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I understand this so poorly that I don’t even know what to Google, so please bear with me. Could somebody explain in really noddy terms why non-ionising radiation is non-ionising please?

I gather that visible light is non-ionising, so my torch isn’t going to ionise anything. The photons don’t have enough energy to do that. Even a bright torch will just produce more photons (is that correct?). But what if my torch goes up to 11? Could it produce a really big photon, or hit an atom with two photons at the same time, thus causing ionisation?

Part of my difficulty is in understanding what a photon is or represents. I think it was all waves when I was at school, which doesn’t help here.

Enough energy to create an ion (ie. remove an electon as a minimum). Radiation is quantised so the photon has to have at least that energy (high UV, X Rays and Gamma).

Wikipedia is always good on science.

Post edited at 13:37

And why can’t two photons hit an atom at the same time?

Post edited at 13:41

You are correct the photons from a torch are in the visible region of the electromagnetic spectrum and do not have enough energy to ionise atoms(depending on the type of torch there might be  infra red emitted too but it is also not ionising)

The energy of a photon is given by E=hv.

E is the energy,

h is Planck's constant, and

v is the frequency of the photon.

For any given photons, eg the photons we detect as, say, yellow light, the frequency is the same so  none of those photons will have more energy than others because they all have the same frequency.

Similarly, photons with a higher frequency, will have higher energies. If the energy is high enough ionisation can occur.

Dave

I quite like this picture.

You can see the ones with higher frequency are on the right.  This goes with what Tringa said about Energy = planks constant x frequency

Post edited at 13:50

A photon is a tiny packet of energy, you can think of it as a wave or a particle (yay physics for keeping things simple, not!).

You never have to consider 2 photons hitting the same thing at the same time when thinking about your issue because it basically doesn't happen.

Providing more power to the sources you are thinking off just makes more photons, not higher energy photons.

Ioniseation is when an atom/molecule is hit by a photon with enough energy to knock an electron off and create an ion. Photons need to have a lot of energy to do this, they need to be x-rays or gamma rays. Photons with lower energy, e.g. visible light, infrared (heat) or radio waves do not have enough energy.

It's probably better not to think of a photon with more energy as being bigger, instead think of it vibrating faster because as a wave, a photon with a higher frequency has more energy.

Thanks. I don’t think I did a good job of explaining what I don’t understand!

Is it just a law of physics that only one photon can hit an atom at once? Even if my torch goes up to 12?

> And why can’t two photons hit an atom at the same time?

No.

But a full explanation of this is far too complex to explain on UKC and it isn't my specialty :D

Two photons can be simultaneously absorbed to promote a particle into a higher energy state than one photon.

I don't know if two photons can cause ionisation by the same process.

The brightness of the light has to be so high to give any appreciable probability of 2 photons being absorbed "simultaneously" that I doubt you could focus your torch to be bright enough; 2 and multi-photon absorption normally uses lasers.  It's common in microscopy and there's a fine line between awesome imaging and cooking the sample.

https://en.wikipedia.org/wiki/Two-photon_absorption

Thanks. I think what confuses me is the treatment of light as discrete photons. When I was a lad it was all waves, and I presume in some instances it is still best to consider light that way?

Imagine waves hitting a harbour wall. Ignoring erosion, it doesn't matter how many 1m high waves hit a 2m high wall they will never get over the top and damage the boats inside the harbour.

Then a 3m wave comes along and easily tops the wall and causes damage inside the harbour.

Photons are the waves, the height of the waves is the photons energy. The height of the harbour wall is the energy required to cause ionisation. The destroyed boats is your DNA / cells being damaged.

the photons simply do not contain enough energy by nature of their wavelength.  If you changed your torch to emit Gamma Ray photons it would be a different matter - the energy wrapped up in the enormously high frequency of oscillation of the photon/wave is very destructive

If you turned it into a gamma ray laser you'd have a very nasty weapon.

Post edited at 14:01

It comes in lots of individual small packets, it isn't a sort of continuous stream. Imagine you look at sand draining through your fingers, you might think it is a continuous stream with no individual particles but if you look closely, you see the particles. The same is true with water streaming past, only the particles are so small (molecules) that you would need very special equipment to "see" them. The same is true with light but the particles (photons) are even smaller than a molecule (much smaller) and it all gets messy because we can't ever "see" the individual particles, only do experiments to show that they must exist in that way.

