A solar gravitational lens will be humanity's most powerful telescope (2022)

Or to put it another way: A gravity lens bends space so that the light from behind an object curves around it while travelling straight.

You are bending the dimension, the light travels straight through a bent dimension thus coming out curved.

I think that's mindblowing.

Stronger gravity around massive objects causes slow down of the part of a light wave closer to object, compared to outer part.

This difference in speed, caused by _interaction_ between the photon and gravitational field of the body, results in the bending of the light's trajectory.

Bending of spacetime is just a simplification of this process to model that easier.

That only matters in areas with _really_ high fields, this effect is negligible for areas far away from a singularity of a black hole.

Instead, it's really the space that curves. The light does not slow down, it always moves at the speed of light. In the general relativity there is no "gravity field", gravity is a fictitious force.

Edit: also, gravitational lensing applies to massive point-like particles as well. For slow-moving particles and weak fields, it's negligible compared to regular Newtonian orbits, but if a particle moves at a speed that is close to lightspeed, it'll be lensed just like the light.

"Bending of spacetime" is just computational trick to increase precision of the model.

Bending of trajectory because of change of speed of light is negligible, yes. It's only visible on light-year long distances.

Photon is very wide. Dual slit experiment show that single single photon interacts with two slits up to millimetre apart. Even small difference in speed/frequency at such large distance will accumulate to noticeable change of course at light year long distances.

I can calculate bending radius, if you wish.

It's the same effect as in reflections, except that speed difference between air and solid objects is much much bigger, which results in sharp turning radius.

Otoh there is no requirement for a wave front to have the same frequency as when it started. A gradient in the gravitational field can cause a gradient in the gravitational redshift and thus "parts of photon" can very well have slightly different frequencies. If you recombine the paths and have the photon to interfere with itself, the interference pattern will capture the shape of such a wave function as affected by the distortion in the gravitational field.

IIRC this is the "standard" way of thinking about what's going on although marrying quantum mechanics and general relativity is still a work in progress.

If you buy into another theory that involves a variable speed of light, I'd love to hear more about what exact theory are you talking about since it seems to me that the burden of proof is on who makes the most extraordinary claims.

Doesn't the entire photon simply exchange momentum with the star, without needing to invoke any higher-order effects? Just as the star exerts a gravitational pull on the entire photon, the entire photon exerts a (very miniscule) gravitational pull on the star.

Light doesn't travel in a straight line because, to change trajectory of photon, photon must interact with something to exchange momentum. You are talking about mathematical model[1].

[1]: https://en.wikipedia.org/wiki/Curved_spacetime

Let's imagine two points in space A and B, that are let's say 10 light minutes distant from each other. A signal going straight from A to B will thus take 10 minutes.

If point A sits in a strong gravitational field (e.g. it's orbiting a very heavy star), the signal will still take 10 light minutes to reach point B. (please tell me if you disagree with this assumption).

Now, let's place another heavy star at the midpoint between points A and B.

How long will it take for a photon emitted by A to reach B? Well, it won't reach it because it will hit the start that's in between.

But another photon whose direction wasn't directly in the path from A to B will follow a longer path, be deflected around the star and reach point B.

It will take longer than 10 minutes to reach point B because it will move along a longer path.

Do you agree this is what would happen?

c is an universal constant and it seems that you're saying that it is not!

No. I'm not forgetting anything. Photons _do_ _not_ change direction. They always move in straight lines (from their "point of view"). It's just that if you step a bit away, these straight lines are not actually "straight" globally.

A classic example is a 2D ant crawling on a surface of a sphere. From the ant's point of view, it moves in a straight line, but a 3D observer will see that a straight line is actually a 3D circle.

Conservation laws are not violated. A photon (or another particle) will cause its own slight bending of the space-time, that in turn will slightly bend the star's trajectory.

It does sound like an interaction between gravitational fields, but the models give different numeric predictions.

> "Bending of spacetime" is just computational trick to increase precision of the model.

It really is not.

> Photon is very wide.

Facepalm. Sorry dude, but you have no idea what you're talking about. Lensing and time dilation also happen for point-like particles like electrons.

I cannot read your mind.

> c is an universal constant and it seems that you're saying that it is not!

Yep, c is universal constant for many physical models.

In physical world, c is constant as long, as properties of physical vacuum (permitivity and permeability) are constant, which in turn depends on α (Fine-structure constant[1]), which, in turn, variates at higher energies[2].

[1]: https://en.wikipedia.org/wiki/Fine-structure_constant

[2]: https://arxiv.org/abs/hep-ph/0201198

That is not true. The "speed of light" in vacuum is not constant for all observers in the _general_ relativity. It is constant only _locally_, Lorentz invariance is a local symmetry in GR. Special relativity thus simply becomes an edge case of GR, where the Lorentz invariance is also a global symmetry.

That's how we get lensing, regions of space near a massive object are more "viscous" and the light moves slower through them.

Now imagine that it's not a star, but a black hole with a small radius to make arguments easier. You shoot a photon slightly off the axis, and it gets deflected.

You can try to treat a photon as a moving object, and integrate the forces acting on it. Taking Lorentz transformations into account, of course.

But the thing is, your calculations will be off, and the experimental results won't match your predictions. You will need to take into account that the lightspeed near massive objects is _slower_ for distant observers.

Another example, suppose that you have a star surrounded by a massive cloud of fog. Somebody shoots a laser beam from one side of the fog bank to another, while you are far away from the star. The fog is there just to allow you to see the beam as it moves, it does not by itself slow the light.

But you will actually see the light moving _slower_ than lightspeed!

Or equivalently, you can take a clock that ticks every second. And if you lower that clock to the surface of a planet, you will see the clock ticking slower. And this is a very real effect, we have to correct for it in the GPS satellites.

The speed of light is the same in both frames of reference. What you think is going affect the speed is actually the slowing of the proper time which effectively causes the photon to redshift.

It's not possible, because EM field doesn't affect all particles.

> Lensing and time dilation also happen for point-like particles like electrons.

If we stick 2E15 electrons together in a long line, then it will start to rotate too due to differences in gravitational field at inner and outer segments of the line. Something like that must happen to an 1mm wide photon too. I'm not talking about orbit of those electrons around object, but about rotation alone.

No. You can drop a ruler onto the surface of Earth and measure from the Moon the time it takes the light to travel from one end of the ruler to the other. It will be slower than the lightspeed from your point of view. This is a real effect, we've measured it.

However, it will be lightspeed from the point of view of an Earth observer.

It's not the EM field, but gravity.

> If we stick 2E15 electrons together in a long line, then it will start to rotate too due to differences in gravitational field at inner and outer segments of the line.

Just look at an individual electron. Why would it curve? It's sufficiently point-like for the gravitational field gradient to be negligible.

And this effect falls out directly from the warping of space-time described by general relativity

am I understanding correctly that you claim that the warping of space-time is just a mathematical trick and that the phenomena are better explained by just postulating they light slows down in a gravity well?

Light slows down in gravity wells because the space-time is "denser" near massive objects. This is not a mathematical trick, this is actually a real effect.

It's also the reason for gravitational lensing, as the shortest path through a gravity well is not a straight line. Light can avoid the slowdown near the massive object, if it instead "goes around" it. The curved path is longer, but faster lightspeed along it compensates for the additional length.