That is a little like suggesting that a sound recorder is just electronics and shouting at any electronics should elicit a response. Bringing it back to the neurons,

• loud enough sound on any neuron will probably excite it
• However the sensitivity of neurons connected in the ear to sound is thousands or millions or billions (not bothering to calculate it) higher than the sensitivity of a random neuron in the brain to sound
• A random neuron responding to sound won't feel like sound. If a pain neuron is activated by sound, it will appear as pain, if a hot neuron activated by sound will appear as heat, etc.

So as hot as the air has to be to excite your cochlear apparatus, and thus the neurons connected to it, it probably has to be thousands or millions times hotter to excite the neurons directly in your brain. And long before it gets to that temperature your brains has been cooked, then dessicated, then burned, and finally decomposed into a plasma of atoms and electrons flying about separately, and probably at the temperatures we are talking about, the protons and neutrons are smashed apart into a cloud of subatomic particles.

If, hypothetically, I tried to catch a terminal-velocity bowling ball with my face, your theory says I would experience the bowling ball doing nonfatal damage and then stopping just before killing me, and my theory says I would experience changing my mind and getting out of the way of the bowling ball.

So from the perspective of a you that I can talk to after the near miss with the bowling ball, your description makes sense. But it also makes sense to me. We are both in the universe where you changed your mind before the bowling ball hit you and you got out of the way.

But from the perspective of me in the world where you got hit by the bowling ball and died in pain, your consciousness did whatever consciousnesses do when people die. Presumably it felt the fear when it noticed he inevitability, felt the impact and then the pain, and then stopped working as the neurons in the brain stopped working, some from immediate injury, others more slowly form loss of viable environment.

The worlds in which people die exist. I am in a world where billions, of people have died. A small number I have seen die with my own eyes, a larger number I have seen soon after they died, a much larger number I know of by reliable report.

This immortality you speak of: if there are identical twins and a the age of 5 they are crossing the street and one is hit by a bus,has not some individual died? If you live in a world with MWI, and at the age of 5 for one conscious version of you the universe splits, and in one of those branches EVERY new universe generated ends in your death at a finite age at least 20 years later, while in the other branch there are some branches where you go on forever, than have there not been at least one conscious version of you which will last 20 or more years, but not infinitely, that will die?

This idea that your consciousness jumps from the dying world to somehow mystically join with the version of you in a different world is anti-intuitive at best, and non-scientific or religious at worst. Nothing else jumps between worlds once they have split, why would consciousness? There is already a consciousness in the world you want to jump to with different experiences than yours as you face your last seconds of life, how is there room for your consciousness to pop on over to the other universe to escape death?

Your theory strikes me as the opposite of timeless. Your theory seems to come down to, if I ask my 10,000 year old self about the worlds, I am always going to get an answer in which I lived at least 10,000 years. But if you ask your 20 year old self about the world, then almost all the answers you get are going to be about worlds in which you live less than 100 years, I say that based on the observation that the people other tyan you that you see, way over 99% of them are dying before age 100.

A QI belief in infinite life seems indistinguishable from any other regligious belief in infinite life, at least in regards to conformity with evidence, logical plausibility, and some amount of wishful thinking.

I'm not certain that I understand your argument, so I may have responded incorrectly. Let me know if you need any clarification.

On re-reading, I actually misunderstood your original point and my argument has nothing to do with your original point.

I would still want to point out a few things that may make what is going on clearer.

First, Brownian motion amplitude rises as temperature rises. So while the Brownian motion of temperatures typically found in the ear, or in the air near the ear, is small enough that the ear can't detect it, as you say, if you were to raise the temperature, the Brownian motion would be higher amplitude and would eventually rise to a point where it was detectable. This is a pretty academic point: the temperatures required to hear the brownian motion would harm the ear so in practical terms your statements are right enough.

If vibrations in the air cause the endolymph to have pressure waves in it which then cause cochlear hairs to move, it is still quite reasonable to describe that as air vibrations making cochlear hairs move. Introducing the endolymph is a clarification at best, not a correction.

Another thing to keep in mind is that at equilibrium, you have thermal excitation everywhere. You might as well ask why you don't hear or see or smell the thermal excitation in your own brain.

I think you are suggesting something like: if I was detecting thermal vibration by the vibration of a membrane due to thermally induced air pressure I wouldn't because the temperature is the same in the air on both sides of the membrane and therefore the thermal air pressure on each side of the membrane is the same and so fails to move the membrane. If this is what you are suggesting it is wrong, and in a basic enough way to merit explanation.

Sound is pressure changing in time. Thermal vibration follows a random distribution. The air on each side of a membrane at the same temperature will have the same statistics of pressure change on each side of the membrane, but not the same instantaneous pressure on each side of the membrane. If the random pressure exceeds p1 25% of the time an is less than p0 25% of the time, then 6.25% of the time there will be a pressure difference of at least p1 - p0 on the membrane, and a different 6.25% of the time there will be an opposite sign pressure difference of at most p0 - p1 where we have chosen p1 to be the higher pressure than p0. So thermal vibrations will absolutely cause a membrane to vibrate randomly. Further, it is the case that the magnitudes of p1 and p0 rise as temperature rises as temperature rises, so we expect the membrane to be moved more when surrounded by hotter air than it does when surrounded by cooler air.

