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A new Stephen Hawking presentation about black holes (washingtonpost.com)
169 points by ColinCochrane on Aug 25, 2015 | hide | past | web | favorite | 95 comments



I don't understand what's new about these theories. Both the holographic principle and Hawking radiation have been known for years, decades even. If there's something about those theories the article doesn't explain what it is exactly.


"Nobel laureate Gerard t'Hooft, who was present for the discussion, has been thinking about information loss in a similar way, and he cited several papers he has published on the subject. It will take more discussion — and much comparing of math equations — to establish what's new about Hawking's theories in relation to t'Hooft's, and whether Hawking has overcome some of the issues associated with earlier iterations of the idea."


And here are the two discussing it today: https://twitter.com/ChancellorFolt/status/636306368861966341


So is Hawking even claiming it's new or was he simply discussing the existing theory but the article misconstrued that?


As I understand it, he is redefining what is called the event horizon, because it is a relativistic concept that doesn't work well with quantum mechanics.

One consequence of that redefinition is that humans could theoretically survive crossing it, contrary to the firewall hypothesis (well, if they were in a sustainable capsule with a large gene pool, and the hole was rotating just right), by waiting long enough on top of it until the hole evaporates (from the capsule's point of view).

Time would indefinitely slow down for them, so they would see the rest of the universe speed up until the hole dies.


An account of the event from an eye witness, with some additional hints: http://backreaction.blogspot.se/2015/08/hawking-proposes-new...


It appears to be mostly a clarification of his previous argument that information is retained in the [incomprehensibly small fluctuations of the] geometry of the event horizon.

http://arxiv.org/pdf/1401.5761.pdf

I'm guessing people are taking it seriously, but as it's not a complete theory of the dynamics it's hard to say "solved" in reference to this development.


> his previous argument

AFAIK that wasn't originally his argument, but he conceded and agreed that it was likely a better hypothesis than Hawking Radiation.

This talk seems to be a brilliant attempt at reconciling, what were previously, two competing hypothesis.


I'm somewhat glad to see so many people in this thread having the same "Didn't we already know this?" reaction to the article.


I don't know if there's something new in there, but Leonard Susskind wrote a book in 2008 exactly on that matter - "The Black Hole War" [0]. He describes his "battle" with Hawking on whether the information is lost in the black hole, and Susskind is the one who's against information disappearing.

[0] https://en.wikipedia.org/wiki/The_Black_Hole_War


I loved that book. One of the best books on cosmology/space-time/quantum-general relativity. Susskind's thought experiments are mind boggling, and he is a great writer, able to explain complicated physics in a clear and thoughtful manner.


Is this a book that someone with no background in the sciences involved can enjoy? Is it for the layman, or requires some knowledge of the facts involved to appreciate enough to read?


It's written in pretty easy way, you just must be interested in this stuff, then you'll enjoy it :)


Wait, hasn't this holographic theory been arround for years? What's new?


"It will take more discussion — and much comparing of math equations — to establish what's new..."


I don't know a thing about black holes, but every time I see him I'm impressed that someone with such severe disabilities is still able to accomplish so much.


In 'A Brief History of Time' he briefly discusses the intellectual advantages of his physical disabilities.


Not that I want to take anything away from his impressive achievements, but it would only be fair to point out that he doesn't work solo on these theories. Much of the maths is done by his team.

I can't even imagine how frustrating his condition must be for him though. To have full mental capabilities but have so little (and slow!!) means of communicating his ideas.


Beyond being slow at communicating his idea, he is also largely limited to his working memory. I can't imagine doing any sort of serious intellectual work without the assistance of paper (or similar technology) to augment my working memory.


In fairness that is something you can get better at with practice.

In IT we tend to train our brains to commit processes (eg ways to research information) rather than facts. But we also have the luxury of being able to be lazy (internet, functioning limbs so can write notes, etc). Heck, before IDEs and the internet I used to remember every function name and order of parameters for any particular programming languages I was proficient in. These days I just learn the function name and let the IDE remind me of the rest (sometimes I don't even learn the function name and just search for it in the package namespace).


And in our defense, I think this is the only sane way to go about programming given that all the APIs and tools we use are just unfathomably deep layers of human constructs. There's no real point in remembering so much detritus. It naturally benefits from a completely different mindset to the problem of studying the deepest mysteries of the universe.


I imagine that would be an effective way to vastly improve his working memory, like strength-training for his brain. Maybe Aristotle or whoever it was that complained about books ruining youth's brains would find Stephen Hawking's memory up to snuff.


> disabilities

He's not at all disabled for what he does, which is think, and he does that better than (I'm guessing here) any of us.


You try thinking productively with only a low bandwidth mechanism to bounce ideas off of other thinkers.

Also no ability to use paper and pencil, and still only a low bandwidth mechanism to store and retrieve notes to increase your effective memory.


