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It’s time to admit quantum theory has reached a dead end

nautil.us

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Re: It’s time to admit quantum theory has reached a dead end

#201

Post by l33tman »

shoyer wrote: post #73

As former quantum physicist, I find it little troubling to read "quantum theory has reached a dead end" in specific reference to the interpretation of quantum mechanics. Most quantum physicists could not care less about how quantum mechanics is interpreted when it makes highly accurate quantitative predictions, and there are still plenty of interesting open problems for quantum theory (e.g., related to the practical…

Decoherence theory is really somewhat orthogonal to the measurement problem. Decoherence explains how the "collapse" happens gradually if you have a non-perfectly isolated system by seeping entanglement. But at some point you still need to activate the stipulation that you as an observer is drawn into the entanglement and at that point whatever's left of the wavefunction "collapses".

In essence the Copenhagen interpretation is still correct as a simplification that can still be OK in most cases. This is reflected by the fact that practising solid state physicists have successfully used this 20's style of QM for 100 years now.

Re: It’s time to admit quantum theory has reached a dead end

#202

Post by l33tman »

akomtu wrote: post #110
8bitsrule wrote: post #28

Even on the macro level there are lots of tricky questions. A bar magnet is surrounded by magnetic flux lines. Apparently it takes no energy to maintain that magnetic field. If I heat the bar in boiling water for long enough, it becomes demagnetized. On the macro level there's no visible/measureable change in the bar. We need a lot of magnification to see that the 'particles are no longer aligned'. On the other hand,…

I've been bothering myself with the same question. This rabbit hole in the wikipedia has led me to a dead-end explanation: those iron filings are pulled by virtual photons created by the magnet, those virtual photons are completely virtual and cannot be observed, except for the fact that they somehow pull the filings despite being virtual (this must be a small incursion of the purely mathematical world into reality).…

By that logic you can't observe anything or explain anything, though. All the force carrying bosons (like the photon) have to interact with fermions (the electrons and quarks that make up your equipment and your body). Some of the interactions happen due to on-shell photons ("light") but most happen due to completely off-shell photons (EM force). They are really the same interaction.

Don't get hung up on the "virtual" in the lingo about particles. All particles are more or less "virtual" depending on how you analyze the problem.

I'm sorry there is not a more intuitive explanation of this.. The underlying QFT building blocks are so far from classical physics. Even concepts like energy and momentum are in some sense emergent properties that lose most of their classical intuition "at the bottom", this is reflected by the Heisenberg uncertainty relationship for example.

Re: It’s time to admit quantum theory has reached a dead end

#203

Post by jiggawatts »

kayamon wrote: post #90
jiggawatts wrote: post #4

Earlier quoted context omitted.

To me, this always felt like programmers excusing abhorrent code with "but it works". Quantum theory isn't. It's not a single, cohesive, consistent theory.[1] There is no recipe you can apply, it's just a bunch of guessworks and heuristics that happen to produce the numerically correct equations if you keep trying long enough. This isn't secret or some sort of external criticism, you'll find this front-and-centre in…

To use the bug collecting analogy, I like the idea that by capturing and describing these particles perhaps we've killed them in the process.

The analogy I like to use is that high-energy particle physics is like trying to learn a game like chess or baseball by analysing the statistics of past games.

That's literally what goes on at CERN. They run effectively trillions of experiments, collect millions of them (after some filter), and then they draw histograms.

Unless you know nothing about mathematics, you know that histograms are inherently statistical beasts. It's meaningless to talk about the histograms of one experiment.

Could you learn to play, say, tennis by drawing histograms of player movements? Ball bounce locations? No? Why not!? It's tons and tons of data! Accurate, scientific data!

Quantum physicists would argue that only statistics exists. I like to point to the fact that one hydrogen atom can exist, and it can have on electron. Or zero electrons. In nice countable, integral quantities. You can place a single gold atom on a crystal lattice with an AFM or even draw cute pictures with them: https://cen.acs.org/analytical-chemistry/imaging/30-years-mo...

Just because some people think only collecting bugs is what biology is all about, doesn't mean that there isn't more to it...

Re: It’s time to admit quantum theory has reached a dead end

#204

Post by l33tman »

TheOtherHobbes wrote: post #61
renox wrote: post #24

Earlier quoted context omitted.

Yes, I never understood why light was considered 'particles' :-( As far as I understand it is because light can only be emitted by atoms with discrete energy levels, but (IMHO again) light is emitted by atoms with discrete energy levels because it is emitted when electrons jumps from one energy level to another and that the possible trajectory of electrons around the atoms aren't continuous, I don't see how this is r…

Light is considered a particle - or more accurately has particle-like behaviours in certain contexts - because you can count individual photons. The energy varies with frequency, and it arrives in discrete lumps at discrete locations. You can't have half a photon. It's all of a photon or no photon. It's one of Einstein's key insights, one of the foundations of quantum theory. In Quantum Field Theory you have quantum…

While seemingly intuitive, the idea of a QFT field as a probability field over space is not correct. It usually comes as a shock to anybody entering QFT when they finally realize the actual "probability space" is infinitely larger..

So as a programmer's example you might have an idea that to simulate a QFT you could have an array of floats over space to describe your "electron field", and you draw in values to reflect a probability distribution and you'll write some update rules to describe how this evolves in time. But this won't work, because it's not how nature works.

