Organisms might be quantum machines
Few of us really understand the weird world of quantum physics – but our bodies
might take advantage of quantum properties
Martha Henriques
If theres any subject that perfectly encapsulates the idea that science is hard to understand, its quantum physics. Scientists tell us that the miniature denizens of the quantum realm behave in seemingly impossible ways: they can exist in two places at once, or disappear and reappear somewhere else instantly.
The one saving grace is that these truly bizarre quantum behaviours dont seem to have much of an impact on the macroscopic world as we know it, where classical physics rules the roost.
Or, at least, thats what scientists thought until a few years ago.
Quantum processes might be at work behind some very familiar processes
Now that reassuring wisdom is starting to fall apart. Quantum processes may occur not quite so far from our ordinary world as we once thought. Quite the opposite: they might be at work behind some very familiar processes, from the photosynthesis that powers plants and ultimately feeds us all to the familiar sight of birds on their seasonal migrations. Quantum physics might even play a role in our sense of smell.
In fact, quantum effects could be something that nature has recruited into its battery of tools to make life work better, and to make our bodies into smoother machines. Its even possible that we can do more with help from the strange quantum world than we could without it.
At one level, photosynthesis looks very simple. Plants, green algae and some bacteria take in sunlight and carbon dioxide, and turn them into energy. What niggles in the back of biologists minds, though, is that photosynthetic organisms make the process look just a little bit too easy.
Its one part of photosynthesis in particular that puzzles scientists. A photon a particle of light after a journey of billions of kilometres hurtling through space, collides with an electron in a leaf outside your window. The electron, given a serious kick by this energy boost, starts to bounce around, a little like a pinball. It makes its way through a tiny part of the leafs cell, and passes on its extra energy to a molecule that can act as an energy currency to fuel the plant.
Photosynthetic organisms make the process look just a little bit too easy
The trouble is, this tiny pinball machine works suspiciously well. Classical physics suggests the excited electron should take a certain amount of time to career around inside the photosynthetic machinery in the cell before emerging on the other side. In reality, the electron makes the journey far more quickly.
Whats more, the excited electron barely loses any energy at all in the process. Classical physics would predict some wastage of energy in the noisy business of being batted around the molecular pinball machine. The process is too fast, too smooth and too efficient. It just seems too good to be true.
Then, in 2007, photosynthesis researchers began to see the light. Scientists spotted signs of quantum effects in the molecular centres for photosynthesis. Tell-tale signs in the way the electrons were behaving opened the door to the idea that quantum effects could even be playing an important biological role.
This could be part of the answer to how the excited electrons pass through the photosynthetic pinball machine so quickly and efficiently. One quantum effect is the ability to exist in many places at the same time a property known as quantum superposition. Using this property, the electron could potentially explore many routes around the biological pinball machine at once. In this way it could almost instantly select the shortest, most efficient route, involving the least amount of bouncing about.
Quantum physics had the potential to explain why photosynthesis was suspiciously efficient a shocking revelation for biologists.
I think this was when people started to think that something really exciting was going on, says Susana Huelga, a quantum physicist at Ulm University in Germany.
Quantum physics had the potential to explain why photosynthesis was suspiciously efficient a shocking revelation for biologists
Quantum phenomena such as superposition had previously been observed mostly under highly controlled conditions. Typical experiments to observe quantum phenomena involve cooling down materials to bitingly cold temperatures in order to dampen down other atomic activity that might drown out quantum behaviour. Even at those temperatures, materials must be isolated in a vacuum and the quantum behaviours are so subtle that scientists need exquisitely sensitive instruments to see whats going on.
The wet, warm, bustling environment of living cells is the last place you might expect to see quantum events. [But] even here, quantum features are still alive, Huelga says.
Of course, just because these quantum features make an unexpected appearance in living cells, it doesnt necessarily mean that theyre playing a useful role. There are theories as to how quantum superposition may be speeding up the process of photosynthesis, but a hard link between this behaviour and a biological function is still missing, Huelga says.
