[♪ 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]
