7 ms·

Physics laws cannot always turn back time

66 points by vo2maxer 7y ago

sarah180 7y ago
This headline is (unsurprisingly) misleading. The argument is actually "the rounding error of this calculation is smaller than the Planck length, and we still get rounding errors that compound to large amounts over time." This is really an argument about the ability to measure and simulate, not about which of the laws of physics are invariant under time reversal.
- empath75 7y ago
  I don't think it's any more about the ability to measure and simulate than anything else related to quantum mechanics is. If you attempt to observe anything at that scale, you will create a black hole. Without a theory of quantum gravity, it's sort of nonsensical to talk about what happens at that scale of space-time.
  - djaque 7y ago
    All of this is carried out in a classical framework though. You may be right that you need quantum at those scales, but that's not what this article is about. It's just talking about our ability to calculate the trajectories of classical bodies with precision. That, and chaos.
- djaque 7y ago
  OK, thanks. The article made it sound like they found a way to show time reversal symmetry breaking and I was waiting for them to drop whatever assumption they used to get there. The underlying theory is symmetric and so it only makes sense that it's just a limitation on numerics.
  - mcguire 7y ago
    "...a limitation on numerics." My understanding is that measuring quantities smaller than the Planck limits is problematic. (IIRC, the energy of a photon needed to make the measurement is large enough to create a black hole.) As a result, it's not just "numerics"; the theory may be symmetric, but the physical system cannot be observed to be.
- crimsonalucard 7y ago
  What does it mean that time can never be observed or measured to be reversible? Does the above mean the exact same thing as saying that the laws of physics are not time invariant period?
- nabla9 7y ago
  The content of the matches the headline. What they do is to increase the accuracy so much that physical perturbations at the quantum level dominate, not numerical accuracy. Irreversible up to the Planck length means fundamentally unpredictable. https://arxiv.org/abs/2002.04029 https://arxiv.org/abs/2002.04029
  - the8472 7y ago
    Would that still hold if the universe operated analytically?
    - nabla9 7y ago
      No. Perturbations would still be chaotic, but the limit would be only the exponential need for accuracy. You would also have to be able to measure the starting point exactly.
  - im3w1l 7y ago
    That made me think of a really strong argument for many-worlds. Copenhagen's collapse notion is not time-reversible.
  - Florin_Andrei 7y ago
    That means the arrow of time is essentially quantum in nature. (I'm not talking about statistical sources such as large particle collectives.)
  - carapace 7y ago
    > The time symmetry is broken because disturbances the size of the Planck length have an exponential effect. Yeah, that's the point right there up front: Butterfly effect at the sub-Planck scale.
    - perl4ever 7y ago
      I guess I'm too dumb to understand what is novel about this? What does it tell me beyond James Gleick's popularization of chaos theory circa 1987? In some other thread, people told me I was wrong about reality having finite resolution. So is there a "sub-Planck scale"? How can simulations tell you if there is or isn't?
- oarabbus_ 7y ago
  > not about which of the laws of physics are invariant under time reversal. If I recall from Brief History of Time, it's the combination of charge, parity, and time all together which are invariant. Some google searching also yields that time crystals were shown to disprove time symmetry/invariance.
- tyre 7y ago
  with regards to compounding rounding, is this not why we have significant digits? I haven't done physics in ten years so my memory is hazy on them, but I remember them being some control on what level of precision we can remain confident with. (Side note: significant digits were a royal PITA)
  - pmiller2 7y ago
    Yes, but certain calculations are inherently numerically unstable, meaning that rounding errors can quickly dominate the calculation.
    - tyre 7y ago
      what does "numerically unstable" mean in this context?
      - lonelappde 7y ago
        that rounding errors can quickly dominate the calculation.
  - zentiggr 7y ago
    If the number of significant digits required to maintain precision takes you past Planck length, you've reached an absolute physical limitation on your measurement ability. If the process being measured is dependent on changes of sub-Planck length magnitude, you have an inherently chaotic system, and that makes it irreversible in practice.
    - tyre 7y ago
      wow that's crazy, thank you for the explanation
- deleted 7y ago
  [deleted]
phkahler 7y ago
The guys should dig up an old copy of "Chaos" and read it. Their simulations exhibit sensitive dependence on initial conditions. The predicted orbits will diverge similarly in the forward-time direction if tiny perturbations are made to the initial conditions. They're also subject to rounding errors. Nothing to see here about time reversal, its just chaos.
- plafl 7y ago
