# Tag Info

## Hot answers tagged gravitational-lensing

40

Contrary to the other answers, I will point out that gravitational lensing due to planets in the solar system is a significant and measurable effect. The measured positions of stars, as seen from a point in the solar system near the Earth are altered by the gravitational deflection due to the fields of the Sun and then, in order of decreasing effect, Earth ...

35

It's pretty easy to see that the gravitational lens doesn't have a focal point. Please excuse my very poor drawing: Rays from the top are focused across a line. A black hole would have a "focus line" but not a focus point.

29

No. Ordinary optical lenses deflect the light ray, at least in a linear polarization, by an angle that is linear as a function of the location $(x,y)$ on the lens: $$\theta \sim ax + by$$ Well, we should really talk about $\vec k$, the wave vector, a two-dimensional angle of a sort. On the other hand, the deflection by the gravitational lens goes like an ...

25

Yes, it does, or to be more precise, Newtonian gravitation predicts photons will be deflected if you assume photons have some mass. However the amount of this deflection is just half of what GR predicts. And the observed amount of deflection is what GR predicts (within experimental error). In particular the way this is studied is by using something called ...

16

The gravitation lens generates smaller angular deflections at large impact factors. A converging lens generates larger angular deflections at larger impact parameters. So, a first guess would be "no way".

