Please explain quantum tunneling. (Can it be "goal-oriented"?)

[Swammerdami]

The "efficiency" of photosynthesis is an ambiguous term: losses occur at various stages of the process, beginning with the incoming photons: Many photons are of the wrong wavelength, or not aimed at a chlorophyll. However one key step in photosynthesis is said to be almost 100% efficient.

The incoming photon strikes the magnesium atom in a chlorophyll molecule, creating an "exciton" (electron-hole pair). This exciton bounces around, transferring via other magnesium atoms (the chlorophyll molecules acting like ping-pong paddles!) and eventually finds its way to the photosynthetic reaction center (featuring manganese ions) where the harvesting of the electron's energy begins. Apparently almost 100% of these excitons do make their way to the reaction center. They have to get there quickly: the exciton's energy would otherwise dissipate as waste heat in less than a nanosecond IIUC.

In classical physics, the exciton would travel a "Drunkard's Walk" and very seldom make its way to the reaction center. But in quantum physics the exciton's path is a superposition of hundreds of random paths, some of which will get lucky and find the reaction center in time. It is one of those "lucky" paths that is instantiated (when the wave function collapses?); this is an example of "quantum tunneling."

My question is . . . WHY? Why does the wave function collapse on that successful path, rather than on some other random path? Articles that write about this, or quantum tunneling more generally, seem to ignore this question. I'm afraid that I am missing something very basic. (It is said that this aspect of photosynthesis is similar to quantum computing algorithms. Is that the connection I should ponder? Or does the arrival at the reaction center just minimize some thermodynamic measure?)

I've had this question for ten years! ... ever since I read Life on the Edge, by Al-Khalili & McFadden. Here's a more recent article on the same topic.

 

[Jarhyn]

The "efficiency" of photosynthesis is an ambiguous term: losses occur at various stages of the process, beginning with the incoming photons: Many photons are of the wrong wavelength, or not aimed at a chlorophyll. However one key step in photosynthesis is said to be almost 100% efficient.

The incoming photon strikes the magnesium atom in a chlorophyll molecule, creating an "exciton" (electron-hole pair). This exciton bounces around, transferring via other magnesium atoms (the chlorophyll molecules acting like ping-pong paddles!) and eventually finds its way to the photosynthetic reaction center (featuring manganese ions) where the harvesting of the electron's energy begins. Apparently almost 100% of these excitons do make their way to the reaction center. They have to get there quickly: the exciton's energy would otherwise dissipate as waste heat in less than a nanosecond IIUC.

In classical physics, the exciton would travel a "Drunkard's Walk" and very seldom make its way to the reaction center. But in quantum physics the exciton's path is a superposition of hundreds of random paths, some of which will get lucky and find the reaction center in time. It is one of those "lucky" paths that is instantiated (when the wave function collapses?); this is an example of "quantum tunneling."

My question is . . . WHY? Why does the wave function collapse on that successful path, rather than on some other random path? Articles that write about this, or quantum tunneling more generally, seem to ignore this question. I'm afraid that I am missing something very basic. (It is said that this aspect of photosynthesis is similar to quantum computing algorithms. Is that the connection I should ponder? Or does the arrival at the reaction center just minimize some thermodynamic measure?)

I've had this question for ten years! ... ever since I read Life on the Edge, by Al-Khalili & McFadden. Here's a more recent article on the same topic.

Why does lightning always find a fairly efficient path to ground?

This could very well be a similar activity of some kind of front expanding in some fundamentally probabilistic way like an A*. There is, in theory, some kind of gradient there it is following.

I wonder if another reaction center would be presented, whether it would have the same probability as, say, a lightning strike, indeed with similar shadows.

 

[Shadowy Man]

My question is . . . WHY? Why does the wave function collapse on that successful path, rather than on some other random path?

I think if you can answer this question, you will have solved one of the greatest mysteries of the universe. Quantum Mechanics is by nature probabilistic precisely because we do not know what governs the collapse of wave functions in particular states. We can only predict the probability of collapse for each possible state. There have been ideas of so-called "hidden variables", but as I understand QM those have largely been ruled out.