All excellent points that I agree with. Absolutely a disability from standard human terms, did not mean to argue that. I only meant his brain is still incredibly sharp, and from that aspect he's at no disadvantage.


yup, that was the only thing I could think about while reading this article


Does physics have some "conservation of information" laws that I missed the memo about? Can anyone recommend some introductory reading about this?


The laws of physics conserve information in that sense that two initially distinct states will always remain, at least in principle, distinguishable for all times, i.e. two distinguishable states A and B will never evolve into the same state C. This is equivalent to the statement that the laws of physics are time reversible, i.e. if you would end up in state C you could not tell whether the system came from state A or B.

If you, for example, start a pendulum with different initial displacements the pendulum will always end up at rest due to friction but if you could take into account the exact states of all the molecules in the air and the joint and so on you could in principle figure out in which exact positions you started the pendulum.

You could actually just very precisely measure the temperature of a pendulum in a box and infer the initial displacement from that because different initial displacements imply different initial (potential) energies and therefore different final temperatures after all the energy became thermal energy and everything in the box had time to reach the equilibrium temperature. You just couldn't figure out whether the pendulum was started to the left or to the right of the rest position with this method.


Fascinating. Do physicists generally believe this? The idea that both the infinite future and the infinite past are all encoded into any snapshot of the universe is blowing my mind.

My layperson's understanding of QM always gave me the impression that it does the opposite: that it gives non-determinism a place to "hide," so-to-speak. My impression was that religious believers could use QM to argue that their unseen God, for example, does actually have a physically plausible mechanism by which he/it could intervene in the physical world. But if I'm understanding this theory correctly, unless God is a physical thing, there is no way for God or any other supernatural force to affect the physical world without itself being physical.

This theory also would seem to deny the existence of free will.


It is the same for quantum mechanics, in technical terms the time evolution of a quantum system is unitary. It is the uncertainty principle that prevents you from learning the exact state of a system and therefore perfectly predicting its evolution into the future or past. The underlying mathematical structure nonetheless allows you to do exactly this assuming you exactly knew the state of the system at one point in time.

There is of course the infamous collapse of the wave function that seems to turn a quantum state into a classical state during a measurement and this with different probabilities for different outcomes. This collapse of the wave function is not unitary and there are no good reasons to assume that a measurement is in any way special and should violate the laws of quantum mechanics which have otherwise been shown to work with incredible precision.

Therefore the collapse of the wave function is probably not a thing but just an apparent artifact of our ignorance of the details of the measurement apparatus which just is a gigantic quantum mechanical systems with billions and billions of particles.

Another way to think about that are symmetries and conserved quantities. The physical laws are believed to have certain symmetries, for example time and space translation symmetry or rotation symmetry, i.e. a physical experiment should yield the exact same result no matter whether you do it here or five meters to the left, today or in three weeks, with the laser beam pointing north or east.

Space translation symmetry, for example, means that if you change all you initial say x coordinates by the same amount the calculations should nonetheless yield exactly the same result as a calculation with the unchanged coordinates besides that the final state will of course have the same offset in its x coordinates.

If you demand that your laws of physics obey such symmetries and do some math you will find that each such symmetry inevitably gives you a quantity that does not change over time, i.e. that is conserved. That is Noether's theorem and extremely general, i.e. (almost) no matter what laws you imagine, a symmetry will always give you a conserved quantity.

The conserved quantity for space translation symmetry is what we call momentum, for rotation in space it is angular momentum and for time translation it is energy. But now that we have conserved quantities it is now impossible that two states differing in any of the conserved quantities will ever evolve into the same state or have ever been in the same state because that would require the change of a conserved quantity.

So to summarize, very basic assumptions like isotropy and homogeneity of space and time inevitably lead to conserved quantities which in turn prevent certain state evolutions that would violate those conservation laws. I am not sure if those constraints force time reversibility onto you but I assume they do.

Another commenter mentioned T-symmetry, that is that the physical laws remain the same if you reverse the time axis, i.e. multiply the time coordinates by minus one flipping past and future. This is of course not the same as time translation symmetry which is about adding to the time coordinates and it is also different from the reversibility I mentioned before, i.e. the ability to retrodict past states which does not require that the evolution into the past is symmetric with the evolution into the future. T-symmetry is believed to be violated for very good reasons but that has not yet been shown experimentally although it follows from the observed CP-violation and the assumed CPT-symmetry.

And I am not a physicist, so while I believe everything I said is accurate or marked as me not being sure or not knowing it may still contain some - hopefully minor - errors.