  Your comment sounds quite disrespectful. I would say that they are perfectly aware of what chaotic behavior is.
  - phkahler 7y ago
    As others have pointed out, I was basing that not on the paper, but the summary linked here. It sounds like some new fundamental physics insight about time reversibility, when in fact it's just the old chaos stuff. Given that impression, it deserved a little snark and should not even be here. But apparently that's actually a lot of what it's about, in which case the authors are well aware - but it's still not very interesting then. The Lorenz attractor is is simpler than a 3-body system and will exhibit similar divergence.
- tom-thistime 7y ago
  The technical content of your comment looks correct, but "Chaos" is the first word in the abstract of the paper: https://academic.oup.com/mnras/article-abstract/493/3/3932/5736044?redirectedFrom=fulltext https://academic.oup.com/mnras/article-abstract/493/3/3932/5... The blurb that's linked to HN is not very lucid, but the scientists who wrote the paper make it very clear that they're just talking about deterministic chaos.
- sarah180 7y ago
  This is all correct—but you should always look at the actual paper and not the summary article that some journalist wrote. The authors are pretty clear about what they've shown and how it fits into ideas about chaos.
gus_massa 7y ago
Wen you use this level of precision, you need to consider the quantum effects. And [the current version] of Quantum Mechanics is time reversible. The problem is that they mix some Classical Mechanics that is time reversible and Quantum Mechanics that is time reversible, and the wrong mix is not time reversible. Autonitpicking: There are some technical details about time reversal, but as far as we know the CPT symmetry is not broken.
- crimsonalucard 7y ago
  I'm no physicist but shouldn't QM be not time reversible or even not repeatable? Based purely on the fact that events happen randomly within some constraint then if you go forward, backward or repeat nothing can ever be the same even when initial conditions are exactly identical?
  - akimball 7y ago
    The evolution of the wave function which determines the probability distribution sampled by observations is deterministic and reversible in all of the quantum theories I have personally studied (which is not many).
    - crimsonalucard 7y ago
      I'm talking about the actual materialization of the particle. Not the probability wave itself. Dunno if it's the right term "materialization" but basically I'm saying when it's no longer a probability wave but it becomes a particle it appears at random places within the probability wave. The location of the particle after the materialization of the particle itself will be different forward or backwards through time? Does someone know?
      - gus_massa 7y ago
        Wavefunction collapse is a huge problem, and taken at face value make QM not reversible. Nobody is sure, but probably when you consider a bigger system that includes the experiment and the observer, the time reversion is restored. https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence (Note that the observer in QM doesn't need magical properties like consciousness, in some experiments it is enough to place a brick in the right position.)
scythe 7y ago
The quantum mechanics of black holes is still open, isn't it? As I recall even Hawking entropy depends on putting the (non-unitary) Second Law of Thermodynamics in as a prior. This turns out to be consistent with (unitary) string theory and LQG, so far at least.
rland 7y ago
I have a question for those more knowledgeable about physics: Everything we know so far points to time invariance * , right? What would happen if we discovered that some things truly aren't time invariant? Would it crucially change the fundamental understanding, or would it just be a new class of phenomena? * except maybe TFA which isn't really right based on the other comments I'm reading.
- lidHanteyk 7y ago
  It would substantially reject a lot of assumptions that we normally not just take for granted, but require as part of the restrictive structure that shapes our models. In particular, time invariance is linked via Noether's Theorem to conservation of energy, and thus via Einstein's work to conservation of matter. Indeed, the modern version of time invariance, CPT symmetry, is as weak as it can be while still maintaining this connection. If CPT symmetry were to fail, then we would need to go find new mathematics and new mechanics. For this reason, we test it constantly; see [0] for an overview. [0] https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi...
- Enginerrrd 7y ago
  Not at all a physicist, but I think I can take a stab at this. I think so far we only see CPT (charge, parity & time reversal) symmetry as being a fully unbroken symmetry of nature within the context of what you're asking. (I believe the weak force breaks a simple time reversal symmetry.) Time symmetry is typically synonymous with energy conservation in classical mechanics. It get's more complicated when you introduce quantum mechanics, and lorentz invariance from relativity. In essence though, there's probably an analogous invariant to energy conservation for CPT symmetry. Maybe this is just lorentz invariance for a quantum field theroy? Not quite sure. Implications of seeing that symmetry broken would be huge since the CPT theorem implies that a quantum field theory obeying lorentz invariance obeys a CPT symmetry. (I'm guessing by an application of Noether's theorem?) SO a violation of that would be either a violation of lorentz invariance (which would be VERY interesting) or some type of violation of the assumptions of quantum field theory. In essence, it would mean new physics!
