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Einstein, Symmetry and the Future of Physics

77 points by theafh 7y ago

followthegoose 7y ago
Quanta is absolutely terrible at illustrating what it's talking about, and why. Borderline clickbait titles, fluffy meandering prose that often completely fails to tell you anything of substance. At first I thought "I'm too dumb for this", but I'm no longer sure who the intended audience even is.
DiogenesKynikos 7y ago
"Noether showed that the symmetries of general relativity — its invariance under transformations between different reference frames — ensure that energy is always conserved." This isn't quite right. Energy isn't conserved in General Relativity. There is a conservation law involving the stress-energy tensor, but as the name implies, it's not energy alone that's being conserved. But more generally, Noether's theorem, which leads to this conservation law, is probably the most beautiful theorem in physics: https://en.wikipedia.org/wiki/Noether%27s_theorem https://en.wikipedia.org/wiki/Noether%27s_theorem. Given that, what's amazing is that Emmy Noether only worked on physics for a relatively short time. She was asked by David Hilbert to work on the problem of invariants in General Relativity, came up with one of the most important theorems in physics, and then moved on to other (extremely important) mathematical work.
- mr_mitm 7y ago
  > This isn't quite right. Energy isn't conserved in General Relativity. Correct, and the reason is that in GR there is (in general) no time translation invariance. The associated conserved quantity to time translation invariance is precisely what we call "energy". In our universe this manifests as the expansion of spacetime. More on this: https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
- Whatitat90 7y ago
  > She was asked by David Hilbert to work on the problem of invariants in General Relativity, came up with one of the most important theorems in physics, and then moved on to other (extremely important) mathematical work. Interestingly I just read about this in a Lawrence M. Krauss book [0] where it was described that David Hilbert insisted on hiring Noether by the university but was overruled by the male majority that didn't like the idea that a female would teach male students. Hilbert commented that "this is university, not a bath-house!" [1]. [0]: I think it was "The Greatest Story Ever Told—So Far" [1]: I'm writing that from memory so excuse some inaccuracies.
  - imglorp 7y ago
    It was University of Göttingen, 1915. https://www.famousscientists.org/emmy-noether/ https://www.famousscientists.org/emmy-noether/
dghf 7y ago
> Specifically, Einstein was puzzled by a difference that didn’t make a difference, a symmetry that didn’t make sense. It’s still astonishing to drop a wad of crumped paper and a set of heavy keys side by side to see that somehow, almost magically, they hit the ground simultaneously — as Galileo demonstrated (at least apocryphally) by dropping light and heavy balls off the tower in Pisa. If the force of gravity depends on mass, then the more massive an object is, the faster it should sensibly fall. Inexplicably, it does not. Is this right? I haven't studied physics formally since I was 16, but I thought that the extra gravitational force due to greater mass was effectively cancelled out by the extra inertia also due to the same greater mass: i.e., more massive bodies are indeed more strongly attracted by the Earth, but because they are more massive they are also harder to accelerate to the same degree, with the result that everything falls at the same rate, regardless of mass. And I also thought this was known long before Einstein: if I had to guess, I'd credit Newton. Or am I fundamentally misunderstanding something?
- hollerith 7y ago
  No you are not misunderstanding. The author of the article doesn't understand elementary physics as well as you do, which means he or she should NOT be trying to write an article about relativity or the future of physics. Welcome to modern journalism. In the paragraph after your quote is another point the author almost certainly gets wrong: >When Einstein realized that a person falling freely would feel weightless, he described the discovery as the happiest thought of his life. I have read elsewhere that this "happiest thought" was that there is no experiment that someone falling freely could do that would reliably distinguish between being stationary and falling freely, with an emphasis on measurements of the speed of light. I am pretty sure that it was common knowledge among physics PhDs back then that people falling freely feel weightless. It is not like physics PhD are incapable of jumping while noticing the sensations in their bodies when the sensations are a straightforward consequence of Newtonian mechanics.
  - ChrisLomont 7y ago
    >The author of the article doesn't understand elementary physics as well as you do Don't be so quick to make such judgments: There is ample scientific interest in the differences between inertial and gravitational mass [1]. That they seem the same is a profound question known to Einstein. He explicitly assumed they were the same in his equivalence principle [2]. So their equivalence is a postulate, not derived from deeper theories. But one day we may find it is not consistent over time. It most certainly is a deep question in physics. There are other theories of gravity, taken seriously, as possible future modifications to relativity, such as Brans-Dicke theory [3], that remove this equivalence principle, yet still agree with all observational evidence to date. Current experimental evidence puts them equivalent to under 1 part in 10^-15 [2], with more experiments still being done. [1] https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitational_mass https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitationa... [2] https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle [3] https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory
vincent-toups 7y ago
I'm reading Weyl's book "Symmetry" now and I have to say, physics looks like it was a lot more fun and loosey goosey back in the day. I think only the Quantum Gravity and Interpretation of Quantum Mechanics people still have approximately this much fun.
gone35 7y ago
As others have commented, lots of physics errors and misunderstandings in this article. The author, K.C. Cole, has no business explicating physics she doesn't understand to the public. Expected more from Quanta Magazine...
- Simon_says 7y ago
  > Expected more from Quanta Magazine... You won't if you keep reading them.

