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It is often stated that the scientists who discovered CMBR in the mid-1960s "heard" the radiation. Did they actually "hear" it (as in sound)? If so, what was it they "heard"?

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Yes, you can indeed hear it, and you do so by essentially the same technique you use to listen to an ordinary (non-digital!) radio: in outline what you do is listen (in the sense of 'having your system detect') to a rather narrow range of frequencies and then shift that range down to audio frequencies, which you can then listen (in the sense of 'making a sound to which your ears are sensitive') to. The very simplest way of doing this (which is pretty much what primitive AM radios do) is to listen (first sense) to a narrow range of frequencies and then essentially rectify what you hear, which gives you audio directly.

I believe that Robert Wilson and Arno Penzias did indeed actually listen (second sense) to what their antenna was hearing (first sense), not least so they could rule out obvious bogons -- if the thing was hearing something like a radar or some other artificial source this would have been immediately apparent.

There are a couple of references to this, behind some of which are audio which you can listen to!

There is a rather famous thing related to this. If you have an old analogue TV and you tune it between channels you see 'snow' -- random noise on the screen. A significant fraction of that snow (I remember 10% but I think it is less -- perhaps 1%?) is the CMB.

I think it's just amazing that you can both hear and see the CMB with really very mundane equipment.


I apologise for the two senses I have used for 'listen' and 'hear': I wrote this without realising I was doing that as these two uses are common, at least among people I worked with.

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  • $\begingroup$ the Brian Koberlein link says "video post missing" $\endgroup$
    – anna v
    Oct 4, 2018 at 3:38
  • $\begingroup$ @annav: yes, the link has been hopelessly mangled when he converted his blog. I've extracted it from the mess on his page and linked it from my answer (I am nervous about linking directly to YouTube, as I hate clicking on things which make a noise!) $\endgroup$
    – user107153
    Oct 4, 2018 at 7:25
  • $\begingroup$ A very helpful, clear, concise & informative response -- thank you. $\endgroup$ Oct 4, 2018 at 13:19
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I think the radar operators most likely actually heard it. The xcorr.net article cited above is a great explanation. Another words, among the humans who can hear it, they don't actually hear it directly but through an indirect effect on the cochlea. I for example can hear CMBR. First, Ive played the piano all my life and am very sensitive to different sounds. Second, I happened to practice a technique in yoga class called yoga nada many years ago that asked us to listen to anything we could hear, starting from obvious noises. Sounds simple but try it yourself! After a little practice, I realized i was hearing very high frequencies and very low frequencies that did not vibrate outside my head ie made no sound in the room. For years i heard an extremely low vibration, no matter where I lived, like a furnace rumbling in the basement. Had no idea what it was until i came across an article this past year (not the one above) that pointed to a small subset of humans that can hear CMBR, from studies in the 1940s. To describe it, it sounds like what white noise on an old tv set looks like, but very low, and I find it rather comforting. I've asked family members if they can hear it, but so far nobody i know can. Guess I just have a lucky head size.

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