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First let's discuss what the heck is quantum tunneling? We know the story of Einstein the Great, his award winning paper on light as particle had received universal acclaims and was plagiarized too many times over the centuries. But it is the wave like nature that allows matter particle to overcome physical barriers, physical barriers here could be anything like electric field you name it.

Now suppose something weird is happening right now, every ongoing experiments showed no sign of tunneling regardless the size of barriers... OMG quantum tunneling is broken! What the worst that can go wrong beside having to reprint all the physic textbook?

P.S. magic tag is to handwave why quantum tunneling don't work anymore... not that we know why it work in the first place but it's kind of an insurance policy ;D

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    $\begingroup$ It would shut down many (most?) chemical reactions. Also stop the normal hydrogen fusion in the sun. So the universe rapidly becomes Dead, then starts becoming Darker, then possibly gets very bright as a collapsing sun goes nova. Does this qualify as "anything Bad"? $\endgroup$ – PcMan Mar 3 at 7:44
  • $\begingroup$ @PcMan, can a nova happen without tunneling? I guess that's how nuclei fuse, right? Tunneling through the nuclear potential barrier $\endgroup$ – L.Dutch - Reinstate Monica Mar 3 at 7:47
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    $\begingroup$ @L.Dutch-ReinstateMonica unsure. Definitely yes for huge stars, but unsure if the Sun is nearly massive enough. Tunneling is only important if the ambient energy level is below the potential barrier strength. Apply enough pressure, make the average energy higher than barrier strength, and stuff will fuse. Something like what happens in a core collapse supernova core during/at the climax of collapse, to fuse higher elements. $\endgroup$ – PcMan Mar 3 at 7:53
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    $\begingroup$ This submission has insufficient prior research. A quick Google finds en.wikipedia.org/wiki/Quantum_tunnelling#Applications which already identifies what real-world effects the end of quantum tunneling would cause. $\endgroup$ – GrumpyYoungMan Mar 3 at 17:45
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    $\begingroup$ Even if it wasn't life threatening at the molecular level, most of our electronic technology would stop working and most of us would starve to death within a few months. Not to worry about the slow death scenario, our teen-agers would probably turn on us as soon as their cell phones stopped working ;) $\endgroup$ – jwdonahue Mar 3 at 18:28
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We wouldn't notice anything because we would be dead. And with us all life forms.

Oh, yeah, that count as "the worst which could happen", I guess.

Why? Quantum tunneling plays a role in cellular respiration

Quantum tunneling is among the central non-trivial quantum effects in quantum biology. Here it is important both as electron tunneling and proton tunneling. Electron tunneling is a key factor in many biochemical redox reactions (photosynthesis, cellular respiration) as well as enzymatic catalysis. Proton tunneling is a key factor in spontaneous DNA mutation.

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    $\begingroup$ This was my first reaction ;-). $\endgroup$ – Vincent Fourmond Mar 3 at 15:43
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    $\begingroup$ "We all die instantly" is basically what happens whenever you change any of the fundamental laws of reality we've discovered. $\endgroup$ – Dragongeek Mar 4 at 15:13
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    $\begingroup$ The emphasis has to be on “We wouldn't notice anything” as even Death becomes meaningless when nothing can change. There’s no metabolism anymore, but also no decay of dead bodies. It’s not even clear whether the corpses can fall down to Earth then. It’s just every chemical reaction and every physical process depending on quantum tunneling to be able to happen. It’s absence would just freeze the entire universe in its current state. $\endgroup$ – Holger Mar 5 at 8:04
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What the heck is quantum tunneling?

This is key to a coherent discussion of the question. Unlike the name suggests, the phenomenon does not necessitate a repeated interaction, nor does it involve the weakening of the potential barrier. Quantum tunneling is a phenomenon well accounted for in modern quantum theory, in which the phenomenon is built-in mathematically.

But it is the wave like nature that allows matter particle to overcome physical barriers, physical barriers here could be anything like electric field you name it.

The issue with this view is that it incorrectly assumes how modern physics views the quantum world. Particles, as described in quantum mechanics, do not have a "wave-like" nature in the physical sense. The Wavefunction describes the probability of each of the possible "states" the particle may be in.

