This year's science Nobel Prizes

[lpetrich]

The official website of the Nobel Prize - NobelPrize.org


The Nobel Prize in Physics 2020

Roger Penrose received 1/2 of it for showing that black-hole formation is a rather general consequence of general relativity.

Reinhard Genzel and Andrea Ghez received 1/4 each of it for their discovering of the supermassive black hole in the center of our Galaxy.

I'll be explaining the significance of their discoveries in my next posts.

The Nobel Prize in Chemistry 2020 - NobelPrize.org

Emmanuelle Charpentier and Jennifer A. Doudna received 1/2 each of them for their discovery of the CRISPR/Cas9 genetic scissors, something very convenient for editing genes in place in cells. This gene-manipulation tool has been a valuable tool in basic research, it has been used to do genetic engineering on crop plants, and it may make possible certain sorts of therapies.

The Nobel Prize in Physiology or Medicine 2020 - Press release - NobelPrize.org

Harvey J. Alter, Michael Houghton and Charles M. Rice received 1/3 each of them for their discovery of the hepatitis C virus. After the discovery of the hepatitis A and B viruses, many cases of hepatitis continued to have an unknown cause. These three researchers were among those who tracked down the virus that caused these mysterious cases.

 

[lpetrich]

First, Roger Penrose's work.

The story goes back to 1905, when Albert Einstein published his papers on special relativity. Not long after, one of his teachers, Hermann Minkowski, made a remarkable discovery about the theoretical structure of SR. Space and time are very closely related in it, and the square of a time interval behaves just like minus the square of a space interval.

That means that space-time intervals fall into five categories:

• Forward timelike (c*|time interval| > |space interval|)
• Forward null (c*|time interval| = |space interval|)
• Spacelike (c*|time interval| < |space interval|)
• Backward null (c*|time interval| = |space interval|)
• Backward timelike (c*|time interval| > |space interval|)

c is the speed of light in a vacuum, and it is now officially a units-conversion factor. In theoretical work, it is often set to 1.

The forward and backward null directions form cones: "light cones".

Einstein originally dismissed Minkowski's work as "superfluous learnedness", but he ended up using it to trying to generalize special relativity to include gravity. The result of his work was general relativity, and he published it in 1915.

Einstein despaired of finding exact strong-field solutions, but when physicist Karl Schwarzschild learned of GR, he soon found one. He imposed spherical symmetry, and he found that the equations for space-time become much simpler. He succeeded in discovering a strong-field solution, a solution for a point mass with infinite density, a "singularity". This solution has lots of interesting properties, and one of them is that there is a spherical surface surrounding the central point called the "event horizon". It is a point of no return for anything going in. Inside of it, the radial direction is timelike, meaning that anything inside it was doomed to continue until it reached the singularity. The distance from the central point to the event horizon is the black-hole radius or Schwarzschild radius. For the Earth, it is about 9 mm, and for the Sun, 3 km.

This was the first black-hole solution, and for a long time, it seemed doubtful that a black hole could come into existence from some larger object collapsing with its surface going past its black-hole radius. Even if one could form, did it require some good approximation of spherical symmetry?

 

[lpetrich]

Then the first possible observational evidence of black holes was discovered. In the 1950's, pointlike radio sources were discovered, and when some astronomers got the precise direction of one, another astronomer, Maarten Schmidt, discovered in 1963 what looked like a star at its position. It turned out to have some noticeably redshifted spectral lines, making it very distant -- and very bright, like a thousand times brighter than our Galaxy.

This and other such objects were named quasi-stellar objects or QSO's or quasars. Not only were they too small to resolve, they varied in brightness over hours or days, giving a strong constraint on their sizes. But with their brightnesses, they had to be very massive to avoid disintegrating under their light's radiation pressure. Their combination of masses and sizes strongly suggested black holes.

