# Tag Info

33

Physics does not answer existential problems. It gathers data and observations and models them with mathematical equations and functions, and then can explain the data with the model and predict new observations. This has been going on for centuries, and what we see if we study the history of physics is that there are regions of validity for the ...

26

For visible stars, the answer is no. In Newtonian physics, a star that would pull something travelling at light speed back to itself, i.e. a star for which the escape velocity were $c$, was called a dark star and seems to have been first postulated by the Rev. John Mitchell in a paper to the Royal Society in London in 1783. The great Simon Pierre de Laplace ...

19

The only experiment I know of was done by the ALPHA team at CERN. The results are published in this paper. The error bounds are huge - all the team were able to say is that the upper limit for the gravitational mass of antihydrogen is no greater than 75 times its inertial mass! However I believe an updated version of the experiment, ALPHA2, is in progress ...

16

The electromagnetic field tensor $F_{\mu\nu}$ which encodes all the information about the electric and magnetic field, certainly contributes to the energy-stress tensor $T_{\mu\nu}$, which appears in the Einstein Field Equations: $$G_{\mu\nu}= 8\pi G T_{\mu\nu}$$ The left hand side of this equation encodes the geometry of spacetime, while the right hand side ...

12

Yes the effect is real, potentially at least, but no it's not measurable. As an aside, the redshift of light by its gravitational interaction with (homogeneous and isotropic) dust is exactly what the FLRW metric predicts, but this clearly isn't what the question means. The gravitational field of a beam of light is calculated in the paper On the ...

10

Magnetic effect on gravity: In technical terms: yes. For practical purposes: no. As Danu states, a magnetic field is a form of energy, and Einstein showed that energy can be equivalent to mass. A sufficiently large amount of energy (of any kind) collected together will produce measurable gravity (aka bending of space-time). This source puts the total ...

9

Gravity definitely does exist. Einstein did not provide a model without gravity, he simply found a new way to think about it. Gravity, as we experience it, is a consequence of bodies moving along so-called geodesics, which can in simple terms described as "shortest paths through spacetime". The effect of gravity now arises if there is motion along geodesics ...

9

The key to answering this question is the Goldschmidt classification of elements. Thirteen of the long-lived elements are siderophilic; they preferentially bind to iron. Those thirteen elements are significantly depleted in the Earth's crust compared to their prevalence on meteors, asteroids, and the Sun. This list of thirteen does includes rhenium to gold, ...

9

The source of gravity is not mass, but stress-energy-momentum, so you are correct that the energy converted in this process already has gravity and that that gravity is only rearranged The change in the gravitational field needs time to propagate, though, and this does indeed happen at the speed of light.

5

Your position that mass measurement is done by measuring gravitational force is not quite correct. A balance measures the mass of an object by comparing the force of gravity on the mass in question to the force of gravity exerted on a reference mass. In some cases there is also a factor based on the geometry of the scale. The measurement is based on the ...

4

I think you are reading a lot into what is a minor distinction. Strictly speaking I suppose the gravitational potential is the energy per unit mass, i.e. $m=1$ in your first equation, while the gravitational potential energy is the potential times the mass. In practice no-one I know has ever bothered to make the distinction because it's usually obvious what ...

4

The closest I know of to this is the C-metric, which describes a pair of black holes accelerating away from each other. I couldn't find any nice articles on the Internet, but the C-metric is described in Chapter 14 of Exact Space-Times in Einstein's General Relativity by Jerry B. Griffiths, Jiří Podolský. You can find a scan on Google books.

4

As well as the antihydrogen experiments, ALPHA, AEGIS and GBAR that were mentioned in other answers, there are a couple of other experiments, though they haven't had any results. In the 60's, they tried the obvious thing of dropping positrons down a metal tube (paper), but it didn't work, for the subtle reason that the electrons in the metal sag under ...

4

The criterion for gravitational radiation is (conjectured to be, pending direct evidence) a changing quadrupole moment in the mass distribution, so an accelerating mass distribution does not always radiate, but can do so if the acceleration changes the quadrupole moment. This is in contrast to electromagnetic radiation, which occurs when the charge ...

3

The gravitational analogue of magnetism is called "gravitomagnetism" (and the general mathematical analogy between Maxwell's equations and gravitation in general relativity is called gravito-electromagnetism), which deals with the gravitational interactions of currents of mass/energy, just as magnetism deals with interactions of currents of charge. According ...

