{"id":13150,"date":"2018-03-16T16:23:59","date_gmt":"2018-03-16T23:23:59","guid":{"rendered":"http:\/\/www.preposterousuniverse.com\/blog\/?p=13150"},"modified":"2018-03-16T16:23:59","modified_gmt":"2018-03-16T23:23:59","slug":"stephen-hawkings-scientific-legacy","status":"publish","type":"post","link":"https:\/\/preposterousuniverse.com\/blog\/2018\/03\/16\/stephen-hawkings-scientific-legacy\/","title":{"rendered":"Stephen Hawking&#8217;s Scientific Legacy"},"content":{"rendered":"<p><a href=\"https:\/\/www.nytimes.com\/2018\/03\/14\/obituaries\/stephen-hawking-dead.html\">Stephen Hawking died Wednesday morning<\/a>, age 76. Plenty of memories and tributes have been written, including these by me:<\/p>\n<ul>\n<li>&#8220;<a href=\"https:\/\/www.nytimes.com\/2018\/03\/15\/opinion\/stephen-hawking-quantum-gravity.html\">Stephen Hawking&#8217;s Most Profound Gift to Physics<\/a>,&#8221; in <em>The New York Times<\/em> &#8212; a piece concentrating on black hole evaporation and the information-loss puzzle.<\/li>\n<li>&#8220;<a href=\"https:\/\/www.theatlantic.com\/science\/archive\/2018\/03\/stephen-hawking-sean-carroll-physics-airport\/555764\/\">Stephen Hawking Was Very Particular About His Tea<\/a>,&#8221; in <em>The Atlantic<\/em> &#8212; more focused on our personal interactions and Hawking&#8217;s human side.<\/li>\n<\/ul>\n<p>I can also point to my Story Collider story from a few years ago, about how I turned down a job offer from Hawking, and eventually took lessons from his way of dealing with the world.<\/p>\n<ul>\n<li>&#8220;<a href=\"https:\/\/www.storycollider.org\/stories\/2016\/1\/1\/sean-carroll-what-would-stephen-hawking-do?format=amp\">What Would Stephen Hawking Do?<\/a>&#8220;<\/li>\n<\/ul>\n<p>Of course Hawking has been <a href=\"https:\/\/www.preposterousuniverse.com\/blog\/?s=hawking\">mentioned on this blog<\/a> many times.<\/p>\n<p>When I started writing the above pieces (mostly yesterday, in a bit of a rush), I stumbled across this article I had written several years ago about Hawking&#8217;s scientific legacy. It was solicited by a magazine at a time when Hawking was very ill and people thought he would die relatively quickly &#8212; it wasn&#8217;t the only time people thought that, only to be proven wrong. I&#8217;m pretty sure the article was never printed, and I never got paid for it; so here it is!<\/p>\n<p>(If you&#8217;re interested in a much better description of Hawking&#8217;s scientific legacy by someone who should know, see <a href=\"https:\/\/www.theguardian.com\/science\/2018\/mar\/14\/stephen-hawking-obituary\">this article in <em>The Guardian<\/em> by Roger Penrose<\/a>.)<\/p>\n<p><a href=\"http:\/\/www.businessinsider.com\/stephen-hawking-fulfilled-dream-experiencing-zero-gravity-in-2007-2018-3\"><img loading=\"lazy\" decoding=\"async\" class=\"aligncenter size-full wp-image-13152\" src=\"https:\/\/www.preposterousuniverse.com\/blog\/wp-content\/uploads\/2018\/03\/hawking-weightless.jpg\" alt=\"\" width=\"600\" height=\"300\" srcset=\"https:\/\/preposterousuniverse.com\/blog\/wp-content\/uploads\/2018\/03\/hawking-weightless.jpg 600w, https:\/\/preposterousuniverse.com\/blog\/wp-content\/uploads\/2018\/03\/hawking-weightless-300x150.jpg 300w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><\/p>\n<p><strong><b>Stephen Hawking\u2019s Scientific Legacy<\/b><\/strong><\/p>\n<p>Stephen Hawking is the rare scientist who is also a celebrity and cultural phenomenon. But he is also the rare cultural phenomenon whose celebrity is entirely deserved. His contributions can be characterized very simply: Hawking contributed more to our understanding of gravity than any physicist since Albert Einstein.<\/p>\n<p>\u201cGravity\u201d is an important word here. For much of Hawking\u2019s career, theoretical physicists as a community were more interested in particle physics and the other forces of nature \u2014 electromagnetism and the strong and weak nuclear forces. \u201cClassical\u201d gravity (ignoring the complications of quantum mechanics) had been figured out by Einstein in his theory of general relativity, and \u201cquantum\u201d gravity (creating a quantum version of general relativity) seemed too hard. By applying his prodigious intellect to the most well-known force of nature, Hawking was able to come up with several results that took the wider community completely by surprise.