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I figured we could use a thread about physics and Science! in general, and no better starting place than this new video on the changing views towards Dark Matter and modified gravity...
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One of the bigger papers in foundational physics in the last few years, the Frauchiger-Renner paradox concerning the impossibility of using quantum theory to describe observers themselves and their actions:
Quantum theory cannot consistently describe the use of itself - Nature Communications
Quantum mechanics is expected to provide a consistent description of reality, even when recursively describing systems contained in each other. Here, the authors develop a variant of Wigner’s friend Gedankenexperiment where each of the current interpretations of QM fails in giving a consistent...www.nature.com
In a common trend recently it's one of those things suspected by Heisenberg, Bohr and other early developers of QM, but only recently proven.
That's cool about virus scanning stuff!Sounds like a quantum version of Godel's incompleteness theorem, which states something to the effect that a formal (i.e. 100% rules-based) system can never be fully self-describing. The most famous application of this is perhaps Turing's Halting Problem, which was a mathematical proof done by Turing in the 1930s where (at least in the general case) you can't predict what an algorithm or program will do by statically analysing the code without running it. This is why virus scanning software is very hard to make and you should assume anybody claiming foolproof computer security is either lying or incompetent.
And I said this:@Seadna, what do you think of M-theory and the notion of a holographic universe?
with this as additional detail:The holographic conjecture is interesting but it's a property of String theories in a universe not like ours. It's cool, but not only is there the ambiguity in whether String theory is right, but also whether its properties in those fictitious universes also hold in ours.
Neils Bohr said:Physics is to be regarded not so much as the study of something a priori given, but rather as the development of methods of ordering and surveying human experience
If you enjoy a bit of a deeper dive.So physics is limited specifically to human ability/perception. That is fascinating.
Or as Pauli described it in a letter to Born:Einstein said:Most of [Bohr, Heisenberg, Pauli, Born, etc] do not see what a risky game they are playing with reality - reality as something independent of what is experimentally established
Pauli said:Einstein's point of departure is 'realistic' rather than 'deterministic', which means that his philosophical prejudice is a different one
As any fule kno, when taken from a non-linear, non-subjective point of view, it's more like wibbly-wobbly timey-wimey stuff.I'm saying this in advance because I imagine stuff like this might be in general media with titles like "time is an illusion". Where as the actual message is only the common everyday experience of time is concrete, older mathematical notions of time invented for calculus are mental constructions.
If you like Hofstadter you might really enjoy "Are Quanta Real?" by J.M. Jauch which Hofstadter himself praised.Reading it reminded me of the statement by Lewontin that a common cause of a lot of modern scientism is a lack of grounding in the philosophy of logic.
I suspect part of this (based off teaching the subject in different countries) is that Anglophone society in particular has constructed a very strong Mysticism/Materialism dichotomy so that if you say materialism is false people immediately think you're saying ghosts, psychic powers etc are real. I noticed much less of this sort of thing in Japan but even places like France where the philosophical tradition is a bit different.Bohr said:We must be clear that when it comes to atoms, language can be used only as in poetry. The poet, too, is not nearly so concerned with describing facts as with creating images and establishing mental connections.
Hideki Yukawa and Richard Feynman in Kyoto in 1954. Yukawa was the first person to explain why the nucleus of an atom didn't just explode immediately since it contains positive charges packed close together. He proposed that protons and neutrons exchanged an additional particle, the pion, which kept them glued together with a strength the electromagnetic force couldn't overcome. He later received the Nobel prize for this work.
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The JUQUEEN supercomputer cluster in 2014, Europe's most powerful at the time, which running at full power with the second most powerful cluster the JUROPA for several days managed to simulate a single proton well enough to compute its mass to within a percent.
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Chen-Ning Yang and Tsung-Dao Lee work out the early details of the Weak Nuclear force. Yang would go on to figure out the fundamental equations controlling all forces except gravity. Both won Nobels.
