Black Holes

Do black holes have singularities?

I think Roy Kerr’s recent paper is important. It’s called Do black holes have singularities? I think it’s important because it challenges an orthodoxy that’s been taken for granted, and because Kerr has the authority to get some attention. That’s because he was in on the Golden Age of General Relativity, and because the EHT collaboration described both M87* and Sagittarius A* as matching the Kerr metric. To set the scene, black hole physics has been around for a long time, but a major development was…

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The gamma bomb

I’ve always been interested in antigravity. It started when I was when I was six, when Fireball XL5 was on TV. Of course, the spaceship itself was a rail-launched rocket with wings. But Steve Zodiac and crew had a "gravity activator" for artificial gravity inside the ship, plus hover bikes. Hover bikes were a recurring feature in Gerry and Silvia Anderson’s futuristic programmes. I think it was because they were cool, and because it wasn’t easy to make those marionettes walk. Gravity generators or compensators were also…

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We have to talk about LIGO

I’ve mentioned LIGO a couple of times in passing. You know, LIGO, the Laser Interferometer Gravitational Wave Observatory. Rainer Weiss of MIT had a lot to do with the idea back in the 1960s, as did Kip Thorne of Caltech. Things got serious in 1980 when the US National Science Foundation funded prototypes at Caltech and MIT, and funded Weiss to lead a study into a full size interferometer. Construction eventually started in 1994 and was finished in 1997. Observations eventually started in 2002 and stopped…

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The black hole charlatans

When you read the Einstein digital papers, you understand how gravity works. Einstein said things like “the curvature of light rays occurs only in spaces where the speed of light is spatially variable”. Note that he never abandoned this, and he never said light curves because it follows the curvature of spacetime. Instead he said a gravitational field is a place where space is “neither homogeneous nor isotropic”, he referred to Huygen’s principle, and he talked about “the refraction of light rays by the gravitational field”. So did Newton, see…

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Most laymen completely misunderstand what a black hole is

I saw Luboš Motl’s blog post Most laymen completely misunderstand what a black hole is. When I read the title my irony meter skipped a beat, so I thought I’d take a closer look. Especially since he said this in his opening paragraph: One of those invalid memes that I want to discuss… is the idea that the point of the black hole is the singularity. That is what makes a black hole a black hole and that's also where the mysteries of black holes hide.…

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A brief history of black holes

I read Sabine Hossenfelder’s latest blog post yesterday. The title was A brief history of black holes. I left a couple of comments. One was a reply to Louis Marmet, and referred to Oppenheimer’s 1939 frozen star black hole. I said I think the black hole grows like a hailstone, from the inside out. The other was addressed to Hossenfelder, and referred to Einstein talking about the variable speed of light. I said that IMHO this had to mean Penrose/Hawking singularity theorems were wrong. I found…

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Misconceptions in gravitational physics

I think it’s safe to say that there are some misconceptions in physics. The issue of course, is how many. That’s where I part company with your average physicist. He’ll tell you that whilst we don’t understand everything, we do understand some things, such as black holes. Only he doesn’t. Take a look at the Wikipedia black hole article. It says this: “a black hole is a region of spacetime exhibiting gravitational acceleration so strong that nothing - no particles or even electromagnetic radiation such as…

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The Big Bang

Big Bang cosmology arguably started in 1917. Vesto Slipher had measured 21 galactic redshifts by 1917. That’s when Albert Einstein wrote his cosmological considerations paper and Willem de Sitter came up with the de Sitter universe. The next year in 1918 Erwin Schrödinger came up with his cosmic pressure. In 1922 Alexander Friedmann came up with a non-static universe. In 1924 he came up with negative and positive curvature, and Knut Lundmark came up with an expansion rate within 1% of measurements today. In 1927 Georges…

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Firewall!

The black hole firewall is a relatively recent idea. On Wikipedia you can read how it “was proposed in 2012 by Ahmed Almheiri, Donald Marolf, Joseph Polchinski, and James Sully as a possible solution to an apparent inconsistency in black hole complementarity”. Their proposal is known as the AMPS firewall, and the title of their paper is black holes: complementarity or firewalls? They cannot all be true They start by saying “we argue that the following three statements cannot all be true: (i) Hawking radiation is…

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The information paradox

The information paradox was first mooted by Stephen Hawking in 1976. For an introduction to the subject, see Brian Koberlein’s black holes tell no tales or do they? Then see Hawking’s paper on the breakdown of predictability in gravitational collapse. Hawking said information is lost down a black hole because the quantum emission is completely random and uncorrelated. He also said “this means there is no S matrix for the process of black-hole formation and evaporation”. The S-matrix is the scattering matrix which is to do…

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