In 1935, when quantum mechanics and General theory of relativity was very young, not very well known Soviet physicist Matvei Bronstein, aged 28 years, made the first detailed study on the harmonization of these two theories in a quantum theory of gravity. This, “perhaps the theory of the whole world,” wrote Bronstein, could displace einsteinova classical description of gravity in which it is seen as curves in the space-time continuum, and to rewrite it in quantum language as the rest of physics.
Bronstein figured out how to describe gravity in terms of quantum particles, now called gravitons, but only when gravity is weak — that is (in General relativity) when space-time was so slightly bent that it will be almost flat. When gravity is strong, “the situation is completely different,” the scientist wrote. “Without a deep revision of classical notions it seems almost impossible to imagine a quantum theory of gravity in this area.”
His words were prophetic. Eighty-three years later, physicists are still trying to understand how space-time curvature manifests itself in the macroscopic scale, arising from more fundamental and presumably the quantum picture of gravity; perhaps this is the most profound question in physics. Perhaps if you had the chance, bright mind Bronstein would speed up the process of this search. In addition to quantum gravity, he also made contributions to astrophysics and cosmology, theory of semiconductors, quantum electrodynamics and has written several books for children. In 1938, he got under Stalin repressions and was executed at the age of 31 years.
Search the complete theory of quantum gravity is complicated by the fact that quantum properties of gravity never appear in the actual experience. Physics don’t see how broken einsteinova description of the smooth space-time continuum, or quantum postanova approaching it in a slightly curved condition.
The problem is the extreme weakness of the gravitational force. While the quantized particles that transmit the strong, weak and electromagnetic forces, strong enough to tightly bind matter into atoms and can be studied literally under a magnifying glass, gravitons individually so weak that labs there is no chance to detect them. To catch a graviton with high probability, the detector of the particles should be so large and massive that it collapses into a black hole. This weakness explains why the need for an astronomical savings of the masses to influence other massive body by gravity, and is why we see gravitational effects on a huge scale.
That’s not all. The universe is apparently subjected to some kind of cosmic censorship: a region with strong gravity, where space-time curves so sharp that Einstein’s equations fail, and should reveal the quantum nature of gravity and space-time — always hide behind the horizons of black holes.
“Even a few years ago, there was a General consensus that, most likely, measure the quantization of the gravitational field in any way is impossible,” says Igor Pikovsky, a theoretical physicist at Harvard University.
And here are a few recently published in Physical Review Letters articles have changed the situation. In these works made a statement that to reach the quantum gravity might be possible — even without knowing anything about it. Works written, Sugata Bose from University College London and Kiara, Marletto and Vlatko Verulam from the University of Oxford, offering technically challenging, but doable experiment that would confirm that gravity is a quantum force, like all the others, without requiring detection of the graviton. Miles Blanco, quantum physicist from Dartmouth College, did not participate in this work, says that such an experiment could detect a clear trail of invisible quantum gravity — “Cheshire Cat smile”.
The proposed experiment will determine whether the two object — group, Bosa is planning to use a couple of microdiamonds become quantum-mechanically entangled with each other in the process of mutual gravitational attraction. Entanglement — a quantum phenomenon in which particles become inseparable intertwined, sharing a single physical description, which defines their possible combined state. (The coexistence of different possible States is called “superposition” and defines the quantum system). For example, a pair of entangled particles can exist in a superposition in which the particle A will with 50% probability to rotate (spin) from the bottom up, B — down, and with 50% probability Vice versa. No one knows in advance what result you get when measuring the spin direction of the particles, but you can be sure that they will have it the same.
The authors claim that two objects in the proposed experiment may be get confused so just in case, if the force acting between them — in this case gravity is a quantum interaction mediated by gravitons, which can support quantum superpositions. “If an experiment will be received confusion, according to work, we can conclude that gravity is quantized,” said Blanco.
To confuse diamond
Quantum gravity is so discreet that some scientists doubted its existence. The famous mathematician and physicist Freeman Dyson, who is 94 years old in 2001, argues that the universe can support a kind of “dual” description in which the “gravitational field described by the General theory of relativity is a purely classical field without any quantum behavior”, with all substance in this smooth space-time continuum to quantized particles, which obey the rules of probability.