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Shin'ichirō Tomonaga, ardent Japanese traditionalist and the first person to correctly combine quantum theory and relativity. Julian Schwinger would do the same within a year in the United States but post-war difficulties prevented communication and neither side would know of each other's work for another year. Funnily enough Tomonaga and Schwinger both mean "Shaker". Richard Feynman later combined quantum theory and relativity using very different methods which are easier to use, but less fundamental than those of Tomonaga and Schwinger. All three would later win the Nobel for this work.
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Very recent and exciting experiment (July 2021)! A machine that pumps positrons into silicon crystals in Bar Ilan university Israel. This provided the first experimental confirmation that even the number of atoms present in a material is observer dependent and non-objective. An effect first predicted by Canadian professor Bill Unruh in British Columbia in 1976.
Stationary detectors perceived the crystal as having the usual number and structure of atoms, but a highly accelerated detector detected a "bath" of super-heated atoms within the crystal. Thus confirming there is no objective answer to "How many atoms are there in an object". Just contradictory subjective answers. Contradictory because if you imagine the superheated atoms are objectively present they should have superheated the stationary detector. The presence of two mutually contradictory truths like this being called by Bohr "Complementarity".
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I know for some scientific simulations they're being replaced by clusters of GPUs, but I don't know much beyond that.Supercomputers are funny things. The first teraflop supercomputer was ACSI Red in 1996. Today I think it would be possible to build a machine off Ebay for maybe £10k or so that would outperform it on a fair variety of workloads.
Pretty much all supercomputers since about 2000 or so have been clusters. Big SMP machines did get some play but the biggest have been clusters since the 1990s or so. You can put GPUs into a cluster. There are a few places where you can get bottlenecks.I know for some scientific simulations they're being replaced by clusters of GPUs, but I don't know much beyond that.
He covers the stuff about the decline of String Theory well, but there are three small things I would expand on. This isn't a disagreement with anything he says, just an expansion since I think it might be of interest.The Babbage podcast from The Economist has a good discussion on the decline of string theory due to evidence from recent experiments (nice to see the string theory skeptics finally winning the day) and something called entropic gravity.
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Schrodinger's issue was that he could set up and describe mathematically a situation where this seemed to apply to a cat's being alive or not. Rather than being dead and alive at the same time, the issue was much stranger. The cat was neither dead nor alive but rather "undefined" until somebody chose to look in the box.Essay titled THOUGHTS ON NON-LOCALITY AND QUANTUM MECHANICS said:in quantum mechanics [...] one cannot regard the system in question as having a value for the quantity of interest in the absence of measurement
Essay titled RESPECTING ONE'S FELLOW said:our definition of an agent is broad: It does not rule out attributing agency to dogs, euglenas, or artificial life. However, it does exclude a computer program that deterministically “chooses” an action from a look-up table
Thanks for the enlightening read; could you please add a paragraph about GPUs and Bitcoin? I've read that part of the general computer chip shortage is a GPU shortage caused by people hoarding GPUs in farms to mine Bitcoin (which, I gather, is mainly finding prime numbers in the bazillion range). All of which is, at least to me, a pretty dark matter in many respects.[…]
TL;DR: If you really wanted to, you could build a PC with comparable throughput to a supercomputer that was state of the art maybe a couple of decades ago.
GPUs can exploit parallelism to do the hashing calculations for crypto mining much faster than a CPU can. You can also now buy dedicated ASICs for mining have the hashing functions built into hardware and are faster than GPUs. The GPU shortage was caused by a couple of factors; demand for GPUs from miners was one, but the other one was the manufacturers underestimating the demand for GPUs, which has spiked with COVID as more people are stuck at home.Thanks for the enlightening read; could you please add a paragraph about GPUs and Bitcoin? I've read that part of the general computer chip shortage is a GPU shortage caused by people hoarding GPUs in farms to mine Bitcoin (which, I gather, is mainly finding prime numbers in the bazillion range). All of which is, at least to me, a pretty dark matter in many respects.
It's all Greek to me.Just a little visual contrast. The Standard Model Lagrangian, i.e. the expression describing matter and the three quantum forces (Electromag and the two Nuclear forces).
