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The same scientific theory that predicted the existence of black holes also predicts the existence of white holes, the opposite of black holes in almost every respect. Whereas black holes are endless takers of matter and energy, white holes (hypothetically) ceaselessly blast energy out into the universe. And since nothing can escape a black hole, nothing should be able to enter a white hole.

While black holes are tough to spot due to their lack of emissions, white holes should be bright fountains of radiation and, theoretically at least, should be difficult to miss. Yet, so far, astronomers havent been able to find any.

But that hasnt deterred many prominent physicists, such as Italian theoretical physicist and science communicator Carlo Rovelli, from positing their existence. This shouldnt be too surprising. After all, general relativity has a good track record of theoretically predicting aspects of the universe well before they are discovered including black holes, gravitational waves, and the deviation of light known as gravitational lensing (which is used by instruments like the James Webb Space Telescope to see objects in the early universe).

Yet, white holes stubbornly remain the unfulfilled prediction of general relativity.

You cant get into white holesif youll excuse the punwithout first thinking about Albert Einsteins magnum opus theory of gravity, general relativity.

General relativity was first introduced to the physics community in 1915 as Einsteins geometric theory of gravity, and it caused quite a stir. Up until then, the best description of gravity was that by Isaac Newton, which still works just fine on small scales but always had considerable failings when it came to explaining physics on massive scales.

The major difference between Einsteins formulation of gravity and that of Newton was whereas the latter saw space and time as the stages upon which the events of the universe played out, general relativity posited that the united four-dimensional entity of spacetime is an active player in this cosmic production.

That is because general relativity suggests that when an object of mass sits in spacetime, it causes its very fabric to warp. The more massive the object, the greater the warp in spacetime it causes, and gravity arises from this warping. That explains why the sun has a bigger gravitational influence than Earth: its warping of spacetime is more extreme. This warping then tells energy and matter how it should move through space.

An illustration of a black hole warping spacetime.

As theoretical physicist John Wheeler astutely put it: Matter tells space how to curve, and space tells matter how to move.

Just a year after the introduction of general relativity, to the surprise of Einstein, physicist and astronomer Karl Schwarzschild found a solution to the complex field equations that define it. Within this solution was the singularity that represents the heart of a black hole, making this development the theoretical birth of the concept of black holes.

In 1960, mathematician Martin David Kruskal expanded on the Shwartzschild solution to consider a version of spacetime that lacks edges, creating what has become known as the Maximaly Extended Version of Schwarzschild Metric. This included creating a reflection of the singularity at the heart of a black holethe interior of a white holethough it would be Soviet cosmologist Ivor Novikov who realized the significance of this four years later in 1964.

Very simply, a white hole could be considered a black hole that runs backward in time. White holes would have some things in common with black holes: they would possess the characteristics of mass, angular momentum or spin, and electric charge.

Like black holes, because they have mass, white holes would attract matter toward them, at least at first. The difference is that when matter and light pass the event horizonthe point at which the gravity is so strong, the escape velocity exceeds the speed of lightof a black hole, it would never actually be able to reach the anti-event horizon of the white hole. It is possible that matter that approaches the anti-event horizon of a white hole could be whipped away with an incredible amount of force.

Space, light, and time are warped by the strong gravity of a black hole, forming an accretion disk of matter on the event horizon.

The major difference between black holes and white holes is their formation. We know, thanks to the work of J. Robert Oppenheimer and collaborators, that when a massive star undergoes a complete gravitational collapse at the end of its nuclear fuel-burning life, its outer layers are blasted away in a supernova explosion while its core collapses to birth a black hole.

Yet if these death throes could somehow be rewound like a cosmic VCRbreaking all the laws of cause and effect in the processthat would not result in a white hole as the mathematics of Kruskal or Novikov surmise. Instead, this cosmic rewind button would just give us back a star on the brink of death.

That means there is actually no physical process in the universe that we know of that could create a white hole.

While considering the possibility of black holes leaking radiation, Stephen Hawking postulated that they would come to a thermal equilibrium, which has an interesting consequence for white holes.

A state of thermal equilibrium is time-reversal invariant, which means that the same laws of physics should apply to a body in thermal equilibrium whether time is running backward or forward. If white holes are just time reversals of black holes, that meant to Hawking that black holes and white holes are reciprocal in structure.

The leaking of this radiation, which would later be termed Hawking radiation, would cause black holes to gradually evaporate, ironically throwing a lifeline to the white hole concept. This is because there is a rule in quantum mechanics that says information cant be destroyed. That means all the information that is carried into a black hole must be preserved.

