# Is there a need to consider the relativistic mass when calculating gravitational potential energy?

The equation for gravitational potential energy is:

$$V(r) ={GMm\over r}$$

In the reference frame where $$M$$ is stationary. I am wondering for mass $$m$$ here do we need to consider its relativistic mass? And what would be the reason behind it?

• There is never a need to consider relativistic mass. It is considered a poor idea these days. In any case, you cannot do gravitational problems the way you are attempting. – m4r35n357 Aug 2 '19 at 17:21
• Relativistic mass is basically an obsolete convention: physics.stackexchange.com/a/133395/4552 – user4552 Aug 2 '19 at 17:50
• – Qmechanic Aug 2 '19 at 22:10

The answer is a big NO. I will show this with one (very naive) counter example.

Assume that we should use relativistic mass in $$v=-\frac{GMm}{r}$$. Let's say S is inertial observer who moves in $$z$$ direction with veloctiy $$u$$. Sun is at the rest at $$(x,y,z)=(0,0,0)$$ and Earth orbits the Sun in $$xy$$ plane with velocity $$w$$. Now, from S's perspective Earth and Sun move in $$z$$ direction with velocity $$-u$$ and based on our assumption we should use relativistic mass for Earth and Sun. The velocity of Earth in $$xy$$ plan is small enough compare to $$-u$$ so it's safe to neglet it for now. Suppose that Sun's invariant mass is $$M$$ and Earth's invariant mass is $$m$$. According to S, their relativistic mass will be $$M'=\gamma_uM$$ and $$m'=\gamma_um$$ respectively. If we put them in $$v$$ we will see that $$v'=-\gamma_u^2\frac{GMm}{r}$$ which is a lot greater than $$v$$. The problem is, Earth's velocity in $$xy$$ plane will become slower by a $$\gamma_u$$ factor from S's point of view. You can see it from velocity addition formula. Now, $$F'=-\nabla_rv'$$ So $$F'_g=\gamma_u^2\frac{GMm}{r^2}$$.

Earth-Sun system is in equilibrium (that's centrifugal force is equal to gravitional force in Sun's frame hence Earth doesn't change its orbit, roughly), So in Sun's frame we have $$F_g-F_c=\frac{GMm}{r^2}-\frac{mw^2}{r}=0$$. According to first special relativity principle, Earth should not change its orbit in other inertial frames like S, that's $$F'_g-F'_c=0$$, but unfortunately it's not the case because $$F'_g-F'_c=\gamma_u^2\frac{GMm}{r^2}-\frac{mw^2}{\gamma_ur}\ne 0$$ clearly. Which shows that assuming relativstic mass is not a good idea.

P.S: Actually, instead of velocity addition formula, one can check Earth's motion in $$xy$$ plane from S's point of view with time dilation formula, That's, Earth orbits the Sun every $$365$$day from Sun's point of view, So from S's perspective it should take $$\gamma_u 365$$day for earth to complete its rotation. That would mean Earth's angular velocity is decreased by a gamma factor. If you want to consider relativistic effect, you will need Einstein field equations.

• Thank you for your response. I got your point but here is one confusion I have: following your argument, in Sun's frame, the Earth is not stationary with respect to the sun, so should we have a gamma factor for the potential energy? I am referring to this line: $F_g-F_c=\frac{GMm}{r^2}-\frac{mw^2}{r}=0$. I am not sure if that changes anything, but is my point valid? – Yiyang Zhi Aug 13 '19 at 22:35
• @YiyangZhi For the sake of counter example (i.e assuming relativistic mass in $v$ although it's wrong) yes your are right, but remember, the velocity of the Earth in $xy$ plane is very small, that's $\gamma_\omega \approx 1$ that's why I didn't write it in sun's frame, I didn't even consider it in S frame (i.e $\gamma_{u+\omega'}\approx \gamma_u$) too. – Paradoxy Aug 14 '19 at 8:11
• Well if we were to consider them, still we could see that the first principle of relativity would be violated. That's why we should only use invariant mass in $v$. The proper way to calculate relativistic force, in SR at least, without considering GR is different. See en.m.wikipedia.org/wiki/Acceleration_(special_relativity) check Acceleration and force section. Because Earth moves at $x,y,z$ axes at the same time w.r.t S, calculation is a bit of hassle. – Paradoxy Aug 14 '19 at 8:11

General relativity does not describe gravity using a scalar potential as in Newtonian gravity. GR's closest equivalent to the Newtonian potential is actually the metric.

No, you don't get a description of the sources of the gravitational field in GR by considering relativistic mass rather than mass. The source of the gravitational field in GR is the stress-energy tensor, not a scalar mass or mass-energy density. The mass-energy density is one of the 16 components of the stress-energy tensor.

I would start with the metric line element for the nonrotating central gravitational field $$ds^2=(1-2m/r)dt^2-(1-2m/r)^{-1}dr^2-r^2(d\theta^2 + sin^2\theta d\phi^2),$$ where $$m=GM/c^2$$. The variation of this is zero, where $$ds$$ is the proper time and is defined at an extremum. It is really a form of action. We can further see this if we divide by $$ds^2$$ with the four-velocities defined by $$U_t=dt/ds$$, $$U_r = dr/ds$$, $$U_\theta = d\theta/ds$$ and $$U_\phi=d\phi/ds$$. This gives $$1=(1-2m/r)U_t^2-(1-2m/r)^{-1}U_r^2-r^2(U_\theta^2 + sin^2\theta U_\phi^2),$$ and clearly the variation of $$1$$ is zero. Now factor out $$U_t=dt/ds$$ $$1=[(1-2m/r)-(1-2m/r)^{-1}v_r^2-r^2(v_\theta^2 + sin^2\theta v_\phi^2)]U_t^2,$$ where we have the coordinate velocities $$v_r = dr/dt$$, $$v_\theta = d\theta/dt$$ and $$v_\phi=d\phi/dt$$.

Now let us for simplicity consider a nice circular orbit so that $$v_r=0$$ and we shall set this on a plane with $$\theta=\pi/2$$ and is constant. Now we have $$1=[(1-2m/r)-r^2\omega^2]U_t^2,$$ where $$v_\phi=\omega$$ the angular velocity. We can see that obviously $$U_t=[(1-2m/r)-r^2\omega^2]^{-1/2}$$. This is a sort of gravitational Lorentz $$\gamma$$-factor.

Now I will start writing this with $$m=GM/c^2$$ and us small $$m$$ for a test mass. I now multiply by this mass-energy $$mc^2$$ $$mc^2=[(mc^2-2GMm/r)-mr^2\omega^2]U_t^2.$$ Now do a little algebra $$\frac{1}{2}mc^2(U_t^2-1) = \left(\frac{1}{2}mr^2\omega^2+GMm/r\right) U_t^2.$$ This is a sort of Lagrangian. For $$U_t = 1$$ we obtain a Newtonian type of result.

So this illustrates some of the connections between spacetime transformations and $$-GMm/r$$. In some ways the potential energy is really replaced with time dilation. The deeper a clock is in a gravity well the slower it appears. This is the main role of $$-GMm/r$$ that in a weak gravity limit is the Newtonian gravity potential.