The Quantum Physics Sequence

post by Eliezer Yudkowsky (Eliezer_Yudkowsky) · 2008-06-11T03:42:14.000Z · score: 36 (36 votes) · LW · GW · Legacy · 26 comments

Contents

  Basic Quantum Mechanics:
  Many Worlds:
  Timeless Physics:
  Rationality and Science:
None
27 comments

This is an inclusive guide to the series of posts on quantum mechanics that began on April 9th, 2008, including the digressions into related topics (such as the difference between Science and Bayesianism) and some of the preliminary reading.

You may also be interested in one of the less inclusive post guides, such as:

My current plan calls for the quantum physics series to eventually be turned into one or more e-books.

Preliminaries:

Basic Quantum Mechanics:

Many Worlds:

(At this point in the sequence, most of the mathematical background has been built up, and we are ready to evaluate interpretations of quantum mechanics.)

Timeless Physics:

(Now we depart from what is nailed down in standard physics, and enter into more speculative realms - particularly Julian Barbour's Machian timeless physics.)

Rationality and Science:

(Okay, so it was many-worlds all along and collapse theories are silly.  Did first-half-of-20th-century physicists really screw up that badly?  How did they go wrong?  Why haven't modern physicists unanimously endorsed many-worlds, if the issue is that clear-cut?  What lessons can we learn from this whole debacle?)

26 comments

Comments sorted by oldest first, as this post is from before comment nesting was available (around 2009-02-27).

comment by gordon_wrigley · 2008-06-11T10:58:18.000Z · score: 3 (3 votes) · LW · GW

I don't suppose you could spend a post or two explaining in your nice easy to understand way exactly what it is a quantum computer is supposed to do and how this might impact the NP problems.

comment by Baughn · 2014-12-18T14:42:38.065Z · score: 2 (1 votes) · LW · GW

It's not exactly known.

Grover's algorithm provides a quadratic speedup over brute-force algorithms for solving NP-complete problems, but that still leaves them as exponential-time. BQP (Bounded error, quantum, polynomial-time; basically the complexity class of quantum computers) is suspected to be disjoint from NP and a superset of P, but neither is known for sure.

comment by Vladimir_Golovin3 · 2008-06-11T12:20:34.000Z · score: 6 (6 votes) · LW · GW

Eliezer -- this post goes straight to my Favorites. Giving concise, straightforward summaries to posts is a great idea.

My attention is very fragmented (thanks to my job), so when reading Quantum Arena, I completely missed the point of the post which you gave in your excellent summary:

"Instead of a system state being associated with a single point in a classical configuration space, the instantaneous real state of a quantum system is a complex amplitude distribution over a quantum configuration space."

This sentence, in fact, has told me a lot more than the entire post -- which, of course, I'm going to re-read now.

Please consider including such summaries / guides for every series of posts you write. It would be even better if you could include such summaries into a dependency graph like the one you posted here earlier -- for example, the summaries might display when a user hovers the mouse cursor over a graph node.

comment by Bill_Mill · 2008-06-11T13:34:41.000Z · score: 3 (3 votes) · LW · GW

Eliezer - will these e-books be edited by a professional editor, a friend, or just yourself?

Gordon - Scott Aaronson gave a wonderful explanation of quantum computing at his blog.

comment by Infotropism2 · 2008-06-11T14:02:10.000Z · score: 1 (1 votes) · LW · GW

Question : money derived from those ebooks, goes it to the singularity institute ? In other words, is buying one of these equivalent to donating money to the institute ?

comment by Tom3 · 2008-06-11T14:06:13.000Z · score: 1 (1 votes) · LW · GW

In the Timeless Physics section:

"The laws of physics are perfectly local; the configuration space is perfectly local."

