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As in history, revolutions are the lifeblood of science. Bubbling undercurrents of disquiet boil over until a new regime emerges to seize power. Then everyone's attention turns to toppling their new ruler. The king is dead, long live the king.

This has happened many times in the history of physics and astronomy. First, we thought Earth was at the center of the solar system an idea that stood for over 1,000 years. Then Copernicus stuck his neck out to say that the whole system would be a lot simpler if we are just another planet orbiting the sun. Despite much initial opposition, the old geocentric picture eventually buckled under the weight of evidence from the newly invented telescope.

Then Newton came along to explain that gravity is why the planets orbit the sun. He said all objects with mass have a gravitational attraction towards each other. According to his ideas we orbit the sun because it is pulling on us, the moon orbits Earth because we are pulling on it. Newton ruled for two-and-a-half centuries before Albert Einstein turned up in 1915 to usurp him with his General Theory of Relativity. This new picture neatly explained inconsistencies in Mercury's orbit, and was famously confirmed by observations of a solar eclipse off the coast of Africa in 1919.

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Instead of a pull, Einstein saw gravity as the result of curved space. He said that all objects in the universe sit in a smooth, four-dimensional fabric called space-time. Massive objects such as the sun warp the space-time around them, and so Earth's orbit is simply the result of our planet following this curvature. To us that looks like a Newtonian gravitational pull. This space-time picture has now been on the throne for over 100 years, and has so far vanquished all pretenders to its crown. The discovery of gravitational waves in 2015 was a decisive victory, but, like its predecessors, it too might be about to fall. That's because it is fundamentally incompatible with the other big beast in the physics zoo: Quantum theory.

The quantum world is notoriously weird. Single particles can be in two places at once, for example. Only by making an observation do we force it to 'choose'. Before an observation we can only assign probabilities to the likely outcomes. In the 1930s, Erwin Schrdinger devised a famous way to expose how perverse this idea is. He imagined a cat in a sealed box accompanied by a vial of poison attached to a hammer. The hammer is hooked up to a device that measures the quantum state of a particle. Whether or not the hammer smashes the vial and kills the cat hinges on that measurement, but quantum physics says that until such a measurement is made, the particle is simultaneously in both states, which means the vial is both broken and unbroken and the cat is alive and dead.

Such a picture cannot be reconciled with a smooth, continuous fabric of space-time. "A gravitational field cannot be in two places at once," said Sabine Hossenfelder, a theoretical physicist at the Frankfurt Institute for Advanced Studies. According to Einstein, space-time is warped by matter and energy, but quantum physics says matter and energy exist in multiple states simultaneously they can be both here and over there. "So where is the gravitational field?" asks Hossenfelder. "Nobody has an answer to that question. It's kind of embarrassing," she said.

Try and use general relativity and quantum theory together, and it doesn't work. "Above a certain energy, you get probabilities that are larger than one," said Hossenfelder. One is the highest probability possible it means an outcome is certain. You can't be more certain than certain. Equally, calculations sometimes give you the answer infinity, which has no real physical meaning. The two theories are therefore mathematically inconsistent. So, like many monarchs throughout history, physicists are seeking a marriage between rival factions to secure peace. They're searching for a theory of quantum gravity the ultimate diplomatic exercise in getting these two rivals to share the throne. This has seen theorists turn to some outlandish possibilities.

Arguably the most famous is string theory. It's the idea that sub-atomic particles such as electrons and quarks are made from tiny vibrating strings. Just as you can play strings on a musical instrument to create different notes, string theorists argue that different combinations of strings create different particles. The attraction of the theory is that it can reconcile general relativity and quantum physics, at least on paper. However, to pull that particular rabbit out of the hat, the strings have to vibrate across eleven dimensions seven more than the four in Einstein's space-time fabric. As yet there is no experimental evidence that these extra dimensions really exist. "It might be interesting mathematics, but whether it describes the space-time in which we live, we don't really know until there is an experiment," said Jorma Louko from the University of Nottingham.

Partly inspired by string theory's perceived failings, other physicists have turned to an alternative called Loop Quantum Gravity (LQG). They can get the two theories to play nicely if they do away with one of the central tenets of general relativity: That space-time is a smooth, continuous fabric. Instead, they argue, space-time is made up of a series of interwoven loops that it has structure at the smallest size scales. This is a bit like a length of cloth. At first glance it looks like one smooth fabric. Look closely, however, and you'll see it is really made of a network of stitches. Alternatively, think of it like a photograph on a computer screen: Zoom in, and you'll see it is really made of individual pixels.

