## Thursday, November 12, 2009

As explained in a previous post, quantum mechanics allows for particles to be entangled. Basically, that means that the quantum state of two particles must be described together; they cannot be described separately.

A concrete example: Two electrons, which we'll name A and B, are emitted from a source in opposite directions. We can use a magnet to measure whether each electron is spin up or spin down. After repeating this experiment many times, we determine that 50% of the time, A is spin up and B is spin down. The other 50% of the time, A is spin down and B is spin up. This is because, before any measurements are made, A and B are in a mixed state of 50% (A up and B down) + 50% (A down and B up). They are entangled.

This basic idea of entangled particles led Einstein, Podolsky and Rosen to formulate the EPR paradox in 1935, which argues that quantum mechanics cannot be a complete theory. Imagine that A and B are entangled as above, and sent to opposite sides of the galaxy. Then I measure the spin of A. Before my measurement, the spin of B was undetermined, in a quantum mixed state. After my measurement, the spin of B is known, even though I never touched it, never even got close to it.

The problem is that it seems the state of B is affected instantly even though it is a hundred thousand lightyears away. Was there some sort of signal sent from A to B faster than the speed of light? If there were such a faster than light signal, this would not merely be strange and counterintuitive, but also paradoxical. According to Relativity Theory, if something travels faster than light, then there exists a reference frame in which it is traveling backwards in time. So now we have a signal traveling backwards in time, violating causality. What's to stop us from sending a message back in time to tell our past selves not to send the message?

The resolution: if there is indeed a signal traveling faster than light, this signal could not possibly transmit any information. If you can't transmit information, you can't send a message to your past self, and you can't violate causality.

Let's say I wanted to send a faster-than-light message across the galaxy. A message is basically composed of zeroes and ones. So let's try to send a short message across the galaxy, a single "1". When I measure A, I have a 50% chance to get spin up and 50% chance to get spin down. From this information, I can know the state of B. But I don't choose the state of A or B. So how can I choose to send a "1" rather than a "0"? I can't send any messages this way. The only thing I can send is random noise, which is exactly what my colleague across the galaxy would have gotten even if I had made no measurements at all.

Of course, I am greatly simplifying the EPR paradox and its resolution. This is all just to say that quantum mechanics escapes paradox by the skin of its teeth. Its position is delicate. And most forms of quantum mysticism just trample all over it.

More specifically, it's said in quantum mysticism that quantum mechanics is non-deterministic, and that we affect outcomes by observing them. Therefore, as the argument goes, observers can choose their own reality. And that's why thinking positively causes good things to come your way, and thinking negatively causes bad things to happen to you. It's not because positive thoughts lead to positive actions. It's quantum mechanics. (Implication: the positive actions themselves are unnecessary.)

From a common sense perspective, the quantum mystic's argument is just riddled with flaws. But as if that weren't enough, there is another flaw from a physics perspective: the EPR paradox returns! If I could in fact choose the outcome of a measurement, then I could use this quantum psychic ability to send coherent messages faster than light. All I have to do is choose to observe electron A as being spin down, and electron B is guaranteed to be spin up. I've just sent a single "1" across the galaxy instantaneously. Repeat the experiment many times, and I could send a bunch of zeroes and ones backwards in time.

It turns out that quantum mysticism doesn't just violate common sense, it violates causality too. So now it's just that much more of an extraordinary claim, and requires just that much more extraordinary evidence.

Mind you, I wouldn't recommend actually using this argument against quantum mysticism, since the target audience probably doesn't understand a word of it. It's all in good skeptical fun to think about it.

An inappropriate time to bring up this argument

Norwegian Shooter said...

Perfect timing! I just finished reading Charles Seife's Decoding the Universe, which is about information theory. The last part of the book explains the no-information-is-sent-between-EPR-pairs point very well. Highly recommend Seife, Zero was also great.

DarkSapiens said...

"…that quantum mechanics is non-deterministic, and that we affect outcomes by observing them. Therefore, as the argument goes, observers can choose their own reality. And that's why thinking positively causes good things to come your way, and thinking negatively causes bad things to happen to you. It's not because positive thoughts lead to positive actions. It's quantum mechanics."

I still remember the first time I was faced with this argument. My reaction was something like "What kind of nonsense is this?? These guys don't even understand what the concept of observation means! Isn't it a bit lame trying to use the words quantum mechanics just to say that if you think positively it's more probable that nice things will happen to you?"

Indeed, when a quantum system is observed this means it has had some kind of interaction with an external "something". Since to receive information from the system you need it to "perturbe" some kind of sensor, if the system is really small this has an appreciable effect. And people doesn't understand this and gets caught in this kind of things…

We need another Carl Sagan.

Regards!