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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.