[HN Gopher] Our reality may be a sum of all possible realities
___________________________________________________________________
Our reality may be a sum of all possible realities
Author : nnx
Score : 248 points
Date : 2023-02-20 01:24 UTC (21 hours ago)
(HTM) web link (www.quantamagazine.org)
(TXT) w3m dump (www.quantamagazine.org)
| ManEatsPants wrote:
| [dead]
| laserbeam wrote:
| If you're going to talk about Feynman's equations and use the
| double slit experiment as an illustration instead of using
| Feynman diagrams (good explanation by pbs space time here
| https://youtu.be/fG52mXN-uWI), I'd say you're doing the concept a
| disservice. Extrapolating that to philosophy is usually risky,
| especially when doing it to a general audience.
|
| Also, I feel we should stop trying to explain quantum physics
| approximately AND do philosophy based on it in short articles.
| It's too hard to convey that "a possible reality" may mean
| something else than you traditionally expect in quantum
| mechanics, or that it's an interpretation of what the math
| implies. A general public always leaves with an inaccurate
| impression of what was written.
| YESIMFUNNY wrote:
| [dead]
| unfriendme wrote:
| [dead]
| pbj1968 wrote:
| [flagged]
| taeric wrote:
| Calling it an "imaginary hack" is frustrating to me. Imaginary
| numbers are called that for derogatory reasons already, leaning
| in on that bias is ridiculous in the modern world.
| biomcgary wrote:
| Articles on this topic always remind me of Leibniz's
| philosophical optimism, which stemmed from considering all
| possible worlds through the lens of his theological motivations (
| https://en.wikipedia.org/wiki/Gottfried_Wilhelm_Leibniz#Theo...).
| This was famously criticized by Voltaire in _Candide_.
|
| But, even earlier, the Scholastics wrestled with alternatives to
| reality as we know it, which seems to have found its best
| articulation by Luis de Molina
| (https://en.wikipedia.org/wiki/Molinism).
|
| Even Calvin seems to resurrect his influence through
| superdeterminism (most recently championed by Sabine
| Hossenfelder).
|
| I'm not claiming that these thinker prefigure modern physics,
| only that intellectual history sometimes rhymes, even if it
| doesn't repeat.
| elecush wrote:
| "the whole is greater than the sum of its parts" - Aristotle a
| long time ago
| mewse wrote:
| Interestingly, Zeno had said the exact opposite, a hundred
| years earlier -- that since a 'whole' could be divided into an
| infinite number of smaller 'parts', which themselves could be
| divided ad infinitum, that these infinite _parts_ when summed
| up must be far greater than the whole. Which was the basis for
| nearly all of his popular paradoxes.
|
| (And then Newton and Leibnitz, some two millennia later,
| spoiled everybody's fun by developing the integral calculus
| such that whole categories of amusing philosophical conundrums
| became utterly mundane mathematical word problems with
| rigorously defined correct answers)
| coolshirtbra wrote:
| [dead]
| bigfootisreal wrote:
| [dead]
| 93po wrote:
| What's up with all the weirdly typed dead comments here? Is
| someone unleashing a GPT bot for commenting?
| babydoodles wrote:
| [dead]
| oprahwindfury wrote:
| According to many, in the future, the simulation of the entire
| universe can be made through a quantum computer, that is, a
| sufficiently powerful computer. So how are we sure it isn't
| already happening to us?
| neom wrote:
| Does that make me the mean me? Am I the most mediocre of me's? Or
| does this mean I'm the best me? Is everyones reality discreet to
| themselves? Or is this the sum of all possible observable
| realities by everyone?
| bbayles wrote:
| For a contrary perspective, see
| https://www.math.columbia.edu/~woit/wordpress/?p=13367
| [deleted]
| osigurdson wrote:
| If true, the most important thing for us is to find out how to
| construct a filter such that the bad realities are suppressed.
| dclowd9901 wrote:
| We can't even agree on an objective version of reality and you
| want someone to determine which ones are "bad."?
| flangola7 wrote:
| I don't see why that should be any harder than the AI
| alignment problem
| osigurdson wrote:
| Everything must be objectively awesome in one of those
| realities. I'd pick that one.
| Rinum wrote:
| Sounds like the TVA -
| https://en.m.wikipedia.org/wiki/Time_Variance_Authority
| lukeplato wrote:
| Peter woit recently wrote on this topic in 'The Trouble With Path
| Integrals':
|
| - https://www.math.columbia.edu/~woit/wordpress/?p=13319
|
| - https://www.math.columbia.edu/~woit/wordpress/?p=13367
| wyager wrote:
| It seems like Woit is kind of intending this to serve as a
| refutation of PIF, but I don't really see why - this sounds
| more like a description of some problems that are hard in PIF,
| rather than anything wrong with the underlying theory.
| consilient wrote:
| As an organizational scheme for approximate numerical
| calculations, the path-integral formulation is fine. But path
| integrals can't serve as a rigorous foundation for QFTs,
| because for the most part they don't actually exist. The
| high-frequency behavior Woit describes is one obstacle; the
| fact that no infinite-dimensional Lebesgue measure is
| possible is another. There are others.
|
| Presumably there's some mathematical object out there that
| captures all the data a path integral ought to have without
| its pathologies, but no one knows what it is yet.
| crazygringo wrote:
| I read Feynman's _QED_ back in high school and was fascinated by
| it, but always had a lingering question.
|
| Sure the math works out by summing all possible paths, and then
| most of them usually cancel each other out, and so you're left
| with the ones that mostly contribute to the actual probabilities,
| as complicated as those can still be.
|
| But I always suspected... _surely_ that just has to be _one_
| mathematical way of looking at it. That at some point we 'd
| discover that turns out to be mathematically identical to
| something else that would be a bit more local, a bit more
| intuitive, a bit more straightforward. That never even had to
| deal in the first place with the infinity of paths that all wind
| up cancelling each other out anyways.
|
| So now that I'm on a forum with smart people, the kind I didn't
| have access to back in high school... has there been _any_
| progress towards something like this at all? Are there any
| directions for this? Or is there a conceptual reason I 'm missing
| why this fundamentally isn't possible, why the sum of all
| possible paths is the _only_ way to account for certain
| behaviors?
| rthomas6 wrote:
| Pilot wave theory
|
| Note: I don't have a good grasp of quantum physics
| prof-dr-ir wrote:
| Theoretical physicists are, on the whole, not a silly bunch and
| there do exist various "mathematically identical" ways to look
| at quantum field theory. But by the very definition of
| "mathematically identical" one can find the path integrals
| _somewhere_ in these alternative descriptions anyway.
|
| Contrary to your stated desire, I actually think that the path
| integral formulation is in some sense the most "intuitive" and
| "local". I apologize for not taking the time to explain that
| here, but let me instead try to convince you that "locality" is
| not at all obvious in quantum field theory.
|
| One intuitively non-local process is the following. If I start
| with a state with one electron at the origin at time 0, what is
| the probability that I measure an electron one lightyear away
| one minute later? Naively this probability is zero, because the
| electron that you started with cannot move faster than light.
| But the actual (and measured, in a less idealized setting)
| probability is not zero, since vacuum fluctuations can create
| an electron exactly one lightyear away and you end up measuring
| that one. That, a physicist would say, is perfectly cromulent
| with locality of the underlying theory.
|
| I hope this gives a glimpse of how subtle causality (and hence
| locality) really is, and why at least some believe that path
| integrals may actually be the most "intuitive" description
| available.
| Y_Y wrote:
| > Theoretical physicists are, on the whole, not a silly bunch
|
| ... at least until the workday ends.
|
| My own feeling is that it's obviously local if you consider
| that there really are virtual particles taking the non-
| critical paths. I know this is a bit interpretation-dependent
| though. GP may be interested to read e.g.
| https://en.m.wikipedia.org/wiki/Double-
| slit_experiment#Inter...
|
| Of course the simple way around this issue is the best: "shut
| up and calculate".
| daniel-cussen wrote:
| [dead]
| inciampati wrote:
| Yes, this always bothered me as well. It happens to annoy some
| physicists too, so there has been a resurgence of interest in
| alternative (less magical?) interpretations of quantum
| mechanics in the past decade.
