179
votes
What is a manifold?
What is a manifold?
A manifold is a concept from mathematics that has nothing to do with physics a priori.
The idea is the following: You have probably studied Euclidean geometry in school, so you ...
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141
votes
Accepted
Would touching a black hole of a small mass (the mass of an apple) cause you to spiral in and get dead?
This is just a quick calculation that shows what would happen to a black hole with a mass equal to the mass of an apple: It is shown that not only the builder of this black hole but also the whole ...
- 4,530
138
votes
If gravity isn't a force, then why do we learn in school that it is?
Because Newtonian gravity, where it indeed is considered a force, is a good enough approximation to the situations you consider in middle school (and beyond).
General relativistic effects are very ...
- 120k
130
votes
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Is there such thing as imaginary time dilation?
Nice discovery! The formula for time dilation outside a spherical body is
$$\tau = t\sqrt{1-\frac{2GM}{c^2r}}$$
where $\tau$ is the proper time as measured by your object at coordinate radius $r$, $...
- 27.6k
109
votes
What is the connection between special and general relativity?
Suppose we start by considering Galilean transformations, that is transformations between observers moving at different speeds where the speeds are well below the speed of light. Different observers ...
- 349k
108
votes
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Is spacetime wholly a mathematical construct and not a real thing?
TL;DR This is a complicated question and anyone who tells you a definitive answer one way or another is either a philosopher or is trying to sell you something. I justify arguments either way below, ...
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101
votes
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General relativity (and other theories) when proven wrong
My question would be, what happens in the scientific community if one experiment proves it wrong
We have already seen what happens in this circumstance by looking at what happened to Newtonian ...
- 91.9k
101
votes
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Does gravity CAUSE the bending of spacetime, or IS gravity the bending of spacetime?
I think the correct answer should be that what we call gravity is a fictional force which we experience due to living in an accelerated reference frame (as opposed to an inertial one). Unlike other ...
- 3,640
91
votes
Accepted
What is really curved, spacetime, or simply the coordinate lines?
Congratulations! You stumbled upon an important question of differential geometry:
How can I know whether the curvature is caused by my choice of coordinates or the space I live in?
As has been ...
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89
votes
Why isn't an infinite, flat, nonexpanding universe filled with a uniform matter distribution a solution to Einstein's equation?
This is a rather subtle question, which confused even Newton. It is very tempting to think that an initially static Newtonian universe with perfectly uniform mass density will not collapse, because ...
- 99.9k
85
votes
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Is there 'friction' in spacetime?
I think the question suggests you are thinking of space-time as if it were e.g. a substance, like a fluid, that we move through. That's not how we view space-time, at least in pure general relativity....
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78
votes
Why is light bent but not accelerated?
You missed a key aspect of general relativity (GR):
The gravitational 'force' we observe is an illusion according to general relativity. Specifically, the 'gravitational force' and 'gravitational ...
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77
votes
Why is the detection of gravitational waves so significant?
Chris' answer provides an excellent explanation as to why gravitational waves are useful to detect in general. Here's my take (as someone who works in the theory of black holes) on what is ...
- 2,213
75
votes
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If the speed of light is constant, why can't it escape a black hole?
The speed $c$ that is constant is so when measured locally relative to a freefalling frame (i.e. one for which all points follow spacefime geodesics wrt to the metric $g$). Local means that the frame'...
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71
votes
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Do photons bend spacetime or not?
Classical electromagnetic fields carry energy and momentum and therefore cause spacetime curvature. For example, the EM field around a charged black hole is taken into account when finding the ...
- 51k
68
votes
Books for general relativity
This list is extensive, but not exhaustive. I am aware that there are more standard GR books out there such as Hartle and Schutz, but I don’t think these are worth mentioning. Books with stars are, in ...
66
votes
Why don't we put satellites into an orbit where there is (almost) no time dilation/contraction compared to Earth's surface?
The higher the satellite's orbit, the more of the Earth it can see (and hence the fewer satellites you need to ensure complete coverage of the Earth). The particular orbits chosen for navigation ...
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66
votes
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Why did we expect gravitational mass and inertial mass to be different?
"isn't there just one property called m and it just appears in
different equations (e.g. Newton's second law and the law of
gravitation)? In a similar way that (say) frequency appears in many
...
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64
votes
What does general relativity say about the relative velocities of objects that are far away from one another?
There's already a good answer, I'll just say it a slightly different way.
Consider two people running at the same speed on a road. Computing their relative speed is easy, we just do vector ...
- 99.9k
64
votes
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Why is light bent but not accelerated?
One thing that the previous answers are missing -- the light is accelerated; it just is accelerated according to the rules of special relativity, which says that it cannot pick up speed when already ...
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62
votes
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Indirectly breaking the speed of light
A black hole of mass $10^{36}$ kg would have a Schwarzschild radius (the distance from the center to the event horizon) of about $1.5\times 10^{9}$ m.
So your choice of "orbital" distance is ...
59
votes
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What if we shine a laser to east and another to west, will they arrive at the same time?
It depends on your frame of reference.
If you are positioned by the laser on the Earth's surface you would observe the two laser beams to have the same speed and your would observe the two targets ...
- 8,640
59
votes
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Why is Andromeda only partially blue-shifted?
I am going to attempt an explanation based on this image that you posted a link to in the comments.
What you are looking at here is a broad-band visible wavelength colour image of the Andromeda ...
- 126k
58
votes
Why does the speed of an object affect its path if gravity is warped spacetime?
You're using the wording "curved spacetime", but you're still only thinking "curved space" with an independent, linear time.
In your curvature model, you're assuming that moving ...
- 2,746
58
votes
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Why don't merging black holes disprove the no-hair theorem?
No. The no-hair conjecture applies to stable solutions of the Einstein-Maxwell equations. In the case of merging black holes, it applies to the end state of the merger into a single quiescent black ...
- 51k
56
votes
Why doesn't Einstein's general theory of relativity seem to work on Earth?
The short answer: general relativistic effects are mostly not noticeable on such a small scale (except in a few cases). For example, one common way to characterize the strength of a gravitational ...
- 640
56
votes
Why is the detection of gravitational waves so significant?
In additions to what Chris White lists, I'd like to point to the fact that, except for a few meteorites and some dust collected on the plates of satellites and rocks from Mars (and cosmic rays and a ...
- 10.6k
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