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If there’s two words in all of physics associated with the strange and unfathomable,
it’s “quantum mechanics”.
Quantum mechanics describes how things behave at the level of molecules or
smaller. And at those scales, things don’t behave in ways that we’re used to.
While everyday objects like tennis balls or springs have simple and
predictable motion, objects at quantum scales are probabilistic.
That means even when you technically know everything about a quantum system,
you can usually only predict the probability of what might happen next
or where an object might be.
The hazy nature of quantum mechanics leads to all kinds of
counterintuitive behavior, like tiny particles acting like they’re in two places at once
or even seeming to pass through walls.
But while all of that sounds pretty bizarre,
the line between life at human scales and the quantum world
is blurrier than you might think --
and quantum phenomena are more familiar than you might assume.
It turns out, lots of quantum effects are directly responsible
for many things we observe in nature.
And some things we don’t yet fully understand may have quantum explanations.
In fact, scientists from outside of physics are increasingly looking to quantum physics
to shed light on mysteries in their own field.
Consider plants, for example.
Plants feed themselves through photosynthesis,
converting carbon dioxide and water into sugar and oxygen.
And that process requires energy, which plants get from sunlight.
Light is initially absorbed by an antenna,
basically a molecular receiver that captures light.
At the other end of the process are the reaction centers,
the chemical engines that actually drive photosynthesis.
What connects the two are special proteins called light harvesting complexes.
In conjunction with light absorbing pigment molecules called chlorophylls,
they collect energy from the antenna in the form of energized electrons
and deliver that energy to the reaction centers -- kind of like a tiny, biological wire.
Amazingly, those complexes are nearly 100% efficient
at transporting the energy they receive.
By comparison, the most efficient solar panels you can buy
are only about 20% efficient.
Scientists think the key to the remarkable efficiency of these complexes
could be a phenomenon called quantum coherence.
Coherence describes quantum events with more than one possible outcome.
But instead of happening one way or the other,
it ends up happening lots of different ways at the same time.
Inside light harvesting complexes are smaller groups of atoms, or sites,
that create stepping stones for electronic energy, hopping from one site to the other.
The arrangement of those sites provides lots of different routes
to get from the antenna to the reaction centers, where the energy needs to be.
And instead of choosing just one path to take,
coherence allows the energy to take lots of different paths at the same time.
In a 2007 study published in the journal Nature,
researchers announced evidence of this happening in photosynthesis.
They were studying a specific light harvesting complex found in green sulfur bacteria,
called the FMO complex.
These bacteria are a useful model for studying photosynthesis,
since their complexes are well known and easy to study in the lab.
After shining light through an FMO complex in the lab, the researchers saw
clear signs of oscillation imprinted on the light coming out the other end.
See, when you’re observing quantum coherence from the outside,
the system seems to switch from taking one path to taking the other,
even when they’re totally different in space.
The amount of energy carried along those paths is a little different too,
so it looks like the system is oscillating between different states of energy.
That oscillation is a tell-tale sign of coherence in the system.
The original study involved an FMO complex at minus 196 degrees Celsius,
but later studies showed signs of coherence even at room temperature --
which is more relevant to actual living bacteria.
And we’re not sure yet, but it could be the secret to why FMO complexes --
and photosynthesis in general -- are so efficient.
Researchers think that coherence encourages the transfer of energy
to happen on a particular time scale, a few trillionths of a second long.
And it turns out other processes, like how sites interact with each other and how the
complexes interact with their surroundings, also occur on the same time scale.
So coherence might synchronise the energy transfer process
to allow all these factors to come together,
creating the perfect conditions to transport energy to the reaction center.
While we’ve yet to study coherence in a living organism,
these early results indicate that certain plants and bacteria
could have evolved to use coherence to improve the efficiency of photosynthesis.
And it’s not just the plants in photosynthesis that have roots in quantum weirdness.
The sunlight they depend on has a quantum story to tell, too.
Sunlight begins with nuclear fusion taking place in the sun’s core.
And fusion involves lighter chemical elements, like hydrogen,
fusing their nuclei to form heavier elements -- like helium.
And in the process, they also generate photons of light,
which becomes the sunshine that reaches our planet.
There’s just one small problem. That process should basically be impossible!
At least as far as ordinary physics is concerned.
It’s a tale of two fundamental forces:
the strong nuclear force, and the electrostatic force.
The nucleus of a hydrogen atom consists of just a single
positively charged particle -- a proton.
Protons fuse if they can get really close to one another,
where strong nuclear forces merge them together.
But those forces only act over tiny distances.
At the same time, there are electric forces, which act over much larger distances.
Because both protons are positively charged, they repel each other.