SO it is the case that generally heating air makes its average pressure rise if it is in a constrained volume, and a membrane will certainly not be displaced on average if it has air on each side at the same average pressure, but it is the temporal or time variations that produce sound, and the time variations on each side of the membrane for most conditions you can create in the lab are uncorrelated, and so the membrane vibrates randomly and with an amplitude that rises as the temperature of the air rises.

Photons with over 1 Million electron volts of energy can create a positron-electron pair, but only when near another massive particle (like the nucleus of an atom). The other massive particle is moved in the interaction but is otherwise not-necessarily changed. https://en.wikipedia.org/wiki/Pair_production. This process has been demonstrated experimentally. The mean free path of the energetic photon near an atomic nucleus is something down on the atomic scale, the experiment I read about used a piece of gold foil and generated lots of positron-electron pairs.

A single photon in otherwise empty space cannot create a pair of particles I was wrong when stating that. However, space with nothing but two photons in it can create matter. Two photons each with a bit over 511 million electron volts of energy can collide and result in the creation of a positron and an electron. https://en.wikipedia.org/wiki/Two-photon_physics Alternatively a single 80 Tera Electron Volt photon can collide with a very low energy photon to create an electron-positron pair. This effect actually makes our existing universe opaque to photons above 80 TeV because our universe is filled with approximately 0.0003 eV photons known as the Cosmic Microwave Background radiation. This background radiation is left-over radiation from the big bang which by now has cooled down to about 3 Kelvin in temperature. I don't know any of the actual mean-free-paths associated with this, just that they are much shorter than interstellar distances.

As far as I remember, you need to hit the resonant frequency of a particular hair to trigger a "sound" response, so frequencies higher than 20KHz might excite them, but if you're not getting resonance, nothing triggers.

No this is wrong. Each hair is excited by the amount of its particular resonant frequeny in the sound hitting it. If a violin note is heard, that note only has a few discrete frequencies in it and so a few hairs are very excited about it and the brain (of the trained violinist with perfect pitch anyway) goes "oh, A 440." If white noise loud enough to hear is hitting the ear, then essentially all the hairs are excited because all frequencies are present in white noise, and the brain goes "sounds like the ocean."

As to excitement by sound above 20 kHz, a very high frequency ultrasound, say at 100 kHz, can be modulated with the vibrations associated with a violin string, much as sound can be modulated on radio carriers. Such ultrasound hitting a human ear can actually cause the appropriate hairs to be excited so that the brain goes "oh, A 440." The phenomenon relies on the non-linear response of cochlear hairs and highly directional speakers based on this effect have been built and demonstrated. See for example http://www.holosonics.com/

As to molecule collisions, I'm not sure vibrations at sufficiently high frequency can be called "acoustic" at all.

Your reasoning here carries useful information. For example, when you are dealing with vibrations whose frequency is so high that the wavelength of the vibration is less than the average spacing between molecules in a gas, or in a solid lattice, then a lot of what you calculate about the detection and interactions with lower frequency vibrations no longer applies.

However, the same limitations apply to electromagnetic radiation. For example we think of vacuum or empty space as transparent to EM radiation, and it is as long as the EM frequency is low enough frequency. But high enough frequency EM radiation, empty space is opaque to it! For example, at high enough frequencies, a single photon has enough energy to create a positron-electron pair in free space. Photons at that frequency don't travel very far before they are destroyed by such a spontaneous generation of particles.

So in principle, EM radiation and acoustic vibrations are the same in this respect: as long as you are considering frequencies "low enough" that they don't rip apart the medium in which the wave exists, they behave in the ways we usually think of for sound and light. But above those frequencies, they rip apart the media they are traveling through, even if that medium is so-called empty space.

The reasoning behind blackbody electromagnetic radiation applies equally well to thermal vibrations in solids and gases. Meaning the spectral limits derived from a quantum consideration of the quantization of electromagnetic radiation (into photons) applies equally well to the quantum considerations of vibrational radiation (into phonons).

"Thermal" photons are indistinguishable individually from photons from other sources. The thing that makes a thing thermal is the distribution and prevalence of photons in time and frequency, those from a thermal source follow a well understood set of statistics, while photons from other sources clearly deviate from that. So a photon arising from a cell phone tower's radio transmitter reacts similarly with a cell phone's radio receiver as a photon at a similar frequency arising from thermal emission from the air. Physics can't distinguish between these two photons which is why it is a major effort in building radio communications to get enough signal-sourced photons compared to the thermal-sourced photons so that the signal-sourced photons dominate, and therefore the signal can be accurately derived from their detection.

Similarly with phonons. Vibrations because something is hot are indistinguishable from vibrations from a vocal cord. It is the statistical distribution of the vibrations in time and frequency that defines a thermal set of vibrations. And again, to hear what someone is saying, it is important to get enough phonons from their vocal cords into your ears compared to the phonons from other sources in order to accurately enough derive the intended information.

Thermal noise or other white noise, and a symphony, have the same kind of phonons and both can be heard by the same kinds of ears. They carry different kinds of information (they sound different) because of their different time and frequency statistics.

A hair cell that was triggered by Brownian motion would be useless. All inner hair cells are tuned to certain vibrations in the endolymph that are greater than those caused by Brownian motion.

Brownian motion is motion of air that, considered as vibrations, has a broad range of frequencies in it. Which means that an ear exposed to air experiencing a sufficiently high level of brownian motion will have many or all of its inner hair cells excited. If your statement was correct, humans would not be able to hear white noise, whereas obviously (to any hearing person who has ever been exposed to white noise) we can.