previously on veritasium we saw how our
understanding of light has changed over
the centuries in the late 1600s Huygens
proposed that light was a type of wave
while Newton considered it a stream of
particles this debate appeared to be
settled in 1801 by Young's double-slit
experiment which showed light passing
through two slits produced patterns like
water waves but by 1900 it was clear
that light energy was not evenly
distributed as expected for a wave
rather on the smallest scales it comes
in lumps called quanta or photons so the
question is how does this affect the
double slit experiment here I have a
more contained double slit experiment
where there is a laser which fires a
beam through a single slit then through
a double slit and onto a screen where
you can see a well-defined
interference pattern there's a series of
bright and dark bands which are much
easier to observe than when I use
sunlight because with only one
wavelength there are no other colors
involved this is what a graph of
intensity versus position would look
like for the interference pattern what
creates that pattern consider the bright
spot in the middle the light from each
slit has to travel the same distance to
reach that point and hence both waves
arrive in phase that means crests with
crests and troughs withdraws so they add
together and create an interference
maximum a bright spot but if you look
slightly to the left there's a dark spot
now that's because light from one slit
has to travel on an angle and it has to
travel an extra half a wavelength
compared to the light from the other
slit which means when this light is
arriving as a crest the light from the
other slit is arriving as a trough and
so they cancel each other out but if you
go further left you see another bright
spot because now the light from one of
the slits has to travel a full extra
wavelength compared to the light from
the other slit and so again they arrive
in phase crest with crests and troughs
with troughs creating constructive
interference and so we see a bright spot
of light and that's how the whole
pattern is created but what if I
decrease the intensity so much that
there wasn't a whole wave of light going
through there there were only single
photons then how could they interfere
with each other because there's only one
in the device at any one
time so would we still see an
interference pattern that is what we're
going to find out in order to make this
work
I had to line up a very faint source and
to see where the light was going I had
to shroud my head in that black cloth
but I finally have the apparatus ready
you can see up here I have a frequency
counter which actually counts the number
of photons received per second at the
detector the detector is a
photomultiplier tube which is capable of
detecting single photons it's like a
very sensitive eye like the frogs eye
now right now there's no light passing
through to the detector but there is
still a bit of a background reading and
that's because I can't block all the
light out of there plus their cosmic
rays passing through this room which
will also set off the detector I'm
plotting a graph of the number of
photons counted as a function of
position across the detector if you have
a look after one second the distribution
seems random there doesn't seem to be
any pattern in the arrangement of those
photons as they hit the detector so
maybe it's true a single photon can't
interfere with itself because it's just
a localized point it has to go through
one slit or the other but just to be
sure let's add up the results over a
period of time and see if any pattern
emerges
look at that you can clearly see the
same interference pattern that we got
when we were sending tons of photons
through but we're getting it out of
single photons we're counting up
individual photons and that pattern is
emerging as we aggregate the results
over time but how could this be
happening how could a single photon pass
through both slits well if we try to
interpret these results in terms of the
objects we experience every day they
don't make any sense a photon is
something different to a macroscopic
object it's not a wave and it's not a
particle it's a quantum mechanical
object and sometimes it seems like it
has properties of a wave and sometimes
it seems like it is properties of a
particle but ultimately it is something
totally different to anything we've
experienced before
and that's what makes this seem so
counterintuitive so what is light wave
or particle the true answer I think is
neither though if you want you could
call it a wave achill
is by the ripples from the two sources
interacting with each other where they
meet up peaks with peaks and troughs
with troughs the amplitude of the wave
is increased that's what we call
constructive interference but if the
peak from one wave meets up with at the
end of the last video I asked why is it
that the interference pattern is made by
blobs rather than thin lines of light as
it was in this experiment and in the
comments I saw two common answers one
was that the blobs were images of the
Sun the other answer was that it was due
to Heisenberg's uncertainty principle
now those were very thin slits so it's
true Heisenberg's uncertainty principle
would be involved and that light passing
through each slit would spread out but
the reason we actually saw blobs was
because they were images of the Sun so
if the sunlight comes through one slit
it diffracts out and would spread out
onto the bottom of the box and light
from the other slit would do exactly the
same and where those two spreads overlap
is where we saw the three main blobs so
there's a diffraction maximum created by
one slit and another diffraction maximum
created by the other slit and those two
overlap but due to the light being in
different phases it cancels out in
certain positions creating those
separate images of the Sun