|
| Perhaps the most striking is experimental evidence that
| particles follow "surreal" trajectories:
| https://doi.org/10.1126/sciadv.1501466
|
| There are other very interesting, and intuitive, features of
| this line of modeling. I think it's better to have a realistic
| mental model of the microworld even if the overall QM
| predictions aren't (obviously, yet) affected. Maybe there's
| more here than immediately obvious to the mainstream physics
| community.
| pithon wrote:
| You can take an amplitude-vs-time representation of a signal
| and via a Fourier Transform instead represent the same
| information as a sum of weighted complex exponentials in the
| frequency domain. It works mathematically, sure- but does that
| mean that- physically, at the core of existence- every RF,
| acoustic, seismic, or financial data ripple is actually a bunch
| of sines and cosines which are getting summed together to
| create the real phenomena?
| dsego wrote:
| I've read that we can typically reconstruct any complex wave
| form from other periodic waves. So it doesn't seem like sines
| are something special.
|
| But apparently, mass on a spring behaves in a sinusoidal
| motion https://youtu.be/n2y7n6jw5d0?t=968
|
| And sines are also an approximation of harmonic motion of a
| pendulum. https://youtu.be/p_di4Zn4wz4?t=370
|
| Many physical systems resonate or oscillate produce quasi-
| sinusoidal motion.
|
| https://ccrma.stanford.edu/~jos/mdft/Why_Sinusoids_Important.
| ..
| 323 wrote:
| Why wouldn't summing all possibilities be the simpler solution?
|
| Thing about Monte Carlo simulations - it's simpler to just try
| all possibilities than to figure out what is the formula of the
| correct result.
|
| Neural networks also work like that - you randomly do things
| and propagate errors with simple rules instead of figuring out
| the complex formula that would give you the answer.
|
| There seems to be a lot of power in summing up all the things.
|
| If our universe is a computation, our practical experience
| seems to suggest that a Monte Carlo like simulation of it would
| be more efficient than a closed-form formula.
| shanebellone wrote:
| "it's simpler to just try all possibilities than to figure
| out what is the formula of the correct result."
|
| Isn't this the premise of quantum computing too?
| kzrdude wrote:
| Summing all the things is similar to just a big "OR" right?
| It lets the important signal through (you don't know which it
| is) and the other ones just have to be suppressed or
| suppressed enough to not interfere. For example all the other
| ones might as well be as if stochastic, so on average you get
| it right.
| oneoff786 wrote:
| The canceling out part is the reason quantum computing works.
| We basically just avoid computing a bunch of bad answers making
| the actual problem faster to solve. So it has some observable
| examples.
| tim333 wrote:
| >is there a conceptual reason I'm missing why this
| fundamentally isn't possible, why the sum of all possible paths
| is the only way
|
| It's not so much a conceptual reason it isn't possible so much
| as that's just the way the universe is. If you have your
| particle going from A to B, changing things at some possible
| other path C simply does change the way it arrives at B, like
| it or not.
|
| Feynman's take was:
|
| >If you will simply admit that maybe [nature] does behave like
| this, you will find her a delightful, entrancing thing. Do not
| keep saying to yourself, if you can possible avoid it, "But how
| can it be like that?" because you will get 'down the drain',
| into a blind alley from which nobody has escaped. Nobody knows
| how it can be like that.
|
| Personally though I actually like pondering how it can be like
| that as an interesting puzzle. But it's the way the universe
| is, not a conceptual thing. My guess as to why is that reality
| at the most basic level is more like a path integral than like
| objects whizzing around.
| naasking wrote:
| Many-Worlds probably fits what you're looking for, that
| conforms with orthodox quantum mechanics. How to bootstrap
| probability seems like the only open question there.
|
| If you want something less orthodox, then superdeterminism is
| seeing a bit of a revival. For instance, Tim Palmer's Invariant
| Set Theory (Invariant Set postulate on Wikipedia).
| whatshisface wrote:
| You are never going to get the locality you're hoping for,
| because of Bell's theorem.
| saboot wrote:
| In the past 10 or so years, a combinatoric approach to particle
| interactions has shown surprisingly good results. Simple
| algebraic calculations have been able to give answers that
| requires hundreds of pages of math through the feynman diagram
| approach.
|
| This is the amplituhedron, led by Nima Arkani-Hamed, here's an
| article also by quanta magazine about it
|
| https://www.quantamagazine.org/physicists-discover-geometry-...
|
| In addition to giving exact simple calculations to particle
| scattering amplitudes, the most important aspect is that it
| does so in a way that does _not_ require notions of space or
| time. Leading to the notion that there are more fundamental
| aspects to reality and space and time may be emergent
| phenomena.
| Rattled wrote:
| This is the third formulation I've come across that sounds
| like this with space and time emerging from particle
| interactions. The other two are from Stephen Wolfram and
| Carlo Rovelli. Are the formulations related do you know?
|
| I've struggled to get beyond their popular science
| descriptions to the actual theories, I find descriptions like
| the amplituhedron being 'a multidimensional jewel'
| distracting, if you have any suggestions where to start for a
| deeper understanding of this one I'd be interested.
| danbruc wrote:
| You can find quite a few lectures by Nima Arkani-Hamed on
| YouTube including some on the amplituhedron. But they are
| mostly not gentle introductions and require some knowledge
| of physics to follow along. I think this one [1] provides a
| lot of background and explains the motivation.
|
| [1] https://www.youtube.com/watch?v=mF_vqDt-ETU
| philipov wrote:
| I used to follow his lectures, but in the past 3 or 4
| years he's become really quiet. I wonder if people
| stopped inviting him because he always overruns his time
| limit, or if he's too busy making progress on his theory.
| philipov wrote:
| It's a common thing to see in theories that try to unify QM
| with GR because in GR spacetime is dynamic, while in QM
| it's static. For QM to produce GR, there needs to be some
| way of generating space and time instead of taking it as an
| assumption.
| Enginerrrd wrote:
| Sean Carrol has a great paper on this too showing emergent
| spacetime from entangled states in hilbert space:
|
| https://arxiv.org/abs/1606.08444
|
| Here's a more conceptual introduction to that concept:
|
| https://web.stanford.edu/~oas/SI/QM/papers/SpaceFromQMCarro
| l...
| DebtDeflation wrote:
| Spacetime is emergent in both string theory and loop
| quantum gravity. Article contrasting how it works in each:
|
| https://www.scientificamerican.com/article/what-is-
| spacetime...
| saboot wrote:
| The biggest clue we have right now is illustrated by last
| year's nobel prize. Particles in an entangled state can
| interact instantaneously, disregarding space and time
| entirely.
| hughesjj wrote:
| Check out any lecture by Leonard Susskind on YouTube
| https://en.m.wikipedia.org/wiki/Leonard_Susskind
|
| These are the coolest ideas ever and they literally relate
| to computational complexity and information theory. It's
| super cool.
|
| ER=EPR and
| [complexity=action](https://en.m.wikipedia.org/wiki/CA-
| duality) especially imo
| TechBro8615 wrote:
| Be warned, I entered a similar rabbit hole about two
| years ago and have still not emerged from it.
| cjfd wrote:
| The summing over all paths is basically already there when you
| do the two slit experiment. In that case you sum over two
| possible paths. So it is already right there in the basis of
| quantum mechanics.
| gpsx wrote:
| When you look at quantum mechanics from Schroedinger's equation
| point of view, it is very simple. The implications of it are
| not, which is true of many simple systems. What is difficult in
| quantum mechanics is trying to understand how those equations
| have anything to do with what we see.
|
| The path integral interpretation actually does give us an
| intuitive picture of why we see classical physics. If you zoom
| out, the stationary point of the Lagrangian is all that
| survives, with other things mostly canceling out as you
| mentioned. This explains why in lagrangian mechanics of
| clasical physics we are told, without reason, that the
| equations of motion are the stationary point of the Lagragian.
|
| You don't say this, but I think people think quantum mechanics
| is weird just because it is different from what they see.
| People also had trouble with the idea of us moving around the
| sun, one reason being because it looked like everything was
| going around us. Of course, if we did go around the sun, it
| would still look like things went around us. In the end though
| neither is really true. The sun in the center is just a better
| model. Likewise, quantum mechanics is probably just a better
| model to "reality" than classical physics.
