I had three semesters of QM in grad school and I still have trouble with it.
"Anyone who says that they understand Quantum Mechanics does not understand Quantum Mechanics" - Richard feynmann (might be paraphrased)
The Trouble with Quantum Mechanics
51–60 of 126 posts
Re: The Trouble with Quantum Mechanics
#52It seems that the scientific method has led us to a theory that is beautiful, accurate, and yet utterly incomprehensible. Until we can explain exactly what happens during a wave function collapse, I think QM will remain that way.
Wave functions do not collapse in the realist view, as explained in the article. The cost, however, is a giant explosion of alternate possibilities all having equal reality, summed in superposition.
But, the point is, there is at least one interpretation of QM where the wavefunction never collapses: it just evolves unitarily according to the Schrodinger equation, forever.
Re: The Trouble with Quantum Mechanics
#53Relevant for those who are interested in the topic: Two rather unusual interpretations if quantum mechanics: - De Brogle-Bohm theory: https://en.wikipedia.org/wiki/De_Broglie%E2%80%93Bohm_theory - Gerard 't Hooft - The Cellular Automaton Interpretation of Quantum Mechanics: https://arxiv.org/abs/1405.1548 (see also https://en.wikipedia.org/wiki/Superdeterminism )
- Loop quantum gravity: https://en.wikipedia.org/wiki/Loop_quantum_gravity
As far as I know Loop quantum gravity is an alternative to string theory and says nothing about how to interprete quantum mechanics (though it better should reproduce its predictions). But convince me to be wrong.
Re: The Trouble with Quantum Mechanics
#54What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL information transfer, and so Bell's inequalities are satisfied. And anyway the other interpretations DO NOT rule out FTL information transfer. For example if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us. Also there is this: https://en.m.wikipe…
I'm not an expert in the Pilot Wave theory but my understanding is that it does not easily deal with many-particle systems. In the Pilot Wave theory the "wave" is a wave in space-time. In Quantum Mechanics, the wave is a wave in an abstract configuration space.
In the case of multiple distinguishable particles this configuration space is just a product of the configuration spaces of each of the individual particles. When there are non-distingushable particles, one needs to cut down this space in various ways, depending on whether the particles are bosons and fermions. Needless to say, the Quantum Mechanics of multiple particle systems has received overwhelming experimental support.
Actually, I have a question of my own regarding the Pilot Wave theory. Can a quantum computer exist in this theory? Is it just as or less powerful than a quantum/classical computer?
Re: The Trouble with Quantum Mechanics
#55Earlier quoted context omitted.
Do you understand the arguments that the ability to send a signal faster than light allows you to send that signal backwards in time? I've never met anyone who who understood special relativity but was OK with that.
How so? Why should faster-than-light travel imply a reversal of the arrow of time? Can you explain the reasoning?
Because events that have a spacelike separation -- that is, they are close enough together in time and far enough apart in space that even travelling at the speed of light you couldn't be present at both -- have no absolute order. Depending on your frame of reference -- how fast you're travelling and in what direction -- you might see two such events (let's call them A and B) as simultaneous, as A happening before B, or as B happening before A.
This is commonly illustrated with the following scenario. Imagine Alice is sitting by the train tracks. Her friend Bob comes past riding on top of a train, sitting exactly in the middle of the car's roof. As he passes Alice, they high-five (presumably Alice is on a raised platform of some sort). A split second later, Alice sees lightning strike each end of Bob's carriage at exactly the same moment. Thanks to some very precise measuring equipment, Alice is able to determine that the two lightning strikes occurred at the very moment that she high-fived Bob. At that point, Alice was exactly halfway between the two points that were struck, so it makes sense that the light from those strikes reaches her at exactly the same time.
Alice also knows that Bob would have seen lightning strike the front of the car before it struck the back of it: the light from the front strike would have passed Bob on its way to Alice, and the light from the rear strike would have passed Alice on its way to Bob.
Now let's look at it from Bob's view. As Alice concluded, he sees the lightning strike the front of the car first. But (a) he's exactly the same distance between the two strike-points, and (b) the speed of light is always the same for any observer. So if he sees the light from the strike at the front first, that means the front was struck first. Bob has a different order of events from Alice.
Which order is the "right" one? Answer: both. Or, if you prefer, neither. There are no grounds to prefer Alice's view over Bob's, or vice versa. You cannot say that one lightning strike "really" happened first, or that they "really" happened simultaneously.
