we're talking about tides today tides on
earth tides in the solar system tides
and galaxies
so tides are simultaneously
very simple and very complicated well
it's a good question what is what causes
the tides on earth it's like what why is
the sky blue it's one of those questions
that kids ask and often stumps grown-ups
you might naively think well it's just
the force of gravity from the moon
pulling on the earth the earth's surface
is mostly water and so that water gets
pulled to one side of the earth and that
causes a high tide
if that were the case though we'd only
have a high tide once a day we actually
have it twice a day every 12 hours
so there's something slightly more
complicated going on and in physics we
talk about tidal forces this is a
generic term
and a tidal force is not the main
gravitational force but it's the
differential force
it's caused by the fact that the pull of
the moon
on this side of the earth
is stronger than the pull of the moon on
that side of the earth which means if
you just think about the earth sitting
here with the oceans around it and the
moon over here this side gets pulled
hard this way the middle the earth
itself gets pulled a little less and
then the stuff on the far side the
oceans on the far side gets pulled even
less
and so it's the difference in the force
of gravity felt in this position versus
that position that leads to the tides so
the net effect of all that is that that
sort of stretches the oceans out and
that's why you end up with two tides two
high tides because the near side is
getting pulled towards the moon and the
far side is actually getting pulled less
than the earth is so it kind of gets
left behind
we've got the moon over here
and we've got the earth over here
this side of the earth is being pulled
quite strongly
the middle
is being pulled a little bit
and the far side is being pulled not so
much so because these are vector forces
we can actually subtract one from the
other and when we do that
we get something that looks like this
we get a net force in that direction on
this side of the earth which is the side
with the moon
and we get a net force a remaining force
in the opposite direction on the other
side
and
it even happens at the top and bottom of
the earth that there's a net force
inwards as well
and because the earth is most the
surface of the earth is mostly water
what happens to the earth
is that it gets slightly squashed and
slightly stretched on both sides the
complicated part is that isn't actually
what happens with tides that if you so a
prediction of that for example is that
would tell you that actually high tide
ought to occur when the moon's directly
overhead because that's when the moon's
pulling the ocean that way or when the
moon's around the far side of the earth
if you actually look at tide tables you
find that's not when high tide occurs
typically it can be several hours out
and that's because the oceans don't
instantly respond to this they don't
actually instantly form this shape
because they have to kind of slosh
around the earth and rearrange
themselves in order to actually produce
the tides themselves so the reality is
that tides on the earth are a
complicated mixture of the driving
physics which is the moon kind of
tugging the the oceans out and then the
way the oceans actually respond which is
the complicated part which is all to do
with the topology around them where the
coastlines are what the sea floor is
like how deep the oceans are and so on i
mean the moon's far away which means
that the pool of gravity of the moon is
actually quite weak which means that the
difference between the pull of gravity
on one side and on the other side is
even weaker so actually tidal force is a
very weak force but it's enough it's
enough to actually distort the ocean if
you think about it for a moment in fact
the pull of gravity of the sun is a lot
stronger than the polar gravity of the
moon because although the moon's a lot
closer to us than the sun the sun's much
much much more massive and so actually
you know that's the reason why we orbit
around the sun we don't orbit around the
moon so in fact the gravitational
influence overall of the sun is far more
important than the moon but because the
sun is so far away that means the
difference in gravity between one side
of the earth and the other side of the
earth is very small because actually the
difference in distance is a tiny
fraction of the total distance to the
sun whereas the moon because it's a lot
closer the difference in distance to the
moon from one side of the earth and the
other side of the earth is a much bigger
factor and that kind of wins out so
although the gravitational influence of
the moon is much weaker than the sun the
tidal influence of the moon is actually
stronger than the sun
well again in the solar system you get
really interesting
configurations going on when you have
large planets and not just one but lots
of moons so of course jupiter is the
largest planet in the solar system it
has many many moons some of which are
almost as big as planets themselves
the moon closest to jupiter orbiting
closest to jupiter is called io and
that's a really interesting object it's
the most geologically active
body in the solar system it's got
volcanoes going off its surface is
constantly being renewed because of all
this volcanic activity all these
earthquakes or moonquakes that are
happening and that's because it's being
tugged in all different directions in
one direction by the huge mass of
jupiter right next door but also by
these other
major what we call galilean moons
also in orbit around jupiter are pulling
it in other directions so its insides
are constantly being squashed and
distorted and it's that
geological heating that's driving this
tidally induced heating that's driving
all of the geological activity on its
surface in my own particular field
mostly is studying galaxies
and tides have a massive influence on
galaxies because galaxies don't exist in
isolation they often exist in clusters
and in groups and so once in a while one
galaxy comes quite close to another and
then the gravitational influence of one
galaxy on the other can actually the
tidal effects can then stretch that
galaxy out there's a very famous pair of
galaxies called the antennae which are
in a very close interaction with one
another and the tidal effects then have
pulled out these long beautiful tidal
tails from these things the leading
tidal tails form sort of a bridge of
material which is kind of obscured by
the fact that they're both colliding
the trailing tales in both cases are
being flung off quite spectacularly
behind each of the galaxies so what
you're seeing in the antenna galaxy is
you've had these two galaxies that came
close together and then so this galaxy
feels the tidal influence of this galaxy
so it feels the gravitational pull but
the gravitational pull on its backside
is weaker than the gravitational pull on
its front side which means that the
stars at the back kind of get left
behind so it got stretched out into this
long tidal tail and then conversely
exactly the same thing is happening to
the other galaxy and it's getting
stretched out by its companion too
because tides are just basically the
difference in pull of gravity being one
side of a thing and the other side of a
thing wherever you've got gravitational
forces influencing extended objects then
you have tides and so for example a
famous example in astronomy is if you've
got a black hole the pull of gravity of
a black hole is incredibly intense which
means that the tidal forces the
difference in pull between one side or
something and the other can become very
intense as well if you're at the event
horizon near the what we call the edge
of a black hole
that's a small black hole you're very
very close into where most of the mass
is
and that means a small change in your
position
will induce
enormous changes in the gravitational
force that you feel so if i'm coming up
to a black hole
and i'm falling in feet first my feet
will feel an
enormously different force compared to
my head simply because the small
difference between them and what would
happen if i were to be so unfortunate as
to cross the event verizon of a fairly
small black hole is that my feet would
be stretched far away from my head and
the term for that is called
spaghettification so you're stretched
into spaghetti
because of the differential force of
gravity