QM does away with any notion of conservation of information. Time reversibility (T symmetry) is usually talked about at macro scales where QMs effects are vanishingly small. IANAP

Regarding free will -- rigorously define it for us, without evoking the supernatural, and as distinct from pure randomness and absolute determinism, and it should be easy to tell whether the law of conservation of information allows for it. Before spending too much time in e it, though, note that such a definition is widely considered impossible. IANAP


Time reversibility exists in quantum mechanics because observables are self adjoint operators. Closed systems evolve unitarily. In simpler terms, you can think of it as the requirement that maps preserve distances and are easily invertible. We need this so that the information describing a system (which we can still talk about in terms of traces), remains invariant with time. In the classical sense, the corresponding violation leads to probabilities not summing to 1! We clearly can't have information shrink and for pure systems, dropping distance preserving maps leads to a really awesome universe (I believe this also ends up highly recommending L2). We literally go from a universe that is almost certainly near the bottom end of the Slow Zone of Thought to the Upper Beyond (https://en.wikipedia.org/wiki/A_Fire_Upon_the_Deep#Setting). We gain non-locality, causality violations and powerful computational ability.

In practice, our confusion about a system does increase with time as classical systems become ever more correlated, losing distinguishability, aka decoherence.


To me, a lack of free will would mean that a mind's future state is entirely determined by its current state. "Free will" would mean that its future state can be altered by some nonphysical entity (whether you call that a soul, consciousness, or whatever).


Free will is an obvious idea for a self aware consciousness to adhere to. The purpose of consciousness is to keep its organism viable. It does that by collecting an essence of past and from that predict an essence of the future. This expectation of the immediate future is then corrected according to actual perceptions and transformed into appropriate actions. The full past is abstracted into an essence state, lots of information is lost to elsewhere, so there is no way to determine what details of the past went into forming the precise state from which the brain makes its actions. Of course it has to feel like at least some of our choices emerges from nowhere. But the only way to know if a brains thoughts are really free is to replay its entire brain state building past in every minute detail and look for alternative actions. An impossible experiment.


I said without evoking the supernatural. The concept of a soul is supernatural. There's no reason to think consciousness is anything other than physical in nature -- but to the extent that you mean it in some kind of supernatural, extra-physical way, it's not part of this conversation.

By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it.


> There's no reason to think consciousness is anything other than physical in nature

Consciousness is not the same thing as agency. It is possible that we are conscious but do not actually have free will / agency.

> By all means go on thinking there might be some supernatural thing called a "free will" if you want. But if it's supernatural, then the utterly natural laws of QM wouldn't have much to say about it.

I am not sure I agree. If QM implies a strict determinism, to me that rules out free will. For free will to be plausible, there has to be a place for non-determinism to "hide." If QM can rule that out, to me that rules out free will.


altered how?

at random? randomness doesn't look 'free' to me, looks.. random.

following some rules? not 'free' anymore.

This "free will" thing doesn't support well scrutiny.


Altered according to the agency of the nonphysical entity. Not randomly or following rules.


When you formulate it so, the problem is the question itself, as, except if we are religious believers, we don't have any reason to consider a "soul" (or a "consciousness") to be a "nonphysical entity." Look at the etymology of the "psyche:" it's "the breath." At the time people explained their environment unscientifically they considered that the existence of "breath" makes the difference between the dead and the living person but also between the conscious and the unconscious bodies. Animals do breathe too. But humans saw themselves as "special" in their religious setup. As a kid would ask "does my dog go to heaven too when he dies?" See, we go deeper and deeper in religious explanations which just postulate something non-existing (once you accept nonphysical soul, it's easier to believe in heaven or reincarnation or whatever). Now go back and ask, if you believe that the only thing we can "see" around us that doesn't have the "physical entity" is the "soul" of humans why do you do so? Are humans by any way "the chosen ones" in the animal world, looking at their physical bodies and the processes in them? The scientific view doesn't see them so. The human "consciousness" is, so observed, more complex than the "consciousness" of the crow on the lawn due to the more complex brain, but both of these can be explained as the function of their physical "hardware" (the body and the brain) and the interaction with the environment. The "consciousness" as a "neurological process by which the animal or the human sees it as special, the center of its attention in order to protect itself" can be seen as a product of evolution. (And now I have a weird feeling of explaining a grown up person that Santa doesn't exist). So it looks that we humans, if we don't destroy ourselves first, will be able to eventually produce a "conscious" computer. (Hal, open the door, please).

(I've mentioned crows just to be able to link to https://www.youtube.com/watch?v=ZerUbHmuY04 )


> Now go back and ask, if you believe that the only thing we can "see" around us that doesn't have the "physical entity" is the "soul" of humans why do you do so?

I don't think this. I don't think that anything about the consciousness or agency of humans is exceptional to humans.

> The "consciousness" as a "neurological process by which the animal or the human sees it as special, the center of its attention in order to protect itself" can be seen as a product of evolution.

