What do Schrodinger’s cat and European robins have in common? In very different ways, they have— quantum physics in common.
"Schrodinger’s cat" is a thought experiment. It has to do with trying to link the behavior of subatomic matter described by quantum physics, to macro-objects.
The European robin is a bird which uses quantum physics for its navigation.
This emerging field is called quantum biology. It is about studying the underlying quantum physics in some discrete biological processes. However quantum biology is not about trying to link the sum of our biological, macro-object behavior with that of subatomic matter that constitutes all of us.
I am neither a physicist nor a biologist. What I bring here are my observations in this interesting crossover field.
Has Frazier left the building?
Erwin Schrodinger, who was a famous physicist, came up with a thought experiment in 1935. It was described in one paragraph in a long paper titled “The Present Situation in Quantum Mechanics”.
The thought experiment was a half serious way to show that quantum behavior - which he called a ‘blurred view’, is not our everyday reality.
The thought experiment involved a box with a small amount of a radioactive matter in a Geiger counter, a relay mechanism, a hammer, a flask of poison and a cat.
This is what he proposed- “One can even set up quite ridiculous cases. A cat is penned up in a steel chamber, along with the following device (which must be secured against direct interference by the cat): in a Geiger counter, there is a tiny bit of radioactive substance, so small, that perhaps in the course of the hour one of the atoms decays, but also, with equal probability, perhaps none; if it happens, the counter tube discharges and through a relay releases a hammer that shatters a small flask of hydrocyanic acid. If one has left this entire system to itself for an hour, one would say that the cat still lives if meanwhile no atom has decayed. The first atomic decay would have poisoned it. The psi-function of the entire system would express this by having in it the living and dead cat (pardon the expression) mixed or smeared out in equal parts.”
“It is typical of these cases that an indeterminacy originally restricted to the atomic domain becomes transformed into macroscopic indeterminacy, which can then be resolved by direct observation. That prevents us from so naïvely accepting as valid a "blurred model" for representing reality. In itself, it would not embody anything unclear or contradictory. There is a difference between a shaky or out-of-focus photograph and a snapshot of clouds and fog banks.”
The whole purport of the thought experiment was to show the unacceptability and absurdity of the scenario that until an observer takes a look, the cat is both alive and dead— that is, the quantum behavior from the radioactive sub-atomic matter carried over through a sequence to the macro-object- the cat.
We know this is not true. This quantum behavior, called superposition, does not carry over.
When does it cease carrying over?
What constitutes an observation or measurement or observer? A human outside of the chamber opening it? Or is it any irreversible process in the sequence?
It is the latter. (That’s what Niels Bohr, another famous physicist, apparently said). The quantum behavior ceases when the Geiger counter registers a particle- or even right when a radioactively decayed particle is emitted irreversibly.
What happens if the radioactive mass does not emit any particle? Then also, no superposition gets transferred over in any sequence.
And so, absolutely yes, Frazier has long since left the building. Actually he was never in the building. We argued that he was.
This didn’t stop Schrodinger’s cat from becoming very famous. People are still amused and mystified if it can be dead AND alive in a superposition of states - dead-cat/with-decay and alive-cat/no-decay.
(By the way, the internet says Schrodinger had a cat. It was apparently called Milton. Though surely, Milton wasn’t put in a chamber with poison, with a good chance to die).
I spy a robin’s eye
We have long known about magnetoreception, the ability of birds to sense the earth’s magnetic field and utilize this for navigation.
More recently there has been a hypothesis that European robins use a type of quantum behavior called entanglement for their navigation.
If this is true, the robins appear to be transferring information or a signal from a quantum state to a macro-state! Which is then being used for the well-being of the bird-- here, for its flight.
This is what is hypothesized -
Birds have a protein in their eyes called cryptochrome. When light enters the robin’s eyes it can cause changes to this protein. The energy from a photon of light can cause an electron from a molecule of cryptochrome to exit and join another molecule. When this happens, a ‘radical pair’ is created - that is, a pair of molecules which have an odd number electrons. Here is where the twist happens: these molecules are said to be entangled. That is, because they were created at the same time with the jump, they are linked and locked. This state is supposed to last long enough- about 100 microseconds. It is supposed that either the entangled state is itself sensitive to the earth’s magnetic field or the molecules flip-flop between the two states and during this, other chemicals get created and the extent of these creates a compass or a map. This is communicated between the bird’s eye and the brain.
Even at night, enough light is said to be present to trigger this mechanism.