To complete the picture, let's imagine Charlie riding another train car on the set of tracks the other side of Bob's, travelling in the opposite direction to Bob. At the exact moment Alice is high-fiving Bob, Charlie is also exactly lined up with them and smacks Bob on the back of the head. For reasons similar to but opposite to Bob, Charlie will first see lightning strike the rear of Bob's car and then the front, which means that in his (equally valid) frame of reference, the rear strike happened first.
Now imagine someone or something used FTL travel to go from the front of Bob's car to the rear, leaving at the moment the front was struck by lightning and arriving at the moment the rear was struck. In Bob's frame, this would be unremarkable, except for the exceptional speed (ignoring for the moment any adverse environmental effects -- Google "what-if xkcd relativistic baseball" for a flavour of what those might be). But in Charlie's frame, this would be travel backwards in time, as the arrival at the rear would occur before the departure from the front.
Re: The Trouble with Quantum Mechanics
#56Earlier quoted context omitted.
the Automaton interpretation sounds like something I would enjoy reading. But considering that I do not have the time this week nor enough knowledge in the subject, could you perhaps provide me with a quick summary as to what it is about?
My understanding is this: Hooft is an "instrumentalist" as described in this article. He does not believe that quantum mechanics describes "what's really going on" and is interested in classical models that explain why QM makes such good predictions. "Superdeterminism" is a very interesting perspective that Hooft feels could resolve this dilemma. Others feel that superdeterminism is not falsifiable, so there's no poi…
Actually the kind of superdeterministic theories proposed by t'Hooft are falsifiable. From [0] "If engineers ever succeed in making such quantum computers, it seems to me that the CAT is falsified; no classical theory can explain quantum mechanics."
By "such quantum computers" he means computers that can run Shor's algorithm. "...but factoring a number with millions of digits into its prime factors will not be possible – unless fundamentally improved classical algorithms turn out to exist."
Re: The Trouble with Quantum Mechanics
#57What is wrong with the Pilot Wave Theory intepretation? I'm ok with FTL information transfer, and so Bell's inequalities are satisfied. And anyway the other interpretations DO NOT rule out FTL information transfer. For example if one entangled electron flies into a black hole then we would be able to know its spin by measuring the other one even if light from it won't reach us. Also there is this: https://en.m.wikipe…
> What is wrong with the Pilot Wave Theory intepretation?
See
> https://www.quora.com/Why-dont-more-physicists-subscribe-to-...
for a discussion where people of both sides answered. For an answer that is rather critical towards the Pilot Wave interpretation see
> https://www.quora.com/Why-dont-more-physicists-subscribe-to-...
Another skepticall answer is
> https://www.quora.com/Why-dont-more-physicists-subscribe-to-...
Re: The Trouble with Quantum Mechanics
#58It seems that the scientific method has led us to a theory that is beautiful, accurate, and yet utterly incomprehensible. Until we can explain exactly what happens during a wave function collapse, I think QM will remain that way.
It doesn't seem that way to me. Decoherence is understood pretty well; although not every detail is completely worked out it explains our experience and the subjective appearance of collapse. It's just approximate though and hence we get quantum interference.
The reason you think it's incomprehensible is that a lot of smart people have trouble accepting what we called reality is only one of many classical "worlds". It's a philosophical concern; technically there are no major issues and it's not that hard to understand.
The reality of QM won't be commonly accepted until we find a way for people to accept that the universe isn't what they expected or we change their expectations.
Re: The Trouble with Quantum Mechanics
#59This article presents a very inaccurate view of the realist approach. The universe does not "split" when you make a measurement.
The measurement problem is a solved problem. The solution is that measurement and entanglement are the same physical phenomenon. Measurement is just entanglement extended to a macroscopic system through a process called "decoherence". The net result is that, when you do the math, you recover classical behavior by taking "slices" of the wave function (the mathematical operation is called a "trace"). This has been known for decades now, and provides a coherent and easy-to-understand picture of what is "really" going on. It is astonishing to me that the physics community still bifurcates into two camps: those who think this is common knowledge, and those who are completely unaware of it (or think it's a crazy idea).
References:
http://www.flownet.com/ron/QM.pdf
https://www.youtube.com/watch?v=dEaecUuEqfc (The same content in a video)
http://blog.rongarret.info/2014/09/are-parallel-universes-re...
http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
Re: The Trouble with Quantum Mechanics
#60Earlier quoted context omitted.
Wouldn't the entanglement just break? The preassure would rip the electron apart to begin with. I'm not sure if that erases entanglement already or if the quarks needed to be annihilated. Either way, are the known laws of physics applicable inside a black hole at all? I don't think so.
An electron is a lepton, it isn't made up of quarks. What does it mean to rip it apart?
That would be an embarrassing blunder of me.