This is an interesting perspective that definitely made me think, but I think it ultimately begs the question. If you're talking about "seeing" or "attention," you're already speaking in concepts that presume the existence of consciousness. I'm not sure this gives us a useful framework to know when any computational process we have created does indeed have "consciousness," or where the line is between animals advanced enough to have it and animals (or even plants) that don't.


> If you're talking about "seeing" or "attention," you're already speaking in concepts that presume the existence of consciousness.

Maybe I wasn't clear enough. The world around us (and animals) is projected in our (and animal's) neurological system as some kind of the model. The question is just if the model is such that favors the representation of the uniqueness of the organism or not. I claim that there's evolutionary advantage in producing a system (hardware and software) where organism cares for itself and where the model is so formed that "me" is in the center, to the level that "me" is "conscious" in the sense, processing the stimuli as much that the dog understands that the paw is "his paw" and that it's dangerous to put the paw in the fire and up to the level of you and me considering "us" "us" and being able to talk about it. Therefore we're not so much different from the rest of the animals.

> I'm not sure this gives us a useful framework to know when any computational process we have created does indeed have "consciousness,"

I'd still like to know how you can define consciousness in a way that it doesn't sound religious. If you can't, then of course we can't progress in our discussion.

Let me give you very "primitive" and "simplified" view of the "free will" subject. I see it as a purely religious construct, based on the following history: initially, gods weren't "almighty and omnipresent." If you've read ancient Greek literature, there are such gems like "Zeus was at that moment in Egypt so he wasn't there when the soldiers he supported lost." If you've read Bible, the oldest myths (I mean, stories in the Bible) are actually based on such concept of god(s). Then the theology "theory" grew more demanding, postulating the "almighty omnipresent" but also the "loving" god. Which made the myths (stories) much absurder than they were as they were written. How can almighty loving god produce the world and then see that it's bad, so that it has to send the flood to destroy it? It's either not almighty or not loving etc. As a rescue, the priests invented the concept of "free will" as in "once god creates humans, they have their own free will (which god can't control!?) and they do what they do so then they get to deserve to suffer, go to hell and all that nice stuff." Most of the "modern" theological concepts are constructed as an attempt to make less absurd the whole "body of work." A kind of "justification by obscurity" which obviously works for believers, giving them easy covertly nonsensical sentences to answer the "hard questions" others would give them.

In reality, we just have what user jonsen formulates here as "the purpose of consciousness is to keep its organism viable. It does that by collecting an essence of past and from that predict an essence of the future."

Now you ask about the "framework to know when any computational process we have created does indeed have "consciousness."" It depends on the definition of he consciousness, but that definition can't involve gods and "nonphysical entities." As soon as that is clear we can measure the "consciousness as what we observe in a dog" or the "consciousness as what we observe in a six months kid," the "consciousness as what we observe in a twenty years old healthy human" and the "consciousness as what we observe in 80 years old human with Alzheimer's disease which progressed this much."

Note that neurologists as the part of their daily routine have to evaluate the level of consciousness in their patients. It's very instructive to read just some examples of he cases they work with.

When we're there, do you think that a human with Alzheimer's disease which progressed so much that he can't remember anybody from his life or what he did just 15 seconds ago has a "soul?" (a lot of healthy animals remember for months different things, or if you talk with the owners of pets, you'll hear that there are animals which have traumas and a lot of the psychological symptoms you'd just associate with humans). So back to the patient, do you consider him "conscious?" If not, at which point did he lost his consciousness which you equate with soul? If yes, where is he different from some good Perl script? There is no "single unique unchangeable" consciousness, there are just different levels of functioning of the model I mention at the start. Of course, when you die, the model completely stops functioning as it needs both the hardware and the software, the brain and the body as the hardware and all these nice electrical impulses and chemical reactions in the living organisms as the software. There's no such thing as the "agency of the nonphysical entity" there.


Not randomly or following rules

There is no other choice. By definition, random means "not following any rule or pattern".


I think it depends on your perspective. From the perspective of my keyboard, my input is random/unpredictable (it doesn't have perfect entropy, but it doesn't follow rules that make it completely predictable). From the perspective of my body, I have complete control over what I am typing, and there is nothing random about it.


No it doesn't depend on your perspective. If it did the conversation would make no sense. The only perspective worth considering is the global objective perspective. Quantum events are globally, objectively random. Nothing else seems to be, which means, by definition, that everything else is deterministic.


The question I'm trying to get at is: from the perspective of the physical realm (which may be, even likely is, all that exists) is there any possible mechanism by which something nonphysical could influence events in the physical realm?

For example, is there any physical property that is not "locked into place" by the laws we know and the observations we can make? Is there any fundamental impossibity (not practical impossibility) of snapshotting the state of the universe and using that snapshot to determine every past and future state of the universe?