Thanks. I knew you’d get to the bottom of my poor understanding. Have you considered working in education?

If I Google “multi-photon ionisation” I get a lot of papers that I don’t understand, but it seems that instead of a torch I need a laser that goes up to at least 12.

> Thanks. I think what confuses me is the treatment of light as discrete photons. When I was a lad it was all waves, and I presume in some instances it is still best to consider light that way?

I'm a graduate physicist and I never fully (to my satisfaction) got my head round it. I learnt enough of quantum physics to get through, but it is non intuitive, and mathematically very heavy.

> I'm a graduate physicist and I never fully (to my satisfaction) got my head round it. I learnt enough of quantum physics to get through, but it is non intuitive, and mathematically very heavy.

That’s refreshing to read. Some people are happy with “it just is”.

According to the Planck equation, E=hf, where E is energy, h is planck's constant and f is frequency

Separately, frequency is related to wavelength by f = c/λ with f being frequency again, c being the constant speed of light (and all other electromagnetic radiation) and λ being the wavelength. Because both are related by constants, this means that for a given wavelength there'll be a specific energy per photon of electromagnetic radiation.

For something with long wavelength (like microwaves), this will have a low frequency because it's inversely related to wavelength. Because of the planck equation, this means it will have a lower energy relative to something with shorter wavelength.

On the contrary to this, something with with short wavelength (like Ultraviolet radiation) will have high frequency and higher energy.

If you think of the atom and the electron surrounding it like a bowl with a marble at the bottom, with the photon of energy hitting this marble. If the photon has low energy it'll bump the marble, but the marble (electron) will then roll back down the sides of the bowl (atom).

However, if the photon has high energy it'll knock the marble out of the bowl entirely, creating the ion as the atom will now be short of the electron, and the electron will be floating about freely.

So microwaves will not ionise things they hit, as they don't have the energy to knock the electron out, while ultraviolet does have enough energy to knock the electron out and so can ionise things.

Hope some of that makes sense! Been a long time since I've thought about this stuff so could definitely have got some it wrong.

Your torch at 11 would create more photons of the same energy rather than a bigger photon with more energy.

You can have two photons interacting with an ion. That's non linear frequency doubling at high intensities.

https://en.m.wikipedia.org/wiki/Second-harmonic_generation

You can also create an ion by creating a plasma with a pulsed laser (non-ionising wavelength) focused down to a high intensity.

https://www.clf.stfc.ac.uk/Pages/Plasma-physics--laser-science-at-extreme-intensities.aspx

Post edited at 14:13

> Thanks. I think what confuses me is the treatment of light as discrete photons. When I was a lad it was all waves, and I presume in some instances it is still best to consider light that way?

Pilot Wave theory is one way to get an intuitive visualisation of the two different aspects of light.

Ultra brief: Waves explore the universe to determine where it is probable for energy to go, and discrete packets of energy surf those waves.  Arguments are still had about the validity of this interpretation but to me - and as I said to my students - the most important thing about an interpretation is often if it’s *useful* to help one learn and develop.

Here’s Morgan Freeman explaining it with the amazing classical realisation of Pilot Wave theory from Yves Couder and his lab.  https://m.youtube.com/watch?v=fnUBaBdl0Aw

> Have you considered working in education?

Not a chance; the UKC pundits have declared me an armchair expert; no idea how such qualifications would get me such a job...

> If I Google “multi-photon ionisation” I get a lot of papers that I don’t understand, but it seems that instead of a torch I need a laser that goes up to at least 12.

Yup.  If you had a torch that went up to 50,000 it'd probably work as well - the process that allows multiple photons to be absorbed simultaneously needs very high intensities on the scale of a few large molecules, and it's much easier to focus a laser to high intensity to a tiny point without melting everything in the way because optically it acts like a "perfect" point source, where as a torch is an extended source with its power spread out everywhere, focus it down and you get a miniature picture of the bulb or LED in the torch where the power is still spread out over way more space than the laser.

