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Astronomers reveal first image of the black hole at the heart of our galaxy

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Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#121

Post by ISL »

kloch wrote: post #114
zeven7 wrote: post #90

Earlier quoted context omitted.

It seems the common expectation is that GR is more likely to be the theory that needs to be modified the most - or more or less tossed out and replaced with a quantum theory of gravity. Is it possible that GR is just "right" and QM needs to be modified to fit into it?

We can't even measure the gravitational constant to more than about 4.5 significant digits. Most other natural constants are in the 7-12 digit range and improving along with our technology. Measurement of big G hasn't convincingly progressed since the vacuum tube era. We have a lot more to learn about gravity, especially at laboratory scale and smaller.

As a laboratory-scale experimentalist, I heartily agree, but must also point out the existence of compelling precision equivalence principle tests.

While we can only measure G to ~10 ppm, the equivalence principle has been tested at the 10^{-14} level (and ~10^{-9} at meter-scales). The EP is the property that makes gravity really special and simultaneously the thing that makes gravity hard to test.

The core assertion that all things fall the same way is axiomatic to GR and has been very well tested. We have everything left to learn about gravity, but at the same time, GR has held up far better than it "should" have against a battery of really great experiments.

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#122

Post by raattgift »

ISL wrote: post #94
zeven7 wrote: post #90

Earlier quoted context omitted.

It seems the common expectation is that GR is more likely to be the theory that needs to be modified the most - or more or less tossed out and replaced with a quantum theory of gravity. Is it possible that GR is just "right" and QM needs to be modified to fit into it?

Whenever we learn more about the connections between GR and the rest of physics, the early corrections are likely to appear as "GR+new stuff" or "QM+new stuff". Both theories have enough predictive power that we'll hang on to them as effective theories for the rest of humanity's existence. That said, the mathematical fundamentals of GR do not directly incorporate notions of the uncertainty principle. That fact alone,…

Just to cause trouble, since we're straying far far from today's presentations (except arguably UCL's Z. Younsi in the ongoing ESO Q&A panel). :-) Can we write down the uncertainty principle in covariant form? Maybe! There is active theoretical work ongoing (e.g.) https://arxiv.org/abs/2110.15951v2> ("... a geometric formalism for the generalised uncertainty principle which is covariant and connects features of the underlying geometry with the deformation of canonical commutator relations ... [and] an elegant interpretation for the standard dispersion relation p^2=−m^2: it describes flat spacetime in the Milne coordinates, with rest mass m giving the measure of geodesic length from origin").

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#123

Post by dahdum »

sillysaurusx wrote: post #108
zeven7 wrote: post #90

Earlier quoted context omitted.

It seems the common expectation is that GR is more likely to be the theory that needs to be modified the most - or more or less tossed out and replaced with a quantum theory of gravity. Is it possible that GR is just "right" and QM needs to be modified to fit into it?

The most likely explanation is that we have no clue, just like every other point in history. Science is always new. Everyone is different, but most scientists seek areas where experiment doesn’t match the predictions. It’s the only way we learn. One such instance is the W Boson Anomaly: https://vm.tiktok.com/ZTdpwrmoj/?k=1 If the equations were right, the experimental error would be zero (modulo uncertainty bars). In…

> As an aside, this is also a great example of TikTok turning a corner. I now have 184 educational videos saved, along with dozens of science videos. I learn more on TikTok than any other source now, which I didn’t expect.

I too have been surprised with the amount of genuinely educational and interesting material on TikTok. It won't replace Wikipedia but it's a great companion to it.

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#124

Post by bldk »

ISL wrote: post #89
mabbo wrote: post #5

> “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity," And ten thousand physicists sighed disappointingly.

Gravitational physicist here. Every time we look in a new place and see that GR still works, it is pretty amazing. Part of this is the fact that everyone in the field works very hard to find a crack in GR's armor, and so far it has resisted everyone. The other part is that GR has a romantic beauty to it, both in structure and in predictions. Each time GR matches reality in a new context, the feeling is akin to watchi…

can't GR be a micro macro scale solution where perhaps that's why the theories do not work out?

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#125

Post by fsh »

hammock wrote: post #113
wthomp wrote: post #91

For those wondering if we could get sharper images with JWST, here’s the previously imaged black hole (same angular size as our own) compared to a single pixel from Hubble’s wide field camera 3: https://twitter.com/alex_parker/status/1116070667068170240?s... JWST will have smaller “pixels” but is in the same ballpark.

How sharp do we expect an infinite-res picture to be?

