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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 star-shine 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, we’ll settle for saying thanks.

And if you’d like to support SciShow too, you can head over to  patreon.com/scishow to get started. [♪ OUTRO].