1
$\begingroup$

This question already has an answer here:

I remember from my basic physics class 3 years ago that (every?) object naturally emits EM radiation. I also remember that temperature is directly proportional to the frequency of the EM waves. Does it then follow that any object with a low enough temperature emits waves with a very low frequency, such as radio waves? If so, are these emissions detectable with current instruments?

$\endgroup$

marked as duplicate by Yashas, ZeroTheHero, Bill N, Michael Seifert, John Rennie quantum-mechanics May 9 '17 at 17:12

This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.

1
$\begingroup$

Just some back of the envelope numbers below to elucidate some of what is involved. I could be off, and don't have an answer. I could also not be treating the main issues, so consider this an intro to an analysis.

Hopefully somebody on this site knows much more than I in this very low temperature regime.

At 1K degrees the black body radiation peaks near about 3000 microns (actually 2897.75 microns), or 3 cms, which is about 10GHz, right in the middle of the the microwave band. At 1 GHz, the peak for 0.1K, it is in the Radio band, and 0.01K is at 100 MHz or the VHF band. Pretty cold temperatures.

The power radiated is also pretty small, at the 0.1K temperature is a total power of about $10^{-12}$ watts/$m^2$, or about $10^{-9}$ milliwatt/$m^2$. That is -90 dBm/$m^2$. That's about a total power of -100 dBm 1 meter away (where you're still pretty close to the near field). For an antenna say 10 cms X 10 cm you'll get if everything is perfect, -120 dBm

The basic spectral numbers are from http://www.spectralcalc.com/blackbody_calculator/blackbody.php The other calculations easy but mine if I didn't make a mistake.

The problem is that's total power. Per GHz or so it's about a tenth, or -130 dBm. You'd need a 1 GHz bandwidth radiometer centered around 1 GHz

I have no idea if detectable, the instrument will probably have to be cooled even lower (or its own noise will overwhelm it) and I don't know that there's any existing radiometers at those freqs/wavelengths, or some other mechanism to detect it. Still, not clear in principle if there's any theoretical reasons why it might not be possible. But I could be totally ignoring something. You'd also need to electromagnetically isolate the whole thing, and make sure the isolator are also super cooled

$\endgroup$
  • $\begingroup$ Blackbody emission does not only consist of emission at the peak of the Planck function. $\endgroup$ – Rob Jeffries Jan 25 '17 at 7:38
  • $\begingroup$ Are you simply saying it's the whole distribution, or something else? $\endgroup$ – Bob Bee Jan 25 '17 at 17:45

Not the answer you're looking for? Browse other questions tagged or ask your own question.