29
votes
Accepted
Why isn't the De-Broglie wavelength of massive particles redshifted in an expanding universe
The de Broglie wavelength of a massive particle is redshifted in an expanding universe.
The de Broglie wavelength is given by:
$$ \lambda = \frac{h}{p} $$
so a red shift of the de Broglie ...
- 363k
18
votes
Accepted
If I live on a planet that is heavy enough, would the CMB get blue shifted to be in the visible spectrum?
The cosmic background radiation has a wavelength about 2000 times longer than visible light. So you would need to be sitting deep in a gravitational well such that local time progresses 2000 times ...
- 2,413
13
votes
Accepted
Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?
The answer to your question is yes, but the gravitational redshift could not be confused with cosmological redshift because it is small.
The basics of gravitational redshift can be grasped by the ...
- 137k
12
votes
Can a photon that is emitted from a denser part of the universe to a less dense part appear redshifted?
Yes, you're quite correct that galaxies in a region of the universe with greater than average density would appear redshifted to observers outside that region.
In fact this is the origin of the Sachs-...
- 363k
12
votes
Accepted
How accurately has general relativity's prediction of gravitational redshift been measured?
A recent test (2018) using atomic clocks aboard two satellites found general relativity's gravitational redshift prediction to be accurate to $(+0.19 \pm 2.48)\times10^{-5}$.
This was not a planned ...
- 275
9
votes
Accepted
Neutron star accurate visualization
To see a neutron star with a similar angular diameter to the Sun, you would have to get much closer than 1 au. They have a radius of approximately 10 km but their apparent radius to a distant observer ...
- 137k
8
votes
Accepted
Rainbow Blackhole?
Gravitation clearly can change wavelength and frequency, and it does that for instance for the cosmological redshift.
But with the speed of light being $c$ locally, independent of frequency or ...
- 14.1k
7
votes
How is the global time coordinate $t$ ("observer at infinity" time) defined operationally e.g. in the Schwarzschild metric?
The meaning of the coordinate t is much broader than that. In fact we might want to talk about events that never send or receive signals from infinity . After all signals to and ...
- 71
6
votes
How is the global time coordinate $t$ ("observer at infinity" time) defined operationally e.g. in the Schwarzschild metric?
You raise good points, which are often not clear in pedagogy. Indeed, there is a big difference between the statements:
"at $r_0 \gg 2M$, $\quad t$ is the proper time", and
"at any event $(t,r,\theta,...
- 1,238
6
votes
What does the $O$ mean in "$O(v^4)$"?
$O(x^n)$ stands for big O notation. It means that you wrote all terms that are proportional to $x$ up to the ones that are proportional to $x^{n-1}$ and that there are some more terms proportional to $...
- 5,851
5
votes
Does the density of a galaxy affect time?
The answer is yes they can but the effect is trivially small.
Observed gravitational redshift just depends on the difference in gravitational potential between source and observer. Roughly,
$$ z \sim ...
- 137k
5
votes
What were the measurements taken when S2 reached periapsis that were the 'best test of GR to date'?
I'm not sure I would agree the S2 observations are the best test of GR, or even the most rigorous test. However they are a test in a regime that we have not been able to directly probe before.
The ...
- 363k
5
votes
Accepted
As light travels upward in the earth’s gravitational field, it loses energy, and so its frequency goes down?
There are several equivalent answers.
Since you already understand gravitational time dilation, that is probably the most direct. Suppose that you have a clock which is deep in a gravity well so ...
- 109k
5
votes
Accepted
How does gravitational redshift not violate the Second Law of Thermodynamics?
This is really similar to a waterfall. You're exploiting the energy available by lowering the item (EM radiation in this case) down the gravitational field.
So the question becomes, how did that ...
- 42.3k
5
votes
Accepted
Do particles at rest gain energy in an expanding universe?
I present this answer in two sections. The first section gets to the point in a pithy way. The second section then expounds more generally how questions of this kind can be considered.
