{"id":640581,"date":"2023-04-23T09:56:11","date_gmt":"2023-04-23T14:56:11","guid":{"rendered":"https:\/\/news.sellorbuyhomefast.com\/index.php\/2023\/04\/23\/researchers-argue-black-holes-will-destroy-all-quantum-states\/"},"modified":"2023-04-23T09:56:11","modified_gmt":"2023-04-23T14:56:11","slug":"researchers-argue-black-holes-will-destroy-all-quantum-states","status":"publish","type":"post","link":"https:\/\/newsycanuse.com\/index.php\/2023\/04\/23\/researchers-argue-black-holes-will-destroy-all-quantum-states\/","title":{"rendered":"Researchers Argue Black Holes Will Destroy All Quantum States"},"content":{"rendered":"
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At Princeton University<\/span> in the early 1970s, the celebrated theoretical physicist John Wheeler could be spotted in seminars or impromptu hallway discussions drawing a big \u201cU.\u201d The letter\u2019s left tip represented the beginning of the universe, where everything was uncertain and all quantum possibilities were happening at the same time. The letter\u2019s right tip, sometimes adorned with an eye, depicted an observer looking back in time, thus bringing the left side of the U into existence.<\/p>\n

In this \u201cparticipatory universe,\u201d as Wheeler called it, the cosmos expanded and cooled around the U, forming structures and eventually creating observers, like humans and measuring apparatus. By looking back to the early universe, these observers somehow made it real.<\/p>\n

\u201cHe would say things like \u2018No phenomenon is a true phenomenon until it\u2019s an observed phenomenon,\u2019\u201d said Robert M. Wald<\/a>, a theoretical physicist at the University of Chicago who was Wheeler\u2019s doctoral student at the time.<\/p>\n

Now, by studying how quantum theory behaves on the horizon of a black hole, Wald and his collaborators have calculated a new effect that is suggestive of Wheeler\u2019s participatory universe. The mere presence of a black hole, they\u2019ve found, is enough to turn a particle\u2019s hazy \u201csuperposition\u201d\u2014the state of being in multiple potential states\u2014into a well-defined reality. \u201cIt evokes the idea that these black hole horizons are watching,\u201d said coauthor Gautam Satishchandran<\/a>, a theoretical physicist at Princeton.<\/p>\n

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John Wheeler\u2019s \u201cparticipatory universe\u201d suggests that observers make the universe real.<\/p>\n

<\/span>Illustration: Samuel Velasco\/Quanta Magazine; adapted from John Wheeler<\/span><\/p>\n<\/figure>\n<\/div>\n

\u201cWhat we have found might be a quantum mechanical realization of [the participatory universe], but where space-time itself plays the role of the observer,\u201d said Daine Danielson<\/a>, the third author, also at Chicago.<\/p>\n

Theorists are now debating what to read into these watchful black holes. \u201cThis seems to be telling us something deep about the way gravity influences measurement in quantum mechanics,\u201d said Sam Gralla<\/a>, a theoretical astrophysicist at the University of Arizona. But whether this will prove useful for researchers inching toward a complete theory of quantum gravity is still anyone\u2019s guess.<\/p>\n

The effect is one of many uncovered in the past decade by physicists studying what happens when quantum theory is combined with gravity at low energies. For example, theorists have had great success thinking about Hawking radiation<\/a>, which causes black holes to slowly evaporate. \u201cSubtle effects that we hadn\u2019t really noticed before give us constraints from which we can glean clues about how to go up toward quantum gravity,\u201d said Alex Lupsasca<\/a>, a theoretical physicist at Vanderbilt University who was not involved in the new research.<\/p>\n

These observant black holes seem to produce an effect that\u2019s \u201cvery arresting,\u201d Lupsasca said, \u201cbecause it feels like somehow it\u2019s deep.\u201d<\/p>\n