If black holes live forever, no problem; that information sits in the singularity in perpetuity. But if black holes leak and shrivel like a cosmic paddling pool before finally exploding as Hawking thought, then what happens to the information they harbor? It cant be carried away by Hawking radiation, as thermal radiation cant be encoded with information. It isnt allowed to get back out past the event horizon, since no signal is permitted to do this. So where does it go?

One possibility is that black holes are connected through spacetime to white holes. Matter entering a black hole could come gushing out of the white hole at the other side somewhere else in the universepossibly in an entirely different galaxy. The connective tissue between the black holes entrance and the white holes exit would be an Einstein Rosen Bridge, more commonly known as a wormhole, a tunnel through spacetime connecting two seemingly disparate points possibly millions of light years apart.

An illustration of a wormhole.

However, the lack of white holes detected in the universe could suggest that wormholes are actually something much more profound than simply a tunnel from one distant galaxy to another.

The seeming lack of white holes in our universe could mean that if a multiverse of universes exists, there could be a universe populated entirely by white holes with a complete absence of black holes.

That could be because time is a one-way system in each universe in the multiverse. In our universe, time can only run forwardthe future is infiniteand that forbids the creation of a white hole. Meanwhile, in our multiverse counterpart, time can only play backwardthe past stretches into infinityand that forbids the existence of black holes while allowing white holes.

An illustration of the multiverse, with entire universes existing alongside each other.

Scientists like Roger Penrose suggest this could mean that there is a universe in the multiverse all the cosmic junk from our universes black holes spills into. Think of it like a multiverse equivalent of the trash compactor scene from Star Wars: A New Hope minus Han, Luke, Chewy, Leia, and a tentacled monster (probably).

Some theoretical physicists also consider that maybe our universe had just one white hole, a tremendous one, right at its very beginning.

The Big Bang certainly sounds a little like a time-reversed black hole, if you consider the rapid expansion of space as being like a sudden eruption of matter at the beginning of the universe. And the forward-flow-of-time rule wouldnt apply if that white hole was here at the very start, before time started rolling forward. Perhaps every universe in the multiverse starts with matter that flows out of a parent universe through a black hole connected to a white hole via a wormhole.

Whatever the case, since black holes have an event horizon that prevents accessing the information sealed behind it, well likely never be able to see into another universe through a white hole even if the two are linked.

Whether they are doorways to other regions of space or other universes entirely, or sealed exits, scientists are not set to stop speculating about white holes any time soon, meaning they will always be a doorway to the imagination.

Robert Lea is a freelance science journalist focusing on space, astronomy, and physics. Robs articles have been published in Newsweek, Space, Live Science, Astronomy magazine and New Scientist. He lives in the North West of England with too many cats and comic books.

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What Is a White Hole, and Do White Holes Really Exist? - Popular Mechanics

Imagine if every time a belligerent wanted to kill civilians or bomb a hospital, all they had to do to be absolved from culpability was to send a text message beforehand? This is effectively what Israeli President Isaac Herzog and US Secretary of State Antony Blinken proposed in a press conference on 3 November.

The press conference was designed to convince the public that the Israeli army was conducting its military campaign in Gaza run by the armed Palestinian group Hamas in line with international law. But this idea is simply not true: Texting civilians before you bomb them doesnt make targeting civilians any less of a war crime. In the context of Israels bombardment of Gaza, it is an effort by powerful nations and entities to skirt their obligations, and to misrepresent these obligations to the general public. Beyond Gaza, this places anyone in any conflict zone anywhere in the world at incredible risk.

For the general public, the idea of laws of war might seem a little incomprehensible. If war is itself an indication that the law has failed, how can such a failure be itself regulated by law? But for people who interact consistently with fighters technically referred to as belligerents or for communities affected by war, these laws are a lifeline.

At their simplest, the laws of war are a complex system of ideas and beliefs, some of which are explicitly written or codified in documents like the Geneva Conventions and the Convention on the Rights of Refugees, and some of which are based on traditional, religious, or cultural practices.

The most well-known formal documents, the First and Second Geneva Conventions, are two of very few international instruments that have been accepted by every single country in the world. As such, these are believed to apply in their entirety to every single conflict in the world.

The concept of laws of war can be simplified as the expression of a global agreement that those who are not party to a conflict either civilians or those who are no longer fighting because they are prisoners of war or injured should be protected from the worst of its negative consequences.