The wave aspect here refers to the built-in limitations on our knowledge of how the particle is going to interact, which is described in probabilistic terms that have the same mathematical form as are those used to describe physical waves (note this does not mean that they are waves, analogous mathematical constructs are common in physics and are never understood to imply a physical connection).

Physics does not describe the electrical potential as forming a "physical" barrier. Instead, the "electrical field" is a mathematical description of the electrical interaction. Now the above distinction may appear pedantic, however, as we shall see it is critical to a proper answering of the question.

Now suppose something weird is happening right now, every ongoing experiments showed no sign of tunneling regardless the size of barriers... OMG quantum tunneling is broken! What the worst that can go wrong beside having to reprint all the physic textbook?

Since the mathematical theory of quantum mechanics necessitates quantum tunneling, if this phenomenon were to suddenly "turn off", either the theory would change, or math would break.

If the theory were to break, the probabilities would change and this would have consequences well beyond the phenomenon of quantum tunneling, instead the entire theory would fall apart. From our theories therefore, it is impossible to say exactly how things would be affected since we would have to reformulate the mathematical theory because the old one becomes broken.

In other words...unless you have a very good understanding of physics (and even then), it is almost never a good idea to modify a physical theory (or even just an equation) and work forwards to the consequences...they will certainly be too numerous and consequential to coherently discuss.

Of course one way to approach the question is to look up all the phenomena that include the phenomenon of quantum tunneling and assume these phenomena wouldn't happen anymore. That is, however, naive from a physical perspective, to understand why it is necessary to look a little at the mathematics of quantum mechanics vs. classical mechanics.

Classical mechanics views all interactions through forces (which can be characterized mathematically by energy potentials, this will become important in a minute) upon "particles", which at the simplest level would be would be described by non-deformable masses which fill a definite volume of space. The most pertinent restriction on how forces and particles interact in classical mechanics is described by the conservation of energy: $E = T + V$. For a single particle the total energy is the kinetic energy possessed by the particle added to the potential energy imposed by the forces acting on each particle through each force's energy potential.

Quantum mechanics, on the other hand, does not view interactions like this. A "particle" is a mathematical description which describes localized "states" (or collections of information), the best we can do is describe this localized collection of states through guessing information. Our guesses are not random however, we educate our guesses through probability and statistics, which has been found to follow the following equation: $E \Psi = (T+V) \Psi$. The difference is apparent, for a single particle we describe the probability of, say, energy. We first specify a given state. For a given state we can compute a particular energy. The states a particle may have are determined by through the wavefunction. Of course this doesn't mean that a particle can have any energy value, or be anywhere. Take for example the (relatively) simple case of the hydrogen atom which is a classic (pun intended) problem in an undergraduate course on quantum mechanics. The electron's wavefunction (and corresponding probability densities shown as spatial distribution across space) is shown here.

To understand this idea of multiple states, think of a ball in a normal sized bedroom. Suppose that you had to try to figure out where the ball was. Now the ball could be anywhere, right? Well, if suppose you knew the owner of this room and they were quite tidy, you could guess that the ball is more likely going to be in the closet and not lying in the middle of the room.

Quantum tunneling is no different. We cannot know definite things about a particle, for instance, its momentum. When an interaction occurs, we can compute the likely energy of the particle. In our analogy, we might find that the ball is in the closet 99/100 times randomly checking throughout the day, or a particle in a nucleus has an energy of -8 MeV, 99.999% in a given second. However, this is not definite. The ball might end up in the middle of the room during the time the owner takes it out of the closet to juggle for fun, or the particle might have 0.8 MeV of energy and fly out of the nucleus.

Thus, since quantum tunneling is an inherent part of quantum mechanics, if this phenomenon were to suddenly disappear, quantum mechanics would falls apart and anything goes.

