1. The problem statement, all variables and given/known data This isn't a homework problem, but a question my physics teacher asked us in school, more than 25 years ago. I've thought about this from time to time, but I never found anyone to ask whether my explanation is at least plausible. Imagine you had a machine that could convert matter into energy and vice-versa. Now you take a mass, say, 1kg of iron, and convert it into energy that you receive as photons. Direct the stream of photons to the ISS, where you use another machine to convert it back to 1kg of iron. Obviously, the 1kg in orbit has a greater potential energy that it had back on earth. Where did this energy come from? 2. Relevant equations None. No exact (numerical) answer is expected. Assume no atmosphere between the earth and the ISS, and assume your machines convert mass to energy and back without losing either during the process. 3. The attempt at a solution It's obvious that the mass on the ISS must be less than it was on earth, so it must have decreased somewhere "en route" while it was in the form of light. The number of photons can't have decreased, so it's their frequency that must have. The beam of light must have been red-shifted, but how? My only explanation is that this happens every time an electromagnetic wave moves away from a source of gravity. Hence, to exaggerate a bit, if you point a ray of blue light at the moon, an astronaut there would see it as red. And, conversely, if this astronaut shines a red light on us, we'd see it as blue. (That's VERY exaggerated; I don't know whether the change in frequency would be noticeable at all to a human eye.) Is this what my teacher was thinking of? Or am I missing something?