Why don’t stars escape from the galaxy? What causes the universe to accelerate its ‎expansion? And how the heck do you combine quantum mechanics ‎with relativity? ‎These are just some of the unsolved riddles of physics.

“Tiger got to hunt, bird got to fly;
Man got to sit and wonder 'why, why, why?'
Tiger got to sleep, bird got to land;
Man got to tell himself he understand.”

-Kurt Vonnegut, Cat’s Cradle

Curiosity is one of the traits that distinguish human beings from other animals, because man has never stopped to ask or explore in order to better understand the world around him. This is particularly evident in physics, in which important discoveries and breakthroughs have been discovered since the beginning of the 20th century; such as the development of quantum mechanics, the theory of general relativity and the big bang theory, to the amazing discovery of gravitational waves last year. But of course there are still a number of unsolved riddles which physicists are still working on today, which might lead us to future breakthroughs.

Dark Matter

In 1687 Isaac Newton published his book "Mathematical Principles of Natural Philosophy" which laid the foundation for classical mechanics. Among other things, he explained the motion of bodies under the influence of gravity and the movement of the stars in the sky. But during the 20th century, and especially in the 70s and 80s, there was accumulated evidence that certain bodies in space travel at higher speeds than proposed by Newton's laws, which even general relativity cannot explain.

Scientists observed some galaxies and the mass distribution within them; depending on the distance between the planets and Newton's laws of motion, they calculated the velocity of stars at the edge of the galaxy. They found these stars were moving much faster than that explained by Newton's Law.

According to Newtonian mechanics, these stars would need to have a more powerful force than what was calculated for them to remain in the galaxy and not escape its gravitational pull. The current explanation for this extra force is that there is more material in galaxies than we can see and it provides the gravity required to keep the stars in the galaxy. This substance is called "dark matter".

According to calculations, the dark matter in the galaxy should be dispersed almost uniformly in order to allow the movement of the stars as we see it. And the amount of dark matter in the universe is supposed to be five times greater than the quantity of material that is composed of the atoms that we see. So far there is no evidence of its existence, but this fact is not surprising at all because it only affects gravity and does not emit any type of electromagnetic radiation, including visible light or radio waves.

The search continues for the particles that make up dark matter, and scientists continue trying to build extremely sensitive detectors to detect even the smallest effect of dark matter. Another possibility is that Newton’s and Einstein’s theories of gravity are in need of some reform to explain the movement of the stars, and in that case dark matter might not exist. For example, in the 80s Professor Moti Milgrom of the Weizmann Institute of Science developed the Modified Newtonian Dynamics (MOND) Theory stating just that.

Dark energy

Another mystery that physicists are trying to crack is dark energy. In 1929, Edwin Hubble measured the velocities of galaxies and found that all galaxies are moving away from us and that the ratio of distance to speed is constant; this is called the "Hubble Constant". His measurements concluded that the universe is not static, but expanding steadily.

In 1998, researchers measured the movement of distant galaxies by observing supernovas. From these measurements they observed the Hubble Constant has increased over time, meaning that the universe not only expands, but is also doing so at a faster pace over time. This discovery won Adam Riess, Brian Schmidt and Saul Perlmutter a Nobel Prize in physics in 2011.

The conventional explanation for the acceleration of the expansion rate of the universe is dark energy that comes from an unknown source. It creates a repulsive force that causes the universe to expand at an accelerating rate. According to calculations it comprises close to seventy percent of the total mass-energy in the universe. One possible theoretical explanation for this spread can be calculated by adding a cosmological constant to the equations of motion of general relativity. Adding the constant to this equation can explain the acceleration, but it's only one explanation and many researchers still disagree.

Theory of everything

These problems of dark matter and dark energy belong to the world of astrophysics and cosmology, where there are huge distances and heavy objects - galaxies, stars and the like. In this situation the dominant force is gravity, described by Einstein’s Theory of General Relativity.

The fundamentals of the situation change when we look at small objects such as atoms, molecules, electrons, etc., and the small distances between them. Here gravity is very weak and the dominant force is the electromagnetic force, the weak nuclear force and the strong nuclear force. All three of these are described in detail in the Standard Model of particle physics and quantum physics.

What is still missing is a "Theory of Everything" - a unified theory that properly explains all the forces at all distances and in all possible situations. Albert Einstein himself tried to solve this question up until the day he died, but was unsuccessful.

In the 70s and 80s of this century, physicists have tried to answer these problems using string theory. This complex theory describes particles and the universe that we know through little strings propagating in a multidimensional universe. Particles that we see are composed of these strings and their properties are explained by vibrations in the strings. Until now, researchers have not succeeded in finding a significant prediction that can be adapted to an experiment, so the problem remains.

So there are still many challenges humankind are going to face in order to understand the reality around them. In 1900 it seemed that physicists were able to explain most phenomena except for a small number of problems that seemed relatively minor. Emerging from the solutions of these problems were quantum mechanics and the theories of special and general relativity, which were huge revolutions that have changed the outlook of scientists in the 20th century. Who knows what unsolved problems the theories of the 21st century will create in our time?