Black Holes and Superpositions<\/p>\n

To understand how a black hole could observe the universe, start small. Consider the classic double-slit experiment, in which quantum particles are fired toward two slits in a barrier. Those that pass through are then detected by a screen on the other side.<\/p>\n

At first, each traveling particle seems to appear at random on the screen. But as more particles pass through the slits, a pattern of light and dark stripes emerges. This pattern suggests that each particle behaves like waves that pass through both slits at once. The bands result from the peaks and troughs of the waves either adding together or canceling one another out\u2014a phenomenon called interference.<\/p>\n

Now add a detector to measure which of the two slits the particle passes through. The pattern of light and dark stripes will disappear. The act of observation changes the state of the particle\u2014its wavelike nature disappears entirely. Physicists say that the information gained by the detection apparatus \u201cdecoheres\u201d the quantum possibilities into a definite reality.<\/p>\n<\/div>\n

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Importantly, your detector doesn\u2019t have to be close to the slits to figure out which path the particle took. A charged particle, for example, emits a long-range electric field that might have slightly different strengths depending on whether it went through the right-hand or left-hand slit. Measuring this field from far away will still allow you to gather information about which path the particle took and will thus cause decoherence.<\/p>\n

In 2021, Wald, Satishchandran, and Danielson were exploring a paradox brought about when hypothetical observers gather information in this way<\/a>. They imagined an experimenter called Alice who creates a particle in a superposition. At a later time, she looks for an interference pattern. The particle will only exhibit interference if it hasn\u2019t become too entangled with any outside system while Alice observes it.<\/p>\n

Then along comes Bob, who is attempting to measure the particle\u2019s position from far away by measuring the particle\u2019s long-range fields. According to the rules of causality, Bob shouldn\u2019t be able to influence the outcome of Alice\u2019s experiment, since the experiment should be over by the time the signals from Bob get to Alice. However, by the rules of quantum mechanics, if Bob does successfully measure the particle, it will become entangled with him, and Alice won\u2019t see an interference pattern.<\/p>\n

The trio rigorously calculated that the amount of decoherence due to Bob\u2019s actions is always less than the decoherence that Alice would naturally cause by the radiation she emits (which also becomes entangled with the particle). So Bob could never decohere Alice\u2019s experiment, because she would already have decohered it herself. Although an earlier version of this paradox was resolved in 2018<\/a> with a back-of-the-envelope calculation by Wald and a different team of researchers, Danielson took it one step further.<\/p>\n

He posed a thought experiment to his collaborators: \u201cWhy can\u2019t I put [Bob\u2019s] detector behind a black hole?\u201d In such a setup, a particle in a superposition outside the event horizon will emanate fields that cross over the horizon and get detected by Bob on the other side, within the black hole. The detector gains information about the particle, but as the event horizon is a \u201cone-way ticket,\u201d no information can cross back over, Danielson said. \u201cBob cannot influence Alice from inside of the black hole, so the same decoherence must occur without Bob,\u201d the team wrote in an email to Quanta<\/em>. The black hole itself must decohere the superposition.<\/p>\n

\u201cIn the more poetic language of the participatory universe, it is as if the horizon watches superpositions,\u201d Danielson said.<\/p>\n

Using this insight, they set about working on an exact calculation of how quantum superpositions are affected by the black hole\u2019s space-time. In a paper<\/a> published on the preprint server Arxiv.org in January, they landed on a simple formula that describes the rate at which radiation crosses over the event horizon and so causes decoherence to occur. \u201cThat there was an effect at all was, to me, very surprising,\u201d Wald said.<\/p>\n

Hair on the Horizon<\/p>\n

The idea that event horizons gather information and cause decoherence isn\u2019t new. In 2016, Stephen Hawking, Malcolm Perry, and Andrew Strominger described<\/a> how particles crossing over the event horizon could be accompanied by very low-energy radiation that records information about these particles. This insight was suggested as a solution to the black hole information paradox, a profound consequence of Hawking\u2019s earlier discovery that black holes emit radiation.<\/p>\n