<\/p>\n<p>By acclimation, Hawking\u2019s most important result is the realization that black holes are not completely black \u2014 they give off radiation, just like ordinary objects. Before that famous paper, he proved important theorems about black holes and singularities, and afterward studied the universe as a whole. In each phase of his career, his contributions were central.<\/p>\n<p><!--more--><\/p>\n<p><strong><b>The Classical Period<\/b><\/strong><\/p>\n<p>While working on his Ph.D. thesis in Cambridge in the mid-1960\u2019s, Hawking became interested in the question of the origin and ultimate fate of the universe. The right tool for investigating this problem is general relativity, Einstein\u2019s theory of space, time, and gravity. According to general relativity, what we perceive as \u201cgravity\u201d is a reflection of the curvature of spacetime. By understanding how that curvature is created by matter and energy, we can predict how the universe evolves. This may be thought of as Hawking\u2019s \u201cclassical\u201d period, to contrast classical general relativity with his later investigations in quantum field theory and quantum gravity.<\/p>\n<p>Around the same time, Roger Penrose at Oxford had proven a remarkable result: that according to general relativity, under very broad circumstances, space and time would crash in on themselves to form a <em><i>singularity<\/i><\/em>. If gravity is the curvature of spacetime, a singularity is a moment in time when that curvature becomes infinitely big. This theorem showed that singularities weren\u2019t just curiosities; they are an important feature of general relativity.<\/p>\n<p>Penrose\u2019s result applied to black holes \u2014 regions of spacetime where the gravitational field is so strong that even light cannot escape. Inside a black hole, the singularity lurks in the future. Hawking took Penrose\u2019s idea and turned it around, aiming at the past of our universe. He showed that, under similarly general circumstances, space must have come into existence at a singularity: the Big Bang. Modern cosmologists talk (confusingly) about both the Big Bang \u201cmodel,\u201d which is the very successful theory that describes the evolution of an expanding universe over billions of years, and also the Big Bang \u201csingularity,\u201d which we still don\u2019t claim to understand.<\/p>\n<p>Hawking then turned his own attention to black holes. Another interesting result by Penrose had shown that it\u2019s possible to extract energy from a rotating black hole, essentially by bleeding off its spin until it\u2019s no longer rotating. Hawking was able to demonstrate that, although you can extract energy, the area of the event horizon surrounding the black hole will always increase in any physical process. This \u201carea theorem\u201d was both important in its own right, and also evocative of a completely separate area of physics: thermodynamics, the study of heat.<\/p>\n<p>Thermodynamics obeys a set of famous laws. For example, the first law tells us that energy is conserved, while the second law tells us that entropy \u2014 a measure of the disorderliness of the universe \u2014 never decreases for an isolated system. Working with James Bardeen and Brandon Carter, Hawking proposed a set of laws for \u201cblack hole mechanics,\u201d in close analogy with thermodynamics. Just as in thermodynamics, the first law of black hole mechanics ensures that energy is conserved. The second law is Hawking\u2019s area theorem, that the area of the event horizon never decreases. In other words, the area of the event horizon of a black hole is very analogous to the entropy of a thermodynamic system \u2014 they both tend to increase over time.<\/p>\n<p><strong><b>Black Hole Evaporation<\/b><\/strong><\/p>\n<p>Hawking and his collaborators were justly proud of the laws of black hole mechanics, but they viewed them as simply a formal analogy, not a literal connection between gravity and thermodynamics. In 1972, a graduate student at Princeton University named Jacob Bekenstein suggested that there was more to it than that. Bekenstein, on the basis of some ingenious thought experiments, suggested that the behavior of black holes isn\u2019t simply <em><i>like<\/i><\/em> thermodynamics, it actually <em><i>is<\/i><\/em> thermodynamics. In particular, black holes have entropy.<\/p>\n<p>Like many bold ideas, this one was met with resistance from experts \u2014 and at this point, Stephen Hawking was the world\u2019s expert on black holes. Hawking was certainly skeptical, and for good reason. If black hole mechanics is really just a form of thermodynamics, that means black holes have a temperature. And objects that have a temperature emit radiation \u2014 the famous \u201cblack body radiation\u201d that played a central role in the development of quantum mechanics. So if Bekenstein were right, it would seemingly imply that black holes weren\u2019t really black (although Bekenstein himself didn\u2019t quite go that far).