First, if all the initial conditions of the Universe at the highest granularity possible were known (10^1000? likely higher), then to calculate the current state of the Universe (10^17 seconds) at every step along the way, once per Planck time (10^44 per second), ... a calculation on the order of 10^1000^10^17^44 states (did I get that right?) No computer which exists IN our universe could hope to tackle such a calculation ABOUT said universe, in any reasonable amount of time. (Of course you could cheat like game developers do, but that's another conversation.)
Second, if a conscious entity is able to calculate and predict step N+1, they can choose to alter that step.
This kind of article and comment is fun to think about -- does it even make sense to ask the question in the first place?
One question arises for me: if no computer IN the universe can calculate this, how are these rules being applied?
I mean, something must be enforcing, there's no "magic" voodoo in the Universe, that's for sure.
So how can these rules keep being enforced, so that the outcomes follow the "predicted" path, if the universe can't possibly be able to "calculate" everything?
Ok no scientific basis for this at all but I often wonder if some future inventions are so potent that their "gravity" for lack of a better word, manifests itself into being.
> In the real world, your past uniquely determines your future. If a physicist knows how the universe starts out, she can calculate its future for all time and all space.
[Layman here] Is this ignoring quantum mechanics? Or is the claim that you can reconstruct the original wavefunction a collapse? I thought randomness was pretty intrinsic to quantum mechanics.
I have a master in physics, but I share your surprise over this paragraph in the OP.
I’m a bit unsure though how widely and quickly the probability mass dissipates over possible future states. E.g. the existence of quantum computers implies that at least in some physical systems the probability mass stays concentrated on a few possible future states. I wouldn’t expect that to hold for the universe as a whole though.
I thought it is mostly accepted (citation needed) that apparent wave function collapse is due to the coupling of the microsystem to the measurement apparatus, together they continue to evolve deterministically
Yes, in my experience (master's in physics) this is a widely held belief among physicists studying foundational issues[0]. In some sense it's the Many Worlds interpretation of QM but all inside one single world.
[0] I specifically remember some podcast by Sean Carroll where he expressed this exact same view.
Oversimplifying, Physics is divided in "General Relativity" and "Everything Else".
Where "Everything Else" includes Quantum Mechanics, Special Relativity, Chemistry, particle Physics in the CERN, Superconductor, Semiconductors, Conductors, ..., and a simplified model of Gravity because people is interested in rocks that fall, airplanes, Astronomy, and other stuff.
And "General Relativity" includes another copy of Special Relativity and an accurate model of Gravity, that is only important if you are studding black holes, gravitational waves at LIGO, some parts of Astronomy, GPS and not too many more applications. This part of Physics ignore the Quantum Mechanics effects.
There are some results that mix both, like Hawking radiation around black holes. The idea is to find a more general theory that joins the two parts of Physics, but it's apparently a very difficult problem because a lot of smart people has tried to solve it and failed.
The research paper is about black holes, so it has a huge invisible disclaimer at the top that says:
fake quote> <huge><red>We are going to use General Relativity and we are going to pretend that nobody discovered Quantum Mechanics and the universe is 100% deterministic.</red></huge>
This disclaimer is actually not written because everybody reading the journal will assume that, so it's not necessary to explain that. The problem is that this simplification leaked to the press article, and the simplification was not well explained. So it looks like the author or journalist are not aware of Quantum Mechanics, but actually they are fully aware, it's just that they need this simplification to be able to do the calculations. It's a standard trick in the area.
Also the "100% deterministic" in may fake quote is a lie, because the problem is that in some parts inside black holes the equations of General Relativity have technical problems and have multiple solutions. The article is about this region and about how many horrible things would happen if you try to visit that part of a black hole. The main result is that in some cases it's not as horrible as expected and you may survive.
If you are willing to take a more many-worlds-like view of quantum mechanics, the past (or even just the present) does determine the future. Specifically, the future quantum state is fully determined. This state will be exceedingly complicated at will encode things like “there’s a 5% chance you get that job,” but this can be fully described in the wave function.
