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Comment by delecti | Hacker News Reader

Gravitational lensing has to be the coolest thing ever. That we can use a black hole as a telescope to see such incredibly distant bodies just leaves me with a childlike giddiness whenever I think about it. What's almost as incredible to me is that someone figured it out before we could prove it.

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It's also nice that we apparently can "take" a black hole millions of times as massive as our own system, and use it for our purposes.

It's likely the single biggest part of a scientific instrument. (Not of an industrial tool... yet.)

There's not really any materials or conditions beyond one's immediate locality that would be worth seeking out in space. Interstellar communication is going to be limited at best, interstellar industry or trade may never be practical, and even if one is willing to accept the idea that there is some purpose for which a black hole is useful, I would still be skeptical that a supermassive black hole would be of more use than a smaller one.

The larger the black hole, the more gravitational lensing there is?

Larger black hole would mean larger mass. Larger mass would mean a larger bulge/dent/bend in the fabric of spacetime. That would imply to me a larger lensing effect. Not sure if it would be similar to larger primary mirrors for telescopes or not. But in my head, it makes sense. Another movie plot begins...

I agree. I don't think the aeroplanes will ever been able to achieve lift. I'm skeptical of anyone who attempts anything that hasn't been done before.

We saw birds flying around when we were cavemen. It's always been clear that heavier-than-air flight is possible. We don't currently know of any space-birds flying around black holes.

It's also reasonable to assume that more and more extreme physics is going to be harder and harder (if not practically impossible for any future humans) to come up with new gee-whiz uses.

New revolutions in science have tended to be less "we were totally wrong" and more "in extreme cases things don't quite fit the models". It's conceivable that FTL communication and travel aren't as impossible as we currently think, but that would require that things we've demonstrated fairly well are completely wrong. The same wasn't true with heavier than air flight, which (as the other comment notes) we had proof of it being possible (in birds). We knew it was possible in theory, but figuring out how to actually work out the mechanics of the process were the sticking points.

https://chem.tufts.edu/answersinscience/relativityofwrong.ht...

There are huge parts of physics that remain a mystery( dark matter, dark energy, the big bang, like how on Earth does inflation make sense, a ton of crazy particle physics that go completely over my head). And the funny part is that a lot of these can't be really fit into our current models in a meaningful way. So until we deal with all those I still have hope for an FTL drive.

Physics is very difficult, and magical thinking is much easier and more fun. Relativity is a description of the geometry of the universe, and any subsequent theory will need to explain the same observations. These observations both prohibit any form of FTL, and suggest that a universe with FTL would almost certainly contain causality violations. The hope that down will someday become up is ill-founded, and the effects of this would not be what you would want.

They’re being built by bicycle mechanics for goodness sake.

Black holes do indeed have practical applications. Small black holes are perfect mass-to-energy engines that don't require storing large amounts of antimatter. You just feed them whatever scraps you have on board and use the Hawking radiation they produce as energy source, e.g. for accelerating your ship.

That (mostly) worked well for the Romulans

Use Hawking radiation as an energy source? How exactly do you see that one working?

There actually is at least one interesting and potentially important difference between a supermassive black hole and a stellar mass one. Counterintuitively, even though the total gravitational effect of a huge black hole is larger, the strength of the tidal forces goes down as mass goes up. Tidal forces are the cause of "spaghettification" (yes, that's the real term!), which is the expected cause of death for anyone getting too close: if you fall feet-first toward a black hole, your feet will feel much greater gravitational acceleration than your head and you'll be stretched into spaghetti. My memory is that for a normal stellar-mass black hole this would happen even outside the event horizon (the radius of no return), while for a very large black hole you might be able to safely get close to the event horizon. So any experiments or applications that involved being close to the horizon could be much easier there.

Your memory is incorrect. The interesting gravitational effects typically happen much closer to the singularity. The event horizon is an otherwise normal region of space, and more boring the larger the black hole is.

My main point was the same as yours: that the event horizon looks more and more "normal" as a region of space the larger the black hole is, so if you want to get close to a black hole's event horizon to test its properties (e.g. to explore whether quantum effects might cause that "normalness" to break down), visiting a very large black hole is your best bet.

I haven't double checked the calculations myself: too much other stuff to do. But glancing around at other people who have looked at the numbers, it seems pretty clear that spaghettification is expected to happen for macroscopic objects well outside the event horizon of a stellar-mass black hole. Wikipedia gives an example of a 10-solar-mass black hole: its event horizon radius is about 30km, but macroscopic objects will be spaghettified at a radius of about 320km. https://en.wikipedia.org/wiki/Spaghettification#Inside_or_ou... (That ratio is roughly reversed for a 10,000-solar-mass black hole.) There are some similar calculations shown in detail on this NASA math worksheet (which is for some reason still using cgs units): https://spacemath.gsfc.nasa.gov/blackh/4Page33.pdf

You're right, I'm wrong.

I assume we just need to find rotating black holes and big mirrors.

I agree. I'm also blown away by the programmers that have done the math on how to unbend the light we see to put it back into an actual image.

You can use whole galaxies as gravitational "telescopes".

And yeah, it's super-cool.

Dumb question but does the lensing distort what we would see if there was no lensing at all? Or does that information get compressed as part of the lensing and we need to "stretch" it out to see it?

This is what an Einstein ring looks like, caused by lensing:

https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hire...

Anyway, the test of GR in the actual article does not appear to use or reference lensing at all. It's about looking at the orbit of a star in an extreme gravitational environment and comparing it to what Newtonian models would predict.

Wouldn't Newtonian models be off in their predictions for those extreme cases?

I assume that's the point?

Indeed that's the point:

> The team compared the position and velocity measurements from GRAVITY and SINFONI respectively, along with previous observations of S2 using other instruments, with the predictions of Newtonian gravity, general relativity and other theories of gravity. The new results are inconsistent with Newtonian predictions and in excellent agreement with the predictions of general relativity.

The conservation of information means that the phase space of a system is incompressable. I believe this is equivalent to saying we can just stretch the image out, within the limits of our ability to resolve detail.

This observation isn't about lensing

The sun could be used as well, the problem is that the focus would be outside the solar system.

One of the early bits of evidence for relativity was it's ability to explain anomalies in the precession of Mercury's orbit.

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