The electromagnetic spectrum

[GrimJosef]

Originally Posted by G8HQP Dave

A 1MHz photon has energy 6.6 x 10^(-28) J. Energy scales with frequency.

Yes. Voyager 1's X-band (8GHz) transmitter runs closer to 1THz than 1MHz but still the photon energy is only a little over 5 x 10^(-24) J. As David says, if you're patient enough you can extract signal despite the SNR being a lot less than 1. But the quantum noise limit still only kicks in at extremely low signal levels.

Cheers,

GJ

[Bazz4CQJ]

I think that the best discussions of the dual properties of light have been presented in one of the series made by Jim Al-Khalili. I think that he's very much the best 'presenter' of science we've had.

B

[G8HQP Dave]

Originally Posted by paulsherwin

As David implied back in #5, this wave/particle duality stuff is just the result of physicists not fully understanding the universe at the subatomic level. Waves and photons are alternative ways of describing phenomena metaphorically (ripples in ponds, billiard balls) and neither is a very satisfactory description of what's actually going on 'underneath'.

Or the universe really is this strange but we find it counter-intuitive. All the evidence is that things really are particles and the same things really are waves i.e. not metaphors but reality.

[Skywave]

Originally Posted by paulsherwin

. . . it may be impossible to explain the universe in a way that can be understood from everyday human experience.

To me, that seems like a reasonable conclusion. Our 'human experience' is ultimately determined by our use of the five senses that we have. And those five senses have limitations in many categories, e.g. bandwidth, sensitivity and S/N ratio, frequency fidelity, etc. Super-sensitive hi-tech instrumentation etc. are all very good and wonderful, but their 'outputs' which we perceive remain subject to our five senses.
In a nut-shell, our human experiences are limited by our intrinsic senses. And perhaps that is just as well.

Al.

[Radio Wrangler]

The moment a physicist decides that smugness is a physical attribute, he will have to invent a carrier particle, the smugon. And then the hunt will be on to find or create one.

The universe is, to our level of understanding, a rather strange place and much that goes on is counterintuitive to expectations learned from our direct observations at our natural scale of things.

Physics is heavily engaged in modelling things at extreme scales and though the models certainly don't fit together ideally, the prediction of a further sort of particle and then subsequently finding it, predicting and finding gravity waves are evidence of some successes.

The cutting edge of science isn't far away, it's inside every subatomic particle of every one of us and everything we see hear or touch.

David

[emeritus]

When I was putting my son to bed one night a couple of decades ago, (he was about 7 or 8 at the time) he asked me how radio waves travel through space. I told him that I didn't think anyone really knew. I said that there was something called Maxwell's equations that I had been taught at college that, if you energised a piece of wire of a certain length with a certain frequency, allowed you to calculate how much voltage would appear across a different wire of a certain length a certain distance away, and that this could be used to design radio and TV aerials. These equations worked, and were based on wave motion in a medium. To support this wave, physicists had come up with the concept of the aether that the waves were propagated in. Other physicists had made experiments that had proved that the aether did not exist, but had not addressed how, if it didn't exist, radio waves were propagated, and why Maxwell's equations worked. I also mentioned that some phenomena were explained by treating beams of light as balls of energy, and others by treating light as a wave, so no-one really knew what light was either.

I then said that, when I was at school, a teacher had observed that , in maths and physics, you occasionally came across things (like the value of Pi) which no-one knew exactly what they were. Specialists just gave them names and then everyone else thought that the specialists knew what they were. He was satisfied with that.

[G8HQP Dave]

We know what Pi is; we just cannot write it down in a finite number of digits in any number system. This is not because we are ignorant, but because Pi is an irrational number. (Actually, it is worse than that: Pi is transcendental, so we can't even give a simple equation for Pi)

We only believed that waves needed a medium because all the waves we knew about before EM had a medium. We therefore jumped to the false conclusion that all waves need a medium. Drop that assumption and there is no need to 'explain' how EM waves can propagate without a medium.

[kalee20]

Agree about pi definitely!

Agree also about waves needing / not needing a medium. (In fact, if you subscribe to the corpuscular theory of light, then photons would travel quicker, faster, farther, with no medium to slow them down. But that's getting very simplistic!)

[G6Tanuki]

I think quite a bit of 'general' comprehension of things is compromised by simplistic analogy - I remember getting horribly confused by the various motor/generator/electromagnetism right/left-hand-rules at school because part way through the Physics O-level course the books we were using switched from using 'conventional current' to 'electron flow'!

The 'holes' concept used to explain simple semiconductor electronics also confused me deeply.

Now, where did I put my bottle of Phlogiston?

[ITAM805]

..he asked me how radio waves travel through space. I told him that I didn't think anyone really knew...

That, I believe is the correct answer to the OP's question ...

[G8HQP Dave]

As EM appears to be fundamental it perhaps cannot be explained in terms of anything simpler. If so, a lack of explanation is not due to lack of knowledge but is a way of expressing the knowledge that it is fundamental. Some people always search for the underlying cogs and wheels, but what if they don't exist? If there were cogs we would have to decide whether they were fundamental and so in no need of explanation.