There are multiple interpretations of quantum physics. According to the "many-worlds hypothesis", the randomness is also deterministic -- it means that the reality branches into multiple futures, different futures containing different outcomes.
Here, randomness means that you cannot predict the outcome, because in the future there will be multiple copies of you, and whatever specific outcome you predicted, some of those versions will see a different outcome. But at the same time, this is fully deterministic, in the sense that there is an equation that describes exactly which futures will happen and which outcomes they will contain.
(There are also other intepretations, which add a new assumption on the top of this, such as "and one of those futures, selected randomly, is the real one, and then the remaining ones disappear". Which is kinda unfalsifiable, because yeah, it could be like this; but it also could be that all the different futures are equally valid, and each of them contains a copy of you saying "oh, so this one is the future that actually happened" -- but of course, different copies of you saying it about a different version of the future.)
EDIT: However, from the perspective of the "many-worlds hypothesis", everyone has an infinite number of futures... so the statement in the article doesn't make sense here either.
So, I am going to assume that the paper actually referred to something else, and the author of the article, trying to simplify it, turned it into something nonsensical.
The math of quantum mechanics itself is reversible. Every quantum operation is defined by a matrix, and it's possible to invert that matrix to run the operation backwards in time. The past uniquely determines the future.
"Collapse" interferes with that. It's not a quantum mechanical operator. It forms a bridge between quantum and classical operations by throwing away part of the wave function. It's able to do that because part of the wave function goes to (effectively) zero during decoherence.
With MWI you ignore that, and continue to treat the whole wave function. That follows the mathematical formalism, so it's more useful for thinking about unitarity, even though experimentally it's the same thing.
So the short answer is that the paper isn't ignoring quantum mechanics. For the purpose of this paper we're hand-waving away the question of what happens when the wave function becomes separable like that, because it's not important here. What's important is the conflict: even if you treat quantum mechanics as predictable (because it is, in that sense), you still run into problems with black holes.
That points in the direction of the problems with fusing relativity to quantum mechanics, which is an open question.
> In the real world, your past uniquely determines your future. If a physicist knows how the universe starts out, she can calculate its future for all time and all space.
No (or only in very special configurations). See for example the n-body problem.[1]
> In the physical literature about the n-body problem (n ≥ 3), sometimes reference is made to the impossibility of solving the n-body problem (via employing the above approach).[citation needed] However, care must be taken when discussing the 'impossibility' of a solution, as this refers only to the method of first integrals
Non deterministic dynamics occurs in Newtonian physics as well. These things are worrying philosophically, but we can take some comfort in that the initial conditions for which these occur are "measure zero" i.e. if you were defining a probability measure over initial conditions, then the non-deterministic initial conditions would have measure zero. It doesn't sound like we have that option here though.
But in my view, this result is good news. We knew GR was unphysical anyhow. Now we've increased our understanding of how unphysical it is, potentially ruling out more replacement theories.
My statement was too strong. By "knew" I meant that it was pretty likely that strong cosmic censorship wouldn't work out [1]. In addition, I think QFT and GR are both probably broken in some way given that they're incompatible and that no one has coerced one of the theories into working with the other. That feels like evidence that the problem is not as simple as just rejecting QM or GR to me. But again, calling that "knowing GR is unphysical" was too strong a statement.
So let me get this straight... you enter the event horizon of a black hole, from which nothing can escape. This "frees" you from your deterministic past so that you have infinite possible futures... but since you're in a black hole these infinite futures all probably occur in the ultrahot ultrasqueezy confines of a black hole.
This article unfortunately imho does a poor job in putting the mathematical modeling and Hintz' effort into greater focus and context. Possibly to make it more exciting for the casual science interested reader.
"Some carefully constructed mathematical objects (similar to observed black holes) erase your past (makes it possible to cross the Cauchy horizon)" in GR would be more precise.
Here [0] is a good article about the kind of mathematical manipulation involved (not directly related).
This [1] quora answer goes right to the obvious issues.
> In effect, all the energy the black hole sees over the lifetime of the universe hits the Cauchy horizon at the same time, blasting into oblivion any observer who gets that far.
