Archive for Event Horizon

What to do if you find yourself inside the horizon of a black hole

Posted in The Universe and Stuff with tags , , , on May 7, 2019 by telescoper

Consider how lucky you are that life has been good to you so far.

Alternatively, if life hasn’t been good to you so far – which, given your current circumstances seems more likely – consider how lucky you are that it won’t be bothering you much longer.

That was the advice given to Ford Prefect by The Hitchhikers Guide to the Galaxy when he looked up `What do if you find yourself in a crack in the ground underneath a giant boulder you can’t move with no hope of rescue’. It seems fairly general advice to me, though. If you want more specific advice on what to do if you find yourself inside the horizon of a black hole then you can find it in an interesting paper on the arXiv with the abstract:

In this methodological paper we consider two problems an astronaut faces with under the black hole horizon in the Schwarzschild metric. 1) How to maximize the survival proper time. 2) How to make a visible part of the outer Universe as large as possible before hitting the singularity. Our consideration essentially uses the concept of peculiar velocities based on the “river model”. Let an astronaut cross the horizon from the outside. We reproduce from the first principles the known result that point 1) requires that an astronaut turn off the engine near the horizon and follow the path with the momentum equal to zero. We also show that point 2) requires maximizing the peculiar velocity of the observer. Both goals 1) and 2) require, in general, different strategies inconsistent with each other that coincide at the horizon only. The concept of peculiar velocities introduced in a direct analogy with cosmology, and its application for the problems studied in the present paper can be used in advanced general relativity courses.

It is advertised as a `methodological paper’ and I don’t know if they are planning experimental studies of this problem. I imagine might be difficult to secure funding.

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Results from the Event Horizon Telescope

Posted in Astrohype, The Universe and Stuff with tags , , on April 10, 2019 by telescoper

Following yesterday’s little teaser, let me point out that there is a press conference taking place today (at 2pm Irish Summer Time, that’s 3pm Brussels) to announce a new result from the Event Horizon Telescope. The announcement will be streamed live here.

Sadly, I’m teaching at the time of the press conference so I won’t be able to watch, but that doesn’t mean that you shouldn’t!

I’ll post pictures and comments when I get back. Watch this space. Or you could watch this video..

UPDATE: Well, there we are. Here is the image of the `shadow’ of the event horizon around the black hole in M87:

The image is about 42 micro arcseconds across. I guess to people brought up on science fiction movies with fancy special effects the image is probably a little underwhelming, but it really is an excellent achievement to get that resolution. Above all, it’s a great example of scientific cooperation – 8 different telescopes all round the world. The sizeable European involvement received a substantial injection of funding from the European Union too!

Other parameters are here:

The accompanying EU press release is here. Further information can be found here. The six publications relating to this result can be found here:

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The Shadow of an Event Horizon

Posted in The Universe and Stuff with tags , , , , , on April 9, 2019 by telescoper

There is a paper on the arXiv written about 5 years ago called Towards the event horizon – the supermassive black hole in the Galactic Center by Falcke and Markoff, the abstract of which reads:

The center of our Galaxy hosts the best constrained supermassive black hole in the universe, Sagittarius A* (Sgr A*). Its mass and distance have been accurately determined from stellar orbits and proper motion studies, respectively, and its high-frequency radio, and highly variable near-infrared and X-ray emission originate from within a few Schwarzschild radii of the event horizon. The theory of general relativity (GR) predicts the appearance of a black hole shadow, which is a lensed image of the event horizon. This shadow can be resolved by very long baseline radio interferometry and test basic predictions of GR and alternatives thereof. In this paper we review our current understanding of the physical properties of Sgr A*, with a particular emphasis on the radio properties, the black hole shadow, and models for the emission and appearance of the source. We argue that the Galactic Center holds enormous potential for experimental tests of black hole accretion and theories of gravitation in their strong limits.

Please note that the black hole in the centre of the giant elliptical galaxy M87 is about 1000 times further away from us than the black hole in the centre of the Milky Way but is also about 1000 times more massive, so its Schwarzschild radius is 1000 times larger. The observational challenge of imaging the event horizon is therefore similar in the two cases.

You may find this useful if, by sheer coincidence, there is some big announcement tomorrow..

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