Quantum physics describes the fundamental behavior of particles and is necessary for an understanding of chemistry. There is, therefore, prima facie reason to expect that it bears an important relation to biology. More than finding quantum phenomena that are exceptions to the classical rule, what is needed are plausible hypotheses that, if true, show that life is importantly quantum. Four such hypotheses are considered, of which it is expected that at least one is true: (1) cells are quantum sensors, (2) cells are quantum batteries, (3) brains have quantum stabilizers, (4) humans have an electromagnetic sense. These hypotheses are discussed. Methods for testing them are described, with notes.
Biology without quantum physics?
For the last several decades, scientists have sought to explain biological systems mechanically, chemically, and, in the case of neurons, electrically. Only very rarely do biologists refer to the laws of quantum mechanics. This raises the question of whether there has been a serious omission. Might the remaining puzzles in biology be solved by importing knowledge from the quantum physics? In short, how quantum is life?
It is universally recognized that quantum physics describes the fundamental behavior of particles, which is necessary for an understanding of chemical bonds. The link between quantum physics and chemistry, therefore, is solid. It is also understood that biological systems, from cells to organisms, depend essentially on chemical reactions, such as the conversion of ATP to ADP, which provides energy for cells.
There are, nevertheless, reasons to suspect that quantum phenomena, which are typically associated with very short length- and time-scales, factor out by the time one reaches the length- and time-scales of biology. Quantum physics is most closely associated with the idea of superpositions—states that can be described precisely in mathematical terms, but which are difficult to understand conceptually—yet superpositions are never directly observed, even through microscopes, and canonical quantum effects, like the famous wave pattern in the two-slit experiment, have no known correlate in biology.
Furthermore, the most sophisticated groups that try to preserve and maintain superpositions, technologists on quantum computing projects, find that superpositions are very fragile and can be preserved for long time periods only with very great difficulty and at extremely low temperatures. It is only natural that biologists, who are typically only faintly acquainted with quantum physics, would doubt that quantum states could last long in the noisy, room-temperature environment of the cell.
These concerns notwithstanding, one might still expect that quantum physics has something to contribute to biology. Something surprising is happening within cells and with organisms as a whole. Despite large steps forward, the chemical, mechanical, and electrical paradigm seems dauntingly far from explaining the whole of biology. If there are misses pieces, one natural place to search is a field with new and potentially unexploited explanatory principles. Quantum physics is certainly one of these.
Biology with quantum oddities?
Quantum physics offers more than superposition. It offers tunneling—the ability for particles to pass through energy barriers without receiving the usual requisite boost of energy first. It also offers spin—intrinsic angular momentum had by certain particles and systems, and which generates a magnetic field. These new features are sometimes referred to by biologists and quantum physicists to explain biological phenomena.
The most famous examples, to date, of putative quantum biological phenomena are photosynthesis and the migration of animals who follow the Earth’s magnetic field. Scientists have argued that photosynthesis is more efficient than classical (i.e., non-quantum) processes allow, and must therefore involve quantum tunneling. Scientists have also proposed that birds, in particular, use the oscillations in chemical reactions that arise from spin superpositions to orient themselves with respect to the Earth’s magnetic field.
These effects are certainly important. However, they are hard to identify as uniquely quantum. Even if photosynthesis gains some efficiency from tunneling, it has not been argued that without tunneling, there would be no photosynthesis. Similarly, animals that may use quantum magnetoception to navigate also use other, more familiar capacities, such as sight and memory, not to mention, in some cases, classical magnetosensory capacities as well.
Greater efficiency in photosynthesis and a better ability to navigate using the Earth’s magnetic field are important. Similarly, there are a slew of phenomena, from mitochondrial function to DNA mutation rates that seem to depend on quantum phenomena. Nevertheless, without some larger effect, quantum phenomena in biology seem like oddities, cases notable precisely because they are exceptions to the classical rule. Yet quantum physics is so fundamental, it is hard to believe that life is essentially classical, and takes advantage of quantum mechanics mostly as an afterthought.
Four quantum hypotheses
Quantum biology is an emerging field. Rather than having reached conclusions, the field is at the stage of formulating and testing hypotheses. Fortunately, there are several hypotheses that are plausible, prima facie, and that if true indicate that quantum physics plays an important and central role in life. These hypotheses include:
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Hypothesis #1: Cells are quantum sensors.
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Hypothesis #2: Cells are quantum batteries.
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Hypothesis #3: Brains have quantum stabilizers.
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Hypothesis #4: Humans have an electromagnetic sense.
In the sections below, we will state and motivate each of these hypotheses. We will then describe how they can be tested empirically. If any of these hypotheses, or ones of comparable magnitude, are true, life will have been shown to be importantly quantum. It is our expectation that at least one of the hypotheses is true.
Hypothesis #1: Cells are quantum sensors.