I don't understand that. It sounds as if the reason all the stuff reaches the horizon "at the same time" is because time slows to a stop at the horizon. But doesn't that also apply to the "observer who gets that far"? How can the observer overtake all the other stuff?
Maybe I should stay clear of articles dealing with GR; I'm clearly not clever enough to understand them.
32 comments
[ 3.6 ms ] story [ 79.8 ms ] threadThis seemingly innocent statement could render a prolific discussion. Branching so many ways...
Which one would you like to pick?
Loss of simultaneity, from relativity theory [1]
Consistent histories and probabilistic future prediction by quantum mechanics [2]
Free will -- upfront this can sub-branch into biology, psychology, philosophy, artificial intelligence... [3] [4] [5]
"The Best Way To Predict The Future Is To Create It", so if we can create stuff, then it's possible to (at least partially) predict the future? [6]
Futurism or Future Studies [7]
[1] https://www.bbc.com/reel/video/p04s223f/physics-suggests-tha...
[2] https://quantum.phys.cmu.edu/CHS/histories.html
[3] https://www.frontiersin.org/articles/10.3389/fpsyg.2013.0013...
[4] https://www.wired.com/story/algorithmic-prophecies-undermine...
[5] https://projects.iq.harvard.edu/predictionx/week-3-free-will...
[6] https://quoteinvestigator.com/2012/09/27/invent-the-future/
[7] https://en.wikipedia.org/wiki/Futures_studies
First, if all the initial conditions of the Universe at the highest granularity possible were known (10^1000? likely higher), then to calculate the current state of the Universe (10^17 seconds) at every step along the way, once per Planck time (10^44 per second), ... a calculation on the order of 10^1000^10^17^44 states (did I get that right?) No computer which exists IN our universe could hope to tackle such a calculation ABOUT said universe, in any reasonable amount of time. (Of course you could cheat like game developers do, but that's another conversation.)
Second, if a conscious entity is able to calculate and predict step N+1, they can choose to alter that step.
This kind of article and comment is fun to think about -- does it even make sense to ask the question in the first place?
One question arises for me: if no computer IN the universe can calculate this, how are these rules being applied?
I mean, something must be enforcing, there's no "magic" voodoo in the Universe, that's for sure.
So how can these rules keep being enforced, so that the outcomes follow the "predicted" path, if the universe can't possibly be able to "calculate" everything?
[Layman here] Is this ignoring quantum mechanics? Or is the claim that you can reconstruct the original wavefunction a collapse? I thought randomness was pretty intrinsic to quantum mechanics.
I’m a bit unsure though how widely and quickly the probability mass dissipates over possible future states. E.g. the existence of quantum computers implies that at least in some physical systems the probability mass stays concentrated on a few possible future states. I wouldn’t expect that to hold for the universe as a whole though.
[0] I specifically remember some podcast by Sean Carroll where he expressed this exact same view.
https://en.wikipedia.org/wiki/Laplace%27s_demon
That's dependent on which interpretation you subscribe to. Some are fully deterministic.
Oversimplifying, Physics is divided in "General Relativity" and "Everything Else".
Where "Everything Else" includes Quantum Mechanics, Special Relativity, Chemistry, particle Physics in the CERN, Superconductor, Semiconductors, Conductors, ..., and a simplified model of Gravity because people is interested in rocks that fall, airplanes, Astronomy, and other stuff.
And "General Relativity" includes another copy of Special Relativity and an accurate model of Gravity, that is only important if you are studding black holes, gravitational waves at LIGO, some parts of Astronomy, GPS and not too many more applications. This part of Physics ignore the Quantum Mechanics effects.
There are some results that mix both, like Hawking radiation around black holes. The idea is to find a more general theory that joins the two parts of Physics, but it's apparently a very difficult problem because a lot of smart people has tried to solve it and failed.
The research paper is about black holes, so it has a huge invisible disclaimer at the top that says:
fake quote> <huge><red>We are going to use General Relativity and we are going to pretend that nobody discovered Quantum Mechanics and the universe is 100% deterministic.</red></huge>
This disclaimer is actually not written because everybody reading the journal will assume that, so it's not necessary to explain that. The problem is that this simplification leaked to the press article, and the simplification was not well explained. So it looks like the author or journalist are not aware of Quantum Mechanics, but actually they are fully aware, it's just that they need this simplification to be able to do the calculations. It's a standard trick in the area.
