# What would happen if the double-slit experiment was done under these conditions:

A gentleman on youtube called Tom Campbel, PhD, claims the following. If we perform the double slit experiment with 2 kinds of detectors, one which records the which path information about the particle and another that detects the pattern formed by the particles in the screen, something very unusual would happen:

If we looked at the screen detector alone, we would see a clump pattern, because there exists data, information, on the which path part of the experiment.

If we looked at the which path device alone, it would show data regarding which path each particle took.

If we looked at none, the result would remain uncertain.

If we would let the which path recording device sealed away for 100 years, never examining it's data, and in the mean time we'd never look at the screen pattern device, and if, 100 years later, we were to completely destroy the information recorded in the which path device so it could never, ever be recovered, when we looked at the daata f the screen pattern it would show an interference pattern, instead of a clump pattern, even though we tood the information on which path. but since we completely and utterly erased that information without first observing it, the which way measurement would be like it never existed in the first place, because there isn't data anywhere in the universe of what it recorded.

Is there any truth to this?

Could any of you conduct such an experiment (no need for the 100 years, but absolute need for the complete destruction of the information in the which path measurement device. Is that even possible? to completely destroy information?)

• A very interesting question. Are you working towards the assertion tbat the decision to destroy or keep the information in 100 years time could change the experimental result at the peesent day?
– user140203
Jan 19 '17 at 7:29
• @ColinPierpoint yes and no. What Professor Campbell states is that nothing exists at all. Reality itself is an illusion, a stream of data we are fed somehow. If there's no data on something, that something doesn't need to be "rendered" as he puts it, and that something is just a potential something. Jan 19 '17 at 19:21
• He once used the following analogy: A man goes into some woods where nobody has ever been before. He sees a fallen tree. He then dies. A second man walks into those same woods, but he will not necessarily see the tree as fallen, nor any of the other things the first man saw. The probabilies of all those things is less than 1 if there is no hard data of those things anywhere. Once there is hard data, the probability becomes 1. He says that building the double slit experiment the way I described would achieve the result I described, and would prove him right. Jan 19 '17 at 19:23

but since we completely and utterly erased that information without first observing it, the which way measurement would be like it never existed in the first place,

This is absolutely wrong.

I blame Schrodinger's cat thought experiment for this type of misunderstanding of how quantum mechanics experiments work.

There exist various physics frameworks, and in these frameworks there are elegant theories which have been tested over and over again and work beautifully within their framework. Classical mechanics works for macroscopic observations, classical electrodynamics for light, thermodynamics, an emergent theory from the many body problems of classical mechanics, also is a self contained framework etc. Quantum mechanics is the framework of the very small , a self consistent framework when the Heisenberg uncertainty has to be obeyed , with its own differential equations and mathematical formulations. It can be shown mathematically that it is the underlying framework for most other frameworks which emerge naturally from the laws of the microcosm.( except gravity, and most physicists hope that will be shown to be quantized in the future).

Paradoxes arise when one mixes frameworks without keeping track of the hypotheses entering the problem. Our measurements and measuring devices are in the classical frameworks. We explore the underlying quantum mechanical framework by using interactions that obey quantum mechanical rules and studying the result in our macroscopic detectors.

As an example, the whole experiment, poison and all, of Schrodinger's cat is a macroscopic detector of what happens at the quantum mechanical level. The quantum mechanical equations enter in calculating the probability of decay, and the dead or alive cat is the measurement of whether a decay happened or not. The "dead or alive" cat is as unknown as the "is there a measurement on the counter". The measurement will be there whether I look at it or not, if a decay happened, because measurement of counters is in the classical framework.

In the case of the double slit , any measurements, photos... are in the classical framework and will be there whether looked at or not. The quantum mechanical uncertainties enter at the level of the QM equations with the boundary conditions "particle impinging on two slits of certain width and distance' and "particle impinging on two slits of certain width and distance + a detector at one of the slits".

The interference pattern observed ( or not) is in the probability distribution from the complex conjugate square of the wavefunction describing the experiment, the quantum mechanical framework. The measurement in the detecting devices is in the classical framework.

P.S. an interesting experiment that shows origin of the difference in the two QM distributions is here.

In short, hiding the data is no different than seeing the data.

Quantum limits how good our meters are but we can always build a bad meter

An "observation" happens any time a Q (a "small" quantum system) interacts with a C (a "large" classical system) enough to perturb the Q. In the double slit the Q is the photon, and the C's are any detector device. If a C gives us some information, it must count as a measurement, so any C that can figure out which slit the photon passed through will destroy the interference pattern. However, we always do worse: a C that erases it's own data after a set amount of time will still count as an observation (destroy the pattern) even it said device is worthless to us. Even putting a brick in front of both slits counts as "observing" which slit each photon went through!

No time travel/faster than light We can't build a C that will allow someone i.e. a lightyear away and a year in the future to choose whether or not the photons going through the slit are observed. It is "observed" whether or not they intercept the i.e beams that tell them the results.

• I think that to be true, but what Professor Campbell says is it is not true. If there was a way to erase the information completely from the universe, when we finally observed the pattern it would be as if no measurement had ever been taken. As if Q never interacted with C at all. His theory is that only information, data, matters. The universe is not objective, and our perception of it is a stream of data randomly generated by statistics. Nothing really exists in the first place, only data representing stuff. And he proposes this variation of the double slit experiment would prove it. Jan 19 '17 at 19:07
• Honestly Campbell's ideas seem pretty wacky. Statements like "Nothing really exists in the first place, only data representing stuff". How would we design an experiment to test this, where if stuff really exists the result will be A and if it really doesn't it would be B? Jan 19 '17 at 22:02
• Like I said, erasing the data about the stuff. He says that's the way to prove it, performing the double slit experiment and somehow completely erasing from the universe the which path information. I think it's crazy too. What he claims is that it's not the act of measurement or interaction with the measuring device that collapses the wave function. The universe doesn't render anything, unless there's data in it about that thing. The act of storing data, either in our minds or anywhere else about the measurement's results is what collapses the wave function. Jan 19 '17 at 23:35
• He is crazy not because he is wrong, but because what he is saying doesn't make predictions that can be tested. Standard QM will predict any of his experiments. Jan 20 '17 at 0:45
• I don't understand what you mean. would you like a link to the man himself speaking on his theory? I know you physicists are busy guys, and his videos tend to be long, Anyway, here it is: youtube.com/watch?v=BhMIz_iJtzQ Jan 20 '17 at 2:23

The question in focus here is known as that of "quantum erasing" and relates to the big interpretation issues of quantum mechanics. One may look e.g. at the Wikipedia article: https://en.wikipedia.org/wiki/Quantum_eraser_experiment or at Preskill's lectures notes http://www.theory.caltech.edu/~preskill/ph219/chap2_15.pdf (section 2.5.4). Basically, the answer to your question ("is there any truth to this?") is yes, although much depends on what one accepts as a bona fide "recording" of the data. If it means intrication with a quantum object (here both for the which path device and the screen), then it can indeed be "erased" and interference recovered. If recording means something like a quantum measurement implying Born postulate ("wave packet collapse" involving a "macroscopic apparatus") then it cannot be erased and interference will not be recovered. What makes the difference (and is there really one) between a quantum object and a "macroscopic apparatus" has been a hot question for ages. On practical grounds, interaction with the environment will cause "decoherence" making "recording" look very much like "measurement", but this leaves open Bell's criticism about "For All Practical Purpose" arguments.