Tuesday, January 24, 2017

Killing the cat?



This graphic from New Scientist, and conversations last night at the Science Museum, got me thinking. Using Schrödinger’s cat as a way to illustrate the differences between interpretations of quantum theory is a nice idea. But it suffers from the flaw that challenges the entire thought experiment. In order to be able to talk about the scenario in quantum terms, we need to be able to express it in quantum terms. But we can’t, because “live cat” and “dead cat” are not well-defined quantum states.

What, you can’t tell a live cat from a dead cat? Nonsense! Well yes, it is; but that’s not we’re asking here. What quantum property is it, exactly, that characterizes the superposition state, and that will enable you, unambiguously and in a single shot, to distinguish the two classical states? Live and dead are not quantum variables, and I’m not at all sure that they can be correlated even in principle with quantum variables that can be placed in superposition states.

Schrödinger’s point was not, in any case, that these are two different states of a macroscopic object, but that they are logically exclusive states. The paradox lies not in “two states at once”, but in “two contradictory states at once”. He was pointing not to “weird behaviour” predicted by quantum theory, but to logical paradoxes.

And this is why the Many Worlds Interpretation doesn’t resolve the problem. Yes, it looks as though it does: both outcomes are true! As New Scientist puts it here, “The universe splits. Your cat is dead, but in a parallel world it remains alive.” (Or, as Rowan Hooper points out, vice versa.) But wait: your cat? Who is you? Whose cat is it in the other world?

Brian Greene, in The Hidden Reality, tells us: that is you too! They are both you. Oh, so that sentence reads “Your cat is dead, but your cat remains alive.” Greene isn’t troubled by the fact that this is not how “you” works. But nevertheless, this is not how “you” works.

David Deutsch and Max Tegmark say, ah language! What should we trust more, language or maths? Contingent sounds, or timeless equations? But here language is articulating something that underpins maths, which is logic. Schrödinger realized that, but his point seems to be forgotten (by some). I don’t have time to go into it here (my forthcoming book will), but individual identity is a logical construct. You can’t wish it away with fantasies about “other yous”. I am trying to resist the topical urge to suggest that the Many Worlds interpretation offers us “alternative facts”, but that is terribly hard to do. So folks, the second option here is far more problematic than it looks.

What about the first? Let me say first of all that in neither the Copenhagen nor the Many Worlds interpretation is the cat “simultaneously alive and dead”. Not only is there no way of expressing that in quantum mechanics (at least, no one has articulated one), but in any event the proper statement of the situation is that “We can say nothing about the state of the cat, other than that live and dead are both possible outcomes of an observation”. That might sound like a pedantic distinction, but it will not be possible to make sense of quantum mechanics without it.

Now, I would hesitate to call the Copenhagen interpretation the “standard” interpretation, since there is no consensus, nor even a majority view, about which is the correct interpretation of quantum mechanics, at least among those who think about foundational issues. What’s more, the “Copenhagen interpretation” is not a single thing: Heisenberg expressed it differently to Bohr, and Wheeler had his own view too, as did others. However, I think Bohr would have said something like this: after observation, we have acquired now information that has changed our view of the cat’s condition (assuming it can be expressed in quantum terms at all) from an indeterminate to a determinate one. Some Copenhagenists, such as Pascual Jordan, spoke of this in causative terms: our observations produce the results. In that view, it seems acceptable to say that “Your measurement killed the cat” (although since we cannot say that it was previously alive, we might need to say more strictly “Your measurement elicited a dead cat”). But I’m not at all sure that Bohr would have seen causation at work in the measurement, as if “wavefunction reduction” is a physical effect that kills the cat. (That’s really the third, “objective collapse” option, which is given the least problematic representation here.) I think Bohr might have said something along the lines that “Observation allows us to speak about the classical state of the cat. And look, it is a dead one!”

So, which way will you vote? Bear in mind, however, that there are other option available, not all of them mutually exclusive. And that you won’t be able to prove that you’re right, of course.

22 comments:

Jayarava Attwood said...

Or more simply: quantum doesn't scale up and almost everyone reifies the "cat" metaphor.

Chemdiary said...

I think it is clear for the cat metaphor. The cat is either alive or dead and the observation does not change the outcome. The cat must have been dead/alive before we open the box.

For particles, what if we find an indirect way to measure the momentum or the position of the particle? Such that it will ALWAYS give you the exact same outcome? How would the theoretical physicists interpret it?

Christian Vogel said...

Wow, great art of thinking.....also your brilliant article about the link between consciousness and QM

My question is: Couldn't the Transactional Interpretation of QM by John Cramer, when time is responsible for decoherence solve the problem ?
https://en.wikipedia.org/wiki/Transactional_interpretation

My concept is that consciousness aka information is crucial for creation and to equate consciousness with waves and fields with speed of light,the wave eqauation and probability. I believe in time to be responsible when coherent probabilty becomes decoherent mater. We can even interprete "....in the beginning was the word...." of the gospel of John to see information and consciousness to be crucial for creation.

We might find a solution for the miracle of creation when we compare the christian trinity of father, son and holy spirit with the scientifical trinity of energy, matter and consciousness/information. The pattern is the same. There is something material: matter and son, something immaterial: energy and father and something mental: consciousness/information and holy spirit.......

Physics need energy for the singularity and the bigbang and to be honest: it needs a little bit more than energy: it needs consciousness, information, equations to build a universe......

We might ask the question why of all things it was necessary to quantize energy to avoid the ultraviolet catastrophe while we consider ultraviolet as an "event horizon" of the visible to the unvisible.

What if "love" is like coherence,like a harmony of waves and fields and to be felt physically as e(nergy)motion, excuted by a move of our mind.

In one of his books Douglas Adams mentions: "The Universe is a product of its own fantasy" and i believe there is no smarter and easier explanation of creation than this sentence......

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