Yes there is a principal in on of the "interpretations" (we really need a better word for it) of QM that the information carrier by a wave function cannot be destroyed, this present a problem in certain cases such as black holes.

There's allot of weird stuff going on on the extreme edges of theoretical physics, to the point in which also a though that some areas of theoretical physics have moved into the "Metaphysics" and Philosophy domains and are distancing them selves from certain scientific principals.

That said I like to treat black holes in the GR/SR manner they are simple massive objects not super dense hot balls of entropy, they don't have some universal cosmic powers, they are "black" simply because they red-shift all radiation to the point of invisibility, they can form, grow and break apart, and we never can see anything actually entering a black hole due to time dilation and anything that comes close to it will be red-shifted to a point in which we won't be able to see what or where it is either.

Since nothing from out POV ever enters the black hole and everything is stuck on the event horizon nothing is lost, and since black holes can evaporate (Doesn't require hawking radiation altough it's the most common explanation since Yakov Zeldovich (which Stephen hawking based his work on) and others have proposed similar mechanisms which are more compatible with physics as it is measured and perceived by hmmm reality...), no firewalls or weird mechanism are needed to protect or explain other weirdness that happens around singularities.

Singularities are well covered by GR and SR prevents naked singularities so we are good there too.

But that's just me :)


Here's another interesting one:

https://en.wikipedia.org/wiki/Landauer's_principle

Landauer's principle is a theoretical lower limit on the amount of energy required to perform a computation that destroys information.


All known physical processes only transform information, it's never actually lost (although it may get "transformed" in a way that it's irretrievably scrambled). Black holes are the only known thing that may cause information to disappear from the universe - or not, that's the open question.


What about quantum mechanics? How can you get a true quantum random number generator if information is preserved?


The many worlds interpretations. Roughly speaking, if there is a quantum system that is in a superposition of states A and B, and we observe it, we then become entangled with it and are then in a superposition of having observed A and having observed B. No information has been lost from the universe, but there is no way for those of us living in the A state to observe anything about the B state.


Quantum information is actually different from classical information (classical entropy). Quantum information is (believed to be?) a conserved quantity; classical is not (it never decreases). A quantum random number generator is just emitting quantum bits: without having had those qubits in the first place, you can't have computed them ahead-of-time, so they're random.


Quantum information is (believed to be?) a conserved quantity

No it's not. Why are so many people making things up? QM does not even define a concept of a quantity called "information".



Something that is hard to mention but is worth knowing is that Hawking is not at the leading edge of his field.


How do you know this?


Maybe from the "Einstein index"? [1]

[1] http://www.science20.com/hammock_physicist/who_todays_einste...


So top 7 =/= leading edge?


Top 7 among those whom the author chose to compute his "Einstein index" for.

Something not accounted for by that illustration is age ~ proportion of realized potential. Maldacena is 47, Randall 53; no longer promising youngsters, but with decades of index-climbing potential left in them. Weinberg and Glashow are both 82, and owe the bulk of their index position to work done in the 60s and 70s. Witten is 64 and peaked in the 80s and early 90s. Polyakov is 69; for him it's 70s and 80s.

Hawking is 73. His index-driving contribution is black hole evaporation, from 1974. After that, he's mainly been a popularizer.


Undergrad in physics, still keep up with my PhD physics pals.


It seems that matter usually doesn't fall through the event horizon: due to time dilution it will take infinite time for matter to reach the horizon from our point of view.

That's what I've got on Physics stack exchange. There's those collapsars that look exactly like black holes but are made of matter around the event horizon, infinitely falling in.

Is this right, wrong, or irrelevant?


That is correct. From the point of view of the infalling object, however, nothing changes as he crosses it except his position in space.

I say who gives a shit about the point of view of the infalling object. :-)


It seems to me, intuitively, that the infalling object should experience inverse time dilation and extreme blue-shifting of the outside universe as it approaches the event horizon. Due to length contraction, that energy would be increasingly perpendicular to the object's trajectory, approaching a point where it is bombarded with enough energy to either knock it off its trajectory or "smear" its particles into orbit around the massive object (basically another sort of firewall).

Presumably the mathematical descriptions tell a different story because I never see it described like this.


> It seems that matter usually doesn't fall through the event horizon: due to time dilution it will take infinite time for matter to reach the horizon from our point of view.

From our point of view, not the matter's. The matter falls through just fine.


It can only do it once it witnesses the end of times. We can't see any information destruction since it's always sometime in the future.


A video of Hawking's presentation is available at https://www.kth.se/en/aktuellt/nyheter/hawking-offers-new-so...


How come quantum indeterminacy doesn't preclude information preservation?


All the information about the state of a particle is contained in its wavefunction. The uncertainty comes from the fact that the wavefunction can only tell you the probability of getting each value of a specific quantity when you make a measurement of that quantity, which doesn't mean you lack information about the state of the particle.


which doesn't mean you lack information about the state of the particle.

