Light is so much stranger than you might think.
Sure, it may seem simple enough, travelling around the universe delivering energy from one place
to another. It helps us see. It provides life to plants, and thus to our planet generally.
It has a reputation for being very fast. And yet, for a source of energy that has
become synonymous with greater understanding, light is surprisingly difficult to understand.
Light helps us see other things better, sure, but when scientists tried to look at light
itself, it was surprisingly difficult. No, I don’t mean that they started staring into
any lamps – please, don’t do that at home - but experiments in the last 200 hundred years
or so have proven that what light appears to be and what light is are actually two different
things, for one simple reason: annoyingly enough, light behaves differently when you’re not
looking at it, compared to when you are. What is the true nature of light? Why is it
behaving strangely when we’re not looking? And what does it say about how the universe
really works? I’m Alex McColgan, and you’re watching Astrum, and in today’s video it’s time we
try and find out. Let’s shed some light on light. Let’s begin with the basics. What is light? In
the early 1700’s Isaac Newton theorised that light was made up of tiny little particles that
he called “corpuscules”, but in 1801 – nearly 100 years later - a man named Thomas Young discovered
that light must actually be more wave-like than particle-like. He proved this using an important
method known as the double-slit experiment. He set up a source of light, and shone it
through two narrow slits onto a board. Young noticed that rather than getting two bands of
light on the other side of the slits, a strange striped pattern was forming. This was known as
an interference pattern, and was incontrovertible proof that light had been travelling as a
wave. Why? Let’s talk about waves for a moment. When waves travel, they oscillate up and down. But
when two waves try to oscillate the same point in space at the same time, you get something known
as interference. Imagine you had a bathtub with a rubber duck sitting on the surface. Two waves
reach the duck at once. One wave tries to raise the duck up at the exact same time the other wave
tries to drop it down. What happens? Provided the waves are of the same magnitude and are perfectly
out of phase, they will cancel each other out, and the duck would not move at all. This is called
destructive interference. Similarly, if the waves both tried to raise the duck up at the same time,
the duck would be raised twice as high. This is known as constructive interference. Because waves
tend to expand in a circle, two waves next to each other will start to both constructively
and destructively interfere with each other. Here are two waves in water. See these lines?
These calmer patches are where the waves are cancelling each other out: This is the effect we
see with light travelling through the two slits. As the light from one slit propagates, it cancels
out the other wave of light at certain points, creating the interference pattern that Young
noticed on the board. So, the mystery was solved. Light was a wave, and not a particle. Except,
there is more to this experiment than meets the eye. Let’s fast forward another 100 years,
to 1905. Scientists around this time had become puzzled by something known as the photoelectric
effect. It turned out that when you shone a light on a metal surface, electron-like particles were
coming off it. This was deduced to be because electrons in the metal were getting knocked off it
by the increased energy the light was imparting. Imagine it like fruit on a tree. If you pull the
fruit off the tree, you need to use a certain amount of energy. Once the energy is greater
than the strength of the fruit’s connection to the branch, the fruit pops off. This was happening
with the light and the electrons. Once the light hit an electron and gave it enough energy to
pass the threshold, it broke free from the metal. However, what surprised scientists was that
if you increased the intensity of the light, they had expected the electrons to be knocked
away faster. If you pulled the fruit off the tree harder, it would come off faster. More
energy = more departing kinetic energy. However, this did not appear to be the case.
Instead, increasing the frequency of the light increased the velocity of the departing electrons.
The intensity of the light didn’t affect the departing electrons’ velocity at all, but did
affect the quantity of electrons being emitted. This was a bit of a puzzler. Albert
Einstein was the man who solved the puzzle. He deduced that light must be travelling in
little packets of energy, so sending more of them – increasing the frequency – was the only
way to increase the energy going to the electrons. He called these packets photons, and later earned
a Nobel prize for his work. Light, it seemed, was more like a particle again. Or both a wave and a
particle at once? Of course, even this is not the full picture. To be honest, we aren’t completely
sure about the full picture even now. Instead, we have more results that are contradictory.
