7 ms·

A Debate Over the Physics of Time

78 points by azuajef 10y ago

empath75 10y ago
Smolin's career seems to basically be entirely comprised of telling the only people who are creating successful theories that their theories can't be right, because he doesn't like the way they make him feel.
- dmreedy 10y ago
  A hunch can be a powerful thing; sometimes, it's the symptom of a nascent theory that hasn't hit the requisite critical model mass to attain that self-sustaining "eureka" moment in the brain yet. Of course, this validates truthers as well...
  - mark_edward 10y ago
    Or it's just nonsense like most hunches. I hope Smolin keeps on trucking but hunch based contrarianism isn't actually a good indicator of truth.
    - maxharris 10y ago
      Have you read Smolin's Time Reborn? I've just finished it, and while I don't agree with certain aspects of some of his ideas, I found it quite interesting and valuable.
- johncolanduoni 10y ago
  The parts of Smolin's career that are easily integrated into a pop science article are entirely comprised of that, but his actual work in physics that doesn't fit into a soundbite does not. He didn't get his position at the Perimeter Institute by talking about how theories make him feel. Just like pop physics tends to distort people's perception of the field as a whole (e.g. giving the impression that 90% of theoretical physics is string theory) it also makes researchers' body of work look very different than it is.
lisper 10y ago
http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...
whiddershins 10y ago
What if whatever force makes time asymmetrical also is at the root of volition and consciousness. So by definition we would only be able to experience time in one direction.
- dmreedy 10y ago
  Scott Aaronson touches on this idea a bit during a discourse with Roger Penrose[1], commenting on the unification of entropy and information: "So on the picture that this suggests, to be conscious, a physical entity would have to do more than carry out the right sorts of computations. It would have to, as it were, fully participate in the thermodynamic arrow of time: that is, repeatedly take microscopic degrees of freedom that have been unmeasured and unrecorded since the very early universe, and amplify them to macroscopic scale." --- [1] http://www.scottaaronson.com/blog/?p=2756 http://www.scottaaronson.com/blog/?p=2756
  - eli_gottlieb 10y ago
    Well, any kind of computation that erases information (for instance, taking two bits and using them to produce one bit in an AND gate) does have to increase entropy, so those two things are already one and the same.
neom 10y ago
I've been watching quantum for the last 5 years, let's just say the next 15 are going to be a fucking trip. ;) <3
- imglorp 10y ago
  It's been a trip since at least 1905 when we started looking into the photoelectric effect. But yeah, I can't wait until we get some more understanding of the current questions. That will reveal more questions, of course!
mathgenius 10y ago
"The laws that underlie these theories are time-symmetric". I don't get how people can say this. Quantum measurement (collapse of the statevector) is not at all time-symmetric. People seem to always leave this out in such discussions. They either view it is a temporary "problem" with the theory that we will one day fix, or just as something that is plainly not a theory. Or who knows what. The wikipedia article on interpretations of quantum physics has a giant table that illustrates the lack of consensus: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparison_of_interpretations https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...
- lostmsu 10y ago
  I am not sure what interpretations have to do with symmetry of equations.
  - marcosdumay 10y ago
    Measurement effects are not captured by any QM equation. The equations are all time symmetric, yes. But the theory is not.
    - mathgenius 10y ago
      Measurement effects are captured by projection operators (at least in standard QM). This is an equation and it is certainly not time-symmetric.
      - danbruc 10y ago
        But who seriously believes the collapse postulate is a fundamentally true law of physics? Unitary time evolution and the collapse postulate are incompatible, so at most one is fundamental and true. And over almost a century we have collected a gigantic amount of experimental data confirming that unitary time evolution is correct. You might argue that the same is true for the collapse postulate and the Born rule but I have to disagree. Yes, it gives the correct answer in as many experiments, but those are parts of the experiments that we don't really understand. We have classical system interact with quantum systems and our measurement devices are just such gigantic quantum systems that we can't treat them quantum mechanically. The collapse postulate is obviously true in a kind of classical limit, but it is really just an emergent law and nothing fundamental. If you could show that a wave function collapse can occurs in quantum system before we measure it, that would change the game. But that is not the case, the wave function collapse only occurs when we throw a gigantic classical system at the quantum system which we can not treat properly due to its complexity. So I argue people are rightly ignoring wave function collapse when they talk about time symmetry because it is just an emergent phenomenon. And there are several ideas that can explain why it looks like a wave function collapse occurs, so we are not simply throwing a corner stone of our theory out of the window.
kordless 10y ago
Upgrade the block universe to a blockchain universe and you should see where time's arrow comes from.
Houshalter 10y ago
I really like this theory: http://lesswrong.com/lw/fok/causal_universes/ http://lesswrong.com/lw/fok/causal_universes/ But regardless, what would it even mean for there to not be an arrow of time? Such a universe would have egg shells on the floor, and then waves of energy that just happen to hit the eggshells, at just the right velocity to propel each piece to assemble an unbroken egg. What a remarkable coincidence that would be. It would be very difficult, perhaps impossible, to make a universe like that. IMO it's just like cellular automata. You start with a "start state" of something simple or random. Perhaps just a single cell. And then it evolves into complex patterns as each time step causes the next time step. There are reversible cellular automata, that in theory don't have an arrow of time either. You could start at any state, and reverse it, just as easily as advance it. But unless you are remarkably clever, or lucky, at arranging the start state, it will tend to increase in entropy over time. Or rather, as the step counter increases.
- gizmo686 10y ago
  >what would it even mean for there to not be an arrow of time? Thermaldynamic soup? Our universe has an entropy based arrow of time because it started in a low entropy state. If a hypothetical universe started in a high entropy state, than forward time would appear no more probable than backward time. Of course, such a universe would also not have eggshells; it would just be a thermodynamical soup.
  - Houshalter 10y ago
    Well humans wouldn't exist in such a universe, and couldn't observe it. But even if you started with a completely random cellular automata, it's often common for order and structure to emerge. You could even say that entropy is going in reverse. But causation and the arrow of time still only flows one direction. Perhaps given enough time, life could evolve in such a universe, and ask why time only goes one direction.

