2020 Nobel prize in physics

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  • #1
martinbn
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Didn't see a thread on this.

https://www.nobelprize.org/prizes/physics/2020/summary/

The Nobel Prize in Physics 2020 was divided, one half awarded to Roger Penrose "for the discovery that black hole formation is a robust prediction of the general theory of relativity", the other half jointly to Reinhard Genzel and Andrea Ghez "for the discovery of a supermassive compact object at the centre of our galaxy."
 
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  • #3
vanhees71
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I find it a great choice. Penrose was overdue being awarded. I had no clue, who might get the Physics Nobel this year. I didn't expect another cosmology/astronomy prize again this year :-).
 
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  • #4
martinbn
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I find it a great choice. Penrose was overdue being awarded. I had no clue, who might get the Physics Nobel this year. I didn't expect another cosmology/astronomy prize again this year :-).
It is also interesting. I, personally, wouldn't have expected for them to give an award for such a theoretical work (I mean the Penrose half), one might even say mathematical work.

Speaking of which, what exactly is Penrose's result that the award is based on? Or is it for his whole contribution to the subject?
 
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  • #5
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Phys. Rev. Lett. has made the following papers free to read after the Nobel Committee cited them:

https://doi.org/10.1103/PhysRevLett.10.66
https://doi.org/10.1103/PhysRevLett.14.57
https://doi.org/10.1103/RevModPhys.82.3121

The first two papers are by Prof. Penrose, @martinbn.

Another set of four papers by Reinhard Genzel and Andrea Ghez that have been made free to read:

https://doi.org/10.1103/PhysRevLett.116.031101
https://doi.org/10.1103/PhysRevLett.118.211101
https://doi.org/10.1103/PhysRevLett.122.101102
https://doi.org/10.1103/PhysRevLett.124.081101
 
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  • #6
berkeman
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You've got to love her enthusiasm! :smile:

https://www.kron4.com/news/national/american-scientist-wins-nobel-prize-for-black-hole-research/

“Today we accept these objects are critical to the building blocks of the universe,” Ghez told an audience at the Royal Swedish Academy of Sciences by phone shortly after the announcement.

Ghez is the fourth woman to be awarded the Nobel Prize for physics, after Marie Curie in 1903, Maria Goeppert-Mayer in 1963, and Donna Strickland in 2018.

“I hope I can inspire other young women into the field. It’s a field that has so many pleasures. And if you’re passionate about the science, there’s so much that can be done,” Ghez said.
 
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  • #7
Amrator
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Finally! Penrose won a Nobel Prize! Ugh, it would have sucked to see him pass away without winning one.
 
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  • #8
robphy
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From https://www.nobelprize.org/prizes/physics/2020/advanced-information/
Schmidt’s discovery prompted Wheeler to reconsider the physics of gravitational collapse and he discussed this with Penrose, who began to think about the problem in late 1964.
...
However, it was far from clear that this could happen in the real world and whether the assumption of spherical symmetry was a prerequisite for gravitational collapse.
...
Penrose set out to analyse the situation without the assumption of spherical symmetry, assuming only that the collapsing matter had a positive energy density. To do this, he had to invent new mathematical methods and make use of topology. The key concept that Penrose introduced was that of a trapped surface.
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After realizing the power of the idea of trapped surfaces, Penrose proceeded to prove that once a trapped surface had formed, it is impossible, within the theory of general relativity and with a positive energy density, to prevent the collapse towards a singularity (Penrose 1965).
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Penrose’s result is heralded as the first post-Einsteinian result in general relativity. It proves that gravitational collapse cannot be stopped after the trapped surface is formed. One should note that its formation happens at a stage in the collapse when density of matter is not very high. (The supermassive black holes of Michell and Laplace have average densities no higher than those of the Sun or Earth.) A few years later Penrose, together with Stephen Hawking, went further to show that similar results also applied to cosmological singularities (Hawking 1965, Hawking & Penrose 1970). Under reasonable assumptions, a past singularity is inevitable in the Big Bang model. Penrose (1969) wrote a beautiful summary of many of these results.
Penrose, R., 1963, “Asymptotic properties of fields and space-times”,
Physical Review Letters, vol. 10, no. 2. pp. 66–68.
https://doi.org/10.1103/PhysRevLett.10.66

Penrose, R., 1965, “Gravitational collapse and space-time singularities”,
Physical Review Letters, vol. 14, no. 3. pp. 57–59, 1965.
https://doi.org/10.1103/PhysRevLett.14.57.

