[♪ INTRO] Physicists have always been  interested in some of the biggest science questions ever, like  “Where did the world come from?” Or, “What is stuff’?” So far, searching for answers has  led to incredible discoveries, from the Big Bang to the standard  model of particle physics.

But these discoveries aren’t the end of the story, because they raise even more questions. We’ve talked about two of the biggest  unresolved questions a lot on SciShow and SciShow

Space: dark matter and dark energy. So, here are four of the other biggest  unsolved mysteries of fundamental physics. First, here’s a question: Why can we  remember the past, but not the future? What makes the two different?

It might sound silly, but the laws of physics  tell us that this is a legitimate question. Basically, the fundamental  laws of physics are equations that can tell you how things change over time. They can tell you what happens to a  system next, given its current state.

But they also work just as well at telling you what happened to that system in the past. Like, you can use the same  information to figure out where a rolling ball will end up,  and where it was a few seconds ago. In other words, the laws of  physics work equally well running the clock backwards and forwards.

But that’s not how we experience everyday life. We only experience time moving  in one direction: forwards. And the reason why is entropy.

Entropy is a physics concept that tracks  how much disorder there is in a system. And in our everyday world, it always increases: Systems tend to get more disordered  and messy as time moves on. That’s because everyday things have  huge numbers of parts interacting with each other, so there’s always  more ways for things to become disordered than to  spontaneously order themselves.

Like, there are just way more ways for globs  of milk to scatter through your coffee, than there are for all the  milk to gather in one spot, so you never see your coffee run  in reverse and un-mix itself. So really, what we experience  as time moving forward is just the universe flowing  from low to high entropy. And that’s where a big,  unanswered question comes in: Where did all of this start?

And why? Like, we know from cosmological evidence  that the whole universe had a very low-entropy beginning: the Big Bang. There, everything was squished  into a tidy, small sphere.

But why was the universe in that state? Why was entropy lower in the past? It should be much more likely for the  universe to be in the maximum entropy state, the most disordered state possible,  to the point where entropy couldn’t rise anymore and the  flow of time wouldn’t be possible.

But instead, the universe  started as highly ordered. Which is such an unlikely  thing to happen by chance that scientists are looking for an explanation. So, basically, to a physicist, “Why  can’t you unmix coffee and milk?” and “Why did the Big Bang happen  the way it did?” turn out to be… kind of the same question.

In terms of proposed solutions to  this, well, they verge into philosophy. Some people say that the question  doesn’t even need an explanation. And even the suggestions we  do have sound pretty wild.

Like, one idea is that our entire, observable  universe is just a little, low-entropy bubble embedded in a much bigger,  more chaotic, ever-inflating whole. But that’s highly speculative… and at  least for now, probably impossible to test. Speaking of inflation… that’s actually  its own unsolved mystery of physics.

About 40 years ago, cosmologists  came up with an idea called inflation to explain many of the problems with  the Big Bang model of cosmology. So now the question is: Did inflation  happen, and if so, what exactly is it? The Big Bang theory says that the universe started off small and dense  and then expanded outward.

The theory of inflation is an add-on to  that, and it says that in the first tiny fraction of a second that the universe  existed, it expanded at a much faster rate. Like, it inflated by a factor of  one followed by twenty-six zeroes… in a timespan of zero-point,  and then 30 zeroes, one seconds. Talk about an early growth spurt.

Most cosmologists believe inflation is  real, because it helps solve a few problems with the Big Bang model, and because they’ve seen most of its predicted effects in telescopes. The Big Bang model predicts that,  on the largest possible scales, we should see notable temperature variations and some warping when we look deep into space. Except, the universe appears really homogenous, and doesn’t appear to have any overall curvature.

Everywhere we look, things are roughly  the same, with similar distributions of things like galaxies, and  similar ambient temperatures. We also don’t see any evidence of space  being warped on the largest scales, the way we see space warping around a black hole. And for all of that, inflation would explain why.

If that extreme growth phase happened, then the full universe is wildly  bigger than what we can see. And if the part of the universe  that we can see is really just a tiny fraction of the whole, it makes  sense that it appears smooth and flat. It’s like how the Earth is clearly  curved and detailed when seen from the International Space Station, but if  you zoom in on a small part of it, it can look flat and featureless.

So really, inflation kind of says that our observable patch of the universe is  the Nebraska of the wider cosmos. But not every scientist is  convinced that this idea is right. Like, the idea of inflation also implies that  there’s a particle called the “inflaton,” but that isn’t predicted by any physics theories, and we have no direct evidence for it.

So, some physicists have proposed  alternatives to inflation, like, the very abstract “conformal  cyclic cosmology model” of Nobel Laureate Roger Penrose, which says that there was an expanding  universe before the Big Bang. The good news is, the next  generation of telescopes and gravitational wave detectors may  be able to identify telltale signals from inflation, and even tell us what  type of inflation, if any, is real. So, of all of the problems in this episode, this is the one we have the  best chance of tackling soon.

