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Physics: Can we craft a theory in which space and time aren’t assumed to exist?

Introversion

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Disclaimer: I understand almost none of this article, even though it’s written for lay people. :Shrug:

In some versions of quantum gravity, time itself condenses into existence.

Ars Technica said:
Over the past decades, a field of physics has developed that postulates the existence of mysterious algebraic entities called spin networks. These networks—proposed as the constituent stuff of space and time—condensed to produce the Universe as we know it. That condensation resulted in the event that we currently call the Big Bang, giving the field its name: condensate cosmology.

It may sound like an odd idea, but we already know that the Universe works in very strange ways.

The idea, technically termed “Group Field Theory (GFT) condensate cosmology,” is a branch of quantum gravity, a field of physics that aims to establish the fundamentals of what everything from light and matter to space and time is made of. It is an idea based completely in theoretical calculations—and it's totally untested for now. Condensate cosmology requires a great deal of abstract reasoning to even try to understand it.

Despite these challenges, quantum gravity has drawn a lot of attention from some of the sharpest minds in all of physics. Its ideas are bold and daring, highly creative, and extraordinarily imaginative.

Quantum gravity has been formulated to tackle one of the greatest problems in all of physics: the need to unite the two great theories of the 20th century—general relativity and quantum mechanics.

The former presents a framework for understanding the world in terms of space and time, and it covers behavior over large distances. General relativity introduces the notion that time is relative and that gravity itself exists because of a curved space-time. As Einstein first realized, a ball does not fall to the Earth because it is attracted to its mass, as Newton told us; it falls because of the existence of a space-time field that permeates the Universe and curves around large objects.

Quantum mechanics is a mysterious yet incredibly accurate theory that describes the world of the very small. It tells us that both particles and fields exist in discrete units that, because of uncertainty, can only be described probabilistically. The theory also describes entanglement, the bewildering phenomenon in which physical systems can be so intertwined with one another that they lose their independent, individual reality and start obeying rules that apply to a collective.

As far as we can tell, these two theories are both right—and in conflict. Their simultaneous existence generates a paradox, meaning physics is, in a sense, in disarray. While quantum mechanics deals with reality in discrete, granular fashion, relativity tells us that space-time, and therefore gravity, is continuous and non-discrete.

One way to deal with this is to give one of the theories precedence. Since we know the world is quantum, general relativity must be an approximation of an underlying quantum description of space-time itself. And this suggests that any unification of the theories requires that gravity become discrete.

Over the past few decades, a branch of quantum gravity called Loop Quantum Gravity (LQG) has shown some potential in solving the challenge of making gravity discrete. LQG begins with Einstein's field equations, but it takes a closer look at what might be hiding beneath the surface of space-time. The mathematics produced myriad discrete geometric objects, including loops, lattices, and polygons, arranged in various constructions called spin-networks and spin foams. Together, they can describe the structure of reality itself—these geometric oddities of LQG do not exist in space and time, but rather they are space and time and therefore the very constituents of gravity itself.

...
 

dickson

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One way to deal with this is to give one of the theories precedence. Since we know the world is quantum, general relativity must be an approximation of an underlying quantum description of space-time itself. And this suggests that any unification of the theories requires that gravity become discrete.

The idea behind the approach described lies in this sentence. At one level, it's actually simple. At any other level . . .

If we want to quantize spacetime, then the spacetime of classical gravitation, i. e. General Relativity, is a classical approximation to some more primitive underlying structure. The elements that comprise garden-variety quantum theory, however, are defined on a four dimensional manifold, either that of Special or General Relativity. (In this context, a manifold is a continuous set that is locally equivalent to the interior of an N-dimensional cube. A one-dimensional manifold looks locally like a subset of the real line. N need not be finite, cue Hilbert space.)

That has left theoreticians scratching their heads about what might make up that underlying structure, and what might its relationship to classical spacetime be. I don't know who first thought about this problem seriously, but Feynman, Roger Penrose, and Bryce DeWitt were early adopters, starting in the 1960's. In the fifties Schrödinger made a sharp critique of the idea that physical theories should be constructed from mathematical objects that take values on a manifold.

The simple part is the idea that in reality, what we call spacetime is instead an emergent property of something that is not spacetime.

Everything else gets hard, really fast!

It does not help that quantum physics is, conceptually and mathematically, a superstructure erected on top of classical physics, for reasons historical, psychological, or cognitive. Nor does it help that there is no unique way to approach the classical limit starting from a quantum-mechanical description. It should be no surprise that attempts to obtain spacetime as a classical limit of . . . something tend to make physicists' eyes water. And not just physicists.
 
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Whenever I approach these eye-wateringly difficult concepts - my main take-away is this: any attempt to impose a construct on reality inevitably leads to error. Reality is too fluid - too infinitely reducible - to subject itself to hard and fast rules unless those rules are conceived within very narrow perspectives. Thus Newton was right, then Einstein was right, then the deeper we look into the Einsteinian rules...they turn out to be wrong.

For me, the best lens to try and understand is gravity and its relationship with dark matter. I can never look at the crema floating in a freshly stirred cup of coffee without seeing a galaxy, and a potential answer.
 

dickson

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. . . Reality is too fluid - too infinitely reducible - to subject itself to hard and fast rules unless those rules are conceived within very narrow perspectives. Thus Newton was right, then Einstein was right, then the deeper we look into the Einsteinian rules...they turn out to be wrong.

Did you perhaps intend "irreducible"? That apart, I must agree with you, even if I am a card-carrying physicist by training and outlook. Every theory concocted by the (fertile!) imaginations of scientists is incomplete, and contains within the seeds of its ultimate failure.

That's the bad news. The good news: Exact same wording!

It is only through demonstrated error than we progress. I've long felt that the greatest service theoreticians in any field can make is to perform a completely rigorous and mathematically watertight calculation pertaining to some aspect of nature that is also unambiguously wrong. (On a related note, Enrico Fermi remarked once after a failed nuclear weapons test "Now we're making progress. We finally set off a dud!")

Newton was right; Newton was wrong; Einstein showed what Newton got wrong--and what he got right, in the bargain. But Eintein was wrong . . . world without end. Smile at the good, frown at the bad. And nothing, absolutely nothing, is more precious than an elegantly conceived and executed experiment that yields a decisive result.
 
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