[PIKAL]

Speaking of radios, I’ve been trying to understand light recently and it’s difficult. I find that for most things, I have a visual intuitive model to reason about things. But I have a hard time visualizing RF light. There aren’t any good videos out there of just a straight forward visualization of RF light. It’s all abstractions with a sine wave, but no visceral visualizations, rendered or otherwise. The closest thing I’ve found is thought emporium visualizing wifi.

And also, why can visible light pass through other visible light without causing interference? Why can’t they make visible light with an antenna? Why are large arrays of RF receivers not more widely used for small-scale RF imaging?

[R0b0t1]

> Why can’t they make visible light with an antenna?

You can. The antennae are the size of molecules. LEDs emit light, your retinas receive them. The retinal dye molecules are tuned for specific frequencies just like you'd tune a macroscopic antenna.

[Gibbon1]

Richard Feynman has a short book on Quantum electrodynamics for dummys. Otherwise known as QED: The Strange Theory of Light and Matter. Read that as many times as you need to break you of any preconceived notions about how light works.

As for sin waves etc. It's better think of light oscillation between it's energy as an electric field and a magnetic field at right angles to each other. Although I think that's kinda gross and wrong.

As for an antenna producing visible light, in theory there isn't any reason. Because radio waves are light. The problem is practicalities.

[duckfang]

> But I have a hard time visualizing RF light.

The best things I can recommend is to play with GnuRadio, IQ data, false color representation of a freq or set of frequencies bound to color/decibel strength, and ParaView and importing said IQ data for graphing.

Think of the frequency of light as its color, and dB strength of how bright it is. However at cm and m wavelengths, voids happen much more regularly. That light can 'bend' around, and go through objects.

> There aren’t any good videos out there of just a straight forward visualization of RF light.

There's quite a few false color images. If you look at hydrogen line radioastrometry, there's a ton of false color images. There's also this HackADay that mapped wifi for a wide area covered by their CNC gantry https://hackaday.com/2015/02/17/mapping-wifi-signals-in-3-di...

> And also, why can visible light pass through other visible light without causing interference?

EM primarily only directly affect things with an electric charge. EM radiation itself doesn't have an electric charge, therefore EM usually doesn't affect other EM. However, if we include gamma (+10^19 Hz), then if those collide, they can create an electron and a positron. But that's only theorized with energy vector diagrams and not actually seen.

> Why can’t they make visible light with an antenna?

You can... You just have to pump enough energy in it to make it glow! /hahaha

> Why are large arrays of RF receivers not more widely used for small-scale RF imaging?

That's primarily a cost issue. Go look how much a single RF frontend chip and an a/d chip costs. (Price gets to stupid levels at, say 24 bit A/D).

Now instead, lets look at human vision. Humans can see (eyes are receptors of radio from 380nM to 720nM). When converted to Hz, we're talking 372.55 THz wide spectrum vision.

My SDR on the table can see 112 MHz, or .0000112 THz

Now, in order to replicate what's going on in the eye, you'd need millions of antennas AND data acquisition (of some sort). And then, even with current SDRs, these generate 60GB/min - you need the disk, memory, and CPU to do stuff with that. It's NOT a trivial problem.

Now there are some RF arrays out there. KerberosSDR is one such array. However, its max bandwidth is 3 MHz @ 8 bit. And it can only do 4 inputs, which is enough to do geographical tracking of radio signals (within 24MHz to 1.7GHz). I know of one person who's trying to do some VR work with a KerberosSDR.

The other problem, once you have the millions of antennas and data acquisition, is a matter of synchronization. Timing is also another stupidly hard area, which increases geometrically with more sensors. And remember that 1nS = 11.8 inches deviation.. So whatever processing you're doing had better be time consistent and local to the device.

Eventually, we'll get to what you're proposing. A lot of us are wanting that. But we're decades away.

[rapjr9]

> Why can’t they make visible light with an antenna?

Why can't an oscillator and antenna emit matter? Is some matter transparent to other matter in the same way that some matter is transparent to light? This idea that matter and energy are equivalent and that the same wave physics underlies it all doesn't seem to be true in reality. Instead it seems that phenomena are divided into regimes and in each regime behavior is very different. Sure it's tough to build an oscillator at the frequency of matter, but is it really impossible to build a matter emitter? I only have a BA in physics but I remember while learning (and in later reviews of physics) that it seems like theory doesn't always scale and there are a lot of very weird assumptions and "explanations".

