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Category Archives: 14. Black holes: the end of space and time

The detection of gravitational waves by the Laser Interferometer Gravitational-wave Observatory (LIGO) opens up a method of observing the universe in a new way, perhaps comparable to the advance that Galileo made in 1609 by turning a telescope on the heavens.

Many awards have been given to Kip Thorne of Caltech, who worked out the theory decades ago, and the people who built LIGO over 40 years until its success in 2015.  The 2017 Nobel Prize in Physics was awarded to Prof. Thorne as well as Barry Barish and Rainer Weiss, half to Weiss for beginning the actual method of the successful observations and one quarter each to the other two.


Credit: Niklas Elmehed/The Nobel Prize Foundation


The October 3, 2017, award of the Nobel Prize was much anticipated and is well deserved.  See, for example, coverage in the following media:
Physics Today
NY Times
The Atlantic provides a thoughtful reflection on the merits of the Nobel prize.


The Laser Interferometer Gravitational Wave Observatory (LIGO) was a topic of NPR’s Morning Edition, August 17, 2016.


Credit: Caltech/MIT/LIGO Lab

Read a full transcript or listen to the broadcast here (5 min 30s).

On June 15, 2016, scientists at LIGO announced they had detected a second pair of black holes merging, at a distance of 1.4 billion light years, releasing the energy equivalent of the mass of the Sun. The consequent ripples in space-time shook the twin detectors at LIGO on December 26, 2015.

This brings the number of confirmed detections by LIGO to two within just four months, giving scientists optimism that more events will follow, enabling quantitative predictions about how frequently these high-energy events occur across space and time.

Read more in this NYT article, including a short video.

An article in the New York Times by Dennis Overbye gives the latest chapter in Stephen Hawking’s saga concerning whether the properties of matter that has fallen into a black hole are lost forever, or whether there is a way out.

Forty years ago, Hawking showed theoretically that black holes were not ‘eternal prisons’ but could leak radiation. There ensued a long-running debate about whether this radiation retained any information or attributes of the original matter. If it does not, this violates a tenet of modern physics, that it is always possible, in theory, to reverse time. This became known as the ‘information paradox’ and was the subject of a famous bet between Hawking and Caltech professor John Preskill. (Hawking conceded defeat 10 years ago, admitting that advances in string theory, had left no room in the universe for information loss.)

In a paper published to be published this week in Physical Review Letters, Hawking and his colleagues Andrew Strominger (Harvard) and Malcolm Perry (Cambridge) announce they have found a clue pointing the way out of black holes. They new results undermine John Wheeler’s famous notion that black holes have “no hair” — that they are shorn of the essential properties of the things they have consumed.

Looked at from the right vantage point — from a far distance in time, technically known as “null infinity” — black holes might not be not be bald at all. A tell-tale pattern of light rays bordering the event horizon contains information about what has passed through. This has been dubbed in their paper a “soft hair” theory.

For a more complete description of this story, see the full NYT article.

In a subsequent article, Dennis Overbye answers questions on black holes submitted by his readers.


An update for Section 13.3f (pp. 352-355) in Pasachoff & Filippenko, The Cosmos, 4th ed:
In what many think is the most important step in astronomy since Galileo first turned a telescope on the heavens in 1609, ripples in space-time known as gravitational waves were detected on September 14, 2015, and reported to the world at large on February 11, 2016. Scientists gathered all over in auditoriums at the National Press Club in Washington, at Caltech, at MIT, in Moscow, and in many other places for the epochal event. “Ladies and gentlemen, we have detected gravitational waves,” said the executive director of the LIGO Laboratory. “We did it!”

