Archive for black holes

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..

Simulation of the binary black-hole coalescence GW170104

Posted in The Universe and Stuff with tags , , , on June 2, 2017 by telescoper

Via the Cardiff University news website, I found this video of a computer simulation of the binary black-hole coalescence that gave rise to the gravitational wave event GW170104 whose detection was announced yesterday, so I thought I’d share it here.

Here’s the  blurb accompanying the video:

The video shows a numerical simulation of a binary black-hole coalescence with masses and spins consistent with the GW170104 observation. The strength of the gravitational wave is indicated by elevation as well as color, with blue indicating weak fields and yellow indicating strong fields. We rescale the amplitude of the gravitational wave during the simulation to show the signal during the entire animation not only close to merger, where it is strongest. The sizes of the black holes are increased by a factor of two to improve visibility. The bottom panel in the video shows the gravitational waveform starting at frequency of 25Hz. The fade in of the video corresponds to a frequency of about 30Hz.

© Numerical-relativistic simulation: S. Ossokine, A. Buonanno (Max Planck Institute for Gravitational Physics) and the Simulating eXtreme Spacetimes project; scientific visualization: T. Dietrich (Max Planck Institute for Gravitational Physics), R. Haas (NCSA).

The colour scheme gives me a headache, and there’s no sountrack, but it’s quite instructive nonetheless.

 

Stephen Hawking’s Reith Lectures

Posted in Biographical, The Universe and Stuff with tags , , , , on January 8, 2016 by telescoper

Yesterday I took off early from work to head up to the Royal Institution in London to attend a recording of the Reith Lectures, this year given by Stephen Hawking.

Here’s a rather crappy phone pic to show I was there.

image

In fact they recorded two of this year’s lectures, as well as a lengthy question-and-answer session. The talks and answers to audience questions did of course have to be pre-loaded into Stephen’s computer before delivery which necessitated some pauses for uploads. This together with the recording of various intros, outros and idents made for quite a lengthy event but I found the whole process fascinating and didn’t mind that at all. I did have three glasses of wine at the drinks reception before the show, however, so was in quite a relaxed frame of mind generally.

In charge of the whole thing was the inestimable Sue Lawley who did her job brilliantly. On a few occasions, Stephen Hawking’s computer had a glitch and made a spontaneous interjection in an inappropriate place. Sue Lawley proved  completely unflappable.

The topic for the series is, not surprisingly because it is what Hawking is most closely associated with, Black Holes. The lectures were enjoyably sprinkled with some very witty asides, but I did get surprisingly technical at a few points; the audience members beside me were visibly baffled on more than one occasion. See what you think yourself when the lectures are broadcast, the first on 26th January and the second a week later, both at 9pm on BBC Radio 4. They will also be broadcast on the BBC World Service.

The Reith Lectures are open to the public. Apparently over 20,000 applied for tickets to attend last night, such is the draw of Stephen Hawking. The capacity of the Royal Institution lecture theatre is only about 400 so many were disappointed. Fortunately for me, owing no doubt to some form of administrative error, I was an invited guest. I was however somewhat relieved to find I was only on the B-list so although I got to use the VIP entrance I didn’t have to sit among the big nobs at the front in reserved seats.

From Darkness to Green

Posted in History, The Universe and Stuff with tags , , , , , , , , , , on March 7, 2014 by telescoper

On Wednesday this week I spent a very enjoyable few hours in London attending the Inaugural Lecture of Professor Alan Heavens at South Kensington Technical College Imperial College, London. It was a very good lecture indeed, not only for its scientific content but also for  the plentiful touches of droll humour in which Alan specialises. It was also followed by a nice drinks reception and buffet. The talk was entitled Cosmology in the Dark, so naturally I had to mention it on this blog!