77 points by theafh 7y ago

Quanta is absolutely terrible at illustrating what it's talking about, and why. Borderline clickbait titles, fluffy meandering prose that often completely fails to tell you anything of substance. At first I thought "I'm too dumb for this", but I'm no longer sure who the intended audience even is.

DiogenesKynikos 7y ago

"Noether showed that the symmetries of general relativity — its invariance under transformations between different reference frames — ensure that energy is always conserved." This isn't quite right. Energy isn't conserved in General Relativity. There is a conservation law involving the stress-energy tensor, but as the name implies, it's not energy alone that's being conserved. But more generally, Noether's theorem, which leads to this conservation law, is probably the most beautiful theorem in physics: https://en.wikipedia.org/wiki/Noether%27s_theorem https://en.wikipedia.org/wiki/Noether%27s_theorem. Given that, what's amazing is that Emmy Noether only worked on physics for a relatively short time. She was asked by David Hilbert to work on the problem of invariants in General Relativity, came up with one of the most important theorems in physics, and then moved on to other (extremely important) mathematical work.

mr_mitm 7y ago
> This isn't quite right. Energy isn't conserved in General Relativity. Correct, and the reason is that in GR there is (in general) no time translation invariance. The associated conserved quantity to time translation invariance is precisely what we call "energy". In our universe this manifests as the expansion of spacetime. More on this: https://www.preposterousuniverse.com/blog/2010/02/22/energy-is-not-conserved/ https://www.preposterousuniverse.com/blog/2010/02/22/energy-...
Whatitat90 7y ago
> She was asked by David Hilbert to work on the problem of invariants in General Relativity, came up with one of the most important theorems in physics, and then moved on to other (extremely important) mathematical work. Interestingly I just read about this in a Lawrence M. Krauss book [0] where it was described that David Hilbert insisted on hiring Noether by the university but was overruled by the male majority that didn't like the idea that a female would teach male students. Hilbert commented that "this is university, not a bath-house!" [1]. [0]: I think it was "The Greatest Story Ever Told—So Far" [1]: I'm writing that from memory so excuse some inaccuracies.
- imglorp 7y ago
  It was University of Göttingen, 1915. https://www.famousscientists.org/emmy-noether/ https://www.famousscientists.org/emmy-noether/

dghf 7y ago

> Specifically, Einstein was puzzled by a difference that didn’t make a difference, a symmetry that didn’t make sense. It’s still astonishing to drop a wad of crumped paper and a set of heavy keys side by side to see that somehow, almost magically, they hit the ground simultaneously — as Galileo demonstrated (at least apocryphally) by dropping light and heavy balls off the tower in Pisa. If the force of gravity depends on mass, then the more massive an object is, the faster it should sensibly fall. Inexplicably, it does not. Is this right? I haven't studied physics formally since I was 16, but I thought that the extra gravitational force due to greater mass was effectively cancelled out by the extra inertia also due to the same greater mass: i.e., more massive bodies are indeed more strongly attracted by the Earth, but because they are more massive they are also harder to accelerate to the same degree, with the result that everything falls at the same rate, regardless of mass. And I also thought this was known long before Einstein: if I had to guess, I'd credit Newton. Or am I fundamentally misunderstanding something?

hollerith 7y ago
No you are not misunderstanding. The author of the article doesn't understand elementary physics as well as you do, which means he or she should NOT be trying to write an article about relativity or the future of physics. Welcome to modern journalism. In the paragraph after your quote is another point the author almost certainly gets wrong: >When Einstein realized that a person falling freely would feel weightless, he described the discovery as the happiest thought of his life. I have read elsewhere that this "happiest thought" was that there is no experiment that someone falling freely could do that would reliably distinguish between being stationary and falling freely, with an emphasis on measurements of the speed of light. I am pretty sure that it was common knowledge among physics PhDs back then that people falling freely feel weightless. It is not like physics PhD are incapable of jumping while noticing the sensations in their bodies when the sensations are a straightforward consequence of Newtonian mechanics.
- ChrisLomont 7y ago
  >The author of the article doesn't understand elementary physics as well as you do Don't be so quick to make such judgments: There is ample scientific interest in the differences between inertial and gravitational mass [1]. That they seem the same is a profound question known to Einstein. He explicitly assumed they were the same in his equivalence principle [2]. So their equivalence is a postulate, not derived from deeper theories. But one day we may find it is not consistent over time. It most certainly is a deep question in physics. There are other theories of gravity, taken seriously, as possible future modifications to relativity, such as Brans-Dicke theory [3], that remove this equivalence principle, yet still agree with all observational evidence to date. Current experimental evidence puts them equivalent to under 1 part in 10^-15 [2], with more experiments still being done. [1] https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitational_mass https://en.wikipedia.org/wiki/Mass#Inertial_vs._gravitationa... [2] https://en.wikipedia.org/wiki/Equivalence_principle https://en.wikipedia.org/wiki/Equivalence_principle [3] https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory

vincent-toups 7y ago

I'm reading Weyl's book "Symmetry" now and I have to say, physics looks like it was a lot more fun and loosey goosey back in the day. I think only the Quantum Gravity and Interpretation of Quantum Mechanics people still have approximately this much fun.

gone35 7y ago

As others have commented, lots of physics errors and misunderstandings in this article. The author, K.C. Cole, has no business explicating physics she doesn't understand to the public. Expected more from Quanta Magazine...

Simon_says 7y ago
> Expected more from Quanta Magazine... You won't if you keep reading them.