If I take away the planets in the solar system and leave the sun alone, would the sun have less energy than before?
That isn't a measure of energy, it's a measure of flux. Depending on how you measure it removing the planets (which might temporarily eclipse the Sun if you are only measuring flux over a small area near the ecliptic plane) could change your results. I still agree with Peter's answer, though, since I wouldn't regard this as changing the Sun's energy. And you could remove any effect anyway by integrating over a sphere enclosing the Sun.sqljunkey said:watts/m^2 at some distance r.
sqljunkey said:I was trying to understand the internal structure of a star or planet when you change gravity.
sqljunkey said:I was thinking that the sun is affected to some degree by the gravity of the other planets orbiting it, like our sea is affected by the moon.
As others have noted "changing gravity" is a very broad notion. You can certainly ask about the effect of a change in gravity due to, for example, a close pass with another star. That seems to be the kind of thing you have in mind when you are talking about the effect of tides on the Sun's luminosity (too small to measure for the case of the planets, but a close encounter with another star would be another matter). The question's probably better asked in Astronomy and Astrophysics, though, because it's about solar physics not relativity.sqljunkey said:Well Ibix I was trying to understand the internal structure of a star or planet when you change gravity. A fun one was trying to imagine what would happen to the Earth if all gravity disappeared
Suppose the ancillary 0.2% mass (see above) is at the ~at the distance of jupiter. And jupiter's orbit is ##10^3## solar radiietotheipi said:but surely it would be very tiny?
etotheipi said:if you were to throw all of the planets out to infinity then part of that should manifest itself as an increase in the mass of the sun
PeterDonis said:No, it wouldn't. Such a process would involve adding energy to the planets (enough for them to escape), not the sun. It would not change the mass of the sun at all.
etotheipi said:I will take your word for it.
etotheipi said:I just remember that in one of my Physics exams we were asked to determine the increase in mass of the Earth-ball system if we perform work on the system to increase its gravitational potential energy. Maybe that is a lie
PeterDonis said:Scenario #2:
hutchphd said:how would your argument in scenario #2 play out if the "ball" were the same size as earth?
etotheipi said:The rest energy of the system
PeterDonis said:In scenario #1, you can't consider the sun and planet to be a "system", because the planet gets moved out to infinity, so there is no way to observe the "system" from outside.
PeterDonis said:In many circumstances it works fine. But it has limitations, which need to be recognized so you don't trip over them.
etotheipi said:I don't know if anyone has calculated a rough estimate of the average total gravitational potential energy of the solar system (or at least only those interactions that include the sun) but if you were to throw all of the planets out to infinity then part of that should manifest itself as an increase in the mass of the sun... but surely it would be very tiny?
And if you move the planets out further away from the sun you likewise increase the mass of the solar system. But you do not increase the mass of the sun.etotheipi said:I just remember that in one of my Physics exams we were asked to determine the increase in mass of the Earth-ball system if we perform work on the system to increase its gravitational potential energy.
sqljunkey said:Why can't you measure mass/energy in a certain region of space in GR?
sqljunkey said:I'm going to add, even though I know it is wrong, that a region of space has to have a set energy because energy is quantized.