# Questions tagged [gravitational-lensing]

Gravitational lensing occurs when light from a distant object is bent as it travels to an observer, due to passing through the gravitational field of a massive object (such as a galaxy, or black hole).

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### Measuring the speed of light with a black hole

is it possible to use the grabity a black hole has to bend the light back to the observer and measure it that way? also saying i made the light go from the right and bend back coming at my left and ...
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### Mutual gravitational acceleration (or deflection) of light beams as a function of the angle between them

Given Einstein's famous equivalence between energy and mass, $E=mc^2$, a light beam with power, $P$, has an energy per unit length of $P/c$ and an equivalent mass per unit length of $P/c^3$. A short ...
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### How to convert from polarization modes ($h_{+}$, $h_{×}$) to obtain spin-weighted spherical harmonic $h_{lm}$ as a function of $h_{+}$, $h_{×}$?

This question arises from a discussion in the thread How to convert from plus and cross polarization modes ($h_{+}$, $h_{×}$) to spin-weighted spherical harmonic $h_{lm}$? I was looking for a ...
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### Does a uniform circular disk make a perfect gravitational lens?

I'm surprised at this. I'm expecting to be corrected, but these are my back-of-the-envelope workings ... Starting out with a cylindrical lens oriented along the line of sight. Assuming the cylinder ...
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### Gaussian curvature in a weird metric

Consider a disformal metric $\tilde{g}_{\mu\nu}=g_{\mu\nu}+h_{\mu\nu}$, with $g_{\mu\nu}$ being the Schwarzschild metric tensor perturbed by $h_{\mu\nu}$, a generic second-rank symmetric tensor. ...
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### What is the gravitational lensing impact on polarity?

Gravitational lens is known for light bending, and gravitation can impact its frequency, but does it any impact on the polarity of this light?
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### Momentum conservation in gravitational lensing?

I have read this question: Every time a photon changes direction, it requires something else to gain momentum in the opposite direction, whether a solar sail or a star bending light by gravity. How ...
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### Can gravitational lenses change over human time scales?

Gravitational lensing is caused by the chance alignment of the observer, the lens, and the source. Obviously these are not permanent events as the earth will move in and out of a focal point as the ...
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### The effect of gravitational lensing during the lunar eclipse

During the lunar eclipse, the Moon turns into bloody colour while the shadow of the Earth is casting over the lunar surface. The red hue can be explained by means of the refraction of light and ...
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### Is the speed of light slow down around gravitational body? [duplicate]

In gravitational lensing effect, there are two types of time delay; geometric delay, gravitational delay. gravitational time delay is not caused by "distance", and speed is "distance/...
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### Does an accelerated frame of reference have the same effect as that of mass in space time

According to the equivalence principle, gravity and inertial forces are similar. And according to general Relativity, If there’s a large object in space-time, it warps the space time’s geometry and ...
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### Does the gravitational field change in the vicinity of a nova?

Since gravity is calculated based on the product of the masses of two bodies and the distance separating them, I initially thought that the orbits of the outer planets would not be affected when our ...
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### If a star gravity can change the apparent location of another star on the sky, can it change the gravitational field vector of that star?

If a star gravity can change the apparent location of another star on the sky can it change the gravitational field vector of the second star? In simple words if a star gravity deflects the position ...
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### Formation of a focal point by gravitational lensing effect

Can a black hole form a focal point of a nearby star at a certain distant point by gravitational lensing? Also, can the BH make a virtual picture of that star to a distant observer so the star looks ...
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### Does the frequency of light have any effect on gravitational lensing? [duplicate]

General relativity considers gravity as the curvature of space-time instead of a force. Therefore, what is bent around an astronomical object is the space-time itself. The light just follows the path ...
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### What is the photon interception radius of a Schwarzschild black hole? [duplicate]

To clarify what i mean by "photon interception radius" I mean the radius of the sphere where if any photons from a distant source enter will also inevitably result in them entering the black ...
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### Light travel time toward and away from a massive object

The curvature of space (or space-time) causes light to follow a curved path. We see this, for example, when we look at an "Einstein Cross" in which light from a distant object, e.g., a ...
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### How does the photon interact with the changing spacetime (say when massive object appers nearby)? How does it know how to "bend"?

How does the photon interact with the curved spacetime? Say, we have the photon travelling through the universe. Then it enters the region of gravitatonal lens (or a curved spacetime created by some ...
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### Is galactic gravitational lensing self-magnification big enough to contribute noticeably to the galaxy rotation curve problem?

Gravitational lenses magnifies the appearance of themselves, so in the case of a galaxy it looks bigger than it is. Thus the outer stars in a galaxy seems to have a higher tangential velocity. For ...
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### Why gravitational lensing appears circular?

I still remember the black hole photo but I know the coffee mug stain is actually the materials orbiting around it appears to be getting pervertedly blended 720 degree around the sphere, but then what ...
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### Do gravitational lensing experience Doppler effect?

I watched dark matter explained by Perimeter Institute on YouTube and they used it as gravitational lenses, they also explained Doppler effect so we know how fast things are relative to us. I wonder ...
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### Are multiple images an exclusive propertiy of gravitational lenses?

Strong gravitational lensing produces multiple images if the object lens and observer are in a specific alignment. This is due to there being multiple stationary points in the time arrival surface for ...
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### Can we use curved spacetime to measure the one-way speed of light?

This is related to Measuring one-way speed of light with gravitational lensing and Measuring the one-way speed of light with a black hole? The idea is to shine a beam of light from a clock towards a ...
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### Can an image formed by gravitational lensing be corrected for the inevitable aberration?