Aren't the laws global?

comment by Michael_G.R. · 2008-06-11T16:35:47.000Z · score: 0 (0 votes) · LW · GW

Will the eBooks also be available as hard copies? I'm probably not alone in preferring to read long texts on paper, and printing them out isn't quite the same.

comment by Boris · 2008-06-11T19:29:41.000Z · score: 0 (0 votes) · LW · GW

Eliezer, thank you very much for all of these posts. I believe that this post is an excellent way of presenting them all in a reasonable sequence.

comment by Emil_Gilliam · 2008-06-12T02:05:58.000Z · score: 0 (0 votes) · LW · GW

I hope you can include this Tom the Dancing Bug cartoon in your e-book, as an example of what you're arguing against.

comment by Grif · 2012-08-02T06:09:53.348Z · score: 2 (2 votes) · LW · GW

Broken link. Here's a new one.

comment by David_Veksler · 2008-06-12T15:34:56.000Z · score: 0 (0 votes) · LW · GW

This link appears to be broken:

http://lesswrong.com/lw/r7/quantum_physics_revealed_as_nonmysterious/

comment by Gherda · 2011-12-19T14:49:49.356Z · score: -4 (4 votes) · LW · GW

Using Quantum Mechanics to identify "possibility" as the smallest of a certain minimum amount of anything (The smallest “thing")

I'm a Banker interested in quantum mechanics and my (tongue in the cheek) conclusion is that it un-doubtfully identifies "possibility" as the smallest of a certain minimum amount of anything (The smallest “thing"). Here it goes...

• The smallest of a certain minimum amount of anything (The smallest “thing”) does not exist in time or space. • Although it does not exist in time or space (attributes of non-locality), it has the potential to exist i.e. to be located at a particular point in space and time.
• The smallest “thing” can thus be identified as a “possibility”.
• A possibility “grows” to become “the smallest thing” when its non-locality attributes are altered to such an extent that it can be observed at a particular point in space and time (located). • Conversion from non-locality to locality occurs when waves (mind waves) use freedom of choice to evaluate all possible locations and then to form an idea of where the smallest thing must be at a particular point in time and space. • This results in the creation of matter in time and space. • An idea is nothing more than a brainwave” which has unlimited access to universal intelligence and freedom of choice to apply consciousness to consider all possibilities and then to form an idea of the desired outcome. • This “possible outcome”” has the capability to exist in any one of the parallel universes at any given time and my understanding of it is as follows:

This smallest “thing” that exists is also the biggest thing that exists. It is a thing called “possibility” which is the sum total of everything that ever has or ever will exist in all the parallel universes.

Possibility is energy with momentum or better explained as waves of energy with the capability of manifesting as a particle i.e.: it is both:

a) A wave (which is everywhere ... all of the time i.e. it is not subject to the concept of time and space); and b) A particle (which has neither mass nor electric charge but possesses energy and momentum to manifests itself as a particle with mass and electric charge) once consciousness is applied.

“It is the wave that determines a precise position for the particle at any particular time”. This means that at its core (in its simplest form) the smallest thing is consciousness applied to a possibility using the freedom of choice of the mind wave to locate (observe) the thing at a specific point in time and space.

The conscious mind can therefore create anything is can possibly think of; in any one of the parallel universes at any given point in time.

comment by Just_existing · 2012-01-16T18:55:14.886Z · score: 4 (4 votes) · LW · GW

This is for everybody who can't wait on the upcoming e-book. I put the Basic quantum explanations in an e-book format myself, which you can download here: http://www.share-online.biz/dl/COXXQR5M6NJ0 The format is for the Amazon Kindle. I created it by copying the html-versions into Word, removing the comments, saving them as .pdf, and finally i created the e-book with callibre. The pictures are sligthly above the place where they are supposed be, apart from that everything should be fine.

Enjoy!

comment by [deleted] · 2012-02-06T09:49:23.609Z · score: 3 (3 votes) · LW · GW

This reminds me, it has been over three years since this was stated:

My current plan calls for the quantum physics series to eventually be turned into one or more e-books.