The next step will be having some quantitative results saying that the efficiency of this biological machine is this due to quantum phenomena.
Quantum effects in biology arent just a quirk of plants and other organisms that do the peculiar job of turning sunlight into fuel. They may also provide an answer to a scientific puzzle thats been around since the 19th Century: how migratory birds know which way to fly.
Quantum effects in biology might explain how migratory birds know which way to fly
In a journey thousands of kilometres long, a migratory bird such as the European robin will often fly to southern Europe or North Africa to escape particularly cold winters. This journey over an unfamiliar landscape would be dangerous, if not impossible, without a compass. Start the journey in the wrong direction and a robin setting off from Poland might end up in Siberia rather than Morocco.
A biological compass isnt an easy thing to picture. If there was some form of tiny magnetic iron needle-like structure spinning deep inside a robins brain or eyes, biologists would almost certainly know about it by now. But no such luck: a biological structure that could do the job has never been found.
Another theory, first proposed in the late 1970s, suggested an alternative way birds might know which way to fly: perhaps they carry a chemical compass that relies on quantum phenomena to tell which way is north.
Peter Hore, a chemist at the University of Oxford in the UK, says that such a chemical compass would work with the help of molecules with excitable lone electrons, known as radicals, and a quantum property known as spin.
Electrons in molecules usually come in pairs, spinning in opposite directions and effectively cancelling out each others spin. A lone electron spinning on its own, though, isnt cancelled out. This means it is free to interact with its environment including magnetic fields.
A lone electron spinning on its own is free to interact with its environment including magnetic fields
As it turns out, Hore says, robins can become temporarily disorientated when exposed to radio waves a type of electromagnetic wave of a particular range of frequencies. If a radio wave has a frequency of just the same rate that an electron spins, it could cause the electron to resonate. This is the same kind of resonance you might experience when you sing in the shower certain notes sound a lot louder and fuller than others. Hitting the right radio wave frequency will make the electron vibrate more vigorously in the same way.
But what does this have to do with the idea that birds use a chemical compass? The theory is that ordinarily, radicals at the back of the birds eye respond to the Earths magnetic field. The magnetic field will cause the electron to leave its spot in the chemical compass and start a chain of reactions to produce a particular chemical. As long as the bird keeps pointing in the same direction, more of the chemical will build up.
So the amount of this chemical present is a source of information, generating signals in the birds nerve cells. As part of many different environmental cues, this information will inform the bird about whether it is pointing towards Siberia or Morocco.
The radio wave observation is an important one because we would expect anything that interferes with electron spin to be able, at least in principle, to disrupt the chemical compass. It can be as useful to study why something sometimes doesnt work as it is to study why it generally does work.
Even so, the quantum compass remains an idea. It hasnt yet been found in nature. Hore has been focusing on finding out how the quantum compass can work in principle, using molecules that theoretically ought to be able to do the job.
The quantum compass remains an idea it hasnt yet been found in nature
Weve done experiments on model compounds to establish the principle that one can make a chemical compass, Hore says. These have helped to pin down some molecules that do seem to be fit for the purpose of detecting magnetic fields, he says. What we dont know is whether they behave in exactly the same way inside a cell in the birds body.
The magnetic compass is just part of a complex and poorly understood system of navigation in birds, Hore says. The quantum theory for how such a compass works may be the best out there so far, but theres still a lot of ground to cover to link up the behavioural patterns of birds with the theoretical chemistry.
There is one field that seems tantalisingly close to demonstrating the reality of quantum biology, though: the science of smell.
Exactly how our noses are capable of distinguishing and identifying a myriad of differently shaped molecules is a big challenge for conventional theories of olfaction. When a smelly molecule wafts into one of our nostrils, no one is yet entirely sure what happens next. Somehow the molecule interacts with a sensor a molecular receptor embedded in the delicate inner skin of our nose.
Exactly how are our noses capable of distinguishing and identifying a myriad of differently shaped molecules? BBC