At the end of the lecture, the vote of thanks was delivered in typically effervescent style by the ebullient Prof. Malcolm Longair who actually supervised Alan’s undergraduate project at the Cavendish laboratory way back in 1980, if I recall the date correctly. In his speech, Malcolm referred to the following quote from History of the Theories of the Aether and Electricity (Whittaker, 1951) which he was kind enough to send me when I asked by email:

The century which elapsed between the death of Newton and the scientific activity of Green was the darkest in the history of (Cambridge) University. It is true that (Henry) Cavendish and (Thomas) Young were educated at Cambridge; but they, after taking their undergraduate courses, removed to London. In the entire period the only natural philosopher of distinction was (John) Michell; and for some reason which at this distance of time it is difficult to understand fully, Michell’s researches seem to have attracted little or no attention among his collegiate contemporaries and successors, who silently acquiesced when his discoveries were attributed to others, and allowed his name to perish entirely from the Cambridge tradition.

I wasn’t aware of this analysis previously, but it re-iterates something I have posted about before. It stresses the enormous historical importance of British mathematician and physicist George Green, who lived from 1793 until 1841, and who left a substantial legacy for modern theoretical physicists, in Green’s theorems and Green’s functions; he is also credited as being the first person to use the word “potential” in electrostatics.

Green was the son of a Nottingham miller who, amazingly, taught himself mathematics and did most of his best work, especially his remarkable Essay on the Application of mathematical Analysis to the theories of Electricity and Magnetism (1828) before starting his studies as an undergraduate at the University of Cambridge which he did at the age of 30. Lacking independent finance, Green could not go to University until his father died, whereupon he leased out the mill he inherited to pay for his studies.

Extremely unusually for English mathematicians of his time, Green taught himself from books that were published in France. This gave him a huge advantage over his national contemporaries in that he learned the form of differential calculus that originated with Leibniz, which was far more elegant than that devised by Isaac Newton (which was called the method of fluxions). Whittaker remarks upon this:

Green undoubtedly received his own early inspiration from . . . (the great French analysts), chiefly from Poisson; but in clearness of physical insight and conciseness of exposition he far excelled his masters; and the slight volume of his collected papers has to this day a charm which is wanting in their voluminous writings.

Great scientist though he was, Newton’s influence on the development of physics in Britain was not entirely positive, as the above quote makes clear. Newton was held in such awe, especially in Cambridge, that his inferior mathematical approach was deemed to be the “right” way to do calculus and generations of scholars were forced to use it. This held back British science until the use of fluxions was phased out. Green himself was forced to learn fluxions when he went as an undergraduate to Cambridge despite having already learned the better method.

Unfortunately, Green’s great pre-Cambridge work on mathematical physics didn’t reach wide circulation in the United Kingdom until after his death. William Thomson, later Lord Kelvin, found a copy of Green’s Essay in 1845 and promoted it widely as a work of fundamental importance. This contributed to the eventual emergence of British theoretical physics from the shadow cast by Isaac Newton which reached one of its heights just a few years later with the publication a fully unified theory of electricity and magnetism by James Clerk Maxwell.

But as to the possible reason for the lack of recognition for John Michell who was clearly an important figure in his own right (he was the person who first developed the concept of a black hole, for example) you’ll have to read Malcolm Longair’s forthcoming book on the History of the Cavendish Laboratory!

Did Hawking Say “There Are No Black Holes”?

Posted in Astrohype, The Universe and Stuff with tags , , , on February 5, 2014 by telescoper

Last week there was a rather tedious flurry of media activity about Stephen Hawking’s alleged claim that there are no black holes after all. Here’s a nice blog post explaining what Hawking actually said. Also, check out the link at the start of this article to a very nice layperson’s guide to the Black Hole Information Paradox.

Matt Strassler's avatarOf Particular Significance

Media absurdity has reached new levels of darkness with the announcement that Stephen Hawking has a new theory in which black holes do not exist after all.

No, he doesn’t.

[Note added: click here for my new introduction to the black hole information paradox.]