Section 1 is the Strong force, Section 2 the Weak and EM forces, Section 3 how matter feels the weak force, Section 4 is matter interacting with the Higgs field, Section 5 is a pile of terms describing the complicated geometry of the fields making up the weak and strong forces:
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This is gravity:
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This must be what science looks like to muggles.Just a little visual contrast. The Standard Model Lagrangian, i.e. the expression describing matter and the three quantum forces (Electromag and the two Nuclear forces).
Section 1 is the Strong force, Section 2 the Weak and EM forces, Section 3 how matter feels the weak force, Section 4 is matter interacting with the Higgs field, Section 5 is a pile of terms describing the complicated geometry of the fields making up the weak and strong forces:
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This is gravity:
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It's all Greek to me.
</dadjoke>
He covers the stuff about the decline of String Theory well, but there are three small things I would expand on. This isn't a disagreement with anything he says, just an expansion since I think it might be of interest.
- The first is where he says General Relativity and Quantum Theory are contradictory. I would more say we consider it "odd" that gravity is still treated as a classical objective force in the 19th century sense and most physicists "feel" it should be quantum like everything else, but that could easily be wrong. Some even think gravity plays a large role in the emergence of objectivity. Regardless keeping gravity as classical causes no contradictions or issues and experimental evidence certainly doesn't rule it out.
- Second, and this is more an expansion on what he says, String theory actually isn't literally about very small vibrating strings. Most quantum theories say the subatomic level can manifest as particles or fields or strings or (loads of others stuff) in our devices. String theory differs from other theories by proposing that the string manifestation is more important for gravity, where as our experience is that the field one was the most important for the other forces. "Most important" here means "theory is mathematically simplest when written in terms of that aspect".
- Finally the thing about qubits being "up and down at the same time". This is worth focusing on a bit, especially using Schrodinger's cat since it goes to the heart of many aspects about QM.
As mentioned before on this thread in QM (to make this shorter) the theory doesn't give a value or description to anything outside of direct perception by observers. To quote Bill Unruh from above:
Schrodinger's issue was that he could set up and describe mathematically a situation where this seemed to apply to a cat's being alive or not. Rather than being dead and alive at the same time, the issue was much stranger. The cat was neither dead nor alive but rather "undefined" until somebody chose to look in the box.
This is really nothing more than applying the same principles in atomic physics to the everyday world. If an atom has no well-defined position until an observer checks, well why not go whole-hog and say nothing is well-defined until an observer checks? Including something as stark as an animal being dead or not.
The "dead and alive" language rather than the more correct "status undefined until somebody looks" arose in the 1980s when some people wanted a way to visualise how a quantum computer works. Imagining it as a suped-up parallel processing computer. It was a noble idea I would say, but we now know it doesn't work. Just like every quantum system a quantum computer isn't given "gears and wheels" as to how the computation is accomplished. It's just "if you do XYZ to a crystal, then measure the crystal in manner ABC, the crystal is 80% likely to spit out the answer to this computational problem encoded in the measurement data". Physical systems can just spit the completed answers for problems back to an observer without any computing or mechanism.
As for Schrodinger's original question as to whether the cat is alive or dead before somebody looks, this really reduces to who is an observer. I don't want to give my own view in detail, which would be just one biased position. However it is worth seeing this quote (different from my own view) from Chris Fuchs the head of the Quantum Information group at UMass Boston and probably the world leading expert on Quantum Information:
The last option has to be excluded due to certain theorems (the Frauchiger-Renner theorem I linked to above is related to this) showing that something controlled by an algorithm can't be an observer, but after that there is little we can say with certainty.
It's certainly not a stupid question. In fact replacing the cat with a human is what Eugene Wigner did in his 1963 "Wigner's Friend" essay to sharpen the question. It'll just take steps to answer, if you don't mind I'll break it into separate posts since it'd just be a massive wall of text otherwise.Forgive me if this is a stupid question but doesn’t the cat itself count as an (self) observer? If the experiment is changed and it is instead a human being in the box, how could they be in a state of uncertainty if they themselves were observing that they were alive and well (or dead)? I understand that the observer outside of the box has no way of determining what is going on inside, but wouldn’t that uncertainty have already been ‘set’ by the observer within?
or is it all completely dependent on individual observances?