Considering this has not happened yet, I'm just wondering if there has been any word if this is still the plan.

comment by ashu_437 · 2013-10-14T00:27:51.453Z · score: 0 (0 votes) · LW · GW

This link seems to no longer work. Can you please upload it somewhere else? Thanks.

comment by ete · 2013-10-17T01:47:32.613Z · score: 0 (0 votes) · LW · GW

I've read all posts in the Basic Quantum Mechanics section, plus many of the links from it and a handful of others (working through the rest, I'm still only three days into this). Quantum mechanics is something I've had vague explanations of from education and discussions with educated people, but it seemed extremely complicated and confusing due to almost precisely the issues touched on in the normal way it's taught. Thank you for putting down the steps needed to walk me through rewriting my basic assumptions of reality to more accurately reflect how reality likely works, it's been very fun and interesting. I'm starting to feel like a native of the quantum universe, and.. it kinda makes sense. Definitely a whole lot more sense than my previous mangled understanding of probabilities and wave/particle duality. Having a base level reality which works very differently from high level phenomenon which feel more intuitive does not seem like a great surprise.

Anyway, one idea I've had which seems interesting to me, but I am not yet in knowledgeable to evaluate properly and would like thoughts on:

Would you, under the many worlds interpretation, be able to experimentally test whether a universe is infinite in time but not space?

I know that infinite time+finite space not a favored model for cosmology currently, but it's still interesting to me if quantum physics testably disproves a whole class of possible universes. And if by this (or similar) reasoning an infinite time/finite space universe is found to be incompatible with many worlds, finding evidence extremely strong evidence of an infinite time/finite space universe (highly unlikely as I understand it) would perhaps bring many worlds into question.

Possible line of reasoning:

  1. In a universe with finite space, there is a finite configuration space (finite amount of physical space, so finite possible universal states).
  2. Any particular blob of amplitude/branch/world will eventually evolve into a state of/near maximum entropy.
  3. Maximum entropy is not entirely stable even if no work can be extracted from it, so it is not a static point in configuration space.
  4. A non-static point in finite configuration space left to move for infinite time will eventually visit all possible arrangements of amplitude (configurations), infinite times. This includes Configuration A, which can be any possible point in configuration space.
  5. In both (particle left, sensor measures LEFT, human sees "LEFT") and (particle right, sensor measures RIGHT, human sees "RIGHT") blobs of amplitude, the universe evolves differently for a vast amount of time after the heat death of the universe, but given infinite time will at some point reach Configuration A with probability 1.
  6. Since both blobs of amplitude will, despite diverging for an unimaginable length of time, arrive at the same configuration as each other with probability 1, they are fully coherent allowing them to interact, and this is testable (and already falsified).

Points one, three, and four seems to me like the most likely weak link, but I'd be interested to know why this is not the case if it is indeed not the case. Perhaps at maximum-entropy each branch gets stuck in a unique infinite loop rather than visiting the rest of configuration space?

If the chain of reasoning holds and leads to the conclusions.. perhaps a stronger version of this argument could perhaps be constructed for a universe infinite in both time and space (depending on whether indefinitely expanding thermodynamic systems will reach all possible configurations given infinite time), though I'm already feeling somewhat out of my depth dealing with the weaker argument.

comment by [deleted] · 2013-10-17T02:33:42.659Z · score: 0 (0 votes) · LW · GW

From a quick glance at your argument, it seems to me that quantum mechanics breaks down on the cosmological scale.

comment by ete · 2013-10-17T03:20:23.372Z · score: 1 (1 votes) · LW · GW

hm, from what I've been taking from the sequence quantum physics seems to apply fully at all levels, and the idea of it working differently/not applying is simply a matter of scale. For example an event causing a "split" affecting significantly macro objects almost entirely decohere, but not perfectly avoiding any kind of hard cutoff. Large systems definitely appear to work differently when you look at them on a large scale, but.. that appearance or classical hallucination is just an emergent property of underlying quantum effects.

Saying the quantum mechanics itself breaks down.. does not fit with the mental picture of reality I've taken from this, reality as entirely locally computable and with higher level effects based entirely on the base level substrate behavior. I'd like you to clarify what you mean by "break down", and preferably how reality would choose where to draw any line between scales where quantum mechanics does and does not break down?

I have read quantum physics has issues with gravity, perhaps that is what you're referring to? If so, I'd be interested in recommended further reading.

comment by [deleted] · 2013-10-17T11:48:30.563Z · score: 0 (0 votes) · LW · GW

from what I've been taking from the sequence quantum physics seems to apply fully at all levels, and the idea of it working differently/not applying is simply a matter of scale.

