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FQXi BLOGS
August 23, 2019

CATEGORY: Blog [back]
TOPIC: Pushing the Quantum Limits [refresh]
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Blogger Vlatko Vedral wrote on Jan. 17, 2009 @ 19:18 GMT
The discovery of quantum mechanics in the mid-twenties was followed by a fierce debate about its meaning and implications. This debate still rages, but there have been many twists and turns since the early days. This post is about the latest such twist.

The fact that quantum mechanics implies that measurements “create reality”—as I described in a previous post, “Is reality really real?”—was very uncomfortable to people like Einstein, who famously said: “Out there is this huge world, which exists independently of us human beings and which stands before us like a great, eternal riddle, at least partially accessible to our inspection.“ In contrast to this stood Bohr who thought that it was not meaningful to take the realistic position of Einstein. Instead he said, and here I quote Bohr: “There is no quantum world. There is only an abstract quantum physical description. It is wrong to think that the task of physics is to find out how nature is. Physics concerns what we can say about nature.“.

According to Bohr, therefore, microscopic objects behave in a strange way, but we should not lose any sleep over it since science only concerns what we can say about nature and not what nature is really like.

All this was profoundly changed by Schrödinger in the mid thirties. He effectively amplified the spilt between a realist, like Einstein, and a pragmatist, like Bohr.

www.animalshaveproblemstoo.com
Schrödinger said something like this: If you think that small objects are weird (and Bohr and Eisntein both agree on this, and so does everyone else) then large objects must be weird too (in spite of appearing to the contrary). He illustrated this with an example that has now entered the realms of scientific classics, a thought experiement involving a decaying particle (small object), a fragile bottle with poison and a cat (a large object), all enclosed into one room.

Quantum mechanics suggests that the decaying particle is typically in the state where it has decayed and not decayed at the same time. If is has decayed then the bottle is broken, the poison released and the cat dead; if the particle has not decayed, the bottle stays intact and the cat is alive. But quantum mechanics suggests that these two possibilities, these two very different worlds, exist at the same time. So the microscopic quantum weirdness implies the simultaneous existence of dead and alive cats, and these are two very different macroscopic possibilities. Therefore it is difficult to maintain Einstein’s naive realism, and it is also surely difficult to maintain Bohr’s ultra pragmatism.

But wait a minute, are there any experiments vindicating Schrödinger? Well, yes and no. There are, of course, no observations of dead and alive cats (yet!), but there are observations of entanglement in solids containing the number of atoms comparable to that making up a cat. So macroscopic entanglement has now become experimental reality and its probably only just a matter of time before we can experiment with entanglement in living creatures. And this, I think, is the biggest recent twist in the debate started by Borh and Einstein.

Micromirror used in quantum experiments (Credit: ARKITEK)
What physicists typically do is as follows: They take a photon (or a bunch of them)—which plays the role of the decaying particle in Schrödinger’s thought experiment—and bounce it off a large mirror typically containing one thousand billion atoms. This mirror plays the role of a cat. If the photon is reflected the mirror has to recoil while if the photon is transmitted the mirror remains stationary. The two states of the mirror are the analogues of the dead and alive cat. Experiments of this type are currently being conducted by Anton Zeilinger in Vienna, with the help of an FQXi grant, and there is little doubt in anyone’s mind that they will corroborate Schrödinger. (Grace Stemp-Morlock’s article, “Quantum Upsizing“, gives more details.)

This type of an experiment is typically very difficult. But we don’t have to work so hard to detect macroscopic entanglement. We can, for example, measure some macroscopic properties of solids, such as their response to the external magnetic field, and this response can tell us if the atoms in the solid are entangled. Currently we have experiments at room temperature and higher, done in Rio in Brazil by Souza et al in Centro Brasileiro de Pesquisas Fisicas, using copper carboxylate and confirming entanglement between its atoms. These experiments use methods of detecting entanglement that I have developed over the last 3 years or so and are not so difficult to perform.

All this tells us that macroscopic entanglement is now beyond any reasonable doubt. To move closer to Schrödinger’s thinking, however, we need to ask if this macroscopic entanglement plays any role in biology. The jury is still out on this one, but if I was you I’d attentively watch this space from now on.

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John Merryman wrote on Jan. 18, 2009 @ 02:10 GMT
But does the mirror split off into many worlds, or does its entangled state collapse into one?

Or did the mirror even have one state to begin with?

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amrit wrote on Jan. 19, 2009 @ 11:39 GMT
Causality problem of Fermi two atoms, see article below

http://prola.aps.org/abstract/PRL/v72/i5/p596_1

can be resolved with a model where time is a measure of motion of atoms an space.

Atoms move in space only and not in time.

