Sign up ×
Physics Stack Exchange is a question and answer site for active researchers, academics and students of physics. It's 100% free.

Dramatic title, I know. But it's shorter than Measuring a person's effective mass through radiation and comparing it to their weighed mass and I figured this would get people's attention.

I just thought of this a while back. If we were to collect the EM radiation a person emits, we could find a measure for their rest energy and therefore we should be able to use $E = Mc^2$ to calculate a corresponding mass $M$. Now I reasoned that this would not yield the same result as an actual measurement of their 'weighed mass' $m$ with a weighing scale due to the fact that humans are living creatures and there's loads of biochemical processes going on inside of us to try and keep our temperature at a certain stable level, among other things.

Then I started thinking about the meaning of the mass we could calculate from the radiation and more specifically about the exact meaning of the difference $M-m$. If all of my above reasoning is correct, I should think this gives us some measure for the amount of energy (maybe power might be a more appropriate quantity) we need to keep us alive. Obviously, in this context it might make more sense to talk about energies (so to transform $m$ to an energy $e$ instead of transforming $E$ to a mass $M$) but hey, they're equivalent and I'm quirky.

Anyway, to get to my question: is there any truth in my reasoning or have I made a mistake? If there is truth to it, I'm guessing other people have explored this already and I'd be interested in any results.

share|cite|improve this question

1 Answer 1

up vote 2 down vote accepted

No, that is not correct. A person will emit thermal radiation that corresponds to his/her surface temperature, and is fuelled by the energy reserves being burned to actively maintain that temperature. As such, measuring the radiated power would, in equilibrium, give you a good idea of how much food someone is burning up. (Beware, though! A sizable fraction of the energy one consumes and stores as ATP goes into building stuff - making proteins and such molecules with a higher internal energy than their constituents - and this will be an additional energy outlet in that budget.)

However, this has nothing to do with the person's total mass. (Consider, for example, someone swallowing a very dense block of lined lead, that does not participate in biological processes. This adds to the total mass but cannot influence the radiated IR.) Schade!

share|cite|improve this answer
That makes sense, I suspected I was oversimplifying matters. And of course I should have realized the radiation would correspond to the surface temperature. I guess I got carried away in my enthusiasm too easily. Oh well, thanks for your answer. :) – Wouter Feb 13 '13 at 22:23

Your Answer


By posting your answer, you agree to the privacy policy and terms of service.

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