Not long after, some rapidly varying X-ray sources were discovered in our Galaxy. Some of them turned out to be neutron stars orbiting other stars that spew out material that falls onto those neutron stars. But some of them were too massive to be plausible neutron stars, like Cygnus X-1 at about 15 solar masses. Black holes there also?

That led physicists to reconsider whether or not a black hole can form, but a further complication discovered back then was Roy Kerr's 1963 discovery of rotating black-hole solutions. These had axial symmetry and equatorial-reflection symmetry, a step down from spherical symmetry but still a lot of symmetry. Was it possible to collapse into one of those?


One of them was Roger Penrose. He decided to avoid assuming spherical symmetry, and he used some very fancy mathematics for doing so. He introduced the concept of a "trapped surface", where light going through it orthogonal to it would end up converging. He found that trapped surfaces will exist inside the event horizons of both Schwarzschild and Kerr black holes, and he found that once a trapped surface forms, it will inevitably collapse to a singularity. He also found that trapped surfaces will do so even in the absence of spatial symmetry, so being asymmetric or low in symmetry will not prevent collapse to a singularity.

He and Stephen Hawking extended his work to cosmology, and they showed that with plausible equations of state of the Universe's material, one gets an initial singularity. One can get around that theorem by using weird equations of state for the Universe's material, like

(pressure) = - (density)*c^2

The - is not a typo: its pressure is negative. When one does so, one finds an "eternal inflation" solution.

 

[lpetrich]

Quasars have the curious property that they were much more abundant in the early Universe, redshift ~ 1 - 2, than they are today. That means that many present-day galaxies have burned-out quasars in them. Many galaxies have "active galactic nuclei" in them, but not enough to be survivals for all the quasars that once existed.

So many galaxies must have black holes in their centers, even if they do not give much evidence of their existence.


Roger Penrose did another interesting theoretical thing. He proposed a process for extracting energy from rotating black holes. A particle coming in along the direction of its rotation can split in two when close enough, and one of the resulting particles can escape faster than the original particle's infall, removing angular momentum from the BH.

 

[lpetrich]

Back to galaxies. In the 1990's, the supergiant elliptical galaxy M87 was discovered to have a very compact object in its center with a mass of 37 million solar masses. A supermassive black hole? Was there also one in our Galaxy?

Reinhard Genzel at the Max Planck Institute in Germany, and Andrea Ghez at UCLA in the US both decided to address that issue. They noted that there was a radio source, Sagittarius A* (Sgr A*) in the direction of the center of our Galaxy. So they decided to look for stars near it. There was no way that they could see anything there in visible light, so they used near-infrared light in astronomers' K band or 2.2 microns. To get very good resolution, they needed to get around the turbulence of our planet's atmosphere. They used "speckle imaging", taking lots of pictures then shifting them to correct for turbulence effects and then adding them together. They found the stars to be noticeably moving relative to each other, just like nearby stars. How fast they were moving relative to Sgr A* fit the 1/(distance)^(1/2) curve that one would expect for Sgr* being very massive.

Their next step was adaptive optics, using a deformable secondary mirror to correct for atmospheric turbulence. It is measured by using some star as a guide star, or else by shooting a laser beam along the telescope's view direction and then observing it. This got resolutions down to the diffraction limit, imposed by the wave nature of light, and it also made possible spectroscopy, to get compositions and radial velocities.

One of the stars, called S2 by Genzel's team, and S02 by Ghez's team, orbits Sgr A* with a period a little under 16 years. Its pericenter distance is 17 light hours or 1400 times the black-hole radius of a 4-million-solar-mass black hole. By comparison, the Sun takes some 200 million years to orbit, and it is 8 kiloparsecs or 26,000 light years away.

Recent observations are good enough to detect S2's GR orbit precession, first observed in planet Mercury's orbit and more recently in the Hulse-Taylor binary pulsar.

Sgr A* makes infrared flares, and high-resolution observation of them shows that they travel at 0.3 c at around 3 - 5 BH radii, close to the closest possible circular orbit for a BH.

So there we have it: a black hole in the center of our Galaxy.