[amluto]

You can (for the most part) produce bosons (e.g. photons, aka light), individually or in big piles, out of thin air. You cannot produce fermions (e.g. electrons, protons, neutrons) out of thin air like this. To make an electron, you need to also produce a positron or do something else that satisfies the various conservation laws. You can make an electron-positron pair in a collider, but doing this in large amounts takes absurd amounts of energy.

[jimktrains2]

> And also, why can visible light pass through other visible light without causing interference?

Radio passing through radio doesn't interfere with thr signal. Its just more likely that the interfering signal will also be picked up by a receiver.

Think about a bright white lighbulb next to a small red LED; you won't be able to see the small LED.

The other part is that out eyes are essentially 2d grids that are fairly large in both directions with respect to thr wavelength of light. Radio receivers are essentially a single point.

We can also bend and focus light with lenses fairly easily with many different substances, due to the refraction index of the material.

If you move the red LED away, you will be able to see it more easily. This is because we can focus the light onto more than just a point receiver so that the the bright light doesn't entirely overwhelm all the sensors. If you do photography, you know that overexposure is problematic and can cause areas around bright areas to be overexposed as well.

While we cant use glass to bend radio waves, they can be be bent in the same way as visible light. Ham radio operators do this all the time by using the ionosphere to bounce radio off of to make long distance contacts that are beyond normal line-of-sight.

Radio and visible waves are also subject to gravitational lensing.

They are also both subject to diffraction, where the light will bend slightly around the endge of an object. Ham radio operators also make use of this to make contacts near the edge of obstructions, like mountains.

Visible light can be used in fiber-optic cables, and waveguides are a similar device used for microwave-frequency light.

> Why can’t they make visible light with an antenna?

In terms of normal radio antenna designs, you could, I think, but antennas need to be sized proportional to the wave length of the light.

However, since visible light is higher energy than radio light, and near many of the transition energy found in atoms, we make extensive use of that additional energy when detecting (e.g. photoreceptors in our eyes and photodiodes), emitting (e.g. LEDs and incadescence), and using (e.g. photo etching and film photography) visible light.

> Why are large arrays of RF receivers not more widely used for small-scale RF imaging?

You can only image something down to a resolution of the wavelength used.

You do see some applications of this, viz millimeter wave imaging at airports.

I can't find the link, but a week or some ago, there was an article on hackernews about radio imaging of cities from satellites? If I find it I'll edit it in. Resolution on the order of meters is adequate for that purpose. (For reference: WiFi is roughly 12½cm. Fm radio stations are roughly 3m. Am radio is roughly 300m.)

Also, this is a limitation of visible light microscopes as well. We can't imagine, say, molecules, because theyre smaller than the wavelengths of visible light.

----

One of the biggest difference between visible light and radio light when it comes to thinking about propogation is what absorbs it. Your WiFi router can be on the otherside of a wooden wall because wood doesn't absorb or reflect substantial amounts of rf energy; a metal wall does. 5ghz doesn't propogate as well as 2.4 because it's absorbed by water in the air and things more than 2.4ghz is. Visible light is absorbed by most things some amount.

So, the big differences are how the physical world affects the light, but in broad strokes it behaves the same way to the same situations, its just that those situations (e.g. what can reflect, refract, diffract, and absorb it) are frequency/wavelength dependent.

[k0htlane]

>> Why can’t they make visible light with an antenna? > In terms of normal radio antenna designs, you could, I think, but antennas need to be sized proportional to the wave length of the light.

There is also one more difference to mention. For antenna design in radio frequencies the metals are taken as perfect conductors—meaning that the fields inside the metal are zero. This assumption relies on the fact that electrons in the metal respond the electric field basically instantaneously. However, the optical frequencies are significantly higher, so the movement of the electron cloud in the metal is no longer instantaneous relative to the outside fields. As a result metals at frequencies of visible light are way less "metal-like" when looking from the perspective of RF antenna design, so you can't directly use the exact same approaches for designing antennas.

[pcdoodle]

Thanks for that explanation. This radio stuff is heavy duty!