Yes it does. Quantum observations (sometimes called "collapse of the wave function") are not time symmetric. Once you've experienced one you can't derive where you were before it. Otherwise you wouldn't be using the word "probability".

This inability to recover a previous state is, by definition, the loss of information.


[deleted]


The article is a filtered version of Hawking's words, which themselves were probably a filtered version of his research. The parent commenter was talking about the quality of the article.

There's no basis to attack Hawking or anyone else. Even the article's author should be relatively immune, because WaPo isn't in the business of writing scientific articles.

If you insist on attacking, then let's aim for the central argument. For example, a decent attack on Einstein in the 1940s would be to remark that if quantum electrodynamics agrees with experiment, then Einstein's goal -- to demonstrate that probability wasn't a fundamental component of the universe -- would be in trouble.

An excellent attack would be to cite the research, and to explain why if X is true then @physicist is wrong.

Are you sure that being mistaken is such a terrible thing? The mistakes are often more illuminating than the discoveries, as they provide a framework upon which to learn how to improve the process of discovery.


It should be noted that QM does not require randomness, you need to pick randomness or sacrifice other assumptions and most people pick randomness as the cleaner option.


Fascinating. May I ask, how does one predict the behavior of an electron without using probability amplitudes?

I'm extremely interested, and the question isn't meant as anything but an inquiry. Finding a gap in one's knowledge is one of the more exciting aspects of life.


First, you need to stop calling them "probability amplitudes".

Amplitudes are complex numbers, out there in the real world (as far as we understand it). This is quite different from probabilities, who are real numbers between 0 and 1 who exist only as a mental constructs. Simply put, while amplitudes share some mathematical properties with probabilities, they are not probabilities.

Now that we're done turning colloquial words into misleading jargon, we can talk about the theory itself. Namely Everett's many-worlds and decoherence.

We could play the "where's the electron" game, but I'd rather play "where's the photon" instead —it's simpler. So you throw a photon through a half-sieved mirror, to be detected by one of to judiciously placed detectors. Oh, and have one detector linked to a kitten murdering system for good measure.

If you repeat the experiment often enough, you will witness a kitten death half the time, with absolutely no way to predict the outcome in advance. The results are the same if you put the kitten in a box, and open that box after the fact. So it certainly looks like the universe is not deterministic.

The equations on the other hand are definitely deterministic. Future amplitude distributions are perfectly predicted by past amplitude distributions —which by the way you can't fully observe, but that's another issue entirely. So, if you look at the amplitude distribution, you'll see that once the photon hit the mirror, there will be a blob of amplitude for both cases: passing through and being reflected. Going further, there will be a blob of amplitude for each of the detectors being hit. Finally, there will be a blob of amplitude for the living kitten, and another one for the dead kitten. Oh, and the equations also says that the blobs quickly cease to interact —that's decoherence.

Basically, what the equations say is that the universe splits itself in two, generating one version with the dead kitten, and one version with the live kitten. The equations also say that the inhabitants of either version don't get to see the other one (they've ceased to interact).

Now the only question left is why we experimentally find ourselves to be in one version and not in another. But never forget that every time you run that Schrödinger experiment, a cat will die. If not in your universe, then the other.

---

The Copenhagen interpretation, which would have the blob of amplitude corresponding to the other universe just collapse into nothingness (that is, set to zero), has no basis in the equations which by now are backed up by mountains of evidence. It is an additional hypothesis layered on top of the equations, conveniently formulated in a way that wouldn't falsify any experiment. On top of that, it violates a number of long standing principles, such as locality.

You could also call those amplitudes "probabilities", but that's just a word trick. It doesn't explain anything.


It's fun that wave-function reduction is "an additional hypothesis layered on top of the equations, conveniently formulated in a way that wouldn't falsify any experiment", while infinities of universes that have "ceased to interact" is not.


Indeed. And it doesn't help that Many-Worlds, despite being much more reasonable than any collapse hypothesis, came decades after the Copenhagen interpretation. Science tends to reject theories that don't make additional predictions. http://lesswrong.com/lw/qa/the_dilemma_science_or_bayes/

But the fact remains that blindly following the equations leads you to Many-Words. You have to modify the results to get to any sort of collapse hypotheses. Also, simply postulating that what you don't see doesn't exist doesn't help. http://lesswrong.com/lw/pb/belief_in_the_implied_invisible/


It seems my comment was a bit too subtle for you. Infinite uncountable infinities of universes is not parsimonious.

Pretend that you're talking to someone who doesn't see many-worlds as an obvious corollary of the current QM theories. What experiments do you propose to indicate the simultaneous existence of e.g. a universe in which the photon chose Slit A and another in which the photon chose Slit B?

