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.