Nobody's resolved the fundamental problems between QM and general relativity (GR). EY comes close to claiming that MWI will do the trick, but it hasn't yet. At one point he even says that (paraphrasing) reality is a dream, and the dream satisfies special relativity -- at which point one has to ask why the dream doesn't satisfy general relativity.

Large systems definitely appear to work differently when you look at them on a large scale, but.. that appearance or classical hallucination is just an emergent property of underlying quantum effects.

See the previous note. Emergence is just another word for "magic."

Saying the quantum mechanics itself breaks down.. does not fit with the mental picture of reality I've taken from this, reality as entirely locally computable

This is an unfortunate side-effect of EY's tone in the QM sequence.

I'd like you to clarify what you mean by "break down", and preferably how reality would choose where to draw any line between scales where quantum mechanics does and does not break down?

The most evident problem in accepting both QM and SR simultaneously is that they substantially disagree on the value of the vacuum energy density.

comment by ete · 2013-10-17T12:38:04.415Z · score: 1 (1 votes) · LW · GW

What I think you're saying, correct me if I'm wrong, is that there's a few big unknowns as to how QM applies to gravity or on cosmological scales, and because of this the answer to my chain of reasoning is "we just don't know"? That there's major unknowns is entirely reasonable/accurate, but.. I'm struggling to see exactly how the very real and important unknowns apply specifically to my reasoning.

Simply put: Where, in the line of reasoning, do you think the unknown of quantum gravity trips up the logic, and why?

It's seems quite possible that in discovering the answers behind the big unknowns we'll change some underlying assumptions and render my reasoning unworkable. But I don't see where in the line of reasoning not knowing vacuum energy density, or quantum gravity, causes a problem. And given that, it seems like working with the best available theory means applying certain aspects of QM at universal scale is not unreasonable, though we should expect we may need to update models once some big unknowns are resolved.

I think my use of emergence does not fall into the emergence/magic trap, since I am not attempting to explain anything about how large scale systems behave through emergence, my statement is purely that whatever the details of how macro systems work the large scale effects are caused by local physics being consistently applied and only appearing to work differently due to taking a larger view. Even though I used the word "emergence", my sentence can be reworded with my intended meaning if you swap it to "emerges from", which is specifically allowed by that post.

Also, you think my picture of reality as locally computable is "an unfortunate side-effect of EY's tone in the QM sequence"? If that's the case, do you dispute reality as locally computable? I'd be interested in sources which coherently argue for reality being non-locally computable.

comment by [deleted] · 2013-10-17T14:50:14.140Z · score: 1 (1 votes) · LW · GW

What I think you're saying, correct me if I'm wrong, is that there's a few big unknowns as to how QM applies to gravity or on cosmological scales, and because of this the answer to my chain of reasoning is "we just don't know"?

We do know -- QM predicts the wrong vacuum energy density by several orders of magnitude. We've measured this value empirically. Read the Baez link; he explains everything pretty clearly.

my sentence can be reworded with my intended meaning if you swap it to "emerges from", which is specifically allowed by that post.

The critical unanswered question is how "underlying quantum effects" generate the (observed) geometry of space-time. If there were a solution to that question, I'd take no issue with you saying it emerges from those quantum effects -- but we don't know if it's actually the other way around, that is, if it's actually relativistic effects on the microscale that generate quantum phenomena. Or if this is just the wrong question entirely, and that both are caused by a third thing.

That's how you fell into the emergence/magic trap.

EY has to spend a lot of time in the QM sequence insisting that QM is natural and fundamental to get over people's preconceptions of it as unnatural. However, that leads to people taking it as the unique baseline physical theory, which it is not.

comment by ete · 2013-10-17T18:15:02.704Z · score: 0 (0 votes) · LW · GW

Okay, I think I see where you're coming from better now. I have read that link, and at least feel like I conceptually understand some of the problems with applying quantum physics to the large scale. However, I'm still very curious as to exactly how the incompatibility in theories applies to this specific argument, and curious as to whether looking at a purely quantum universe (making the assumption that there is some way to derive relativistic experimental results from QM that we've missed, rather than that QM needs major changes) would give the results I'm describing, or whether I'm misunderstanding something about amplitude or thermodynamics in a heat death.