Penrose, R., 1969, “Gravitational collapse: the role of general relativity”,
Nuovo Cimento Rivista Serie, vol. 1. p. 252.
( reprinted as
Penrose, R. “Golden Oldie”: Gravitational Collapse: The Role of General Relativity.
General Relativity and Gravitation 34, 1141–1165 (2002).
https://doi.org/10.1023/A:1016578408204
)


(Other references in the quoted document of possible interest:

Hawking, S. W., 1965, “Occurrence of singularities in open universes”,
Physical Review Letters, vol. 15, no. 17. pp. 689–690.
https://doi.org/10.1103/PhysRevLett.15.689.

Hawking, S. W., and Penrose, R., 1970, “The singularities of gravitational collapse and cosmology”,
Proceedings of the Royal Society of London Series A, vol. 314, no. 1519. pp. 529– 548.
https://doi.org/10.1098/rspa.1970.0021

Hawking, S. W., 1972, “Black holes in General Relativity”,
Communications in Mathematical Physics, vol. 25. pp. 152.
( https://projecteuclid.org/euclid.cmp/1103857884 )

Hawking, S. W., 1975, “Particle creation by black holes”,
Communications in Mathematical Physics, vol. 43, no. 3. pp. 199–220.
https://doi.org/10.1007/BF02345020. ( https://projecteuclid.org/euclid.cmp/1103899181 )

)

Other references of possible interest:


José M. M. Senovilla, David Garfinkle
"The 1965 Penrose singularity theorem"
https://arxiv.org/abs/1410.5226
https://iopscience.iop.org/issue/0264-9381/32/12



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  • #9
martinbn
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So, more accurately it is for the singularity theorem, not that "..black hole formation is a robust predictions of the theory of general relativity..".
 
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  • #10
vanhees71
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It is also interesting. I, personally, wouldn't have expected for them to give an award for such a theoretical work (I mean the Penrose half), one might even say mathematical work.

Speaking of which, what exactly is Penrose's result that the award is based on? Or is it for his whole contribution to the subject?
I watched the announcement on youtube. The 1/2 prize for Penrose was given for the fact that he established mathematically the inevitability of the existence of black holes from the General Theory of Relativity. The point was that it's not only for the special case of spherical symmetry (Schwarzschild black hole) but also for less symmetrical. Penrose then checked it for the (then brand new) Kerr solution of a rotating black hole and also gave a general proof for the inevitability of singularities in GR. I think it's a well deserved prize.

Of course, that's a theoretical argument only, and the other 1/2 prize thus was given to Genzel and Ghez, who established the existence of a supermassive object in the center of our galaxy. To the best of our knowledge that can only be a black hole, and thus the evidence is pretty overwhelming. They observed the motion of stars close to it for decades. It's also a well-deserved Nobel for them.
 
  • #11
PeterDonis
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he established mathematically the inevitability of the existence of black holes from the General Theory of Relativity
But, strictly speaking, that isn't what the singularity theorems establish.

The singularity theorems establish that, if certain conditions are met and a trapped surface is present, then a singularity must also be present.

However, a black hole is not defined by whether a trapped surface is present, but by whether an event horizon is present, i.e., whether there is a region of spacetime that is not in the causal past of future null infinity. The singularity theorems say nothing about that.

It is true that, if the energy conditions are met and a trapped surface is present, there must be an event horizon at or outside the trapped surface; but the theorem that establishes that is not one of the singularity theorems.

To be clear, none of this means that Penrose didn't deserve the prize; he did.
 
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  • #12
pervect
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I am a bit surprised that the observational scientists didn't get more of the prize. For instance Penzias and Wilson did not share the prize with Dicke , Peebles, or Wilkins, or any other theoretician related to the CMB discovery, though they did share it with Pyotr Leonidovich Kapitsa for his unrelated work in low temperature physics, according to Wiki.
 
  • #13
Vanadium 50
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The prize is not awarded by figuring out what, then who. It is closer to who, then what.
 
  • #14
hutchphd
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I am a bit surprised that the observational scientists didn't get more of the prize. For instance Penzias and Wilson did not share the prize with Dicke , Peebles, or Wilkins, or any other theoretician
Historically I believe the maximum number of awardees is three. Five is more than three.......so it may have been that simple.
 