Dyson, who helped develop quantum electrodynamics (the theory of interactions between matter and light) and is honorary Professor at the Institute for advanced study in Princeton, new Jersey, believes that quantum gravity is necessary to describe the unattainable depths of black holes. And he also believes that the detection of the hypothetical graviton may be impossible in principle. In this case, he says, quantum gravity is metaphysical, not physical.
He’s not the only skeptic. The famous English physicist sir Roger Penrose and a Hungarian scholar Lajos, Diosi independently assumed that space-time could not support superposition. They believe that it is smooth, solid, fundamentally classical nature prevents the bending of the two possible ways at the same time — and this rigidity leads to the collapse of superpositions of quantum systems like electrons and photons. “Gravitational decoherence”, in their opinion, allows to happen is one, solid, classic reality that can be felt in the macroscopic scale.
The ability to find the “smile” of quantum gravity, it would seem, refutes the argument of the Dyson. She also kills the theory of gravitational decoherence, showing that gravity and space-time really support superposition.
Suggestions Bose and Marletta appeared simultaneously and completely by chance, although experts say that they reflect the spirit of the time. Experimental quantum physics laboratory around the world are putting increasingly large microscopic object in quantum superposition and optimize test protocols of entanglement of two quantum systems. A proposed experiment would need to combine these procedures, while requiring further improve the scale and sensitivity; perhaps it will take ten years. “But the physical dead end there,” says Pikovsky, who also explores how laboratory experiments could probe the gravitational phenomena. “I think it’s difficult, but not impossible.”
This plan is discussed in more detail in the work of Bose and co. — ocean’s eleven experts for different stages of the proposal. For example, in his laboratory at the University of Warwick one of the authors Gavin Morley is working on the first step, trying to put microalgas in quantum superposition in two places. For this he will make an atom of nitrogen in microalgae near the vacancy in the structure of diamond (so-called NV centre, or a nitrogen-substituted vacancies on the diamond), and charge it microwave pulse. Elektron, rotating around of the NV-center, and at the same time absorbs the light, and there, and the system goes into a quantum superposition of two spin directions — up and down — like the spinning top, which with a certain probability rotates clockwise and with a mind. Microalgas loaded with this spin superposition is subjected to a magnetic field which causes a top spin move to the left, and lower right. The diamond itself is split into a superposition of two trajectories.
In the full experiment, scientists must do all of this with two diamonds — red and blue, for example — located near Verhalten vacuum. When the trap holding them off, two microalgae, each in a superposition of two positions, will fall vertically in a vacuum. As the fall the diamonds will feel the gravity of each of them. How strong is their gravitational attraction?
If gravity is a quantum interaction, the answer is: depending on what. Each component of the superposition of a blue diamond will experience a stronger or weaker attraction to the red diamond, depending on whether the last branch of the superposition, which is closer or farther. And gravity that will feel each component of the superposition of the red diamond, in the same way depends on the state of blue diamond.
In each case, different degrees of gravitational attraction affect the evolving components to the composition of diamonds. The two diamonds become interdependent because their status can be determined only in combination — if this, then that — so, ultimately, the direction of the spins of the two systems NV-centers will be correlated.
Once the MicroBlaze will fall side by side for three seconds, this is enough to get lost in gravitation, they will pass through another magnetic field, which will again combine the branches of each superposition. The last step of the experiment Protocol of “confused knowledge” (entanglement witness), developed by the Danish physicist Barbara Teral and others: blue and red diamonds are included in different devices, which measure the direction of the spin systems of the NV-centers. (Measurement leads to the collapse of superpositions in certain States). Then the two results are compared. Conducting the experiment again and again and comparing many pairs of spin measurements, scientists can determine whether or not the spins of two quantum systems are correlated among themselves more often than specifies the upper limit for objects that are not quantum-mechanically entangled. If so, gravity really confuses diamonds and can support superposition.
“What’s interesting in this experiment is the fact that you don’t need to know what the quantum theory,” says Blanco. “All that is necessary is to assert that there is some quantum aspect in this field, which is mediated force between two particles”.
