We often think of quantum mechanics as only affecting only the smallest scales of reality, with classical reality taking over at some
intermediate level. But in his 1944 book, What is Life?, the quantum physicist Erwin Schrödinger suggested that “incredibly small groups of atoms, much
too small to display exact statistical laws, do play a dominating role in the very orderly
and lawful events within a living organism.” Schrodinger was a visionary - and perhaps
very specifically in this case. Because it turns out we might need all the
weirdness of quantum mechanics to explain birds. In the summer of 1943, a spy from the UK parachuted into Vienne, a French town 20 kilometers south of Lyon. There, the spy learned critical information
from the French Resistance about la Milice, a Vichy paramilitary group with orders to
round up Jewish people for concentration camps and to assassinate Resistance leaders. To deliver the information back to London
securely and rapidly, the spy was sent with an unusual piece of technology: a pigeon named All Alone. The spy attached his message to All Alone and set her free. With unerring accuracy and speed, All Alone
covered the 481 miles back to her perch in Staines, England in less than 24 hours. For her service, she was awarded the Dickin
Medal—the Victoria Cross for animals. While All Alone’s flight was undoubtedly
heroic, she was not unique in her navigational abilities. By some estimates, carrier pigeons were 98% reliable in delivering messages during the war. And pigeons are far from alone, and plenty of other species of migratory birds also have the uncanny ability to make their way home across hundreds, or even thousands of miles. How do they do it and what does quantum mechanics have to do with all of this? Over short distances and on familiar paths, birds can use familiar benchmarks for guidance. Across long distances, birds often rely on
the sun or even the stars. But All Alone and other birds manage just
fine at night and when it’s overcast, seemingly pulled to their destinations like iron filings
to a magnet. And that may not be so far from the truth. The first proposal that birds have a “magnetoreception” that they navigate by the Earth’s magnetic field came from the Russian zoologist Alexander von Middendorf back in 1855. A hundred years later, definitive evidence
came in the 1960s when German biologists Wolfgang Wiltschko and Friedrich Merkel applied magnetic fields to enclosures with European robins, preventing them from navigating properly. How exactly birds detect magnetic fields remains an open question. Some theories have suggested that birds have iron structures in their beaks which help them orient; others have proposed electrically charged fluids sloshing around in the inner ear. But today we’re going to talk about an increasingly favored hypothesis, and surely the most interesting one: the idea that proposes birds can in a sense see the Earth’s magnetic field due to quantum weirdness happening inside their eyes. Before we get into all the cool quantum stuff, a quick review on Earth’s magnetic field is in order. For a more thorough explanation, we have an episode on how that field sometimes flips direction - which I guess drives birds crazy. This “geomagnetic” field is generated
by the convective motion in Earth’s outer core - which is a churning liquid mass of
white-hot nickel and iron. The result is a dipole field, similar to that
of a bar magnet: two poles connected by force lines forming a sort of cage around the planet. At the poles, the force lines are roughly
vertical compared to the surface; at the equator, they’re parallel. At any point on the surface of the earth,
our geomagnetic field can be described with just a few properties. There’s their vertical orientation or “inclination”, so whether they’re parallel to the ground or point into the ground. And their horizontal orientation, or angle
relative to lines of longitude. This “declination” points towards the
magnetic poles, which are offset from the true poles defined by Earth’s rotational
axis. There’s also a directional arrow attached
to the lines - as in one direction pointing “north” and the other “south”. And finally there’s an intensity of the
field, represented by how close together the field lines are. At Earth’s surface it’s about 30 microTesla,
which is about 100 times weaker than a fridge magnet. As far as we know, birds can sense the orientation of the field lines, but NOT their polarity arrows. So they can tell the direction to the nearest
pole, but don’t know WHICH pole it is. In principle it’s easy to come up with ways
to sense a magnetic field. Magnetic fields exert a force on a moving
or rotating charged particle. An electron, for example, can be thought of
as a spinning charge, and magnetic fields can cause that spin to flip direction. Typical bar magnets like the one found in
a compass needle aure ferromagnets, and their magnetic fields come from countless electrons
with aligned spins. External magnetic fields tug on those electrons
resulting in a force that can swivel the compass needle. But you need a lot of electrons to register
Earth’s extremely weak field - far more than you could fit into the microscopic structures within a bird’s eye. And this is where quantum mechanics comes in. The insight came back in 1978, when a German biophysicist named Klaus Schulten was studying “radical pairs.” A radical is any atom or molecule with a lone electron in an outermost or valence shell. A radical pair is, well, a pair of radicals
