man a Cigar! This is the way I have always understood it to work. It would suck if I have been totally off base all these years.

[adolfadsermens]

Schrödingers cat anyone ?

[PheliarearY]

This whole thing tickled a vague memory. I think that at least at one time, many worlds theory was dragged into this as a possible explanation rather than the wavefunction collapse of the Copenhagen interpretation.

I have fallen out of current events on this sort of thing, (Dont get to read as much anymore)so correct me if things have moved along on a differant track since then.

(Ps: I always loved Shrodingers cat, even when I thought it was far-fetched)