Your appeal to BitII fails because that concerns e.g. conservation laws that are regularly observed to be true, and which therefore can be assumed true when a particular situation makes observation impossible. Many-worlds has never been shown by experiment, so it is not eligible for such treatment.


First, a hint about where I'm coming from: http://lesswrong.com/lw/r5/the_quantum_physics_sequence/ (I recommend you read the whole thing, it's interesting, insightful, and even fun.)

We already agree that there is no experimental difference between collapse/Copehagen interpretation, and the Many Worlds interpretation. In the name of what are you demanding experimental evidence for one interpretation, and not the other?

> Infinite uncountable infinities of universes is not parsimonious.

You need to remember that those universes are in no way postulated. They are derived —from the equations. So you can't use that impressive infinity to claim that the Many World interpretation would somehow have a higher Kolmogorov complexity than Copenhagen.

Many Worlds just takes the equations and run with them. It's Copenhagen that makes additional assumptions by manipulating the results of the equations: They're not real, or there's a collapse… Either way, that's an additional hypothesis on top of those equations (which by the way have massive amounts of experimental evidence behind them, and are accepted by everyone as the current best guess). That additional hypothesis is not very parsimonious, don't you think?

(You have to remember how Occam's razor really works. From the Wikipedia, "The principle states that among competing hypotheses that predict equally well, the one with the fewest assumptions should be selected." As a simple matter of fact, Many Worlds makes strictly fewer assumptions than Copenhagen. Parsimony is not measured by the size of the universe predicted by the theory. Parsimony is measured by the number of core assumptions. Not the same thing at all.)

To get a feel of how utterly ridiculous collapse postulates are (in 20/20 hindsight, I don't want to make fun of physicists), I must quote Eliezer Yudkowsky: http://lesswrong.com/lw/q6/collapse_postulates/

If collapse actually worked the way its adherents say it does, it would be:

1. The only non-linear evolution in all of quantum mechanics.

2. The only non-unitary evolution in all of quantum mechanics.

3. The only non-differentiable (in fact, discontinuous) phenomenon in all of quantum mechanics.

4. The only phenomenon in all of quantum mechanics that is non-local in the configuration space.

5. The only phenomenon in all of physics that violates CPT symmetry.

6. The only phenomenon in all of physics that violates Liouville's Theorem (has a many-to-one mapping from initial conditions to outcomes).

7. The only phenomenon in all of physics that is acausal / non-deterministic / inherently random.

8. The only phenomenon in all of physics that is non-local in spacetime and propagates an influence faster than light.

WHAT DOES THE GOD-DAMNED COLLAPSE POSTULATE HAVE TO DO FOR PHYSICISTS TO REJECT IT? KILL A GOD-DAMNED PUPPY?


I'll read that eventually, but I won't be surprised if it turns out to be like much of the rest of LW: a sophisticated, entertaining, ultimately unconvincing apology for EY's particular brand of mysticism [EDIT:], layered over many perfectly cromulent observations about probability[/EDIT]. I don't need the truth so badly that I would swallow the best truth-substitute I can find. I'm perfectly content to categorize something as a "known unknown".

If I were to stipulate that the Copenhagen interpretation(s) is (are) silly philosophical daydreaming, could you do the same for many-worlds?


> EY's particular brand of mysticism

Funny how different people perceive his writings. To me, most of what he writes feels obvious —at least in retrospect. "Obvious" doesn't feel very mystic to me. I do understand however how many people would be distrustful of his casual writing style. He tends to sound like a lowly blogger, not like a respectable academic. (Personally, I don't care for those status signals.)

> I'll read that eventually, but I won't be surprised if it turns out to be like much of the rest of LW

Hmm, if you're already familiar with this material, then don't bother just yet. Start with the first few chapters of E.T. Jayne's Probability Theory: the Logic of Science. That's more basic, less crazy sounding, and more generally applicable.

Now I don't exactly know how physicists deal with quantum mechanics. I asked one, and he didn't even bother with any interpretation, sticking with the observable consequences of the equations (a more prudent attitude than either Copenhagen or Many-World).

Something however bothers me deeply: insisting on calling amplitudes "probabilities", while they're anything but. That only makes teaching harder. Seriously, I was tempted to mass delete every occurrences of "probability" from the "Probability Amplitude" article in the Wikipedia.

> If I were to stipulate that the Copenhagen interpretation(s) is (are) silly philosophical daydreaming, could you do the same for many-worlds?

Err, it doesn't work like that. If I were to convince you, I would have learned nothing, and believe then what I believe now. I can only give you my current best guess.

Which is, many-worlds is by no means certain. At the very least we don't have a Theory of Everything, and we could miss something. Collapse postulates however are just crazy. Rejecting the existence of the amplitude you didn't observe is just as insane (no less, no more) as rejecting the existence of a photon which just passed the limits of our observable universe.

