uh brady's challenged as in physics
to do something with an egg
and coming from the world of particle
physics what can you do with a cadbury's
cream egg and i thought about it i don't
want to miss out on this opportunity
and then i recalled something where the
shape of an egg is important in particle
physics
and that's in trying to explain why the
cosmological constant is zero so the
cosmological constant is a term that's
present in einstein's equations and its
effect
is to
cause the universe to
accelerate today until about the mid-90s
we didn't even know the universe was
doing this the universe we thought was
decelerating so that although these
galaxies were moving apart they're
actually slowing down because the force
of attraction due to gravity was slowing
them down
and so we were all very happy that this
term that could be there
the cosmological constant was actually
zero it wasn't playing any role
but at the turn of the 1990s and then
towards the end of the 1990s it became
clear the universe seems to have this
term not zero it is there but with a
very small value still close to zero but
a small positive value and the big
problem is actually to try and explain
this value so where does the egg come in
so in in the world of particle physics
one of the approaches that we take is
that where we let our imaginations run
free and we say maybe there's more than
four space-time dimensions we usually
think of four we think of time and then
three space x y and z
but in some scenarios especially those
coming from string theory there can be
as many as 10
and
but a simpler version that has been put
forward
as a toy model really not to try and
really explain the universe but to try
and get some of the features of the
extra dimension is one where there are
six that's called the six-dimensional
gauged chiral supergravity model
or the cadbury's egg model
so that's perhaps what they should have
called them because of course they're
extra dimensions so here's my hand
here's my my hand is representing the
four space-time dimensions that we that
we live in that we perceive
of course i've i've had to get rid of
two of them because i've only got two
here so i've got rid of a time and i've
got rid of a position for every point
on this surface which is called a brain
every point on this surface there is
another two dimensions coming out
instead of the universe being four
dimensional in other words you have time
x y and z i'd have two more coordinates
i could call them the typically called
row and theta and these extra two
coordinates so every place in in the
universe there's six coordinates
explaining that particular point
so here at this point here there'd also
be these two extra dimensions coming out
with a particular value of rho and a
particular value of theta
and and and rho takes me from this end
of the egg to that end of the egg and
theta takes me around the egg so those
are the extra coordinates so the
importance of the egg shape as opposed
to a soccer ball
which is a sphere
is that there's a particular curvature
associated with this egg a particular
way that it's curved around and it's
that particular shape which allows
the
energy associated with that curve space
to cancel out the cosmological constant
leaving me with zero cosmological
constant in the four dimensions
and then what i then use is
quantum mechanics to generate a very
small cosmological constant but the key
ingredient of the egg is that it
actually provides me with a nice
mechanism to cancel out the underlying
cosmological constant leaving me with
zero on the on the
four dimensions that we live in and then
what i can use is the world of quantum
mechanics to actually generate
a very small cosmological constant we
haven't yet solved that properly
there's that's still going on the
cosmological constant problem you know
why the universe has the value it has
for the cosmological constant still
remains one of the the big problems in
theoretical physics in order for the um
cancellation to occur you can actually
work out
and to occur at the right value and then
to give me the the the masses of the
particles that we see
you can work out what the typical size
of this egg would be
and the typical size of this egg or of
the extra dimensions is about 10 microns
that's really big
remember that the the
size of an atom is about an angstrom
that's 10 to the minus 10 meters this is
about 10 to the minus 5 meters so it's
easily within the range that we can see
so why have we not ruled this out egg
idea out already so the reason this
hasn't yet been ruled out is that the
forces of nature that we experience
that's the strong force the weak force
the electromagnetic forces
they are all confined to live on the
brain they can't propagate into this
extra dimension in particular
electromagnetism can't so light can't
light can't get into this extra
dimension the only force that's left
that we're aware of that can get into
the extra dimension is gravity and it
leaks from the brain into the extra
dimensions so we'll have to probe for
this extra dimension through
gravitational effects
and the con and the constraints on these
models from gravity are very weak and so
believe it or not it is possible that
there could be an extra dimension around
us that has a scale of about 50 microns
and we haven't yet
detected it but you guys are pretty good
at measuring gravity aren't you you mean
you you know
you guys measure gravity all the time in
the course of your work why wouldn't you
be able to measure this seeping gravity
which is escaping out into the eggs
um because it's such a weak force
so gravity is is a very weak interaction
and that this is one of the explanations
actually why it is weak that some of
it's sleeping out and what we're left
with is is the gravity in in in the
brain