Yes, inverse problems are hard. And not always possible in practice. See, for example, https://www.ams.org/publicoutreach/feature-column/fcarc-199706 https://ww

Yes, inverse problems are hard. And not always possible in practice. See, for example, https://www.ams.org/publicoutreach/feature-column/fcarc-199706 https://www.ams.org/publicoutreach/feature-column/fcarc-1997... for a case where one isn't possible. That said, the gravity technique is one that actually gets used today. With better precision, it can be even more useful than it already is.