Even in the hot, dense core of the sun where they have lots of energy,
it’s really hard for two protons to overcome that repulsion and
get close to one another, let alone fuse into a new element.
And that sets up a barrier between them.
Over very large distances, the electrical repulsion doesn’t play much of a role.
Right up next to each other, protons are also fine
because even though their electric forces are trying to push them apart,
their much stronger nuclear forces are bringing them together.
It’s the middle distance that the protons can’t cross.
It’s a lot like there’s a big hill in the way that they
don’t have enough energy to climb over.
But in quantum mechanics, they don’t don’t have to climb it.
Oddly enough, so long as the protons would have enough energy
to sit next to each other on the other side of the hill,
they can find a way to tunnel straight through it.
Physicists call this quantum tunnelling.
It happens because protons are small enough
that the probabilistic element of quantum mechanics plays a big role.
Instead of thinking of protons as perfect, hard spheres in one particular place,
quantum mechanics tells us they’re more like a wave of... proton-ness in space.
The wave tells us the probability of finding a proton at any given location.
And while the odds favor the protons being on opposite sides of the hill, the math
tells us there’s a small chance that they could both end up on the same side.
And it turns out, just that mere probability means it’s possible for the protons
to actually end up right next to each other.
And at that point, their nuclear forces can take over.
So even without the energy to overcome their electric forces,
the quantum tunnelling lets protons get close enough to each other for fusion to take
place, generating the starshine that makes life possible here on Earth.
While that process is fundamental for our existence,
it might seem like it’s pretty far away from everyday life.
But scientists think there’s a quantum phenomenon which is as close as they come.
In fact, you don’t even need to look past the end of your nose to find it.
Our sense of smell could be sensitive to the quantum nature of the world.
Your sense of smell centers on proteins in your nose called olfactory receptors.
And the receptors act like landing sites for molecules that enter your nose.
There are about four hundred different types of receptors
that help us detect different smells.
When a receptor detects its corresponding molecule,
it passes along a signal that transmits the sensation of smell to your brain.
And the prevailing theory behind how this works is based in molecular biology,
not quantum physics.
It’s thought that receptors are sensitive
to the exact shape and size of scent molecules.
In this model, scent molecules fit their receptors like a lock and key.
But some researchers propose that there’s more to the story.
For example, a 2011 study of fruit flies indicated that they had
different preferences for a certain molecule with infinitesimal chemical tweaks --
just a few extra neutrons here and there.
The neutrons didn’t significantly affect the molecule’s shape or size.
But the extra neutrons do affect the way the molecules vibrate.
And some researchers think that could be the key to the process.
The vibrational theory of smell suggests that receptors
are also sensitive to the quantum states of a molecule.
Inside a molecule, individual atoms carry some kinetic energy.
They’re basically vibrating back and forth relative to one another.
But they can’t vibrate in any old way they like.
Molecules only vibrate at certain, well-defined levels of energy.
This is actually a key feature of quantum mechanics,
and what gives it the name. That unit of energy is called a quantum.
And each molecule has its own specific range of vibrational energy levels --
because each molecule is made of different combinations of atoms.
So the theory suggests that’s what smell receptors are actually picking up on.
According to the vibrational model,
olfactory receptors carry an electron in a high-energy state.
But there’s also a possible low-energy state for that electron.
And the theory proposes that if the electron drops into that low-energy state,
that’s what sends a smell signal to the brain.
But the electron can’t jump from the high- to the low-energy state on its own.
It needs something to take that energy from it.
And remember, that energy has to be a very specific amount.
That’s where scent molecules come in.
In the vibrational theory, it’s only when the right kind of molecule
lands on the receptor that the electron can bridge the gap, by giving up
precisely the amount of energy it needs to land in the low energy state.
And the well-defined vibrational energy states within the molecule
correspond to that gap.
If the molecule goes from vibrating in a low energy state to a high energy state,
it can absorb just the right amount of energy the electron needs to lose
for the receptor to pass its signal along.
Different receptors, then, would have different energy gaps for the electrons to cross,
depending on what molecules they had evolved to sense.
That way, there’d still be a bunch of different molecules --
but they’d be sensing vibrational energy as well as shape!
But like with so many quantum phenomena, there’s still a lot of work to be done
before we know for sure that this is what happens in olfactory receptors.
Quantum mechanics is still pretty young as scientific theories go --
right around the century mark.
So we’re still discovering how these bizarre,
counterintuitive phenomena affect our everyday experience, from sunlight to smell.
But thanks to a quantum perspective,
researchers may at least be on the right scent.
Thanks for watching this episode of SciShow,
and thanks to our President of Space SR Foxley for helping us bring it to you.
Your support means a lot,
and since we can’t quantum tunnel straight to you to shower you with confetti,
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