First, Hawking does not have a new theory… at least not one he’s presented. You can look at his paper here — two pages (pdf), a short commentary that he gave to experts in August 2013 and wrote up as a little document — and you can see it has no equations at all. That means it doesn’t qualify as a theory. “Theory”, in physics, means: a set of equations that can be used to make predictions for physical processes in a real or imaginary world. When we talk about Einstein’s theory of relativity, we’re talking about equations. Compare just the look and…

View original post 979 more words

The Student Education Paradox

Posted in Education, The Universe and Stuff with tags , , , on January 24, 2014 by telescoper

An exciting new paper by a leading theoretical physicist prominent educationalist has just appeared on the arXiv. In it the author addresses the important question of whether information is destroyed in black holes students actually learn anything during lectures.

Until recently it was generally believed that any information falling into a black hole entering the mind of a student was lost forever even though black holes do evaporate students do take examinations after a finite time. This belief is motivated by the properties of Hawking radiation produced by black holes observations of examination scripts written by students, which some claim to be entirely random, i.e. devoid of any information content whatsoever.

This picture has however been challenged by a number of educationalists theorists with a variety of counter-arguments. For example, some have argued for a statistical interpretation in terms of the multiverse a very large class; although information may be destroyed in individual black holes students, in a infinite multiverse large enough class, there may be a finite number of examples in which some information is retained.

The latest article (referred to above) offers a different resolution of the Black Hole Information Student Education Paradox which rests on the idea that information radiated by black holes examination scripts written by students are not in fact entirely random, just produced so chaotically that, although information is present, for any practical purposes such information is so garbled that it is impossible to decipher.

This intriguing suggestion has led to a number of interesting, if somewhat speculative, extensions. Some have even argued that there may after all be some information present in the speeches of Education Minister Michael Gove, though this idea obviously remains highly controversial.

Stephen Hawking is 72.

LHC Hasn’t Destroyed Earth Yet (via Today’s New Reason to Believe)

Posted in The Universe and Stuff with tags , , , , , , on June 19, 2011 by telescoper

I thought I’d reblog this as it relates to the pronouncements about the LHC by Otto Rössler I mentioned yesterday.

LHC Hasn’t Destroyed Earth Yet As I predicted nearly two-and-a-half years ago, it looks like the Earth will survive the most powerful accelerator ever built. A recent article validates my prediction. On September 8, 2008, I recorded a Science News Flash podcast addressing concerns that the Large Hadron Collider (LHC) would produce a shower of black holes that would, in turn, consume Earth from the inside out. I predicted that Earth would not only survive the experiments perfor … Read More

via Today's New Reason to Believe

You might also want to read this older blog post about the kerfuffle when the LHC was switched on. I quote:

Rössler turns out to be quite a strange fellow. He is an MD who stayed in academia, moved into biochemistry, and then made a name in the relatively new field of chaos theory. He seems to think of himself as a visionary, having founded a new field of physics called “endophysics,” which is supposed to take into account the observer’s inner state. Or something like that. Have you heard of it? Neither had I.

Recently, at the age of sixty-eight, Rössler, despite having no particle physics or blackhole physics credentials, announced that he had found important new results, alarmingly relevant to the destructive potential of microscopic black holes in LHC proton-proton collisions. Rössler variously estimates the likelihood of such blackhole production by LHC as being from 10% to 50% though he appears to have pulled these numbers out of a hat.

And there’s also this most excellent video that John Butterworth told me about because he’s in it…

Black Hole

Posted in Poetry, The Universe and Stuff with tags , , , on November 24, 2009 by telescoper

After an exceptionally trying day, I’ve been relaxing by dipping into a collection of poems called Dangerous Driving by Chris Woods. He’s an interesting character who works part-time as a GP in Lancashire and tries to balance medicine, family and writing. His poetry has appeared in numerous magazines and newspapers and has also been broadcast on BBC Radio and Channel Four Television (including a series called Six Experiments that Changed the World, to which I was also a contributor).

Anyway I’ve developed  a bit of a habit of putting up poems with vaguely astronomical themes so when I found this one, I decided to put it up here not least because it made me think of the person who has been causing me so much hassle over the past few days….

