If you divide the matter in the universe into smaller and smaller constituents, you will eventually reach a limitation when faced with a fundamental and indivisible particle. All macroscopic objects can be divided into molecules, even atoms, then electrons (which are fundamental) and nuclei, then into protons and neutrons, and finally there will be quarks and gluons inside them. Electrons, quarks and gluons are examples of fundamental particles that cannot be further separated. But how is it possible that time and space itself have the same limitations? Why do Planck values exist at all that cannot be further divided?
To understand where Planck's quantity comes from, it is worth starting with two pillars that govern reality: general relativity and quantum physics.
General relativity connects matter and energy that exist in the Universe with the curvature and deformation of the fabric of space-time. Quantum physics describes how various particles and fields interact with each other within the fabric of space-time, including on a very small scale. There are two fundamental physical constants that play a role in general relativity: G is the gravitational constant of the universe, and c is the speed of light. G arises because it sets an indicator of space-time deformation in the presence of matter and energy; c - because this gravitational interaction propagates in space-time at the speed of light.
In quantum mechanics, two fundamental constants also appear: c and h, where the latter is Planck's constant. c is the speed limit for all particles, the speed at which all massless particles must move, and the maximum speed at which any interaction can propagate. Planck's constant was incredibly important in describing how quantum energy levels are quantized (counted), interactions between particles, and all possible outcomes of events. An electron revolving around a proton can have any number of energy levels, but they all appear in discrete steps, and the size of these steps is determined by h.
Combine these three constants, G, c and h, and you can use different combinations of them to build a scale for length, mass, and time period. These are known, respectively, as Planck length, Planck mass and Planck time. (Other quantities can be plotted, for example, Planck's energy, Planck's temperature, and so on). All of this is, by and large, a scale of length, mass, and time at which - in the absence of any other information - quantum effects will be significant. There are good reasons to believe that this is the case, and it is fairly easy to see why it is.
Imagine that you have a particle of a certain mass. You ask the question: "If my particle had such a mass, how small should it be compressed to make it a black hole?" You may also ask: "If I had a black hole of a certain size, how long would it take for a particle moving at the speed of light to cover a distance equal to this size?" The Planck mass, Planck length, and Planck time correspond exactly to these quantities: a black hole with a Planck mass will be Planck length and intersect at the speed of light in Planck time.
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But Planck mass is much, much more massive than any particles we've ever created; it is 10 (19 power) times heavier than a proton! Planck's length, likewise, is 10 (14 power) times less than any distance we have ever sounded, and Planck time is 10 (25 power) times less than any directly measured. These scales have never been directly available to us, but they are important for another reason: Planck energy (which you can get by putting Planck mass in E = mc2) is the scale at which quantum gravitational effects begin to take on importance and significance.
This means that at energies of this magnitude - either time scales shorter than Planck's time, or length scales less than Planck's length - our current laws of physics must be violated. The effects of quantum gravity come into play, and the predictions of general relativity are no longer reliable. The curvature of space becomes very large, which means that the "background" that we use to calculate quantum quantities also ceases to be reliable. Uncertainty in energy and time means that the uncertainties become higher than the values we know how to calculate. In short, the physics we are used to no longer work.
This is not a problem for our universe. These energy scales are 10 (15 degrees) times higher than those that the Large Hadron Collider can reach, and 100,000,000 times larger than the most energetic particles created by the Universe itself (high energy cosmic rays), and even 10,000 times higher than the indicators reached by the Universe immediately after the Big Bang. But if we wanted to explore these limits, there is one place where they might be important: at singularities located at the centers of black holes.
In these places, masses that significantly exceed the Planck mass are compressed to a size theoretically less than the Planck length. If there is a place in the Universe where we bring all lines into one and enter the Planck mode, then this is it. We cannot access it today because it is obscured by the black hole event horizon and is inaccessible. But if we are patient enough - and it takes a lot of patience - the universe will give us that opportunity.
You see, black holes slowly decay over time. The integration of quantum field theory into the curved spacetime of general relativity means that a small amount of radiation is emitted in space outside the event horizon, and the energy for this radiation comes from the mass of the black hole. Over time, the mass of the black hole decreases, the event horizon contracts, and after 10 (to the 67th power) years, the black hole of solar mass will completely evaporate. If we could get access to all the radiation that left the black hole, including the very last moments of its existence, we could doubtless be able to piece together all the quantum effects that our best theories did not predict.
It is not at all necessary that space cannot be divided into even smaller units than Planck's length, and that time cannot be divided into units smaller than Planck time. We just know that our description of the universe, including our laws of physics, cannot go beyond these scales. Is space quantifiable? Does time really flow continuously? And what do we do about the fact that all known fundamental particles in the universe have masses much, much less than Planck's? There are no answers to these questions in physics. Planck scales are not as fundamental in limiting the universe as in our understanding of the universe. So we keep experimenting. Perhaps, when we have more knowledge, we will receive answers to all questions. Not yet.
ILYA KHEL