The limits of physics

There are two kinds of limits in scientific research:

1.      Theoretical or intrinsic limits: when these limits exist, no matter how many scientific discoveries may be made in the future, they won’t be exceeded.

2.      Practical limits: they appear when, in theory, a problem can have a solution, but there are practical reasons that make it impossible, at least for the time being. In these cases, we cannot affirm that the problem won’t be solved in the future.

Sometimes we don’t know if a given limit is theoretical or practical. In these cases, what will happen in the future is open. If the limit turns out to be theoretical, it will never be exceeded. If it is practical, it will be exceeded if our technical capabilities exceed the technical needs for its resolution, being possible that this will never happen. Take, as an example, the inherently difficult math problems I mentioned in the previous post.

As I pointed out in another post, in the last centuries we have discovered that the weft of the universe can be tackled on different levels. While the next level has not been found, what happens in the preceding level cannot be explained, it can just be described. In consequence, we cannot explain the lowest level known at any point, we can just describe it. At the risk of repeating myself, I’m going to summarize what I said there:

·         First level: chemical substances. The founder of modern chemistry (Anton Laurent Lavoisier) wrote a book, Traité élémentaire de Chimie (1789), that may be considered a catalog of the properties of the chemical species known at that time. It is a description, not an explanation.

·         Second level: atoms of chemical elements. John Dalton’s atomic theory explained the properties of chemical substances, but the properties of atoms couldn’t be explained, they could only be described. Mendeleev’s periodic table was just a catalog of atoms.

·         Third level: elementary particles. Ernest Rutherford’s atomic model explained the properties of atoms, which are made of three types of elementary particles: electrons, protons and neutrons (the latter discovered later). The properties of elementary particles could just be described. Nobody knew why protons have a positive charge and neutrons have no charge.

·         Fourth level: quarks and bosons. To put some order in the jumble of elementary particles being discovered (baryons, mesons, positrons, heavy electrons, neutrinos, photons...), Murray Gell-Mann proposed in 1964 the quark theory, which explains the properties of hadrons, a family of elementary particles that includes baryons and mesons. But no one knows why quarks and the other family of elementary particles, leptons (which includes the electron and the neutrino) have the properties they have. We can only describe them.

Lavoisier

These are the four levels of matter that we know today: molecules, explained by atoms; atoms, explained by elementary particles; certain elementary particles (hadrons), explained by fundamental particles (quarks and bosons); and fundamental particles, which up to now cannot be explained, they can just be described. To these we must add leptons, which are still in the third level, because their structure and existence has not yet been explained.

What about the future? Perhaps one day we will be able to explain the behavior of fundamental particles by discovering a fifth level, but then we won’t be able to explain this level (whatever it is), it shall only be described. And so on, and so forth. The last level discovered will always be unexplainable, until the next one is found.

The conclusion is evident: physical science will never be able to explain everything. This limit is theoretical, rather than practical, for there will always exist a lowest level in our understanding of matter and of physical nature. Therefore, the theory of everything that physicists are always talking about will always remain out of our reach.

Einstein

A further unsolved physical problem is the nature of time. The arrow of time (the fact that time is a directional dimension, which always goes from the past to the present, and then to the future) is strongly established by human experience, by the second principle of thermodynamics, and by the existence of many irreversible processes. However, in a glaring opposition to these arguments, leading scientists such as Einstein denied the existence of the arrow of time, just because in their theory time seems to be reversible. This provides us with a magnificent example of how even a genius can forget that, in science, facts must prevail over theories.

Will we ever understand the nature of time? Is this a theoretical limit, or a practical limit of our theories? At this moment I would not dare to assert either one thing or the other.

The same post in Spanish

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Manuel Alfonseca