Let’s go back to the double-slit experiment. Armed with the knowledge of photons, physicists
once again took a look at the double-slit experiment. Experimental techniques had improved
in the last 100 years, and it was now possible to emit a single photon of light at a time. So,
the double-slit experiment was done again. This time, only a single photon would be sent
through the slit, onto a detector on the far side. When this was done, the detector registered the
arrival of the photon at just a single point. So, light was behaving like a particle again. But
then, why had it interfered with itself in the previous version of the experiment? Scientists
had an idea. They sent through multiple photons, one at a time, and plotted the results on the
detector. And this is where the result became really strange. Once again, the detector started
seeing the photons arriving at single points, one at a time. But bafflingly, the arriving
photons started creating a pattern: It was the interference pattern. The
proof that light behaved like a wave. But strangely enough, this was occurring only
when a single photon was going through at a time. Somehow, the single photon – which was leaving
the detector like a particle and was arriving at its destination as a particle – was apparently
in some way travelling through both slits at once, enough to then interfere with itself
on the other side, like a wave. If light was just a particle, then when it went
through the slits, you wouldn’t see this pattern. You would see only two blobs of light – one
for particles that went through the one slit, and one for particles that went through the other.
And yet, here was the interference pattern with its multiple lines of light, disproving that.
Scientists tried to pin light down. They set up the experiment, but this time with two more
detectors at the slit, so that scientists could observe whether it was indeed passing through both
at the same time. It didn’t. But at the same time, it stopped creating an interference
pattern on the furthermost detector. And from this, scientists began to realise
something. Light cared about being observed. To be clear, it didn’t matter whether it
was observed by a human eye or a machine. The moment light was interacted with in some
way, by any particle – which is the only way we can detect light, there’s no other way to
observe it - it started behaving differently than if it hadn’t been detected at all. It was
as if light was snapping into focus any time the universe asked it the question of where exactly
it was, when without that scrutiny it appeared to relax into something a little more nebulous.
Bizarrely enough, this seems to imply that light actually is more like a wave of probability,
rather than any discrete particle or wave. Any time it was asked where it was, it confidently
provided a definitive answer – it WAS at this point on the detector, it WAS NOT at any other
point. But with no-one checking up on it, light seems to be travelling in all directions at
once, in accordance with certain probabilities. If you ran the experiment multiple times, you
could quantify those probabilities, discovering that it was more likely to be on the bands of the
interference pattern, and less likely to be in the gaps. But any time a single photon of light was
asked, it gave an answer that was 100% concrete. This is highlighted through something
known as the three-polariser paradox. Consider for a moment a pair of polarising
sunglasses. Obviously, these reduce the amount of light that can pass through them; usually by
about 50%, depending on the type of lens and the wavelength of light. They work by being formed
of thin chains of molecules that run lengthways across the lens. Any light that oscillates in the
same orientation as this lens gets absorbed. Any that is perpendicular to the chains can pass
through without trouble. The interesting case occurs when a single photon is passed through
in an orientation that’s diagonal to the lens. In this case, you don’t get half a photon going
through. Apparently you can’t just absorb the part of the oscillation that is parallel to the lines
and let through the part that is perpendicular. Instead, the photon “snaps” into either the
one orientation, or the other. It either is completely absorbed, or passes through entirely
– but now with a new, perpendicular polarisation, to match what it would have had to have been
to pass through easily. How do we know that the photon wasn’t this orientation all along? Because
of what happens when you start adding more lenses. When you place a second lens behind the
first, you can block out the light entirely, provided the two polarisations are perpendicular
to each other. Let’s say, we rotate the second lens 90 degrees compared to the first one. Any
light that gets through the first lens has a 0% chance of getting through the second, like trying
to post a letter through a chain-linked fence. As a result, we only see black. But add a
third lens, and place it at a 45-degree angle between the other two, and bizarrely light
starts making it through all 3 lenses again. This may seem counter-intuitive – how does adding
more blockages increase the amount of light that makes it through? But this result actually rules
out the possibility that the light has a fixed orientation. It must be snapping into focus at
each new lens, rolling a quantum dice each time to see if it was the right orientation all along
or not. If it makes it through the first lens (a 50% chance), it only did so because it was
oriented perfectly perpendicular to the lens’s polarisation. Which means once it reaches the
second, it’s coming at it from a polarisation that’s diagonal. So, once again there is
a 50:50 chance that it makes it through. It rolls its quantum dice again, and once again
has a 50:50 chance of proceeding. If it gets through this hurdle too, then it again snaps
to the new orientation, as if it were that new orientation all along (which it obviously wasn’t).