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  • #15
atyy
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It is true that, if the energy conditions are met and a trapped surface is present, there must be an event horizon at or outside the trapped surface; but the theorem that establishes that is not one of the singularity theorems.
Who showed that?
 
  • #16
PeterDonis
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Who showed that?
I haven't been able to find a specific reference. I think the theorem is mentioned in Wald, but I don't have my copy handy to check.
 
  • #17
martinbn
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Who showed that?
I think that is not proven yet. Unless you take weak cosmic censorship for given.
 
  • #18
martinbn
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Just to make it clear I was just being pedantic, I do think that he deserves the award. In fact I think he deserves any of the existing maths/physics awards.
 
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  • #19
ChrisVer
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Yesterday I tried to read the "theory" part of scientific background of the Nobel Prize:
https://www.nobelprize.org/uploads/2020/10/advanced-physicsprize2020.pdf

Could someone with better knowledge/expertise on the topic explain to me the following:

As I understood from the article, Lifshitz and Khalatnikov proved that a singularity cannot occur in a realistic model, because there is no spherical symmetry for an arbitrary distribution of matter (the "because" part is what I inferred).
However, later Penrose later proved that it could occur (given positive energy density)? Aren't the two statements exclusive? Or under which conditions was Lifshitz and Khalatnikovs' conclusion correct?

I also struggled a little to understand the definition of trapped surfaces... "all light rays orthogonal to the surface converge when traced toward the future":
By orthogonal to the surface they meant the vectors pointing out from the surface? And how is the direction of the "future" defined? Is it referring to something like a "closed" light cone?
 
  • #20
martinbn
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Yesterday I tried to read the "theory" part of scientific background of the Nobel Prize:
https://www.nobelprize.org/uploads/2020/10/advanced-physicsprize2020.pdf

Could someone with better knowledge/expertise on the topic explain to me the following:

As I understood from the article, Lifshitz and Khalatnikov proved that a singularity cannot occur in a realistic model, because there is no spherical symmetry for an arbitrary distribution of matter (the "because" part is what I inferred).
However, later Penrose later proved that it could occur (given positive energy density)? Aren't the two statements exclusive? Or under which conditions was Lifshitz and Khalatnikovs' conclusion correct?
They didn't prove it, they thought it was true, but they were mistaken.
 
  • #21
ChrisVer
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They didn't prove it, they thought it was true, but they were mistaken.
It is that I can't have access here at the moment (apart from the abstract) which I think is the cited work by the prize editors:
https://inspirehep.net/literature/8888
 
  • #22
martinbn
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I also struggled a little to understand the definition of trapped surfaces... "all light rays orthogonal to the surface converge when traced toward the future":
By orthogonal to the surface they meant the vectors pointing out from the surface? And how is the direction of the "future" defined? Is it referring to something like a "closed" light cone?
This is more dificult to imagin, you need the maths. Think of a sphere that flashes in an instant. Some of the light rays will go outward, some will go inward. The outgoing ones will diverge, the ingoing will coonverge. For a trapped surface you need both to converge.
 
  • #23
julian
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Here is a link to Hawking's chapter's of the pop science/ bit of math book "The Nature of Space and Time" that was coauthored with Penrose:

https://arxiv.org/abs/hep-th/9409195

This has the first chapter that introduces the singularity theorems, but it doesn't have the chapter written by Penrose on the "Structure of Spacetime Singularities" which introduces, amongst other things, the cosmic censorship hypothesis.
 
  • #24
julian
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I thought it was a maths result because the singularity theorems prove (that under physically reasonable conditions) there exist geodesics that come to an end - incomplete geodesics. It is usually assumed that the characteristic of a singularity is that the curvature divergences. However, this is not exactly what the singularity theorems imply.

I think there has been progress in the direction of addressing this issue. I dont know if this might be a reason why it has taken so long for Penrose to receive a noble prize. I dont know the status of this field.
 
  • #25
f95toli
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I think there has been progress in the direction of addressing this issue. I dont know if this might be a reason why it has taken so long for Penrose to receive a noble prize. I dont know the status of this field.
The simple answer is that you can't really get the prize for purely theoretical work; the predictions of the theory must be well established and have been confirmed by experiments. This is one reason why the prize is shared with experimentalists.
I suspect recent observations of black holes by e.g. LIGO and VIRGO; and the fact that the observations seem consistent with theory is another reason for why the prize was awarded now.
 
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