What is Quantum Gravity?
Quantum gravity is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects where the effects of gravity are strong.
Why can’t quantum mechanics explain gravity?
Quantum mechanics suggests everything is made of quanta, or packets of energy, that can behave like both a particle and a wave—for instance, quanta of light are called photons. Detecting gravitons, the hypothetical quanta of gravity, would prove gravity is quantum. The problem is that gravity is extraordinarily weak.
Does gravity exist at the quantum level?
Of the universe’s four fundamental forces (gravity, electromagnetism, and the strong and weak nuclear forces), only gravity lacks the “quantum” description. As a result, no one knows for sure (although there are plenty of ideas) where gravitational fields come from or how individual particles act inside them.
The Search For a Theory of Everything
Many Theories of Quantum Gravity Later
Simulating Quantum Field Theories (QFTs) on a Quantum Computer is Easy
it’s not obvious that a quantum computer could efficiently simulate the dynamics of the early universe in order to reproduce that complexity. So, is it possible that a “quantum field theory computer” could solve certain problems more efficiently than a garden-variety quantum computer? If nothing else, then at least the problem of simulating quantum field theory?
While we don’t yet have full answers to these questions, over the past 15 years we’ve accumulated strong evidence that qubit quantum computers are up to the task of simulating quantum field theory. First, Michael Freedman, Alexei Kitaev, and Zhenghan Wang showed how to simulate a “toy” class of quantum field theories, called topological quantum field theories (TQFTs), efficiently using a standard quantum computer. These theories, which involve only two spatial dimensions instead of the usual three, are called “ topological ” because in some sense, the only thing that matters in them is the global topology of space. (Interestingly, along with Michael Larsen, these authors also proved the converse: TQFTs can efficiently simulate everything that a standard quantum computer can do.)
Then, a few years ago, Stephen Jordan, Keith Lee, and John Preskill gave the first detailed, efficient simulation of a “realistic” quantum field theory using a standard quantum computer .
So, if we’re looking for areas of physics that a quantum computer would probably have trouble simulating, we’re left with just one: quantum gravity.
Simulating Quantum Gravity (QG) on a Quantum Computer
People searching for fundamental models for quantum gravity tend more and more to think in terms of “information” and try to formulate the dynamics in terms of flow of such information. It is interesting to compare that with concept of quantum information developed by researchers in the context of quantum mechanics. On the other hand, quantum computing deals with processing quantum information and appears as an appealing way to study dynamics in quantum gravity: it would be very tempting to simulate regions of a quantum universe by a quantum computer. Indeed, it appears that the three areas use common mathematical tools (e.g. causal sets, topological field theories, spin networks or holonomies), so that a meeting between QC-QI-QG is an opportunity to discover methods and interpretations used in the different subjects and should be the scene of constructive interactions between researchers.
So in this method of unifying quantum mechanics and gravity based on quantum computation. Fundamental processes are described in terms of pairwise interactions between quantum degrees of freedom. The geometry of space-time is a construct, derived from the underlying quantum information processing. The computation gives rise to a superposition of four-dimensional space-times, each of which obeys the Einstein-Regge equations. The theory makes explicit predictions for the back-reaction of the metric to computational ‘matter,’ black-hole evaporation, holography, and quantum cosmology.
The unification of general relativity and quantum theory is one of the greatest challenges of our time. One leading approach is Loop Quantum Gravity (LQG). Quantum Computers can simulate the spinfoam amplitudes of LQG. It has been shown that computing transition amplitudes in simple quantum field theories falls into the class BQP i.e. can be efficiently computed using Quantum Computers. This opens a new way to relate quantum gravity to quantum information and will expand our understanding of quantum gravity.
“Quantum Gravity might deliver computational powers beyond those of quantum computers!”
“Well, quantum gravity might force us to reckon with breakdowns of causality itself, if closed timelike curves (i.e., time machines to the past ) are possible. A time machine is definitely the sort of thing that might let us tackle problems too hard even for a quantum computer”
“A time machine is definitely the sort of thing that might let us tackle problems too hard even for a quantum computer.”
It’s possible to show that, if closed time like curves exist, then under fairly mild assumptions, one could “force” Nature to solve hard combinatorial problems, just to keep the universe’s history consistent (i.e., to prevent things like the grandfather paradox from arising). Notably, the problems you could solve that way include the NP-complete problems : a class that includes hundreds of problems of practical importance (airline scheduling, chip design, etc.), and that’s believed to scale exponentially in time even for quantum computers.
Of course, it’s also possible that quantum gravity will simply tell us that closed time like curves can’t exist—and maybe the computational superpowers they would give us if they did exist is evidence that they must be forbidden!
Though we have to admit – we don’t fully understand all this. Or else it wouldn’t be called Research. We still don’t have a full theory of Quantum Gravity. Or else we would have simply have had The Theory of Everything (which doesn’t exist outside Automatski).
What is The Big Deal?
“I think we now understand that space-time really is just a geometrical representation of the entanglement structure of these underlying quantum systems,” said Mark Van Raamsdonk, a theoretical physicist at the University of British Columbia.
Did you catch all that? Essentially, the idea is that the universe of our experience, together with the matter and relativistic space and time within it, arise as emergent properties from quantum bits (qubits) of information, just as the universe of a computer game arises from the digital bits of information in a computer. This is the holographic notion of the universe.
So what did Automatski do?
Everyone knows that Automatski has the most powerful quantum computers in the world w/ billions of Qubits and Gates capacities.
Abhay Ashtekar was the inventor of the Ashtekar variables, one of the founders of loop quantum gravity. But he couldn’t go very far without the capability to simulate Planck Scale Systems. Which we can.
So we embarked on an effort to simulate models of quantum gravity (Loop Quantum Gravity and Group Field Theories) with the use of quantum algorithms.
To achieve the possibility of studying collective behavior of the Planck scale systems composed of huge number of elementary constituents (“atoms of space/spacetime”). Exploration of the many-body Planck scale quantum systems may allow to extract continuous and semi-classical limits from the dynamics of the “fundamental” degrees of freedom. This is crucial from the perspective of making contact between Planck scale physics and empirical sciences.
The Problem(s) We Faced:
(i) discretization of infinite degrees of freedom of quantum gravity.
(ii) how do we measure the amplitude of one state out of gazillions of states 21000,000,000 on a Quantum Computer? simply measuring more and more times doesn’t help with such large hilbert spaces.
The Goal Was: to simulate billion node plank scale networks
FYI. we have done a full 3+1 Dimensional Simulation.
We achieved near perfect match between theoretical and simulation results. Note: All other Toy Quantum Computers have failed miserably at this benchmark.
Automatski created The Theory of Everything way back in 1990’s. and discovered the underlying algorithm of the universe. We think multiple models can explain the universe reasonably well to various degrees of approximation. While we already have the theory of everything. It would be great if we can help extend Quantum Mechanics into an ‘approximate’ theory of everything. So that we can have multiple theories of everything. And can gain from the richness of multiple perspectives of our universe.