Which means that it’s now polarised diagonally relative to the third lens, meaning that it
now has a final 50% chance of getting through. Of course, some photons do not make it through
all 3 of these probabilistic gauntlets. Only about 12.5% of them make it. But that’s more than
0%, which is what was happening previously when you had only two lenses. Light likes to behave in
discrete quantities. It is “quantum”. It seemingly snaps to a discrete value when observed. And
honestly, we don’t really know why. If you think about a wave, there is no reason why you couldn’t
simply have half a wave. You could halve it again and again an infinite number of times and still
have an answer that makes mathematical sense. And yet it seems that down on a low enough quantum
scale, you can’t halve light past a certain point. You can’t have half a photon, or even one and a
half photons. And if you try to do so, the photon instead snaps to one or the other nearest integer,
based on probabilities: but only when it’s asked. Otherwise, it’s quite content to exist
probabilistically, interfering with itself like a wave as it travels along, before jumping
to an answer when later asked exactly where it is. What is going on here? This is
still being theorised about. The closest comparison we have to it is something
known as harmonics, where on a bounded string, only a certain number of waves can exist. On a
guitar string, you can have one wave, or two, or more, but never any number that isn’t
a whole number . It seems that light works in the same way. Perhaps something pinches the
beginnings and the end of the path light travels down – although what this might be, and what
mechanisms drive it, are unknown as of now. Fundamentally, though, perhaps the craziest thing
about all of this is that this isn’t just about light. Although we’ve focused on light behaving
like a wave, and behaving probabilistically, all particles of matter do the same. Light is
just another form of energy, and energy and matter are linked. Particles of matter – atoms and
even complex molecules – have been shown to have wavelengths. Electrons are just as quantifiable
and just as driven by probabilities a s photons are. We are apparently all driven by probability,
if you scale things down small enough. So, what is everything truly made
of? What makes up energy and matter, that causes it to behave in the way that
it does? What Is going on under the hood of reality? Why is the universe behaving
different when looked at compared to when not? And what does it imply to think that
even you are on some level probabilistic? What this all means is anyone’s guess. The person
who figures it out will be the Einstein of our time. But for now, all we can say is that when
it comes to reality, it seems the universe is playing dice. You and the world around you might
be a lot less certain than you might have thought. Sometimes when I learn about complex topics
in physics, it can feel a little bit like I’m listening to people speaking in another
language. While Babbel – the sponsor of today’s video – might not be able to help me with that,
they can certainly help if I’m actually listening to people speak in another language. Babbel is one
of the top language-learning apps in the world, and can help you start speaking a new language
in just 3 weeks – an activity that can make a big difference. I should know – I met my wife abroad,
and speaking in her language was a massive help. Babbel teaches you real-world conversations,
for travel, business, and relationships. So, if you want to learn a language this
summer to really open up the world for you, try out Babbel to learn your danke’s from
your merci’s. And you can get 60% off your subscription if you click my link in
the description below! Schau vorbei! Thanks for watching. If you liked this video,
you’ll love this one about the shape of the universe. A big thanks to my patrons and
members for your support. If you want to support too and get a host of perks, check the
link below. All the best and see you next time.
