4 ms·

This headline is (unsurprisingly) misleading. The argument is actually "the rounding error of this calculation is smaller than the Planck length, and we still g

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
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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

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