The problem was that Hawking radiation drains energy from black holes, causing them to completely evaporate over time. This process would appear to destroy any information that has fallen into the black hole. But in doing so, it would contradict a fundamental feature of quantum mechanics: that information in the universe can\u2019t be created or destroyed.<\/p>\n<\/div>\n

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The low-energy radiation proposed by the trio would get around this by allowing some information to be distributed in a halo around the black hole and escape. The researchers called the information-rich halo \u201csoft hair.\u201d<\/p>\n

Wald, Satishchandran, and Danielson were not investigating the black hole information paradox. But their work makes use of soft hair. Specifically, they showed that soft hair is created not only when particles fall across a horizon, but when particles outside a black hole merely move to a different location. Any quantum superposition outside will become entangled with soft hair on the horizon, giving rise to the decoherence effect they identified. In this way the superposition is recorded as a kind of \u201cmemory\u201d on the horizon.<\/p>\n

The calculation is a \u201cconcrete realization of soft hair,\u201d said Daniel Carney<\/a>, a theoretical physicist at Lawrence Berkeley National Laboratory. \u201cIt\u2019s a cool paper. It could be a very useful construction for trying to make that idea work in detail.\u201d<\/p>\n

But to Carney and several other theorists working at the forefront of quantum gravity research, this decoherence effect isn\u2019t all that surprising. The long-range nature of the electromagnetic force and gravity means that \u201cit\u2019s hard to keep anything isolated from the rest of the universe,\u201d said Daniel Harlow<\/a>, a theoretical physicist at the Massachusetts Institute of Technology.<\/p>\n

Total Decoherence<\/p>\n

The authors argue<\/a> that there is something uniquely \u201cinsidious\u201d about this kind of decoherence. Usually, physicists can control decoherence by shielding their experiment from the outside environment. A vacuum, for example, removes the influence of nearby gas molecules. But nothing can shield gravity, so there\u2019s no way to insulate an experiment from gravity\u2019s long-range influence. \u201cEventually, every superposition will be completely decohered,\u201d Satishchandran said. \u201cThere\u2019s no way of getting around it.\u201d<\/p>\n

The authors therefore regard black hole horizons as taking a more active role in decoherence than was previously known. \u201cThe geometry of the universe itself, as opposed to the matter within it, is responsible for the decoherence,\u201d they wrote in an email to Quanta<\/em>.<\/p>\n

Carney disputes this interpretation, saying that the new decoherence effect can also be understood as a consequence of electromagnetic or gravitational fields, in combination with rules set by causality. And unlike Hawking radiation, where the black hole horizon changes over time, in this case the horizon \u201chas no dynamics whatsoever,\u201d Carney said. \u201cThe horizon doesn\u2019t do anything, per se; I would not use that language.\u201d<\/p>\n

To not violate causality, superpositions outside the black hole must be decohered at the maximum possible rate that a hypothetical observer inside the black hole could be collecting information about them. \u201cIt seems to be pointing toward some new principle about gravity, measurement, and quantum mechanics,\u201d Gralla said. \u201cYou don\u2019t expect that to happen more than 100 years after gravity and quantum mechanics were formulated.\u201d<\/p>\n

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Illustration: Merrill Sherman\/Quanta Magazine<\/span><\/p>\n<\/figure>\n<\/div>\n

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Intriguingly, this kind of decoherence will occur anywhere there is a horizon that only allows information to travel in one direction, creating the potential for causality paradoxes. The edge of the known universe, called the cosmological horizon, is another example. Or consider the \u201cRindler horizon,\u201d which forms behind an observer who continuously accelerates and approaches the speed of light, so that light rays can no longer catch up with them. All of these \u201cKilling horizons\u201d (named after the late-19th- and early-20th-century German mathematician Wilhelm Killing<\/a>) cause quantum superpositions to decohere. \u201cThese horizons are really watching you in exactly the same way,\u201d Satishchandran said.<\/p>\n