Idea of causality is linked with idea of events running in time, idea of dynamic equilibrium is linked with idea of motion in space where motion tends to the dynamic equilibrium.

see on file attached

attachments: 1_Time_is_a_Measure_of_Motion_Sorli__2009.pdf

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Blogger Vlatko Vedral wrote on Jan. 19, 2009 @ 16:14 GMT
A very brief answer to the question above is this. An entangled state already represents a superposition of many different worlds. So, if no photon hits the mirror, then this represents one world (in which the mirror does not move). If one photon hits the mirror, this creates another world in which the mirror has moved a bit. When two photons hit the mirror we have yet another world where the mirror has moved twice as much and so on. All these exist at the same time. It is exactly the same as the dead and alive cat but with many more options (i.e. worlds) present.

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Buck Field wrote on Jan. 19, 2009 @ 19:21 GMT
Vlatko,

In the Bohr quote, he claims there is no real, natural "quantum world", but then states "physics concerns what we can say about nature". Why is this not a "stolen concept fallacy", assuming nature's existence in order to deny it?

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Blogger Vlatko Vedral wrote on Jan. 20, 2009 @ 01:37 GMT
Dear Buck,

I believe you are right. All physicists are realists deep down. We think there is something out there that we then try to understand. However, if you define reality as values of certain quantities existing prior to measurement (so called hidden variables) then quantum physics rules some of these realities out (but by no means all). It is in this (very narrow, one might say) sense that quantum physics challenges (some specifically defined form of) reality.

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Brian Beverly wrote on Jan. 20, 2009 @ 19:31 GMT
The current mystical mechanics seems to be centered around the use of imaginary time. I would appreciate it if someone would explain how imaginary time is justified.

The measurement problem in physics is where it is implied that imaginary time is ordered:

(...[-itn,...,-it2,-it1,0,it1,it2,...,itn]...)

The mathematical axioms tell us that complex numbers can not be ordered.

Order Axioms:

1) A number can not be less than itself

2) x > y, x < y, or x = y

3) if x > 0 and y > 0, then xy > 0

4) if x < y, then for all z, x + z < z + y

5) if x < y, then for all z, xz < yz

set x = i and y = 2i and z= 2 + i

1) makes sense

2) i < 2i makes sense

3) a bit tricky:

0 = 0 + 0i and i = 0 +1i therefore i>0 and 2i>0

(i)(2i) > 0 ---> -2 > 0 FALSE!

4) 2 + 2i < 2 + 3i (complex # is of the form a + bi)

5) This is the key axiom!

xz = what exactly? xz or x*z (* is complex conjugate i*=-i)

If we distribute xz as we do for real numbers then axiom 5 is false. If we take the complex conjugate x*z then axiom 5 is true.

Quantum mechanics relies on C* algebra which is ordered. What is the big idea of C* algebra? C*C, multiply a complex number by a complex conjugate and you end up with a real ordered/countable number.

By the axioms of math the measurement problem should not exist in physics and neither should mystical mechanics.

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Don Limuti (www.zenophysics.com) wrote on Jan. 21, 2009 @ 06:52 GMT
Vlatko,

Your statement: "All physicists are realists deep down" is at the core of the many worlds thesis (in my opinion). There is an unacknowledged assumption that the mirror and the photons all exist continuously. If they do exist continuously the many worlds logic makes some sense. I believe there are reason to suspect that the all objects are not in continuous existence and that entangled objects come into existence simultaneously. With this viewpoint the many worlds thesis is not needed and only makes things more complex and mystical.

Of course you may say that objects not being in continuous existence is very mystical. I would counter that this mysticism is the essence of the Schrodinger wave equation. And that the objects existence (appearance and disappearance) is at the objects wavelength and period.

Perhaps our measurements are things that happen at discrete times and places and objects are also things that occur at discrete times and places.

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John Merryman wrote on Jan. 21, 2009 @ 19:49 GMT
Vlatko,

Doesn't this assume a universal perspective? In that all states/worlds co-exist because this absolute point of view is all seeing, as opposed to one state exists from one perspective, but doesn't exist from another perspective in which another state does exist?

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John Merryman wrote on Jan. 22, 2009 @ 10:46 GMT
If you and I were staring at the same mirror, we would see two different images, just as Schrodinger and his cat have two different perspectives. The point of my observation was questioning whether there really is "one world" in the first place, or if that isn't just the assumption of a universal perspective which doesn't exist?

I think western civilization has a monolithic fixation that is politically powerful and religiously inclusive, but breaks down on the scientific level. We confuse absolute and ideal.

