There are many things we don't know about
the universe, but there is one thing that is particularly bizarre. Why is gravity so
much weaker than the other known forces? Now it might not seem obvious that gravity
is incredibly, ridiculously, weak, but I can illustrate the fact with this simple magnet
and paper clip. If I put the paper clip on the magnet, a jaw dropping thing happens.
The paper clip sticks to the magnet. Now I know what you're saying. You're saying
"Well... um.. yeah?" But it might be that you've not thought through exactly what that
means. This paperclip is feeling two forces: the force due the magnet pulling it up and
the gravity of the Earth pulling it down. I'll say that again a bit slower. A tiny little
magnet is pulling the paperclip upwards and the gravity of an entire planet is pulling
the magnet downwards. And the magnet wins! If you think about it a bit, this means that
gravity is really an incredibly weak force. So why is it that gravity is so much weaker
than the force of magnetism? Well, the simple fact is that nobody knows. The answer to that
question is one of the biggest mysteries of modern physics and the reward for figuring
that out is one of these babies here. It would look awfully good on a shelf in my
den, don't you think? While nobody knows the answer, there are some
ideas that scientists are kicking around. And one of the coolest ideas is the idea that
gravity might inhabit extra dimensions - more dimensions than the three with which we are
familiar. The problem is that the idea of dimensions
has been used frequently in science fiction to mean many things, for example an alternate
universe. I mean, you don't have to be much of a science fiction buff to have encountered
a movie in which history was different, for instance a history in which the British won
the war for American Independence. Even if you don't watch many movies, I can
help you understand the idea a bit better with the help of some of my physicist colleagues.
For example, we all know that physicists are simply the hippest kind of scientists on the
planet, as you can see from my two colleagues here, Brian and Jason.
Sup. In the alternate reality idea, it could be
that there are universes in which physicists aren't hip. For instance, there could be a
parallel dimension in which Brian is a total geek or even an alternate reality in which
the super hip Jason is actually a giant talking rabbit called Mr. Snuffles.
What do you guys think? Does this seem weird to you? Brian?
Why not at all, this is indubitably the very paradigm of normality.
And Mr. Snuffles? Actually this is pretty comfortable. And I
don't have to wear any pants. You know, I find this all pretty disconcerting.
Let's change back to our own reality. I mean, the idea of parallel dimensions is kind of
interesting, but I'm pretty sure that physicists are the very definition of hip in all universes.
Thanks for helping out, guys. You got it.
These concepts of alternate realities are not what physicists mean when they talk about
extra dimensions. Instead, what we mean are literal extra dimensions
of space. Just as we know of the familiar three dimensions of left, right, up, down
and forward and backward, we are postulating one or more additional dimensions.
Now the idea of additional dimensions seems just total ludicrous. I mean here I am, existing
in a three dimensional world. I mean, it is completely understandable if you think the
whole idea is, well, completely... But what if - and bear with me here - just
what if the fact that gravity is so much weaker than magnetism can be explained by the idea
that gravity exists in more dimensions than magnetism. How would that work?
We can understand the concept by using the example of billiards as an analogy. So you
know the basic idea- if you play properly, the billiard balls are constrained to move
in two dimensions, specifically around the surface of the table. A ball can move across
the table or it can move along the table, but it doesn't leave the surface.
However, the sound of the impact of collision of two balls can radiate in three dimensions.
You can hear the sound above the surface of the table. While the balls move in two, sound
moves in three dimensions. So suppose that gravity is similar. Electromagnetism
and the strong and weak nuclear forces that hold matter together might move in three dimensions,
but maybe gravity can move in four or maybe even more. Under this idea, if gravity can
spread into more dimensions, maybe gravity is really strong - it's just that some of
its strength moves into the other extra dimensions. Now there is a reason why this idea sounds
pretty silly. We actually know the number of dimensions gravity spreads into in our
common experience. The way we know this is actually pretty compelling and it's easy to
understand. In 1687, Sir Isaac Newton published his law of gravity and what it said is that
if you take a small object, the gravitational field it exerts gets weaker in a manner that
is proportional to one over the square of the distance from the object.
Why the world should be this way is simple. The gravitational field from the object spreads
out equally in all directions and it passes through the surface of a sphere surrounding
the object. We know that the surface of a sphere is 4 times pi times the radius of the
surface squared. The square of the radius from the area of a sphere and Newton's radius
squared is the same. Since a sphere is a three dimensional object,
this pretty much proves that gravity goes into three dimensions. However, there is a
catch. Scientists have tested Newton's law of gravity
over huge size scales, from the size of the solar system or even bigger down to the sizes
that are a twentieth of a millimeter or so. However, people haven't directly tested how
gravity works for even smaller sizes. What if there were additional small dimensions?
Can that idea save us? Well, the answer is maybe, and, given the
importance of the question, scientists have come up with some ideas about how that might
work. If there are smaller dimensions, then they
can't be like our familiar ones. The familiar ones are infinitely big and if even if the
new ones were smaller, a smaller infinity is still infinity.
But what if the smaller dimensions were different? Maybe at every point in ordinary space there
are small additional dimensions that wrapped back in on themselves.
Now, this idea sounds, well, pretty crazy, but let's use another analogy to get the basic
idea. I am talking about, of course, tightrope walking.
A tightrope walker - well - a successful one anyway, can move only in one dimension - forward
and back. However, an ant on the same rope can not only move forward and backward, it
can move around the rope itself. The second dimension is around the circumference of the
rope and it is much smaller than the long dimensions to which the tightrope walker has
access. Similarly, perhaps there are other smaller
dimensions that gravity can enter. So that's where measurements at particle accelerators
come in. If one simply makes the assumption that extra
small dimensions of space is the reason that gravity is so weak, it is straightforward
to work out the consequences of that idea. Then scientists can look at their data to
see if those behaviors are observed. If they are, we can begin to be confident that the
idea of extra dimensions could be right. So far, there is no indication in the data
that this idea is the explanation for the weakness of gravity, but that doesn't discourage
scientists. With the resumption of operations of the Large Hadron Collider in 2015, physicists
can study even smaller dimensions than before. It could be that the LHC could have a huge
new message to teach us, and I'll let you know if I hear anything exciting.