Civilian deaths in conflict are not unprecedented, but the startling death rate of civilians and particularly children during Israels bombardment of Gaza a response to Hamas attacks that Israeli officials say killed 1,400 people, the majority of them civilians is a painful reminder of why these rules exist.

According to Palestinian health officials, as of 6 November at least 10,000 people about half of them children have been killed in bombing that has destroyed schools, hospitals, and plenty of civilian infrastructure.

Several UN experts have sounded the alarm. The International Committee of the Red Cross (ICRC), the custodian of the Geneva Conventions and what is known as international humanitarian laws that apply during conflicts, is a neutral organisation. Despite its repeated warnings that hostage taking alongside the bombing of hospitals, schools, and civilian infrastructure are direct violations of the Geneva Conventions, the bombing continues.

Various UN agencies, beginning with the UN secretary-general and the directors general of the World Health Organization and the United Nations Refugee Works Agency (UNRWA) that oversees UN work in the Occupied Palestinian Territories, have warned that the IDF is systematically undermining the laws of war by targeting the civilian infrastructure in Gaza.

Global experts are warningthat, based on the language of senior Israeli politicians, Palestinians are facing the risk of genocide. These warnings seem to be landing on deaf ears. The weeks of expert alarm culminated in the press conference in Tel Aviv that added more confusion about what international law demands from those who engage in it.

Its tempting to look at this situation and think that these rules are therefore pointless. However, they serve an important function that must be reinforced by anyone who has power.

They are providing a baseline for judging conduct that has in the past been too easily swept under the rug of politics. They are giving us a basis on which to categorically declare that the targeting of civilians and civilian infrastructure is a moral wrong, without having to get into long-winded and frankly distracting debates that dont help us get any closer to saving peoples lives.

These laws are allowing governments and institutions that are not directly involved in a conflict to gauge an appropriate diplomatic response. They are necessary, but insufficient to protect civilian life. That threshold of sufficiency can only be achieved if those who hold power within global politics react to warnings, in this case by demanding a ceasefire.

Given how slow this action has come, we are already witnessing an alarming erosion of the idea of international law well beyond Gaza.

In Sudan, for example, the two main belligerents in the civil war have accelerated their attacks on civilians, taking advantage of the inability of global actors to pay attention to more than one thing at a time.

Those in Omdurman the countrys second largest city who did not have money or the opportunity to evacuate are trapped in their homes as food runs out because of heavy bombing within the city: the use of starvation as a weapon of war is forbidden in international law. In El Geneina, the Rapid Support Forces that were once implicated in the genocide in Darfur are rounding up and killing civilians again almost at will: The list of war crimes committed there in the last week alone is difficult to summarise in a single sentence.

At its simplest, international law in war is an agreement between those who wield power on behalf of nations and armed groups that they will not allow their animosity to descend into anarchy in which everyone and everything is a target.

This is the vexing puzzle at the heart of the international legal system. We cant pick and choose the moments in which it will apply without undermining the validity of the entire structure.

All societies have ideas about how they should treat those who are captured or not directly involved in the fighting. Some of these ideas are that everyone is fair game and a target; others explicitly caution that those who are not directly fighting should be spared.

At its simplest, international law in war is an agreement between those who wield power on behalf of nations and armed groups that they will not allow their animosity to descend into anarchy in which everyone and everything is a target. There is an international criminal court that is supposed to work as an enforcer of the rules, but even that is dependent on the good faith cooperation of the countries that signed its treaties.

International law is a network of reciprocity and relation that depends on the participation of all nations and communities. It doesnt require 100% compliance almost no law, domestic or international, does but it does require a critical mass of the worlds countries to willingly comply with a critical mass of its requirements, or, at the very least, not to undermine them.

The current formal body of international law represents not just the dominance of Western thought, but also the fact that some of the most devastating international wars fought in history were fought by Western nations.

These laws began as their mutual agreement to stop being so terrible towards each other. The rest of the world opted in because being less terrible during war made sense. Which raises the question so many people are asking: Given how publicly countries like the United States and various European nations have undermined the very idea of international law in defence of Israel in the past month, how are people in Sudan and elsewhere supposed to demand belligerents already disconnected to any other global system of accountability and sanction stand down? If Western nations act in ways that are destructive to structures that so many nations have opted into, doesnt that mean the very notion of international law itself must be abandoned?

