want more sleep space episodes check the pinned comment or stay until the end of the
video for an announcement on them. If there’s one thing I’ve learned about
light, it’s that for unthinking energy, light seems to love messing with us. For years,
scientists debated about whether it was a particle, or a wave, because it keeps exhibiting
elements of both, seemingly unable to settle. It behaves one way when you’re looking at
it, but a different way when you’re not. But at least its speed is consistent.
Light travels at the speed of light. No matter your frame of reference,
that one thing remains the same... Except, no, it turns out the constancy of
light’s speed might not be right either. Light might go slower than physics would
predict in certain circumstances. And no, I’m not just talking about light slowing down
in denser mediums like glass, although that’s what I originally intended this video to be
about. We have an explanation for that. I’m saying that in some circumstances, light seems
to travel a path through time and space that has it either going slower or faster than the speed
of light, even if dense mediums aren’t present. But the really weird thing is that it ends up at
the same destination in time and space anyway. I’m Alex McColgan, and you’re
watching Astrum. Allow me to show you what I mean. Thanks to a new
take on the double slit experiment, I want to show you that your understanding of
light is either incomplete, or, well, wrong. Light travels at 299 792 458 m/s. According
to relativity, this is the only speed light can travel at, and interestingly seems to stick to
that number regardless of your frame of reference. Two people could be travelling through space
– one at 1% the speed of light, and the other at 50% the speed of light – but if they both
look at the same beam of propagating photons, they will see them travelling at the same
speed. Time and distance would seemingly rather warp than allow you to see anything
other than light travelling at light speed. Of course, when scientists say this, they are
only talking about light travelling in a vacuum. We’ve known for a long time that as soon as you
get matter involved, light gets bogged down and travels slower. Light travelling in air only goes
at 299,705,000 m/s, a full 87,458 m/s slower than light in a vacuum. Light in water goes around
225,000,000 m/s. Light going through glass caps out at around 200,000,000. The reasons for
this are intriguing, but fairly well understood, and certainly not physics-breaking. When light
travels through matter, its constantly waving electromagnetic fields gets the electrons within
the matter to also start moving, like ships bobbing on water. But, as electrons moving up and
down also generate an electric field that in turn creates a magnetic field, a second light wave is
created by these moving particles that crucially overlaps the waves of the original light, albeit
one that waves at a slightly different pace to the original light (exactly what speed varies
depending on the material). When two waves meet, they interfere with each other – they take an
average, sometimes interfering constructively to build each other up, and sometimes working
against each other. So, when you take the grand total of all the ups and downs of each wave,
you actually end up with a new wave – one that travels at a different speed to the other two,
and one that goes slower than the speed of light. Eventually, this propagating wave can reach the
edge of the blocking material and, without those electrons interfering anymore, you’re left with
just the original light again, which is then free to travel along its original path again at its
original speed as if nothing had ever happened. Scientists have had a lot of fun
with this concept over the years. Researcher Lene Hau at Harvard in 1999 was
able to slow down light to an astonishing 61 km/h by sending it through a cloud of sodium
atoms that had been cooled to 1 billionth of a degree above absolute zero. 2 years later,
Hau managed to slow down light’s speed to 0, before warming up the cloud and
sending it on its way again. You might find that result surprising. However, strange things have happened
in the opposite direction too. In 2000, researchers at the NEC
research institute in Princeton, New Jersey sent a pulse of light through a cloud
of cesium atoms. Alarmingly, when they timed to see how quickly the pulse exited the cloud, it
seemed that it exited before it had entered. While this might appear to mess with causality –
how can you leave a building before you go inside it, after all? – fortunately there was a simple
explanation that saved us from creating too many paradoxes. Although the light pulse travelled
faster than light, the light itself did not. This was more an optical illusion than
a refutation of Einstein’s relativity. Let’s take a closer look at a photon of light.
