All Questions
30
questions
1
vote
1
answer
89
views
How does gravity act and propagate in a 2+1D universe?—Newtonian versus general relativity
In a hypothetical 2+1D universe:
if we apply the Newtonian concept of gravity, we might expect that the gravitational force between two mass points with a distance of $r$ would diminish linearly with ...
- 618
0
votes
3
answers
170
views
When does Newtonian physics fail?
When does Newtonian physics fail?
The answer by Zo the Relativist to the question How accurate is Newtonian Gravity? includes the statement:
The key point is that Newtonian physics fails when, ...
1
vote
3
answers
197
views
Equivalence Principle in Newtonian Physics vs GR: A Thought Experiment
I have a question regarding the equivalence principle as it applies in Newtonian Physics and General Relativity. Consider a thought experiment involving a free-falling elevator.
Inside the elevator, ...
- 21
5
votes
1
answer
272
views
ONLY non-relativistic limit of general relativity
From my study of GR I learnt that to reach the “Newtonian” limit of the Einstein field equation we have to assume:
weak field $g_{\mu\nu} = \eta_{\mu\nu} + \epsilon h _{\mu\nu}$ with $\epsilon <&...
- 191
2
votes
2
answers
126
views
Reason for differentiation between inertial and gravitational mass
To my knowledge, inertial and gravitational mass are equivalent according to the weak equivalence principle. I believe modern experiments have shown that $\lvert \frac{\mu}{m}-1\rvert\leq 10^{-15}$, ...
- 39
0
votes
3
answers
95
views
How can weight be equal in the equivalence principle?
According to Einstein, weight will be the same as here in earth in a rocket going at 1G far from gravity fields. If we propulse an object here it will feel two forces, F=mg and F=ma of the thrust, but ...
user189508
0
votes
2
answers
86
views
Does general relativity predict different forces on aircraft than Newtonian mechanics?
When taught how airplanes fly, we are probably always given the Newtonian mechanism: that four forces act on an airplane; lift and weight on the vertical axis, and thrust and drag on the horizontal ...
- 303
0
votes
1
answer
316
views
Newtonian limit of geodesic equation and Euler-Lagrange equations
As far as I know the Euler-Lagrange (EL) equations $$\frac{\partial L}{\partial q^m}-\frac{d }{dt}\frac{\partial L}{\partial \dot{q}^m}=0 $$ are covariant time dependent coordinate transformations,
$$...
-1
votes
2
answers
181
views
Has the Cavendish Experiment ever been Conducted in Zero Gravity?
Given the shortcomings of applying the gravitational equation to distant astral objects, I am wondering if it has ever been attempted to replicate the Cavendish Experiment in a zero gravity scenario (...
- 109
1
vote
0
answers
96
views
Deriving Newtonian Gravity from General Relativity? [duplicate]
Let me provide some insight, I was studiying about General Relativity, and I read that it "encapsulates" Newtonian Gravity. I understand this perfectly, because, Einstein used lots of ideas ...
- 1,525
0
votes
1
answer
40
views
Making the distinction between position and time in a gravitational system
Say we have a large spherical shell of thickness $R$ floating in space. The shell has a hole of negligible radius punched into one side that allows for a ball of mass $m$ to pass through. Say now that ...
- 103
1
vote
1
answer
766
views
Formula of black hole gravitational sphere of influence
Well,
I want to derive the formula
$$ r = \frac{GM}{\sigma^{2}} $$
which happens to be the radius of the gravitational sphere of influence of a supermassive black hole inside a galaxy. How can I do ...
- 55
1
vote
0
answers
33
views
Can a system have a negative energy? What is the energy of black hole compared to the normal star? [duplicate]
I thought about gravitational field. In Newtonian mechanics, gravitational energy between two matter is $U=-G\frac{M_1 M_2}{R^2}$ when mass of each matter is M1 and M2, having a distance R. With this ...
- 295
2
votes
1
answer
143
views
Rigorous derivation of non-relativistic limit in cosmology?
The standard way to derive the non-relativistic limit and its corrections in general relativity is to formally expand Einstein equation in powers of $1/c$ around a background space-time. The non-...
- 21
7
votes
0
answers
339
views
Question about the Newtonian limit of general relativity
I ran into something peculiar while attempting to carefully derive the Newtonian limit of general relativity, specifically for the geodesic equation.
To set it up, we assume that the curve $q:[a,b]\...