I think not. I think it is a moment for the exact opposite reinforcing and rallying around the idea that being less terrible to each other during war is a universal good that must be emphatically defended. The idea that civilians should not be dragged into war should survive this moment of global failure and shame.

Wars are often fought in the context of nationalist or ethnonationalist passion, fuelled by national myths of singularity and led by politicians and generals who will never know what it is to be on either side of a gun or a bomb. The secondary impacts of conflict the physical destruction of so many bodies, not to mention the psychological harm on those who survive are almost never part of the calculus.

The idea of having laws of war is that this heated context can somehow be contained so that only those who actively choose to participate as soldiers or in politics should suffer the consequences. That idea must survive.

The question for those of us imagining a post-colonial, egalitarian future is what form that law should take so that it is no longer so intimately connected to the choices of powerful and indeed, Western nations. International law cannot and should not be abandoned to the vagaries of Western liberalism.

And in case youre still wondering, texting civilians before dropping bombs on them doesnt make dropping bombs on civilians okay. Its still a war crime.

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International law matters even when the West abandons it - The New Humanitarian

Is biological life common in the universe, or should we be looking for artificial, robotic intelligence in the search for alien life?

An increasing number of scientists suspect that if we ever do make contact with alien life, we will be communicating with a computer.

This thinking revolves around an event called the singularity. This term, borrowed from mathematics, signifies a point where our knowledge of math and physics breaks down and we can no longer accurately characterize what we're trying to describe. A black hole singularity is a good example of this.

Related: Could AI find alien life faster than humans, and would it tell us?

In computer science and technology, the singularity describes the moment when artificial intelligence develops so fast that it results in a superintelligence an artificial general intelligence, as opposed to the very specific machine-learning algorithms we have today that experiences runaway growth in computing power and intellectual ability. This superintelligence would grow so far ahead of us, so quickly, that we would lose the ability to understand or explain it.

Computer scientists have been speculating that the singularity could come soon; most predictions seem to agree on the period between 2030 and 2045. What happens beyond the singularity is anyone's guess.

There's no guarantee that the singularity will come to pass; many academics remain skeptical. However, if it does, the timescales would be remarkable, given that it is predicted to occur just 250 years after the Industrial Revolution, 130 to 140 years after the Wright brothers' first powered flight, a century after the atom was first split and 50 years after the invention of the World Wide Web. If we are a typical civilization in the galaxy, the singularity would seem to happen early in the life of a technological species.

Now, consider the age of the universe: 13.8 billion years. Assuming that life has been able, in theory, to develop and evolve for the vast majority of that history, alien species could be billions of years older than our solar system and many billions of years older than Homo sapiens. They would have had plenty of time to pass through the technological singularity, which is why so many researchers studying the search for extraterrestrial intelligence (SETI) are convinced that technological aliens will be artificial intelligences.

"This is very much at the vanguard of thinking in some sections of the SETI community," Eamonn Kerins, an astrophysicist and SETI researcher at the Jodrell Bank Centre for Astrophysics at the University of Manchester in the U.K., told Space.com. "We ourselves are very close to realizing artificial general intelligence (AGI), and there's an expectation that once you reach that point, it can then accelerate away at a very fast rate and quickly outstrip ourselves in intelligence."

Suppose alien life was some form of superintelligence that had gone way past the singularity. What would it mean for SETI?

SETI focuses on searching for radio signals, the same kind that humans transmit. There are still very good reasons for searching the radio spectrum: Radio waves can permeate the Milky Way galaxy, they're a relatively simple means of signaling, and aliens would suspect that our astronomers were already studying the universe in radio waves and would therefore be more likely to spot a radio signal.

A superintelligence billions of years older than us, however, might have long since moved past radio and might not even care enough to attempt to contact primitive life-forms on Earth.

Beyond looking for signals, recent SETI efforts have been considering the broader concept of technosignatures evidence for extraterrestrial technology or engineering possibly on an enormous scale for it to be noticeable to us. This might be one way of detecting an artificial superintelligence since the search for technosignatures is agnostic about why the aliens are doing what they're doing. Beyond the singularity, such reasons might be difficult for us to discern.

"Some of this [discussion about superintelligences] almost doesn't matter from the point of view of doing the search, if you build a good enough anomaly detector," Steve Croft, a radio astronomer who works on the Breakthrough Listen project for the Berkeley SETI Research Center at the University of California, Berkeley, said in an interview with Space.com. "We can figure out what they're up to afterwards we may never comprehend what they're up to."