| lmm wrote:
| The only progress I would say is more and more people realising
| it's a waste of time. Why should we expect reality on a scale
| very different to everyday experience to behave like everyday
| experience?
|
| (And frankly I've never found it particularly counterintuitive.
| It's just a wave bro. It behaves like any other wave)
| nomel wrote:
| Flash storage uses QM [1]. Who knows what other advancements
| could be made with a better grasp of reality, at these
| scales. It doesn't seem like a waste of time.
|
| 1. https://www.linkedin.com/pulse/contributions-quantum-
| mechani...
| lmm wrote:
| Grasping reality is better done by grappling with it
| directly than by trying to force it into your everyday
| intuitions. I'd bet the people who made flash work were
| many-worldsers.
| sfmike wrote:
| Welcome to philosophy
| bmacho wrote:
| > So now that I'm on a forum with smart people, the kind I
| didn't have access to back in high school... has there been any
| progress towards something like this at all? Are there any
| directions for this? Or is there a conceptual reason I'm
| missing why this fundamentally isn't possible, why the sum of
| all possible paths is the only way to account for certain
| behaviors?
|
| It certainly not the only way. One easy trick: you put it under
| multiple layers of definition, and at the lowest layer you just
| do the integral without calling anything "paths" or so.
| (Equivalent theories can be seen as bastardization of each
| other, that's why none of them says anything meaningful about
| ontology.)
| Jach wrote:
| I'm tempted to go check, I read that too around the end of HS
| or very start of college, but isn't _QED_ the one that starts
| off with a story about counting physical beans in jars to do
| familiar arithmetic operations like summing and multiplying?
| Then the rest of _QED_ is presented as describing how _QED_
| works using a similar counting-beans approach that everyone can
| understand vs. the more sophisticated methods that working
| physicists use that require a few years of undergrad training
| at least. That both end up having infinities at the core is an
| important part of being able to accurately (mathematically)
| describe nature, like counting beans vs. just using a
| calculator both involve numbers at the core to accurately
| describe to other humans what 's going on -- and numbers can
| seemingly be "canceled out" when removing beans from a jar! (Or
| perhaps adding a different color bean does the canceling.)
| Without thinking about it further I suppose this falls under
| the umbrella question of whether reality is fundamentally
| continuous (as we assume when we model things with calculus) or
| discrete.
| wyager wrote:
| The path integral formulation is possibly the _most_ local
| theory you could come up with - it 's incredibly simple and
| does not require any nonlocal effects! The relativistic
| lagrangian is even more elegant than the classical one - it's
| just mass! What _kind_ of theory would be _more_ intuitive and
| straightforward?
| froeb wrote:
| It isn't the only way. I would argue that the "sum over paths"
| is more of a convenient language for categorizing QFTs than an
| actual statement of physical reality. Path integrals have the
| benefit that they can be easily connected to correlation
| functions by a Wick rotation, are naturally invariant under
| relativity, and work (kinda) well with gauge theories (well
| better than any other way we know).
|
| However, in practice they are not used directly. Instead, the
| path integral is either treated perturbatively giving you
| Feynman diagrams, or by working out the Hamiltonian
| prescription (probably closest to what you are thinking of), or
| by using Monte Carlo approaches (there are probably other
| approaches I don't know of too). All of these approaches are
| easier to calculate with, but are a less natural language for
| abstractly describing a QFT.
| evanb wrote:
| "Monte Carlo approaches" like lattice QCD _do_ use the path
| integral directly!
| froeb wrote:
| Alright fair enough, I'm not an expert on how those work. I
| was under the impression that there is quite a bit of work
| that has to be done to turn a path integral into something
| amenable to Monte Carlo though.
| evanb wrote:
| Well it depends on what you mean by "quite a bit of
| work"! Wick rotate, focus on a finite volume of
| spacetime, and discretize the spacetime. That's all. The
| first step changes a difficult-to-sample distribution
| exp( i S ) into an easy-to-sample exp( - S ). The other
| two steps keep you from needing infinite RAM.
|
| The discretization and finite-volume approximations may
| be removed by doing calculations with different spacings
| and volumes and extrapolating. The Wick rotation is more
| subtle, and typically means only some observables are
| accessible. To study the path integral with real time
| causes a very difficult sign problem [essentially: you
| need an exponentially large number of samples in order to
| get cancellations under control], which is why real-time
| quantum dynamics is an exciting potential application of
| quantum computers.
| jiggawatts wrote:
| This!
|
| 99% of modern physics was developed for pen & paper
| mathematics, not numeric simulations. Even when computers
| became readily available it was seen as somehow "less than"
| symbolic solutions. More practically, special-case solutions
| that were tractable symbolically require far too much computer
| power if approached from a fully general "local" simulation
| perspective.
|
| QED is essentially a Monte Carlo simulation, just like ray
| tracing in computer graphics. The difference is that QED will
| correctly model diffraction (and other more complex physical
| effects) that the simplified graphics-only model doesn't bother
| with.
|
| Both QED and raytraced computer graphics rely on a mathematical
| dualism where a wavefront can be tracked either as a surface
| _OR_ as a vector perpendicular to the wavefront. The whole
| "summing up but mostly cancelling" is just accounting for the
| wavefronts. The use of complex numbers is mostly also just a
| method for tracking the cyclic nature of wavefronts.
|
| If you skip the "waves" part, it becomes traditional computer
| graphics. Less accurate, but essentially the same.
|
| Computer graphics _could_ also simulate light transport by
| using a 3D volume, which is roughly speaking how some diffuse
| light simulations work in real-time games like the Unreal
| engine. The problem is that to get decent accuracy, you need a
| lot of memory to store that 3D volume. For low-resolution scene
| lighting this is fine.
|
| Monte Carlo enables a solution to be built up incrementally
| with much less memory required, needing only a 2D accumulator
| in the image plane instead of a 3D simulation in object space.
| This allows a vastly more complex and detailed simulation to be
| calculated, which is important for the tiny wavelength of
| physical light instead of some non-diffracting idealisation of
| it.
|
| For a physics problem the memory requirements of any non-
| trivial system would be gigantic, that's why everyone just
| ignores this approach even though it makes a lot more intuitive
| sense.
| orbifold wrote:
| While in principle you can evaluate certain path integrals by
| using MCMC, in practice you run into all kinds of issues like
| the sign problem, which even with things like complex
| Langevin dynamics can't be resolved to this day completely.
| BrandoElFollito wrote:
| > it was seen as somehow "less than" symbolic solutions
|
| This reminds me of an old prof during my PhD studies
| (physics) who was always telling how immature and simple
| minded computer users were.
|
| I told him once that using a computer requires intelligence
| and aptitudes that some people who criticize indeed lack.
|
| It did not go well.
| Karellen wrote:
| > It did not go well.
|
| Insulting people rarely does.
|
| Even if the insult is unintended, or is plausibly deniable,
| if the person ends up being insulted anyway, then it is
| unlikely to go well.
|
| In totally unrelated news, it might be worth noting that
| "being right" is not a relevant defence against the charge
| of "being an asshole". It's entirely possible to be one
| without being the other.
| danaris wrote:
| And why are you pointing this at the GP, and not sparing
| any of it for the professor who insulted him to his face
| first?
| BrandoElFollito wrote:
| See, there is a tendency for students to feel like they
| have to take on them any insults (or similar) coming from
| a higher-up.
|
| I understand that you are fine being called an idiot and
| that the proper response is a smile and nod of
| understanding. Some do not. I do not.
|
| Have you worked in academia, especially in the ones where
| you have a medieval approach to hierarchy?
|
| EDIT: something liake
| http://amandabauer.blogspot.com/2011/08/how-academics-
| see-ea... (the humorous way of looking at it)
| Karellen wrote:
| > I understand that you are fine being called an idiot
| and that the proper response is a smile and nod of
| understanding. Some do not. I do not.
|
| Because the only possible responses are "smile and nod of
| understanding" and "call the professor an idiot in
| return", and no middle ground exists where you can draw
| attention to someone's mistaken beliefs without
| escalating the situation, or even without putting them on
| the defensive?
|
| I don't believe you believe that.
| [deleted]
| consilient wrote:
| > that's why everyone just ignores this approach even though
| it makes a lot more intuitive sense.
