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The whole point of physics has always been
to understand what
the universe is made of and how the stuff
in it interacts.
You know, like how Isaac Newton wanted to
figure out why
an apple always falls straight toward the
ground.
And we’ve come a really long way over the
years.
These days, after about a century of incredible
research in
fundamental physics, we have a pretty good
idea of
the building blocks that make up everything
— and the rules
that describe how they interact.
In fact, our fundamental picture of the universe
seems
so nearly complete that it’s led some people
to suggest
that we’re arriving at some version of “the
end of physics.”
And for sure, physics is at a turning point,
but before researchers pack it up and head
home,
it’s worth understanding what the so-called
“end of physics”
is really all about.
As far as anyone can tell, every single thing
in our entire world
is made up of a small handful of elementary
particles,
like electrons and quarks.
And they obey very strict rules when they
interact with each other.
Starting with those basic particles and the
rules they follow,
you can build up to all sorts of things—like
the physics of baseball,
the chemistry of pie-baking, or the biology
of cell division.
Of course, it doesn’t make much sense to
explain
how something like the brain works using elementary
particles like quarks.
It usually makes more sense to describe reality
with bigger things, like molecules or cells.
But the point is, no matter what unit makes
the most sense to use,
that unit still obeys the same basic principles.
Like, you’re not going to need
a brand-new
elementary particle to make sense of some
everyday thing,
like how a bird flies.
Most of the basic rules that describe the
stuff
in our everyday world are part of a framework
called
the Standard Model.
This framework is essentially a set of math
and physics principles
that describe the fundamental structure of
the world as we know it.
It includes three of the world’s four fundamental
forces
and the couple dozen particles related to
them.
Those three forces are the electromagnetic
force, the strong force,
and the weak force, and they fit neatly into
the Standard Model.
But there is one more fundamental force: gravity.
And it’s a little bit of an oddball because
it’s not neatly described
by the physics of elementary particles, like
the other forces are.
So it doesn’t fit into the Standard Model.
And that’s part of the reason there’s
no theory of everything
that neatly ties up all the forces and particles
in the universe.
But, even without a theory of everything,
the Standard Model
and general relativity do a solid job of describing
almost
everything in our world.
Which is a pretty tall order, all things considered.
These theories are the culmination of a century’s
worth
of research into fundamental physics.
And it can be tempting to see fundamental
physics
as a puzzle with almost all the pieces in
place.
Which kind of sounds like the end of the road
for physicists.
But before anyone calls it a day or, like,
converts to a biologist, there are a few things
to consider.
First of all, there were plenty of times throughout
history
when scientists thought physics was basically
complete…
and each time, they were extremely wrong.
For instance, in the 1920s, even after we
discovered the mysteries
of quantum mechanics and relativity, the physicist
Max Born still had the nerve to say that
“physics, as we know it, will be over in
six months.”
Spolier! It wasn’t.
Then, there was that time at the end of the
19th century
when the physicist Albert Michelson said,
“It seems probable that most of the grand
underlying principles
have been firmly established.”
This guy was ready to call it quits before
we even knew about
quantum stuff and relativity!
Physicists at the time had been so used to
the laws of motion
discovered by Isaac Newton that they expected
them to work forever,
for everything.
The thing was, by the time he’d said that,
Michelson himself
had already conducted an experiment that would
go on to prove him
—and the whole Newtonian worldview—wrong.
The Michelson-Morley experiment, as it’s
called,
provided strong evidence that there isn’t
any universal,
absolute reference frame that everything can
be measured relative to,
as Newton’s way of thinking suggested.
That experiment was crucial in paving the
way for relativity.
And the move from an absolute to a relativistic
way of thinking
about motion totally upended what we’d thought
for centuries
about how the universe works.
As a result, Einstein had to come in and invent
a whole new way to describe space and time.
So, back in Michelson’s day, the “grand
underlying principles”
of physics still had a long way to go.
Basically, every time someone like Born or
Michelson thought
they had all the answers, they realized that
their framework
was just a really good approximation of reality.
Once you got out to certain extremes, that
approximation
started to break down.
In other words, their frameworks did a good
job of describing
the world under certain conditions, but they
weren’t
perfect explanations.
And the same thing is likely true of the Standard
Model
and general relativity.
They seem fundamental because they describe
the universe really well,
but in extreme environments like black holes
or the Big Bang,
those frameworks still seem to break down.
And there are still some problems with the
laws we call fundamental.
I mean, just the fact that gravity doesn’t
mirror the other three
fundamental forces suggests that some piece
of the story
is still missing.
And there are other imperfections that constantly
remind us
that our fundamental theories are just a really
good
approximations of reality.
Meaning it’s almost certainly not the end
of the road.