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Buck Field wrote on Jan. 22, 2009 @ 20:20 GMT
Dear Vlatko,

I appreciate your confirmation that I’m not completely missing the general cognitive structure of physical science. Philosophically, I would resist defining "reality as values of certain quantities existing" as in your hypothetical unless such definitions' value was strongly supported and situation specific. For applicable cases, that support seems to be mathematical convenience, (not to be under-valued), but from a project management perspective, it seems inadequate caution has been taken regarding the combination of epistemology and ontology for advancing the math. This seems to have lead to claims of “unknowability” that appear problematic in structure, and backed only by fallacious support. For example, Bohr claimed, "An experiment can show the particle-like properties of matter, or wave-like properties, but not both at the same time". This assumes three unchangeable constants: the understanding & definition of matter, waves, and constraints on experimentation. AFAICT, an assumption like that ought to meet a very high standard of evidence – if for no other reason than the progress of science is a road paved with the crushed expectations based on similar claims.

What I can’t explain is why there is no detectible uproar in the community over seemingly well-accepted and yet unjustified assumptions. Thoughts?

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Daniel wrote on Jan. 23, 2009 @ 16:35 GMT
No uproar.... well (a deep subject)

OK one last "time" luv the fact that everyone likes psudomystical interpretations but when it comes to reality Schrodinger wasn't quantifying the cat being in both states so much as the role of the observer as part of the experiment. For further evidence please see young's double slit experiment ...

ps are there any real (AKA researching having research funding etc.) here I am just curious. Anyone willing to ask the question and seek the answers is a scientist!

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Brian Beverly wrote on Jan. 24, 2009 @ 01:06 GMT
Here is an article about quantum "uncollapse" where a weak measurement is made partially collapsing a wavefunction. After the weak measurement stops the wavefunction "uncollapses":

http://www.rochester.edu/news/show.php?id=3220


Has anyone heard of this experiment? Does it rule out the MWI? Is it possible any experiment can disprove the MWI?

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Buck Field wrote on Jan. 29, 2009 @ 20:01 GMT
Daniel,

I would disagree that anyone willing to ask questions and seek answers is a scientist, since this would include shamen on a vision quest.

Rather, I would claim that it is the method used to approach the situation, formulate the question, and seek the answer. If the person is commited to using the most reliable scientific method to understand objective truth, and follows through on that, then we may judge them a scientist.

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Lawrence B. Crowell wrote on Feb. 1, 2009 @ 20:02 GMT
Suppose there are two mirrors and a beam splitter. A photon is split so it has a 50% probability of traversing either arm to the two mirrors. Now orient the interferometer so one path is vertical and the other horizontal. The "Schrodinger cat" here is the motion of the two mirrors. In this case the two states also involve a superposition in the gravitational potential due to the displacements of the mirrors.

The uncertainty relationship would appear to involve the change in the gravitational potential &E = g&h, for a displacement of the vertical mirror by &h. This change in the potential is an energy functional associated with different states of a mirror.

Lawrence B. Crowell

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Buck Field wrote on Feb. 2, 2009 @ 20:11 GMT
Dear Brian Beverly,

I am not familiar with why the MWI is considered a useful model, can someone provide links as to how this interpretation is justified as having greater utility in some situations?

In Schrodinger's claim that: "If small objects are weird, then large objects must be weird too" AFAICT, this statement commits the fallacy of composition, doesn't it?

Steven Weinberg asks: "So where do the probabilistic rules of the Copenhagen interpretation come from?" Don't they come from the probabalistic nature of measuring energy using matter-based instruments that detect > ©¤ quanta? Non-locality and super-positioning seem to confirm this to the casual outsider...

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Buck Field wrote on Feb. 2, 2009 @ 20:14 GMT
CORRECTION: that should have appeared as "...greater than h-bar quanta..."

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Lawrence B. Crowell wrote on Feb. 2, 2009 @ 22:08 GMT
When you stop to think about it the quantum world is not at all strange. You have nice Hilbert spaces of states, unitary and Hermitian operators, linear wave equations for solutions over the states and so forth. It it the simplest thing in the world. What is really strange is the classical or macroscopic world!

A rule of thumb for thinking about the classical/quantum dichotomy is that the macroscopic world is due to dechoherence. Draw a circle, and then by random means put dots on the circle. Where ever there is a dot think of an arrow connecting the center to the dot. In the complex plane this is the phase of a state, or a particle. Now if we put thousands of dots on the circles, draw the arrows it is clear that a vector sum of these is nearly zero. These phases all cancel out or destructively interfere. For the most part the system has no phase or wave behavior. Consider a system with a mole of atoms or modes.

Now take a transparency, draw the same circle and put one dot on it. That is the phase of your system prepared in a pure state. The other circle is a statistical distribution of states. Now lay the transparency over the first cirle, which means your system is coupled to a statistical reservior. The phase of your system couples into the reservior and is buried.