<\/p>\n<p>To address this problem seriously, you need to look beyond general relativity itself, since Einstein\u2019s theory is purely \u201cclassical\u201d \u2014 it doesn\u2019t incorporate the insights of quantum mechanics. Hawking knew that Russian physicists Alexander Starobinsky and Yakov Zel\u2019dovich had investigated quantum effects in the vicinity of black holes, and had predicted a phenomenon called \u201csuperradiance.\u201d Just as Penrose had showed that you could extract energy from a spinning black hole, Starobinsky and Zel\u2019dovich showed that rotating black holes could emit radiation spontaneously via quantum mechanics. Hawking himself was not an expert in the techniques of quantum field theory, which at the time were the province of particle physicists rather than general relativists. But he was a quick study, and threw himself into the difficult task of understanding the quantum aspects of black holes, so that he could find Bekenstein\u2019s mistake.<\/p>\n<p>Instead, he surprised himself, and in the process turned theoretical physics on its head. What Hawking eventually discovered was that Bekenstein was right \u2014 black holes <em><i>do<\/i><\/em> have entropy \u2014 and that the extraordinary implications of this idea were actually true \u2014 black holes are not completely black. These days we refer to the \u201cBekenstein-Hawking entropy\u201d of black holes, which emit \u201cHawking radiation\u201d at their \u201cHawking temperature.\u201d<\/p>\n<p>There is a nice hand-waving way of understanding Hawking radiation. Quantum mechanics says (among other things) that you can\u2019t pin a system down to a definite classical state; there is always some intrinsic uncertainty in what you will see when you look at it. This is even true for empty space itself \u2014 when you look closely enough, what you thought was empty space is really alive with \u201cvirtual particles,\u201d constantly popping in and out of existence. Hawking showed that, in the vicinity of a black hole, a pair of virtual particles can be split apart, one falling into the hole and the other escaping as radiation. Amazingly, the infalling particle has a negative energy as measured by an observer outside. The result is that the radiation gradually takes mass away from the black hole \u2014 it evaporates.<\/p>\n<p>Hawking\u2019s result had obvious and profound implications for how we think about black holes. Instead of being a cosmic dead end, where matter and energy disappear forever, they are dynamical objects that will eventually evaporate completely. But more importantly for theoretical physics, this discovery raised a question to which we still don\u2019t know the answer: when matter falls into a black hole, and then the black hole radiates away, where does the information go?<\/p>\n<p>If you take an encyclopedia and toss it into a fire, you might think the information contained inside is lost forever. But according to the laws of quantum mechanics, it isn\u2019t really lost at all; if you were able to capture every bit of light and ash that emerged from the fire, in principle you could exactly reconstruct everything that went into it, even the print on the book pages. But black holes, if Hawking\u2019s result is taken at face value, seem to destroy information, at least from the perspective of the outside world. This conundrum is the \u201cblack hole information loss puzzle,\u201d and has been nagging at physicists for decades.<\/p>\n<p>In recent years, progress in understanding quantum gravity (at a purely thought-experiment level) has convinced more people that the information really is preserved. In 1997 Hawking made a bet with American physicists Kip Thorne and John Preskill; Hawking and Thorne said that information was destroyed, Preskill said that somehow it was preserved. In 2007 Hawking conceded his end of the bet, admitting that black holes don\u2019t destroy information. However, Thorne has not conceded for his part, and Preskill himself thinks the concession was premature. Black hole radiation and entropy continue to be central guiding principles in our search for a better understanding of quantum gravity.