i thought i'd talk about super luminal
motion
what does that mean so superluminal
means above light so faster than light
motion this is not possible everyone
knows that well i can show you a picture
would you like to see a picture of some
superluminal motion so here's a picture
so this object
v838 monocerotis in about 2002 suddenly
went into outburst and then there's a
series of pictures from the hubble space
telescope and as you can see it's
expanding and this is kind of as ever
when you take a picture you're kind of
measuring the angular size of it but
because we know how far away this object
is you can translate that angular size
into a real size and see how fast the
object is actually increasing in size
over time and it works out it's
expanding at about a light year per
month now given that the cosmic speed
limit is the speed of light which is a
light year per year kind of by
definition a light year per month is
about 10 or 12 times the speed of light
what actually happened it's a very
bizarre object so this is a very
uninspiring star i have to say oh it
used to be a very uninspiring star in
that so monaco artists is means it's in
the consolation of monosaurus which is
the unicorn which is not a constellation
many people have heard of because it's
not a particularly exciting
constellation and v838 means this was
the 838 variable star found in this
particular constellation so again it's
not exactly top of anyone's list of
anything until 2002 when all hell broke
loose it brightened up incredibly
quickly in a matter of days it was 10
000 times brighter than it had been at
the point where it brightened up it was
about a million times as bright as the
sun the classical thing that does this
is a thing called a nova and there it's
kind of probably a thermonuclear
reaction happening on the surface of a
white dwarf or something like that this
doesn't fit the bill it doesn't have the
properties of anova something else very
peculiar happened to this star it's
likely that it ate something so likely
that another star merged with it or a
planet merged with it so something fell
into the star which really stirred it up
and heated it up and made it very bright
for a fairly brief period of time but
pretty big incident in the history of
the star system it is i mean that's the
interesting thing is that the star's
still there so nothing totally
catastrophic has happened to it but
somehow it managed to get incredibly
bright for a little while then after
dinner it just calmed down again it's
faded back down again to kind of the
level it was at before yeah and the
other thing that sort of became clear is
actually there's a binary star in the
middle here so again it might be
something to do with the properties of
the fact that it's a binary star in the
middle but no one's really quite clear
what made it do this catastrophic
brightening there was this thing that
that appeared around it which was indeed
expanding faster than the speed of light
the natural way people think about these
things as well it's been a big explosion
so it's just the stuff going out from
the explosion that clearly can't be the
case because the stuff going out for the
explosion is restricted by this speed
limit of the speed of light so it can't
actually be a physical thing which is
expanding and what it actually is is a
thing called a light echo so it's not
actually a physical object expanding at
all it's just in some sense it's an
optical illusion let me give you a kind
of simple example of the same sort of
thing which i can do here in the in the
room okay so if i take a laser pointer
if i shine the laser at the wall and
then just move my wrist backwards and
forwards a little bit the beam moves
backwards and forwards on the wall
now if i move back further away from the
wall and do exactly the same thing so
move my wrist by about the same amount
you see that the dot moves a lot further
on the wall just a geometric effect that
you know i'm further away from the wall
so actually although the angle that it's
moving through is the same the distance
the dot moves on the wall is further now
you could imagine if i would add a
powerful enough laser pointer as i move
further and further away eventually i'd
reach a point where just moving that
little bit would actually make the dot
move along the wall faster than the
speed of light because it's just going
to keep going faster and faster there's
nothing to stop it and we're not
breaking any laws of physics by doing
that because actually you know there's
nothing physical going on there it's not
like there's a particle moving along the
wall faster than light it's not like
there's any information moving along the
wall faster than the light but
nonetheless you can have something which
you know where the appearance of the dot
actually moves faster than light on the
wall it's possible to have things that
appear to move or actually do move
faster than light right that dot really
wouldn't be moving faster than light if
you were far enough away but again
you're not breaking any of the speed
limits and that's kind of what's going
on in this object here um let me draw
you a picture it's probably the easiest
thing to do okay so here's the story so
here's us over here with our telescope
pointing up into the sky and here's the
thing which suddenly went extremely
bright so the distant object that