Carl Zeiss would not be impressed with your average gravitational lens. Compared with familar optical lenses that are generally used to form sharp undistorted images, gravitational lenses make quite a ...
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### Why a gravitational lense makes multiple distinct pictures of a distant object rather than making a symmetric ring?

I cannot imagine how a group of galaxies may produce pictures of a distant object on a ring-like region that is not symmetric. Why there are empty parts of that ring where there are no pictures of the ...
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### Is polarization of light preserved in gravitational lensing?

Space should not have torque since the stress energy tensor is symmetrical. That would imply that gravity can not turn a polarization plane of an electromagnetic wave. Have any changes of ...
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### Plausibillity of using a black hole as a telescope?

I recently learned about the proposed FOCAL telescope, which would use the Sun's gravity as a lens to observe exoplanets (or whatever it's aimed at) with incredibly high resolution. Obviously, the Sun ...
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### Observing starlight bending in high gravity

I note that there is a fair amount of light-bending questions, but I don’t really see the observer effect I am asking about. I apologize there is an answer already. I wonder if the Earth’s gravity – ...
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### Gravitational lensing: derivation of magnification

In gravitational lensing, the image magnification is defined as the image area over the source area. But many texts also give it as the inverse of the determinant of the jacobian, A, of the of the ...
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### Measuring one-way speed of light with gravitational lensing

This recent video from Veritasium (https://youtu.be/pTn6Ewhb27k), explored the problem of measuring one way speed of light and covered a few possible (and practical) solutions. However my ...
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### How much energy from the Sun could we get if we use Jupiter as a gravitational or atmospheric lens?

How much energy (watts) from sunlight could arrive to the focal point if we use Jupiter as a gravitational lens? and if we use it as an atmospheric lens by using refraction? How far the focal point ...
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### Do gravitational lenses act as prisms?

Light creates gravity, and the greater the light's frequency, the greater this gravitational effect is. It stands to reason then that light of different colors would react slightly differently to ...
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I'm trying to understand the deflection of light due to an axially symmetric gravitational lens following chapter 2.3 of these Heidelberg lecture notes. In doing so, I encounter the integral (2.12 a) $... • 329 2 votes 1 answer 78 views ### Are gravitational waves subject to gravitational lensing? [duplicate] Can gravity be focused by large massive objects? 1 vote 1 answer 26 views ### Speed of light vs wavelength due to gravitational lensing - straight path versus curved path Non-physicist asking. Note: I see that there are similar questions but none that I can find that addresses this specific query. Exaggerated diagram Question If the two beams of light leave the ... 7 votes 4 answers 5k views ### Why no gravity lens around planets? If large mass causes a curvature around spacetime, then why don't we see a gravity lens around our planets? • 138 2 votes 5 answers 860 views ### Why do photons follow the geodesic curvature of the gravitational field instead of the spacetime curvature? [closed] If mass merely 'curves' spacetime, why do photons follow the geodesic path of the gravitational field (path A) instead of the spacetime curvature itself (path B)? It seems, as if, the gravitational ... • 241 7 votes 3 answers 381 views ### Why does a gravitational field permanently alter the direction of photons? Given we observe that light do follows path A (gravitational lensing) instead of path B, is there any direct empirical evidence about how photons and gravity interact, other than stating that photons ... • 241 0 votes 1 answer 84 views ### Gravitational light bending and refraction? [duplicate] I've trying to read up on the evidential merit of general relativity and am rather curious as well as skeptical of early as well of possibly later more rigorous measurements of light (electromagnetic ... 0 votes 0 answers 60 views ### Gravitational lensing in the form of optical equation$\frac{1}{p} + \frac{1}{q} = \frac{1}{f}\$? [duplicate]

Gravitational lensing and light deviation is a well known subject in General Relativity. I'm wondering if the null geodesics equation could be recast into the classical lenses equation: \begin{...
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### Why is light affected by time dilations in space-time curvatures

My previous question on this site gave me an answer to what affects light - space warping or time warping. The answer is- both. But what now doesn't make sense to me is why light is affected by time ...
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### Is light affected by space warping or time warping?

Gravity, according to the General Theory of Relativity, is simply the curvature of space-time. Objects in the universe move through space-time in geodesic paths. Also, the most interesting part is ...
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### Visual simulation of gravitational lensing

Is there a way to generate an accurate visual computer simulation of gravitational lensing?
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### Travel/peek back in time with gravitational lensing?

I wonder if it would be possible to peek directly in our past-self by using gravitational lensing in such a way where we could (would be able to) calculate/triangulate the exact path and point a very ...
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### Gravitational lensing, indices of refraction and the size of the universe

Been pondering the following question for over 15 years now. Is it possible that the size of the universe has been over estimated due to the effects of gravitational lensing and boundary indices of ...
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### How do we know the quantity of dark matter in the universe?

I have been reading on dark matter and it is given specific numbers of its quantity, i.e, ~27%, in the universe. How we come up with that number as we have no strong opinion what it is made of? We ...
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### Gravitational waves interference lensing

Is it possible to detect gravitational waves interference pattern in space, using a telescopes alone or a telescope coupled by gravitational lensing?
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### Gravitational lensing redshift around a Kerr black hole

Light from a source passes by a Kerr black hole on two sides at the equator and converges at the observer. The axis of rotation of the black hole is perpendicular to the direction of light. Two rays ...
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### Could a black hole and a white hole cause gravitational waves, if orbiting one another?

I have been puzzled by an idea that I came across while reading about white holes. But I've only just started learning about white holes, so my knowledge is severely limited. It seems I also can't ...
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