Also the "100% deterministic" in may fake quote is a lie, because the problem is that in some parts inside black holes the equations of General Relativity have technical problems and have multiple solutions. The article is about this region and about how many horrible things would happen if you try to visit that part of a black hole. The main result is that in some cases it's not as horrible as expected and you may survive.
Here, randomness means that you cannot predict the outcome, because in the future there will be multiple copies of you, and whatever specific outcome you predicted, some of those versions will see a different outcome. But at the same time, this is fully deterministic, in the sense that there is an equation that describes exactly which futures will happen and which outcomes they will contain.
(There are also other intepretations, which add a new assumption on the top of this, such as "and one of those futures, selected randomly, is the real one, and then the remaining ones disappear". Which is kinda unfalsifiable, because yeah, it could be like this; but it also could be that all the different futures are equally valid, and each of them contains a copy of you saying "oh, so this one is the future that actually happened" -- but of course, different copies of you saying it about a different version of the future.)
EDIT: However, from the perspective of the "many-worlds hypothesis", everyone has an infinite number of futures... so the statement in the article doesn't make sense here either.
So, I am going to assume that the paper actually referred to something else, and the author of the article, trying to simplify it, turned it into something nonsensical.
The math of quantum mechanics itself is reversible. Every quantum operation is defined by a matrix, and it's possible to invert that matrix to run the operation backwards in time. The past uniquely determines the future.
"Collapse" interferes with that. It's not a quantum mechanical operator. It forms a bridge between quantum and classical operations by throwing away part of the wave function. It's able to do that because part of the wave function goes to (effectively) zero during decoherence.
With MWI you ignore that, and continue to treat the whole wave function. That follows the mathematical formalism, so it's more useful for thinking about unitarity, even though experimentally it's the same thing.
So the short answer is that the paper isn't ignoring quantum mechanics. For the purpose of this paper we're hand-waving away the question of what happens when the wave function becomes separable like that, because it's not important here. What's important is the conflict: even if you treat quantum mechanics as predictable (because it is, in that sense), you still run into problems with black holes.
That points in the direction of the problems with fusing relativity to quantum mechanics, which is an open question.
No (or only in very special configurations). See for example the n-body problem.[1]
[1] https://en.wikipedia.org/wiki/N-body_problem
Quantum mechanics, as several other commenters have observed, is where to look for the fundamental unpredictability.
I wonder how long it takes for the quantum uncertainty of a planet to become macroscopic due to the chaotic behaviour in N-body problems…
Non deterministic dynamics occurs in Newtonian physics as well. These things are worrying philosophically, but we can take some comfort in that the initial conditions for which these occur are "measure zero" i.e. if you were defining a probability measure over initial conditions, then the non-deterministic initial conditions would have measure zero. It doesn't sound like we have that option here though.
But in my view, this result is good news. We knew GR was unphysical anyhow. Now we've increased our understanding of how unphysical it is, potentially ruling out more replacement theories.
We did? That is news to me. Would you mind elaborating?
[1]https://en.wikipedia.org/wiki/Cosmic_censorship_hypothesis
Cowards...
"Some carefully constructed mathematical objects (similar to observed black holes) erase your past (makes it possible to cross the Cauchy horizon)" in GR would be more precise.
Here [0] is a good article about the kind of mathematical manipulation involved (not directly related). This [1] quora answer goes right to the obvious issues.
[0]https://www.quantamagazine.org/to-test-einsteins-equations-p...
[1]https://www.quora.com/Is-the-violation-of-strong-cosmic-cens...
I don't understand that. It sounds as if the reason all the stuff reaches the horizon "at the same time" is because time slows to a stop at the horizon. But doesn't that also apply to the "observer who gets that far"? How can the observer overtake all the other stuff?
Maybe I should stay clear of articles dealing with GR; I'm clearly not clever enough to understand them.