The Biggest Unanswered Questions In Physics Today

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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.

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