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3h
answered Is expanding universe adding potential energy?
2d
reviewed No Action Needed How common are exoplanets in distant orbits?
Jul
26
comment Do all orbiting bodies eventually collide?
@Mehrdad their refocusing and absorption by the system is of nigh-infinitesimal probability. But to answer your question, the formula given is based on an a circular orbit in an otherwise empty and asymptotically flat spacetime. The contributions to the emitted radiation has "instantaneous" terms (really dependent on the retarded position) and "non-local" terms (dependent on prior history), which are smaller. Ignoring the latter and taking the leading-order post-Newtonian approximation of the should get us the result in the answer.
Jul
23
reviewed No Action Needed First time visit to dark sky stargazing site: what do I need?
Jul
23
reviewed No Action Needed Can we see Earth by looking into space?
Jul
23
reviewed No Action Needed Where might a semi proficient amateur analyst participate in meaningful astronomical efforts
Jul
16
reviewed No Action Needed Real images (not “artist concept”) by NASA
Jul
16
reviewed Reviewed How many galaxies are there in the Hercules–Corona Borealis Great Wall?
Jul
16
comment How do you determine which star, between two, is more luminous?
Consider Stefan–Boltzmann law. But the problem as stated seems highly artificial.
Jul
14
reviewed Leave Open Official Definition of Satellite?
Jul
14
reviewed No Action Needed Why are retrograde orbits more stable than prograde ones?
Jul
14
reviewed No Action Needed If Venus and Mars changed places, would we then have 3 habitable planets?
Jul
13
reviewed No Action Needed How to calculate the moon phase
Jul
13
reviewed Close Energy required for light speed
Jul
13
comment Energy required for light speed
Short answer: it 'would take' an infinite amount of energy for a massive object to move at lightspeed. But photons have zero mass.
Jul
9
comment How small would you have to crush an object for it to become a black hole?
Thank you, that's awesome!
Jul
8
reviewed No Action Needed Can magnetism escape a black hole?
Jul
8
comment How small would you have to crush an object for it to become a black hole?
If you have some assumptions about the matter and object, you can put more detailed limits. For example, if your matter cannot support superluminal sound waves, then a shell of it would only need to be smaller than $25/24$ of its Schwarzschild radius in order to necessarily become a black hole, and a uniformly-dense ball of it smaller than $4/3$ of its Schwarzschild radius. Outside toy problems like that, though, it becomes a tough question. ... I wonder how small, e.g., a neutron star can be given a fixed mass before it necessarily collapses, and how this depends on assumed EOS, etc.
Jul
6
reviewed No Action Needed What is the axial tilt of a planet measured relative to?
Jul
6
reviewed No Action Needed What is the axial tilt of a planet measured relative to?