This is about the best qualitative description I can give right now for a measurement without getting into the real physics.

Lawrence B. Crowell

Lawrence B. Crowell

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Buck Field wrote on Feb. 3, 2009 @ 01:17 GMT
It seems very hard for us to remember that our constructed models of reality, such as linguistic constructs or mathematical models are attempts at accurate representations of reality. To manage risk, and avoid wasting our limited resources, we are wise to be careful with our assumptions, as they always have one or more flaws. If we mentally distinguish and explicitly acknowledge when we are discussing ontological "reality" of what exists, phenomenal "observation" that indicates that extant reality, and our epistemological processes for creating a particular model, we may be able to avoid centuries of pursuing dead-end investigations. This plodding adherence to defined categories is often tedious, a seemingly nit-picking game for those who just want to argue, but adhering to well-established cognitive practices and standards for information systems change are very good at purchasing us new knowledge abilities if we are willing to put in the plodding grunt work.

For example: if our species is going to do things impossible under current cosmology, like enable supraluminal travel, we must remember that quantum mechanics ideas like a Hilbert space of states is not anything that exists. It, the equations, operators and solutions are descriptions of concepts: ideas we have built with the hope that sufficient isomorphism exists between them and the dynamics of whatever reality gives rise to our observations so that we can predict what we will observe in the future and interpret past observations. Of course, this means that we must also honestly admit the uncomforable truth that these concepts are not even models of reality but of observations, which makes virtually everything in particle physics harder. For example: as far as we know, there is no more reason to regard current assumption of the reality of the “particle” concept as any more "real" than the old "central position of the earth” concept, and both conceptions have similar observational anomalies, and shared increasingly complex additions to those concepts in an effort to resolve observational anomalies. Therefore, while investing significant resources in studying “particles” as a ontological extant, such as spending hundreds of researcher-years studying “decoherence” may appear not only quite reasonable, but the best option, in fact it is a very risky allocation of our all-too-limited lives from an historical perspective. This is what troubles me about the bulk of what I can understand in current physics research.

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John Merryman wrote on Feb. 3, 2009 @ 03:56 GMT
Lawrence,

"The phase of your system couples into the reservior and is buried."



Just because we are the center point of our three dimensional coordinate system doesn't make it objective. The problem is that an objective perspective is an oxymoron. When we try to see the whole, it's just increasing chaos, because we need perspective to give it description. To describe is to limit. To limit is to describe.

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John Merryman wrote on Feb. 3, 2009 @ 04:03 GMT
Buck,

Science means to take our best models, find their weak spots and see if they break. Religion is to keep patching over the weak spots in order to save the model. Science is bottom up. Religion is top down.

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Lawrence B. Crowell wrote on Feb. 4, 2009 @ 13:59 GMT
The example I gave pertains to the complex plane for the phase of a quantum system. The unit circle and vectors from the origin are abstract constructions.

The quantum world is not that mysterious. You have a linear state space, unitary and Hermitian operators for transformations and observables, a linear wave equation and the rest. Things could not be any simpler. What is mysterious is...

view entire post


attachments: confession.jpg

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John Merryman wrote on Feb. 5, 2009 @ 18:36 GMT
Lawrence,

It just seems our minds are fundamentally linear, but reality is not, so we keep bundling these webs of activity into some causal logical sequence, ie, equations, but only get further lost in the detail. What's the old saying, "The more you know, the more you know you don't know."

Not that I make any pretense of being a physicist, but I'm forced to live in this world and find fantasies, no matter how enticing, to be dead ends. Which requires me to make some effort to understand the reality. Thus my love/hate relationship with physics.



One idea that had been running through these conversations that I do seem to have some grasp of is entropy. For all its foundational status, it seems a particularly crude not terribly valid concept. For one thing, doesn't gravity essentially refute it? If the universe were entirely entropic, it seems there would be an fairly even distribution of mass/energy, yet gravity seems intent on accumulating these concentrations of the stuff. I know the expanding universe is supposed to fade out as everything flies apart, but that goes back to my point about the relationship of the geometry of space vs. lightspeed and we don't seem to come to a conclusion on that, so even if the Big Bang universe happened, there has to be some law of increasing returns which created the low entropy singularity in the first place and we go back around the circle anyway.

So it seems to me that physics is like very old software that has had many generations of patches and it's about time to do a complete rewrite. You are particularly good at piloting that old software and likely don't want a rewrite, but maybe it's time to stop trying to rivet everything together and start taking it apart to see what is good and what is junk. Maybe it really is the best way, but just needs cleaning up.

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