“What is space-time made of?”, The physicist Aaron Wall of the Stanford Institute for Theoretical Physics wonders. During the past, there are no physicists in different ways trying to comprehend the riddle of space-time, considering it not just as an empty background against which the history of the Universe unfolds, but rather as a stream of quantum information flowing from one point to another. Wall and his colleagues are increasingly convinced that such a representation of space-time can be the key to developing a theory that can explain gravity using the principles of quantum mechanics. Physicists have been dreaming of this since the days of Albert Einstein.
Peter Zenchikovsky from the Institute of Nuclear Physics of the Polish Academy of Sciences asks the same question as Wall. Is space-time absolute, unchanging, forever and always present arena in which events unfold? Or perhaps it is a dynamic creation, arising as if on a certain scale of distance, time or energy? The mention of the absolute is not welcome in modern physics. It is believed that space-time is emergent, that is, it originates from somewhere. It is not clear where.
What is space time?
Most physicists believe that the structure of space-time is formed in an incomprehensible way within the scale of Planck, that is, on scales close to one trillion of a trillionth of a meter. However, there are some beliefs that call into question the uniqueness of this interpretation. There are many arguments in favor of the fact that the emergence of space-time can occur as a result of processes that are much closer to our reality: at the level of quarks and their conglomerates.
“Mathematics is one thing, the relationship with the real world is another,” says Zenchikovsky. “For example, the value of Planck’s mass seems suspicious. One would expect her to have a value more characteristic of the world of quanta. Meanwhile, it corresponds to about 1/10 of the mass of a flea, which is definitely a classic object. ”
Most physicists tend to assume that space-time is created on Planck scale, at distances close to one trillionth of a trillionth of a meter (~ 10-35 m). In his article in Foundations of Science, Zenchikovsky systematizes the observations of various authors on the formation of space-time and argues that the hypothesis of its formation on the scale of quarks and hadrons (or quark aggregates) is quite reasonable for several reasons.
Questions about the nature of space and time have puzzled humankind since ancient times. Can time be separate from matter, creating a “container” for movements and events that occur with the participation of particles, as suggested by Democritus in the 5th century BC? Or maybe all these attributes of matter cannot exist without it, as Aristotle suggested a century later?
Despite the fact that a thousand years have passed since then, these issues have not yet been resolved. Moreover, both approaches – despite their obvious difference – are deeply rooted in the pillars of modern physics. In quantum mechanics, events occur in a rigid arena with a uniformly current time.
Meanwhile, in the general theory of relativity, matter deforms the elastic space-time (stretches and twists it), and space-time tells the particles how to move. In other words, in one of the theories, actors enter the already prepared stage to play their roles, and in the other they create a scene during the performance, which, in turn, affects their behavior.
In 1899, the German physicist Max Planck noticed that with certain combinations of certain constants in nature one could get the most fundamental units of measurement. Only three constant – the speed of light c, the gravitational constant G and Planck constant h – and we get the units of distance, time and mass equal to (respectively) 1.62 x 10-35 m, 5.39 x 10-44 s and 2, 18 x 10-5 g. Based on modern beliefs, space-time should be born on the Planck length. But there are no significant arguments in favor of the rationality of this hypothesis.
Both our most complex experiments and theoretical descriptions reach the scale of quarks at a level of 10-18 m. How do we know that on the way to the Planck length – over a dozen consecutive and even smaller orders of magnitude – space-time acquires its structure? We do not even know if the concept of space-time at the level of hadrons is rational! The separation cannot be done endlessly, because at a certain stage the question of the next minor part simply ceases to make sense. A perfect example would be the temperature. This concept serves perfectly on macroscales, but with successive fissions of matter, we reach the scale of individual particles and the concept of temperature loses its meaning.
“At present, we first seek to construct a quantized discrete space-time and then“ populate ”it with discrete matter. But if space-time is the product of quarks and hadrons, the dependence will be inverse: the discrete property of matter should enhance the discreteness of space-time, “says Zenchikovsky and adds:” Planck relied on mathematics. He wanted to create units of the smallest possible constants. But mathematics is one thing, and the relationship with the real world is different. The value of the Planck mass seems suspicious. One would expect her to have a more suitable characteristic for the world of quanta. But it corresponds to about 1/10 of a flea mass, which is definitely a classic object. ”
Since we want to describe the physical world, we must rely on physical rather than mathematical arguments. And therefore, when we use the Einstein equations, we describe the Universe on a large scale and there is a need to introduce an additional gravitational constant, known as the cosmological constant “lambda”. If, in constructing the fundamental units, we expand our initial set of three constant lambda, in the case of mass, we get not one, but three fundamental values: 1.39 x 10-65 g, 2.14 x 1056 g, and 0.35 x 10 -24 g. The first can be interpreted as a quantum of mass, the second – the mass level of the observable Universe, and the third one resembles the mass of hadrons (for example, the neutron mass is 1.67 x 10-24. Similarly, taking into account the lambda, the unit 6 will appear , 37 x 10-15 m, very close to the size of hadrons.
“Games with constants can be risky, because a lot depends on which constants we choose. For example, if space-time really was a product of quarks and hadrons, then its properties, including the speed of light, would also have to be emergent. And this would mean that the speed of light cannot be among the main constants, ”Zenchikovsky notes.
Another factor in favor of the formation of space-time on the scale of quarks and hadrons are the properties of the elementary particles themselves. The standard model, for example, does not explain why there are three generations of particles, where their masses come from, or why there are so-called internal quantum numbers that include isospin, hypercharge, and color. In the picture presented by Professor Zenchikovsky, these values may be associated with a specific six-dimensional space created by the position of the particles and their momenta. Space constructed in this way equally respects the position of the particles (matter) and their movements (processes). It turns out that mass properties or internal quantum numbers can be a consequence of the algebraic properties of a six-dimensional space. Moreover, these properties also explain the impossibility of observing free quarks.
“The emergence of space-time can be associated with changes in the organization of matter occurring on the scale of quarks and hadrons, in a more primary six-dimensional phase space. However, it is not entirely clear what to do next with this picture. Each subsequent step will require going beyond what we know. And we do not even know the rules of the game, according to which Nature plays with us, we still have to guess them. However, it seems reasonable that all constructions begin with matter, because it is physically and experimentally accessible. In this approach, space-time will be only our idealization of relations between elements of matter, ”Professor Zenchikovsky summarizes.