The resolution of these telescopes is limited by diffraction, not by the number of pixels on the sensors. The achievable angular resolution is roughly the wavelength divided by the aperture diameter [1]. JWST works in the few µm wavelength range and has a 6.5 m aperture, such that the angular resolution is ~0.1 arcsec. The EHT works with 1.3 mm wavelength and has an effective aperture of roughly the earth diameter (~13000 km). This leads to an angular resolution of a few ten µarcsec which is more than 1000 times higher than that of JWST.

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

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#126

Post by KAMSPioneer »

kloch wrote: post #57
mabbo wrote: post #52

Earlier quoted context omitted.

> The only way to get higher angular resolution is to use higher frequencies (which they are working on) or use radio telescopes in space to get longer baselines than the diameter of the Earth. Would it be possible to use the same trick in space? IE: get a baseline the diameter of Earth's orbit (roughly)?

For Interferometry to work the data from different baselines has to be collected at the same time so waiting 6 months does not help. Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech. Even on Earth they resort to shipping cases of hard drives instead of transferring over the Internet. One technique where simply wait…

So I work at one of the participating institutes, and you're very much dead-on about transfer speeds/logistics limiting options here. I'm not an astronomer, so grain of salt, but the data is generated at a bit of an unwieldy pace: there are four collector nodes at each site, each of which generates ~16Gbps of raw data (though read speeds from disk after observation is more like 8Gbps). This, as you say, forces most locations to ship physical drives, and also makes centrally planning these observations rather tricky, as there's very little visibility into each station's observations until some time afterwards.

But I'm optimistic that some institutions (hopefully including mine!) will be able to transfer these data over the network after the next run of EHT observations. Exciting stuff for sure.

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#127

Post by monocasa »

mhandley wrote: post #67
mabbo wrote: post #5

> “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity," And ten thousand physicists sighed disappointingly.

Any evidence supporting general relativity at these scales is presumably also evidence in favour of dark matter actually being matter, as opposed to gravitational theory needing modification. Seems like we can still learn quite a bit from ruling out alternatives, even if it's not the quick answer we might like.

The Schwarzschild radius of a 4M solar mass black hole like Sagittarius A* is only (lol) about 7M km. Most of the MOND theories I've seen are about fall off curves at very large distances (ie. galatic scales) being more complex than GR suggests, so I'm not sure this is really proof either way. (And I say this as not a MOND adherent).

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#128

Post by lordnacho »

zeven7 wrote: post #66
mabbo wrote: post #48

Earlier quoted context omitted.

Quantum Physics and General Relativity make different predictions in specific circumstances which are beyond our ability to test to figure out which one is wrong. They can't both be perfectly correct. But in every test we can make, both seem to model reality really well. The best place to look for new data where we might find reality disagreeing with either model is in the extreme parts of the universe, like black ho…

Relatedly, before general relativity, people already knew something was wrong with Newton's theory of gravity from observing the orbit of Mercury, which didn't follow the theory accurately. At that time Mercury, being so close to the Sun, was an extreme condition that showed the flaws in the theory. The fact that general relativity continues to hold up to every observation we can make is remarkable.

Could it happen that the universe simply never provides us with the required evidence? Eg if there's an anomaly but only when something that never happens happens. Like a large enough mass/charge/etc or some flavor of unlikely coincidental occurrence, like an eclipse is a bit of a coincidence.

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#129

Post by kloch »

KAMSPioneer wrote: post #126
kloch wrote: post #57

Earlier quoted context omitted.

For Interferometry to work the data from different baselines has to be collected at the same time so waiting 6 months does not help. Spacecraft could theoretically be used to extend the baselines but the volume of data to be transferred is prohibitive with current spacecraft comm tech. Even on Earth they resort to shipping cases of hard drives instead of transferring over the Internet. One technique where simply wait…

So I work at one of the participating institutes, and you're very much dead-on about transfer speeds/logistics limiting options here. I'm not an astronomer, so grain of salt, but the data is generated at a bit of an unwieldy pace: there are four collector nodes at each site, each of which generates ~16Gbps of raw data (though read speeds from disk after observation is more like 8Gbps). This, as you say, forces most l…

How many seconds of observation data are required for one of these images?

Re: Astronomers reveal first image of the black hole at the heart of our galaxy

#130

Post by f1codz »

ISL wrote: post #89
mabbo wrote: post #5

> “We were stunned by how well the size of the ring agreed with predictions from Einstein’s Theory of General Relativity," And ten thousand physicists sighed disappointingly.

Gravitational physicist here. Every time we look in a new place and see that GR still works, it is pretty amazing. Part of this is the fact that everyone in the field works very hard to find a crack in GR's armor, and so far it has resisted everyone. The other part is that GR has a romantic beauty to it, both in structure and in predictions. Each time GR matches reality in a new context, the feeling is akin to watchi…

How long does it take to train to fully appreciate this like you do, if that is even possible for someone in a full time job in an unrelated field.
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