15

The deflection angle for a light ray that is just grazing the surface of a planet or star (so the maximum observable deflection) is $\displaystyle \theta = 2 \left( \frac {v_e} {c} \right)^2$ where $\theta$ is in radians, $v_e$ is the escape velocity of the planet or star and $c$ is the speed of light (see this Wikipedia article). For a planet, $\frac {v_e}{... 13 Of course Coulomb's law has to be adapted! And it is therefore fortunate that there exist manifestly covariant formulations of electromagnetism that do not care how spacetime is curved. However, we should first briefly observe that Coulomb's law is not one of the fundamental laws of electromagnetism, though it has played a great role in its inception: ... 13 Summary This is a partial answer which cannot deal with general gravitational waves, but there is a simple answer, namely, yes gravitational lenses do work on gravitational waves, for a certain class of waves, i.e. those with small wavelengths and small amplitudes. Specifically, the components of a gravitational wave of small enough amplitude that it can be ... 12 I think the distinction that you are making, with concerns about the intermediate blueshifts, redshifts, and time dilation, is too technical for someone whose education is outside of physics altogether. I would start with something simpler and add complexity as the responses of your audience suggest appropriate directions to go. When I explain about ... 11 For the 1st part of your question:- You need light to see anything. Firstly I will emphasize on my comment. If you reach the event horizon of a black hole safely (where light can get into orbit around the black hole), then since the light is in orbit, the light from the back of your head would go around the black hole and come back to reach the front of your ... 10 In Newtonian gravity the acceleration due to gravity is independant of the mass of the object - a falling elephant accelerates downwards at the same speed as an accelerating gnat (ignoring air resistance). That means the orbit of an object does not depend on the object's mass (provided the object is much lighter than the star). The deflection of an object ... 10 Milgrom's simple Newtonian MOND cannot, as it is just a modification of newtonian dynamics (which is the acronym for MOND, after all). Jacob Bekenstein, however, has worked out a relativistic generalization of MOND called TeVeS that does account for gravitational lensing and a variety of other effects: https://en.wikipedia.org/wiki/TeVeS TeVeS is ... 9 Did you notice the title of the video? Former NASA Physicist Disputes Einstein’s Relativity Theory At the moment main stream physics, which is what we discuss here, does not accept all the various theories trying to unseat General Relativity . Usually these people have an obsession with Einstein or other prominent physicist as personalities . Look at the ... 9 Thinking simply about a point source such as a black hole, the equivalent optical lens created by such an object looks like the image below (taken from this article). That is to say it falls off as$\frac{1}{r}$as pointed out by Lubos Motl in his answer. This object has some interesting optical properties. The$\frac{1}{r}$potential will act as a ... 9 To ask what do physicists expect to accomplish with gravitational lensing is nowadays somewhat like asking what do biologists expect to accomplish with looking at things with microscopes. Gravitational lensing is a well established method used across astronomy and the main challenges the field itself has to tackle are mainly technicalities. But I will try ... 9 No, you could not make this work. A black hole acts like somewhat like a lens. The effective focal length for beams that get bent by 180 degrees would be rather short, so the nearly parallel beam emitted by you standing far away would be scattered long before it made it back to you. In practice it would be practically impossible to make even a real mirror ... 8 Yes, it's not only possible but relatively easy. For a gravitational lens the deflection angle at a distance$r$from the black hole is approximately proportional to$1/r$. So your lens is going to look something like a hyperboloid of revolution: Note that the light rays are bent increasingly strongly as you approach the axis. This type of lens has been ... 8 In a sense, the photons do travel in a straight line. They follow what are called geodesics, which are the shortest paths between two points in a general curved space-time. For the case of a flat space-time, indeed the path that photons would follow would be the familiar straight line, but when the space-time is curved, then their trajectories differ from ... 7 The problem with your proposal is the distance to the black hole. Even if a black hole is only one light year away, then a telescope probe that changes the angle of return for the image of the earth by even one degree would have to be quite far from Earth. How far? If the Earth-telescope distance looks like a 1° angle "as seen from the black hole", then it's ... 7 There is indeed a light path from any object (outside the horizon) to your viewpoint. However, there is still a "dark circle" around the hole: a range of angles that no photon will be coming from towards your viewpoint. For this reason it seems wrong to call the hole "transparent", even though it does not obscure anything. : actually multiple light ... 6 Is it true that gravitational lensing only occurs for objects made of plasma? Most of space contains a plasma, or at least some ionisation, at some density or other, making the claim hard to dispute at that level. Also, gases and/or plasmas will tend to exist with higher densities in deeper gravity wells. What is the merit of claim that bending could be ... 6 I believe you've already spotted the answer to your question with this sentence: And a black hole will shift the trajectory entirely. This is all dependent on the proximity to the source of gravity. You can "shift light" (bend its trajectory) as much as you want with as little mass as you want using a black hole. Just let the light get arbitrarily close ... 6 If you assume that light "drops" at the gravitational acceleration$g$, and calculate the angle by which light is deflected using Newtonian mechanics, you end up with the formula: $$\theta = \frac{2GM}{c^2r}$$ where$M$is the mass of the deflecting object and$r$is the distance of closest approach. A quick Google should find the calculation, or see ... 6 Yes. When you look at a (Schwarzschild) black hole with a sufficiently good telescope (according to GR), you see arbitrarily (infinitely) many "Einstein rings". Between each concentric pair of rings, there is an image of the entire surrounding sky. These images correspond to the light paths (null geodesics) that are unbound but very close to the photon ... 6 The only possible answer that can be given here is that those gridlines are not an accurate representation of spacetime curvature. It's unfortunate, because we would all love to have a graphical way of understanding general relativity, but it's true. Therefore, it doesn't really make sense to draw conclusions based on it. The -time part in spacetime ... 6 In general relativity, any test particle's worldline (whether the particle is massive or massless) is determined entirely by its initial four-velocity$u^\mu\$, and not by any other properties. This is manifested by the geodesic equation $$\frac{d u^\mu}{d\tau} = \Gamma^{\mu} {}_{\rho \sigma} u^\rho u^\sigma.$$ From the properties of ordinary differential ...

5

Weak Lensing, Strong Lensing and Microlensing are all forms of gravitational lensing---in which observed images of a distant object are affected by the gravitational field of an intervening, more nearby object. Strong Lensing: Is when the lensing is pronounced enough that multiple images, or an Einstein ring is apparent. Weak Lensing: is when the image ...

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