Question

1. Since you are made mostly of water, you are very efficient at absorbing microwave photons. (As in the Ryden 2.1 formative question, you can assume that you are spherical.)

a. What is the rate, in watts, at which you would absorb energy from the CMB?

b. If you were in intergalactic space, how many CMB photons would you absorb per second?

Answer 1

First, we will solve part (b)

the number density of CMB photons is around 4.11e8 / m³

so,

number of photons / second = n * A * c

where A is area and c is speed of light. Let's take area as 1 m²

then

number of photons / second = 4.11e8 * 1 * 3e8

number of photons / second = 1.233e17 / second

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Now, to solve part (a)

the mean energy per CMB photon ( as per textbook) is 1.02e-22 J

so,

rate of energy absorbed = 1.02e-22 * 1.233e17

rate of energy absorbed = 1.257e-5 watts