What you need is that electron field array (and a photon field array to make any kind of non-trivial observations), but in 4D (space + time), and you need to iterate over all possible values in all array bins for all fields, calculate a magic number for each configuration, and weigh all these magic numbers together to figure out the actual probabilities for any configuration.

Even for a 16x16x16x16 array, that is 65536 bins, even with only 2 levels in each bin this is 2^65536 combinations.

All of practical QFT (theoretical, perturbative or non-perturbative lattice methods) is about doing this calculation with (obviously radical) simplifications.

That we can do these enormous simplifications and still get usable results, can be a very good sign that the underlying reality is in fact simpler than what QFT implies but nobody has figured this out yet.

Re: It’s time to admit quantum theory has reached a dead end

#205

Post by tluyben2 »

adriand wrote: post #115
shoyer wrote: post #73

As former quantum physicist, I find it little troubling to read "quantum theory has reached a dead end" in specific reference to the interpretation of quantum mechanics. Most quantum physicists could not care less about how quantum mechanics is interpreted when it makes highly accurate quantitative predictions, and there are still plenty of interesting open problems for quantum theory (e.g., related to the practical…

I read somewhere that quantum mechanics is the most tested of all scientific theories. And it has been shown to be right, every time. Hawking espoused this idea he called “model dependent realism”. The idea is that every human understanding of reality is model-dependent, that is, it is not “reality” that we truly understand (we can’t) but rather in every case we have some model of reality that is useful in particular…

We are human and prefer elegance, so enough will continue to try to unify models anyway. I don’t buy that we cannot understand nature but this model dependent realism is fine as a practical way of working until we do understand it.

Re: It’s time to admit quantum theory has reached a dead end

#206

Post by majewsky »

alar44 wrote: post #196
dekhn wrote: post #119

Earlier quoted context omitted.

The problem is ill-posed, not that we lack a solution.

Can you elaborate on why it's ill-posed?

When you say:

> it's as solved as it can really be as far as human understanding is concerned

You handwave away several interesting parts of the problem without providing real justification for why that would be necessary or acceptable.

It's like saying "politicians are made up of atoms, and since we know how atoms interact, our understanding of politics is reasonably complete".

Re: It’s time to admit quantum theory has reached a dead end

#207

Post by rpz »

Maybe it is time that we review the fundamentals? Could Planck and Einstein have made a mistake?

I find this to be the simplest explanation of light: E=htf where h is in joules per cycle, t is measurement time and f is cycles per second.

Every wavelength of light has the same energy, regardless of frequency. That energy is 6.26e-34 Joules.

Every “photon” is actually 1 seconds worth of light energy at some frequency f.

Once you see that the time variable was hard coded to one second in planck’s constant of action it starts to make sense. It’s not surprising that single photons cause diffraction patterns because photons aren’t real particles.

forgottenphysics.com

https://img1.wsimg.com/blobby/go/e266e5e7-739c-437c-89c3-eed...

Re: It’s time to admit quantum theory has reached a dead end

#208

Post by majewsky »

ahmedsaad1977 wrote: post #166

How is this: The electron loses its energy as it approaches the final destination, this energy (of wave nature obviously) passes through the slits and forms interference pattern. I know that for every problem there is a solution that is simple, neat—and wrong. And above proposition is most probably the same. But still I'll be glad to read the answers. (Disclaimer: I'm a novice)

You're falling into the very common trap of assuming that "energy" is some kind of substance (like movies/series tend to depict it as for practical reasons), when really it is a mathematical property of a physical object. Like other similar properties (such as spin, momentum and angular momentum), energy is relevant because fundamental symmetries of the universe require it to be constant. Therefore "losing energy" in one part of a physical system (such as the electron) is only possible when another part of the system simultaneously gains the same amount of energy. When you say:

> this energy (of wave nature obviously) passes through the slits

The question then is what type of object would possess this energy as it passes through the slits and hits the screen behind it. Since the screen is specifically set up to detect electrons, the simplest explanation is that it is really the electrons performing the interference.

(Footnote, just to be safe: Energy conservation can be broken on very very short timescales, but this is not relevant for the double-slit experiment.)

Re: It’s time to admit quantum theory has reached a dead end

#209

Post by orbifold »

konschubert wrote: post #194
japanuspus wrote: post #190

Earlier quoted context omitted.

As a former quantum physicist who has just decided to go back into quantum computing, this was my take as well: Introductory quantum physics courses may still include wave-function collapse and all that nonsense, but I have not met many physicists who use this as a mental model. To be a bit more specific as to how _decoherence_ solves this, one way to see it is that classicality (i.e. observables having specific valu…

Okay, cool, I’m 100% with you. But could we then please stop teaching the collapse nonsense to first year students? The logical inconsistencies of the collapse interpretation are an insult to their intellect.

The collapse just stands for the unknowable details of the interaction with the environment during measurement, a good quantum physics course will explain that and include experiments that make that clear. For instance the Stern-Gerlach experiment illustrates this well.

Re: It’s time to admit quantum theory has reached a dead end

#210

Post by goldorak »

drujensen wrote: post #169

As former amateur physicist, who read a couple books over the years :) To me it's quite simple. We haven't detected what is causing the waves that the particle of light is riding on. The particle of light is like a surf board, riding a wave and will always hit the shore in the interference pattern. Einstein and Bohr were both right. What has been the wrong assumption over the years is that the light is generating the…

That's why time slows down near heavy objects, there is more calculus to be done and don't want to drop frames.
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