Post edited at 14:23

Really interesting video thanks. I’m not convinced though that Morgan really understands it properly either

Not bleaching the chromophore is why we sometimes use two photon excitation using lower energy infrared photons to excite green fluorescent protein rather than the normal blue lasers. But yes, you do not want to get your hands or eyes in the beam path of the IR laser!

CB

> Thanks. I think what confuses me is the treatment of light as discrete photons. When I was a lad it was all waves ...

Were you born before the 1920s?

Just as an aside - when I was at university the first time, before the internet existed, if I didn’t properly understand something then there were few alternative avenues to the textbooks or lecturers. Lecturers sometimes turn out to not understand things properly either, but just repeat the explanation they were once given. Nowadays there is a wealth alternatives. I once revisited some of the things that I originally didn’t understand about thermodynamics and understood it within ten minutes. Maybe some comfort to students who will be learning remotely.

Ha! Somewhat younger, but I really don’t recall any mention of the whole photon business. Continental drift was only starting to become fashionable though.

> Not bleaching the chromophore is why we sometimes use two photon excitation using lower energy infrared photons to excite green fluorescent protein rather than the normal blue lasers.

The other benefit is the increased z-sectioning of the sample by limiting fluorescent emission only to the very thin plane where the IR light is focused sufficiently to cause multi photon absorption.  I spent some time working on some funky beam correction optics for a multi photon microscope; more than once the sheer intensity of the laser started cooking the samples in the process.

> But yes, you do not want to get your hands or eyes in the beam path of the IR laser!

Indeed - I've worked with a 5W frequency doubled Nd:YAG before and at least you can see the blinding light coming for you with that - for the OP, this is a process where two low energy photons are combined to make a higher energy photon and is how a lot of green laser pointers work - IR laser diodes stimulate a frequency doubling garnet crystal to make green.

Post edited at 14:37
In reply to Thugitty Jugitty: I didn't read the last couple of entries but surely if you have two sets of photons constructively interfering then you can achieve more energy, - perhaps next to  a star

That is the way that one metre waves can combine to swamp a 2 metre sea wall

> Imagine waves hitting a harbour wall. Ignoring erosion, it doesn't matter how many 1m high waves hit a 2m high wall they will never get over the top and damage the boats inside the harbour.

> Then a 3m wave comes along and easily tops the wall and causes damage inside the harbour.

> Photons are the waves, the height of the waves is the photons energy. The height of the harbour wall is the energy required to cause ionisation. The destroyed boats is your DNA / cells being damaged.

Nice analogy; I wonder if a more simple one might be to consider trying to breach the hull of a tank (the atom you want to ionise) by spraying lots of air pistol pellets (the non-ionising radiation) at it - they just won't penetrate. You need an armour-piercing shell (ionising radiation).

Not only would both photons have to hit at exactly the same time but they’d have to have the same phase. Laser light is coherent. Ie polarised in the same axis and also single frequency so more likely to happen. Light from a torch is all sorts of frequencies and all sorts of polarisations. Even if it did happen once - it could well do due to probability, it would happen so infrequently that ionisation wouldn’t be detectable or significant.

Remember if something is red, it’s because it is absorbing light in that particular frequency of red and all other frequencies of light are being reflected off it.

A lot of the problem here is that QM is so counter-intuitive that if we do what we so often do in trying to understand things - deploy our monkey-derived brains to tell us what experiences we have had that will provide some sort of analogy - we end up getting things completely wrong.  Very often the right answer is <insert horrendous maths here> and if you don't want to spend many years learning this maths then all we are left with is poor analogy.

A photon is not a wave, and it's not a particle either.  It's just a photon and it's doing what a photon is meant to do.  Sometimes that looks wave-like, and sometimes it looks particle-like.  The best I can manage is to make sure I pretend it's a wave in situations where it behaves in a wave-like fashion, and vice versa.

This is also true of electrons, and indeed of pretty much everything else!

My only real (A-level teaching) explanation of why two photons cannot 'team up' to allow visible radiation to carry out ionisation is that they are moving very very fast, so the odds that two would strike a point-like particle truly simultaneously is vanishingly low.  This is not the right answer but it's useful enough.

edit: I'm afraid the waves striking the harbour wall analogy is not a good one, because as waves they can perform constructive interference.  If well timed, two 1m waves can indeed overwhelm a 2m wall.