78 points by azuajef 10y ago

Smolin's career seems to basically be entirely comprised of telling the only people who are creating successful theories that their theories can't be right, because he doesn't like the way they make him feel.

dmreedy 10y ago
A hunch can be a powerful thing; sometimes, it's the symptom of a nascent theory that hasn't hit the requisite critical model mass to attain that self-sustaining "eureka" moment in the brain yet. Of course, this validates truthers as well...
- mark_edward 10y ago
  Or it's just nonsense like most hunches. I hope Smolin keeps on trucking but hunch based contrarianism isn't actually a good indicator of truth.
  - maxharris 10y ago
    Have you read Smolin's Time Reborn? I've just finished it, and while I don't agree with certain aspects of some of his ideas, I found it quite interesting and valuable.
johncolanduoni 10y ago
The parts of Smolin's career that are easily integrated into a pop science article are entirely comprised of that, but his actual work in physics that doesn't fit into a soundbite does not. He didn't get his position at the Perimeter Institute by talking about how theories make him feel. Just like pop physics tends to distort people's perception of the field as a whole (e.g. giving the impression that 90% of theoretical physics is string theory) it also makes researchers' body of work look very different than it is.

lisper 10y ago

http://blog.rongarret.info/2014/10/parallel-universes-and-arrow-of-time.html http://blog.rongarret.info/2014/10/parallel-universes-and-ar...

whiddershins 10y ago

What if whatever force makes time asymmetrical also is at the root of volition and consciousness. So by definition we would only be able to experience time in one direction.

dmreedy 10y ago
Scott Aaronson touches on this idea a bit during a discourse with Roger Penrose[1], commenting on the unification of entropy and information: "So on the picture that this suggests, to be conscious, a physical entity would have to do more than carry out the right sorts of computations. It would have to, as it were, fully participate in the thermodynamic arrow of time: that is, repeatedly take microscopic degrees of freedom that have been unmeasured and unrecorded since the very early universe, and amplify them to macroscopic scale." --- [1] http://www.scottaaronson.com/blog/?p=2756 http://www.scottaaronson.com/blog/?p=2756
- eli_gottlieb 10y ago
  Well, any kind of computation that erases information (for instance, taking two bits and using them to produce one bit in an AND gate) does have to increase entropy, so those two things are already one and the same.