3

This is due to spinning forces, such as centrifuged force. Remember that galaxies form (at least the regular ones) from the spinning of matter around a galaxy nucleus. So, you're not wrong. Gravity is isotropic, but in this cases the spinning forces are the definition to the galaxy form.

3

Hint: You're far enough from the ground that the formula $mgh$, which is an approximation, doesn't really apply anymore. What is the true formula for gravitational potential energy?

3

To be exact Einstein made a claim that it is gravity that curves space-time. You can follow his reasoning in his "Relativity: The Special and General Theory." Einstein started off with comparing acceleration caused by gravity to acceleration in a lift (assuming it moves with accelerated motion) going up. He claimed that these two accelerations are ...

3

Here are couple of references that describe professional uses of a post-Newtonian formalism to model the planets and the Earth's Moon: Standish, et al. "Orbital ephemerides of the Sun, Moon, and planets," Explanatory Supplement to the Astronomical Almanac (1992): 279-323. The relevant equation is 8-1 on page 3. Petit and Luzum (eds.), "IERS Technical Note ...

3

Ultimately, the only thing that can explain a physical theory is a better theory. Newton's force law used to be the entire theory of gravitation up until the early 1900's. Newton himself had no explanation for what "caused" the force. Einstein gave us General Relativity, which describes how gravity is the geometry of space and time. In GR, energy and ...

3

The first definition of $\mu=GM$ is the standard definition of the SGP. The second one comes from the velocity of a circular orbit. If you have an object in a circular orbit of radius $r$ and velocity $v$ around a body of mass $M$, then the velocity is given by $$v=\sqrt{\frac{GM}{r}}$$ From this you can see that $rv^2=GM$ for circularly orbiting objects. ...

3

Newton's first law does not apply to objects, but to observers. If you are an inertial observer, then you will see everything that is not acted upon by a force travelling in a straight line. There's no qualifier on the everything here - if it is not travelling in a straight line, it has a force acting upon it. Non-Newtonian fluids derive their name almost ...

3

I too was confused by this difference between gravity and electromagnetism. Hopefully the following clears things up. The gravitational potential a distance $r$ from a mass $M$ is $$\phi_g=-\frac{GM}{r},$$ the gravitational field is $${\bf g} = - \nabla \phi_g,$$ and the gravitational potential energy (of two masses $M$ and $m$ separated by a distance ...

3

It depends on what direction you assign to be positive in your coordinate system. To avoid confusion, just remember which direction acceleration is acting and which direction you assigned to be positive.

3

Actually, they are still currently sinking to the core. Earth's internal heat comes from a number of sources, and one of these is the release of gravitational energy from the heavy elements migrating further toward the center. A similar statement holds for other planets. This isn't the majority of the source of heat. Other sources are the original thermal ...

2

I realised why this happens. Objects moving at different speeds have different trajectories in spacetime, so they naturally follow different geodesics. This is explained very well here: http://curious.astro.cornell.edu/question.php?number=649

2

I) In Palatini $f(R)$ gravity, the Lagrangian density is $${\cal L}~=~ \sqrt{-g} f(R),$$ with $$R~:=~ g^{\mu\nu} R_{\mu\nu}(\Gamma),$$ and where $\Gamma^{\lambda}_{\mu\nu}=\Gamma^{\lambda}_{\nu\mu}$ is an arbitrary torsionfree$^1$ connection. II) As OP mentions, the word Palatini refers to that the metric $g_{\mu\nu}$ and the connection ...

2

Since gas molecules are affected by gravity, wouldn't that make gas molecules at higher than average elevation slower (at the top of their ballistic parabola) and thus colder than air molecules accelerating to the ground? In non-relativistic theory no, because in thermodynamic equilibrium temperature has to be the same everywhere. The slowing down does ...

2

I'm not sure whether these theoretical ideas are is included in what you have in mind. They are only good (and the first , as far as I know, only in theory) for fundamental particles and not for measuring masses of everyday things, but here goes. The second - inference from cross coupling co-efficient between otherwise dispersionless, massless states - is ...

2

The following has ben found via Wikipedia page “Gravitational interaction of antimatter”. Another experimental test has been provided by the supernova SN1987a (anti)neutrinos, and this has been published in two brief reports in Phys. Rev. D in 1988 [1] and 1989 [2]. After the explosion of this supernova, 19 antineutrinos have been detected at IMB and ...

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