Post edited at 15:00

... and the increased imaging depth due to the reduced scattering of the longer wavelength excitation beam. However, that was never that important for my personal imaging purposes, as my samples were small enough.

edit: Forgot to mention, the best way to fry your sample is the depletion laser in a STED microscope...

CB

Post edited at 15:10

> A photon is not a wave, and it's not a particle either.

I would argue that we just don't know what a photon is, and therefore we can't say what it isn't.  A wide open gamut of interpretations exists with no known scientific way to choose between them.  It's a photon.  It has wave-like and particle-like properties; I'd limit myself to that.

> edit: I'm afraid the waves striking the harbour wall analogy is not a good one, because as waves they can perform constructive interference.  If well timed, two 1m waves can indeed overwhelm a 2m wall.

Doesn't that make it a near-perfect analogy for the actual process?

You could describe the excitation or ionisation process as a result of interaction of matter with the electromagnetic field; what matters is the time evolution of that field; and under the right circumstances two small waves can interfere to produce the same effect as one large wave.  With waves nebulously pegged to particles, and particles travelling in discrete packets the issue becomes that it's very improbable that two wave packets will be in the the same place at the same time, unless a very bright and focused source is used.

> the best way to fry your sample is the depletion laser in a STED microscope...

Which is why it's never really taken off big-time I think.  Technically very clever stuff - one of my most highly cited contributions is on an early Laguerre-Gaussian optics paper.  I'd intended to publish a short paper on using them for high power laser driven space-sail propulsion as a way of countering the dynamically unstable nature of the sails (if off-centre a gaussian beam torques the sail in a way that reflects the beam in a direction that drives the space-craft more off centre, the hole in the middle of an LG beam gives a stable torque/thrust coupling) but someone beat me to it with a paper on a technicality less elegant beam of four offset gaussians.

My all time favourite high resolution method has to be PALM/STORM for the sheer cleverness of it and the awesome results - on the subset of samples where it's applicable.

"radiation" comes in two types: classical "particles" (eg alpha particles or beta particles) or electro-magnetic waves (or photons, the quantum mechanical particle of EM waves).

Alpha particles are (essentially) helium-4 nuclei usually created by nuclear decay of heavy nuclei. They are heavy, they have 2+ charge and they carry a lot of kinetic energy when they are emitted.  They are ionising and therefore can create ions (charged atoms) by knocking electrons off of neutral atoms. They has a range of a few cm in air and a ~mm in tissue.

Beta particles are fast moving electrons, usually emitted as a side product of nuclear decay.  They are light and carry -1 charge (although they can be positrons with +1). The can penetrate Al foil and travel ~1m in air.  They are ionising.

Electromagnetic waves (or photons) cover a wide range of wavelengths and energies from radio waves (low energy photons) through to infra-red light, visible light, UV, x-rays and then gamma rays.

The energy carried on each photon of radiation is inversely proportional to the wavelength.  In order to be ionising the energy carried on each photon (particle of light) has to be enough to knock an electron off an atom.  This depends on the atom/molecule a bit but is generally regarded as to start occurring in the ultraviolet and extends to x-rays and gamma rays.

So:

Ionising: alpha, beta particles etc UV light, x-rays, gamma rays

Non ionising: all forms of E-M radiation with wavelengths longer than UV including visible light, IR light, microwaves, radio waves etc etc

Examples of things you encounter:

Alpha particles (very) old luminescent watch dials (pre 1960s), smoke alarms, internal radiation treatment for cancers, scientific instruments.

Beta particles: radiation therapy, PET scans. You can get high energy electrons by other means but historically "beta particle" is the term used when resulting from nuclear decay.

UV: sun beds, the sun

X-rays: when you get an x-ray or CT scan, bag scanners in airports

Gamma rays: you'll not really find these in normal life except for a small dose caused by cosmic rays and they really only when flying.

