Kind: captions
Language: en
[Applause]
today I'm doing an experiment that
demonstrates Heisenberg's uncertainty
principle so here I have a green laser
and I'm firing it down towards the front
of the room through a narrow slit now
that slit can be adjusted so it could be
made narrower or wider and the laser
spot is projected onto a screen behind
it so what do you think is going to
happen to this spot on the screen as I
narrow the slit well let's have a look
you see exactly what you'd expect the
spot gets narrower and narrower the
sides are getting cut off by the slit
makes complete sense and if you stopped
there you would never realize that
Heisenberg's uncertainty principle is at
work but if you keep going something
strange happens as you make the slit
even narrower the spot starts to spread
out isn't that incredible you're making
the slit narrower and yet the spot on
the wall is getting wider the narrower
you make it the wider that spot on the
wall
becomes to understand this we have to
look at Heisenberg's uncertainty
principle Heisenberg's uncertainty
principle is normally written as Delta X
Delta p is greater than or equal to H on
4 Pi so what does this mean well it's
about the position and the momentum of a
particle so X is the position of the
particle and P is its momentum so Delta
X is the uncertainty in position and
Delta p is the uncertainty in the
momentum now if you multiply those two
quantities together they must always be
greater than or equal to H on 4 Pi now H
is Plank's constant and that deserves a
video all to itself like this one by 60
symbols but for our purposes it's just a
very small number so in our everyday
lives we don't come up against this
uncertainty relation because everything
is much much bigger than H but as we
narrowed the slit we were decreasing
Delta X for those photons so we were
getting more and more precise about
where the photons were passing through
that slit and at a certain point you
come to this limit so that if you narrow
this any further you're going to break
this uncertainty relationship so what
needs to happen is the uncertainty and
momentum needs to go up I should specify
this is uncertainty and momentum in the
X Direction in the horizontal Direction
so if before photons were going
perfectly straight now they must Veer
off to the left to the right to ensure
that we don't break Heisenberg's
uncertainty relation and the more you
decrease your uncertainty in position
the more narrow you make that sliit the
more the uncertainty in momentum has to
go up and so if these photons are going
to the left and the right that's going
to produce a much wider beam it's really
really non-intuitive but it's the way
the world
works that it must be the sun playing
tricks with my mind the what about here
is really going to test you right tell
me about what's next well this is only a
few blocks from here with you expect we
see October I've got to say a big thank
you to Professor Walter Luen at MIT he
inspired me to make this video and I
also have to say a big thanks to the
University of Sydney for letting me use
their equipment and especially to Tom
and Ralph for helping me set this all up
oh and just one more thing I should
point out that this explanation of the
experiment is slightly controversal
at least in that Henry from minute
physics and I have been debating whether
it's really that counterintuitive I mean
if you see light as a wave then all
light is doing here is diffracting
that's the phenomenon where if a wave
passes through a slit it bends at the
corners and radiates out in all
directions and that explains the
spreading of the beam but that goes to
the very nature of light is it made of
waves or particles that's something that
I would like to explore in the coming
weeks so stay tuned for that but Henry
has an excellent video about
Heisenberg's uncertainty principle which
I think makes it more intuitive and less
spooky so if you want to check that out
click on The annotation it's a really
good
video