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    $\begingroup$ +1 for the pointing out that "wave-particle duality" is a mathematical model only. Your average photon doesn't particularly care whether it is modelled as a particle or wave--its nature doesn't change $\endgroup$ – nzaman Mar 3 at 10:23
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    $\begingroup$ @nzaman have you performed a survey of photons? Can you cite where someone has? How many photons have you interviewed and how do you know what they care about? A bit presumptuous of you isn't it? Modeling is a big industry. It pays well. They may care how they are modeled. wink. Sorry, i couldn't resist. $\endgroup$ – Dave Harris Mar 4 at 22:05
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    $\begingroup$ @DaveHarris: Sadly, photons involved in modelling get robbed. The photographer. camera maker, lighting crew, even the electrons get, if not paid, at least accounted for. Photons are treated as free labour, despite being the most critical component of the industry. Since they don't get paid, they don't really care. $\endgroup$ – nzaman Mar 5 at 13:46
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    $\begingroup$ I appreciate this answer because it points out that you can't break (or even discuss really) quantum tunnelling in isolation from the rest of our understanding of Quantum Physics. But the answer gets too long and a bit meandering; you lost me around the middle (and I'm a physicist) $\endgroup$ – cecilian - MonicaWasMistreated Mar 5 at 15:45
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End of the physical matter, as we know it. Including, but not limited to, the matter our bodies are made of.

The nuclear force (the residual strong force, the force holding protons and neutrons in the atomic nuclei together) pretty much requires quantum tunnelling in order to work. No more atoms. Only hydrogen and free neutrons, unable to decay as they usually do.

Quantum tunnelling is not a separate phenomenon. It is just an unexpected (from our macro-world viewpoint) manifestation of how the wave-particles in the quantum world behave. That said, it is not really easy to predict what else will break in order to get the tunnelling killed. But I think the above is enough.

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All nuclear fusion would instantly stop - with catastrophic results

Quantum tunnelling is responsible for all fusion processes, including those within our sun.

As this is then responsible for the constant release of energy and 'outward pressure' to keep in balance the forces within the sun, this outward release of energy suddenly stopping will enable the gravitational forces of the sun to compress without any outwards pressure, and the sun would go dark and suddenly compress to a very small size.

From the earth the sun will blink out after 8 minutes after the phenomenon suddenly occurs, plunging the earth into cold darkness and killing almost all life on Earth. Not to mention all stars in the universe too.

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    $\begingroup$ of course it will be many thousands of years for us to notice - fusion only happens in the sun's core, it will take a long time for the sun to cool off. $\endgroup$ – ths Mar 3 at 10:30
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    $\begingroup$ @ths And the gravitational collapse of the sun will also release energy that will keep it hot for much longer than that — it would still be shining in ten million years if fusion turned off today. $\endgroup$ – Mike Scott Mar 3 at 11:57
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    $\begingroup$ @ths I don't think it works like that. Stopping fusion immediately removes a source of pressure in the core, shifting the balance between pressure and gravity that keeps the sun in hydrodynamic equilibrium. This means the core will instantly collapse into a new denser configuration. The surrounding layers, now left unsupported, will follow in a cascading sequence, very much analogous to what happens in a supernova, just considerably less energetic. $\endgroup$ – JohannesD Mar 4 at 18:20
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    $\begingroup$ @JohannesD No, nothing happens immediately. It takes tens of thousands of years for the photons created by fusion reactions to work their way out of the sun’s core, so 99% of them are still in there for centuries to come. $\endgroup$ – Mike Scott Mar 4 at 21:03
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    $\begingroup$ The core pressure is a result of the temperature, not a direct result of the fusion reactions. Stopping fusion only stops the source of energy keeping the temperature up. The star won't suddenly collapse when fusion stops, it'll slowly cool and shrink into something like a white dwarf, just with more hydrogen than usual. If big enough, it would normally collapse into a neutron star, but that may not happen with no tunneling. $\endgroup$ – Christopher James Huff Mar 6 at 14:23
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Most likely nothing will happen.

Quantum tunneling is just a theory. It is the way we think the universe works. But time and time again the universe has its way to prove us wrong.

So if somebody finds in an experiment that quantum tunneling is "not working" anymore, then somebody else will come up with yet another theory explaining why.

The OP question could have been asked two centuries ago as "would anything bad happen if gravity stopped working overnight?"

Einstein figured out gravity (as explained at the time) didn't work sometimes and everything was just fine.

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Given that you want to change physics anyways, there is always the possibility to go back in the history of physics to the point where quantum tunneling was not known. Most of everyday physics was explained so you don't really have a problem. You just need to find plausible explanations for the few unsolved puzzles scientists where thinking about back then. If they matter to you at all.

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