All that would matter is that we could potentially detect these intelligent life-forms' activities, even if we don't fully understand what they're doing. In some cases, though, we might understand.

A superintelligence would need a lot of energy to facilitate the computations of its CPU. In 1964, Soviet astrophysicist Nikolai Kardashev proposed what would become known as the Kardashev scale, in which increasingly technologically developed civilizations harness the total energy of first a planet (Level I), then a star (Level II) and then an entire galaxy (Level III).

In principle, the latter two levels would be achievable via Dyson swarms of solar-energy collectors around a civilization's home star, and then around every star and black hole in their galaxy. According to the Kardashev scale, a Type II civilization could harness 4 x 10^26 watts; a Type III civilization could reach 4 x 10^37 watts.

A superintelligence might even opt to live inside a Dyson swarm for example, in a "Matrioshka brain," a series of nested shells of Dyson swarms in which the innermost shell absorbs sunlight, uses the energy for processing and then emits the residual heat energy for the next shell to pick up, and so on.

What would a superintelligence do with all that energy? "Maybe they smash neutron stars together for fun and those are the fast radio bursts!" Croft said, only half-jokingly. "If you do have command of ridiculous amounts of energy, if you've achieved a Kardashev Type II or III level, then what might you do with your spare time? One thing we've seen through human societies over millennia is art, and it drives a lot of our endeavors, creating beautiful things, and I wonder whether a superintelligence might make art and whether that's something we could spot."

Spotting alien art might not be so easy; art is cultural, so we would not know what is beautiful to them. However, the scale of the potential art projects we could detect might make life easier. A superintelligence might push stars around, for example. One theoretical way of doing this is via a Shkadov thruster, which is essentially a giant concave mirror facing a star at a distance where the gravitational attraction that the mirror feels from the star is balanced by the stellar wind trying to push the mirror outward. The mirror would reflect the stellar wind and the star's own light back toward the star. And because photons and particles can carry momentum, the reflected radiation would push the star in the opposite direction. Over millions of years, it could, in theory, move the star many light-years.

If an alien superintelligence has an artistic leaning, it may wish to assemble geometric shapes out of stars, such as a Klemperer rosette. This is a gravitationally stable system of six objects in this case, stars perhaps alternating in mass between large and small, all moving around a common point on the same orbit. Such a star system could not form naturally, and if we found one, it would be evidence for a powerful extraterrestrial intelligence. An alternative concept would be to place all of the planets in a system on the same orbit around their star; a recent study showed how it might be possible to fit 24 planets on the same orbit without them colliding.

However, all of these are brute-force projects. Superintelligence may be more focused on the loftier goal of just thinking, or running virtual reality programs. Processing information requires a lot of energy, and the more a superintelligence thinks, the more energy it will require. And the less ambient heat there is, the more efficiently the computations run.

The interior of the Milky Way galaxy is a warm place, so superintelligences might relocate to the outskirts of the galaxy, where the ambient temperature drops, thus allowing more efficient information processing. Some researchers have even proposed that superintelligences might go into hibernation for tens of billions of years while the universe around them cools to just a fraction of a degree above absolute zero, which would permit more efficient computations. (Currently, the universe or, more specifically, the cosmic microwave background, the leftover radiation from the Big Bang is 2.73 kelvins above absolute zero.).

What would they be thinking and calculating? That's not a question we can answer, but we don't need to. All we have to do is find evidence for their presence whether in a Dyson swarm, a Shkadov thruster, a Klemperer rosette or activity on the edge of the galaxy. And perhaps, if our own AIs arrive at the singularity too, that could give us some insight into what the great intelligences of the universe spend their time doing.

Follow Keith Cooper on Twitter @21stCenturySETI. Follow us on Twitter @Spacedotcom and on Facebook.

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In the search for alien life, should we be looking for artificial ... - Space.com

One of the most important advances in all of physics was the development of Einsteins general relativity: our greatest and most predictively powerful theory of gravity. Replacing the idea of a gravitational force that acts on objects that never physically touch one another with the notion that all objects exist within the fabric of spacetime, and that the curvature of spacetime determines how those objects will move, is a concept that many even professionals still struggle to wrap their heads around. However, it comes along with consequences: certain configurations of matter-and-energy within spacetime inevitably lead to a condition that marks an effective end or beginning to spacetime itself, more commonly known as a singularity.

