We've made ourselves pretty comfortable
here in this Universe with its laws of physics.
What if the laws of physics were a little bit,
or maybe even a lot, different?
I'm Jared Kaplan and this is "Did the Universe have
to be the way that it is?"
We've only seen one Universe, our own,
and we don't know what other kinds of Universes
could exist, or might really exist, in a multiverse.
However, we can try to extrapolate
based on the laws of physics that we do know.
Let's imagine a Universe where gravity is a lot stronger.
Gravity at a quantum level is actually
a very, very weak force.
We have this weak gravity to thank
for the size of the Earth, the size of the Sun,
and our orbit, which keeps the Earth wonderfully habitable.
If gravity were a lot stronger, we
could be living on a "Little Prince" kind of planet.
If gravity were stronger still, stars
could be the size of apples, and perhaps the whole Universe
would only be the size of the Earth.
This is because the curvature of spacetime
would be a lot greater, causing matter to collapse in on itself
and making celestial objects rather petite.
That might sound charming, but with much smaller planets
we wouldn't have very much space to roam.
And smaller stars would have less nuclear fuel,
meaning that they would burn up more quickly.
That means that the process of evolution
and the development of life would be greatly curtailed.
But what if the opposite were true
and gravity were much weaker?
It might not be able to overcome the expansion of the Universe.
Since matter wouldn't be pulled together,
the Universe would be a complete mess, like it didn't clean up
its bedroom, or planets and stars could still form,
but they might be much bigger.
In fact, a planet like the Earth might be shaped like a potato.
And on such a planet, because of the weak gravity,
we'd be able to jump astonishingly high, maybe
so high that we could jump into orbit.
Let's see what would happen if we changed some
of the other laws of physics.
A funny fact about our Universe is
that neutrons and protons, which make up atomic nuclei,
have nearly identical masses.
However, if we were to fiddle just a little bit
with the masses of the quarks, we could make protons heavier
than neutrons.
In a Universe like that, protons could decay to neutrons.
That would mean that atomic nuclei themselves would decay,
and all the elements in our Universe would be unstable.
In such a Universe, chemistry, and life as we know it,
would be completely impossible!
If we were to remove the strong nuclear force,
not only would nuclei break apart,
but even protons and neutrons would fall apart
into their constituent quarks.
In a Universe like that, electrons and quarks
would be flying around freely.
We don't really know what kind of structures would form,
but certainly life as we know it wouldn't be possible.
Now let's try something different,
keeping the forces as they are, what
would it be like if we lived in more or fewer spacetime
dimensions?
We live in a Universe with three spatial dimensions.
After all, you can go forward and backward,
up and down, left to right and that's it.
But living in a 2D world would be
more like living inside a piece of paper.
A two-dimensional Universe would be very, very different.
For example, the electromagnetic force would be quite different.
It would be a lot stronger, functioning a lot
like the strong force in our Universe.
The gravitational force in two-dimensions
would also be very different.
Although there would still be space time curvature,
there wouldn't be a conventional long range force.
That means that matter wouldn't necessarily clump together,
and planet and star formation would
be nothing like what we see in our Universe.
Imagining what it would be like to live
in a four-dimensional Universe is a lot harder.
In fact, it's basically impossible to visualize.
However, there are some things we know
based on the laws of physics.
For example, in higher dimensions,
orbits wouldn't be stable.
That means that a four-dimensional Earth
would go crashing into its Sun.
So why did the Universe end up the way that it is?
We really don't know the answer, but one possibility,
or at least food for thought, is that maybe the Universe
is the way it is because if it were different,
we wouldn't be here to talk about it.