Consciousness – By Ignacio Cirac

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Some philosophers use the measurement postulate to suggest consciousness and Quantum Mechanics are somehow linked. What do you think about this kind of use philosophy sometimes makes of science?

There are two types of uses. There are some which are completely incorrect, which are not only not based on, but actually against Quantum Mechanics, even though the ones who put them forward say they agree with it; then, there are others which are open questions within Quantum Physics and which, for the moment, we don’t know how to test, but one may have several interpretations and Philosophy may have its own view, which is perfectly valid.

In that sense, when one speaks about the measurement postulate and consciousness, one can see these two trends I mentioned: there are people who say one can decide what’s going to happen, just by using their consciousness, which is against what Quantum Physics says. Quantum Physics does not allow one to manipulate measurement outcomes. One cannot decide what the result is going to be, what’s going to happen. One cannot modify the future. But, on the other hand, there is a problem in Quantum Physics with the measurement postulate, which is when the so-called wave-function collapse happens, when the properties of objects become determined. We know that happens when an observation is made, but at what time? One may think it’s when we see it, when we feel it, when… there are several possibilities and some people think it has to be related to the fact that we are aware of it. All of those are different interpretations which, from the physicists’ point of view, are just as valid, because none of them have been falsified by an experiment. One can perfectly speak about that and I have absolutely no problem with it.

Do you think the wave-function collapse postulate is justified? Or is it explained by decoherence theory?

Decoherence theory does not explain the fact that, after doing a measurement, we obtain a result, and I guess every scientist who works on Quantum Physics will say the same. Decoherence explains other things, which we understand very well, but not the fact that we obtain a result after measuring. That is not explained. There are, however, physicists who have a different interpretation of Quantum Physics in which the measurement postulate is unnecessary, because they understand Quantum Physics from the point of view of information. According to them, what we do is simply to receive information from outside. An we can’t ask ourselves whether the outside world exists or whether its properties are defined or not. It’s not necessary, since all we do is receive information. If one looks at Quantum Physics from that vantage point, there is no need for any postulate. Even classically, when we acquire information, our perspective, our description of the world, changes. One has a certain probability of events happening; when one learns something, that probability changes. It’s what is called the Bayes theorem in probability theory. The measurement postulate is basically the same. That is, if one forgets about the rest of the universe and only thinks that we are receiving and processing information, there’s no need for any conflict, as far as the measurement postulate in Quantum Physics goes.

Now, if we want to speak about things beyond ourselves, if we assume that the facts we observe -of which we have some information- are happening there, that there is a reality beyond which we want to talk about, about whether that reality is defined or not, etc. then we need to assign that reality an entity and that’s what makes the measurement postulate necessary. There’s a small contradiction inside Quantum Physics because, on one hand, it tells us physical processes happen according to Schroedinger’s equation -some laws- except for the measurement process, which is described by other laws. And one thinks: why are there two different laws if we ourselves should be described by the laws of Quantum Physics and, therefore, should be subject to the former? This is the controversy around the measurement postulate and one of the proposed solutions is saying: “let’s forget about objects and speak only about information, so there’s no contradiction”, but I think that’s burying the problem instead of solving it.

Is there a way of solving it?

Not that we know of. What would be interesting would be to design some experiment that would allow us to falsify some of the theories: there’s the many-world interpretation, the interpretations based on the wave-function collapse, there are orthodox interpretations, there are many kinds of interpretations within Quantum Physics, but the problem is that all of them make the same physical predictions, so we cannot find out what the correct one is. It’s an arguable question, something which we physicists don’t like: we like answers. So what we need is somebody who figures out a way to be able to tell between the different interpretations but, for the moment, we don’t know how to.

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