Friday, December 05, 2008

What if... #5

What if we found a way to predict the outcome of quantum measurements?


This post is part of the 2008 advent series "What if..."

23 comments:

Phil Warnell said...

I would say it would allow us to predict the future. The question then would be if we would be able to change it or would such a change produce yet another version of reality? The former is called destiny the latter many worlds or perhaps free will if you prefer to have consciousness have some reality as to have worth.

Albert said...

we should all throw a party on Einstein's behalf

Phil Warnell said...

Oh yes I forgot to add if consciousness did lend some power to alter reality and as there are more then one which can; does this infer there are many private realities or the resultant sum total of a collective one?

Phil Warnell said...

I would just like to further point out that with the current political events taking place in my nation (Canada) indicating that there is little likely hood that consciousness is of a collective nature and thereby suggests perhaps reality truly may indeed be both multiple and private:-)

Daniel de Fran├ža MTd2 said...

I would try to be a Jedi.

Giotis said...

Then we 'll call these measurements classical.

fh said...

FTL communication, breakdown of relativity and, eventually, instantaneous space exploration

Anonymous said...

It would be a sad day for the scientific community ...

Rae Ann said...

I can't explain why, but I think time would stop somehow.

Peter Shor said...

Suppose you predicted the outcome in one basis, and then changed your mind and measured in a different basis. You'd be able to effectively measure in two non-commuting bases simultaneously. It's well know that this lets you communicate faster than the speed of light, which is equivalent to being able to send messages backwards in time. So, the consequences would be that physicists would get very rich day-trading on the stock market.

Uncle Al said...

Blood Music, Greg Bear.

The only thing saving mankind is its inability to predict the future. Science fiction exists to enumerate and discard bad futures.

Be careful what you do not wish for, too.

andy.s said...

We'd know if the moon was there without actually looking at it, which always bugs me cuz then I have to crane my neck at an awkward angle.

Bee said...

Hi Peter,

You hit the nail on the head. Evidently, it follows from what you say that if you'd want to avoid major problems with causality, you can't change your mind without spoiling the predictions. Or, to put it differently, the settings of the measurement are part of the input necessary to make the predictions. Best,

B.

Neil' said...

This would have to be a way to predict the outcome of a "general" quantum measurement, since we can already predict the outcome of idealized same-basis measurements already. For example, if I produce a horizontal polarized photon I can predict it will near definitely pass a horizontal filter. But that's only from knowing how the photon was produced in the first place, and I wouldn't know what result I'd get from the H photon hitting a 20-degree filter. The latter would be a matter of statistics.

I have come up with a possible way to predict at least the outcome of measurements of polarization "circularity" of a photon. The measurement would take advantage of the tendency for CP light to impart angular momentum to half-wave plates it passes through. Since a HWP flips the spin sense of a photon, that imparts AM to the plate a little bit each time a photon passes (even if it was the same photon!)

On average that would be proportional to "circularity" so that elliptical pol. light would have intermediate effect, and linear pol. light no effect.

You'd think that couldn't be done for a single photon, but perhaps it can: run the photon through two HWPs: the second one to revert spin back to original value. Then use mirrors to direct the photon back through the same two plates. Eventually, enough spin is built up in a plate to be detected, and it should distinguish between RH, linear, LH, and maybe even some levels of elliptical. It would also be a way to predict further measurements of circularity, since the photon comes out (of the second HWP) with the same degree of CP it started with.

This isn't complete measurement of photon polarization, so I don't know if weird consequences like FTL would follow. But others have considered pushing the envelope of what we can measure in QM: look into "weak measurements" by Y. Aharonov (the same of Aharonov-Bohm effect fame.)

Bee said...

Hi Andy,

Well, that is an interesting point which touches on a much more difficult question, that is how to combine the collapse of the wave-function with gravity. Suppose the moon was not 'there' if you wouldn't look at it (measure it). What would happen to it's gravitational field? Best,

B.

Bee said...

Hi Phil,

I would think that if we could predict the future, we can not change it. The one is incompatible with the other. It's the same thing that puzzles me about horoscopes. Why do people read them? They either say what is going to happen and you can't change it, then what's the point of knowing it anyway. Or you can change it, then it's not a useful prediction.

Anyway, the question of predictions is actually more complicated, since it might be 'in principle' possible to predict something, but not 'in practice'. I.e. even if the world was purely classical I doubt you could sensibly predict something as complex as a human being, and that doesn't even touch on the question of chaotic systems.

Either way, my thoughts were that such a conclusion would be faced with major sociological resistance up to the point of denial, since it would mean we have no free will. Best,

B.

Arun said...

Maybe a horoscope operates like a QM wave function? It tells you what is possible, but doesn't guarantee a particular outcome.

Phil Warnell said...

Hi Bee,

“Either way, my thoughts were that such a conclusion would be faced with major sociological resistance up to the point of denial, since it would mean we have no free will.”

All good points including this last one. The way I look at it something can be unknowable while still being determinant. This means simply that although cause still leads to effect, being able to know is a question of what the system allows for. I would say what then defines as being conscious is it is what things possess who make the self determined attempt.

Free will is often associated with an individual’s ability to change the destiny of the system, where I would offer it is limited at best to how it can modify its own destiny in the wake of such change. You might then better refer to this as being a “private will” rather then a “free” one.

Best,

Phil

Shawn Wilkinson said...

I thought the probability density was already a way for us to predict the outcome of a measurement. Was something else meant based off of the question that i simply don't see?

Bee said...

I meant non-probabilistically

Peter Shor said...

Can I muddy the waters further by pointing out that we already have a way to predict the results of a quantum measurement. Suppose I hold one of a pair of entangled photons in the EPR state, and you hold the other. Theoretically (assuming ideal experimental equipment) I can predict the results of any measurement you make on your photon. This experiment has indeed been done, and I think the only real consequence was to get the experimentalists a publication of a paper, and to foment further debate among philosophers of foundations of physics.

Plato said...

And if you were to understand what Peter Shor just said, some might even use it now like Susskind did to ponder in a thought experiment the relevance of the horizon and "a elephant" to wonder about the inside of a blackhole:)

Best, :)

Count Iblis said...

If A is an observable, then so is

A(t)= exp(i H t/hbar)A exp(-i H t/hbar)

So, quantum mechanics does allow you to measure now who will be the president of the US in the year 2013. :)