If you divide the matter in the Universe into smaller and smaller components, you will eventually reach the limit, 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 protons and neutrons, and finally inside them are quarks and gluons. Electrons, quarks and gluons are examples of fundamental particles that cannot be separated even more. But how is it possible to have the time and space were the same restrictions? Why are the Planck values, which can not be divided further?
To understand the origin of Planck size, it is worth to start with the two pillars that govern reality: the General theory of relativity and quantum physics.
The General theory of relativity relates matter and energy existing in the Universe, with curvature and deformation of the fabric of space-time. Quantum physics describes how particles and fields interact with each other within the fabric of space-time, including in a very small scale. There are two fundamental physical constants that play a role in the General theory of relativity: G is the gravitational constant of the Universe, and c is the speed of light. G occurs, since it provides a measure of the deformation of space-time in the presence of matter and energy; c — because it is the gravitational interaction is distributed in space-time at the speed of light.
In quantum mechanics too, there are two fundamental constants: c and h, where the latter is Planck's constant. c is a limiting speed for all particles, the rate at which have to move all massless particles, and the maximum rate at which can spread any interaction. Planck's constant was incredibly important, as are quantized (calculated) quantum energy levels, the interactions between particles and all possible outcomes of the event. An electron rotating around a proton may 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 will be able to use different combinations for the scale of length, mass and time. They are known respectively as the Planck length, Planck mass and Planck time. (It is possible to construct other quantities, for example, the Planck energy, the Planck temperature, and so on). All this, in General, the scale of length, mass and time, which — in the absence of any other information — will be significant quantum effects. There are good reasons to believe that this is the case, and quite easy to understand why.
Imagine you have a particle with a certain mass. You ask the question: "If my particle had a mass in as small a volume it must shrink to become a black hole?". You can still ask: "If I was a black hole of a certain size, how much time a particle moving at the speed of light would cover distance equal to this size?". Planck mass, Planck length and Planck time correspond exactly to these values: black hole the Planck mass is the Planck length and intersect at the speed of light in Planck time.
But the Planck mass is much, much more massive than any particles we've ever created; it is in 1019 times heavier than a proton! A Planck length, in the same way in 1014 times smaller than any distance that we have ever tested, and the Planck time, 1025 times smaller 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 placing the Planck mass in E = mc2) is the scale at which quantum gravitational effects begin to acquire importance and significance.
This means that if energy of such magnitude or temporal scales shorter than the Planck time or length scales less than the Planck length — our current laws of physics must be violated. Come into play effects of quantum gravity, and the predictions of General relativity cease to be reliable. The curvature of space becomes very large, and therefore the "background" which we use to calculate quantum units, also ceases to be reliable. The uncertainty of energy and time means that the uncertainties are higher values that we know how to calculate. In short, familiar to us in physics is no longer working.
For our Universe is not a problem. These energy scales in the 1015 times higher than those which can reach Large hadron Collider, and 100 000 000 times more than the most energetic particle that creates the universe itself (cosmic rays high energy), and even 10 000 times higher than achieved by the universe immediately after the Big Bang. But if we wanted to explore these limits, there is one place where they can be important: the singularities are located in the centers of black holes.
In these places the mass, far exceeding the Planck mass, shrink in size, theoretically less of the Planck length. If in the Universe there is a place where we bring all the lines into one and included in the Planck regime, this is it. We can't access it today because it is covered by the event horizon of a black hole and inaccessible. But if we are patient enough and need a lot of patience — the universe will give us that opportunity.
You See, black holes eventually disintegrate slowly. Integration of quantum field theory in curved space-time of General relativity means that a small amount of radiation is emitted in the 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 shrinks, and 1067 years black hole solar masses has evaporated completely. If we could get access to the entire radiation leaves a black hole, including the last moments of its existence, we certainly could gather together all the quantum effects, which are not predicted by our best theories.
It is not necessary that the space cannot be divided into even smaller units than Planck 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 can't go beyond those levels. If space is quantized? Whether time flows continuously, really? And what are we to do with the fact that all known fundamental particles in the Universe have masses much less than the Planck? To these questions in physics have no answers. The Planck scale is not so fundamental in limiting the Universe as to our understanding of the Universe. So we continue to experiment. Perhaps when we have more knowledge we will receive answers to all questions. Yet....
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