|
| - No one ignores the correspondence between QFTs and
| stochastic processes (which is what I assume you're talking
| about, since the claim that QED is just Monte Carlo is
| extremely false if taken literally), any halfway decent intro
| QFT class will cover it.
|
| - It only "makes more intuitive sense" because you've papered
| over all the gory details. There's very little intuitive or
| natural about analytic continuation in C^{\infty}, for
| instance.
| trashtester wrote:
| Stochastic processes is one thing. But most calculations in
| QFT is done using various perturbation theories (kind of
| like Taylor series expansion), instead of by using exact
| calculation.
|
| That could be interpreted kind of like ray tracing with
| finite resolution and a finite number of reflections.
|
| EDIT: Feynman diagrams serve as an attractive way to
| rationalize the various perturbations, but it may be that a
| "true" theory needs to be able to solve the (for us)
| unsolvable integrals directly.
|
| Especially since the perturbation theory integrals (that we
| can solve) diverge, and require renormalization to avoid
| infinities.
| philipov wrote:
| It kind of feels like the epicycles that were used to make
| geocentrism work. Sure, they make the math work out correctly,
| but the equations for the motion of the planets become
| needlessly more complex than if you just take the sun to be the
| center.
| elteto wrote:
| I think the current state is very much "the formulas work,
| don't get too philosophical with them though". IANAP.
| andreareina wrote:
| That's more about Copenaghen vs Many Worlds vs Pilot Wave...
|
| The infinite path integral is heavy mathematical machinery so
| it's avoided if possible.
| evanb wrote:
| The path integral is used for computational approaches all
| the time.
| ladyattis wrote:
| I think the fact that the oddest approaches work aren't exactly
| a measure of how the particles actually work in physics. I
| think it proves that math isn't the language of the universe no
| matter how much mathematicians and physicists want to say it
| is. It just proves we're good at modeling but not good enough
| to actually know what we're seeing/measuring (a natural limit
| to our knowledge). I don't know why this position is considered
| controversial or out of the mainstream when it seems to be the
| logical answer.
| naasking wrote:
| I don't see why it's logical to conclude that difficulties
| modeling hard problems means we can't model them using math.
| These difficulties are not uncommon in the history of
| science, and we've eventually solved them all before with
| math, and we should expect such problems to become more and
| more difficult as the low hanging fruit has been plucked. I
| see literally no reason to jump to the conclusion that the
| core problem is trying to use math at all.
| ladyattis wrote:
| >I don't see why it's logical to conclude that difficulties
| modeling hard problems means we can't model them using
| math.
|
| That's not what I'm stating though. I'm stating that the
| math involved is a model but it isn't ever going to be
| identical to the thing being modeled. Meaning that math
| isn't the "language of nature" as we don't have a direct
| means to truly comprehend it (direct realism has a whole
| has been discarded by philosophers for a long time now).
|
| >I see literally no reason to jump to the conclusion that
| the core problem is trying to use math at all.
|
| Again, that's not what I said, please read my post again.
| zeroonetwothree wrote:
| Physics is a _model_ of the way the universe works. There 's no
| reason to be sure that it does work the way that we model it.
| Practically speaking though it doesn't really matter as long as
| the model gives us accurate predictions. At that point you are
| entering the realm of philosophy.
| inciampati wrote:
| A model can become a cage. Although the QM "infinite
| trajectories" interpretation is fine at explaining what
| happens in a quantum system from the perspective of an
| observer. But it doesn't provide any intuition about how the
| systems really behave. There may be ways to get access to
| more realistic interpretations. If we had them they might
| help us work with quantum systems.
| stevenhuang wrote:
| Well, a proposed model should firstly match or exceed the
| explanatory power of current models.
|
| Until that bar is met, any such model is more in pursuit of
| beauty and pure mathematics than physics.
|
| Lastly while mathematical beauty and intuitive elegance is
| generally a good rule of thumb, reality need not conform to
| our human sense of aesthetics.
| taylodl wrote:
| Models answer the questions _What?_ and _How?_ They do not
| answer the question _Why?_
|
| The simplest model explaining all observed behavior, and
| even better, making new predictions that are then
| confirmed, is the model we tend to call "reality." But it's
| just a model. When you step back to think about it -
| _everything_ is a model: colors, sounds, the table you 're
| sitting at, the chair you're sitting on. We've just become
| so accustomed to these models we think of them as "reality"
| and it's super convenient to do so. I think that's a big
| point the Buddha was trying to get across.
| cvoss wrote:
| That's a fascinating philosophical presupposition you have
| there: That there is such a thing as a difference between
| the observable* behavior of a system and its real*
| behavior. I won't agree or disagree with you. Just pointing
| it out because it's really interesting to me.
|
| *You used essentially those words ('observer', 'really'). I
| won't try to pin you down into a particular definition of
| them, but I'm guessing we at least approximately agree on
| what they mean.
|
| You could make a similar but less controversial statement
| by substituting "low-fidelity observer" & "high-fidelity
| observer" for "observer" & "reality". I wonder if that's
| what you mean, or if you meant what you said?
| TeMPOraL wrote:
| Hm... I suppose this could be mathematically/logically
| equivalent, but the thing that bugs me in GP's answer
| (and the way I hear QM talked about in general) is the
| very concept of an _observer_. I 've never heard a good
| explanation of why an _observer_ would be a thing - I 'm
| not sure if there could be, without creating some kind of
| magical realm for it to live in, detached from the rest
| of physics.
| Folcon wrote:
| I could be very wrong about this, but I always thought
| observation was a property of measurement?
|
| IE to get any info, we have to interact with the system,
| so we "observe it", by say colliding a photon or
| something.
|
| This action interrupts the existing system.
|
| Once again this could be not the right way of looking at
| it, but that's always how I've interpreted the observer
| to not be this magical thing.
| trashtester wrote:
| > But it doesn't provide any intuition about how the
| systems really behave.
|
| In physics, intuition is much more of a cage than the
| models.
|
| In fact, I would argue that our intuition itself is such a
| model. It's just one that vertebrates evolved over 100s of
| millions of years that enabled their brains to optimize
| Darwinian fitness.
|
| It has layers upon layers of simplifications and heuristics
| to reduce complexity to a levels it can use to compute
| viable behaviour.
| WastingMyTime89 wrote:
| We, humans, are limited in our understanding of the worlds
| by what we can perceive and conceive. What does it means to
| understand how a system _really_ behave when we are so
| limited? That's an interesting metaphysical question but we
| have left the realm of sciences by asking it. Sciences care
| about what can be experienced.
|
| The aim of sciences is to accurately predict the results of
| experiments. A model doesn't have to be simple or
| intuitive, just accurate.
|
| Obviously everyone would like a simpler model predicting
| the same things but not because it would be more real, just
| because simpler is nicer.
| qqtt wrote:
| Feynman's QED talks pretty extensively about this as
| well. An intuitive mental model that humans can picture
| in their heads is a nice to have, not a requirement.
| Understanding how a system "really works" is also not
| really a goal of physics - the science is about
| predicting physical phenomena - and we generally choose
| whatever is the simplest equation we have currently
| available that does that. If you want to define the bar
| for explaining how a "system really works" as being able
| to give you an intuitive human-centric mental model you
| can fully understand and capture in your head - well, you
| are moving the goal posts over what physics really is and
| quantum physics has already left you far behind.
| uoaei wrote:
| > We, subjective agents, ...
|
| Let's not pretend only humans can comprehend their
| observations ;)
| WastingMyTime89 wrote:
| My post is not a statement about animal sentience. Until
| we can have a discussion about empiricism with another
| kind of sentience, I think beginning sentences with "we,
| humans" will be fine. I would appreciate if you could go
| farm PC points somewhere else than in reply to my
| comments and kept condescending winks out of it.
|
| Unless you are making a pun about AI bots and I'm just
| having a bad day, in which case, I will have to ask you
| to be kind enough to bear with my rudeness.
| floweronthehill wrote:
| If we had to design the universe, could we use our models or
| are there better, "truer" formulas?
|
| Apology for the bad analogy but we can write the same program
| in python and java. The output would be the same but
| internally they work differently.