The sun, light bulbs, TV, mobile phone, bluetooth, wifi, cordless phones, microwave, airport body scanners, MRI scanners, radio and TV transmitters, warm objects, RFID tags, google/apple pay, mobile phone towers (inc 5G) etc etc

Mostly NI radiation does very little to biology except warm things. However, visible light (especially at the blue end of the spectrum), while it doesn't carry enough energy on its photons to cause ionisation, can drive chemical reactions and can cause tissue damage (other than heating).

Someone above mentioned multiphoton absoption - ignore this - it only applies is special cases when the optical power is very high - it's not relevant outside niche areas in some scientific applications.

> edit: I'm afraid the waves striking the harbour wall analogy is not a good one, because as waves they can perform constructive interference.  If well timed, two 1m waves can indeed overwhelm a 2m wall.

Granted the analogy is overly simplistic and your right I should have stated to ignore constructive interference. As a visual concept however I think it's quite accessible to describe relative energy levels.

Yes, this is why I only ever published one single figure in a paper that involved STED, even though I had access to a Leica demo model for free and would have enjoyed a head start...

PALM/STORM is great, but unfortunately most of my stuff has to be done on live samples, so for me it is rather useless. Nowadays I mainly use FLIM/FRET microscopy because I am more interested in the conformation and interactions of my molecules rather than their precise, subcellular localization.

It’s a long time since I tried to understand the wave particle duality.  As I recall, sometimes light behaves like a wave and sometimes it behaves like a particle.

I remember something about a cat that was somehow alive and dead at the same time being involved somewhere.

My brain decided to file it under “things that are odd that you just accept and forget trying to understand, use it as a working model and stop worrying about it”

I seem to remember a good book on it by John Gribbin, In search of Schrodiger’s cat.  It made sense at the time.

Allegedly Feinmann said something along the lines of if you think you understand it you are crazy or an idiot.

Perhaps I am guilty of approaching this with my A level blinkers on.  Two water waves combining is something I would expect my students not to consider negligible, whereas two photons combining would be considered negligible (in fact, taught as simply impossible).

> PALM/STORM is great, but unfortunately most of my stuff has to be done on live samples, so for me it is rather useless.

Some time ago but were people starting to combine organelle motion tracking with the image reconstruction just as I drifted out of high-precision organelle tracking towards whole organisms.  I don't think live-cell PALM/STORM is completely impossible but getting it running robustly for an end user would be a technological tour-de-force, perhaps with localised tradeoff between pseudo-exposure and image quality.  I was keen on the idea of field dependant brightness to allow for localised tradeoff between the epi- and PALM/STORM ends of the spectrum using real time analysis of the images.  Whole organisms make for a nice change though.

> Nowadays I mainly use FLIM/FRET microscopy because I am more interested in the conformation and interactions of my molecules rather than their precise, subcellular localization.

Great data but it hardly makes a picture suitable for the journal's front cover!

There are some right clever bastards on this forum.

Post edited at 15:59

> When I was a lad it was all waves

When I was a lad it was all fields...

it's even more fields now than ever

> Thanks. I think what confuses me is the treatment of light as discrete photons. When I was a lad it was all waves, and I presume in some instances it is still best to consider light that way?

Just to be helpful it is generally accepted that light has both wave like and particle like properties.

> Just to be helpful it is generally accepted that light has both wave like and particle like properties.

...but is not actually either.

I tend to think it is a wave but that we don't have a full understanding of what waves are and, as particles are the result of measuring waves in a particular way, that goes for the waves/vacuum energy behaving as matter too when they are in the state of chasing their own tails rather than travelling at c through space, chaisng their own tails at c instead.

Post edited at 12:55

Given that it has both properties isn't it a given? Or mabe it just depends how you choose to define such things.

> it's even more fields now than ever

I recently had the pleasure of throwing out the textbook order and teaching basic EM from a “fields first” perspective.  It grinds my gears the way most textbooks derive and present fields as a way of visualising Coulomb’s law (which up to this point is rather unintuitive and just presented as-is) rather than as the best linguistic and mathematical match to WTF is actually going on as well as a great foundation for developing an intuitive bridge from maths to reality.

There’s never been a better time to teach “fields first” with the wealth of visualisations you can do with computers these days, yet textbooks are still full of shit diagrams.

