Einstein theory passes black hole test

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The black hole at the centre of our galaxy has helped astronomers confirm a key prediction of Albert Einstein's ideas.
By observing a cluster of stars near the hole, they were able to confirm a phenomenon known as "gravitational redshift".
It's when the wavelength of light gets stretched out in response to a gravitational field.

https://www.bbc.co.uk/news/science-environment-44967491
 

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I thought we knew this already, since it's similar to gravitational lensing? Or have I misunderstood?
 

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That's what I thought!

N.
 

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I thought we knew this already, since it's similar to gravitational lensing? Or have I misunderstood?


It's not the same effect. Gravitational lensing refers to the path of light being altered by a gravitational field. Gravitational redshift refers to the wavelength being altered by a gravitational field.
 

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I thought we knew this already, since it's similar to gravitational lensing? Or have I misunderstood?

You have misunderstood.

What was confirmed now is a different effect:

If you have a photon that is emitted deep inside a gravitational well, as the photon travels up the well, it actually loses energy and its wavelength increases.

Of course, the opposite is true when a photon falls into the gravitational well. Gravitational lensing happens when light from behind a black hole is bent towards us. In that case, the light fell into the gravitational well, got redirected and then exited the gravitational well, so the "falling in" and "climbing out" effects cancel out.

But this time the star itself entered the black hole's gravitational well and once there, it emitted photons. Those photons appeared to be redder than they should have been because they only had to "climb out".


The effect gets more obvious in stronger gravitational fields, so it's very difficult to confirm unless you have a black hole and a star handy.

---------- Post added at 14:45 ---------- Previous post was at 14:43 ----------

It's not the same effect. Gravitational lensing refers to the path of light being altered by a gravitational field. Gravitational redshift refers to the wavelength being altered by a gravitational field.

Wavelength, frequency and energy, because they're all connected. The gravitational field saps energy from the photon.
 

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It's not the same effect. Gravitational lensing refers to the path of light being altered by a gravitational field. Gravitational redshift refers to the wavelength being altered by a gravitational field.

Welllll...

I think the two effects are actually related: The parts of the wavefront that pass closer to the gravitating object travel through a region where time travels more slowly. Since light always travels locally at c, the light deeper in travels at an apparent speed less than c as observed from upwell (or, alternatively, covers a greater distance), distorting the wavefront., and also causing the wavelength to shorten as you get deeper in.

The wavelength observed is the same as the wavelength emitted if the source and observer are at equal potentials, but the change of wavelength does come into play.
 

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It's hard to fathom that he was able to make such stunning theories 100 years ago, with no technology to support him. Just crazy impressive.
 

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Welllll...

A photon is not divisible, so we can't talk about wavefronts here. You can talk about two different photons travelling in parallel, but if they enter the well in parallel, they won't exit it like that, because the one close to the black hole will get bent more.

The effects are related through relativity, sure, but not directly. Light bends not because of some gravitational attraction or potential, it bends because it travels in straight lines and the straight lines are bent by the black hole. This doesn't apply to photons only, but to any massless particle. We can even take a very high energy neutrino as an approximation: It travels so quickly that when it passes past a planet or star, it travels in a pretty much straight line. But when passing a black hole, it no longer can, because the straight lines aren't straight for an outside observer.

The second effect does come into play because of potentials.



It's hard to fathom that he was able to make such stunning theories 100 years ago, with no technology to support him. Just crazy impressive.

Einstein didn't just poof his theory into existence without facts that conflicted with previous understandings:
- Speed of light was measured to be constant in all frames or reference by Michelson and Morley using a Michelson interferometer.
- Slowdown of the decay of particles entering Earth's atmosphere, requiring technology to detect and characterize said particles.

You're conflating lack of computers with lack of technology.
 

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A photon is not divisible, so we can't talk about wavefronts here. You can talk about two different photons travelling in parallel, but if they enter the well in parallel, they won't exit it like that, because the one close to the black hole will get bent more.

A photon is not divisible, but we *can* talk about wavefronts. Each individual photon propagates as a wave according to Maxwell's equations. You can't meaningfully talk of two photons travelling in parallel unless you set up detectors to track them every step of the way, in which case you alter the way they propagate.

Light bends not because of some gravitational attraction or potential, it bends because it travels in straight lines and the straight lines are bent by the black hole.

The point of General Relativity is that both cases are equivalent.
 
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