People have created sensors that use quantum states—electron spin superpositions, in particular—to achieve a higher degree of sensitivity in measurement. What if cells are using electron spin superpositions to achieve a higher degree of sensitivity to their environment? In other words, what if cells are themselves natural quantum sensors?
Electron spin superpositions are thought to occur in chemistry. In particular, there is a mechanism, known as the “radical pair mechanism,” where excited electrons can become entangled with one another, oscillating back and forth between a state known as a “singlet” state and a state known as a “triplet” state. In the triplet state, the electrons pass through a superposition of three different spin states, two of which are less liable to participate in chemical reactions.
The question, then, is whether the radical pair mechanism can operate in the environment of the cell without being disrupted, and if so, whether it can serve as a useful sensor. The radical pair mechanism is triggered by the introduction of an external magnetic field, like the Earth’s magnetic field, and yields internal oscillations in the availability of electrons for chemical reactions. It thus has an sensor input, external magnetic fields, and a sensor output, variable chemical reaction rates.
Notably, one might imagine that some of the observed effects of magnetic fields in biology can be explained by the radical pair mechanism. In essence, this hypothesis is a generalization of the explanation of avian magnetoception: it’s not just birds that use the radical pair mechanism, it’s everything.
Hypothesis #2: Cells are quantum batteries.
When placed in a hypomagnetic chamber, organisms develop differently. Tadpoles experience accelerated development, killifish get stuck in a particular developmental state for longer, and E. coli experience an extended lag phase. Why? One possibility is that external magnetic fields provide cells with some important type of input. The input may be energy.
In particular, the Earth’s magnetic field applies torque to all magnetic particles within a cell, communicating to them angular momentum. (Conservation of angular momentum means that the Earth’s core rotates a very tiny amount less as a result.) The Earth’s magnetic field, however, puts many of the magnetic particles within a cell into superposition, where the superposition states allow the smooth exchange of angular momentum, in the form of spin, throughout the cell without the particles needing to directly bump into one another.
The result is that the magnetic particles within the cell, which includes all protons, neutrons, and electrons, receive and exchange angular momentum. Individually, this may not be significant. In aggregate, it’s an increase in energy—which may qualify the cell as a type of quantum battery. There would need, of course, to be a way to store the energy in usable form, e.g., chemical bonds. The cell would then operate as a reverse motor, receiving angular momentum and storing it as chemical energy. (In this connection, it is worth noting that ATP, which stores chemical energy in the cell, has been observed to rotate more than one hundred times per second.)
The idea that magnetic fields impart energy to cells may shed some light on the origin of the first cell. One key question is how the first cell acquires its energy. Some mechanisms, like photosynthesis or the consumption of other organisms, only become available after evolution has had time to work. These were obviously unavailable to help form the first cell. Regular physical mechanisms, like gravity, electricity, and heat are too likely to crush or scatter the parts of the proto-cell; angular momentum imparted by magnetic fields may thus be the most attractive option.
Hypothesis #3: Brains have quantum stabilizers.
The impressive computational power of the brain has led some to postulate that the brain is itself a quantum computer. There are a variety of difficulties with this proposal, one of which being it is hard to see what evolutionary purpose genuine quantum computation would serve. (Genuine quantum computation would make it possible to break public-key encryption, but this presumably did not provide a fitness advantage in the ancestral environment.)
That said, a quantum element may be necessary, simply to support classical computation in the human brain in everyday contexts. As people move around, their brains move through the roughly static magnetic field of the Earth. This should produce current within the electrical circuits of the brain. If the current is great enough, motion through the Earth’s magnetic field may cause many neurons to fire, possibly even randomly, as electromotive force is received throughout the brain.
Cognition is highly sensitive. Conceptual structures have been linked to small numbers of neurons. Misfires from moving through a magnetic field, therefore, may disrupt regular patterns of thought unless, of course, the brain has some sort of compensatory mechanism. One possibility is a sub-neuronal substrate, with entangled particles that can receive and distribute the received electromotive force. In short, a quantum stabilizer for classical computation.
Building out this hypothesis would require identifying the appropriate substrate. Microtubules and ferritin are two structures that have been suggested. The idea would then be that the brain, which in humans is capable of classical computation, depends on a quantum sub-structure, not to perform genuine quantum computation, but to perform a genuine quantum function (in fact, a type of “error correction”) without which classical computation may not be possible.
Hypothesis #4: Humans have an electromagnetic sense.
There are strong prima facie reasons to expect humans to have some ability to sense electromagnetic fields. First, many animals can sense electrical or magnetic fields, whether by a quantum or classical mechanism. It is, therefore, broadly feasible on a biological level. Second, it is arguably evolutionarily beneficial to be able to detect such fields. Third, it seems easy to evolve such abilities; according to evolutionary biologists, life has evolved electroreception at least eight separate times.