<\/p>\n<p><strong><b>Quantum Cosmology<\/b><\/strong><\/p>\n<p>Hawking\u2019s work on black hole radiation relied on a mixture of quantum and classical ideas. In his model, the black hole itself was treated classically, according to the rules of general relativity; meanwhile, the virtual particles near the black hole were treated using the rules of quantum mechanics. The ultimate goal of many theoretical physicists is to construct a true theory of quantum gravity, in which spacetime itself would be part of the quantum system.<\/p>\n<p>If there is one place where quantum mechanics and gravity both play a central role, it\u2019s at the origin of the universe itself. And it\u2019s to this question, unsurprisingly, that Hawking devoted the latter part of his career. In doing so, he established the agenda for physicists\u2019 ambitious project of understanding where our universe came from.<\/p>\n<p>In quantum mechanics, a system doesn\u2019t have a position or velocity; its state is described by a \u201cwave function,\u201d which tells us the probability that we would measure a particular position or velocity if we were to observe the system. In 1983, Hawking and James Hartle published a paper entitled simply \u201cWave Function of the Universe.\u201d They proposed a simple procedure from which \u2014 in principle! \u2014 the state of the entire universe could be calculated. We don\u2019t know whether the Hartle-Hawking wave function is actually the correct description of the universe. Indeed, because we don\u2019t actually have a full theory of quantum gravity, we don\u2019t even know whether their procedure is sensible. But their paper showed that we could talk about the very beginning of the universe in a scientific way.<\/p>\n<p>Studying the origin of the universe offers the prospect of connecting quantum gravity to observable features of the universe. Cosmologists believe that tiny variations in the density of matter from very early times gradually grew into the distribution of stars and galaxies we observe today. A complete theory of the origin of the universe might be able to predict these variations, and carrying out this program is a major occupation of physicists today. Hawking made a number of contributions to this program, both from his wave function of the universe and in the context of the \u201cinflationary universe\u201d model proposed by Alan Guth.<\/p>\n<p>Simply talking about the origin of the universe is a provocative step. It raises the prospect that science might be able to provide a complete and self-contained description of reality \u2014 a prospect that stretches beyond science, into the realms of philosophy and theology. Hawking, always provocative, never shied away from these implications. He was fond of recalling a cosmology conference hosted by the Vatican, at which Pope John Paul II allegedly told the assembled scientists not to inquire into the origin of the universe, \u201cbecause that was the moment of creation and therefore the work of God.\u201d Admonitions of this sort didn\u2019t slow Hawking down; he lived his life in a tireless pursuit of the most fundamental questions science could tackle.<\/p>\n<p>&nbsp;<\/p>\n","protected":false},"excerpt":{"rendered":"<p>Stephen Hawking died Wednesday morning, age 76. Plenty of memories and tributes have been written, including these by me: &#8220;Stephen Hawking&#8217;s Most Profound Gift to Physics,&#8221; in The New York Times &#8212; a piece concentrating on black hole evaporation and the information-loss puzzle. &#8220;Stephen Hawking Was Very Particular About His Tea,&#8221; in The Atlantic &#8212; [&hellip;]<\/p>\n","protected":false},"author":4,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[28],"tags":[],"class_list":["post-13150","post","type-post","status-publish","format-standard","hentry","category-science"],"jetpack_featured_media_url":"","_links":{"self":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13150","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/users\/4"}],"replies":[{"embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/comments?post=13150"}],"version-history":[{"count":2,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13150\/revisions"}],"predecessor-version":[{"id":13153,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/posts\/13150\/revisions\/13153"}],"wp:attachment":[{"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/media?parent=13150"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/categories?post=13150"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/preposterousuniverse.com\/blog\/wp-json\/wp\/v2\/tags?post=13150"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}