- connected in a very special way. Their unpaired valence electrons are entangled. A quick review of quantum entanglement is
in order here, although we’re talked about it before.. When two particles are entangled, it means
one or more of their quantum properties are correlated. This can lead to all sorts of weird effects,
including an apparent faster-than-light influence - measure the property of one particle and
you instantaneously influence the entangled partner. Today we’re interested in a different aspect
of the weirdness of entanglement. The entangled properties are the quantum spins of the two valence electrons in two separate radical molecules. There are four possible combinations for the spins: the first is the so-called singlet state, where the spins
are pointing in opposite directions - we'll call them up and down. We don't know which electron is up or down, and this is undefinedness is part of the whole entanglement deal. Simply “opposite each other” becomes this single state - hence “singlet”. The other three states are when the electrons have the same spin direction - either both up, both down, or a quantum superposition
of both at the same time. These three are energetically equivalent, so together they make up a triplet state. If you have just one radical, its valence
electron spin tends to stay fixed until disturbed by its environment. And Earth's magnetic field isn't strong enough to influence spin in that time. But in a radical pair spin states will oscillate between the singlet and triplet states. They do that evenly in the absence of a magnetic field - 75% of the time in the triplet state and 25% in the singlet. But even a weak magnetic field like the Earth's can affect the amount of time the radical pair spends in these states. If that field has the correct orientation,
the system will spend more time in the triplet state - with both electron spins aligned in
the same direction, and less in the singlet state
where they have opposite alignments. OK, so we have a mechanism to influence two tiny electrons - but a few questions remain: how is the radical pair produced, how long
does the entanglement need to last in order to be influenced by Earth's magnetic field,
and how does the simple slipping of electron spins go on to give the bird magnetovision? Klaus Schulten and colleagues proposed the mechanism that has remained mostly unchanged to this day. It goes like this: birds have some protein
in their eyes. When light hits the protein, it knocks an
electron off an attached molecule that goes onto an adjacent molecule. The two molecules now share a pair of entangled electrons—they become a radical pair for a short period of time. Then they quickly react to produce some chemical byproducts. But there’s the key - those byproducts are
sensitive to the spin state of the valence electrons at the time of the reaction. So during the short lifespan of the radical
pair, its valence spin state can be modified if the bird changes the orientation of its
head relative to the Earth’s magnetic field. That leads to a changing yield of different
possible byproducts across the bird’s eyes. That could lead to a true visual sense of
magnetic field orientation. We won’t delve too far into the biology
or chemistry here, but there is a protein that does all of this - it’s called cryptochrome. Experiments since Schulten’s proposal have shown that it’s possible to affect cryptochromes with a weak magnetic field and get that characteristic change of rate of chemical reaction. Notably, fruit flies without the gene for
cryptochromes are unable to navigate. Although birds do have cryptochromes, the
mechanism itself has not been directly observed in a bird, and it remains only the most likely
explanation. This “avian compass” presents a tantalizing
possibility of quantum biology. It’s strange to think of quantum effects
being relevant in living organisms. Normally, to observe the strange behavior
of the quantum world we need to perform incredibly careful experiments in highly controlled environments - ideally isolated systems of very few particles, perhaps in a vacuum or near absolute zero
temperature. Not in the warm, wet, and messily macroscopic environment of a living organism. Quantum entanglement is very quickly destroyed in such environments - but birds may have found a workaround. The radical pairs only need to stay entangled for a microsecond for this mechanism to do its job - because after the entanglement is
destroyed the subsequent chemical reactions remember the quantum state, and so remember the magnetic field. So is this true quantum biology? Earlier this year, Peter Hore, a physical
chemist at Oxford co-authored a paper that seems to answer that. The team’s calculations showed that only
a full quantum description of the process could produce the required sensitivity to
magnetic fields. If, for example, the valence electrons were
just interacting due to their magnetic fields - so-called spin-spin interactions - rather
