Physicists are interested in big scientific questions, such as where did the world come from? So far, the search for answers has led to incredible discoveries: from the Big Bang to the Standard Model of particle physics. But these findings are not the end of the story, as they raise more questions. So here are four of the biggest mysteries of fundamental physics yet to be solved.
The first question
First of all, the first question is: Why can we remember the past but not the future? This may sound silly, but the laws of physics tell us it's a legitimate question. Basically, the fundamental laws of physics tell us how things change over time. They can tell us what will happen next to the system, given its current state. But they also tell us a lot about what happened to the system in the past. For example, we can use the same information to determine the final position of a rolling ball and its position a few seconds ago. In other words, the laws of physics apply equally to the clock running back and forth.
But this is not how we experience our daily lives. We only experience time movement in one direction, and the reason is entropy. Entropy is a physical concept used to track the degree of disorder in a system. In our daily lives, it is always increasing: over time, systems tend to become more disordered and chaotic. That's because everyday things have tons of interacting parts, so there are always more ways for things to get out of order than to spontaneously organize themselves.
So, over time, all we experience is the flow of the universe from low entropy to high entropy. This is where the big, unanswered question arises: Where did it all start? As we know from cosmological evidence, the entire universe had a very low-entropy beginning: the Big Bang . There, everything is squeezed into a neat little sphere. But why is the universe in that state? Why is entropy lower in the past?
It should be more likely that the universe is in a state of maximum entropy, such that entropy no longer rises and time no longer flows. But instead, the universe began as highly ordered. This is something that cannot happen by chance, and scientists are looking for a reasonable explanation for this. Therefore, for physicists, "Why can't broken mirrors spontaneously reunite" and "Why did the Big Bang happen like this" turn out to be similar questions.
Second question
About 40 years ago, cosmologists proposed an idea called inflation to explain many problems with the Big Bang model of cosmology. So the question now is: did the boom actually happen? If it happened, what exactly was it?
The Big Bang theory says that the universe started small and dense and then expanded outward. Inflation theory complements this by saying that during the first small fraction of the universe's existence, it expanded at a faster rate. Most cosmologists believe inflation is real because it helps solve some problems with the Big Bang model, and they have seen most of its predicted effects in telescopes.
The Big Bang model predicts that at the largest possible scales we should see significant temperature changes and some warping. However, the universe looks really uniform and doesn't seem to have any overall curvature. Things are roughly the same everywhere we look, things like galaxies are distributed similarly, ambient temperatures are similar, and we don't see any evidence that space is curved at the largest scales.
For all of this, the boom could explain why. If that extreme phase of growth had occurred, the entire universe would have been much larger than what we see. If the universe we can see is really just a small part of the whole, it makes sense that it would look smooth and flat. It's like the Earth is curved as seen from the space station , but if you zoom in on a small part of it, it looks flat.
But not every scientist is convinced this idea is correct.The concept of inflation also implies the existence of a particle called "inflation," but this is not predicted by any physical theory and we have no direct evidence for it. As a result, some physicists have proposed alternatives to inflation, such as the very abstract "conformal cyclic cosmological model" proposed by Nobel Prize winner Roger Penrose .
The good news is that next-generation telescopes and gravitational wave detectors may be able to identify the signature of inflation and even tell us what type of inflation is real.
The third problem
Next is the fine-tuning problem, which has to do with constants. Whenever scientists nail down a physical theory, they always find some hard-hitting numbers. For example, the speed of light is 300,000 kilometers per second, but we don't know why. There are many such constants, from the mass of electrons to the strength of gravity.
According to statistics, there are 26 fundamental constants, which cannot be explained as coming from deeper theories and can only be measured experimentally, which raises all kinds of questions. Because if some of those numbers were slightly different, life as we know it wouldn't exist. For example, if gravity were slightly stronger or weaker, stars would not be able to produce the diversity and abundance of elements necessary for life to function.
So, what’s going on here? There may be a more fundamental theory that we don't know about yet that explains why these constants have the values they do. Or maybe the answer lies elsewhere. Some researchers speculate that there is a multiverse in which constants have different values and physics works differently. Others go further and believe that we only exist because we are in a universe suitable for life. That last one is a very speculative idea, though, and since it's not something we can actually test and provide evidence for, it also borders on being unscientific.
The truth is, physicists just don't know what's going on here. To learn more, we need to better understand the basics of physics. This brings us to the final question, the holy grail of fundamental physics: the pursuit of a theory of everything.
The fourth question
The question is: Is there one theory that can explain all physics? Physicists believe there should be a theory of everything because many of the breakthroughs in physics over the past two centuries have been some kind of unification. Now, we have two important theories: general relativity and quantum mechanics , which are somewhat incompatible.
General relativity describes how gravity works, and quantum field theory describes the other three fundamental forces: the electromagnetic force, the strong nuclear force, and the weak nuclear force. Incredibly, everything in the entire universe can be explained by one or more of these four forces. But general relativity and quantum field theory sometimes actually contradict each other, and the results can conflict over issues such as extremely short distances or high energies. So the goal here is to find a deeper underlying structure that looks like quantum field theory in some cases and general relativity in other cases.
The most popular theory of everything idea is called string theory , which says that everything is made up of one-dimensional vibrating "strings". All the forces and types of matter arise from their different vibrations. We could spend hours talking about string theory, or other ideas like loop quantum gravity. However, there is no evidence to support any of these theories.