Each photon represents a tiny packet of waves, moving up and down. The speed the waves inside the
packet propagate is known as its phase velocity, while the speed at which the packet as a whole
is travelling is known as the group velocity. You can also have a wavefront velocity, which
is how fast the first photon in a wave of photons can travel. This is a little heavy in its
terminology, so let’s explain it with an example. Think of a crowd of people doing a Mexican
wave. The wave that the people are doing is the phase velocity. You can see the
wave travelling along through the crowd, it might look like it’s travelling quickly. But
the crowd itself isn’t going anywhere, so our wave’s true speed is 0. These people are the group
velocity, or possibly the wavefront velocity. Let’s imagine that we wanted to send
our crowd marching. They could do so, and could keep doing a Mexican wave as they
travelled. But although their waving hands might make the wave go really fast in the direction of
their travel, it would vanish whenever it reached the front of the crowd. Information exchange
couldn’t go faster than the walking speed of the crowd itself, regardless of how fast the
peaks in the waves seemed to be travelling. Einstein in relativity never claimed that
phase velocity couldn’t exceed light speed. He just claimed that information
couldn’t travel faster than light. And if you’re trying to deliver
a message to someone by sending a crowd of Mexican wavers in their direction,
it really doesn’t matter how fast they’re waving. Until the first person in the crowd
arrives, no information can be delivered. Still, this difference between the waves within
light and the speed of light itself will become interesting in our next experiment. And this
is where things start to get a little weird. Oh, you thought it was weird already? Oh,
no, this is the really physics-defying part. Let’s go back to an experiment we’ve looked at
before, the double slit experiment. I go more into depth on this experiment in my video here. If
you haven’t watched it already, I really recommend you do so, as I’m going to assume you know what
this experiment is for the part that comes next. This experiment is a fascinating exploration
of how light can sometimes behave like a wave, and sometimes like a particle. However, in
2023 researchers from the Imperial college figured out a way to separate the slits of
this experiment, not in space, but in time. The way they did this was simple. They took a
transparent material called indium-tin-oxide, that under specific conditions can be made to
be reflective. Indium-tin-oxide is the stuff they use in most mobile phone screens. They
fired a laser at it, and then rapidly changed the material from transparent to reflective
and then back again. This left only a slim window – a few femtoseconds – where the laser
was reflected. They called this a “time slit”. They recorded what the laser looked like after it
had been reflected, and found that its frequency had spread out a little bit in the process, but
other than that nothing too crazy had happened. The weird thing was what happened when they sent
two laser pulses through these “time slits”, in rapid succession. The position of the
emitter, the mirror and the receiver remained the same – the only thing different
was the time the lasers went through. Oddly enough, when two went through,
an interference pattern happened. This was not an interference pattern in the same
sense as with the regular 3D space double slit experiment, though. This was an interference
pattern that affected the laser’s frequency. Certain frequencies of light within the
laser faded out, exactly in line with the way intensity faded out in the regular
version of the double slit experiment. To visualise why this might be happening, let’s
draw out this experiment in regards to time. The time slit experiment can be drawn in a
similar way to the double slit experiment, except we’re going to need to visualise the
change in the experiment over time. To do that, let’s create a 4D graph where space is along
the x-axis and time is along the y-axis. This is easy enough to do – it just looks like this:
The photon leaves the emitter to the left, hits the time slit, is reflected and arrives at
the receiver. I’ve drawn this as a continuous line just to make things simpler later,
but the idea works just as well either way. Later, a second photon is
released from the emitter, it reflects and arrives at the
receiver at a slightly later time, represented in how it takes place higher up
(further into the future) on our time graph. If light behaved normally, travelling along
at the speed it was supposed to go at, this would be the end of it. Instead, light
is interfering with itself. This means it must be travelling along a path that takes
it through the other slit as well as its own. This is the only way that light would come
in with the pattern that we see. And just like in the double slit experiment, it’s likely
happening on the other side of the slits too: As for why it’s frequency and not intensity
that’s being messed with here, think about the implications of what you might see if light did
indeed come in at a different angle like this. Photons come in little packets of
waves, as I’d previously mentioned. Now, look at what happens if you change
the angle at which those waves arrive. Here’s how it normally might look:
I’ve added a black timeline here, and have highlighted every time the
receiver sees a new peak in the wave. Here’s what happens when you alter
the direction of the wave’s arrival: Suddenly, the peaks are coming in much more
frequently. The frequency of a wave over time is very much connected to the colour
we perceive light to be. Low frequency light is redder in colour, while increasing the
frequency shifts light’s colour towards blue. So, this colour variation makes sense.
What makes less sense is what’s going on with the paths this light is
taking through time. Remember, the straight lines we started with represent the
299,792,458 m/s that we see light travelling. So what can we say about the photons
that are travelling along these paths? For some parts of their journey, they are
travelling slower than the speed of light, taking more time to arrive at a destination that’s
the same distance away. And yet, for other parts of their journey, they are travelling faster than
causality ought to allow. From their perspective, they are travelling backwards in time. As a
reminder, these two emitters on the left are actually the same one, just at different points
in time. The same for the receivers on the right. It is a mind-bending result. And yet, according
to the results of this experiment performed by a research team at the Imperial College
in London, this is what is occurring. The implications of this are startling. Light
always travels the path of least time – the route that allows it to arrive at its destination
along the path closest to 299,792,458 m/s, the fastest anything in the universe apparently
can go. And yet, it seems that in its efforts to locate exactly what path that might involve,
light is testing the waters – putting out feelers that check to see if other paths, and
seemingly other paths through time itself – might present a more viable solution. These feelers are
interfering with photons that travel alongside it, but also with photons that travel a
little ahead or behind it in time. To be clear, we never actually detect photons
taking any of these other paths. We don’t see photons coming in from the future. We never see
photons travelling slower than the speed of light, provided there is no supercooling gases
providing an explanation for why they slow down. And yet, for interference patterns to occur,
to at least some extent light must be trying out alternative routes through time. Perhaps
it’s like lightning, testing many different directions to find the optimal path to its
destination, before finding the one that works and collapsing down that path in one giant
boom, all other feelers vanishing and collapsing: Or perhaps some other phenomena is at play.
Who can say? For now, all we know is that light
has proved once again that it doesn’t play by anyone’s rules. At least,
not rules that we can figure out.
