The Biggest Unanswered Questions In Physics Today

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Physics mysteries are everywhere we look. From the nature of reality to why the shower curtains blow in, there is so much that we don’t know. And while ignorance is part of the human experience, so is knowing things. We are also bothered when people (or in this case nature) keep things from us. Here are some of the most intriguing and complex questions that physicists are trying to solve.

How are we going to reconcile quantum mechanics and relativity?

Two pinnacles of human ingenuity were both formulated at the beginning of the 20th century. These are quantum mechanics and Einstein’s theory of General Relativity. Over the last 100 years, they have been expanded upon and tested and they continue to serve scientists extremely well. But there’s one problem. They don’t work well together.

Relativity explains the large and massive, and its realm is gravity. Quantum mechanics describes the other three fundamental forces: electromagnetism, and the two nuclear forces, and everything it involves is small. The two theories are formulated and constructed in different ways so at present it’s not possible to use one to explain the other’s domain. Problems arise when they have to work together and usually we end up with nonsensical answers.

This tells us that there should be a better theory out there. One that can include both relativity and quantum mechanics. String theory and quantum gravity have been proposed to be such a theory, which is usually dubbed the theory of everything. So far we have not been able to test either of them to confirm these claims, due to limitations of technology. While the theoretical edifice of such a theory is being built, we also lack observations that push our trusty science to the limit. Nothing seems to properly violate either relativity or quantum mechanics. Finding such limits will help us make the theoretical efforts better directed to their goal. 

What are dark energy and dark matter?

Everything you see, the Sun, your phone, and the town of Lllanfairpwllgwyngyllgogerychwyrndrobwllllantysiliogogogoch in North Wales are all made of regular matter. And regular matter is only 4 percent of the entire matter content of the universe. The rest is made by two unknown components – dark matter and dark energy

Dark matter was proposed almost five decades ago to explain how galaxies spin on their axis. If we take account of all the mass that we can see in stars and gas, their rotation doesn’t make sense. Dark energy was instead proposed 20 years ago to explain the accelerated expansion of the universe.

The effects of these two components are, in broad terms, well understood. We have used them in many simulations and they have produced predictions in agreement with what we observe in the universe. And yet after many years of study, we are none the wiser about their true nature.

Dark matter Is thought to be made of weakly interacting massive particles (WIMPs) that formed a few instants after the Big Bang. Dark energy seems to be a cosmological constant or a vacuum energy, a property of every single bit of space-time. We have yet to find direct evidence of either.

There are alternative ideas that try to explain the universe without these two components. Some are corrections to the equations of general relativity, and some use more complex ideas. The reason why most physicists stick with the "dark universe" is simple: because when dark energy and dark matter are put together in the standard model of cosmology, they work exceptionally well. To paraphrase Dr Katie Mack, dark matter and dark energy are still the worst theory. Except for all the other ones.

A flaring black hole. NASA/JPL-Caltech

What happens inside a black hole?

Nothing, not even light, can escape a black hole. But what would we see inside if it could? Long story short is we have no idea. All the mathematical and physical tools we have are really not good at describing the extreme world within the event horizon, the surface that separates a black hole from the rest of the universe, and the point at which light can no longer escape.

Before being discovered as actual physical objects, black holes were a particular set of solutions from Einstein’s general relativity. The math is utterly fascinating because it takes us deeper than we could possibly go in real life, all the way down to the center of the black hole, the singularity, a point of infinite density. So, we have already a problem. We have something with a huge mass and a tiny size. So we would need both quantum mechanics and relativity but we know that won’t happen.

But even if we steer away from explaining how the singularity would work, there are still oddities that come out based on the maths. Time seems to behave like space and vice versa. There’s even the case of a rotating black hole where the singularity is a ring and not a point. And even more weirdly, if you pass through that ring then time and space get back to “normal”, as if there’s an exit point if you go through the black hole just at the right angle.

While it is fascinating to consider these mathematical views, they have no physical explanation. We don’t know if the singularity is a ring or even a point. We don’t know if black holes form other universes or whether they simply eat to their heart's content with impunity. Answering this question might be seen as an exercise in futility. None of us will end up in a black hole and even if we did, we wouldn’t survive to tell the tale. Nonetheless, black holes are one of those extreme objects that push quantum mechanics and relativity to the limit. Understanding them might tell us how to make them work together.

What kind of physics lays beyond the standard model?

Most of the discoveries of quantum mechanics have been enshrined in the standard model of particle physics, which has allowed physicists over the last 50 years to predict and organize the fundamental particles and the force carriers. The Higgs Boson, which was discovered in 2012, formed one of these predictions.

The standard model is a phenomenal achievement. And yet we know it is limited. It doesn’t contain gravity. It has no mentioned of dark matter and dark energy. It expects neutrinos to be massless. And it implies that matter and anti-matter are exactly the same under the laws of physics. And we know that’s not the case because if it was, the universe wouldn’t exist as all the particles would slam into antiparticles and annihilate themselves. Even so, to date we have no better alternatives.

There are several processes, among which is a well-known particle decay, that hint at the new physics and several predictions that have not been witnessed in the real world. But so far researchers have come short of an actual discovery that could break the standard model.

Researchers are unsure when such a finding will be uncovered. Labs around the world are working to find such an event and hope to find it soon, but it might still take years. And while it might be revolutionary, it might not turn physics upside down overnight. Scientists have been researching alternative ideas for decades and the first discoveries might just help to refine which alternative theories are closer to reality. Or maybe physics will throw us a curve ball and truly send us back to the board. That’s definitely fun to think about.


Why does time have a direction?

Time is a quantity that is central to our lives. We either have too little of it or we end up wasting it. We wish it away or hope it'll stand still. In physics, it’s just one of the four dimensions of the space-time continuum, and yet it is special because unlike space it can only be explored in one direction.

The problem is known as the arrow of time. In our everyday experience time is not symmetric, but a lot of physical laws and processes don’t care about the direction of time. Physicists are focusing on everything that seems to act in a different way as time passes to find a possible explanation for the preferential direction that time takes.

Certain particle decays have been proposed to explain the arrow of time. The expansion of the universe has also been suggested. And the second law of thermodynamics is a fan-favorite too. Entropy increases with time, after all. These explanations might suggest a deeper link between time and other physical quantities, or their evolution might just be a natural consequence of a universe in which time has a direction.

To solve this conundrum, we need to move beyond our current physics. And breakthroughs in other areas of physics might lead to a better understanding of what time is and what makes it special.


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