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 ...
Martin's user avatar
  • 15.4k
141 votes

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
  • 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 ...
ACuriousMind's user avatar
  • 120k
130 votes

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
  • 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 ...
John Rennie's user avatar
108 votes

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
  • 8,332
101 votes

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
  • 91.9k
101 votes

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
  • 3,640
91 votes

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
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 ...
knzhou's user avatar
  • 99.9k
85 votes

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
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 ...
KareemElashmawy's user avatar
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 ...
user_35's user avatar
  • 2,213
75 votes

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
71 votes

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
  • 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 ...
Eric Smith's user avatar
  • 7,670
66 votes

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
  • 4,426
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 ...
knzhou's user avatar
  • 99.9k
64 votes

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

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

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
  • 8,640
59 votes

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
  • 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 ...
Ralf Kleberhoff's user avatar
58 votes

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
  • 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 ...
QiLin Xue's user avatar
  • 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 ...
pela's user avatar
  • 10.6k

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