38
votes
Accepted
It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?
If I interpreted this article correctly, then the answer to my question should be: 8 minutes is what we perceive, whereas for the photon the journey is instantaneous, due to the fact that it travels ...
- 629
26
votes
Accepted
Why are there not yet any instruments dedicated to registering time dilation caused by passing gravitational waves?
General relativity predicts that there are only two possible polarizations of gravitational waves, the so-called "tensor" polarizations $+$ and $\times$. It turns out you can show that the ...
- 37.3k
24
votes
From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?
First off, there's no real time dilation effect. In coordinate systems that cover the event horizon (Kruskal-Szekeres, Eddington-Finkelstein, Gullstrand-Painlevé, Kerr-Schild), objects fall through it ...
- 3,914
23
votes
It takes light roughly 8 minutes to travel to Earth from the Sun, but based on which perspective?
When people talk about the time taken for light to go from the Sun to the Earth, they're generally just considering classical Newtonian mechanics, not relativity. So we simply divide the distance by ...
- 975
22
votes
Accepted
How do two black holes merge?
The "event horizon" is defined as the point (or surface) from within which light rays can never (ever) reach a distant observer. To find the location of the event horizon implies that you ...
- 156k
14
votes
Accepted
Depth of gravitational well within our local Virgo supercluster?
The size of the relative gravitational time dilation effect (when it is small), compared to a clock at infinity, is $\sim \Delta \phi/c^2$, where $\Delta \phi$ is the change in potential
You can then ...
- 156k
12
votes
Accepted
How do I estimate the difference in the speed of time flow between Earth and a distant star?
Nearby stars
For such a small distance of $\sim 100$ light years, you can approximate that the Galaxy's gravitational field $g$ is uniform. On average, stars near the Sun orbit at speeds $v\simeq 220\...
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12
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From an outsider's perspective, how can a black hole grow if nothing ever crosses the event horizon?
Due to time dilation, an outside observer never sees a falling object actually cross the event horizon.
This is correct, but perhaps not quite in the way that you visualize it.
It's trivially true, ...
- 2,626
9
votes
How do two black holes merge?
If you have two black holes, then the event horizons are distorted.
As event horizons are regions of spacetime (not just spheres in space) there is no issue with "time stopping". That is ...
- 126k
8
votes
What is the oldest thing?
Pretty much every hydrogen atom that's in a glass of water has a proton that dates from 1 / 1000000 seconds after the big bang. That's older than the cosmic microwave background, which dates from ...
- 3,455
8
votes
Can anyone one show how speeds greater than c cause a paradox on a space time diagram?
Suppose there was a magic gun that fired a bullet at ten times the speed of light relative to the firer.
If I have the only such gun, and I don't move then there is no paradox.
But now suppose ...
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8
votes
Accepted
How much time passes in the outside universe when falling into a black hole?
You are essentially asking the following: if someone falls from the Earth from some way beyond the event horizon of a black hole, how long after they have left can an observer on Earth still signal to ...
- 156k
7
votes
Could a star closely orbit a black hole long enough for the star to have lost 0.5B+ years to time dilation?
Yes, but not very likely.
The closest orbit that does not require constant expenditure of energy to maintain it is the prograde equatorial ISCO. For a Kerr black hole the time dilation factor on this ...
- 15k
7
votes
How does gravity affect a physical clock?
If it has to do something with gravity affecting the inner working of wind up and pendulum clock will this also affect digital clock ?
As astrosnapper noted in his comment, gravitational time ...
- 34.1k
7
votes
Accepted
How does gravity affect a physical clock?
I'm going to give a longer, slightly less accurate answer because this is a fun question.
I'll start with special relativity because it's easier to understand.
Using a thought experiment the ...
- 24.1k
7
votes
Why are there not yet any instruments dedicated to registering time dilation caused by passing gravitational waves?
The answer by @HDE 226868 addresses the current attempts by LIGO/Virgo and PTAs to detect alternate gravitational wave (GW) polarization states, which have not been detected. In that answer, this SE ...
- 4,533
6
votes
Accepted
Could a star closely orbit a black hole long enough for the star to have lost 0.5B+ years to time dilation?
In the comments of the other answer, the question came up whether decay of the orbit would limit the time amount of time dilation. The answer is of course yes. But by how much? This question can be ...
- 2,720
6
votes
Cosmological redshift - How do we know it's not caused by the observer's time dilation?
The reason we think that the cosmological redshift is caused by the metric expansion of space is 1) that there is a well-known, physical mechanism that can cause this effect, and 2) that this ...
- 38.9k
6
votes
Accepted
If you are in a deep gravity well, where time goes by more slowly, do you see the unfolding of a cosmic event at a different rate?
Time dilation is related to differences in gravitational potential in General Relativity.
Observing a clock situated deep in a potential well, a distant observer would see it running slow. Vice-versa, ...
- 156k
5
votes
If you looked at Earth while travelling at relativistic speeds, will you see sped-up activities?
It doesn't have to be a visible telescope, any electromagnetic radiation will act in the same way.
I'll give you the answer, but if you want the details you need to look at a description of the ...
- 156k
5
votes
How does gravity affect a physical clock?
A physics professor once described the following exchange, and I found it immensely helpful. It's not an exact answer, but the formatting requires an answer post.
Question:
What is time?
Answer: (...
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5
votes
Accepted
Time in 0 gravity points
makes time pass more slowly for us
This is a fundamental misunderstanding of time dilation, which only says anything about the relative rates that clocks run compared to a clock that is in your own ...
- 156k
5
votes
Planck epoch and time dilation
We don't really know at what energy scale general relativity actually breaks, but according to simple dimensional analysis arguments, the Planck scale is actually a plausible candidate.
Quantities at ...
- 4,471
5
votes
Do black holes "store" ancient light?
Infalling objects pass right through the event horizon. They don't freeze there. The vicinity of the event horizon is locally just like any other part of spacetime.
If a light-emitting or light-...
- 3,914
5
votes
Measurement of Planetary Aberration (similar to stellar aberration)
I. The statement about planetary aberration quoted in the question, and for which the quesioner seeks some authority, wasn't actually called a 'definition' by the questioner : it's probably best ...
- 1,329
5
votes
How do I estimate the difference in the speed of time flow between Earth and a distant star?
There's no significant difference over a distance of a hundred light years or so.
The gravitational time dilation can (ignoring rotational terms) be calculated as the velocity time dilation of the ...
- 126k
5
votes
How do I estimate the difference in the speed of time flow between Earth and a distant star?
As others point out, the differential effects of the different positions in the Galactic potential are of order 1 part in a billion.
A bigger effect is the gravitational potential of the star. If the ...
- 156k
4
votes
What is the oldest thing?
If we could detect the cosmic neutrino background, then I would class those neutrinos as "things" and there should be lots of them! I guess whether they are detected or not, they are extremely likely ...
- 156k
4
votes
Must time pass more slowly, relative to our inertial reference frame, inside galaxies that are currently located at half way to the Hubble Horizon?
The point of the unification of space and time in relativity is that there's no sense in asking what's happening "right now" at a different position. It makes as much sense as asking what's ...
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4
votes
Why are there not yet any instruments dedicated to registering time dilation caused by passing gravitational waves?
In Cartesian coordinates, the flat spacetime interval can be written in terms of invariant proper time $\tau$ as
$$c^2 d\tau^2 = c^2dt^2 - dx^2 - dy^2 - dz^2\ ,$$
where $t$ is some universal time ...
- 156k
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