# Tag Info

### Is rotation invariant under gravitational time dilation?

is vertical rotation an invariant in general relativity or not? The answer is no. First let us consider a small flywheel. If you were to place this flywheel in an elevator and descend with it, into a ...
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### Definition and mathematical description of comoving reference frame and locally inertial frame

A will choose a "moving coordiante",which is a series of coordinate system,and not "a" coordinate system in which the metric tensor can be written as η. This is not correct. Time ...
• 103k

### Photonic black holes

The existing answers don't mention a crucial fact, which is that the $M$ in the Schwarzschild metric is an invariant parameter, whereas a photon's energy is frame dependent. So a single, free photon ...
• 9,611

### What is the gravitational field of a hole in an infinite perfect crystal?

This can be used for reactionless propulsion. Am I missing something or have I discovered (effective) antigravity? You are forgetting the role of the mind in all of this, which, a unified field ...
1 vote

### A question about Quantum entanglement, 45° Light Polarization, and faster than light communications

When the photons leave the crystal, their polarization is in superposition. They have no definite polarization. When they hit a polarizer, 50% of them pass through, regardless of any entanglement. ...
1 vote

### Distance and luminosity distance

Using the inverse square law we can deduce that the flux F(watts/m^2) received from a source with an intrinsic luminosity L(watts) diminishes as a function of distance squared so $F \propto L/D_e^2$ ...
• 6,217

### Necessity of Singularity in General Relativity

Well the famous singularity theorems show (very very roughly speaking) that in the theory of classical GR, collapse beyond horizon implies a singularity. Classical GR is not the true theory of physics,...
• 1,456

### Can perfectly stable orbits exist in GR?

In the 1979 paper "Time without end", Freeman Dyson calculates a time in the order of 1020 years until the earth would fall into the sun due to gravitational decay alone, based on the ...
• 355

### Can perfectly stable orbits exist in GR?

As you say, GR implies that all orbits lose energy (very slowly) over time, due to gravitational waves. Also the vacuum of space is not true vacuum and there is some drag from the intergalactic medium ...
• 6,217
1 vote

### Necessity of Singularity in General Relativity

What is it that preludes the predicted field, even at $r<r_{Schwarzschild}$, from simply terminating at the surface of a collapsar of dense matter, and taking some other form inside it? This is ...
• 12.7k

### We know there is no aether, so what is being dragged in frame dragging?

Michaelson Morley did not disprove the Aether theory; it just showed a null result. If MM had Ligo as a detector or something more sensitive, how would the result have been interpreted? MM never took ...

### Can perfectly stable orbits exist in GR?

In the literature, the orbits you are looking for a called “floating orbits”. Floating orbits are not possible in plain GR (See e.g. 1302.1016).
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1 vote

### Boundary conditions on transition maps on general relativity

There's nothing fancy going on with transition maps. What a mathematician means by a "transition map", in the setting of general relativity, is nothing more or less than a coordinate change ...
• 291

### How exactly does curved space-time describe the force of gravity?

If we ... use a fictional device to cancel all the inertia of the particle, it is obvious that the curve of space-time is towards the star but what is not obvious is what would make the particle begin ...
• 6,217
1 vote

### Hawking Temperature of the BTZ Black Hole

Another option for finding the Hawking temperature of the BTZ black hole is by the formalism of the surface gravity $\kappa$ which connected to the Hawking temperature via: $$T_H=\frac{\kappa}{2\pi}$$ ...

### Is the size of a black hole singularity smaller than a fundamental particle?

We know that black hole is infinitely densed. More exactly: The theory of general relativity predicts that the center of a black hole is infinitely dense. This theory predicts very well everything ...
• 40.1k

### Is the size of a black hole singularity smaller than a fundamental particle?

While @paulina's answer: we don't know is correct, because quantum gravity is not understood, I'll answer for a classical Schwarzschild blackhole as described by Kip Thorne. The size is zero, however ...
• 35.5k
Accepted

### Is the size of a black hole singularity smaller than a fundamental particle?

The very short answer to this is: We have no idea. General relativity predicts that the singularity of a Schwarzschild black hole (which I assume is what you mean by "actual black hole") is ...
• 1,897
1 vote

### Two contradictory derivations of Killing equation

I think the problem lies in the notation. I guess Tong is treating the components $\xi_\mu$ as scalars for which we have $\frac{\mathrm{d}}{\mathrm{d}\tau}=u^\alpha\partial_\alpha$. And the ...
• 425

### Spatial Curvature of Universe at recombination vs now

We do not use CMB data to directly measure the curvature of the universe. Instead, we use CMB data to estimate the average density (as well as the size of density fluctuations) in the early universe. ...
• 56.5k

### Does black hole formation contradict the Pauli exclusion principle?

In actuality, there is no such thing as this so called singularity... It's a convenient mathematical invention to represent something of which we understand very little. Although quantum physics can ...

### Do you always experience the gravitational influence of other mass as you see them in your frame?

These are excellent questions! They involve the interrelation of various physics concepts. And we would expect physics to be internally consistent, generally speaking. But since your insights are not ...
• 1,148

### As per Friedmann Equations, is big-bang singularity necessary?

In a closed (unlikely, but still possible within the measurement accuracy) dark energy dominated universe where $\Omega_{\rm K}=1-\Omega_{\Lambda}$ (which it was during inflation if there was any, ...
• 12.4k
1 vote

### What is the gravitational field of a hole in an infinite perfect crystal?

... overdensity attracts underdensity and underdensity repels overdensity ... Yes, in an empty universe a pair of matched positive and negative masses initially at rest would be self-accelerating ...
• 15.9k