# Why don't we emit radio waves

If our temperature is already above the necessary temperature to emit radio waves? At our temperature we emit mainly infrared right?

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cjl
We do emit radio. It's just much, much fainter than the infrared we emit. Similarly, the sun emits much more infrared per square meter than we do, even though it peaks in the visible.

At our temperature, we mainly emit infrared radiation correct?

Then why do we see each other in visible light? Here's my hypothesis:
Left to ourselves, meaning no light present (at night) we are not seen because we don't have enough energy in us to emit visible light, we emit infrared at the most. That is why even if there is no light, we can still see if we have infrared goggles.

We do emit radio. It's just much, much fainter than the infrared we emit. Similarly, the sun emits much more infrared per square meter than we do, even though it peaks in the visible.
Then we are basically antennas? Why can't we be used as antennas then?

Then we are basically antennas? Why can't we be used as antennas then?
Lemme rephrase that, why do antennas have the ability to send out radio waves, if we emit radio waves as well?

To suggest that biological systems, emitting IR, must ALSO emit lower frequencies is false.
There is no natural requirement for a biological system emitting a specific wavelength to be required to emit all the other sub-wavelengths down to zero.

To suggest that biological systems, emitting IR, must ALSO emit lower frequencies is false.
There is no natural requirement for a biological system emitting a specific wavelength to be required to emit all the other sub-wavelengths down to zero.
Took it right outta my mouth, why is this?

cjl

That's reasonably accurate.

The reason we see each other in visible light is because we see the light reflected off of each other, not because we see the light emitted. The reason this is in the visible range is because that's where the sun's emission peaks.

Andy Resnick
If our temperature is already above the necessary temperature to emit radio waves? At our temperature we emit mainly infrared right?
How do you know we don't?

Humans can be clearly imaged using the 8-12 um (LWIR) band, which corresponds closely to the peak in thermal emission.

I know that humans are opaque to millimeter waves (new airport full body scanners), and o we emit in the millimeter wave as well. But at some point we become transparent, and so by Kirchoff's law we won't emit in that waveband.

That's reasonably accurate.

The reason we see each other in visible light is because we see the light reflected off of each other, not because we see the light emitted. The reason this is in the visible range is because that's where the sun's emission peaks.
Great, I have some questions I would like to ask you instead of starting a new thread, would you mind?

How do you know we don't?

Humans can be clearly imaged using the 8-12 um (LWIR) band, which corresponds closely to the peak in thermal emission.

I know that humans are opaque to millimeter waves (new airport full body scanners), and o we emit in the millimeter wave as well. But at some point we become transparent, and so by Kirchoff's law we won't emit in that waveband.
Kirchoff's law BRB

Took it right outta my mouth, why is this?
As opposed to phonon's(sound waves), photons do not have interactive, potentially harmonic properties.

Andy Resnick
Kirchoff's law BRB
I don't understand?

I don't understand?
I went to research it.

To suggest that biological systems, emitting IR, must ALSO emit lower frequencies is false.
There is no natural requirement for a biological system emitting a specific wavelength to be required to emit all the other sub-wavelengths down to zero.
We emit infrared because we aren't at absolute zero, not due to any unusual property of biological systems. The radiation our bodies emit more or less follows a black body spectrum, and we do indeed emit at lower frequencies, all the way down to zero. Your statements suggest the IR emissions are special, when it is really the gaps and other variations due to our deviations from black body emitters that are special. Your statement applies to bioluminescence, but not to body heat.

Lemme rephrase that, why do antennas have the ability to send out radio waves, if we emit radio waves as well?
The question doesn't make sense. Why would the fact that we emit radio waves make you think antennas can't?

But anyway, note that there's two types of emission here. You and an antenna both emit thermal radiation in a range of frequencies including the radio band but peaking in the infrared spectrum. Heat an unpowered antenna up to body temperature in the sun, and its emission spectrum will be quite similar. These emissions are very weak, though. An object at human body temperature won't be emitting particularly large amounts of power, and most of that will be in the infrared range.

Radio transmitters do not produce radio waves by thermal emission. They do it far more directly, by literally pushing the electrons in the antenna back and forth at the frequency desired. Accelerating charges radiate, and by doing this you can emit very large amounts of power in a very narrow portion of the spectrum.

Then we are basically antennas? Why can't we be used as antennas then?
Because it could trigger painful sparks every now and then so no, thank you.

We should be emitting radio waves too , or at least i think so. It's just we emit much more infrared and infrared has a lot more energy than radio waves, infrared dominates.

To be totally fair, most night-vision goggles are based around photomultipliers. They also have some IR sensitivity, but that is mainly so that they can use an IR light source to "illuminate" an area without being seen. But that's an engineering point. You are more or less correct about the underlying emission phenomenon.

You are conflating two concepts here:

An antenna is something that transmits and receives COHERENT radio transmissions. This is done by applying electromotive force of the appropriate phase and frequency, blah blah blah.

Thermionic emission is what the human body does, with respect to radio waves. Just the heat of the body causes some ultra-long-wave IR (AKA RF) waves to come off. They are not, in any way, coherent. Similarly, the body is not a coherent absorber. The energy we absorb from RF waves is just dissipated as heat, not conducted down our length to a detector. A lot of the art of antenna design is optimizing it for sensitivity to what we are interested in.

berkeman
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(Moderator note -- 2 similar threads merged)