Kind: captions
Language: en
[Applause]
the speed of light is meant to be the
ultimate speed limit in the universe
according to Einstein's special theory
of relativity nothing should move
through space faster than light but that
doesn't stop people from trying every
day I get a lot of messages proposing
ways to go faster than the speed of
light there is the classic method where
you shine a laser at the Moon if you can
flick that beam across the moon surface
in less than a 100th of a second which
is not hard to do then that laser spot
will actually move across the surface of
the Moon faster than the speed of light
imagine what that would look like if you
were standing on the moon if you were
quick enough to perceive it you would
see this laser spot move faster than the
light coming out of your own laser how
is that even possible well in truth
nothing here is really traveling faster
than the speed of light the individual
particles of light the photons coming
out of my laser are still traveling to
the moon at the speed of light it's just
that they're Landing side by side in
such quick succession that they form a
spot which moves faster than the speed
of light but really it's an illusion
nothing is actually going faster than
the speed of light so you couldn't
transmit any information this way Dan
asked what if instead of a laser we used
a long rigid stick instead now surely if
you flick your wrist the tip of the
stick must move across the surface of
the Moon faster than the speed of light
well unfortunately this won't work
either as we learned in the slinky drop
experiment the fastest of force can
propagate through an object is the speed
of sound as because each atom needs to
bump into the one next to it to transmit
that force and this is a lossy process
so you you'd be lucky if any of the
energy you put in at the start actually
made it to the tip you'd be lucky if the
tip moved at all now this is a really
sophisticated idea Gerard writes a very
special Space Age engine would need to
be designed that is capable of doing
10,000 plus RPM in outer space with very
high torque consult Elon Musk for this
as the engine is spinning it slowly
deploys two very long tethers made from
carbon Nano tubes on opposing sides
eventually each carbon Nano tube tether
reaches an aming length of 285 km at
this point the end of the tether will be
traveling at the speed of light can you
point out some reasons as to why it
would not work yes Gerard yes I can
first any object going in a circle
requires a force pulling it in towards
the middle of that Circle that's called
centripetal force and you can feel it
when you whirl a ball above your head
now that force is dependent on the speed
of the object squared so if that gets to
be too great The tether breaks now if
you had a single gram rotating at 99%
the speed of light the amount of force
required to pull it towards the center
would be 300 Mega newtons that is the
weight of 6,000 fully grown African
elephants but you're right carbon nanot
tubes are tremendously strong if you had
a fiber just 8 cm wide you could support
all of that Force but now the problem is
if you have less than a centimeter of
that fiber it adds another gr to the tip
of your tether and so now you need a
thicker fiber in order to support that
additional force and that would happen
all the way to the base the fiber would
need to get thicker and thicker and
thicker all the way back to the motor
and if you do the calculation you find
that basically 30 m from the tip the
fiber already has to be as wide as the
observable universe in order to support
all of that Force it's nuts but it gets
worse as an object moves faster its
inertia actually increases that means it
requires more Force to accelerate it in
fact that one gr Mass going 99% the
speed of light would require seven times
the amount of force we calculated before
and so the tether would have to be even
thicker but things get even more
problematic if you think about speeding
up the tip of the tether that extra 1%
to the speed of light I mean since the
inertia keeps getting greater and
greater it requires more and more Force
to accelerate it and in fact to speed it
up that extra little bit to go the speed
of light would require an infinite
amount of energy okay well putting the
infinite energy aside let's say we could
create an incredible motor and we could
find a material much stronger and
lighter than carbon nanot tubes is it at
least in principle possible that the tip
could go faster than light no there is
one final problem which is
insurmountable which is that a tether
like anything is held together by the
electromagnetic interaction that is the
attractions between all the tiny little
charges that make up the material now
the problem is electromagnetism is a
force carried by photons I mean the way
that something knows that another thing
is there to attract it is is by The
Exchange change of photons these Force
carrying particles the problem is the
photons themselves move at the speed of
light so even if you could create this
incredible apparatus with ridiculously
strong materials and spin it up with
Infinite Energy it still wouldn't go the
speed of light because the force
carrying particles that hold the whole
thing together only go the speed of
light the speed of light really is the
ultimate speed limit in the
universe hey did you see I made a video
about the problem with Facebook over on
my second Channel really seems to have
struck a chord so you should check it
out if you haven't already now I want to
thank Audible for supporting this
episode of veritasium they are a leading
provider of audio books with over
150,000 titles in all areas of
literature from fiction to non-fiction
and periodicals now this week I wanted
to recommend the book by bill bryen
called a brief history of nearly
everything when this book first came out
I really want to dislike it because I
felt like it was just piggybacking on
Steven Hawkings Brief History of Time
but what Bill Bryson has done is
something truly different and
extraordinary I really think it's a
great summary and a great sort of
Investigation of what happens in science
it is a a brilliant thing to listen to
also if you go to audible.com/veritasium
you can download this book for free or
another of your choosing now they
actually have this book in an Abridged
form read by Bill bryen himself it's
really interesting to hear the author's
voice voice to me he sounds a little bit
like cgp gray but with a hint of a
British accent so you should really
check that out just go to
audible.com/veritasium all right thanks
for watching and thanks to Audible for
supporting me but there are some things
which are going faster than the speed of
light relative to us there's some
distant galaxies which are receding out
a velocity faster than the light so we
will never be able to see the light that
they emit but this doesn't violate
Einstein's theory of relativity because
they're not moving through space faster
than light it's just that the space
between and them is expanding so quickly
that their effective velocity is greater
than the speed of light