I really want to make 30 spare days to sit down and write Wintertree’s Picturebook of Electromagnetism; it would be the most dyslexic friendly way of getting to maxwell’s equations.  I’d try and get Oliver Heaviside’s biographer to produce some annotations on his input as well.  It always upsets me that so few people don’t know his step function by name but recall Dirac’s Delta, when the later is purely derivative work (get it?).

It’s not impossible, and its likelihood does indeed scale with intensity (squares). It’s called multiphoton ionisation and requires very powerful lasers in general. There is a related technique called resonance enhanced multiphoton ionisation which works at much lower (but still laser) intensities by exciting the atom to a long lived excited state with one photon and then ionising it with a second. I used to make that happen for a living (and a PhD).

takes me back..

Whilst correct for a vacuum or hypothetical one state system (which doesn’t exist), This is not true if the induced polarisation has non zero lifetime (which it always does because there are resonant states somewhere in the spectrum which contribute non zero terms to the coherence)

You have reminded me that I wrote this, aeons ago.

https://rpubs.com/mbh038/659061

I like your analogy. Unfortunately it is wrong. At least when it comes to electromagnetic radiation. Here frequency and amplitude are inversely related. So, for a high amplitude wave the frequency is low.  On the other hand, the higher the frequency the higher the energy. And it is the high energy photons that are capable of ionising atoms and molecules.

> Given that it has both properties isn't it a given? Or mabe it just depends how you choose to define such things.

I think many people will view light actually as either a particle or a wave, not as something which has some of the properties of a wave or a particle.

Yes that's a fair point, E=hf and the analogy is wrong in technical terms. The idea was to give the OP a starting point one could relate to then perhaps look into it a little further if desired.

Like most physics and chemistry concepts taught at school the basics are often imperfectly described but I always found abstract theories a little bewildering at first until I could picture it in my head.

> Nice analogy; I wonder if a more simple one might be to consider trying to breach the hull of a tank (the atom you want to ionise) by spraying lots of air pistol pellets (the non-ionising radiation) at it - they just won't penetrate. You need an armour-piercing shell (ionising radiation).

Oh well it works for me and everyone I’ve trained in basic xray systems operation.

> Remember if something is red, it’s because it is absorbing light in that particular frequency of red and all other frequencies of light are being reflected off it.

I’ve spent 33 years thinking it was the other way round!

Lovely, can I use that and show my students, not water, RF.  Credit will be given!

Toby

Of course, no credit needed. I like the way it shows how the circular particle motion becomes linear in the standing wave, rotating between vertical motion at the antinodes, and horizontal at the nodes.

> > Remember if something is red, it’s because it is absorbing light in that particular frequency of red and all other frequencies of light are being reflected off it.

> I’ve spent 33 years thinking it was the other way round!

Me too!

edit: A fair bit more than 33 years!

Post edited at 16:50

I’m not quite sure what I meant there. I think I was getting muddled with absorption spectroscopy. Probably distracted at the time.

That is wonderfully hypnotic.

Can you put it online when it's done?

> Pilot Wave theory is one way to get an intuitive visualisation of the two different aspects of light.

> Ultra brief: Waves explore the universe to determine where it is probable for energy to go, and discrete packets of energy surf those waves.  Arguments are still had about the validity of this interpretation but to me - and as I said to my students - the most important thing about an interpretation is often if it’s *useful* to help one learn and develop.

> Here’s Morgan Freeman explaining it with the amazing classical realisation of Pilot Wave theory from Yves Couder and his lab.  https://m.youtube.com/watch?v=fnUBaBdl0Aw

> Not a chance; the UKC pundits have declared me an armchair expert; no idea how such qualifications would get me such a job...

> Yup.  If you had a torch that went up to 50,000 it'd probably work as well - the process that allows multiple photons to be absorbed simultaneously needs very high intensities on the scale of a few large molecules, and it's much easier to focus a laser to high intensity to a tiny point without melting everything in the way because optically it acts like a "perfect" point source, where as a torch is an extended source with its power spread out everywhere, focus it down and you get a miniature picture of the bulb or LED in the torch where the power is still spread out over way more space than the laser.

