Einstein's general theory of relativity, in which gravity is born out of the curvature of spacetime, is remarkable. It has been verified with an incredible level of accuracy, in some cases up to fifteen decimal places. One of her most interesting predictions was the existence of gravitational waves: ripples in spacetime that propagate freely. Not so long ago, these waves were caught by the LIGO and VIRGO detectors.
And yet there are many questions for which we do not yet have answers. Quantum gravity could help find them.
We know that general relativity is incomplete. It manifests itself well when the quantum effects of space-time are completely invisible, which is almost always the case. But when the quantum effects of spacetime get large, we need a better theory: a theory of quantum gravity.
An illustration of the early universe, consisting of quantum foam, when quantum fluctuations were huge and manifested on the smallest scale
Since we have not yet formulated a theory of quantum gravity, we do not know what space and time are. We have several suitable theories for quantum gravity, but none of them is widely accepted. Nevertheless, based on existing approaches, we can assume what can happen to space and time in the theory of quantum gravity. Physicist Sabine Hossfender has collected ten startling examples.
1) In quantum gravity, there will be wild fluctuations in space-time even in the absence of matter. In the quantum world, vacuum is never at rest, as are space and time.
At the smallest quantum scale, the universe can be filled with tiny, microscopic black holes with low masses. These holes can connect or expand inward in a very interesting manner.
2) Quantum spacetime can be filled with microscopic black holes. Moreover, it can contain wormholes or infantile universes can be born - like little bubbles that break away from the mother's universe.
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3) And since this is a quantum theory, spacetime can do it all at the same time. It can simultaneously create an infant universe and not create it.
The fabric of space-time may not be a fabric at all, but consist of discrete components, which only seem to us to be a continuous fabric on large macroscopic scales.
4) In most approaches to quantum gravity, spacetime is not fundamental, but consists of something else. These can be strings, loops, qubits, or variants of space-time "atoms" that appear in condensed matter approaches. Individual components can be disassembled only with the use of the highest energies, far exceeding those that are available to us on Earth.
5) In some approaches with condensed matter, space-time has the properties of a solid or liquid body, that is, it can be elastic or viscous. If this is true, the observed consequences are inevitable. Physicists are currently looking for traces of similar effects in wandering particles, that is, in light or electrons, that reach us from distant space.
Schematic animation of a continuous beam of light scattered by a prism. In some approaches to quantum gravity, space can act as a dispersive medium for different wavelengths of light
6) Spacetime can affect how light passes through it. It may not be completely transparent, or light of different colors may move at different speeds. If quantum spacetime affects the propagation of light, this too can be observed in future experiments.
7) Fluctuations in space-time can destroy the ability of light from distant sources to create interference patterns. This effect was sought and never found, at least in the visible range.
Light passing through two thick slits (top), two thin slits (center), or one thick slit (bottom) exhibits interference indicative of its wave nature. But in quantum gravity, some of the expected interference properties may not be possible.
8) In areas of strong curvature, time can turn into space. This can happen, for example, inside black holes or in a big bang. In this case, the space-time known to us with three spatial and dimensions and one temporal can turn into a four-dimensional "Euclidean" space.
The connection of two different places in space or time through a wormhole remains only a theoretical idea, but it may not only be interesting, but also inevitable in quantum gravity
Spacetime can be connected non-locally with tiny wormholes that permeate the entire universe. Such non-local connections must exist in all approaches whose underlying structure is not geometric, such as a graph or a network. This is due to the fact that in such cases, the concept of "proximity" will not be fundamental, but implied and imperfect, so that distant regions can be accidentally connected.
10) Perhaps in order to combine quantum theory with gravity, we need to update not gravity, but the quantum theory itself. If so, the consequences will be far-reaching. Since quantum theory is at the core of all electronic devices, revising it will open up entirely new possibilities.
Although quantum gravity is often viewed as a highly theoretical idea, there are many possibilities for experimental verification. We all travel through space-time every day. Understanding him can change our lives.
Ilya Khel