Hm, how to explain clearly.. It seems like what's being said is QM is at odds with observation (vacuum energy density) and at odds with our other best theory, relativity, (event horizon, thanks for chiming in shminux), so QM is wrong or incomplete in some way. I accept this as a likely conclusion, though I do not understand either theory deeply enough to be able to follow the arguments for inconsistency in full.

However, dismissing a thought experiment about a widely used theory with some possible implications (if I've not missed anything and have understood various things better than I'd guess I have, that chain of reasoning could show a certain interpretation (MW) is incompatible with finite space+infinite time, while a different interpretation (collapse) would not be), due to the underlying theory (QM) being wrong/incomplete for other reasons seems.. limiting. Even if the line of reasoning only holds meaning with the assumption that the universe is fundamentally quantum, local, and macro effects are all explainable in principle by the laws which govern the smallest parts, I'm interested in whether or not it holds.

I'm primarily trying to refine my mental model of how decoherence works with these thoughts, and an answer focused on whether in a quantum universe would, from our current understanding of quantum physics, do as I suppose (that is, in finite space+infinite time, it could never even slightly decohere due to probability 1 arriving at an identical configuration eventually), or have I made some error in my reasoning which can be explained and would allow me to improve my model of decoherence?

comment by shminux · 2013-10-17T17:48:08.303Z · score: 3 (3 votes) · LW · GW

To chime in as a person with grad-level training in the subject matter: there is a glaring tension between Quantum Field Theory and General Relativity in the low-energy macroscopic-size limit, which is very bad. How bad? Imagine "proving" that 1=2 in Peano arithmetic, something like that. The issue is the black hole: firewall or horizon? question. GR says that there is nothing locally special about crossing the event horizon (and must be applicable, since GR has been tested in this low-curvature regime), while QFT says that, after a while, the horizon becomes a high-energy incinerator (and must be applicable, since quantum entanglement has been tested in this low-energy regime). The best physics minds on the planet are at a loss to explain the problem. Last time something like that happened in physics, a completely new and unexpected theory eventually resulted. Odds are, we are in for a similar paradigm shift some day, hopefully soon.

comment by Zaq · 2017-05-24T01:38:25.956Z · score: 0 (0 votes) · LW · GW

I see three distinct issues with the argument you present.

First is line 1 of your reasoning. A finite universe does not entail a finite configuration space. I think the cleanest way to see this is through superposition. If |A> and |B> are two orthogonal states in the configuration space, then so are all states of the form a|A> + b|B>, where a and b are complex numbers with |a|^2 + |b|^2 = 1. There are infinitely many such numbers we can use, so even from just two orthogonal states we can build an infinite configuration space. That said, there's something called Poincare recurrence which is sort of what you want here, except...

Line 4 is in error. Even if you did have a finite configuration space, a non-static point could just evolve in a loop, which need not cover every element of the configuration space. Two distinct points could evolve in loops that never go anywhere near each other.

Finally, even if you could guarantee that two distinct points would each eventually evolve through some common point A, line 6 does not necessarily follow because it is technically possible to have a situation where both evolutions do in fact reach A infinitely many times, but never simultaneously. Admittedly though, it would require fine-tuning to ensure that two initially-distinct states never hit "nearly A" at the same time, which might be enough.

comment by rot head · 2019-02-25T15:37:27.594Z · score: -9 (1 votes) · LW · GW

Yeh so your just pointing the finger, how do you make a quantum computer that actually works is what i want to hear. (but that defys sense in the fact that everyone would be fully armed with exponential logic, and its such a great idea to just spill the beans.)

comment by Raemon · 2019-02-25T21:28:09.253Z · score: 4 (2 votes) · LW · GW

Hey rot head,

In the past day you've posted a lot of comments, some of which were downvoted a bit. Reading over them, many of them seem a bit off topic, or joking in a way that wasn't quite appropriate to the conversation.

I would recommended aiming to post one comment a day for awhile, and put more individual effort into those comments.

This is your first warning. If you continue to make low value comments you'll be given a temporary ban.