Quantum Mechanics is weirder than I thought

[cdave]

I recently read "The Fabric of the Cosmos" by Brian Greene. It contained a rather brilliant description of the Delayed Choice Double Slit experiment, in which effect precedes cause. Rather damaging to the notion of free will I thought.

This led to me think about an Asimov story where such an experiment takes place. They wait until the experiment says that water was/will be poured on this in 24 hours, and then seal the container. They find that the universe conspires to ensure that there always was/will be someone to do just that. Be it a Janitor spilling something, or a junior lab technician not understanding, or something more.

I was wondering what would happen if the same thing was done with this Delayed Choice test. So I started writing it up as popular science essay, that I was aiming at being understood by anyone without a science background.

I was comparing this to the Bell's inequality experiment, when I finally realised what I'd missed. In the Bell's inequality tests, non-local quantum entanglement cannot be used to send information faster than light because it involves random processes, and you need to combine information from detectors on both sides of the experiment before you can see the non-local connection.

Similarly, there is no way to send information back in time using the Delayed Choice test, as the only way to show that effect has preceded cause is to combine information from both detectors at the cause and effect time of the experiment before you can see the non-temporal connection.

{ETA} This makes the weirdness of quantum mechanics really clear to me. I knew that entanglement meant that entangled particles have some faster-than-light spacial connection, but hadn't made the obvious connection that they therefore must have some faster-than-light chronological connection.

I think this implies that all events must be predetermined.

Comments

[shpalman.livejournal.com]

Well it's either that or believe that influence can be sent
backwards in time. I expect it comes out looking much the same though.

[cdave]

Hmm. That is another way of looking at it. ... ... Makes my head hurt, but I keep coming back to the future not being in flux.

[shpalman.livejournal.com]

There's nothing random in the equations of quantum mechanics.

[cdave]

I thought that the general consensus with quantum mechanics is that things don't have definite values until you make the measurement, and there is no way to determine which value it will be beyond a statistical probablity.

That is what I meant by random in other comments.

Here's a clearer version of what I meant by "future not being in flux".

Previously my understanding of the universe was that even quantum mechanics turns out to be deterministic, there's no way to make completely accurate predictions of the future of the whole universe, as there's no way to make all the calcuations needed fast enough to keep up with the rest of the universe. In other words even if the future attributes of every particle ever is already determined those attributes are not "real" as the have not happened yet.

Except that if they can spread influence backwards, they must have happened.

[shpalman.livejournal.com]

I'm not sure - I think that idea may be going out of fashion. If measurement itself is a quantum interaction then it has to be itself described by the time-dependent Schrödinger equation.

It depends also on what you mean by attributes - the wavefunction exists (as a complex, nonobservable entity) even if some of the more intuitive "attributes" don't.

[cdave]

But AFAIK (and I could be wrong) there is no known equation that describes how a wavefunction is transformed by measurements, and that's precisely transition is what the Delayed Choice Double Slit is really weird with.

[shpalman.livejournal.com]

That's because “measurements” is too general - but it doesn't mean that the interactions which are going on during any particular measurement are beyond the scope of quantum mechanics.

[cdave]

Okay, let's take the interactions in a standard double split.

Wavicle is emitted.
It is not detected at first screen so must have passed through slits.
The interaction of the waveform from both slits can then be used to make predictions about the likelyhood of finding a particle at any point.
Once the second screen registers an impact the probability of finding it anywhere else is zero.
This is not time invariant. At one instant there is a non zero probability space filling waveform, the next there is not. Reversing this doesn't isn't possible.
Of course neither is measuring this probability wave. I guess the only question is if you saw a particle leap off the rear screen, and saw it arrive at the `source` could you tell this was time reversed, and the answer would be no.

I'm not trolling here, I genuinely understood that the collapsing waveform is not time reversable.

[sharikkamur.livejournal.com]

I don't think it means that all events must be predetermined. I think it means that we almost certainly don't really understand QM yet. The very existance of 'hidden variables' is a dead giveaway. :)

[cdave]

I think it's just as likely that we don't understand time yet.

As I understood it the point of Bell's inequality is that it proves that 'hidden variables' are not enough and that there must be non-locality.

[offensive-mango.livejournal.com]

Isn't it because time is an illusion, and all future and past and current things will/have/are happen/happened/happening in the same moment? Or, um, something?

[cdave]

Time appears to as much of an illusion as that two coloured spiral that's been all over the place this week. It appears different when you look at it close up.

Maybe the universe is like a violin string. There's an infinite number of modes of vibration, but they almost all destructively interfere leaving you with only a few harmonics. That doesn't mean that any of those notes actually existed. It's just a mathematical trick. The laws of physics allow for many things that could happen, but most of them never will.

[cdave]

When you have an hour or so to spare go read Liz's link. There's aspects of it that I think would chime with you.

[offensive-mango.livejournal.com]

Ack! Proof of NPCs! Hurray?

That was awesome and got awesomer all the way through. I don't understand why people didn't understand the ending.

[cdave]

A lot of people were unclear. Then ending is punchy and clearly supposed to be laden with meaning. The problem is that people weren't clear what had just happened.

Firstly there were the people who we're'nt clear with which way round the implications would be if envelope had contained an interference pattern or a diffuse spread. That's understandable, apparently the magazine had diagrams at the appropriate point in the exposition.

The last line: "a series of shaded semi-circles" put me (and all but 1 of the other commenters) in mind of an interference pattern. Which implies one of at least three things happen next.

However the author apparently meant the opposite, and the envelope contained only a diffuse pattern, which only has a single, depressing, meaning.

[cdave]

One of the key themes in FotC is that all known physics (with the exception of certain exotic meson decay paths) are time reversible (if you mirror charge, etc. at the same time). This includes the Schrödinger 'probability waves' that define how a wave/particle travels.

The only thing that isn't is the waveform collapse when a measurement takes place. And to this day no-one seems to to know why, how, or even what defines a measurement.

[despotliz.livejournal.com]

HAve you read Divining Light, by Ted Kosmatka? Also interesting fiction about free will and the double slit experiment, and there's a long discussion about what it means here.

[cdave]

Thanks for the links, will try and read today.

[cdave]

Good read! Thanks. Yep, I agree with most of those comments as to what the final line meant. Ian's came closest.

[andrewducker]

Seems reasonable to me. I don't believe in free will anyway.

[emmajking.livejournal.com]

This is the sort of reason I love QM. I shall definitely add the delayed-choice double slit experiment into the next talk I give on the subject!

In SR/GR, FTL -> backwards in time, so perhaps this isn't entirely surprising. Although I could be mixing my theories in an imappropriate fashion there.....

I think that, especially with things like QM, it's dangerous to think we have a good idea of what is actually going on in the universe, just because we have a good mathematical model which accurately predicts the outcome of experiments.

I certainly don't think we have a good handle on time yet, and hence IMO the pre-determinedness of events is still, um, undetermined..... ;)

Free will, however, is a different matter. My Dad's thoroughly convinced it doesn't exist, simply because the way we behave is largely due to our natures - could any of us, given any particular situation, genuinely do other than we do? - and I'm often tempted to agree with him....

[cdave]

I haven't thought that free will exists for some time; I had thought that the universe unravels according to set laws, and an initial input.

However this changes that such that the whole of spacetime may well exist as a whole, with time as a complete illusion. Which befuddles me a bit.

[cdave]

{ETA} Whether those rules are deterministic or random didn't really bother me.