- 7y ago
tom-thistime 7y ago
As usual with physics links, the layperson's blurb sounds kind of silly and is easy to criticize. But the actual paper is linked and you can look at it: https://academic.oup.com/mnras/article-abstract/493/3/3932/5736044?redirectedFrom=fulltext https://academic.oup.com/mnras/article-abstract/493/3/3932/5... General HN comment: I think our discussion on physics topics will really be improved if we comment on the physics papers (start with the abstracts) themselves. The (completely understandable) trend right now is to focus on the press release or science journalism that accompanies the link to the physics. Unfortunately that normally leads away from the basics of what's actually happening in the physics. (Metaphor here: If you're a programmer, imagine reading about some CS topic in Wired magazine, then looking at 200 comments from people who agree or disagree with the pop sci writeup.)
- moron4hire 7y ago
  I don't have to imagine. That's still what HN does.
- Florin_Andrei 7y ago
  I'm a Physics major and I thought the article was a perfectly fine summary in plain language. But I agree, it would be nice to have the primary source handy, right here on this page.
- leetcrew 7y ago
  New garbage collection algorithm makes manual memory management obsolete! Kernel devs hate it!
  - diegoperini 7y ago
    You deserve reddit gold!
    - leetcrew 7y ago
      thanks, I really appreciate the kind words!
  - 7y ago
codesnik 7y ago
well, system of black holes would lose some energy via gravity waves, this isn't something that could be reversed, yes?
- Florin_Andrei 7y ago
  In a simulation you would simply flip a sign on that one and it should go back. I've done plenty of simulations myself where waves radiate from a source, and those are perfectly reversible. The article is pointing at a more subtle issue related to a fundamental small-scale fuzziness which affects chaotic systems and breaks reversibility.
  - chrisshroba 7y ago
    Probably a naive question, but why is this a subtle issue? Isn't this basically the well-known butterfly effect?
    - Florin_Andrei 7y ago
      In a nutshell, yes, it kinda is a sort of butterfly effect. But look, you go to school and they drill into you until the cows come home that quantum mechanics is time-reversible, and gravity is time-reversible, and you just assume this is always true. But turns out, all that's needed is a bit of small-scale fuzziness due to QM effects, and then particle ensembles of 3 or more are suddenly not time-reversible anymore, though each of the basic laws still are. For someone like me this was a small shock.
      - perl4ever 7y ago
        Ok, but if you have the idea that "Planck scale" means something, then how can it be a shock then? Often understanding something better makes it less obvious, but at my level, I don't understand what's non-obvious about this.
zelphirkalt 7y ago
Perhaps this is a philosophical question in the end again. There are open questions: Why would their accuracy be sufficient? Is there a proof of it being sufficient to make such a claim? Then we have: Is their simulation correct? Can we even make a correct simulation inside a computer, about 3 black holes and in addition to that, make general statements about laws of physics, based on the results of that simulation? Perhaps to make a perfect simulation, to be really sure, we would need an apparatus of as much matter as fell into those black holes. So I remain sceptical.
canjobear 7y ago
This only shows that time evolution on the observer‘s branch of the wave function is not reversible. Considering the whole wave function, the time evolution is still unitary.
- danbruc 7y ago
  This was not a quantum mechanical simulation, the wave function is not relevant for this.
deleted 7y ago
[deleted]
SeanFerree 7y ago
Awesome article!
jng 7y ago
Isn't this paper more about simulation techniques than about reality itself? It looks like time reversibiliy (sub-planck length or otherwise) should be studied in closed form or at least in non-lossy mathematical ways, not simulated with imprecise digital computers. Montecarlo simulation to calculate pi is just not a good way to know whether the digits of pi ever repeat or they don't.
fallingfrog 7y ago
Mm, no I think what this shows that in just the same way as the future is uncertain (you don’t know which path you will observe an electron to take), the past is uncertain in the same way (you can’t know which path an electron took). They’ve just expanded that principle to a cosmic size (there are several past histories, diverging on the Planck scale, that lead to the same present day observations). That doesn’t mean physics is not reversible, it just means there isn’t a unique way to reverse it. Which, we already knew.
saadalem 7y ago
To travel through time, you’ll have to apply the laws of special relativity. Time Travel Is Possible, But We’ll Need Lots Of Negative Energy

