Tag Info

8

It is correct that the Kerr black hole solution of GTR allows travel between universes. However, that does not mean that if you actually jump into any kind of black hole that you could go to another universe. To motivate the resolution to this conundrum, let's start off very easy: suppose you stand on the ground with a ball in your hand, and you throw it ...

8

Gravity doesn't affect the speed of light. It affects the space-time geometry and hence the paths of light. However, this can have a similar effect. Light emitted at source $S$ to pass a massive object $M$ that is very close on the otherwise (if M weren't there) straight path to an observer $O$ has to "go around" $M$, which takes longer than following the ...

7

This has been considered long ago (Here's a paper talking about this). Wormholes are not forbidden by physics, but the creation of wormholes is iffy ground. THere are two possible paths one can take to create a wormhole: Choose a pre-existing wormhole in the quantum foam and "expand" it by feeding it exotic matter. "Tear and sew up" space — we're ...

6

As Walter says, gravity doesn't bend light. Light travels along null geodesics, a particular type of straight path. Since (affine) geodesics don't change direction by definition, geometrically light trajectories are straight. Moreover, the speed of light in vacuum is $c$ in every inertial frame, regardless of whether or not spacetime is curved, although a ...

6

Stellar clusters around supermassive black holes are systems in which relativity likely plays a role. Currently, only bright stars can be seen in our own galactic center because there is a ton of neutral gas between us and the galactic center that obscures it. As a result, we only have a few "test particles" out of the many stars that actually orbit the ...

5

My topological defect cosmology is a little rusty, but I'm pretty sure this is how it goes. Start with the fluid equation, $$\dot{\rho} + 3 {\dot{a} \over a} \left( \rho + p \right) = 0,$$ and the equation of state, $$p = w \rho.$$ Plug the equation of state into the fluid equation, assume a constant $w$, and you'll find $$\rho \propto a^{-3(1 + w)}.$$ ...

4

You have many questions. I only answer the first. It's doesn't only matter how heavy a star is, but also how big. For ordinary stars, the effect is neglible (work it out by yourself -- it's a useful exercise). Even for compact stars, such as white dwarves or neutron stars, the effect is small. However, what astronomers commonly refer to as (stellar-mass) ...

4

I think the image you posted is not quite reallistic. On it, objects are just inverted from some radius on, while what you can expect from a real black hole seen from near enough is a combination of these: a) an accretion disc b) a companion being sucked c) Hawking's radiation d) X-Ray burst from the poles (really starting out of the event horizon) You ...

4

Newtonian gravity of a point-source can described by a potential $\Phi = -\mu/r$. If we suppress one spatial dimension and use it to graph the value of this potential instead, we get something that looks very close to this illustration, and is indeed infinitely deep at the center--at least, in the idealization of a point-mass. And farther away from the ...

4

I will make a small calculation here, but please proceed to the results if you may like to. Calculation Stars are spherical and static, so metric near their surface (photosphere) and outside on is Schwarzschild. Hence time-time metric component on the surface is: $$g_{44}=1-\dfrac{R_{grav,*}}{R_*}$$, where $R_*$ is the radius of the star and $R_{grav,*}$ ...

4

"Allowed for" does not mean "necessarily cause". What the professor implied is that the solutions look, from a mathematical p.o.v., just like what you would expect from a bridge between universes - IF multiple universes exist, and IF the bridge is passable. That's all there is to it. A mathematical solution that looks like a bridge. But has it ever been ...

4

This book by Andrew Liddle is fantastic for explaining the basic concepts and giving the reader a good mental picture of the ideas, but it may be a little more basic than what you're looking for (I still keep it as a good reference when reading more advanced texts). If you're looking for something more advanced Peter Coles & Francesco Lucchins guide ...

3

There are a few different features of black holes that researchers look for: 1) An accretion disk 2) Mass transfer from companion star 3) X-ray emission 4) Absorption of gas and other matter around it 5) Gravitational lensing You mentioned #3 in your question; quite a few black holes, including Cygnus X-1, emit X-rays. 1 is a common feature around ...

3

I don't know any specific text but I can recommend you a course I took last year at Coursera.org. It says "introduction" but I can assure you it is really complete, 12 week that include a lot of mathematics and physics. https://www.coursera.org/course/introastro

3

That's a lot of not quite trivial questions! I'll try to answer part of it. First, the red-shift can be composed of relativistic Doppler-effect and gravitational red-shift. When neglecting the gravitationl part, we get a higher radial velocity. The radial velocity can be used to calculate a distance estimate via the Hubble "constant". So for low velocities ...

