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> 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

> 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.

totetsu 5y ago
Is this refering to simulations to recreate The Large Scale Structure, by specifying initial conditions?
bjornsing 5y ago
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.
deleted 5y ago
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byteware 5y ago
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
- codethief 5y ago
  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.
tempodox 5y ago
Isn't that quote just Laplace's demon in other words? That pipe dream is over 200 years old. https://en.wikipedia.org/wiki/Laplace%27s_demon https://en.wikipedia.org/wiki/Laplace%27s_demon
optimalsolver 5y ago
>I thought randomness was pretty intrinsic to quantum mechanics That's dependent on which interpretation you subscribe to. Some are fully deterministic.
gus_massa 5y ago
Yes, this is ignoring Quantum Mechanics. 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.
- deleted 5y ago
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amluto 5y ago
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.
Viliam1234 5y ago
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.
jfengel 5y ago
It's complicated. 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.

totetsu 5y ago

Is this refering to simulations to recreate The Large Scale Structure, by specifying initial conditions?

bjornsing 5y ago

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.

deleted 5y ago

[deleted]

byteware 5y ago

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

codethief 5y ago
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.

tempodox 5y ago

Isn't that quote just Laplace's demon in other words? That pipe dream is over 200 years old. https://en.wikipedia.org/wiki/Laplace%27s_demon https://en.wikipedia.org/wiki/Laplace%27s_demon

optimalsolver 5y ago

>I thought randomness was pretty intrinsic to quantum mechanics That's dependent on which interpretation you subscribe to. Some are fully deterministic.

gus_massa 5y ago

Yes, this is ignoring Quantum Mechanics. 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.

deleted 5y ago
[deleted]

amluto 5y ago

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.

Viliam1234 5y ago

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.

jfengel 5y ago

It's complicated. 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.