But are these singularities necessarily physically real, representing something profound thats occurring within the Universe? Or might there be some way to avoid them, perhaps signaling a very different scenario than space and time themselves ceasing to exist? (At least, as we understand them.) Thats what Patreon supporter Cameron Sowards wants to know, as he writes in to ask:

Why do we believe that the pre big bang state was not a singularity when it is a much higher concentration of energy than a black hole could possibly have since the pre big bang universe was not a singularity, could the same mechanisms that prevented it from being a singularity apply to the interior of black holes?

Theres a tremendous amount to unpack here, so lets try and do this question justice!

Once you cross the threshold to form a black hole, everything inside the event horizon crunches down to a singularity that is, at most, one-dimensional. No 3D structures can survive intact. However, one interesting coordinate transformation shows that every point in the interior of this black hole maps 1-to-1 with a point on the outside, raising the mathematically interesting possibility that the interior of each black hole gives rise to a baby universe inside of it, and the possibility that our Universe itself may have arisen from a black hole in a pre-existing universe prior to our own.

The Big Bang and the question of a first singularity

If you start with just two basic observations that the Universe is full of matter and energy, and also, is expanding today you might think theres no way out of an initial singularity. Indeed, this was first put together nearly a hundred years ago, all the way back in the 1920s. As soon as you recognize that your Universe, on the largest of cosmic scales, is roughly the same in all locations and in all directions (what astrophysicists call homogeneous for the first and isotropic for the second), then theres a particular exact solution (and metric for spacetime) that applies within the context of general relativity: the FLRW (FriedmannLematreRobertsonWalker) metric.

This metric, which describes the spacetime of the Universe as well as its relationship to the matter and energy within it, mandates that the Universe cannot be static, but must either expand or contract. Given that observations of the recession speed (or redshift) of distant galaxies is directly proportional to their measured distance from us, this indicates that the Universe is expanding today.

If its expanding today, and full of matter and radiation, then that implies that in the past, the Universe was smaller but contained the same amount of stuff within it. Therefore, it was denser and hotter as well. The farther we extrapolate back in time, the smaller the Universe gets. And if we go all the way back to the moment where it reaches 0 for its size, we arrive at a singularity.

As a balloon inflates, any coins glued to its surface will appear to recede away from one another, with more distant coins receding more rapidly than the less distant ones. Any light will redshift, as its wavelength stretches to longer values as the balloons fabric expands. This visualization solidly explains cosmological redshift within the context of the expanding Universe. If the Universe is expanding today, that means it was smaller, hotter, and denser in the past: leading to the picture of the hot Big Bang.

This picture held sway for most of the 20th century, having been bolstered by what are known as the four observational cornerstones of the Big Bang theory.

With these four pillars supporting the hot Big Bang, there was no doubt that this theory in contrast to all other competing models accurately describes our cosmic origins.

In the top panel, our modern Universe has the same properties (including temperature) everywhere because they originated from a region possessing the same properties. In the middle panel, the space that could have had any arbitrary curvature is inflated to the point where we cannot observe any curvature today, solving the flatness problem. And in the bottom panel, pre-existing high-energy relics are inflated away, providing a solution to the high-energy relic problem. This is how inflation solves the three great puzzles that the Big Bang cannot account for on its own.

But just because this story describes our past doesnt necessarily mean its chapter 1 of the story of our Universe. There are a great many unexplained puzzles that come along with the hot Big Bang, including:

In the standard hot Big Bang, there are no explanations for this. You have to simply assert that these are the initial conditions of the Universe with no explanation, or as Lady Gaga might say, the Universe was simply born this way.

However, theres a wonderful scientific mechanism that can set up these conditions if we hypothesize an early phase to the Universe that preceded the hot Big Bang: cosmological inflation. This theory, first proposed in 1980, not only provides explanatory power for all three of these observations, it also made an incredible new set of predictions that differ from that of a hot Big Bang without inflation, including some really weird ones, that have since been observationally confirmed.

The quantum fluctuations inherent to space, stretched across the Universe during cosmic inflation, gave rise to the density fluctuations imprinted in the cosmic microwave background, which in turn gave rise to the stars, galaxies, and other large-scale structures in the Universe today. This is the best picture we have of how the entire Universe behaves, where inflation precedes and sets up the Big Bang. Unfortunately, we can only access the information contained inside our cosmic horizon, which is all part of the same fraction of one region where inflation ended some 13.8 billion years ago.

Whereas the original hot Big Bang demanded a singularity, however, the situation now becomes a lot murkier with cosmic inflation added to the mix. Whereas an expanding Universe filled with matter-and-radiation can be traced back to a singularity, in the case of an expanding Universe thats dominated by some sort of vacuum energy which is the case for cosmic inflation the question of a beginning is much less clear.