Black Hole

You turn all the lights off
but never sleep,
pace round the edge of yourself,
never communicate.
A million dark years distant,
you suck in light like spaghetti.

You got too big for yourself
and collapsed,
but ferocious energy remained
and now you’re back, muscling in
carving out your own space
and time.

You smash up your neighbourhood,
pull the light off stars.
Masked,
you are far outside our laws,
giving nothing away,
stealing everything from everything.

(reproduced with the kind permission of Comma Press).

You can also see a video based on the poem Black Hole from Ghost Code on Vimeo although I have to admit I could make neither head nor tail of it.

The Last Experiment

Posted in Science Politics, The Universe and Stuff with tags , , , on September 16, 2008 by telescoper

I’ve launched myself into the blogosphere just a bit too late for the feeding frenzy surrounding the switching on of the Large Hadron Collider at CERN last week. Obviously the event itself was a bit of a non-event as it will take years for anything interesting to come out the other end of its multi-billion-dollar tunnel. There are a couple of things worth saying in retrospect, though, now that the dust has settled.

The first is about all this nonsense concerning the creation of black holes that could destroy the Earth. If it were possible to create black holes in the LHC then they would be very puny ones, not capable of destroying anything very much at all. The phrase “black hole” conjures up Hollywood-style images of dead stars rampaging through the Galaxy devouring planets and costing a fortune in special effects. But not all black holes are massive enough to be stars in movies. If the LHC could make black holes it would only make very titchy ones. Since the gravitational effect of a black hole depends on its mass – and these little ones have very little of that – any that did pop out of an event in the LHC would be more of a pin-prick than a hole…

Moreover, energetic particles in the form of cosmic rays are constantly raining down on the Earth’s atmosphere, colliding with hadrons as they do so. The most extreme cosmic rays have energies far in excess of the limit that can be reached by the LHC. If an energetic hadron collision were going to produce a black hole that could destroy the planet, it would have happened a very long time ago and we wouldn’t be around to discuss the possibility.

So how did this daft idea come to dominate the news coverage surrounding the switch-on of the LHC? The press are never reluctant to peddle the doomsday scenario whenever they can as it appears to sell newspapers. But there is probably a bit more to it than that. I think part of it is a side-effect of the exaggerated language used by particle physicists in their attempt to use the LHC to capture the public imagination. “The Big Bang Machine” is just one example. If the experiment were really attempting to recreate the Big Bang, then there would indeed be much to be scared about. But the fact of the matter is that the LHC doesn’t reach energies anything like those reached in the Big Bang (nor even in the many smaller bangs that our Universe indulges in from time to time, such as supernova explosions).

The maximum energy reached by the LHC is going to be about 7 TeV (roughly equivalent to the energy of a bumble bee in flight). Although the very earliest stages of the Big Bang itself are not well understood, we are pretty sure that the primordial fireball started off with energies at least a million million (i.e. 1,000,000,000,000) times larger than this. It is doubtful (to say the least) that we’ll ever be able to build a device capable of reaching such energies, so the only “Big Bang Machine” there will ever be is the one we happen to be living in.

This is perhaps the reason why particle physicists are so desperate to glean maximum publicity for the LHC. It’s cost – though not extreme when compared to, for example, military spending – far exceeds that of any other scientific experiment. When it is over, will it be possible to build an even bigger experiment to probe even deeper into the subatomic world? Funding of such experiments generally comes from the public purse and it seems more than likely that the taxpayer will draw the line very soon. Although it won’t destroy the world, perhaps the LHC is nevertheless the end of the line for experimental physics of that kind.

So by all means let’s celebrate the LHC. It’s a wonderful demonstration of what international cooperation can achieve. It is also a response to the need all humans have to ask questions about our Universe. But let us not forget that our ability to probe the inner space of particles with experiments will always be limited, while the outer space beyond the stars offers much wider horizons.

PS. I can’t resist adding this link, as the best example of the worst of the hysteria about the LHC.

PPS. And this one, which explains why the LHC really is safe.