 

[bilby]

My understanding is that particles always take the path of least action, even when quantum effects make calculating this path more complex than classical physics would anticipate.

https://www.livescience.com/47221-quantum-particle-path-most-traveled.html

 

[Loren Pechtel]

Finding it's target puts the result into a lower energy state and thus is favored.

 

[steve_bank]

As an example the quantum efficient of a simple photo detector is photons in versus electrons out.

Efficiency implies losses.

Streetman's Solid State Electronic Devices is a good general intro to QM. More practical Han theoretical.

An older book apparently now in the public domain as a PDF. Used to have a copy. Tunneling is in there.

https://rrsdce.files.wordpress.com/2018/07/sspd-eee-swapnil.pdf

Lightning from sky to ground occurs when the electric potential exceeds the breakdown voltage of the air, plasma. Same as putting a high voltage across two metal plates separated by air, aN air capacitor.

In fact the atmosphere is a giant capacitor.

As I remember it Newtonian is the point at which the number of particles makes an object seem continuous. The moon at any point in time is the aggregate sum of sttates of all particles in the moon.

The moon is subject to discrete states and probabilities, but the density of states makes the moon appear continuous, aka Newtonian.

One gas particle buncing arid in a tanl has a [rbaility of hoyting the tanl of the wall. A lot of gas patcles boucj araoud in a tank and at any point in time htere is an averafe numberr of particles hitting the wall of the tank, which we descibe in Newtonian continuous terms as Newton/m^2 or ptessure in Pascals.

I don't think there is an answer to why a system goes to one state but not another, but we can model how a quntun sytesm behves ad esre it. Hemce trasistors and lasers.

I think you ed up den\bating causlaity and infinite regression back in time.


'Hidden varia' give me a chuckle. We looked for hidden varvbles and foud none therfore there are no hidden variables.

 

[Swammerdami]

I thank all of you very much for the responses. I will try to contemplate and see if this finally makes sense.


I still recommend the book Life on the Edge mentioned in OP. It gives other examples of quantum effects exploited by living creatures.. Of these the simplest is the "swipe card" model of smell. Chemical bonds' vibrations have specific energies. Suppose an olfactory receptor has a region which wants to donate an electron with A units of energy and another region which wants to accept B units of energy. If an odorant (which wants C units of energy) passes between those two regions, the energy transfer will occur if C ≈ A-B. Thus that olfactory receptor detects any molecule with a bond whose vibrational energy is (A-B).

This hypothesis about odor detection is probably simpler to consider than photosynthesis, but it still leaves me confused. I guess the two-step transfer A-->C-->B minimizes action and/or enthalpy.

 

[Bomb#20]

My understanding is that particles always take the path of least action, even when quantum effects make calculating this path more complex than classical physics would anticipate.

No, always taking the path of least action is pretty much the definition of classical particles.

https://www.livescience.com/47221-quantum-particle-path-most-traveled.html

"An infinite number of superpositions exist between these two states, so the researchers repeated the experiment 1 million times to determine the most commonly occurring path.

The results revealed that the particles most frequently travel a convex curve. The equation is simple, and it's fairly easy to calculate the path the particles are most likely to take, Jordan said."

 

[Bomb#20]

We can only predict the probability of collapse for each possible state. There have been ideas of so-called "hidden variables", but as I understand QM those have largely been ruled out.

'Hidden varia' give me a chuckle. We looked for hidden varvbles and foud none therfore there are no hidden variables.

They haven't exactly been ruled out, but there's a lot more to it than they must not exist because we didn't find any when we looked. What were ruled out were local hidden variables, i.e., variables whose effects propagate from one place to another by transferring energy from point A to point A plus infinitesimal epsilon. The math of quantum mechanics requires that in any hidden variable explanation, the effect of a hidden variable must get from point A to point B instantaneously, without first traversing the path between A and B at the speed of light or slower. So the reason people say hidden variables are ruled out is because relativity implies information can't travel faster than light. Most people aren't prepared to believe relativity is wrong.

Of course, maybe the math of quantum mechanics is wrong; but back around 1980 a guy named Aspect did an experiment to actually measure how fast information was passing between two entangled particles on the assumption that they were correlated due to hidden variables. Sure enough, faster than light. Got the 2022 Nobel Prize for it.

 

[Swammerdami]

Entanglements that defy common sense, like the GHZ experiment or the simple experiment underlying Bell's Theorem, have a very simple interpretation to bypass paradox!