Next up is the fine-tuning  problem, and it’s about constants. Whenever scientists settle on physics theories, they always find numbers that  appear as just... brute facts. Like, take Einstein’s E equals m c  squared, which relates to mass and energy.

The “c” stands for the speed of light. And we know that’s about 300,000 kilometers  per second, but we don’t know why. And there are loads of constants like that, from an electron’s mass to gravity’s strength.

By one count, there are 26  constants that are fundamental: They can’t be explained as coming  from a deeper theory, but just... are. And that raises all kinds of questions. Like, if those numbers are truly random, there’s quite the coincidence going on here,  because if some of these numbers were even slightly different, life as  we know it wouldn’t be able to exist.

For instance, if gravity was a bit stronger  or weaker, stars wouldn’t be able to produce the variety and abundance of  elements needed for life to function. And there are other weird coincidences,  too, like that fundamental particles are less massive than it seems like they  should be, given the forces acting on them. So, what’s going on here?

It could be that there’s a more  fundamental theory we don’t   know yet that explains why these  constants have the values they do. Or maybe the answer lies elsewhere. Some researchers even speculate that  there’s a kind of multiverse where constants have different values and  where physics works differently.

Others take this idea even  further and suggest that we only exist because we’re in  the universe suited to life. That last one is an extremely speculative  idea, though, and since it’s not something we could ever really test and provide  evidence for, it also borders on unscientific. The truth is, physicists just  don’t know what’s going on here.

To learn more, we need to understand the  fundamentals of physics a lot better. And that brings us to our final problem, the  holy grail of the foundations of physics: the quest for a Theory of Everything. The question is this: Is there one  theory that can explain all of physics?

Because right now, we have  two, and they’re kind of… completely incompatible with each other. Physicists think there should  be a theory of everything, because lots of physics breakthroughs  over the last two centuries have been some kind of unification. Like, Isaac Newton showed that the  ‘thing’ that makes apples fall from trees is the same ‘thing’ that makes  planets orbit the

Sun: gravity. He demonstrated that two different  phenomena in two different situations were governed by the same underlying principle. And that spirit of unification  has also linked space and time, electricity and magnetism, and more. In fact, over the years, physicists  have unified their theories down to two fundamental ones: Einstein’s  general relativity, or GR, and Quantum Field Theory, or QFT.

GR describes how gravity works,  and QFT describes the other three fundamental forces: electromagnetism,  the strong nuclear force, and the weak nuclear force. And the incredible thing is, everything  in the entire universe can be explained using one or more of these four forces…  even if it’s super hard to do in practice. But general relativity and  quantum field theory actually contradict each other at times,  with results that can clash over things like extremely short  distances or high energies.

So, the goal here is to find a deeper,  underlying structure that looks like QFT in some circumstances, and like GR in others. Basically, one structure that explains both… and by extension, explains all of physics. That’s a Theory of Everything,  and there are lots of contenders.

Each of them propose that there’s  one fundamental “thing” that the universe is made of, and  everything manifests from that. The most popular idea is called String  Theory, and it says everything is made from one-dimensional vibrating “strings.” And all the forces and types of matter  come from their different vibrations. That said… we could spend hours talking  about String Theory, or other ideas like Loop Quantum Gravity... but there’s  zero evidence for any of these theories, and no prospects of that changing any time soon.

To find evidence for a Theory  of Everything, we’d need to find where quantum field theory and  general relativity break down, in other words, where they  give the wrong predictions. That would hint at something deeper in physics. Except... those theories are too good, so it’s super difficult to  find evidence against them.

We do get rare hints, like the Muon g-2  experiment, or dark energy in cosmology, which currently doesn’t have a  good explanation in either theory. But they’re basically drops  of water in a vast desert. Then again, finding a Theory of Everything  was never going to be an easy task.

A structure like that would also include  dark matter, those hypothetical inflatons, and answers to things like  the fine-tuning problem, so, it’s like asking to solve all  of fundamental physics at once! Overall, there’s a theme running  through all these problems. For each of them, physicists use insights  from the study of the smallest things in the universe and the largest things.

They need to know about  particle physics and cosmology. And there are loads more unsolved problems we could’ve talked about  where that’s the case, too. It’s a sign of how progress in  physics is always interdisciplinary, and how the path forward is  so long and daunting that we can only chart it by working together.

Thanks for watching this episode of SciShow, which was brought to you as always  with the help of our patrons. You guys truly make it possible for me  to stand in front of this green screen and talk about impossible physics problems and bring the results to  the whole entire internet. So, thanks.

To learn more, check out Patreon.com/SciShow. [♪ OUTRO]