If we make a square box with lead thick walls (1 meter thick or more), can me have a space inside this box 100% (or 95%) free of Cosmic microwave background radiation or it is something which we cannot isolate and it is property of the space and even in the box like mentioned above it will appear again somehow?


Clarification: I assume that I can distinguish between CMB and blackbody radiation of the box itself (maybe it is not possible, but it is theoretical questions anyway. Let's assume that I can :)). I can make box temperature high (500K, 1000K) or low (0.1K), that blackbody radiation of the box itself will not interfere with CMB.

  • $\begingroup$ What is the temperature of your box? $\endgroup$
    – user137289
    Aug 10, 2018 at 10:12
  • $\begingroup$ I put the clarification in UPDATE 1. I think it answers your question. $\endgroup$
    – Zlelik
    Aug 10, 2018 at 11:20
  • $\begingroup$ Boxes (and whole rooms) like that are a commercial product: ramayes.com/Shield_Rooms.htm They are not made of lead. They typically are made of sheet metal, or lined with electrically conductive foil, and then there is an additional lining of ferromagnetic tiles. $\endgroup$ Aug 10, 2018 at 11:43

1 Answer 1


The Cosmic microwave background (CMB) has spectrum of blackbody radiation, at about T=2.7K. The lead box will equalize to T, and so there will be blackbody radiation of 2.7K inside the cavity.

The CMB is not a property of space. It's a leftover from hot Big Bang.

You could artificially cool the lead box to some temperature lower than T (but not quite all the way down to zero). There would then be a blackbody spectrum inside the box at that lower T.

  • $\begingroup$ Thanks for reply. I do not mean blackbody radiation from the box itself. I mean real CMB, which left from Big Bang. Do I correctly understand, that your answer is yes, inside this box, there will be no CMB (or most of it will be isolated)? For example, if my box has temperature 300K, then inside the box there will be blackbody radiation for 300K, but almost no blackbody radiation with 2.7K. Or if I make this box very cold like 0.1K, then it will be almost no radiation inside, except this 0.1K blackbody radiation. $\endgroup$
    – Zlelik
    Aug 10, 2018 at 11:15
  • $\begingroup$ Yes, Zlelik, that's correct: the spectrum in the cavity reflects only the temperature of the box itself. If you leave the box alone (no more heating/cooling), then eventually you get the same spectrum as the CMB, which has a blockbody spectrum of 2.7K. $\endgroup$
    – bernander
    Aug 10, 2018 at 11:29
  • 3
    $\begingroup$ To stop microwaves of this intensity aluminium foil is enough. $\endgroup$
    – my2cts
    Aug 10, 2018 at 11:41

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