Because an inflationary spacetime expands exponentially, it cant be traced back to a singularity; only back to a progressively smaller and smaller but still finite and non-zero size.

Whereas a non-inflationary expanding Universe (the classical Big Bang scenario) has all of its geodesics inevitably meet at a single point in the past, rendering it a past-timelike complete spacetime, some geodesics go back an infinite amount in inflationary spacetimes, while others pathologically blow up and/or result in curvature singularities, indicating that inflationary spacetimes are past-timelike incomplete. This suggests that something very likely preceded cosmic inflation, and although its the subject of a lot of interesting ongoing research, the jury is still out on whether those spacetimes must include a singularity or not.

In other words, inflation probably wasnt chapter 1 of our Universes story either, and it is not presently 100% established whether our Universe began from a singularity or not.

In a Universe that isnt expanding, you can fill it with stationary matter in any configuration you like, but it will always collapse down to a black hole. Such a Universe is unstable in the context of Einsteins gravity, and must be expanding to be stable, or we must accept its inevitable fate.

Black holes and their inevitable singularities

On the other hand, the situation is very different when it comes to black holes. In fact, it was Einstein himself who first noted that if you took any initial configuration of mass that started off at rest (what relativists idealize as pressureless dust) within an otherwise static spacetime, it must inevitably collapse. Not collapse and form a dust cloud, but collapse all the way down until it became point-like: until it formed whats known as a Schwarzschild (non-rotating) black hole.

In the case of a spacetime that contains Schwarzschild black hole, what happens is that far away from the black hole itself, it behaves as any other mass would: deforming and distorting the fabric of spacetime, causing it to curve from its presence, the same way that any other equivalently-valued mass (whether a gas cloud, a planet, star, white dwarf, or neutron star) would deform it.

But unlike those other cases, where the mass is distributed over a large volume of spacetime, in the case of a Schwarzschild black hole, all of that mass collapses down to a single point: a singularity. Around that singularity exists an invisible boundary a mathematical surface known as an event horizon, which itself marks the dividing line between where an object, even one moving at the speed of light, can or cannot escape from the gravitational pull of this hole in spacetime.

Both inside and outside the event horizon of a Schwarzschild black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. At the event horizon, even if you ran (or swam) at the speed of light, there would be no overcoming the flow of spacetime, which drags you into the singularity at the center. Outside the event horizon, though, other forces (like electromagnetism) can frequently overcome the pull of gravity, causing even infalling matter to escape.

And calling it a hole really is appropriate in this instance. In general relativity, we often consider the behavior of what are known as test particles, which is to say, something that we can drop down with any property we dream up [mass (including massless), charge, spin, position and speed (including, for massless particles, the speed of light) and a direction for that speed], and ask how it evolves/behaves in the presence of this spacetime. If you want to know what happens within your spacetime and whether you have a singularity or not, and whether your spacetime is timelike-complete in either the future or past dropping a series of test particles, including massless ones, is one great way to find out.

In the Schwarzschild spacetime, you can have stable orbits well beyond the vicinity of the event horizon just as you can have planets orbit the Sun or stars move around a galaxy. However, if you get too close to the event horizon, thats no longer the case. Any quantum of anything that crosses over the event horizon, regardless of its other properties, gets inevitably drawn into the central singularity in a finite (and brief) amount of time. There are no paths around this fate, and nothing that can save you from it.

In fact, the greatest contribution of famed Nobel Laureate Roger Penrose to physics, and in fact the contribution that earned him the Nobel Prize, was the demonstration of how realistic matter, from a collapsing star, actually creates an event horizon and results in a future-complete spacetime that ends in a singularity.

One of the most important contributions of Roger Penrose to black hole physics is the demonstration of how a realistic object in our Universe, such as a star (or any collection of matter), can form an event horizon and how all the matter bound to it will inevitably encounter the central singularity. Once an event horizon forms, the development of a central singularity is not only inevitable, its extremely rapid.

Wiggle room and the chance for a way out

A black hole even the earliest, simplest conception of a black hole meets all the necessary criteria for being a complete spacetime that does, in fact, terminate in a singularity. At that location, theres a finite, non-zero amount of mass/energy that exists within a single point of infinitesimal size, and that means all the things youd normally calculate, like density or temperature, would simply blow up and go to infinity. Thats what happens at a singularity, and it truly is a place where pathological behaviors are all that you encounter.