The interesting observation here is that - at no point is the quantum entanglement itself transferred to any macro-object. The quantum state, which here is the radical-pair entanglement state, lasts for a very short time (but long enough to create a signal) and then ceases. It is information from the signal that is transferred and interpreted.
If you’re the Cause.. what is the Effect?
I’d like to visualize these two - one a thought experiment and other a hypothesis, in a very different and simplified way.
Let’s say, what we experience in our everyday lives and WHAT WE LIVE BY- like a ball thrown against a wall will bounce back, a car will stop when we apply the brakes… and so on.. have a cause and effect familiar to us which we’ll put under the umbrella of C_E-known. Let’s put the cause and effect of subatomic matter, which lie outside our everyday experience under the umbrella of C_E-unknown.
Schrodinger’s cat thought experiment can be depicted as-
C_E-unknown ———> (trying to transfer to) C_E-known through a sequence of actions.
What is said to happen in a molecule in a robin’s eye can be seen as-
C_E-known ——> C_E-unknown (entangled radical pair state) ——> C_E-known
(this process happens repeatedly)
Can you see in this way?
While we’ve been intrigued and entertained for decades by the thought-experiment argument if C_E-unknown (from atomic decay) can be transferred to C_E-known (cat), biological processes have long been going on where starting with C_E-known (a molecule in the eye) a subtle extract or result of C_E-unknown (entangled state) transfers to C_E-known (other molecules in eye and then to brain), as if in flashes or at a certain rate while never transferring the C_E-unknown state. This has been going on ceaselessly around us.
Let's bring in bacteria
In 1998, Johnjoe MacFadden, a microbiologist, and Jim Al-Khalili, a quantum physicist, both from the University of Surrey, published a paper on adaptive mutations in bacteria.
The experiment and result were like this- bacteria (e-coli here) had an option of a mutation going in one direction or another. In a neutral environment, the chance of this mutation was 50-50 in either direction. But if the environment was helpful, which in this case was with glucose present in the environment, the mutation wasn’t random any more! It was higher in the direction of the mutation which could use the glucose in the environment.
How did the bacteria know that mutating in one direction had more energy that was advantageous to its survival?
You see how interesting this experimental observation is? The researchers proposed a theory to explain this: It involves the mutated and un-mutated states being in a state of quantum coherence or quantum superposition states. This state holds for a sufficient biological time scale. When the environment contains a utilizable substrate, here glucose, a rapid decoherence is induced and the superposition is lost. This faster rate of decoherence in the presence of the environment with glucose explains the higher rate of production of one mutated state.
So one next question would be - why does an environment favorable for the survival of the organism induce a faster collapse of a quantum state towards a macro-state (the mutation) that can use the favorable condition?
Surely you can see the implications of this novel experimental result in other scenarios.
Other biological processes where quantum behavior is being studied are photosynthesis, enzymatic activity, DNA mutation, and a few others.
Diamond cuts diamond
All in all, quantum biology is an exciting and an intriguing new field. I hope new research in this area will give us answers to many mysteries, including the question Schrodinger himself had written about- What is Life?
My key question is - what is the role of C_E-unknown in the world known to us as- C_E-known?
We are limited in our understanding of this intersection area and in our ability to find proof. We’ve barely scratched the surface.
The experiments to figure these quantum biological processes are increasingly sensitive and complex because we we are trying to see and understand C_E-unknown through C_E-known.
It’s like we trying to see very small stuff, like water molecules, with a flash light. We need something on the same scale. To scratch a diamond we need another diamond.
We need to be able to comprehend C_E-unknown through C_E-unknown. But how? C_E-unknown collapses to C_E-known as soon as we try to observe it!
1. The Present Situation in Quantum Mechanics, by Erwin Schrodinger, 1935, Translated by John D. Trimmer. https://archive.is/20121204184041/http://www.tuhh.de/rzt/rzt/it/QM/cat.html#sect5
2. A bird's eye view of quantum entanglement by Katherine Wu, in Nova. https://www.pbs.org/wgbh/nova/article/birds-quantum-entanglement/
3. In the blink of bird's eye, a model for quantum navigation by Lisa Grossman. https://www.wired.com/2011/01/quantum-birds/
4. Quantum Biology, explained by Jim Al-Khalili. https://www.youtube.com/watch?v=asps5mZ4Kp8
5. A quantum mechanical model of adaptive mutation, J. McFadden and J Al-Khalili. https://pubmed.ncbi.nlm.nih.gov/10400270/