When we do find a theory of everything, I bet the equations will predict the existence of things that can't be observed —not even in theory. I just hope people won't see that as a licence to not believe in those things at all.


Thanks for the cordial discussion, but I think the physicist you mention has it right. "Prediction" of phenomena that can never be observed seems like an oxymoron.


Yeah, but there is one practical application, if we ever conquer the stars: if we send colonists so far away that they eventually cross the boundary of our observable universe, we care a great deal about their continued survival, even though we will never observe it directly.

Other unobservable stuff may or may not be incredibly important. (Though at a first glance, QM interpretation isn't.)


I so appreciate this explanation, as I have wondered about this for years. Thanks!

My understanding of (real) probabilities is that they're really about our ignorance of a system. E.g., if I'm flipping a coin but put very little spin on it, it's easy for a human to predict what side will show. As we put more spin on it, the probability of guessing correctly goes down, but that's about perceptual and cognitive limitations.

My (layman's) understanding of quantum mechanics, though, is that there are no hidden variables. In which case, I couldn't understand why anybody would call the quantum stuff "probability". It's a relief to know that they're fundamentally different.


I read once that you can look at QM without randomness by comparing it to interference waves, similar to what happens if you sprinkle water on the surface of a pond -- the result looks chaotic but it is 100% deterministic, and QM can be viewed in this way too.


An amplitude is not a complex number. It is the real number one gets taking the root of the sum of the squares of the real and imaginary part of a complex number. In QM they will be between 0 and 1 and are probabilities.


From memory of quantum class the amplitude is a complex number and are not directly probabilities:

Most convenient reference:

> In quantum mechanics, a probability amplitude is a complex number ...

https://en.wikipedia.org/wiki/Probability_amplitude#A_basic_...


No matter, that's still a physical quantity. Probability is in the mind.

http://lesswrong.com/lw/oj/probability_is_in_the_mind/


Nice explanation.

I don't understand this part: "Now the only question left is why we experimentally find ourselves to be in one version and not in another"

Surely there are different 'we' observing in both?


Yes there are. So to an "outside observer" (like an omniscient being from outside our universe), it's definitely deterministic.

From the inside however, we still have the Born probabilities to contend with: apparently, our subjective probability to observe one version or another is tied to the square root of the relevant amplitudes. In the case of a half-sieved mirror, that's one chance in two. But we can easily bias this with a stronger (or weaker) mirror.


>The mistakes are often more illuminating than the discoveries

God yes. Sadly, they don't tend to help you get more funding.


Being a good science communicator and doing significant serious research are not mutually exclusive.


[flagged]


What the hell are you talking about?


Apparently I'm kicking the hornets nest by questioning some hypothesis and showing flaws that some critics have found in modern Physics.

I forgot to cite sources and my comment is closed to editing.

Why hawking is wrong about black holes: http://www.universetoday.com/108870/why-hawking-is-wrong-abo...

Why the Higgs Boson found may be an impostor: http://www.technologyreview.com/view/428428/higgs-boson-may-...

Hawking admits he is wrong on black holes, makes corrections to his theories http://www.nature.com/news/stephen-hawking-there-are-no-blac...

Why General Relativity and Quantum Mechanics aren't compatible: http://www.quora.com/Why-are-general-relativity-and-quantum-...

Black Holes are like Wormholes: http://mkaku.org/home/articles/blackholes-wormholes-and-the-...

Einstein when he talks about God it is Spinoza's God: https://en.wikipedia.org/wiki/Spinozism

It is a key to understanding his theories.

Einstein argues with quantum theory scientists: http://www.amazon.com/Shaky-Game-Science-Conceptual-Foundati...

Black Holes do not destroy matter: http://guardianlv.com/2015/04/black-holes-do-not-destroy-mat...

I could be wrong, my citations could be wrong, but this is the way we learn by testing things out.


You seem to think there is some sort of schism in physics, with one side on GR insisting it is "correct" and one side on QM insisting their model is what the universe "really is". That's not the case at all and I'm not sure what's leading you to that conclusion - everyone knows both models are incomplete and it is not at all controversial to say as much.

You're not really kicking a hornet's nest, rather you're just talking nonsense. You're so off base it's impossible for anyone to take you seriously enough to be upset. "You're not even wrong" as the saying goes.


Thank you for correcting me. I didn't understand why I was being downvoted. I've confused the incompleteness of both theories with being conflicting. I've learned from it now.


>I could be wrong

first time you've been right.

> this is the way we learn by testing things out.

that's not at all what you're doing. you're pushing an anti-science, pro-creationist agenda.


"You wouldn't technically lose any information if you kept all of the ashes in one place, but you'd have a hard time looking up the capital of Minnesota."




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