Instead, we’re in a kind of weird, unprecedented
era
where physicists know our theories aren’t
complete, but they also
have very little evidence for anything beyond
it.
That makes it much harder to make progress
now than it was
a hundred years ago.
Because the thing is, Michelson and Morley
were able to disprove
the fundamental laws of their time using a
lab experiment
that can now be done in a college classroom.
But the times have changed.
People have plucked all the low-hanging fruit.
If you want to make discoveries about fundamental
physics these days,
you need really big experiments.
In 2012, the Large Hadron Collider at CERN
in Switzerland
discovered the last particle missing from
the Standard Model,
the Higgs boson.
That was a gargantuan international collaboration
involving
billions of dollars, thousands of scientists
and engineers,
and an army of support staff.
You’re simply not going to find an undiscovered
particle
without that kind of tech, because you need
something
that can create conditions way more extreme
than you get
in everyday life.
And as much as theoretical physicists hoped
that
the Large Hadron Collider would find evidence
of physics beyond
the Standard Model, it simply hasn’t.
But the fact that it’s difficult doesn’t
mean
that there is nothing left to find.
And the good news is, modern physicists understand
this better
than people like Michelson did back in the
day.
So they’re not claiming that the job is
done.
They know that there are still “grand underlying
principles”
that haven’t been discovered, and tons of
ways
that our current theories are incomplete.
Just like what happened with relativity back
in the day,
the solution to the problem will likely be
a whole new theory
that only looks like the Standard Model or
general relativity
under the right conditions.
Fundamental physics has reached a very high
plateau
with our current theories.
But without enough experimental evidence to
guide it higher,
it’s also kinda stuck in a rut.
The good news is, there are still lots of
ways
theoretical physicists are pushing forward.
For one, finding ways to unify the fundamental
laws of physics
is still a huge area of research.
In some cases, physicists know where the fundamental
laws
break down—like in black holes.
There, both relativistic effects and quantum
mechanical effects
are relevant, but they don’t agree on what
should happen.
General relativity says black holes should
evaporate into nothing,
but quantum mechanics says that’s not possible.
So we simply don’t know what’s right.
But scientists have begun figuring out ways
to research
those kinds of environments.
Like, to get around the fact that they can’t
exactly study things
like black holes in labs, they sometimes
use what are called
analogous physical systems.
So rather than study, say, a black hole directly,
physicists study a system that has similar
properties.
For instance, one of the most important properties
of black holes
is that they don’t let light escape.
So physicists found a way to make a similar
system in a lab
—except instead of trapping light, their
system traps sound.
This is called an acoustic black hole.
And these things actually reproduce properties
we’d expect to see
in real black holes.
Scientists are hoping that they can help
figure out the real fate
of black holes, since relativity and quantum
mechanics disagree.
Another team of researchers found a way to
put ultracold helium atoms
in a state where they behave like Higgs boson
particles,
and they were able to use that to study properties
of the Higgs
even before they had discovered the Higgs
boson itself.
In general, you can use the concept of analogous
systems
to invent all sorts of unusual environments
that’ll
make particles behave in ways they normally
wouldn’t,
and that’s been one way for physicists to
push the limits
of fundamental physics.
But these days, it’s not the only way to
study
the fundamentals of reality.
As computers get more and more powerful, simulations
of
physical systems have become much more common
in fundamental physics research.
And simulations have been a total game-changer,
because in the past,
just knowing the basic rules that govern a
system
wasn’t enough to tell how the system would
behave in practice.
For instance, it would take way too much number-crunching
for a human to figure out how the laws of
physics would play out
over billions of years of galactic evolution.
But with simulations, computers do the work
of figuring out
how the laws of physics play out under certain
conditions
—in a fraction of the time.
Based on the results, scientists make predictions
about
how those systems behave in the real world.
And that can get us a long way!
Like, predicting the chaotic movement of weather
systems
would be close to impossible without advanced
computer simulations,
no matter how well you understand how particles
work.
And simulations are currently the only way
to test theories
about things like the evolution of the early
universe.
So even without new particles or forces, fundamental
physics
is still pushing forward.
Clearly, theoretical physics isn’t done.
But it has changed, for sure.
The next plateau in our search for the theory
of everything
isn’t going to be reached by a lone maverick
working alone in a lab.
It’s going to take contributions from thousands
of researchers
across the globe, doing everything from writing
equations
to examining astronomical data to programming
computers.
The plateau we’ve reached with our latest
theories is exciting,
but in some ways the things a theory can’t
explain
are more exciting than the things that it can.
Thanks for watching this episode of SciShow!
If you’re interested in learning more about
fundamental physics,
you can check out our four-part series that
covers each one
of the fundamental forces.
It begins with the strong force, and you can
get started
with that video right after this.
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