The singularities are expected to vanish in a more fundamental theory of gravity that Einstein’s space-time picture merely approximates — a theory of quantum gravity.
But do these phenomena truly represent space-time singularities?
So a more universal theory that can predict that fate — very likely a quantum theory — must take over.
In a semiclassical world, space-time guides the journeys of quantum particles, but it cannot react to their presence.
In that case, the still-unknown fundamental theory of quantum gravity would not kill singularities but demystify them.
Physicists suffer from two blind spots: the center of a black hole and the beginning of the universe. Although the former might seem like a point in space and the latter like a moment in time, the typically interwoven threads of time and space appear to end abruptly in both situations. We refer to these enigmatic locations as singularities.
The general theory of relativity, developed by Albert Einstein, predicts singularities. The force of gravity is induced by the curvature of the space-time fabric toward itself, which is caused by clusters of matter or energy, according to this theory. According to Einstein’s equations, space-time will curve infinitely steeply in a small enough area if enough stuff is packed there, increasing gravity to an infinitely strong level.
However, the majority of physicists don’t think Einstein’s theory explains much of what actually occurs at these points. Instead, singularities are commonly viewed as “mathematical artifacts,” rather than things that “occur in any physical universe,” according to physicist Hong Liu (opens a new tab), of the Massachusetts Institute of Technology. There, general relativity breaks down. In a more fundamental theory of gravity, known as quantum gravity, which Einstein’s space-time picture only approximates, the singularities should disappear.
However, singularities are proving difficult to eliminate as physicists move closer to that more accurate and comprehensive theory by combining general relativity and quantum physics. In the 1960s, British mathematical physicist Roger Penrose was awarded the Nobel Prize in Physics for demonstrating that singularities would exist in an empty universe composed solely of space-time (opens a new tab). This understanding has been expanded into more practical situations by more recent studies. One study found (opens a new tab) that singularities would also exist in a universe with quantum particles, but it only looked at the scenario in which the particles don’t bend the space-time fabric at all. Earlier this year, a physicist demonstrated that these flaws are present even in theoretical universes that are very similar to our own and in which quantum particles do slightly alter space-time itself (opens a new tab).
It is possible that singularities are more than just mathematical illusions, and this trio of proofs forces physicists to consider this possibility. According to their hints, there might be places in our universe where space-time becomes so distorted that it is unrecognizable. Clocks stop ticking and no objects can pass. The singularity theorems challenge scholars to consider the nature of these ideas and seek a more basic theory that can elucidate what might go on if time really does stop.
The deadly dangers of space-time.
In 1916, only a few months after Einstein’s publication of General Relativity, Karl Schwarzschild made the initial discovery of a singularity in the arrangement (opens a new tab) of space-time. The strange characteristics of the “Schwarzschild solution” were not understood by physicists for years. The shape of space-time resembles a whirlpool, with walls that spiral steeper as you go deeper; at the bottom, space-time’s curvature is infinite. Anything falling inside the vortex, including light rays, is trapped by its spherical boundary, making it impossible to avoid.
It took decades for physicists to believe that these unthinkable objects—later dubbed black holes—might be real.
The J. In 1939, Robert Oppenheimer and Hartland Snyder calculated that a perfectly spherical star’s matter would become so dense that it would stretch space-time into a singularity if it gravitationally collapsed to a point (opens a new tab). However, physicists questioned whether the nonspherical shapes of real stars would prevent them from forming singularities because they bubble and churn, particularly during implosion.
The requirement for geometric perfection was abolished by Penrose in 1965. He used two presumptions in his seminal proof. A “trapped surface” that prevents light from escaping is necessary first. If you place light bulbs on this surface and turn them on, the light beams will fall inward more quickly than they can leave. It’s important to remember that this shell of light will contract whether it began as a perfect sphere, a golf ball with dimples, or something more asymmetrical.
Second, space-time ought to be curved so that light beams never diverge but instead bend in the direction of one another. In other words, as long as energy is always positive, gravity ought to be appealing.
Penrose demonstrated the mortality of at least one of the trapped light rays with these two conditions. Its otherwise endless journey through time and space must come to an end at a singularity, where the fabric of space-time vanishes and the light ray has no more future to travel into. The Schwarzschild solution’s infinite curvature was not the same as this new definition of a singularity. Because of its generality, Penrose was able to demonstrate in just three page (opens a new tab) of mathematics that singularities will unavoidably form under his two assumptions.