| lmpdev wrote:
| The "truest" would capture everything in its most
| parsimonious representation
| amai wrote:
| See
|
| https://en.wikipedia.org/wiki/Consistent_histories
|
| and the last paper from Murray Gell-Mann (published posthumously)
|
| https://arxiv.org/abs/2110.15471
| fuzzfactor wrote:
| >Our reality may be a sum of all possible realities
|
| OTOH I would say there's a number of possible realities that
| never did add up.
|
| Some of the actual realities also.
| k__ wrote:
| That's what the article says.
|
| If they're too different, they cancel out.
| deathgripsss wrote:
| Quanta really is one of the best science websites around these
| days
| fluffycookie wrote:
| [dead]
| indigodaddy wrote:
| As if I'm a ten year old, because I kind of am when it comes to
| this kind of stuff... how in the world could adding up all
| alternate realities end up with a single reality? How would that
| work?
| slewis wrote:
| It's also surprising at first that infinite series can add up
| to a specific, finite number. If I go halfway to X, halfway
| again, and so on, where do I end up?
|
| All next realities are weighted based on their probability
| given the current one. The probabilities are based on our
| knowledge of the equations of physics. Some realities are more
| likely than others and adding this all up results in an average
| outcome. At the smallest scales the average outcome is not
| actually what happens, but time is a great equalizer.
| osigurdson wrote:
| Lots of things are like that. An infinite number of sine
| waves are needed to make a square wave for example. Taylor
| series expansion, etc.
| CamperBob2 wrote:
| _An infinite number of sine waves are needed to make a
| square wave for example._
|
| And even then it only really works if the waveforms are
| infinite in duration.
|
| Is there a Gibbs Phenomenon equivalent for path integrals?
| [deleted]
| jemmyw wrote:
| The problem here is the word "realities" because it doesn't in
| any way mean an alternate reality where you're there doing
| something else.
|
| This is from the math model where the resulting path of a
| particle can be figured out by finding all possible paths. Then
| what follows is the conjecture that because the math works
| maybe those particles really do travel by every possible path.
| There's no proof of this, and it's very hard to test because
| the result has to be the same.
|
| So maybe our reality is the sum of particles taking every
| possible path. Or may be it isn't. Wake me up when the article
| title is "Our reality IS the sum of all possible realities."
| nine_k wrote:
| "Our" reality is not necessarily the only one, or is somehow
| special: under the many-worlds interpretation, every "quantum
| event" forks the universe. It should look somehow similar to
| "updating" a typical immutable data structure (a tree, a list)
| so that a new copy is created, sharing most data with the old
| copy.
|
| https://en.m.wikipedia.org/wiki/Many-worlds_interpretation
| wyager wrote:
| You wouldn't necessarily end up with a single reality, but if
| there was some kind of process "sampling" a finite number of
| realities from the wavefunction (either continuously, e.g. if
| quantum collapse is ontic, or e.g. at some time in the distant
| future god takes a look at the state of the wavefunction) where
| the sample probability was calculated in the normal way (square
| of the wavefunction magnitude), that would explain how
| probability "gets into" quantum mechanics. This is kind of an
| open question, though - you can find plenty of papers (mostly
| bad) wondering out loud how you get the appearance of
| "discrete" realities with things like probability when quantum
| mechanics appears to be continuous.
| User23 wrote:
| Most of them cancel out. Think of it like summing an infinite
| array of an equal number of 1 and -1 and then a 2. The sum of
| that array is 2.
| travisjungroth wrote:
| Might be easier if you start with the 2 :P
| whatshisface wrote:
| If we station ourselves outside of the quantum system, then the
| answer is that each reality puts a complex number on every
| outcome, which get summed into a single complex number. Then,
| the magnitude squared of that sum becomes the probability with
| which we will observe that outcome.
|
| If we put ourselves inside the system then saying anything
| about it comes with a lot of implied philosophy about what "we"
| are.
| jwuphysics wrote:
| I don't have a good answer for you, but the best advice I'd say
| is to read Feynman's book "QED: the Strange Theory of Light and
| Matter" (it's really just a four part lecture series).
| krohling wrote:
| The path integral suggests that all of the possible realities
| are like waves. When these waves are added up, they can
| interfere with each other in a way that only one of the
| possible realities can be seen. It's like when you throw two
| stones into a pond at the same time. The waves created by the
| stones will interfere with each other and create a single
| pattern on the surface of the water. In the same way, the path
| integral suggests that all of the possible realities can
| interfere with each other and create a single reality.
| thriftwy wrote:
| Alternative realities may have alternative people living in
| them - will they observe different laws of physics since
| their reality is not a sum of all realities, as opposed to
| ours? Or would they have the same conclusion about sum of
| realities?
|
| A) these realities can't have people - they're not real
| enough to be called that
|
| B) physics will be different - that would be a great
| scientific revelation
|
| C) physics will be the same - making the premise meaningless
| indigodaddy wrote:
| Great explanation, quite fascinating thanks!
| krohling wrote:
| Thanks for the kind response. I actually generated that
| comment by feeding parts of the article into ChatGPT and
| asking it the previous comment.
|
| If you would like to ask it more things about this article
| I have it searchable here: https://app.conifer.chat/threads
| /37321fdc-6a04-4c56-a857-21b...
| nomel wrote:
| There should Be a disclaimer that this could be complete
| fiction, unless you've verified that it's true.
| 93po wrote:
| I mean it's all theory anyway. By definition, theories
| could be complete fiction.
| kim_chi wrote:
| [dead]
| superposeur wrote:
| There's an aspect of path integrals that's always bugged me. They
| rely on an underlying classical field configuration space to
| integrate over. But, this seems problematic to me, since properly
| the objects of classical physics should emerge as limits of
| quantum physics. Shouldn't we be able to start with "pure"
| quantum mechanics (a state space, a Hamiltonian operator), with
| no mention even of "paths" and get out classical physics? I.e. it
| seems backward to speak of a particle trying out different paths
| since a "path" is a classical notion; it's as though quantum
| mechanics is merely a superstructure on top of an underlying
| classical layer rather than the other way around. Know what I
| mean?
| mlloydw wrote:
| Quantum and Newtonian physics are both just conceptual models
| to help ourselves understand and predict the things around us.
| They're useful for describing reality but neither of them are
| accurate representations of the real deal. All models break
| down at some limits.
|
| To imagine one model "being merely a superstructure" on another
| implies that the limit doesn't exist and the two models are, in
| fact, the same thing. If you expand that idea to all other
| models, they would all eventually merge into one grand unified
| theory that accurately explains every single occurrencence in
| the universe. A nice idea conceptually, but we're at best no
| where near that and at worst have brains that will find the
| nuance of the universe impossible to comprehend in totality.
|
| Any model that is useful for understanding and predicting the
| universe is worth sticking with, as long as you understand the
| limits where it applies. If path integrals happen to unlock
| some new insight that seems evidentially correct, it doesn't
| really matter if it ties into other theories.
| adamnemecek wrote:
| Sounds like a convolution.
| lukeplato wrote:
| Indeed, then the fundamental object would be a kernel much like
| the cosmological polytope or amplituhedron
| adamnemecek wrote:
| Function composition is convolution of unit impulses.
| consilient wrote:
| This is approximately right, time-evolution of a spatially
| homogeneous system is convolution of its initial state against
| the propagator `K = \int e^{iS[x]} Dx` over all of space - with
| the caveat that none of this is quite true. These aren't really
| integrals, `Dx` isn't really a measure, and fully rigorous
| interacting QFTs in 4d may or may not exist.
| juliangoldsmith wrote:
| Each year is just another round of stochastic gradient descent,
| optimizing for the darkest timeline.
| iNeed2p wrote:
| According to physicists, our universe is governed by such simple
| laws, which we call sutras And the reader himself is an example
| of how many types of complex entities are created. Do you
| understand the amazing similarity of the game of life with our
| three-dimensional world?
| User23 wrote:
| It was Feynman's excellent _QED_ (basically the ELI5 for quantum
| physics) that introduced me to the concept that the photon I
| shine on my wall with a flashlight did a few orbits around
| Jupiter too, it just cancelled out. I believe the "dippy" process
| to do that is called renormalization. Feels like it's time for a
| reread.
