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183 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 ...
Martin's user avatar
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143 votes
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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 ...
SG8's user avatar
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129 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 $r$, $...
Javier's user avatar
  • 28.3k
111 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 ...
John Rennie's user avatar
110 votes
Accepted

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, ...
Bob Knighton's user avatar
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103 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 ...
Kai's user avatar
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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 ...
Dale's user avatar
  • 103k
92 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 ...
knzhou's user avatar
  • 103k
90 votes
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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 ...
Lukas Juhrich's user avatar
86 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....
StephenG - Help Ukraine's user avatar
77 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 ...
KareemElashmawy's user avatar
76 votes
Accepted

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'...
Selene Routley's user avatar
72 votes
Accepted

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 ...
G. Smith's user avatar
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68 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 ...
Eric Smith's user avatar
  • 9,611
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 ...
silverrahul's user avatar
  • 4,466
66 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 ...
knzhou's user avatar
  • 103k
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 ...
Zo the Relativist's user avatar
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 ...
62 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 ...
Ralf Kleberhoff's user avatar
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 ...
M. Enns's user avatar
  • 8,803
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 ...
ProfRob's user avatar
  • 133k
58 votes
Accepted

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 ...
G. Smith's user avatar
  • 51.7k
55 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 ...
QiLin Xue's user avatar
  • 630
55 votes
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Do planets orbiting stars emit gravitational waves?

Yes, but undetectably. The Earth-Sun system radiates a continuous power average of about 200 watts as gravitational radiation. As Wikipedia explains, “At this rate, it would take the Earth ...
G. Smith's user avatar
  • 51.7k
54 votes

Would touching a black hole of a small mass (the mass of an apple) cause you to spiral in and get dead?

No. A black hole created from something like an apple will still have the gravitational pull of an apple, so it's not going to suck you into it. If you swiped it it would pass through your hand, ...
Eric Smith's user avatar
  • 9,611
53 votes

If gravity is a pseudoforce in general relativity, then why is a graviton necessary?

While it's common to describe gravity as a fictitious force we should be cautious about the use of the adjective fictitious as this is a technical term meaning the gravitational force is not ...
John Rennie's user avatar
53 votes
Accepted

Why did the neutron star merger signal last for so much longer than the black hole merger signals?

It is not that the merger of two neutron stars takes longer, the inspiral and merger of a pair of neutron stars just spends a longer time in the frequency range where LIGO is most sensitive. Let me ...
TimRias's user avatar
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