Geoff Penington (opens a new tab), a physicist at the University of California, Berkeley, stated that, aside from Einstein’s original paper, Penrose’s paper was likely the most significant work in general relativity ever written.
Stephen Hawking quickly applied Penrose’s theory to the early universe, demonstrating that a universe that fits the description of general relativity had to have originated from a single point during the Big Bang (opens a new tab). If you were to rewind the universe’s history, light rays would collide with a wall at the beginning of time, which is how this cosmological singularity is similar to a black hole.
Numerous pieces of evidence have been gathered by physicists over the years to support the existence of black holes and the idea that the universe started with an event that closely resembles the Big Bang. But are space-time singularities actually represented by these phenomena?
For many physicists, the very possibility of such points exists. General relativity falters and provides infinitely possible answers when you attempt to determine what would happen to a particle that is getting close to the singularity. Liu stated, “The singularity implies a lack of predictability.”. “Your theory simply fails. “.”.
In the real world, however, the particle must have some kind of destiny. Thus, a more all-encompassing theory that can forecast that outcome—likely a quantum theory—must take precedence.
Space-time assumes a single, unchanging shape because general relativity is a classical theory. Matter, on the other hand, is quantum mechanical, which means that it can exist in multiple states simultaneously. This property is called superposition. Any matter particles in a superposition of occupying two different locations should force space-time into a superposition of two distortions, according to theory, because space-time reacts to the matter within it. Specifically, gravity and space-time ought to adhere to quantum principles as well. However, the rules are still unknown to physicists.
Into the onion.
Theorists tackle the task of developing a quantum theory of gravity layer by layer, much like they would when peeling an onion. Every layer depicts a universe theory that is only a rough approximation of the actual one. You can capture more of the interaction between space-time and quantum matter the deeper you go.
Penrose worked on the onion’s outermost layer. He completely disregarded quantumness and instead applied the general theory of relativity. He essentially demonstrated that the space-time fabric contains singularities when there is no quantum matter present at all.
Someday, physicists hope to reach the core of the onion. They will discover a theory in it that describes matter and space-time in all of their quantum splendor. This theory would not have any blind spots; all computations ought to produce insightful findings.
However, what about the intermediate layers? Would physicists be able to resolve Penrose’s singularities by switching to a slightly more realistic and quantum model?
“The obvious hypothesis was that the singularity should be fixed in some way by quantum effects,” Penington stated.
In the late 2000s, they made their initial attempt. Penrose’s proof was limited to the outermost layer due to the presumption that energy is always positive. Quantum mechanics does not support that, but it does in everyday, classical scenarios. In quantum phenomena like the Casimir effect, where two metal plates are attracted to one another in a vacuum (as demonstrated by experiments), energy goes negative, at least temporarily. The idea that black holes radiate particles before “evaporating” completely is also influenced by negative energies. This unusual energetic behavior would be present in all of the onion’s deeper, quantum layers.
Aron Wall (opens a new tab), a physicist currently at the University of Cambridge and formerly at the University of Maryland, was the one who peeled back the top layer. Wall seized upon a theoretical finding by Jacob Bekenstein in the 1970s in order to break through into the quantum realm and reject Penrose’s energy assumption.
Bekenstein was aware that the contents of a particular region of space become increasingly jumbled over time. In other words, the second law of thermodynamics states that entropy, a measure of this mixing, tends to rise. When the physicist was thinking about a black hole region, he discovered that there are two sources of entropy. One common explanation is the quantity of possible configurations for quantum particles in the vicinity of the black hole. However, entropy also exists in the black hole, and its magnitude is determined by its surface area. The black hole’s surface area plus the entropy of the surrounding quantum matter add up to the region’s overall entropy. The “generalized” second law was born out of this observation (opens a new tab).
According to Berkeley physicist Raphael Bousso (opens a new tab), Wall “made it his mission to understand the generalized second law.”. “Compared to everyone else on the planet, he had a far clearer and better way of thinking about it.”. “”.
It would be necessary to allow for negative energy and the existence of quantum particles in order to reach the onion’s quantum layers. According to the generalized second law, Wall could do this by adding the entropy of those particles to any surface area in general relativity. In order to prove his singularity theorem, Penrose used the trapped surface. Wall therefore enhanced it to a “quantum trapped surface.”. And it held when he reformulated Penrose’s singularity theorem in this manner. Even when quantum particles are present, singularities can still form. Wall’s research was published in 2010 (opens a new tab).