The observational existence of gravitational radiation had been established by the changing period of the Hulse-Taylor “double pulsar” at a rate that matched the energy loss expected from the emission of gravitational waves (Section 13.3f, pp. 352-354), but the gravitational waves themselves – distortions of space-time – had not been detected directly until the Advanced LIGO (Laser Interferometer Gravitational Wave Detector) picked up a signal in its engineering run on September 14, 2015.  The signal, which could be heard as an upwards “chirp” in the audio range, resulted from two black holes 1.3 billion light years away, each containing about 30 solar masses, spiraling rapidly into each other and merging to become a single black hole with almost the same total mass but with 3 solar masses of material converted into energy in the form of gravitational waves.

Advanced LIGO has about 3 times the sensitivity of the earlier LIGO, and this event was within that advance of sensitivity. A further gain of 3x is expected with Advanced LIGO in the future.

Scientific papers were published February 11, 2016, in the Physical Review Letters and in the Astrophysical Journal Letters.

A few days later, I [JMP] attended a two-hour evening session at Caltech’s main auditorium to hear the principals of the LIGO project speaking, and I had the feeling that it was just like being at a meeting of the Lincei Academy in Florence in 1610 to hear Galileo speak about his new discoveries with the [optical] telescope.

There are many links to discussions and animations available. Here are some of them:

Popular and academic press:
New York Times coverage (includes movie); also NYT Opinion piece and an NYT Editorial a few days later, justifying scientific inquiry; a congressman’s letter, and sound bites from scientists.
Nature coverage (with explanatory graphics)
Science magazine in depth (requires subscription to access full text)
New Yorker article (with an account of LIGO’s inception and development)
Physics Today comparison of gravitational waves and sound waves
The Wall Street Journal and Michio Kaku’s desciption.

Societies and organizations:
American Physical Society Viewpoint
The Kavli Foundation Scientific spotlight
Caltech (press releasesillustrations, movies and animations)
NSF press release
AAPT resources on gravitational waves
STFC (UK) press release
Cornell Chronicle and media statement
University of Texas Rio Grande Valley press release
ESA congratulations
CSIRO (Australia) news release

LIGO websites:
LIGO labs (Observatories: Livingston | Hanford); Advanced LIGO; LIGO Scientific Collaboration; LIGO Partner Experiments and Collaborations






From a press release of the European Space Agency:

Over the past week, ESA’s Integral satellite has been observing an exceptional outburst of high-energy light produced by a black hole that is devouring material from its stellar companion.


Credit and copyright: ESA/ATG medialab

X-rays and gamma rays point to some of the most extreme phenomena in the Universe, such as stellar explosions, powerful outbursts and black holes feasting on their surroundings. In contrast to the peaceful view of the night sky we see with our eyes, the high-energy sky is a dynamic light show, from flickering sources that change their brightness dramatically in a few minutes to others that vary on timescales spanning years or even decades.

On 15 June 2015, a long-time acquaintance of X-ray and gamma ray astronomers made its comeback to the cosmic stage: V404 Cygni, a system comprising a black hole and a star orbiting one another. It is located in our Milky Way galaxy, almost 8000 light-years away in the constellation Cygnus, the Swan. In this type of binary system, material flows from the star towards the black hole and gathers in a disc, where it is heated up, shining brightly at optical, ultraviolet and X-ray wavelengths before spiralling into the black hole.

The V404 Cygni black hole system has not been this bright and active since 1989, when it was observed with the Japanese X-ray satellite Ginga and high-energy instruments on board the Mir space station.

Link: the full ESA press release.

