# What is the experiment where subatomic particles appear to foresee the future?

I've seen a documentary, whose name I don't remember but I'm curious because it suggests that subatomic particles are able to "foresee the future".

I'll try to describe it here:

Some particles are shot through two slits in a plate, if the particles are not interfered by trying to measure its trajectory, they form an interference pattern on a wall where the beam is being projected, otherwise they just hit the wall in a single point according to the measured trajectory.

The following image depicts what is described above:

The dotted red lines are where waves are canceled, blue ones are where waves overlap.

Then, they modify the experiment, so that they sort of turn on the detector after the particles have crossed the slit, where particles have allready being "forced" to behave as waves or single particles, to see if they change its behaviour after the measurement, just to find that the particles were measured as if they always knew that they would be measured, and they crossed the slit as particles and not waves.

Update:

This is the link to the documentary, it's titled "Microscopic Universe", is part of this TV series. They describe the experiment about 17 minutes in.

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Would you be able to be more specific? Possibly include a link to the documentary or a related article about the experiment. – Argus Dec 19 '12 at 15:17
This sounds like the quantum eraser experiment. No actual foreseeing of the future takes place in it. – Peter Shor Dec 19 '12 at 16:07
@PeterShor: That's what I thought as well. But it's a bit vague--if he can be more specific, I'd reopen this question.. – Manishearth Dec 19 '12 at 16:30
@Manishearth: sounds good. – Peter Shor Dec 19 '12 at 16:33
@rraallvv: If you can be more specific about the experiment you heard of, ping me/another mod (Use @Manishearth in a comment or flag the post for ♦ attention) and I'd be happy to reopen it for you :) – Manishearth Dec 19 '12 at 16:37

The answers given by @annav and @ChrisCharles, got me to the answer that the documentary possibly refers to the most recent experiment of the Wheeler's delayed choice experiment, because it's called the first "clean" experimental test of Wheeler's ideas, and one posible interpretation of the experimental data is that they were able to change retroactively the particle/wave behaviour.

Also this page has a pretty good description of the experiment, and explains its interpretations.

This is the abstract of the article on Science Magazine, the full text is free upon registration.

As a personal note: Maybe the subatomic particles behave as both, waves and particles, and by meassuring we just change the image projected onto our perceived reality of some hiper-particle that in essense remains basicaly unaltered in the hiper-space.

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In the wiki link provided also by @ChrisCharles there is a paragraph on a more recent experiment where the interference pattern appears even if the experiment is set up in such a way that the slit through which the electrons passed is known.

It is a clear demonstration that one is talking of a quantum mechanical probability distribution and not of an electron matter "wave". The "waviness" is in the mathematical wave function the square of which gives the probability of finding an electron on the (x,y) of the screen, when there are two slits through which it could pass.

Usually a measurement disturbs the boundary conditions which define the solution of the quantum mechanical problem. In this experiment care was taken to minimize the disturbance and still know from which slit the electron passed. In this sense the measurement above contradicts the usual narrative, that if one measures the path the interference disappears, it should be if one measures badly and disturbs the electron path the interference disappears.

Now in the video they are exaggerating for impression the effect of "bad measurement", imo.

Every experimental setup establishes boundary conditions that really pick up which solution of the quantum mechanical equations refer to the experiment. To go on and say that the setup projects backwards in time is wrong. The set up picks up the whole solution which does have solutions for -t, but it is the setup that does it, nothing mysterious. In a similar way one knows that the train that arrives at the station was moving on the tracks ten minutes before, so is that backwards in time or an extrapolation of the solution : "train at station" ?

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