According to Penington, “Aron’s paper was a monumental breakthrough in combining quantum mechanics and gravity in a more precise way.”.
Wall reached a slightly quantum layer, which physicists refer to as semiclassical, after peeling back the classical outer layer of the onion, where energy is always positive. In a semiclassical world, space-time directs quantum particles’ motions but is unable to respond to their existence. As a result of particles experiencing space-time warped into a black hole shape, a semiclassical black hole will, for example, radiate particles. For all eternity, however, even as the radiation leaks energy into the void, the black hole itself, or space-time, will never truly shrink in size.
It’s not quite the same as what occurs in the real world. For a hundred years, you could observe a black hole emitting particles without witnessing it shrink by even a single nanometer. However, if you could observe for a much longer period of time—many trillions upon trillions of years—you would witness the black hole evaporating into nothing.
The next layer of onions drew near.
Increasing the Quantumness.
Upon reexamining Wall’s proof, Bousso discovered that he could go a bit further. The space-time fabric can respond to quantum particles in the world where black holes contract as they radiate.
Despite the increased quantumness of his scenario, Bousso discovered that singularities still exist using more sophisticated mathematical tools created by Wall and others since 2010. In January, he published his unpeer-reviewed paper (opens a new tab).
There are still significant differences between the world of Bousso’s new theorem and our own. Some physicists question whether this third layer, with its roughly 17 known particles, more accurately depicts reality than the second layer because he made the unrealistic assumption that there is an infinite variety of particles for mathematical convenience. At the University of California, Santa Cruz, physicist Edgar Shaghoulian (opens a new tab) stated, “We don’t have an infinite number of quantum fields.”.
Despite its irrational particle abundance, some experts believe that Bousso’s work provides a satisfying conclusion to the Penrose and Wall singularity story. Singularities are unavoidable, even in space-times with weak reactions to quantum matter, it is established. The singularity cannot be avoided by merely making minor quantum corrections, according to Penington. The work of Wall and Bousso “provides a fairly definitive answer to that.”. “.”.
The Actual Singularity.
The form singularities will take at the core of the onion, that is, in the final quantum theory of gravity, is still unknown despite Bousso’s theorem.
The possibility that the dead ends will eventually disappear remains. What appears to be a singularity may actually have a connection to another location. Perhaps the light rays from a black hole wind up in a different universe. “A black hole could give birth to a baby universe,” Bousso stated. When you fall into a black hole, everything appears to be falling apart, but then all of a sudden, a new world appears and everything begins to expand once more. “”.
Additionally, the absence of a Big Bang singularity could suggest that the universe started with a “Big Bounce.”. The theory goes that a former universe somehow avoided the creation of a singularity and instead bounded into an era of expansion as it collapsed under the force of gravity. The second layer of the onion is where most physicists working on bounce theories operate. They use semiclassical physics, which takes advantage of negative-energy quantum effects, to avoid the singularity that the Penrose and Hawking theorems demand. They will now have to delve further into the onion and contend that the more accurate theories they discover there may defy the generalized second law’s requirements in light of the more recent theorems.
Surjeet Rajendran (opens a new tab) of Johns Hopkins University, a physicist who studies bounces, claims he is unfazed. Even the generalized second law, he notes, is not absolute truth. Any gaps would allow for space-time continuations and the avoidance of singularities.
However, Bousso and other physicists who share his suspicions believe that the generalized second law applies to the entire onion, meaning that the core theory and our universe should continue to have dead ends. The boundaries of the map where space stops and clocks cannot tick would be marked by the beginning of the universe and the centers of black holes.
MIT physicist Netta Engelhardt, who has collaborated with Wall, was adamant that there is a concept of singularity within black holes.
So singularities would not be eliminated but rather demystified by the as-yet-undiscovered fundamental theory of quantum gravity. With this more accurate theory, physicists could still pose questions and derive insightful answers, but the terminology used would be very different. A singularity may not be adequately described by space-time quantities such as position, curvature, and duration. Other numbers or ideas may need to be used in its place where time stops. According to Penington, “whatever quantum state describes the singularity itself does not have a notion of time,” he said. “.”.