66 points by vo2maxer 7y ago

This headline is (unsurprisingly) misleading. The argument is actually "the rounding error of this calculation is smaller than the Planck length, and we still get rounding errors that compound to large amounts over time." This is really an argument about the ability to measure and simulate, not about which of the laws of physics are invariant under time reversal.

empath75 7y ago
I don't think it's any more about the ability to measure and simulate than anything else related to quantum mechanics is. If you attempt to observe anything at that scale, you will create a black hole. Without a theory of quantum gravity, it's sort of nonsensical to talk about what happens at that scale of space-time.
- djaque 7y ago
  All of this is carried out in a classical framework though. You may be right that you need quantum at those scales, but that's not what this article is about. It's just talking about our ability to calculate the trajectories of classical bodies with precision. That, and chaos.
djaque 7y ago
OK, thanks. The article made it sound like they found a way to show time reversal symmetry breaking and I was waiting for them to drop whatever assumption they used to get there. The underlying theory is symmetric and so it only makes sense that it's just a limitation on numerics.
- mcguire 7y ago
  "...a limitation on numerics." My understanding is that measuring quantities smaller than the Planck limits is problematic. (IIRC, the energy of a photon needed to make the measurement is large enough to create a black hole.) As a result, it's not just "numerics"; the theory may be symmetric, but the physical system cannot be observed to be.
crimsonalucard 7y ago
What does it mean that time can never be observed or measured to be reversible? Does the above mean the exact same thing as saying that the laws of physics are not time invariant period?
nabla9 7y ago
The content of the matches the headline. What they do is to increase the accuracy so much that physical perturbations at the quantum level dominate, not numerical accuracy. Irreversible up to the Planck length means fundamentally unpredictable. https://arxiv.org/abs/2002.04029 https://arxiv.org/abs/2002.04029
- the8472 7y ago
  Would that still hold if the universe operated analytically?
  - nabla9 7y ago
    No. Perturbations would still be chaotic, but the limit would be only the exponential need for accuracy. You would also have to be able to measure the starting point exactly.
- im3w1l 7y ago
  That made me think of a really strong argument for many-worlds. Copenhagen's collapse notion is not time-reversible.
- Florin_Andrei 7y ago
  That means the arrow of time is essentially quantum in nature. (I'm not talking about statistical sources such as large particle collectives.)
- carapace 7y ago
  > The time symmetry is broken because disturbances the size of the Planck length have an exponential effect. Yeah, that's the point right there up front: Butterfly effect at the sub-Planck scale.
  - perl4ever 7y ago
    I guess I'm too dumb to understand what is novel about this? What does it tell me beyond James Gleick's popularization of chaos theory circa 1987? In some other thread, people told me I was wrong about reality having finite resolution. So is there a "sub-Planck scale"? How can simulations tell you if there is or isn't?
oarabbus_ 7y ago
> not about which of the laws of physics are invariant under time reversal. If I recall from Brief History of Time, it's the combination of charge, parity, and time all together which are invariant. Some google searching also yields that time crystals were shown to disprove time symmetry/invariance.
tyre 7y ago
with regards to compounding rounding, is this not why we have significant digits? I haven't done physics in ten years so my memory is hazy on them, but I remember them being some control on what level of precision we can remain confident with. (Side note: significant digits were a royal PITA)
- pmiller2 7y ago
  Yes, but certain calculations are inherently numerically unstable, meaning that rounding errors can quickly dominate the calculation.
  - tyre 7y ago
    what does "numerically unstable" mean in this context?
    - lonelappde 7y ago
      that rounding errors can quickly dominate the calculation.
- zentiggr 7y ago
  If the number of significant digits required to maintain precision takes you past Planck length, you've reached an absolute physical limitation on your measurement ability. If the process being measured is dependent on changes of sub-Planck length magnitude, you have an inherently chaotic system, and that makes it irreversible in practice.
  - tyre 7y ago
    wow that's crazy, thank you for the explanation
deleted 7y ago
[deleted]

phkahler 7y ago

The guys should dig up an old copy of "Chaos" and read it. Their simulations exhibit sensitive dependence on initial conditions. The predicted orbits will diverge similarly in the forward-time direction if tiny perturbations are made to the initial conditions. They're also subject to rounding errors. Nothing to see here about time reversal, its just chaos.