The trouble is that when LQG physicists say small, they mean really small. These defects in space-time would only be apparent on the level of the Planck scale around a trillionth of a trillionth of a trillionth of a meter. That's so tiny that there would be more loops in a cubic centimeter of space than cubic centimeters in the entire observable universe. "If space-time only differs on the Planck scale then this would be difficult to test in any particle accelerator," says Louko. You'd need an atom smasher a 1,000-trillion-times more powerful than the Large Hadron Collider (LHC) at CERN. How, then, can you detect space-time defects that small? The answer is to look across a large area of space.

Light arriving here from the furthest reaches of the universe has traveled through billions of light years of space-time along the way. While the effect of each space-time defect would be tiny, over those distances interactions with multiple defects might well add up to a potentially observable effect. For the last decade, astronomers have been using light from far-off Gamma Ray Bursts to look for evidence in support of LQG. These cosmic flashes are the result of massive stars collapsing at the ends of their lives, and there is something about these distant detonations we currently cannot explain. "Their spectrum has a systematic distortion to it," said Hossenfelder, but no one knows if that is something that happens on the way here or if it's something to do with the source of the bursts themselves. The jury is still out.

To make progress, we might have to go a step further than saying space-time isn't the smooth, continuous fabric Einstein suggested. According to Einstein, space-time is like a stage that remains in place whether actors are treading its boards or not even if there were no stars or planets dancing around, space-time would still be there. However, physicists Laurent Freidel, Robert Leigh, and Djordje Minic think that this picture is holding us back. They believe space-time doesn't exist independently of the objects in it. Space-time is defined by the way objects interact. That would make space-time an artifact of the quantum world itself, not something to be combined with it. "It may sound kooky," said Minic, "but it is a very precise way of approaching the problem."

The attraction of this theory called modular space-time is that it might help solve another long-standing problem in theoretical physics regarding something called locality, and a notorious phenomenon in quantum physics called entanglement. Physicists can set up a situation whereby they bring two particles together and link their quantum properties. They then separate them by a large distance and find they are still linked. Change the properties of one and the other will change instantly, as if information has traveled from one to the other faster than the speed of light in direct violation of relativity. Einstein was so perturbed by this phenomenon that he called it 'spooky action at a distance'.

Modular space-time theory can accommodate such behavior by redefining what it means to be separated. If space-time emerges from the quantum world, then being closer in a quantum sense is more fundamental than being close in a physical sense. "Different observers would have different notions of locality," said Minic, it depends on the context. It's a bit like our relationships with other people. We can feel closer to a loved one far away than the stranger who lives down the street. "You can have these non-local connections as long as they are fairly small," said Hossenfelder.

Freidel, Leigh, and Minic have been working on their idea for the last five years, and they believe they are slowly making progress. "We want to be conservative and take things step-by-step," said Minic, "but it is tantalizing and exciting". It's certainly a novel approach, one that looks to "gravitationalize" the quantum world rather than quantizing gravity as in LQG. Yet as with any scientific theory, it needs to be tested. At the moment the trio are working on how to fit time into their model.

This may all sound incredibly esoteric, something only academics should care about, but it could have a more profound effect on our everyday lives. "We sit in space, we travel through time, and if something changes in our understanding of space-time this will impact not only on our understanding of gravity, but of quantum theory in general," said Hossenfelder. "All our present devices only work because of quantum theory. If we understand the quantum structure of space-time better that will have an impact on future technologies maybe not in 50 or 100 years, but maybe in 200," she said.

The current monarch is getting long in tooth, and a new pretender is long overdue, but we can't decide which of the many options is the most likely to succeed. When we do, the resulting revolution could bear fruit not just for theoretical physics, but for all.

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Was Einstein wrong? Why some astrophysicists are questioning the theory of space-time - Space.com

We dont know very much about our universe. Were fairly certain it exists, but we dont know how it got here, how long its been here, or how big it is. Heck, we dont even know if our universe is unique.

Ever since Albert Einstein came up with the theory of relativity and other scientists realized that classical physics and quantum mechanics dont really line up, weve been trying to reconcile those worlds.

Many theoretical physicists believe that bridging the gap between obvious reality (classical physics) and the wacky quantum realm could help us finally understand the true nature of our universe.

As far as we know, theres no such thing as a gods eye viewof the universe. We cant just zoom out in space and time and figure out whats going on like were dealing with a 3D model.

Instead, we have to use math to describe all the features of the universe beyond those we can directly measure with sensors and observations. Basically, scientists take the cosmic events they can observe and measure, and use them as data-points to help inform hypotheses about all the things that could happen beyond our field of observation.

And, when it comes to describing the universe, we need a theoretical framework that can unify classical and quantum physics with an explanation that makes sense of mysterious occurrences in both worlds. Thats where singularities come in.