| froeb wrote:
| This is a pretty common misconception of path integrals from
| how Feynman explains them to a non-physicist audience. The path
| integral formulation cannot work from a particle-based
| approach, where you integrate over all possible trajectories of
| electrons, photons... . Instead, you are actually integrating
| over all possible configurations of fields, the electron field,
| the photon field... . As far as I know, there is no way to view
| the universe as being fundamentally made of particles that is
| compatible with our modern understanding of QFTs.
|
| Also, the path integral really shouldn't be taken too
| literally. It's not reality, it's just an equation.
| evanb wrote:
| > The path integral really shouldn't be taken too literally.
| It's not reality, it's just an equation.
|
| While it's always important to point out that the current
| paradigm might be overthrown tomorrow, it is ALSO important
| to understand different ways of describing what the current
| paradigm says!
|
| The "bizarre" trajectories that contribute to the path
| integral are not optional; they're _really there_ and you can
| arrange interference experiments to make their physical
| importance clear.
| dvt wrote:
| > the photon I shine on my wall with a flashlight did a few
| orbits around Jupiter too
|
| This is a very confusing way to essentially say that an
| integral of exp(-iS) is taken over all possible paths for the
| action S. Your flashlight's photons don't _literally_ do a few
| orbits around Jupiter. In fact, photons are quantum objects
| (not billiard balls) and they are "smoothed out" over
| spacetime, meaning they don't really have paths to take in the
| first place.
| CyberDildonics wrote:
| You think that when you turn on there are actual photon
| particles that traveled 48 minutes to jupiter and back?
| evanb wrote:
| One absolutely astounding aspect of the path integral is that
| it ascribes amplitudes to trajectories that don't obey the
| (classical) equations of motion. If you like: the classical
| paths are at turning points of the action (loss function) but
| all other paths have some associated imaginary loss. In the
| path formalism we sum every possible path, weighted by the
| imaginary loss.
|
| Since the classical paths are at turning points, small
| deviations around that path tend to sum coherently, while
| large deviations tend to have neighboring paths that
| contribute with opposite phases.
|
| If you write the amplitude for a photon going from your
| flashlight to your wall, the path integral tells you to sum
| up _all possible paths_ , not just the classical ones. So
| yes, the path integral formalism _really does say_ that a
| photon goes from the flashlight, around Jupiter, and to the
| wall. But not in 48 minutes! In time = (distance to wall) /c!
| It goes every which way without caring about the classical
| law. The "insane" ones cancel.
|
| Well, if all the insane ones cancel, what's the point of all
| this? Just forget about them, they're a huge surplus of
| nonsense in the theory! However, it's perfectly possible to
| construct experiments where it's clear that photons really do
| "smell out" alternatives. Diffraction gratings, the double-
| slit experiment, etc., all show this quantum weirdness.
| raattgift wrote:
| > a huge surplus of nonsense in the theory
|
| It gets even worse if one starts thinking about the
| flashlight's filament or semiconductor, the wall,
| stratified Jupiter with its optical depth, the flashlight-
| wielder's visual system, and everything in between all
| being (sets of) quantum objects.
|
| Maybe the thing to do in this example is to assume
| correspondence and think classically, and insist on using a
| fewer-quantum-number example if one wants to think
| quantumly. However I tend to admire _detailed_ wrestling
| with everything-is-quantum in an example like this. Maybe
| start by remembering that the spot-on-wall-to-retina is
| relevant, not just the flashlight-to-wall.
| CyberDildonics wrote:
| Are there any other household appliances that emit photons
| that circle around any other planets?
| evanb wrote:
| "Emit" usually means as radiation. The photons that take
| every path are not on-shell.
|
| When your refrigerator is open the light you see contains
| an amplitude of the photon going around Uranus.
| CyberDildonics wrote:
| Flashlight -> Jupiter
|
| Refrigerator -> Uranus
|
| What planet does the oven radiation go around?
| raattgift wrote:
| > What planet does the oven radiation go around?
|
| _All_ of them, including planets outside our observable
| universe.
|
| Same with the flashlight and refrigerator.
|
| Each photon also goes into and through _all_ the planets,
| and near each of their electrons in an infinite number of
| ways. They also go through all the stars, all the vacuum,
| etc. And with all the speeds, all the accelerations, ...
|
| Most of these infinite possibilities are a negligible
| contribution to the final probability that photon-
| detected-at-B was photon-emitted-at-A. The non-negligible
| contributions come from the (still infinite) trajectories
| allowed by relativity and mostly from those near the
| lightlike geodesic.
| raattgift wrote:
| There are two different "circle around distant bodies"
| here that are probably causing some confusion.
|
| Classically, if near your appliance you can see stars and
| Mars through the window on a cloudless night, a flash of
| light from your appliance can in principle be seen (e.g.
| by a really good telescope orbiting one of those bodies).
| Indeed, some of the light from a laser pointer shining
| out your window will go to infinity, and has some
| astronomical chance of being stuck the photon ring around
| a black hole. It works just like you being able to see
| through your window starlight or (in principle) a bright
| flash on the surface of Mars. However, this is not the
| sort of "goes around Jupiter" that we are talking about;
| we are talking about something like the probability of
| one microscopic component of your appliance's flash of
| light scattering off or being absorbed by a microscopic
| component of said window (for example, in part of the
| spectrum in which the window is not transparent), where
| calculating that probability considers an infinity of
| possible paths.
|
| Indeed, an infinity of the path-integral trajectories of
| _one_ such photon (that classically is definitely
| intercepted by the window) will go out to infinity, an
| infinity of those trajectories will go out to Jupiter, an
| infinity of them will go to somewhere very near the
| window pane, an infinity of them will speed up and slow
| down along the way rather than travel at the speed of
| light at all times, etc. We then want to assign a
| probability to each of these paths.
|
| The idea is that trajectories that are not like a
| classical path between appliance and window are
| suppressed because (somewhat technically) they [a] have
| the same magnitude of probability amplitude, but [b] have
| different phases, [c] different changes in trajectory
| have different changes in phase, and [d] for "longer"
| paths compared to the classical one, a small change of
| trajectory makes a big change in the action compared to
| Planck's constant, so [e] for sets of small changes in
| "longer" paths the phases change rapidly and thus are
| highly likely to cancel out. So most of those infinities
| are effectively irrelevant and we can focus on the
| smaller infinities that make a marginal-to-relevant
| change to the total probability amplitude and figure out
| a way to calculate it from them (e.g. by sampling).
| averagestudent3 wrote:
| [dead]
| sashank_1509 wrote:
| Well if it's about QED I'm always drawn to the paper : "There's
| something rotten about the state of QED" by Oliver Consa
| (https://arxiv.org/pdf/2110.02078.pdf). Since I'm in CS, not
| physics could anyone let me know if the paper above is making
| stuff up. If not, then it does paint QED in a very bad light and
| indicates something wrong with that field
| naasking wrote:
| Yes, there are historical issues with that calculation, and
| field theories in general. I've become very suspicious of
| continuous formalisms period:
|
| https://arxiv.org/abs/1609.01421
|
| I think many problems in physics will be solved by moving to
| more discrete formalisms.
| puzzledobserver wrote:
| I skimmed the article, and admittedly didn't understand much.
| Action principles and path integrals are unfortunately one step
| above where my knowledge of physics stops.
|
| But seeing a headline that reads, "Our reality may be a sum of
| all possible realities", I wonder whether the point is to (a)
| explain new experiments, (b) develop simpler mathematics, (c)
| close the gap between intuition and theory, or (d) whether this
| is all unfalsifiable speculation.
|
| A charitable interpretation might be (c), but a skeptic within me
| thinks that this might be (d).
| skissane wrote:
| I think one problem here - physics and the philosophy of
| physics are two different things, but a lot of people (even
| professional physicists) fail to treat them as distinct in
| practice. So long as we are talking about math, observation and
| experiment, that's physics. But many of these "multiverse
| theories" are moving beyond that into philosophy, because they
| are not about the math, but rather how to _interpret_ it; not
| only are existing observations or experiments are insufficient
| to determine their truth or falsehood, but we often have no
| idea of any practically feasible way to test them.
|
| There's nothing wrong with doing philosophy-but let's be honest
| when we are doing it, rather than pretending it is still just
| physics. Furthermore, your average professional physicist has
| indubitable expertise in physics, their expertise in philosophy
| (even the philosophy of physics) is much more variable.