suddenly uh brightened enormously due to
whatever this peculiar phenomenon is the
extra bit we have to add to this picture
is that before it went incredibly bright
probably in some previous explosion of
some kind it threw out a shell of
material which is now just hanging
around in a kind of a sphere so this is
a shell which is you know light years
away so it must have been something that
happened a long time ago nothing at all
to do with this present explosion it
might even have been left over from the
formation of the star in the first place
but it's some shell of material hanging
out out there the thing suddenly
brightened up in 2002 and it basically
lit up that sphere it's like a lampshade
lit up around it but we didn't see the
lampshade all lit up at the same time we
saw it the first bit that was lit up was
a bit that had the shortest path to us
then another bit a bit further out and
so kind of a ring of material in this in
this debris field and then a third and
so on so over time you'll see this
expanding ring of material
of course the material itself isn't
expanding at all it's just the sphere of
material sitting there but it's because
it's being lit up in the sequential way
you'll see it expand this expanding ring
of material that's getting lit up and
that's really all that you're seeing
here
now it's a little bit more complicated
because you can see there's lots of
structure in this thing so probably
there's actually a whole series of these
spheres of material around it but the
basic picture's the same it's just the
whole series of things all getting lit
up in this sequential fashion just as
with the laser on the wall you know it's
just down to the geometry of the
situation that if you set things up you
know the difference between this path
and this path isn't actually very big
and so actually the time delay between
the direct route and the next route out
isn't actually all that long and so the
you know the time for it to to go from
lighting up just the central bit to
lighting this up this much bigger ring
can actually be quite short and just by
playing around with the geometry you can
make that speed anything you want it to
be including faster than the speed of
light
if you were here on earth pointing a
really powerful laser pointer at the
moon you could presumably create this
effect on the surface of the moon where
the laser point is moving across the
surface of the moon faster than light
yeah does that mean if i was standing on
the moon this point of light on the
surface would come towards me and then
pass me it faster than the speed of
light it would but of course but there's
not it's not like it's a physical thing
it's not like it's a particle or a car
or a rocket going past you faster than
the speed of light it's just a series of
light you know lighter rivals that that
shoots past you faster than the speed of
light so if i replace that powerful
laser pointer with a really big metal
rod then you can't do it because then
there's a physical entity which is
actually traveling and that you can't do
if you were trying to accelerate the end
you're trying to by doing that you're
trying to accelerate the end of the rod
to fasten the speed of light and the
laws of special relativity say actually
in order to do that you'd need an
infinite force to do it so you wouldn't
need just a strong wrist you didn't need
an infinitely strong wrist in order to
do it so the so where it falls down is
the mask becomes so great you just can't
move the mess it really is yeah i mean
the yeah it just is so the inertia of
the thing becomes so great that actually
the laws of physics guarantee you can't
do it with a physical thing the way you
can with a beam of light does this
optical illusion this this deception
cause problems for astronomers it seems
like something that could catch you guys
out from time to time you've got to be
careful right in that if you were to you
know you might try and do a naive
calculation of trying to figure out how
far away this object was if you didn't
have any other constraints as to how far
it was by saying well i know that this
thing can't be expanding faster than
speed of light i can see how fast it is
expanding on the sky that then tells me
how far away the object can be so indeed
you could get completely confused by
making the wrong assumption that you're
actually seeing a physical thing
expanding and using that to infer
something about the properties of the
object you're looking at do light echoes
occur anywhere else other than in gas
shells surrounding stars or binary stars
i think they're the main place you see
them so we've seen them so for supernova
1987a for example they're seeing light
echoes still from the explosion from
supernova in 1987a but you really need
something which produces a pulse in
order to get a kind of an identifiable
echo from different directions and the
the main things that produce those kind
of pulses are exploding stars in one
form or another
so that leads nicely to the issue of
second generation moon trees i'm sure
you can figure out what they are
these are the sons and daughters of
original moon trees or first generation
moon trees just take the seeds plant
them somewhere else okay we don't know
exactly where the trees were planted it
was somewhere here at the space and
rocket center so in order to figure out
what the whole ground looks like
veronica space shot