than true entangled states - their spin state wouldn’t be sensitive enough to detect Earth’s field. So Erwin Schrodinger’s ideas about quantum mechanics influencing living organisms may be right. And quantum magnetoreception in birds isn’t the only example of what we sometimes call “quantum biology”. We know for sure that it happens in some cases - like the quantum tunneling that drives enzyme catalysis. There are other contentious, but intriguing
cases - like the idea that long-range quantum coherence may drive photosynthesis. And there are some highly contentious ideas - like quantum entanglement in the brain’s microtubule proteins as a key ingredient in
human consciousness. The quantum magnetoreception of the avian compass sits somewhere in the middle - not yet proved, but more and more favoured. Klaus Schulten’s microtubules must have
been working overtime to hit on such a brilliant insight. He may still be proved wrong, but it’s a
beautiful idea: pigeons and geese and albatrosses, swallows - european especially - birds of
many a feather using quantum physics to flock together to navigate the hidden lines of a geomagnetic space time. As always, a huge shoutout to our supporters on Patreon. Thanks for having our back, guys. And today I want to give an extra extra special thank you to Hank S, who supports us at the quasar level. Hank - you once asked me a question about
space contraction in special relativity and whether or not a spaceship could travel so
fast it bumped into its own rear end. I couldn't give you a satisfactory answer
at the time - and so to show our appreciation I'm not going to give you one now. Instead I'm going to do a whole episode on
some of the weirder paradoxes in relativity, that I think will make it all totally clear. Or confuse matters far worse- it remains to
be seen. Anyway, your support means everything to us, so thank you. I just wanted let you know about PBS's new
science show Weathered. If you feel like the weather is getting weirder
lately, well, you’re right. And if you want to know the science of why
that's the case, join meteorologist and host Maiya May on Weathered to explore why hurricanes, wildfires and other extreme weather is more frequent and intense than ever, as well as what you can do to prepare. Weathered is on PBS Terra and you can check out the link in our description. Okay. Last week we talked about a possible new type of supernova - the black dwarf or iron star supernova, which may be the very last explosions at the end of our universe. I was gratified that you were all so excited
about this thing that no one will ever, ever observe. Kroy H correctly summized there there are
still more types of supernova to talk about. Indeed there are - many in fact. You have types 1 and 2, each with multiple
sub-types, then the rarer types 3 through 5. In typical astronomy style, the classifications are entirely based on the observational properties - what elements are observed as emission or absorption lines, the way the light decays over time, and other things. But really there are two main scenarios - either the core of a very massive star collapses after expending its fuel - that’s a type
2 supernova - or the remnant of a lower mass star - a white dwarf - gains extra mass and
explodes - that’s a type 1a. For more detail we would indeed need a whole episode Quantum fields asks what about neutron stars. What are their final fates? This all depends on some physics that we haven’t nailed down yet. There are two broad scenarios - first, if
protons do NOT decay then quantum tunneling will cause the neutron star to collapse into
a black hole over an absurdly long timescale of 10^10^20-70 or so years. And then those black holes evaporate into
radiation on a comparitifly short timescale. On the other hand if protons DO decay then
the small proton content of neutron stars will decay into pions and neutrinos, leaking
away some of the mass of the neutron star. The neutron star will then expand, allowing
some neutrons to convert back into protons, which can themselves decay. The Star may go through a phase as a white
- or perhaps now black dwarf and will continue to evaporate due to proton decay. That last option is described in the book
five ages of the universe by Fred Adams and Gregory Laughlin Broken silence asks whether all this means
that Bertrand Russell’s teapot will also come to an end. For those who aren’t familiar Russell’s
teapot is a hypothetical piece of chinaware that orbits the sun between the earth and
mars, and which science has yet to prove doesn’t exist. So yeah, Russell’s teapot either evaporates
as its protons decay, or quantum tunnels into an iron teapot - is that a kettle? Many of you pointed out how hard it is to
wrap your heads around the sort of timescales we talked about in that episode. Like 10^32000 years for the smallest iron
dwarf to explode. C’mon guys, it’s easy. Step one - first imagine the passage of 10^31999 years. Then imagine that happening 10 times. How do you imagine 10^319999 years you ask? Easy, start by imagining 10^31998 years and then do THAT 10 times. Anyway, you get the picture.
You know every time a new episode comes out it makes my week. Thanks.
I believe there's a typo at 7:08 - the superposition triplet state should be (|▲,▼〉 + |▼,▲〉)/sqrt(2) for orthogonality.
I still need to view this episode, but it's coincidental that I came here to ask if quantum entanglement occurs with cellular molecules during mitosis?
Perhaps this episode will shed some light on the subject.