Einstein and his longtime research partner Roger Penrose spent a lot of effort trying to figure out singularities because theyre among the most powerful, exoticobjects in existence that we know of. They literally bend light, space, and time. If we can figure out whats really going on inside a black hole, well be well on our way to determining whats happening everywhere else in our universe.

The problem: We have absolutely no idea how to physically study a black hole. As far as we know, anything that gets close enough to slip over the event horizon of a singularity is gone forever.

Scientists have long posited that black holes could contain exotic space materials that could have been present at the universes genesis event most commonly thought to be the Big Bang.

But, thats just a guess. As to whats actually inside of them: we can only theorize.

M-theory, or string theory, has long been considered a strong candidate for unifying quantum and classical physics. At the risk of grossly oversimplifying, string theory is exactly what it sounds like: instead of being made up of infinite particles, the universe is made up of strings that connect everything to everything else.

And then theres superstring theory. This adds supersymmetry to the mix which, again grossly oversimplified, accounts for fermions and bosons, particulate objects that are essential to quantum mechanics.

An international team of researchers recently published a pre-print paper that uses superstring theory to posit a unified explanation of classical and quantum physics that not only explains the origin story for our universe, it also theorizes the existence of innumerable other universes.

And it all relies on black holes.

Per the teams paper:

We show that an S-Brane which arises in the inside of the black hole horizon when the Weyl curvature reaches the string scale induces a continuous transition between the inside of the black hole and the beginning of a new universe.

This provides a simultaneous resolution of both the black hole and Big Bang singularities.

And there you have it, in one fell swoop weve figured out that rather than being destructive vacuums from which nothing can escape, black holes are objects of creation. Theyre pregnant with young universes that, one far away day, could mature to contain stars and planets and life just like our own.

No. Not really. I mean, maybe. The scientists arent saying any of this is true. In fact, this pre-print paper isnt actually saying anything is true: its positing mathematical possibilities that could explain why black holes act the way they do.

One the one hand, they could just be sucking everything into them, as Einstein figured, because of regular old gravity-related stuff. Thats pretty much what relativity is; the more massive something is, the more powerful its gravitational pull should be. And black holes are incredibly massive.

But, if they are just acting out extreme classical physics, then we have no way of explaining how they function in the quantum realm. And the problem with that is, were pretty sure quantum mechanics drives the machinations of black holes.

So we need a better answer.

And even though superstring theory and the idea that black holes exist to feed energy (or dark energy maybe?) to other universes might seem unbelievable, it does make a modicum of sense.

The bulk of the paper is dedicated to describing the theory in mathematical terms, so the physicistsdo show their proverbial work. But, because this is a pre-print, its still awaiting recognizable peer-review. And we should take everything it says with a grain of salt until then.

Ultimately, this is a pretty wacky take on the typical theory of everything. But Occams Razor tells us the simplest explanation is often the correct one. And when you see a giant tear in the fabric of the universe that appears to be pouring unfathomable amounts of energy somewhere, it makes sense to make the basic assumption its a portal.

H/t: Interesting Engineering

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New superstring theory says black holes may be portals to other universes - The Next Web

Astronomers have known that galaxies across the universe are behaving badly. Some are spinning too fast, while others are just way too hot and still others glommed into super structures too quickly.

But they don't know why. Perhaps some new hidden particle, like dark matter, could explain the weirdness. Or perhaps gravity is acting on these coalescing clusters of stars in a way scientists hadn't expected.

For decades, astronomers have debated the possibilities. While most astronomers believe that dark matter exists, some still think that we need to modify our theory of gravity. However, new research has found a critical flaw in modified gravity theories: They allow for effects to occur without causes and for information to travel faster than the speed of light. This is bad for modified gravity.

Related: The 15 weirdest galaxies in our universe

"It may change this research area considerably, forcing it in rather new directions," lead researcher and Tufts University astrophysicist Mark Hertzberg told Live Science.

Something funny is going on in the universe. For instance, based on what scientists would predict based on the masses of galaxies, stars orbit around the centers of them far too quickly; the temperature of the gas inside of galaxy clusters is far too hot; and large structures appeared in our universe far too soon.

At galactic and cosmological scales, either astronomers' understanding of the force of gravity is totally off, or there's a new ingredient in our universe that exerts gravity but is otherwise invisible. The latter idea is known as cold dark matter (CDM), which is the name given to a hypothetical form of matter that is as yet unknown to physics. The "cold" is there to note that whatever exotic particle might be responsible for the dark matter, it moves relatively slowly, in contrast to other potential dark matter candidates like the neutrino an example of a candidate for hot dark matter particles.