neom 10y ago

I've been watching quantum for the last 5 years, let's just say the next 15 are going to be a fucking trip. ;) <3

imglorp 10y ago
It's been a trip since at least 1905 when we started looking into the photoelectric effect. But yeah, I can't wait until we get some more understanding of the current questions. That will reveal more questions, of course!

mathgenius 10y ago

"The laws that underlie these theories are time-symmetric". I don't get how people can say this. Quantum measurement (collapse of the statevector) is not at all time-symmetric. People seem to always leave this out in such discussions. They either view it is a temporary "problem" with the theory that we will one day fix, or just as something that is plainly not a theory. Or who knows what. The wikipedia article on interpretations of quantum physics has a giant table that illustrates the lack of consensus: https://en.wikipedia.org/wiki/Interpretations_of_quantum_mechanics#Comparison_of_interpretations https://en.wikipedia.org/wiki/Interpretations_of_quantum_mec...

lostmsu 10y ago
I am not sure what interpretations have to do with symmetry of equations.
- marcosdumay 10y ago
  Measurement effects are not captured by any QM equation. The equations are all time symmetric, yes. But the theory is not.
  - mathgenius 10y ago
    Measurement effects are captured by projection operators (at least in standard QM). This is an equation and it is certainly not time-symmetric.
    - danbruc 10y ago
      But who seriously believes the collapse postulate is a fundamentally true law of physics? Unitary time evolution and the collapse postulate are incompatible, so at most one is fundamental and true. And over almost a century we have collected a gigantic amount of experimental data confirming that unitary time evolution is correct. You might argue that the same is true for the collapse postulate and the Born rule but I have to disagree. Yes, it gives the correct answer in as many experiments, but those are parts of the experiments that we don't really understand. We have classical system interact with quantum systems and our measurement devices are just such gigantic quantum systems that we can't treat them quantum mechanically. The collapse postulate is obviously true in a kind of classical limit, but it is really just an emergent law and nothing fundamental. If you could show that a wave function collapse can occurs in quantum system before we measure it, that would change the game. But that is not the case, the wave function collapse only occurs when we throw a gigantic classical system at the quantum system which we can not treat properly due to its complexity. So I argue people are rightly ignoring wave function collapse when they talk about time symmetry because it is just an emergent phenomenon. And there are several ideas that can explain why it looks like a wave function collapse occurs, so we are not simply throwing a corner stone of our theory out of the window.

kordless 10y ago

Upgrade the block universe to a blockchain universe and you should see where time's arrow comes from.

Houshalter 10y ago

I really like this theory: http://lesswrong.com/lw/fok/causal_universes/ http://lesswrong.com/lw/fok/causal_universes/ But regardless, what would it even mean for there to not be an arrow of time? Such a universe would have egg shells on the floor, and then waves of energy that just happen to hit the eggshells, at just the right velocity to propel each piece to assemble an unbroken egg. What a remarkable coincidence that would be. It would be very difficult, perhaps impossible, to make a universe like that. IMO it's just like cellular automata. You start with a "start state" of something simple or random. Perhaps just a single cell. And then it evolves into complex patterns as each time step causes the next time step. There are reversible cellular automata, that in theory don't have an arrow of time either. You could start at any state, and reverse it, just as easily as advance it. But unless you are remarkably clever, or lucky, at arranging the start state, it will tend to increase in entropy over time. Or rather, as the step counter increases.

gizmo686 10y ago
>what would it even mean for there to not be an arrow of time? Thermaldynamic soup? Our universe has an entropy based arrow of time because it started in a low entropy state. If a hypothetical universe started in a high entropy state, than forward time would appear no more probable than backward time. Of course, such a universe would also not have eggshells; it would just be a thermodynamical soup.
- Houshalter 10y ago
  Well humans wouldn't exist in such a universe, and couldn't observe it. But even if you started with a completely random cellular automata, it's often common for order and structure to emerge. You could even say that entropy is going in reverse. But causation and the arrow of time still only flows one direction. Perhaps given enough time, life could evolve in such a universe, and ask why time only goes one direction.