The Morgan Freeman narrated video is fascinating. When I was at school in the 1960's we were taught wave-particle duality: that matter at minute scales displays both wave-like and particle-like properties. My rather excellent Physics teacher emphasised the *-likes*, and said if you visualise wave too much like everyday waterwaves and particles like billiard balls, you may get yourselves tied up in knots. I've always had problems with the standard Copenhagen interpretation of quantum theory in which one can only observe wavelike behaviour or particle-like behaviour at any one time, but not both at once. However, the greatest minds in history, such as Planck and Einstein, could never quite get their heads fully around wave-particle duality. IIRC, Einstein favored de Broglie's Pilot Wave theory, in which both the particle and the wavelike manifestations exist at the same time. And, as a layman of quantum theory, I find that easier to swallow, without knowing whether it is any more correct or not.

Recently, I have been delving into phonon theory, which underlies  heat conduction in solids. The elastic wave energy of warm solids is quantised into phonons by analogy with the photons of EM energy. What makes phonon theory even more confusing is that phonons are not regarded as any type of real particle but as pseudo-particles. From your very good descriptions in this thread, I think you would probably be able to help me with that.

It's far harder to explain quantum than to understand it. Frustration tutoring 2nd year chemists at Oxford put me off becoming a teacher, and they are above average in capability!

I wouldn't get bogged down in wave/particle, and can recommend Mr Tompkins in Paperback for those interested in exploring the oddities of quantum and relativity.

I agree completely, but it I always get frustrated if I don't fully understand some part of science and I go on trying to find some enlightenment.

I have much less of a conceptual problem with relativity than quantum theory. The mathematics of Special Relativity is elegantly simple and enlightening. Intuitively, we all accept the Gallilean principle of relativity, though applying this to the (constancy of the) speed of light regardless of the frame of reference *is* counter-intuitive.

Overall, though, I totally accept that nature does things in different ways at different time and length scales. In fact, this is one of the messages I put over when teaching geoscience courses. What amazes me is how nature always seems to do things in the most efficient ways possible at all scales. Although, I think what is really going on is that nature tries everything and the most successful prevail - a kind of Darwinism at the particle or molecular levels.

> However, the greatest minds in history, such as Planck and Einstein, could never quite get their heads fully around wave-particle duality. IIRC, Einstein favored de Broglie's Pilot Wave theory, in which both the particle and the wavelike manifestations exist at the same time. And, as a layman of quantum theory, I find that easier to swallow, without knowing whether it is any more correct or not.

I prefer not to get caught up in which is "correct"; for all we know everything we see and experience is actually a manifestation of the thermodynamic interaction of information; "reality" is a slippery concept at the best of times.  I personally am drawn to the elegance of Pilot Wave theory and was blown away to see it realised to some degree in a classical mechanics system.  I've seen professors get quite agitated when arguing about what "really happens" in a double slit experiment for example - sad really as neither of them actually know and the end result is the same either way.  A quantum microcosm of many internet debates.

>  What makes phonon theory even more confusing is that phonons are not regarded as any type of real particle but as pseudo-particles.

I think the word pseudo- makes stuff seem way more complex than it really is.

You've presumably experienced a compression wave driving on a motorway, where suddenly free-flowing traffic bunches up?  Seen from above a physically solid wall of densely packed cars moves against the flow of the traffic.  It isn't "real" - it's just made of cars, yet it is something that has one mechanic and behaviour that emerges from the different mechanic and behaviour of the individual cars.  It doesn't exist in any tangible way, but you can see it from a helicopter and you can write rules that describe how it propagates and behaves, that are abstracted away from the rules that describe how individual cars work.  You could treat the point of maximum compression as a pseudo-particle, and write equations describing it and pretend it's real.  The equations would tell you what happens.

Watch this -  https://www.youtube.com/watch?v=Suugn-p5C1M& - find the compression wave, put your finger on it and follow it - your finger moves counter-clockwise whilst the cars move clock-wise.  Your finger represents the pseudo-particle.

Phonons are just like that, except that they emerge from atoms, not cars, and they emerge in a regime where the behaviour of the atoms is quantised, so they are quantised.  What's interesting is that the equations that you find for these pseudo-particles aren't so different from those for real particles.