Related: The 11 biggest unanswered questions about dark matter

"If one gives up the principles of causality and locality, then it means we are essentially unable to explain the structure of the Standard Model of Particle Physics and General Relativity."

By filling galaxies with a form of matter that is invisible to light, the CDM hypothesis is wildly successful at explaining the majority of observations of galaxies and the larger universe. It is by far the most commonly accepted explanation for why the universe behaves as it does.

But the CDM hypothesis isn't perfect. Whatever it is, it sits outside the Standard Model of particle physics, meaning we have no idea what it is. Also, it has difficulty explaining something called the Baryonic Tully-Fisher Relation. The observed relationship shows that the total mass of normal matter, called baryonic matter, of a galaxy is proportional to the fourth power of the rotation speed. But CDM models predict that the relationship should be to the third power, predicting that galaxies spin slower for a certain amount of mass than they actually do.

What else could be going on?

An alternative to the whole CDM idea is a modified understanding of gravity. The simplest models fall under a class called MOND, for Modified Newtonian Dynamics. These models replace Newtonian physics (think Force = mass x acceleration) with other relationships that match the observed rotation rate of stars inside galaxies. While these models were popular when dark matter was first discovered in the 1970s and 1980s, they have failed to account for observations of galaxy clusters and the larger universe; as such, most scientists have all but rejected these models.

But the inadequacies of CDM to explain internal galactic dynamics provide an opening for MOND to survive. If a "MONDian" theory wants to compete on the galactic stage, however, it must be compatible with our other theories of physics, such as the special theory of relativity and quantum mechanics. So that's exactly what Hertzberg and his team set out to do. The results of their study were published in May to the preprint database arXiv, so the study hasn't been peer-reviewed.

"The only possibility to obtain something new [within the framework of relativity and quantum mechanics] is to add new degrees of freedom," Hertzberg told Live Science. In other words, in order to get MONDian theories to work with known physics, you have to add a whole bunch of funky stuff to theories. In examining that funky stuff, Hertzerg and collaborators found "some theoretical problems lurking in these attempts."

For instance, Hertzberg and his collaborators examined whether MONDian theories protect two principles: locality and causality. Locality is the concept that objects are directly influenced only by their surroundings in order for one object to influence another, it must transmit that influence via something like a force that travels at a finite speed. Causality is the simple notion that all events have a cause.

If a theory violates locality and/or causality, it is unlikely to fit in with our theories of physics, which do protect both principles

"If one gives up the principles of causality and locality, then it means we are essentially unable to explain the structure of the Standard Model of Particle Physics and General Relativity, as they are some of the central principles that go into constructing these theories in the first place," Hertzberg said. "In other words, if causality were badly broken in nature, we likely would have seen it already in various corrections to particle physics in the lab or tests of gravity in space."

In other words, we should've noticed by now.

Since all available evidence indicates that locality and causality are preserved (at least at macroscopic scales), then they should be obeyed by any new theory of physics. The team of physicists put MONDian theories to the test and found that they contain features that allow for non-locality and acausality. In other words, if MONDian theories are correct, then it's possible for events to happen without a cause and for effects to travel instantaneously, which violates the speed-of-light limit in the universe.

"Since we found that the existing proposals for radically new dark matter and MOND-like theories have some form of acausality, then it suggests they may not be embedded into fundamental physics, at least in their present form," Hertzberg said.

It might indeed be possible for locality and causality to be violated on galactic scales, but this would be extremely difficult to reconcile with everything else we know about physics.

As to the future of MONDian theories, Hertzberg speculated, "it motivates attempts to try to construct some classes of similar models that somehow maintain causality, but this looks difficult to achieve. In our paper, we show that a generalized form of these models fails the above tests for consistency."

Still, the "cold dark matter" paradigm has difficulty explaining the details of galactic physics. But there could be far more mundane reasons for this rather than upending all known physics. Modeling how galaxies form and evolve, even just accounting for all the messy processes where normal matter plays a role, is very difficult. Perhaps, a more sophisticated understanding of galaxies will provide an explanation for the observed Baryonic Tully-Fisher Relation.

And CDM is by far the best explanation we have.

"What is great about CDM is that it is theoretically on firm ground, and passes all the above theoretical consistency tests, even though it is not part of the Standard Model of Particle Physics," Hertzberg said. "The reason I say it is on firm ground is that there is no known theoretical reason why there shouldn't be some stable, neutral particles out there in the universe that don't couple to us very much. So CDM is reinforced, for now, as the leading idea."

Originally published on Live Science.

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How a weird theory of gravity could break cause-and-effect - Livescience.com