| evanb wrote:
| If you have a quantum-mechanical system that you describe by
| specifying a Hamiltonian, I can always write down an exactly
| equivalent path integral. A failure of equivalence between
| these two descriptions would be extremely exciting, as it'd
| be evidence of some paradigm beyond quantum mechanics.
| skissane wrote:
| The mathematical equivalence of Hamiltonians and path
| integrals, by itself, doesn't tell us anything about a
| "multiverse", however.
|
| Since the mid-20th century, many philosophers have come to
| analyse everyday talk about possibility in terms of
| "possible worlds" or "possible universes". However, those
| who invented that language (such as Carnap and Kripke)
| weren't claiming that all those possible worlds were just
| as actual as this one - that claim only arose later (David
| Lewis). Similarly, the originators of the path integral
| formulation of QM (Dirac and Feynman) weren't claiming that
| all those paths were "equally real". By contrast, the
| majority of proponents of "multiverse" theories, are
| claiming those other "universes" are ultimately just as
| real as this one, as as opposed to merely being some
| theoretical construct, a mathematical metaphor.
|
| You can understand these "paths" in two ways - either as
| just a useful mathematical tool for analysing past
| observations and experimental results, and predicting
| future ones, but passing no judgement on their ultimate
| ontological status; or, as a claim that in some sense, all
| these paths "really exist". And, if you adopt the later
| reading, you can then argue whether they are all "equally
| real", or maybe one of them is "actual" and the others
| "non-actual", or even just maybe that some are "more
| actual" than others. Nothing wrong with those debates - but
| when we start engaging in them, we are no longer doing
| physics proper, we are doing the philosophy of physics.
| evanb wrote:
| Agree with your first paragraph and the first half of
| your second paragraph. But if you listen to Feynman talk
| about the path integral, he emphasizes that all the paths
| _are_ real, are in fact critical to describing
| experimental results, and that it is simple to construct
| experiments to get those paths to coherently add to
| produce classically-unexpected results! I don 't know
| about Dirac's "philosophy"; what you say of his stance
| may be correct.
|
| I don't think Feynman would say any path is actual or
| non-actual. It's not the case that ONE of them "really"
| happens and the others are a calculational trick. It's
| that the thing that happens is a sum of alternatives. The
| sum is real, full stop. If you leave out terms in the sum
| you get the wrong answer. Is that enough to mark them as
| "real"?
|
| THAT is philosophy. Certainly you may find another
| description for the same sum which does not discuss a sum
| over paths at all. Perhaps that means that the terms in
| the sum are not real? But we can all agree that the sum
| itself comes out right, whichever picture of the world
| you want.
| skissane wrote:
| > But if you listen to Feynman talk about the path
| integral, he emphasizes that all the paths are real, are
| in fact critical to describing experimental results, and
| that it is simple to construct experiments to get those
| paths to coherently add to produce classically-unexpected
| results!
|
| Okay, but is he claiming that the paths are "real" in the
| exact same sense as the actual experimental results are?
| We (in principle) add up an infinite number of paths, and
| that infinite sum produces a probability amplitude, which
| gives us a probability, and then we expect that if we
| repeat the experiment enough times, and sufficiently
| control the sources of experimental error, the observed
| probability should converge to the calculated one. Even
| if he says that all those paths are "real", I don't think
| he is claiming that they are "real" in quite the same
| sense as the actual experimental results are "real".
| Whereas, a proper "multiverse theory" would be claiming
| something like that.
| evanb wrote:
| OK, I think I appreciate the issue: "multiverse" is
| overloaded. When people talk about the many worlds of
| quantum mechanics, or alternate histories, they're
| talking about the terms in the path integral sum.
|
| But multiverse can also mean, separately, at least two
| additional things. First, different bubble universes that
| may be a part of cosmology. Second, alternate quantum
| field theories (with different actions, or fundamental
| parameters, or different gauge groups, or whatever).
|
| The standard quantum-mechanical path integral is a sum
| over histories / a many-world theory in the sense that
| the different histories / parts of the wave function can
| be detected via interference experiments. Other meanings
| of multiverse are speculative (though maybe a cosmologist
| will come and tell me that bubble universes are a direct
| prediction of inflationary cosmology and are not optional
| either).
| skissane wrote:
| > When people talk about the many worlds of quantum
| mechanics, or alternate histories, they're talking about
| the terms in the path integral sum.
|
| Hugh Everett's original "many worlds" theory wasn't just
| "talking about the terms in the path integral sum" - he
| saw all the "worlds" as "real" in exactly the same sense
| that the actual experimental results are "real". Everett
| believed in "quantum immortality", which only makes sense
| if you interpret the worlds in a much more literal sense
| than mere "terms in the path integral sum" suggests. By
| contrast, even though Feynman calls the paths "real", it
| isn't clear if he means them to be "as real" as the
| actual experimental results they are used to predict.
| People who reinterpret many-worlds as just "terms in the
| path integral sum" are cutting the theory down to be
| something far less metaphysically grandiose than its
| originator intended.
|
| To quote Max Tegmark:
|
| > Atheist or not, Everett firmly believed that his many-
| worlds theory guaranteed him immortality: His
| consciousness, he argued, is bound at each branching to
| follow whatever path does not lead to death --and so on
| ad infinitum. (Sadly, Everett's daughter Liz, in her
| later suicide note, said she was going to a parallel
| universe to be with her father. [149a])
|
| https://space.mit.edu/home/tegmark/everett/everett.html
| skissane wrote:
| > To quote Max Tegmark:
|
| Correction: that's on Tegmark's website, but it isn't
| Tegmark's words. It is authored by Eugene Shikhovtsev,
| edited by Kenneth W. Ford
| peteradio wrote:
| Sadly? You presume she is not actually living in a
| parallel universe!
| ImHereToVote wrote:
| Can you do that for two systems that interfere? I think
| that is where the multiverse stuff comes from.
| evanb wrote:
| Well, it depends on what you mean by "two systems". If
| they are described by one Hamiltonian the result is one
| path integral. In a relativistic quantum field theory,
| for example, one may describe systems of any number of
| particles. The path integral captures all the
| interference effects.
| wyager wrote:
| At some level this is just the application of wave mechanics to
| reality. "A vibration on a guitar string is just the sum of the
| vibrations of its harmonics." You are decomposing a system into
| a mathematically favorable basis, performing a calculation in
| that basis, and recovering the original basis (by linearity).
|
| You can do this in a hamiltonian way (find the eigenvectors of
| the hamiltonian, decompose your system into those, step them
| forward [easy] and add them back together) or in a lagrangian
| way (treat each point in the system as the source of a sort of
| spherical wavefront). Feynman is doing the latter - he's just
| asking "what if we treated other fields the same way we know we
| can treat the EM field via Huygens-Fresnel?" Calculate the
| progression of wavefronts through the field, using euler-
| lagrange to find the parts of the wavefront that don't fade
| away into random noise.
|
| So, in the framing of your question, I think it's mostly B and
| C, with maybe a bit of D (it's not clear if we have any kind of
| tool besides aesthetic preference to distinguish between
| extrinsically equivalent models). However, your framing might
| be a bit uncharitable.
| evanb wrote:
| If we falsified quantum mechanics we might expect some
| formalisms (Hamiltonian, Lagrangian, path integral, whatever)
| to continue to work while others don't. So it's perfectly
| possible to falsify the idea that path integrals do not
| describe the world; doing so would demonstrate a failure of
| quantum mechanics and would be _extremely exciting_ to
| physicists.
| whatshisface wrote:
| Path integrals are for (a) and (b), but take that with the
| knowledge that Feynman came up with them maybe 70 or so years
| ago, so that anything we say about it now tends towards (c) and
| (d) by virtue of a lot of the (a) and (b) questions already
| being taken care of.
| babyshake wrote:
| Sometimes the discussion I observe among scientific experts
| resembles the type of discourse I've seen from Marvel fans
| debating the finer points about multiverses and incursions
| within their comic book canon.