The primary questions, if humans can sense electromagnetic fields, are how we do so and why it is difficult to tell that we have the relevant sense. One possible answer, though difficult to establish, is that humans have entangled electromagnetic senses. Here, “entanglement” is meant both figuratively and literally: since superpositions spread at the speed of light and yield entanglement until the point of collapse, a superposition would need to last only 10 nanoseconds in order to yield entanglement between two people standing 10 feet apart.
In layman’s terms, this means that if humans detect electromagnetic fields via a quantum mechanism, it is possible that people spatially proximate to one another will have sensory mechanisms whose states depend on each other. This is less interesting if they simply detect the presence, absence, or direction of an electromagnetic field, but much more interesting if the sensing is combined in some way with internal physiological or cognitive processes.
Testing the hypotheses experimentally
The foregoing hypotheses are, to varying degrees, striking. If any of them are true, life is quantum to an appreciable degree. Quantum physics would then provide mechanisms that would substantially enhance the capacities of life, or even allow life or crucial elements of life to exist in the first place. A complete description of biology would, even at a relatively high level, require reference to quantum physics.
More than striking, however, the hypotheses described are also testable empirically. Here is how one might go about testing them.
Hypothesis #1. To test whether cells are quantum sensors, one can find fluorescent magnetosensitive proteins which are native to cells and subject the cells to a magnetic field. If the magnetic field reduces the fluorescence of the proteins by the right amount, that gives strong prima facie reason to believe that the cells are receiving input in the manner of quantum sensors. If one can determine the function of the proteins in the cells, one can test whether the magnetic fields affect the output in the right ways. If so, the proteins may be acting as full-fledged quantum sensors.
Hypothesis #2. To test whether cells are quantum batteries, one can take identically prepared cells which are designed to remain in a state of suspended animation, place one set inside a hypomagnetic chamber, and place the other in an incubator with similar lighting, humidity, and temperature conditions. One can then wait for a suitable time to see how the absence of the putative quantum battery effect affects the cells in the chamber as compared to those outside it.
Hypothesis #3. To test whether brains have quantum stabilizers, one may approach the problem in two different ways. First, one may identify the best candidate particles, such as microtubules or ferritin, extract them, and try to use them to create an example quantum circuit. Second, one might see whether artificial neural nets, which are modeled at least generally on the pattern of neurons in the human brain, suffer from problems when subject to realistic perturbations from motion through a simulated geomagnetic field. If so, this may suggest the need for a stabilizer.
Hypothesis #4. Testing whether humans have an electromagnetic sense is tricky. If we do have such a sense, there is certainly some factor preventing it from being generally recognized, which may be more than the lack of obvious sensory organ. One possible place to start is to look at studies of people’s “innate” directional sense, which may have an electromagnetic component. Another place to look is subjective reports of electromagnetic sensitivity and try to identify a reliable subset.
To be clear, these tests, as specified, have limitations. The test for quantum sensors supposes that adequate proteins can be identified; of course, if such proteins cannot be identified, that may be evidence against the quantum sensor hypothesis. The quantum battery test may encounter issues pertaining to duration; running a test for ten days is much easier than running the same test for ten thousand. The point is that, there are ways of gathering evidence for or against each of the above hypotheses, which is necessary if the question of quantum phenomena in biological systems is to be suitably resolved.
It is also important to emphasize that the above tests, even if confirmatory, may not give everyone what they want. Advocates of quantum consciousness will still have a ways to go, even if the brain is shown to have a quantum stabilizer. Advocates for full-blown telepathy will also still have to wait, even if humans are shown to have entangled electromagnetic senses. The process of science leads to exciting conclusions, but also progresses incrementally. This incremental progress, even in the context of revolutionary new ideas, is part of the scientific process. Patience must also be as well.
Biology and quantum physics
Quantum physics and biology are both large domains. It is likely, therefore, that there are many potential connections between quantum physics and biology beyond those described above. What matters, if life is importantly quantum, is that some important and deep connection hold, so that when the nature of life is explained, quantum mechanics must also be explained in some regard.
The above four hypotheses are examples of potential important and deep connections between quantum physics and biology. Are cells quantum sensors? Or quantum batteries? Do brains have quantum stabilizer mechanisms to prevent them from being thrown off by random neuron firings in the context of the Earth’s magnetic field? Do humans have an electromagnetic sense with a quantum basis? An affirmative answer to any of these questions implies that life is, in fact, meaningfully quantum.
Of course, it is unlikely that all of the above hypotheses are true. Part of the advance of science is refuting hypotheses as well as tentatively confirming them. And even if some hypotheses connecting quantum physics and biology end up being confirmed, as we expect there will be, our journey towards understanding life, and how intricately it is woven into the fabric of our quantum universe, will still be just beginning.