https://www.youtube.com/watch?v=GzbKb59my3U
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
