Steven S. Gubser: Thunder and Lightning from Neutron Star mergers

As of late 2015, we have a new way of probing the cosmos: gravitational radiation. Thanks to LIGO (the Laser Interferometer Gravitational-wave Observatory) and its new sibling Virgo (a similar interferometer in Italy), we can now “hear” the thumps and chirps of colliding massive objects in the universe. Not for nothing has this soundtrack been described by LIGO scientists as “the music of the cosmos.” This music is at a frequency easily discerned by human hearing, from somewhat under a hundred hertz to several hundred hertz. Moreover, gravitational radiation, like sound, is wholly different from light. It is possible for heavy dark objects like black holes to produce mighty gravitational thumps without at the same time emitting any significant amount of light. Indeed, the first observations of gravitational waves came from black hole merger events whose total power briefly exceeded the light from all stars in the known universe. But we didn’t observe any light from these events at all, because almost all their power went into gravitational radiation.

In August 2017, LIGO and Virgo observed a collision of neutron stars which did produce observable light, notably in the form of gamma rays. Think of it as cosmic thunder and lightning, where the thunder is the gravitational waves and the lightning is the gamma rays. When we see a flash of ordinary lightning, we can count a few seconds until we hear the thunder. Knowing that sound travels one mile in about five seconds, we can reckon how distant the event is. The reason this method works is that light travels much faster than sound, so we can think of the transmission of light as instantaneous for purposes of our estimate.

Things are very different for the neutron star collision, in that the event took place about 130 million light years away, but the thunder and lightning arrived on earth pretty much simultaneously. To be precise, the thunder was first: LIGO and Virgo heard a basso rumble rising to a characteristic “whoop,” and just 1.7 seconds later, the Fermi and INTEGRAL experiments observed gamma ray bursts from a source whose location was consistent with the LIGO and Virgo observations. The production of gamma rays from merging neutron stars is not a simple process, so it’s not clear to me whether we can pin that 1.7 seconds down as a delay precisely due to the astrophysical production mechanisms; but at least we can say with some confidence that the propagation time of light and gravity waves are the same to within a few seconds over 130 million light years. From a certain point of view, that amounts to one of the most precise measurements in physics: the ratio of the speed of light to the speed of gravity equals 1, correct to about 14 decimal places or better.

The whole story adds up much more easily when we remember that gravitational waves are not sound at all. In fact, they’re nothing like ordinary sound, which is a longitudinal wave in air, where individual air molecules are swept forward and backward just a little as the sound waves pass them by. Gravitational waves instead involve transverse disturbances of spacetime, where space is stretched in one direction and squeezed in another—but both of those stretch-squeeze directions are at right angles to the direction of the wave. Light has a similar transverse quality: It is made up of electric and magnetic fields, again in directions that are at right angles to the direction in which the light travels. It turns out that a deep principle underlying both Maxwell’s electromagnetism and Einstein’s general relativity forces light and gravitational waves to be transverse. This principle is called gauge symmetry, and it also guarantees that photons and gravitons are massless, which implies in turn that they travel at the same speed regardless of wavelength.

It’s possible to have transverse sound waves: For instance, shearing waves in crystals are a form of sound. They typically travel at a different speed from longitudinal sound waves. No principle of gauge symmetry forbids longitudinal sound waves, and indeed they can be directly observed, along with their transverse cousins, in ordinary materials like metals. The gauge symmetries that forbid longitudinal light waves and longitudinal gravity waves are abstract, but a useful first cut at the idea is that there is extra information in electromagnetism and in gravity, kind of like an error-correcting code. A much more modest form of symmetry is enough to characterize the behavior of ordinary sound waves: It suffices to note that air (at macroscopic scales) is a uniform medium, so that nothing changes in a volume of air if we displace all of it by a constant distance.

In short, Maxwell’s and Einstein’s theories have a feeling of being overbuilt to guarantee a constant speed of propagation. And they cannot coexist peacefully as theories unless these speeds are identical. As we continue Einstein’s hunt for a unified theory combining electromagnetism and gravity, this highly symmetrical, overbuilt quality is one of our biggest clues.

The transverse nature of gravitational waves is immediately relevant to the latest LIGO / Virgo detection. It is responsible for the existence of blind spots in each of the three detectors (LIGO Hanford, LIGO Livingston, and Virgo). It seems like blind spots would be bad, but they actually turned out to be pretty convenient: The signal at Virgo was relatively weak, indicating that the direction of the source was close to one of its blind spots. This helped localize the event, and localizing the event helped astronomers home in on it with telescopes. Gamma rays were just the first non-gravitational signal observed: the subsequent light-show from the death throes of the merging neutron stars promises to challenge and improve our understanding of the complex astrophysical processes involved. And the combination of gravitational and electromagnetic observations will surely be a driver of new discoveries in years and decades to come.