B <------- A -------> C​

Entangled particles are created at event A (Monday), one travels to B where it is measured on Tuesday; the other to C where it is measured on Wednesday. The apparent paradox is that the measurement at B is seen to affect the measurement at C even though the two events seem to have no way to communicate

Paradox disappears when an arrow is reversed:

B -------> A -------> C​

The Tuesday measurement affects Monday's event A, and hence the particle at C. All you need to accept is that the direction of causality is not sacrosanct. And (setting aside the 2nd Law of Thermodynamics, which is a statistical deduction rather than a "Law") the laws of physics are all time-reversible. The apparently fixed arrow of causality is flexible: Causality can flow in both directions.

Top theoretical physicists including Feynman, Penrose, and Hawking are all quite aware of this interpretation. But they seem to ignore or reject this viewpoint. I think they should embrace it.


This is one reason I try to understand quantum processes like that in OP. Does time-reversed causality help any explanation?

 

[Bomb#20]

The apparently fixed arrow of causality is flexible: Causality can flow in both directions.

Top theoretical physicists including Feynman, Penrose, and Hawking are all quite aware of this interpretation. But they seem to ignore or reject this viewpoint. I think they should embrace it.

Penrose (provisionally) embraced it in The Road to Reality.

 

[lpetrich]

Quantum tunneling is due to the wavelike nature of elementary particles. If a potential barrier is too high for them to travel across in the classical limit, it's nevertheless possible for the particles' wavefunctions to spread into the barrier.

Quantum tunneling explains alpha decay. If one runs it backward in the classical limit, the ejected helium-4 nucleus will stop well outside its ejector, because of their electrostatic repulsion, but from quantum mechanics, it can spread through that potential barrier to the ejector.

 Quantum tunnelling and Alpha Particle Tunneling This effect also explains  Spontaneous fission

Wave-particle duality is weird, with elementary particles having both wavelike and particlelike properties. At macroscopic scales, one sees only wavelike or particlelike properties, breaking this duality.

 

[steve_bank]

Tunnel diode - Wikipedia

en.wikipedia.org

A tunnel diode or Esaki diode is a type of semiconductor diode that has effectively "negative resistance" due to the quantum mechanical effect called tunneling. It was invented in August 1957 by Leo Esaki when working at Tokyo Tsushin Kogyo, now known as Sony.[1][2] In 1973, Esaki received the Nobel Prize in Physics for experimental demonstration of the electron tunneling effect in semiconductors.[3] Robert Noyce independently devised the idea of a tunnel diode while working for William Shockley, but was discouraged from pursuing it.[4] Tunnel diodes were first manufactured by Sony in 1957,[5] followed by General Electric and other companies from about 1960, and are still made in low volume today.[6]

Quantum tunnelling - Wikipedia

en.wikipedia.org

In physics, quantum tunnelling, barrier penetration, or simply tunnelling is a quantum mechanical phenomenon in which an object such as an electron or atom passes through a potential energy barrier that, according to classical mechanics, should not be passable due to the object not having sufficient energy to pass or surmount the barrier.

Tunneling is a consequence of the wave nature of matter, where the quantum wave function describes the state of a particle or other physical system, and wave equations such as the Schrödinger equation describe their behavior. The probability of transmission of a wave packet through a barrier decreases exponentially with the barrier height, the barrier width, and the tunneling particle's mass, so tunneling is seen most prominently in low-mass particles such as electrons or protons tunneling through microscopically narrow barriers. Tunneling is readily detectable with barriers of thickness about 1–3 nm or smaller for electrons, and about 0.1 nm or smaller for heavier particles such as protons or hydrogen atoms.[1] Some sources describe the mere penetration of a wave function into the barrier, without transmission on the other side, as a tunneling effect, such as in tunneling into the walls of a finite potential well.[2][3]

Tunneling plays an essential role in physical phenomena such as nuclear fusion[4] and alpha radioactive decay of atomic nuclei. Tunneling applications include the tunnel diode,[5] quantum computing, flash memory, and the scanning tunneling microscope. Tunneling limits the minimum size of devices used in microelectronics because electrons tunnel readily through insulating layers and transistors that are thinner than about 1 nm