oh you don't look too enthused
i am knocking i'm working on an article
with a deadline of um well effectively
five o'clock this evening so
can you tell us what the difference is
between special and general relativity
there's various differences but i mean
the simplest one the simplest difference
is that special relativity is relatively
easy and general relativity is
incredibly difficult special relativity
is to do with the idea that the speed of
light is constant no matter which frame
of reference you're in so if you're
traveling in a train the speed of light
you'll measure will be the same as if
you're stationary
einstein then realize
that there's something funny in a
gravitational field
and it took him ages to work this out
but i mean this was the greatest piece
of work he ever did so again classic
example people use is to talk about
uh being in a lift and you can't
actually tell if you feel yourself push
to the floor of the lift you can't tell
if that's because the lift is
accelerating upwards or whether it's
because the pull of gravity is pulling
you downwards onto the floor of the lift
and general relativity is all about the
equivalence of those two things special
relativity you can't consider
accelerations at all everything has to
be traveling at constant speed and so
you don't have these reference frames
that accelerate you don't have things
like lifts accelerating you have to
think about just things traveling at
constant speed and he realized that mass
was equal to energy and energy could be
attracted to anything which has a mass
through the gravitational force so the
idea was put in his head that light
doesn't actually travel in a straight
line at the speed of light it can
actually travel in a curved path because
it can be bent by the gravitational
field
and so he tried to incorporate
into
the theory of relativity the idea that
you could have
a distortion of space-time and the light
will then travel in the curve path and
that's the field of general relativity
special relativity came first einstein
came up with special relativity um and
then actually spent quite a long time uh
dealing with general relativity coming
up with general relativity and the
reason is that actually the maths you
need to do special relativity is
actually pretty trivial you need
pythagoras theorem the square on the
hypotenuse is equal to the sum of the
squares on the other two sides if you
know that you know all the mass you
really need to do special relativity
whereas general relativity the mass you
need is truly scary i look at all the
maths for that and it's really
frightening
but he then predicted what should be
seen and in about 1919 there was an
expedition with eddington that confirmed
his predictions they were slightly
suspect experiments but this made a big
headlines at the time because here was
german science being vindicated by
british experiments and so he became the
first superstar of science because of
that to put things in perspective we can
teach special relativity to first-year
undergraduates and they love it because
it doesn't use any math that they
haven't already seen but the results you
get out of it are truly astounding you
know the difficult thing about special
relativity isn't the math it's
understanding what comes out of that
matter at the end whereas general
relativity final year undergraduates
still struggle with it special is in
designed for situations where there is
no mass around and just light going
around in a straight line
general then tries to incorporate
incorporate the ideas of special
relativity in a gravitational field
so does general relativity sort of take
special relativity into the real world
yes that's perfect that's a perfect
answer if you could change the value of
any constant what would it be and why
i mean it'd be fun to see what would
happen if i changed the value of either
of uh planck's constant
newton's constant i've mentioned what
the damage potential damage changing
newton's constant could be don't change
any of them because if we change them
too much it may be very very unfavorable
for life plans constants affecting
things on the very small
newton's constant affecting things on
the very large
and so slight modifications of these
could have dramatic effects on on either
of these scales so i think the world is
not perfect but let's not fiddle around
with them too much
i think we better not even ask that
question
oh
um
well it's tough because they're all
combined with other things so you change
one and you don't really know how the
other things would get affected i guess
it would probably be planck's constant
just because planck's constant is the
thing that kind of dictates all the
quantum mechanics in the universe and
the quantum universe is such a bizarre
thing you know this strange property
that particles can be wave like and a
single particle can simultaneously pass
through two different apertures and all
sorts of weird things the trouble is it
only happens on the tiny microscopic
scale so you never get to see it
probably planck's constant again because
then you could
okay
very hand wavingly if we took planck's
constant instead of it being 6.63
by 10 to the minus 34 joules second if
we made it much much larger then the
sort of weird quantum effects that we
experienced right down at the scale of
atoms and molecules would become
apparent in in the visible world see i i
don't think i'd go messing around with a
good thing i think the universe is
pretty interesting the way it is
it's pretty fine-tuned
so if we start messing about with
something even if we could do that
it's probably likely that we'd
obliterate ourselves out of existence if
i could mess around with plank's
constant so i could actually see some of
those weird quantum effects but actually
just in the lab or you know on the table
in front of me that would be a great
experiment to be able to do
being able to to walk through walls or
tunnels through walls and diffract and
interfere that might possibly not be the
greatest thing it would certainly lead
to a very different world a very
different set of physics if i could
change anything really it would be
direct's constant which is h with a
little line through it if you come
across that it's planck's constant
divided by two pi and i would put
everything in those units and drive
everybody else crazy because then you
get these 10 to the 34s coming all over
the place or the ratio between the
gravitational force and the
electromagnetic force it's
some ridiculously
huge number
um and and we don't know why or how so
if that changed a bit it would it would
change whether or not stars could form
and what their lifetimes would be so
sure shake things up change that ratio
there are some people by the way believe
the constants are not constant
and that they have been changing
over time and and and so there are lots
of observational tests to see whether
these constants in particular newton's
constant
and the charge of the electron it
changes that would be fun change the
charge on the electron a bit and see
what happens to atoms as they get a bit
bigger
so yeah i'll stick with the universe as
it is with fundamental constants that we
have thanks
you