Technical difficulties — weight. The largest object that was placed in superposition in two places before was a 800-atomic molecule. Every Michaelmas contains over 100 billion atoms of carbon — enough to accumulate appreciable gravitational force. Unpacking quantum-mechanical nature require low temperatures, high vacuum and precise control. “A lot of work is to set up the initial superposition and running,” said Peter Barker, a member of the experimental team, which will improve the methods of laser cooling and capture of microdiamonds. If it could be done with one diamond, adds the Lord, “the second won’t be a problem.”
The uniqueness of gravity?
Researchers of quantum gravity do not doubt that gravity is a quantum interaction, can cause confusion. Of course, gravity is something unique, and still have much to learn about the origin of space and time, but quantum mechanics must be involved, the researchers say. “Well, really, what’s the point in theory, in which most quantum physics and gravity is a classic,” says Daniel Harlow, a quantum gravity researcher at MIT. Theoretical arguments against mixed quantum-classical models is very strong (though not undisputed).
On the other hand, the theorists were wrong before. “If you can check, why not? If this will shut up these people who are questioning kvantovoi gravity, that would be great,” said Harlow.
Reading work, Dyson wrote: “the Proposed experiment is certainly of great interest and requires in terms of this quantum system”. However, he notes that the direction of thought of the authors of the quantum fields are different from him. “I wonder whether this experiment to resolve the question of the existence of quantum gravity. The question I asked — whether the observed individual graviton is another question, and it may have a different answer.”
The direction of thought the Bose, Marletto and their colleagues about quantum gravity stems from the work of Bronstein in 1935. (Dyson called the work of Bronstein “great work”, which he had not seen before). In particular, Bronstein has shown that weak gravity, we give birth to low weight can be approximated by the Newton law of gravitation. (This is the force that acts between the composition of microdiamonds). According to Blanco, calculations of the weak quantized gravity is not particularly carried out, although are certainly more relevant than the physics of black holes or the Big Bang. He hoped that the new experimental proposal will encourage theorists to search for subtle refinements to the Newtonian approximation, which is a future of desktop experiments could try to check.
Leonard Susskind, a famous theorist of quantum gravity and strings at Stanford University, saw the value of the proposed experiment, because “it provides observations of gravity in the new range of masses and distances”. But he and other researchers stressed that the MicroBlaze can’t reveal anything about the complete theory of quantum gravity or space-time. He and his colleagues would like to understand what is happening in the center of the black hole and the Big Bang.
Perhaps one of the clues to why quantize gravity so much harder than all the rest, is that other forces of nature have so-called “locality”: of a quantum particle in one area of the field (photons in the electromagnetic field, for example) “independent of other physical entities in another region of space,” says mark van Raamsdonk, theorist of quantum gravity from the University of British Columbia. “But there are many theoretical evidence that gravity isn’t working”.
The best sand models of quantum gravity (with simplified space-time geometries) it is impossible to assume that tape space-time fabric is divided into independent three-dimensional pieces, said Wang Raamsdonk. Instead, modern theory suggests that the underlying, fundamental constituents of space “is organized rather two-dimensional”. The fabric of space-time may be like a hologram or a video game. “Although the picture is three-dimensional, information is stored in a two-dimensional computer chip”. In this case, the three-dimensional world is illusa in the sense that its various parts are not so independent. In analogy with a video game, a few bits on the two-dimensional chip can encode the global function of the entire gaming universe.
And this difference matters when you are trying to create a quantum theory of gravity. The usual approach to quantization of something is to define its separate parts, the particles, for example, and then the use of quantum mechanics. But if you do not specify the correct components, you will get the wrong equation. Direct quantization of three-dimensional space, which wanted to make Bronstein, works to some extent with weak gravity, but is useless when the space-time is strongly curved.
Some experts say that witnessing the smiles of quantum gravity may lead to motivation of this kind of abstract reasoning. In the end, even the most high-profile theoretical arguments about the existence of quantum gravity are not supported by experimental facts. When van Raamsdonk explains his research at the Colloquium of scientists, he says, usually starts with a story about what gravity you need to rethink quantum mechanics because the classical description of space-time breaks down at black holes and the Big Bang.
“But if you conduct this simple experiment and show that the gravitational field was in a superposition, the failure of the classical description becomes apparent. Because it is an experiment, which implies that gravity is quantum”.
According to the materials of the Quanta Magazine
Physicists have found a way to see the “smile” of quantum gravity
Ilya Hel