[Radio Wrangler]

But there might be something there... we just need the funding for the new 17 peta-electron-volt Zappatron to find out. Only lack of results from infinite effort will prove that there is nothing more. Something being fundamental is very difficult to prove.

David

[G8HQP Dave]

Probably impossible to prove. However, at present electrons, photons etc. appear to have no structure so they seem not to be made of anything else.

[kalee20]

Originally Posted by G6Tanuki

I remember getting horribly confused by the various motor/generator/electromagnetism right/left-hand-rules at school because part way through the Physics O-level course the books we were using switched from using 'conventional current' to 'electron flow'!

When I was doing O-levels (1978), at school, current flowed from positive to negative. But I had been educating myself in (valve) electronics and all the books I'd read worked in electron flow, from negative to positive. Outside school I work in electron flow. I just couldn't stomach the idea of current flow from anode to cathode in a valve - it's a totally alien concept. So I ignore it - still do - all my thinking is in terms of a flow against the arrow in diodes, LED's, transistors. (It follows the arrow in Zener diodes!).

I don't know if there was a few years (1940's - 50's) where an attempt was made to have currents move in the direction of electrons - and it failed?

Originally Posted by G6Tanuki

The 'holes' concept used to explain simple semiconductor electronics also confused me deeply.

I was fine with holes - just electrons moving the other way. Until I came across Hall effect. In p-type semiconductors the stupid voltage is entirely the wrong polarity to explain by electron movement; it's opposite to metals and n-type material. Still worries me.

Originally Posted by G8HQP Dave

Probably impossible to prove. However, at present electrons, photons etc. appear to have no structure so they seem not to be made of anything else.

John Dalton had the plum-pudding, uniformly constructed atom; Dave you've got your plum-pudding photon! And the plum-pudding electron is soaked with a spoonful of charge. Easy!

[Dave Moll]

When I was at school, being the rebel that I was, I risked the wrath of the physics teachers by refusing to use "conventional current" - for much the same reason as kalee20 describes. It seemed to make no sense in the context of thermionics. After all, how could heating a cathode of a valve cause "current" to travel from the anode to the cathode. At least I managed to pass my physics "O" level with a respectable grade and managed a pass at "A" level, so I don't know whether I was docked any points if I used electron flow in any electronics questions.

[superjust]

Electrons are fundamental particles ie they are not made of any other bits. Unlike other particles such as protons and neutrons (and lots of other particles called Mesons) which are made up of Quarks. Don't get me started on Strangeness and Charm. Suffice to say that Particle Physics and Quantum Physics is a different world altogether where the rules are very complicated. As for conventional current, it was originally thought that it was the positive charges that moved, but later it was discovered that it was really the electrons. However, we stayed with the conventional current even though we know technically it is incorrect.

[G8HQP Dave]

Those who find holes and conventional current difficult to swallow may be interested to know that Paul Dirac predicted the positron when he found negative energy solutions for his new theory of electrons. In essence, he said that a positron is a negative energy electron state which happens not to have an electron in it - very much like a hole in a semiconductor. Nowadays positrons are also sometimes thought of as an electron going backwards in time - unconventional current?

[GrimJosef]

I understood that the assignation of positive and negative to the charges of the fundamental particles was due, at least in part, to a simple error made by Zeeman in the experimental investigation of the effect which carries his name https://en.wikipedia.org/wiki/Zeeman_effect.

I remember actually doing the experiment on my undergraduate physics course. Completing it successfully gained you a credit of 12 hours towards the course 'practical' requirement. It was infamous for taking either 6 hours, if no-one had laid a finger on the Fabry-Perot etalon since the previous week's students had finished, or 18 hours if they had - the thing was such a nightmare to line up and keep stable.

Anyway, the point of the experiment was to determine the polarisation of the light emitted in the Zeeman-split components. Based on that and a knowledge of the direction of the applied magnetic field, you could in theory work out whether, in a planetary model of the atom, the heavy bit in the middle (nucleus) and the much lighter orbiting bits (electrons) were positive and negative respectively, or vice versa. In practice the polarisation preparation and analysis set-up was a fiendish combination of waveplates and retarders, all of which had their own directions to take into account. Get just one of them wrong and the answer would be wrong too. Poor Zeeman did. So his original publication ended up declaring the electrons to be positive. The following issue of the journal included an embarrassed correction.

Cheers,

GJ

[SteveCG]

This may interest:

If our eyes were sensitive to radio waves instead of light waves then the sky would seem very different. The Sun would be a dim object - with just occasional bursts of activity - likewise nearly all of the stars and planets would fade away from sight. Instead our galaxy would appear as a glow across the sky, with the two brightest objects being 1) the remnants of a Supernova explosion (called Cass A) in our galaxy and 2) An active galaxy that is millions of light years away (called Cyg A). The amounts of energy being radiated by these objects make anything we do on the Earth look rather puny...

[kalee20]

Originally Posted by SteveCG

If our eyes were sensitive to radio waves instead of light waves then the sky would seem very different.

But it set me thinking, what would an electric "light" be like? Probably the business end could be a coil of wire, with AC going through it. Not a lot different to now, really!