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>But the laws of physics are not only symmetrical in space but also in time. Can someone explain what this means? It makes sense that physical laws are symmetr

by klunger 10y ago

>But the laws of physics are not only symmetrical in space but also in time. Can someone explain what this means? It makes sense that physical laws are symmetrical in space. But in time? I thought that would not be the case because of entropy/irreversible processes etc. What am I missing? The whole article rests on this premise.

jbmorgado 10y ago
Thermodynamics (from where you get entropy) are a statistical description of physical laws taking process over many particles. When you look at each particle in the thermodynamic system, you can easily see there is indeed simmetry in time.
proxy176590 10y ago
Noethers' theorem: https://en.m.wikipedia.org/wiki/Noether%27s_theorem https://en.m.wikipedia.org/wiki/Noether%27s_theorem Groundbreaking stuff. But it is not something you would want to go over in a weekend.
cyphar 10y ago
"The arrow of time" which you're referring to (entropy increasing in closed systems) is something that is IMO still puzzling in the field of physics. To be clear, entropy isn't a "force" its just a statistical effect (a very large number of particles in a system are always going to be more likely to be in a macrostate that has the most microstates -- meaning that the system will tend towards more "disordered" states). However, it should be noted that the arrow of time only applies to the macroscopic universe. All of the laws of physics (in this article's case it's quantum mechanics) are time symmetric (if you play the experiment in reverse you couldn't tell it was being done in reverse -- which is what you can see in Feynman diagrams). So at the heart of it, the universe is very symmetric in time. To quote Primer: "What's the one variable in all of the equations of heat and matter? What's the one variable that you can turn negative and still get rational answers? Its not energy, it's not mass..." [It's time].
JonnieCache 10y ago
Here's a link to the videos from FXQi's Setting Time Aright conference, where there are lots of talks addressing the relationships between cosmology, QM and entropy with regards to time, with David Albert, Sean Carroll, Max Tegmark, Tim Maudlin, etc etc. https://www.youtube.com/watch?v=QR9reQrumS0&list=PL1FA30AE298BA990D&index=27 https://www.youtube.com/watch?v=QR9reQrumS0&list=PL1FA30AE29... Spolier alert: they don't really know whats going on, and they disagree pretty strongly. Decoherence seems related to the thermodynamic arrow of time: you can't un-measure something, but famously we don't know how to interpret that, many worlds blah blah.

by klunger 10y ago

jbmorgado 10y ago

Thermodynamics (from where you get entropy) are a statistical description of physical laws taking process over many particles. When you look at each particle in the thermodynamic system, you can easily see there is indeed simmetry in time.

proxy176590 10y ago

Noethers' theorem: https://en.m.wikipedia.org/wiki/Noether%27s_theorem https://en.m.wikipedia.org/wiki/Noether%27s_theorem Groundbreaking stuff. But it is not something you would want to go over in a weekend.

cyphar 10y ago

"The arrow of time" which you're referring to (entropy increasing in closed systems) is something that is IMO still puzzling in the field of physics. To be clear, entropy isn't a "force" its just a statistical effect (a very large number of particles in a system are always going to be more likely to be in a macrostate that has the most microstates -- meaning that the system will tend towards more "disordered" states). However, it should be noted that the arrow of time only applies to the macroscopic universe. All of the laws of physics (in this article's case it's quantum mechanics) are time symmetric (if you play the experiment in reverse you couldn't tell it was being done in reverse -- which is what you can see in Feynman diagrams). So at the heart of it, the universe is very symmetric in time. To quote Primer: "What's the one variable in all of the equations of heat and matter? What's the one variable that you can turn negative and still get rational answers? Its not energy, it's not mass..." [It's time].

JonnieCache 10y ago

Here's a link to the videos from FXQi's Setting Time Aright conference, where there are lots of talks addressing the relationships between cosmology, QM and entropy with regards to time, with David Albert, Sean Carroll, Max Tegmark, Tim Maudlin, etc etc. https://www.youtube.com/watch?v=QR9reQrumS0&list=PL1FA30AE298BA990D&index=27 https://www.youtube.com/watch?v=QR9reQrumS0&list=PL1FA30AE29... Spolier alert: they don't really know whats going on, and they disagree pretty strongly. Decoherence seems related to the thermodynamic arrow of time: you can't un-measure something, but famously we don't know how to interpret that, many worlds blah blah.