I await someone to tear in to my noddy description with a better one!

> Can you put it online when it's done?

Sadly I don't think it's going to happen any time soon; I've packed in the teaching on account of having more exciting things to do, so I won't be revising the material this year as previously planned, and I want to try and get my collection of children's short stories published first.

Promise not to divulge my identity and I can send you the lecture slides for critique however.

Many thanks for your excellent analogy. I do get the fact that phonons are an excellent abstractions that simplify some problems. One of the great things about heat conductive phonons is that they can be treated as a "phonon gas" to which the kinetic theory of gases can be applied!

Phonons are a great illustration of the general physics principle that any time you have a field that can sustain a vibration, those vibrations must be quantised (their energies can only come in discrete values related to the natural frequency of the vibration), and each quantised unit corresponds to a particle.  So photons are the quantised units of the vibrations of the electromagnetic field (i.e. light waves) and  phonons are the quantised units of sound waves.

Quantum field theory takes the position that every fundamental particle arises in this way, from the quantisation of a field.

You can think of phonons as particles in the sense that they're packets with an energy and a momentum, so, just as you say, you can think of their interactions with each other and with electrons as collisions between particles that conserve energy and momentum, so you can build a theories of heat conduction and electricity conduction along the lines of the kinetic theory of gases.

> ...the general physics principle that any time you have a field that can sustain a vibration, those vibrations must be quantised...

they sound a bit like the famous "holes" that were said to move in semiconductors - Once I dropped that idea things made more sense!

I don't have an opinion as to *why* the universe is as it appears to be, but here's my best attempt to say how we know that the electromagnetic field must be quantised.   If it wasn't, every time you opened the door to your microwave you'd be instantly fried by a flood of x-rays and gamma rays.

If you imagine a metal lined box, we know that the atoms that make up the walls are constantly jiggling around because of their heat energy, and because of this they radiate em waves.  These end up making standing waves inside the box, and when things settle down the walls must end up emitting as much energy as they absorb from each type of standing wave, so the energy contained in each standing wave ends up a constant amount proportion to temperature.  The problem is that the shorter the wavelength of the standing waves, the more of them you can fit in the box, so on this argument the shorter the wavelength, the higher the total energy, so a box at room temperature would be expected to be full of high energy UV, x and gamma rays.  Obviously this isn't the case - a metal box at room temperature will be filled with em waves, but only with infra-red radiation.

Max Planck at the end of the 19th century realised that this problem wouldn't arise if the standing waves are only allowed to have energies that are whole numbers of a "quantum" proportional to the frequency of the wave.  These packets of energy are what we now call photons.  The quantum theory of the electromagnetic field developed from this is so successful and so accurate that physicists have developed other similar (but more complicated) theories to explain all the other fundamental particles as the quantised vibrations of a whole bunch of other fields.  Of course, having so many different fields is a bit untidy, hence the quest for a unified field theory.

That helps. Thanks for taking the time.

Which, when you think about it, is basically the Special and Inevitable Anthropic Principle.

Another take rather more out there take would be that the computer simulation our universe runs on is bound by finite memory and finite precision maths...

I'm just a grubby experimental condensed matter physicist, so I know that all the messy paradoxes and infinities that show up when we do field theory are just artefacts that arise because we're making the approximation than things are continuous when we know that the underlying reality is discrete (because stuff is made of atoms). Far be it from me to suggest that the same applies to *real* reality.

Don't get me started on Nick Bostrom!

I’m not having a dig at you or your explanation!  I’ve used a similar one when it comes to help students consider why the induced E and B fields are in opposing “directions“ - if they weren’t, energy wouldn’t be conserved and a single fluctuation would run away and destroy everything.  So the Special and Inevitable Anthropic Principle is again behind the most compelling explanation.

> Don't get me started on Nick Bostrom!

Simulation or God, it just proxies the problem...

Post edited at 20:11

Was doing my best to distinguish between "why" and "how do we know" questions.

About a decade ago I spent too much time talking to transhumanists, until it became clear to me that it was essentially a religious movement.

Often not much good comes of pondering the why.  I’m always cautious of someone who thinks they know why.

There’s certainly one or two people who consider themselves messianic amongst the transhumanists.