Quantum Mechanics for Dummies #2: Observation
Well, it's been a while, but an e-mail exchange not long ago prompted me to get back to this series. If you haven't yet read it (or need a refresher), I recommend you go back and read my first post, on the Wave Nature of Matter. This time, I'm going to be talking about something that can happen to waves and is a very important part of Quantum Mechanics: the collapse (which is closely tied into the concepts of a measurement and an observation).
But before that, we need to cover the concept of eigenstates. (For anyone who already knows about it, this is a vastly simplified explanation, so you're probably safe just skipping past.) Unlike particles, waves don't have discrete positions they'll be in, but rather a range of possible positions they could be found at at any time. Since waves travel, the probability distribution of where it will end up will often change with time. However, there are ways to trap waves, such as a beam of light between two mirrors. In these cases, the wave can only take on discrete stable patterns so that it doesn't end up interfering with itself. These discrete patterns are what are known as eigenstates.
But the probability distribution for a wave won't always fall perfectly into one eigenstate. Often its probability distribution will be a linear sum of multiple eigenstates. Due to interference between the waves of the different states, this pattern won't be perfectly stable and will change somewhat with time, but it will generally do so in a periodic fashion.
This also isn't limited to cases such as light waves. For instance, all particles have a property known as "spin" (if you take that exactly as it sounds, you're close enough). If you measure the spin of a particle such as an electron, you will always get one of only two values, regardless of the orientation of your measuring device: +h/(4π) and -h/(4π) (called "spin up" and "spin down." Think of it as spinning clockwise or counterclockwise).
Both spin up and spin down are eigenstates of the electrons spin, but it's not necessary for the electron to be exactly in one of these states. To generate this, say you take an electron that you just measured to be spin up, then made a second measurement at a right angle to the first. Classically, you would expect to measure a spin of zero, but this isn't one the allowable results. Instead, you'll end up with a 50% chance to measure spin up and a 50% chance to measure spin down.
And here's where things get a bit strange: After you measure it once, if you go back and measure it the same way, you'll always get the same result. It's no longer a 50/50 split. However, if you go and measure it back in the initial direction, it's a 50/50 split here. The implication here is that by measuring the spin of the electron, you somehow changed it - in this case so that it was in a spin up eigenstate with respect to your new measurement.
What's happened here is known as wavefunction collapse. After your first measurement, the electron was spin up in the first direction, which corresponded to a 50/50 split when measuring from the second direction. This 50/50 split was a combination of two eigenstates for the electron. When you then measured it in this direction, one of those states was randomly selected and the electron then became 100% in that state.
And this when the misinterpretations start to happen. The reason for the misinterpretations is the fact that quantum physicists happened to use one particular word in describing it: "observation." In the sense of what happens, an observation simply entails measuring the wavefunction of something, causing said wavefunction to collapse.
But that's not how the word "observation" sounds to the layman. When many hear it, they then think, "So, does this mean that reality is unresolved until I look at it?" People started to believe that quantum states wouldn't resolve until the information from them had filtered its way to a human mind. Even when it came to quantum physics, people wanted to put the human mind on some special pedestal in the universe.
This argument wasn't limited to laymen however. At first, the best quantum theorists couldn't decide themselves what exactly caused the wavefunction to collapse. All they knew was that if they weren't measuring particles, the wavefunction wouldn't collapse, and if you were, then it would. (They tested this by means of the double-slit experiment, which I mentioned in my previous post in this series.)
So, you have scientists not knowing the answer and using a word which heavily implies that human consciousness actually is the answer, and what do you expect happens? People pick it up and start extrapolating, saying that we then must create reality with our minds and be able to control it as we see fit. Even if we were to accept the premise (that being processed by a human mind is what caused the collapse of a wavefunction), this in no way implies that the human mind actually creates or can control reality. The results are still inherently random, whatver the processing mind may wish.
Beyond that, there's actually good reason to believe that it isn't a human mind that causes the collapse of a wavefunction. Let's go back to the case of the double slit experiment, which is our best way of determining whether or not a wavefunction has collapsed. In this case, we'll be shooting electrons from an initial source through one of two slits. One foot beyond the slits is a detector screen. We know that if the electron's wavefunction is uncollapsed at the slits, we'll end up seeing an interference pattern on the screen, while if it's collapsed, we'll see the sum of two diffraction patterns.
If we just let the experiment run, without any detectors, we end up seeing the interference pattern (nothing's causing the wavefunction to collapse). If, instead, we put in detectors at each slit that will notify us if the electron passes through (say by blinking a light on the left or right side), we see the sum of diffraction patterns on the detector screen. Now, what if you were to try this: Have the detectors at the slits and turned on, but don't look at the lights. You could simply disable the feature that has it flashing the lights on the detectors and not store data of which slit the electron passed through, so no human could ever know which way it went.
In this case, we can expect to see one of two outcomes: Either we see an interference pattern, which means that without a human observing it, the wavefunction wouldn't collapse, or we could see a sum of diffraction patterns, which would mean the interaction of the electron with the detector (or some process within the detector after the detection) caused the collapse. This has in fact been done, many times. Very frequently, scientists did experiments using a detector but didn't care which slit was detected, and so they didn't set it up to tell them this data. The result of these tests? The wavefunction collapsed anyways, so human consciousness is not necessary to cause the collapse of a wavefunction.
Why then, do we still see people claiming that it is? Mostly it's do to a poor understanding of the subject. They see words like "observation" and interpret it to mean human observation. Even some professors of quantum physics (including one I had as an undergrad) made this mistake, and then taught it as accepted fact to their students. The only way you can really know for sure on something like this is to go to the experiments themselves. Fortunately, this particular phenomenon is testable, and it has been tested.* Unfortunately, this won't stop some people; I raised this issue up with my professor at one point, and he said testing it was a waste of time because he knew that human consciousness had to be involved. Well, you can't convince everyone.
*Edit to add: Unfortunately, no one seems to have a link to the studies that actually test this, most likely because they were done so long ago that they were never published online anywhere. Actual science has moved on far past this point, while popular science is just starting to get interested in it. I've asked a few people in the know about it, and while some, like the professor mentioned above, still hold to the view that it's human consciousness that does the collapsing, the consensus is pretty clear that the collapse happens long before any human mind looks at whether the particle was detected in either slit.
In fact, even when humans are looking at this, there's so much processing that goes on in the technology that interprets the data that the particle as already traveled and hit the plate at the end before information about which slit it passed through reaches the mind of a human (the "flashing light" is just a metaphor, it's not what's actually done in these experiments). So if it was human thought that was controlling these outcomes, then this result would also have to reach back in time to collapse the particle's wavefunction at some point before it hit the plate.
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Other posts in this series:
Quantum Mechanics for Dummies #1: Wave Nature of Matter
