Newly discovered black hole 'speed limit' hints at new laws of physics | Hacker News Reader

Newly discovered black hole 'speed limit' hints at new laws of physics

livescience.com

62 pointsperihelions12 comments

This title is seriously misleading. There are no "new laws of physics here", just old laws (GR) applied to new regimes (black hole collisions).

Think I read somewhere that the GR prediction on the speed of the colliding black holes doesn't match the observations from LIGO.

AFAIK, Gravitational waves are out first method of "seeing" the universe that isn't just capturing incoming photons from the object under observation. The only exception is neutrino detectors, which have hilariously bad resolution.

It seems reasonable to expect new phenomena to be uncovered this way.

This seems like a good example of that.

Also neutrino detectors and gravitational wave detectors allow to observe the very early universe. Until something like 300K years after the Big Bang the universe was too hot for atoms to form and so it was full of particles that block light. But the universe became clear for neutrinos a lot earlier and for gravitational waves even earlier (think it's milliseconds past the Big Bang).

These methods also allow to observe the same event in different ways. This is known as "multi-messenger astronomy":

https://en.wikipedia.org/wiki/Multi-messenger_astronomy

Very interesting. I did not imagine that there would be a theoretical maximum on the recoil velocity of such a collision below the speed of light, and I definitely didn't expect it to be so much smaller than the speed of light

I guess the maximum comes from the gravity's inability to keep them together, when their relative speed gets too high.

The article seems to claim a speed limit of 0.1c for matter... but isn't this already disproven by the speeds reached in our particle accelerators?

It's not a "speed limit of 0.1c for matter". This limit would apply to the scenario they investigated -- colliding black holes where the spacetime is massively affected by the mass and movement of the black holes.

And this extreme scenario is where probably any errors in GR can be observed, as quantum gravitational effects will become visible.

No, GR is expected to work fine in this regime since there is no expectation of any quantum effects.

A quantum theory of gravity is only needed when we expect there to be significant quantum effects, for example maybe in a neutron star and definitely in situations where our measurement results depend on a superposition of "spacetimes". The reason we don't have a consensus on a quantum theory of gravity yet is that gravity is so weak that it's usually only detectable in situations that involve huge amounts of matter, and it's very rare to have huge amounts of matter together and still have the system behave in a way that requires quantum mechanics to describe it

It’s not talking about the maximum speed of matter under any conditions. It’s just calculating the maximum speed of a black hole when it merges with another one

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