3

Let's restrict to special relativity, meaning two inertial frames moving in a Minkowski spacetime. A clock in the first inertial frame ticks slower, when seen from the second. A clock in the second inertial frames ticks slower, when seen from the first. Now assume, that you are fixed to one of the two inertial frames. Usually you measure velocities within ...

3

Since the astronomers are using radio telescopes and not optical telescopes, I'd like to point out why they are doing so - The centre of the Milky Way is a very dusty place. Wavelengths from the millimeter to optical get easily absorbed by all this dust, so it's very difficult to see the centre of the galaxy in the optical spectrum. But radio waves do not ...

2

Gravity is sometimes described as a curvature in space-time. Due to relativity, doesn't this imply that gravity doesn't propagate? There's a fairly precise sense in which gravity propagates: if you have a spacetime and you perturb it a bit, then you can think of the new spacetime as the old "background" spacetime with a small change on top of it. Then ...

2

Black holes distort the geometry of spacetime. So you need to take care about that. Changes of gravity propagate with the speed of light. That's thought to occur for accelerated mass, e.g. a binary of black holes. A second fundamental point: Speeds don't add up in an additive way, but in a subadditive way: speed of light + speed of light = speed of light. ...

2

Adding to @Guillochon's answer, there are even a number of general relativistic tests in our solar system, the most famous being the precession of the perihelion of Mercury. In short, the location of the point of closest approach to the Sun (perihelion) for the planet Mercury is a changing quantity. Essentially, given one full revolution, it doesn't trace ...

2

At 99% the speed of light the behaviour would be almost completely determined by special relativity. The scenario is well-investigated for synchrotrons. In principle a synchrotron or a storage ring, e.g. around the equator of Earth, could be built. At 99% the speed of light the frequency $f_s$ of the circling object should occur red-shifted by a factor of a ...

2

Without some reference to compare to, time passing more slowly makes no sense at all. But every clock has its own proper time that measures time along its own worldline. In cosmological models, the cosmological time is the proper time of a certain kind of ideal observer: one comoving with the Hubble flow. In other words, imagine space filled with observers ...

2

I think, you are trying to describe a world line.

2

Indeed space is not a physical entity (as far as we know). Saying space expands is another way to say that galaxies recede away from each other at a rate proportional to their distance. The "expanding space" picture is unfortunately a source of endless confusion and misconceptions, as can be seen on the page of this very question: mpv mentioned that "The ...

2

One of the predictions of General Relativity is that certain objects can give off energy in the form of gravitational radiation. This means that over time, the orbit of the two neutron stars should "decay", and they should come closer to each other. Using General Relativity, it is possible to predict the changes in the orbits and the energy of the emitted ...

2

16-Year Long Study Tracks Stars Orbiting Milky Way Black Hole "Undoubtedly the most spectacular aspect of our long term study is that it has delivered what is now considered to be the best empirical evidence that supermassive black holes do really exist. The stellar orbits in the Galactic Centre show that the central mass concentration of four million ...

2

Particles are generated and destroyed all the time. This is obvious for photons, but also holds for massive particles. Today most particles are stable and long-living, but shortly after the big bang the universe was so hot that particle generation and destruction were in some equlibrium and all particles participated in this process (for photons this ...

1

This is a tough question to answer because the dimensions of your proverbial cube would be affected by the mass inside (as dimensions can only exist in space, so anything that affects space will also bend your cube), and mass does not bend space, it bends spacetime. You have to keep in mind that when talking about special and general relativity, you are ...

1

Is it not enough with geometry to calculate volumes? Is knowledge of the mass (and its distribution?) also necessary? To answer this question. Mass distribution does make a difference. For example, if a star collapses into a black hole, the mass of the black hole is less than the original mass of the star due to some matter lost during the supernova. ...

1

The expansion of space means that objects in cosmological distances are receding away from each other. You may ask: is it the same thing like if there were 2 rockets in empty space (stationary with respect to each other) and then one of them starts its engines and accelerates away? The answer is no. The above described case is not expansion of space. It is ...

Only top voted, non community-wiki answers of a minimum length are eligible