plafl 7y ago
Your comment sounds quite disrespectful. I would say that they are perfectly aware of what chaotic behavior is.
- phkahler 7y ago
  As others have pointed out, I was basing that not on the paper, but the summary linked here. It sounds like some new fundamental physics insight about time reversibility, when in fact it's just the old chaos stuff. Given that impression, it deserved a little snark and should not even be here. But apparently that's actually a lot of what it's about, in which case the authors are well aware - but it's still not very interesting then. The Lorenz attractor is is simpler than a 3-body system and will exhibit similar divergence.
tom-thistime 7y ago
The technical content of your comment looks correct, but "Chaos" is the first word in the abstract of the paper: https://academic.oup.com/mnras/article-abstract/493/3/3932/5736044?redirectedFrom=fulltext https://academic.oup.com/mnras/article-abstract/493/3/3932/5... The blurb that's linked to HN is not very lucid, but the scientists who wrote the paper make it very clear that they're just talking about deterministic chaos.
sarah180 7y ago
This is all correct—but you should always look at the actual paper and not the summary article that some journalist wrote. The authors are pretty clear about what they've shown and how it fits into ideas about chaos.

gus_massa 7y ago

Wen you use this level of precision, you need to consider the quantum effects. And [the current version] of Quantum Mechanics is time reversible. The problem is that they mix some Classical Mechanics that is time reversible and Quantum Mechanics that is time reversible, and the wrong mix is not time reversible. Autonitpicking: There are some technical details about time reversal, but as far as we know the CPT symmetry is not broken.

crimsonalucard 7y ago
I'm no physicist but shouldn't QM be not time reversible or even not repeatable? Based purely on the fact that events happen randomly within some constraint then if you go forward, backward or repeat nothing can ever be the same even when initial conditions are exactly identical?
- akimball 7y ago
  The evolution of the wave function which determines the probability distribution sampled by observations is deterministic and reversible in all of the quantum theories I have personally studied (which is not many).
  - crimsonalucard 7y ago
    I'm talking about the actual materialization of the particle. Not the probability wave itself. Dunno if it's the right term "materialization" but basically I'm saying when it's no longer a probability wave but it becomes a particle it appears at random places within the probability wave. The location of the particle after the materialization of the particle itself will be different forward or backwards through time? Does someone know?
    - gus_massa 7y ago
      Wavefunction collapse is a huge problem, and taken at face value make QM not reversible. Nobody is sure, but probably when you consider a bigger system that includes the experiment and the observer, the time reversion is restored. https://en.wikipedia.org/wiki/Quantum_decoherence https://en.wikipedia.org/wiki/Quantum_decoherence (Note that the observer in QM doesn't need magical properties like consciousness, in some experiments it is enough to place a brick in the right position.)

scythe 7y ago

The quantum mechanics of black holes is still open, isn't it? As I recall even Hawking entropy depends on putting the (non-unitary) Second Law of Thermodynamics in as a prior. This turns out to be consistent with (unitary) string theory and LQG, so far at least.

rland 7y ago

I have a question for those more knowledgeable about physics: Everything we know so far points to time invariance * , right? What would happen if we discovered that some things truly aren't time invariant? Would it crucially change the fundamental understanding, or would it just be a new class of phenomena? * except maybe TFA which isn't really right based on the other comments I'm reading.

lidHanteyk 7y ago
It would substantially reject a lot of assumptions that we normally not just take for granted, but require as part of the restrictive structure that shapes our models. In particular, time invariance is linked via Noether's Theorem to conservation of energy, and thus via Einstein's work to conservation of matter. Indeed, the modern version of time invariance, CPT symmetry, is as weak as it can be while still maintaining this connection. If CPT symmetry were to fail, then we would need to go find new mathematics and new mechanics. For this reason, we test it constantly; see [0] for an overview. [0] https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_violation https://en.wikipedia.org/wiki/Modern_searches_for_Lorentz_vi...
Enginerrrd 7y ago
Not at all a physicist, but I think I can take a stab at this. I think so far we only see CPT (charge, parity & time reversal) symmetry as being a fully unbroken symmetry of nature within the context of what you're asking. (I believe the weak force breaks a simple time reversal symmetry.) Time symmetry is typically synonymous with energy conservation in classical mechanics. It get's more complicated when you introduce quantum mechanics, and lorentz invariance from relativity. In essence though, there's probably an analogous invariant to energy conservation for CPT symmetry. Maybe this is just lorentz invariance for a quantum field theroy? Not quite sure. Implications of seeing that symmetry broken would be huge since the CPT theorem implies that a quantum field theory obeying lorentz invariance obeys a CPT symmetry. (I'm guessing by an application of Noether's theorem?) SO a violation of that would be either a violation of lorentz invariance (which would be VERY interesting) or some type of violation of the assumptions of quantum field theory. In essence, it would mean new physics!
7y ago