1. Quick answer
...
- 61.9k
5
votes
How do you intuitively understand of the Einsteinian portion of the gravitational lensing equation?
The second term (which, unlike the first, is independent of the speed) comes from the angular defect in the spatial geometry around the sun, the same thing that causes the anomalous precession of ...
- 28.7k
4
votes
Accepted
Black holes and red light
He would see red because the light wave coming from the person falling would get stretched due to the massive gravitational pull of the black hole. Since the wave length is increasing, it becomes red. ...
4
votes
Accepted
Consequences of infinite redshift in the event horizon of a Schwarzschild black hole
The error in your premise is that you have an observer who is hovering at the event horizon which is impossible. No observer can hover at the event horizon, once they touch the event horizon they will ...
- 4,119
4
votes
Consequences of infinite redshift in the event horizon of a Schwarzschild black hole
Suppose that you start with a black hole of mass M and infinitely far away from the black hole a spherical pulse of light is directed in towards the horizon. Suppose further that the total energy of ...
- 109k
4
votes
Accepted
Gravitational lensing redshift around a Kerr black hole
Kerr metric is stationary and has a Killing vector field $\xi_t=\partial _t $, time-like outside of the black hole, representing stationarity. This means that the quantity:
$$ E = g_{\mu\nu }\xi^\mu_t ...
- 16.6k
4
votes
If I live on a planet that is heavy enough, would the CMB get blue shifted to be in the visible spectrum?
The extreme time dilation proposed can only come if you were on a planet that was orbiting a black hole.
The time dilation for a circular orbit around a Schwarzschild black hole is given by $(1 - 3r_s/...
- 137k
4
votes
Has anyone considered a gradual change in Gravitational Potential across the universe as a cause for cosmic red shift?
Therefore, for the exact same reason our "observable" universe is limited by the travel distance of photons since the CMB, our local gravitational potential shifts over time for the same ...
- 28.7k
4
votes
Clocks in gravitational field
Gravitational time dilation is about time, not about clock mechanisms. Wristwatches, atomic clocks, and biological processes like aging all proceed "faster" (relative to the ground) when ...
- 10.7k
4
votes
Accepted
Why the CMBR redshift is so higher than the redshift of the most distant therefore oldest galaxies in the universe?
The relationship between redshift and look-back time is highly non-linear!
Here's a plot from https://astronomy.stackexchange.com/a/14836/2531 which goes nearly far enough (the redshift of the CMB is ...
- 137k
4
votes
Neutron star accurate visualization
It would look like a bluish, very bright star. I'll give a rough, order of magnitude calculation.
Its diameter is about $10^{-5}$ of the Sun, so its solid angle would be about $10^{-10}$ of the Sun. ...
- 21.9k
3
votes
Accepted
Is the event horizon of a black hole wavelength-specific?
No, it's all the same.
GR does not care about the mass of something, nor does it care about its energy, when it calculates its path. It's always a geodesic and for massless particles it's always a ...
- 14.1k
3
votes
Gravity's effects on photons moving away from source
For the first question: Sure, light emitted by a galaxy is affected by the gravitational redshift, but the effect is small and independent of the distance of the galaxy from us. (See also the question ...
- 160
3
votes
What does the gravitational potential of the Milky Way do to the CMB?
The gravitational potential of the Milky Way will cause a blue shift not a red shift. This happens because relative to an observer far from the Milky Way the gravitational potential within it causes a ...
- 363k
3
votes
Can a photon circle a black hole indefinitely?
A photon, being a quantum of EM radiation, is not only a particle but also a wave. Basic solutions of wave equation around the black hole are quasinormal modes, which are exponentially decaying in ...
- 16.6k
3
votes
Black hole, gravitational redshift, Doppler effect
The blue light would be yellow, orange or red, depending on how close to the event horizon the spaceship was. The longer interval between signals is mainly because of gravitational time dilation, not ...
- 1,748
Only top scored, non community-wiki answers of a minimum length are eligible