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 Jan 13th 2018, 11:02 PM #1 Senior Member     Join Date: Feb 2017 Posts: 205 Simple explanation. It is really simple putting 2 + 2 to get 4. The Danish astronomer, Olaus Roeme in 1676, first successfully measured the speed of light. So it was not einstein who measured the speed of light. So the logic is simple see when it was proved that gravity is a fictitious force by equating the two equations F=ma and universal law of gravitation. So gravity formula was F=ma. So what does that prove? It proves that there has to be a constant motion in a straight line. Straight line by observing when you throw a ball in air vertically it falls down in a straight line. Will get back to this later on. Now constant speed: When can there be a constant speed it can only be there if acceleration is zero. Last edited by avito009; Jan 13th 2018 at 11:08 PM.
 Jan 13th 2018, 11:34 PM #2 Senior Member     Join Date: Feb 2017 Posts: 205 Later. Need time to reflect.
 Jan 14th 2018, 12:57 AM #3 Senior Member   Join Date: Nov 2013 Location: New Zealand Posts: 552 Newton's law of gravitation is F = GmM/r^2 which is an approximation compared to Einsteins formula but still useful for calculating orbits and most things. F=ma is newton's second law for changing an objects inertia and is not explicitly relating to a gravitational field. It could be an electric field or magnetic field even. Its just a law that explains how "resistant" an object with mass is to having its velocity changed. __________________ Burn those raisin muffins. Burn 'em all I say.
Jan 14th 2018, 05:35 AM   #4
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 Originally Posted by avito009 It is really simple putting 2 + 2 to get 4. The Danish astronomer, Olaus Roeme in 1676, first successfully measured the speed of light. So it was not einstein who measured the speed of light.
Who ever said Einstein was the first to measure the speed of light?

[quote]So the logic is simple see when it was proved that gravity is a fictitious force by equating the two equations F=ma and universal law of gravitation. So gravity formula was F=ma.[quote]
"F= ma" is true for any force, not just gravity or what you are calling a "fictitious force" (I think you are using "fictitious force" in a non-standard way. "Centrifugal force" is a "fictitious force". Even in general relativity, gravitational force is not.).

 So what does that prove? It proves that there has to be a constant motion in a straight line.
A "constant motion of what in a straight line"? Motion is at a constant speed in a straight line as long as there is no external force.

 Straight line by observing when you throw a ball in air vertically it falls down in a straight line. Will get back to this later on. Now constant speed: When can there be a constant speed it can only be there if acceleration is zero.
Yes, the definition of "acceleration" is "rate of change of speed". It there is no change in speed then there is 0 acceleration.

Jan 14th 2018, 08:52 AM   #5
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 Originally Posted by avito009 It is really simple putting 2 + 2 to get 4. The Danish astronomer, Olaus Roeme in 1676, first successfully measured the speed of light. So it was not einstein who measured the speed of light. So the logic is simple see when it was proved that gravity is a fictitious force by equating the two equations F=ma and universal law of gravitation. So gravity formula was F=ma. So what does that prove? It proves that there has to be a constant motion in a straight line. Straight line by observing when you throw a ball in air vertically it falls down in a straight line. Will get back to this later on. Now constant speed: When can there be a constant speed it can only be there if acceleration is zero.
I'm not trying to get on your case or bury you with notation but I don't think you are truly appreciating the complexities here. So...

The Einstein field equations in GR are as follows:
$\displaystyle R_{\mu \nu} - \frac{1}{2} R~g_{\mu \nu} = \frac{8 \pi G}{c^4} T_{ \mu \nu}$

I'm going to ignore T, the energy-stress tensor in what follows. (That means there's no mass in the region I'm defining but mass is somewhere in a larger space.) For convenience as well as the fact that I'm not terribly comfortable with it.

The scalar curvature is
$\displaystyle R = R^{\mu}_{\mu}$

and the Ricci tensor is
$\displaystyle R_{\mu \nu} = R^{\kappa}_{ \mu \kappa \nu}$

and the Riemann tensor is
$\displaystyle R^{\alpha}_{\beta \gamma \delta} = \Gamma ^{\alpha}_{\beta \delta,\gamma} -\Gamma^{\alpha}_{\beta \gamma , \delta} + \Gamma^{\mu} _{\beta \delta} ~ \Gamma ^{\alpha}_{\mu \gamma} - \Gamma ^{\mu}_{\beta \gamma} ~ \Gamma^{\alpha}_{\mu \delta}$

and the connection is
$\displaystyle \Gamma ^{\alpha} _{\mu \nu} = \frac{1}{2} g^{\alpha \delta} \left ( \partial _{\nu} g_{\delta \mu} + \partial _{\mu} g_{\delta \nu} - \partial _{\delta} g_{\mu \nu} \right )$

where $\displaystyle g_{\mu \nu}$ are the components of the metric. (I hope I've got all the components right!)

The topic drives even the relativists nuts sometimes. The base of the derivation of the field equations comes from principle of equivalence, which is very simple but there's really not much simple about the field equations.

-Dan

PS Gravity is not a fictitious force. We can look at it from the standpoint that gravitational force is nothing more than a bunch of equations that mimic the non-Reimannian structure shown above or we can say that matter actually bends space-time. This is an ongoing Philosophical topic.
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