 

BlackSteven S. Gubser is professor of physics at Princeton University and the author of The Little Book of String TheoryFrans Pretorius is professor of physics at Princeton. They both live in Princeton, New Jersey. They are the authors of The Little Book of Black Holes.

Read like a Nobel Prize-winning physicist

This morning Princeton University Press was thrilled to congratulate PUP author and celebrated physicist Kip Thorne on being a co-winner of the Nobel Prize in Physics for 2017. Dr. Thorne’s research has focused on Einstein’s general theory of relativity and astrophysics, with emphasis on relativistic stars, black holes, and especially gravitational waves. The latter observation, made in September 2015, validated a key prediction of Einstein’s general theory of relativity. Princeton University Press is honored to be the publisher of Dr. Thorne’s Modern Classical Physics, co-authored with Roger Blandford, and the new hardback edition of the renowned classic, Gravitation, co-authored with Charles Misner and the late John Wheeler, forthcoming this fall.

Over the years, we’ve published several Nobel winners, including:

  • Einstein
  • Richard Feynman (QED)
  • P.W. Anderson (the classic and controversial Theory of Superconductivity in the High-Tc Cuprates)
  • Paul Dirac (General Theory of Relativity)
  • Werner Heisenberg (Encounters with Einstein)

Interested in learning more about physics yourself? We put together the ultimate Nobel reading list. Click the graphic for links to each book.

PUP congratulates Kip S. Thorne, joint winner of the Nobel Prize in Physics

New Books Gravitation and Modern Classical Physics Publishing in October 2017

Princeton, NJ, October 3, 2017—Upon today’s announcement that Dr. Kip S. Thorne is the joint winner of the Nobel Prize in Physics for 2017, Princeton University Press would like to extend hearty congratulations to the celebrated physicist.

The Royal Swedish Academy recognizes Dr. Thorne, along with Rainer Weiss and Barry C. Barish, for decisive contributions to the LIGO detector and the observation of gravitational waves”.

Feynman Professor of Theoretical Physics, Emeritus at the California Institute of Technology, Dr. Thorne has focused his research on Einstein’s general theory of relativity and on astrophysics, with emphasis on relativistic stars, black holes, and especially gravitational waves. The latter observation, made in September 2015, validated a key prediction of Einstein’s general theory of relativity.

Princeton University Press is honored to be the publisher of Dr. Thorne’s Modern Classical Physics, co-authored with Roger Blandford, and the new hardback edition of the renowned classic, Gravitation, co-authored with Charles Misner and the late John Wheeler, publishing in October 2017.

According to Christie Henry, director of Princeton University Press, “Dr. Thorne’s creativity and brilliance have been as grounding to Princeton University Press’s publishing program in the physical sciences as gravitation is to the human experience.  His recently released Princeton University Press contributions, Gravitation and Modern Classical Physics, are vital to our mission of illuminating spheres of knowledge to advance and enrich the human conversation, and today we celebrate his commitment to science with the Nobel committee and readers across the universe.”

Since the publication of Albert Einstein’s The Meaning of Relativity in 1922, Princeton University Press has remained committed to publishing global thought leaders in the sciences and beyond. We are honored to count Dr. Thorne’s work as part of this legacy.

Thorne

For more information, please contact:

Julia Haav, Assistant Publicity Director

Julia_Haav@press.princeton.edu, 609.258.2831

 

Neil DeGrasse Tyson & Stephen Colbert: Make America Smart Again

On November 9, Neil DeGrasse Tyson joined Stephen Colbert on The Late Show to talk about Welcome to the Universe and to blow his own mind. Watch the clip here:

 

Gravitational waves making waves at Princeton

Today marks a new era in cosmology, astronomy, and astrophysics. The main page of the Einstein Papers Project website reports, “Gravitational waves do exist, as has been announced today with great joy by the scientists of the LIGO collaboration, after more than two decades of intensive experimental work.”

The cosmic breakthrough, which proves Einstein’s 100 year old prediction, has resulted in a tremendous response across the scientific community and social media. Scientific websites everywhere are already debating the meaning of the discovery, the #EinsteinWasRight hashtag has been bantered about on Twitter; You Tube featured a live announcement with over 80,000 people tuning in to watch (check it out at 27 minutes).