| [deleted]
| malthaus wrote:
| i'm way too dumb to understand quantum mechanics and all its
| implications but i love to believe in quantum immortality and
| surfing all those possible realities to infinity - the most
| optimistic, while somewhat grounded in science, approach to
| thinking about life & death imho
| HeartTicker wrote:
| That's right, the universe is so complex that programming it into
| a computer is close to impossible for even the most advanced
| civilizations, at least it seems to us. But actually there is no
| need to program the entire universe. Just enough programs to fool
| us, the characters in the simulation.
| 93po wrote:
| Alternatively, just program the characters to be incapable of
| perceiving the gaps in the simulation. Which is totally
| possibly happening considering we are far from understanding
| the entirety of how the universe works.
| vouaobrasil wrote:
| This title really irks me. Basically it is saying that quantum
| states or possible paths are realities. But the very definition
| of reality is what we experience, so how can a quantum state, be
| a separate reality? The article mentions reality 5 times, not
| explaining what it even means by the word "reality".
|
| While there may be some philosophical content to the statement,
| it is not a well-defined physical statement and moreover, the
| article doesn't show any "evidence" for it assuming that there is
| even a valid way to interpret the title.
|
| I've come to expect this type of artilce from Quanta Magazine
| though.
| 3np wrote:
| The title irks me for a different reason. An improvement would
| be "Reality Considered As a Sum of All Possible Realities".
|
| This is an interpretation/model you can choose. It doesn't seem
| falsifiable or provable.
|
| "The sweater I wear today may be a sum of all sweaters is my
| wardrobe" - yeah, it is if you choose to think of it that way.
| Rury wrote:
| It irks me, because I find the title reads similar to: "Our
| possibilities may be a sum of all possible possibilities". Or
| shorter "What's possible is the sum of all that's possible".
| It's like, well no duh?
|
| If it doesn't read like that, then I too find it irksome for
| the aforementioned reasons.
| layer8 wrote:
| One way to look at this is that there is no difference between
| possibility and reality. If there was, then the question would
| arise why only this one particular possibility is real, while
| the other possibilities aren't. If there is a necessary reason
| for why this particular possibility is real, then that would
| mean that the other possibilities aren't actually possible
| after all. If, on the other hand, there isn't any such reason,
| then that would mean that the fact that this particular
| possibility happens to be real is _fundamentally_ arbitrary.
|
| Personally, I find it more plausible, in a parsimonious Occam's
| razor sense, that everything that is logically possible
| actually exists, because the only other option is
| arbitrariness, the lack of any explanation even in theory.
|
| Whether summing up path integrals makes sense, is however a
| different question.
| [deleted]
| Rury wrote:
| The problem with that, is how can you ever be certain that,
| there isn't _always_ such a reason? Just because no reason is
| found (yet), is not proof that there does not exist any such
| a reason.
|
| To me, accepting that everything logically possible actually
| exists, is accepting arbitrariness, as all of logic is
| fundamentally built upon arbitrary assumptions. It seems like
| rather accepting that things aren't inherently arbitrary (
| _that not all logical possibilities exist / there always is
| such reasons_), is the only way to avoid arbitrariness. It's
| just a matter of finding the theory that explains it.
| goatlover wrote:
| > But the very definition of reality is what we experience
|
| If you're an anti-realist when it comes to science. I suspect a
| lot of scientists think there's plenty of reality we don't
| experience, such as quarks, dark matter or the interior of
| black holes. Or what lies beyond our light cone (probably a lot
| more of the same), given the flat topology that's been
| measured.
|
| Sean Carol has argued for the Many Worlds Interpretation being
| a true description of reality, even though we can only
| experience the decohered branch (world) we're on.
| RyEgswuCsn wrote:
| My interpretation of the double-slit experiment has been that the
| presence of physical matters in a certain configuration imposes
| some sort of probabilistic "reality field" that propagates at
| light-speed. The superposition of these "reality fields" at a
| certain point in space(-time) determines the probability of the
| particles' behaviour at that point, not unlike how support vector
| machines with a RBF kernel work.
|
| This interpretation has the benefit (I think) of bypassing the
| problems with the "observers" --- any apparatus that is able to
| discern the path a particle had taken just so happens to impose a
| field such that, when superposed with the original "reality
| field" the double-slit imposes, the mixture field no longer
| exhibits the interference pattern.
|
| Of course I am not a physicist by training, so the above could be
| completely nonsense (or it could have already been an established
| theory).
| 93po wrote:
| I would love for someone to clarify this for me: My
| understanding is that the physical act of observing, as in,
| with the specific machine we use, is what causes photons to
| suddenly behave different. Like if decided to gently touch a
| marble rolling down a ramp in darkness to measure how fast it
| was rolling, the act of touching changes how the marble rolls.
| It's not that we're actually observing the marble's behavior,
| is that's we're interfering with its behavior when we measure
| it.
|
| Is this accurate? Or is the observation of the photon not
| physically impacted by our observation, and through some magic
| of quantum behavior, it somehow "knows" it's being observed and
| changes because of that?
| RyEgswuCsn wrote:
| I'd thought direct observation of the photo would mean
| physically interact with it (and thus changing its
| behaviour). My understanding is that there are also other
| experiment designs which allow identifying which path a
| photon went after it passed the slits; in such cases the
| interference pattern disappears too.
| consilient wrote:
| > It's not that we're actually observing the marble's
| behavior, is that's we're interfering with its behavior when
| we measure it.
|
| There's no difference: observing something without
| interacting with it isn't possible.
| hartator wrote:
| Can someone explain why "the future is already written" theory
| doesn't work to explain quantum effects?
|
| If we see the world as a book and we are in the middle of it, we
| don't need string theory, multiverse, or this new theory to
| explain quantum effects.
| peteradio wrote:
| It's not a predictive solution. It could very well be revealed
| to be true by some god type creature, but it defies the
| fundamentals of conducting the scientific method.
| naasking wrote:
| Why does it defy the fundamentals of the scientific method? I
| don't see what fundamentally changed just because that search
| is deterministic. Given all finite theories are recursively
| enumerable, and we clearly have enough degrees of freedom to
| enumerate all such theories, what's the problem exactly?
| peteradio wrote:
| The scientific method is hypothesis testing. You must be
| able to make a hypothesis that is testable. What method are
| we using to make testable hypotheses?
| naasking wrote:
| Abductive reasoning, which is compatible with
| determinism. Even if we had to resort to blind trial and
| error after enumerating all possible theories, science
| would still work, just less efficiently. Like I said, I
| don't see the problem.
| peteradio wrote:
| I think you might be joking. I assert that we cannot in
| fact enumerate all possible theories, even if you could
| write down a representation, they must be enumerated in
| meatspace to conduct an actual comparison.
| naasking wrote:
| Of course we can enumerate all possible theories, in
| principle. This is a trivial corollary of the fact that
| we can enumerate all Turing machines.
|
| It's not practical but that's not the point. The point is
| that if we are free enough to enumerate all possible
| theories in a deterministic universe, then science can be
| conducted. Clearly we can enumerate all possible
| theories, therefore even if our universe is
| deterministic, we can do science.
| ladyattis wrote:
| If you mean why is quantum mechanics probabilistic? That's
| because it's the best model we have to handle many of the
| momentary variations in physical systems. But I'll say that
| determinism could still be true even if we can't make
| predictions. But equally, I believe that determinism can be
| false even if the observable universe seems well ordered. Chaos
| doesn't just mean random things happening without a cause that
| could be found. It just means that not all chains of causality
| can link backwards and forwards perfectly in time (i.e. some
| chains of causality maybe emergent at best).
| fallingfrog wrote:
| Well, that seems vague, but here are some of the things about
| quantum physics that people find odd:
|
| 1) We intuitively think of the past as certain and the future
| as unwritten. You seem to be suggesting that both are written.
| But quantum physics says that both are _unwritten_. So it 's
| exactly the opposite. Just as there are many possible futures
| from this point, there are also many pasts leading to this
| point, and that's why you get interference patterns and so
| forth.
|
| So, it's not that there is one past and many futures, nor is it
| the case that there is one past and one future. There are many
| pasts and many futures, and they are all mathematically,
| measurably, equally real. (But not all equally probable)
|
| 2) Probabilities in quantum physics have a phase angle, which
| is why they can sum or they can cancel out.
| SaintBroseph wrote:
| [dead]
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