General, Research, Technology

Like light, spacetime can create a rainbow

When white light passes through a prism, a rainbow onthe other end demonstrates a rich palette of colors. Theorists at the Faculty of Physics at the University of Warsaw have shown that models of the Universe using any quantum theory of gravity should also have a kind of “rainbow” consisting of different versions of space-time. This mechanism predicts that instead of a single and common space-time, particles of different energies should sense slightly modified versions of it.


We all probably saw the experiment: when white light passes through a prism, it decays to form a rainbow. This is because white light is a mixture of photons of different energies, and the higher the photon energy, the more it is deflected by the prism. Thus, we can say that a rainbow arises because photons of different energies feel the same prism as having different properties. For many years, scientists have suspected that particles of different energies in the models of the quantum universe essentially sense different structures of space-time.

Warsaw physicists used cosmologicala model containing only two components: gravity and one type of matter. Within the framework of the general theory of relativity, the gravitational field is described by space-time deformations, while matter is represented by a scalar field (the simplest type of field in which each point in space has only one value).

“There are many competing theories today.”quantum gravity. Therefore, we formulated our model in the most general terms so that it can be applied to any of them. Someone may suggest one type of gravitational field - which in practice means space-time - proposed by one quantum theory, another may suggest another. Some mathematical operators in the model will change, but not the nature of the phenomena occurring in them, ”says Andrea Dapor, graduate student at the University of Warsaw.

“This result is simply amazing. We start with the fuzzy world of quantum geometry, where it’s even hard to say what time is and what space is, but the phenomena occurring in our cosmological model seem to occur in ordinary space-time, ”says another graduate student Mehdi Assaniussi.

Things got even more interesting when physicists lookedon scalar field excitations that are interpreted as particles. Calculations showed that in this model, particles that differ in terms of energy interact with quantum space-time in a different way - like photons with different energies interact differently with a prism. This means that even the effective structure of classical space-time is perceived differently by individual particles depending on their energy.

The appearance of an ordinary rainbow can be described in terms ofrefractive index, the magnitude of which depends on the wavelength of light. In the case of a similar space-time rainbow, similar relationships are proposed: beta function, a measure of the degree of difference in the perception of classical space-time by different particles. This function reflects the degree of non-classicality of quantum space-time: under conditions close to classical, it tends to zero, while in true quantum conditions it tends to unity. Now the Universe is in a classic-like state, so the beta value is close to zero, physicists estimate it as not exceeding 0.01. Such a small value of the beta function means that the rainbow of space-time is currently very narrow and cannot be detected experimentally.

The study of theoretical physicists of WarsawUniversity, funded by grants from the National Science Center of Poland, led to another interesting conclusion. A space-time rainbow is the result of quantum gravity. Physicists generally agree that the effects of such a plan will be visible only at gigantic energies close to the Planck energy, which is millions or billions of times higher than the particle energy, to which the Large Hadron Collider is now accelerating. However, the value of the beta function depends on time, and at moments close to the Big Bang, it could be much higher. When beta approaches zero, the space-time rainbow increases significantly. As a result, under such conditions, the rainbow effect of quantum gravity can be potentially observed even at particle energies hundreds of times lower than the proton energy on a modern LHC.

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