Here is a consolidated list of errors from the text’s first printing. Many of these have already been posted here as separate chapter updates. (Our publisher will make the necessary corrections to the printed book at the earliest opportunity.):

p. 25, Figure It Out 2.3: The last paragraph (about Fraunhofer) shouldn’t be there. Instead, it should be at the end of the caption of Figure 2-4 on p. 26.

p. 64, Q34: 1 Angstrom should be listed as 1010 m, not 108 m.

p. 64, Q41: Ditto

p. 78: There is an error in the equation relating the apparent magnitude and brightness of stars in Figure It Out 4.1.  In this equation, 2.512 should be raised to a power equal to (mB−mA).

p. 92, Q1: We could more clearly say “On the top picture” instead of just “On the picture” – since there are now two pictures on the opening page of the chapter (and the stars are somewhat too dense for individual clarity in the bottom picture).

p. 190: First sentence of Section 7.4d: “a little larger” should be “a little smaller” for the relative sizes of Triton and the Moon.

p. 309, Q53, there is a printing error when going from the bottom of column 1 to the top of column 2. At the top of column 2, the “(e)” should be boldface, there should be a period after “1/16”, and the remainder of the text should be deleted.

p. 363, column 1, second line from the bottom: When referring to the event horizon: “1/3” should be “2/3”, i.e. the sentence should read “Its radius is exactly 2/3 times that of the photon sphere…”

p. 391, column 2, second line from the bottom: for Spitzer, “Section 3.8c, Figure 3-32a).” should say simply “Section 3.8c).”

p. 392, column 1, line 5: At the end, add “(See Section 3.8c, Figure 3-32a.)”

Appendix 3C, column 3 header: “105 km” should be “106 km”

Adapted from a UCLA press release, November 3, 2014.

For years, astronomers have been puzzled by a bizarre object in the center of the Milky Way that was believed to be a hydrogen gas cloud headed toward our galaxy’s enormous black hole. (See Section 15.5, Chapter opener figure, p. 382, and Figure 15-5, p. 388.)

Having studied it during its closest approach to the black hole this summer, UCLA astronomers believe that they have solved the riddle of the object widely known as G2.

A team led by Andrea Ghez determined that G2 is most likely a pair of binary stars that had been orbiting the black hole in tandem and merged together into an extremely large star, cloaked in gas and dust – its movements choreographed by the black hole’s powerful gravitational field. The research is published today in the journal Astrophysical Journal Letters.

Astronomers had figured that if G2 had been a hydrogen cloud, it could have been torn apart by the black hole, and that the resulting celestial fireworks would have dramatically changed the state of the black hole. However, G2 survived and continues on its orbit unaffected.

G2 appears to be just one of an emerging class of stars near the black hole that are created because the black hole’s powerful gravity drives binary stars to merge into one. In our galaxy, massive stars primarily come in pairs. The star suffered an abrasion to its outer layer but otherwise will be fine.

Keck Observatory

Credit and copyright: Ethan Tweedie Photography

The team utilized the Keck Observatory’s adaptive optics technology, a powerful technology that corrects the distorting effects of the Earth’s atmosphere in real time to more clearly reveal the space around the supermassive black hole.

Links: full UCLA press release, Keck press release.

There is a typographical error on p. 363, column 1, second line from the bottom, when referring to the event horizon: ‘1/3’ should be ‘2/3’ in the sentence:
“Its radius is exactly 2/3 times that of the photon sphere, or 3 km for each solar mass.”

Stephen Hawking, very famous for his ideas about black holes seeming to emit radiation (dubbed ‘Hawking radiation’) and also well known for his ability to function as a theoretical astrophysicist in the face of tremendous personal disability, has made some recent statements in a new paper that seem to challenge the nature of black holes’ event horizons (see Section 14.3, p. 363).

Credit and copyright: Alain Riazuelo

Marek Demianski, a cosmologist at Warsaw University and often a visiting professor at Williams College, evaluated the controversy as follows (February 5, 2014):”Yes, I read this short paper of Hawking. It is really nothing new; when one takes into account quantum effects a black hole is not black any more. Unfortunately we do not have yet a quantum theory of gravity so all these considerations are only hypothetical. From our practical point of view, quantum effects do not influence properties of astrophysical black holes.”

It looks like this theoretical debate is set to continue.

Links: accessible New Scientist article and overview of the Firewall Paradox; New Republic overview; Nature news article; Preprint of Stephen Hawking’s paper.