tom-thistime 7y ago

As usual with physics links, the layperson's blurb sounds kind of silly and is easy to criticize. But the actual paper is linked and you can look at it: https://academic.oup.com/mnras/article-abstract/493/3/3932/5736044?redirectedFrom=fulltext https://academic.oup.com/mnras/article-abstract/493/3/3932/5... General HN comment: I think our discussion on physics topics will really be improved if we comment on the physics papers (start with the abstracts) themselves. The (completely understandable) trend right now is to focus on the press release or science journalism that accompanies the link to the physics. Unfortunately that normally leads away from the basics of what's actually happening in the physics. (Metaphor here: If you're a programmer, imagine reading about some CS topic in Wired magazine, then looking at 200 comments from people who agree or disagree with the pop sci writeup.)

moron4hire 7y ago
I don't have to imagine. That's still what HN does.
Florin_Andrei 7y ago
I'm a Physics major and I thought the article was a perfectly fine summary in plain language. But I agree, it would be nice to have the primary source handy, right here on this page.
leetcrew 7y ago
New garbage collection algorithm makes manual memory management obsolete! Kernel devs hate it!
- diegoperini 7y ago
  You deserve reddit gold!
  - leetcrew 7y ago
    thanks, I really appreciate the kind words!
- 7y ago

codesnik 7y ago

well, system of black holes would lose some energy via gravity waves, this isn't something that could be reversed, yes?

Florin_Andrei 7y ago
In a simulation you would simply flip a sign on that one and it should go back. I've done plenty of simulations myself where waves radiate from a source, and those are perfectly reversible. The article is pointing at a more subtle issue related to a fundamental small-scale fuzziness which affects chaotic systems and breaks reversibility.
- chrisshroba 7y ago
  Probably a naive question, but why is this a subtle issue? Isn't this basically the well-known butterfly effect?
  - Florin_Andrei 7y ago
    In a nutshell, yes, it kinda is a sort of butterfly effect. But look, you go to school and they drill into you until the cows come home that quantum mechanics is time-reversible, and gravity is time-reversible, and you just assume this is always true. But turns out, all that's needed is a bit of small-scale fuzziness due to QM effects, and then particle ensembles of 3 or more are suddenly not time-reversible anymore, though each of the basic laws still are. For someone like me this was a small shock.
    - perl4ever 7y ago
      Ok, but if you have the idea that "Planck scale" means something, then how can it be a shock then? Often understanding something better makes it less obvious, but at my level, I don't understand what's non-obvious about this.

zelphirkalt 7y ago

Perhaps this is a philosophical question in the end again. There are open questions: Why would their accuracy be sufficient? Is there a proof of it being sufficient to make such a claim? Then we have: Is their simulation correct? Can we even make a correct simulation inside a computer, about 3 black holes and in addition to that, make general statements about laws of physics, based on the results of that simulation? Perhaps to make a perfect simulation, to be really sure, we would need an apparatus of as much matter as fell into those black holes. So I remain sceptical.

canjobear 7y ago

This only shows that time evolution on the observer‘s branch of the wave function is not reversible. Considering the whole wave function, the time evolution is still unitary.

danbruc 7y ago
This was not a quantum mechanical simulation, the wave function is not relevant for this.

deleted 7y ago

[deleted]

SeanFerree 7y ago

Awesome article!

jng 7y ago

Isn't this paper more about simulation techniques than about reality itself? It looks like time reversibiliy (sub-planck length or otherwise) should be studied in closed form or at least in non-lossy mathematical ways, not simulated with imprecise digital computers. Montecarlo simulation to calculate pi is just not a good way to know whether the digits of pi ever repeat or they don't.

fallingfrog 7y ago

Mm, no I think what this shows that in just the same way as the future is uncertain (you don’t know which path you will observe an electron to take), the past is uncertain in the same way (you can’t know which path an electron took). They’ve just expanded that principle to a cosmic size (there are several past histories, diverging on the Planck scale, that lead to the same present day observations). That doesn’t mean physics is not reversible, it just means there isn’t a unique way to reverse it. Which, we already knew.

saadalem 7y ago

To travel through time, you’ll have to apply the laws of special relativity. Time Travel Is Possible, But We’ll Need Lots Of Negative Energy