Exactly what it means for the edge of the known universe to watch everything inside the universe isn\u2019t entirely clear. \u201cWe don\u2019t understand the cosmological horizon,\u201d Lupsasca said. \u201cIt\u2019s super fascinating, but way harder than black holes.\u201d<\/p>\n

In any case, by posing thought experiments like this, where gravity and quantum theory collide, physicists hope to learn about the behavior of a unified theory. \u201cThis is likely giving us some more clues about quantum gravity,\u201d Wald said. For example, the new effect may help theorists understand how entanglement is related to space-time.<\/p>\n

\u201cThese effects have to be part of the final story of quantum gravity,\u201d Lupsasca said. \u201cNow, are they going to be a crucial clue along the way to gleaning insight into that theory? It\u2019s worth investigating.\u201d<\/p>\n

The Participatory Universe<\/p>\n

As scientists continue to learn about decoherence in all its forms, Wheeler\u2019s concept of the participatory universe is becoming clearer, Danielson said. All particles in the universe, it seems, are in a subtle superposition until they are observed. Definiteness emerges through interactions. \u201cThat\u2019s kind of what, I think, Wheeler had in mind,\u201d Danielson said.<\/p>\n

And the finding that black holes and other Killing horizons observe everything, all the time, \u201cwhether you like it or not,\u201d is \u201cmore evocative\u201d of the participatory universe than the other types of decoherence are, the authors said.<\/p>\n

Not everyone is ready to buy Wheeler\u2019s philosophy on a grand scale. \u201cThe idea that the universe observes itself? That sounds a little Jedi for me,\u201d said Lupsasca, who nevertheless agrees that \u201ceverything is observing itself all the time through interactions.\u201d<\/p>\n

\u201cPoetically, you could think of it that way,\u201d Carney said. \u201cPersonally, I\u2019d just say that the presence of the horizon means that the fields living around it are going to get stuck on the horizon in a really interesting way.\u201d<\/p>\n

When Wheeler first drew the \u201cbig U\u201d when Wald was a student in the 1970s, Wald didn\u2019t think much of it. \u201cWheeler\u2019s idea struck me as not that solidly grounded,\u201d he said.<\/p>\n

And now? \u201cA lot of the stuff he did was enthusiasm and some vague ideas which later turned out to be really on the mark,\u201d Wald said, noting that Wheeler anticipated Hawking radiation long before the effect was calculated.<\/p>\n

\u201cHe saw himself as holding out a lamp light to illuminate possible paths for other people to follow.\u201d<\/p>\n

Original story<\/em><\/a> reprinted with permission from<\/em> Quanta Magazine<\/a>, an editorially independent publication of the<\/em> Simons Foundation<\/em><\/a> whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.<\/em><\/p>\n<\/div>\n<\/div>\n

Read More<\/a>
\n Thomas Lewton<\/p>\n","protected":false},"excerpt":{"rendered":"

At Princeton University in the early 1970s, the celebrated theoretical physicist John Wheeler could be spotted in seminars or impromptu hallway discussions drawing a big \u201cU.\u201d The letter\u2019s left tip represented the beginning of the universe, where everything was uncertain and all quantum possibilities were happening at the same time. The letter\u2019s right tip, sometimes<\/p>\n","protected":false},"author":1,"featured_media":640582,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[92762,5016,46],"tags":[],"class_list":["post-640581","post","type-post","status-publish","format-standard","has-post-thumbnail","category-argue","category-researchers","category-technology"],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/posts\/640581","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/comments?post=640581"}],"version-history":[{"count":0,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/posts\/640581\/revisions"}],"wp:featuredmedia":[{"embeddable":true,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/media\/640582"}],"wp:attachment":[{"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/media?parent=640581"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/categories?post=640581"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/newsycanuse.com\/index.php\/wp-json\/wp\/v2\/tags?post=640581"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}