 

 

Princeton University Press authors Jeremiah Ostriker and Kip Thorne had a bet about gravitational wave detection in the 80s. Today when we contacted him, Ostriker, author of Heart of Darkness, was ebullient:

“The LIGO announcement today and the accompanying papers are totally persuasive. We all believed that Einstein had to be right in predicting gravitational waves, but to see them, so clean and so clear is marvelous. Two independent instruments saw the same signal from the same event, and it was just what had been predicted for the in-spiral and merger of two massive black holes.

A quarter of a century ago I had a bet with Kip Thorne that we would not see gravitational waves before the year 2000 – and I won that bet and a case of wine. But I did not doubt that, when the sensitivity of the instruments improved enough, gravitational waves would be found.  Now the skill and perseverance of the experimentalists and the support of NSF has paid off.

Hats off to all!!!”

But was Einstein always a believer in gravitational waves? Daniel Kennefick, co-author of The Einstein Encyclopedia says no:

“One hundred years ago in February 1916, Einstein mentioned gravitational waves for the first time in writing. Ironically it was to say that they did not exist. He said this in a letter to his colleague Karl Schwarzschild, who had just discovered the solution to Einstein’s equations which we now know describe black holes. Today brings a major confirmation of the existence both of gravitational waves and black holes. Yet Einstein was repeatedly skeptical about whether either of these ideas were really predictions of his theory. In the case of gravitational waves he soon changed his mind in 1916 and by 1918 had presented the first theory of these waves which still underpins our understanding of how the LIGO detectors work. But in 1936 he changed his mind again, submitting a paper to the Physical Review called “Do Gravitational Waves Exist?” in which he answered his own question in the negative. The editor of the journal responded by sending Einstein a critical referee’s report and Einstein angrily withdrew the paper and resubmitted it elsewhere. But by early the next year he had changed his mind again, completely revising the paper to present one of the first exact solutions for gravitational waves in his theory. So his relationship with gravitational waves was very far from the image of the cocksure, self-confident theorist which dominates so many stories about Einstein. Because of this, he would have been thrilled today, if he were still alive, to have this major confirmation of some of the most esoteric predictions of his theory.”

Here at Princeton University Press where we recently celebrated the 100th anniversary of Einstein’s theory of general relativity, the mood has been celebratory to say the least. If you’d like to read the Einstein Papers volumes that refer to his theory of gravitational waves, check out Document 32 in Volume 6, and Volume 7, which focuses on the theory. Or, kick off your own #EinsteinWasRight celebration by checking out some of our other relevant titles.

Traveling at the Speed of Thought: Einstein and the Quest for Gravitational Waves
by Daniel Kennefick

j8387

Relativity: The Special and the General Theory, 100th Anniversary Edition
by Albert Einstein

relativity 100 years

The Meaning of Relativity: Including the Relativistic Theory of the Non-Symmetric Field
by Albert Einstein

j484

Einstein Gravity in a Nutshell
by A. Zee

Zee_EinsteinGravityNutshell

The Road to Relativity: The History and Meaning of Einstein’s “The Foundation of General Relativity” Featuring the Original Manuscript of Einstein’s Masterpiece
by Hanoch Gutfreund & Jürgen Renn.

The Road to Relativity

The Curious History of Relativity: How Einstein’s Theory of Gravity Was Lost and Found Again
by Jean Eisenstaedt

the curious history of relativity jacket

 An Einstein Encyclopedia
by Alice Calaprice, Daniel Kennfick, & Robert Sculmann

Calaprice_Einstein_Encyclopedia

Gravitation and Inertia
by Ignazio Ciufolini & John Archibald Wheeler

gravity and inertia jacket

Einstein’s Jury: The Race to Test Relativity
by Jeffrey Crelinsten

einstein's jury jacket

What Does a Black Hole Look Like?
by Charles D. Bailyn

black hole

Dynamics and Evolution of Galactic Nuclei
by David Merritt

dynamics and evolution of galactic nuclei

The Global Nonlinear Stability of the Minkowski Space (PMS-41)
by Demetrios Christodoulou & Sergiu Klainerman

the global nonlinear stability of the minkowski space

Modern Classical Physics: Optics, Fluids, Plasmas, Elasticity, Relativity, and Statistical Physics
by Kip S. Thorne & Roger D. Blandford

modern classical physics

The Collected Papers of Albert Einstein, Volume 7: The Berling Years: Writings, 1918-1921
by Albert Einstein

albert einstein