You might try and argue that the Universe, in reality, isnt described by idealized Schwarzschild black holes. You can instead attempt to add more realistic ingredients, like angular momentum (or spin), and the fact that all of the realistic black holes weve observed seem to not only be spinning, but spin at speeds that are quite relativistic, or an appreciable fraction of the speed of light.

And that will get you somewhere: into a different spacetime known as a Kerr spacetime, rather than a Schwarzschild spacetime. A bunch of interesting things happen in this spacetime that dont occur in the case of non-rotation, including that the event horizon splits in two, into an inner and outer event horizon. Theres also a new in-between region, outside the outer event horizon, known as an ergosphere: where energy and mass can be extracted from just beyond the event horizon.

In the vicinity of a black hole, space flows like either a moving walkway or a waterfall, depending on how you want to visualize it. Unlike in the non-rotating case, the event horizon splits into two, while the central singularity gets stretched out into a one-dimensional ring. Nobody knows what occurs at the central singularity, but its presence and existence cannot be avoided with our current understanding of physics.

However, theres still a singularity at the center. While it changes, becoming no longer a point but rather a 1-dimensional object thats smeared out into a circular ring, its still a singularity: a line of infinite density, where again those same pathologies arise, and the laws of physics break down. That attempt to wiggle out wont get you anywhere.

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You can try to imagine that somewhere, inside the event horizon but before you get to the singularity, theres some compact collection of matter that refuses to collapse further. But that, too, fails due to a fact of Einsteins relativity: no signal, interaction, or force can move faster than the speed of light. If you wish to have a particle thats closer to the singularity (from within the event horizon) push back on an outermore particle and keep it from falling in any further, it must propagate back away from the singularity. But all paths from inside the event horizon only lead further down and closer to the central singularity; youd have to propagate faster than the speed of light to push backward. Unless we throw out relativity altogether, theres no hope there.

Which leaves only two places left to turn if we want to try and wriggle out of this fate:

From outside a black hole, all the infalling matter will emit light and is always visible, while nothing from behind the event horizon can get out. But if you were the one who fell into a black hole, your energy could conceivably re-emerge as part of a hot Big Bang in a newborn Universe.

There are many good reasons to hold out hope for the second one, as theres an interesting mathematical mapping between:

In other words, its possible that any infalling material into a realistic black hole will, in some sense (after being ripped apart due to tidal forces and converted into a soup of fundamental quanta), emerge once again into what it perceives as a new Universe, and might potentially experience a hot Big Bang and the resultant cosmic evolution all over again.

However, those are our only two realistic and best hopes for avoiding encountering a central singularity within every black hole. Either quantum gravity will save us (and good luck figuring that one out, as its perhaps the most difficult holy grail problem in all of theoretical physics), or theres the possibility that falling into a black hole will chew you up and spit your remnants out in a newborn Universe on the other side. Either way, as long as were stuck in our Universe, and as long as the laws of general relativity hold, it appears that a singularity at the center of each black hole really is inevitable.

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Original post:

Ask Ethan: Are singularities physically real? - Big Think

Are humans progressing morally as well as materially? What does it mean to be human in the cosmos? On a new episode of ID the Future, we bring you the second half of a stimulating conversation between Dr. David Berlinski and host Eric Metaxas on the subject of Berlinskis book Human Nature.

In Human Nature, Berlinski argues that the utopian view that humans are progressing toward evolutionary and technological perfection is wishful thinking. Men are not about to become like gods. Im a strong believer in original sin, quips Berlinski in his discussion with Metaxas. In other words, he believes not only that humans are fundamentally distinct from the rest of the biological world, but also that humans are prone to ignorance and depravity as well as wisdom and nobility. During this second half of their discussion, Berlinski and Metaxas compare and contrast the ideas of thinkers like psychologist Steven Pinker, author Christopher Hitchens, and physicist Steven Weinberg. The pair also spar gracefully over the implications of human uniqueness. Berlinski, though candid and self-critical, is unwilling to be pigeonholed. Metaxas, drawing his own conclusions about the role of mind in the universe, challenges Berlinski into moments of clarity with his usual charm. The result is an honest, probing, and wide-ranging conversation about the nature of science and the human condition. Download the podcast or listen to it here.

This is Part 2 of a two-part interview. If you missed it, listen to Part 1.

Cross-posted at Evolution News.

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Are We Approaching the Singularity? - Walter Bradley Center for Natural and Artificial Intelligence