
In a photo of a black hole, a possible key to mysteries - Hooke
https://news.harvard.edu/gazette/story/2020/04/black-hole-imaging/
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xvilka
If anyone is not a physicist but wants to dive a bit deeper into cosmology and
quantum theory without insurmountable math - I can recommend PBS Spacetime[1]
YouTube channel and their Discord community. I was pleasantly surprised to see
the most updated information about the current knowledge, including the
results from 2019.

[1]
[https://m.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g](https://m.youtube.com/channel/UC7_gcs09iThXybpVgjHZ_7g)

~~~
SkyMarshal
What's the Discord community link? Not seeing it on their website or YT
channel.

~~~
xvilka
It requires Patreon $2 sponsorship at least [1].

[1]
[https://www.patreon.com/pbsspacetime](https://www.patreon.com/pbsspacetime)

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fernly
Alright, what the bloody hell is this text, and Peter Galison, attempting to
say?

> A black hole hoards images of the past. Light is composed of photons, and
> each one carries a bit of the image of whatever it hits. So when you see a
> tree the light hits the tree, bounces to your eye, and your brain eventually
> pulls it all together like a mosaic. Light stuck in a black hole’s
> gravitational pull can loop once, twice, or an infinite number of times,
> depending on its angle of approach, Lupsasca said. Those[sic] that finally
> escape in the direction of Earth carry a reflection of what the universe
> looked like when they entered the black hole’s pull. The longer light was
> held captive, the earlier in the past their image shows.

> “As we peer into these rings, first, second, third, etc., we are looking at
> light from all over the visible universe; we are seeing farther and farther
> into the past, a movie, so to speak, of the history of the visible
> universe,” said Peter Galison, the Joseph Pellegrino University Professor of
> the History of Science and of Physics...

Does anyone assert that any photon carries information about what it
interacted with in the past? Does a photon have any information content other
than its energy/wavelength? So "each one carries a bit of the image of
whatever it hits" is total (ok, be nice now) poetic blarney.

So then, you observe some photons coming away from a black hole, how the AF
are you supposed to be able to know which ones circled it once, twice, 42
times, whatever? And in what possible sense, Prof. Galison, is any of that
like a "movie of the history of the universe"?

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pdonis
_> Does anyone assert that any photon carries information about what it
interacted with in the past?_

Yes. Each photon carries one bit of information. (I don't know if Galison
intended "a bit" to mean literally a bit of information, but it's true.)

 _> Does a photon have any information content other than its
energy/wavelength?_

Yes, its polarization. In laboratory experiments in quantum information,
photon polarization is what encodes the information.

 _> how the AF are you supposed to be able to know which ones circled it once,
twice, 42 times, whatever?_

By putting together the images conveyed by very large numbers of photons and
looking for patterns in them. Yes, the information can be hard to decode, but
it's there.

~~~
cryptonector
> Each photon carries one bit of information.

More than a bit. Between its wavelength, its presence (or absence),
polarization, and timing, a photon can carry a great deal more than one bit.

~~~
pdonis
You can't measure all of these things simultaneously (the operators don't
commute), so even if all these observables exist for a photon, they don't all
carry independent information. You can only "read out" what you can get from
one observable.

In most quantum information experiments, that observable is polarization (more
precisely, polarization in some particular direction), which is an exact one-
bit measurement result.

In radio telescope observations, it's typically amplitude and wavelength that
are measured. I don't know that "the information content of a single photon"
is really meaningful for such observations since they don't observe single
photons, they observe the overall intensity at different wavelengths.

~~~
Strilanc
Even accounting for the uncertainty principle, the amount of information you
can transfer in principle with a single photon is far more than one bit. For
example, you can distinguish the single photon hitting a screen at more than
two different times, or at more than two different places, or at more than two
different energies.

~~~
pdonis
_> you can distinguish the single photon hitting a screen at more than two
different times, or at more than two different places, or at more than two
different energies._

You can't transfer information by any of these means because you can't control
the time or the location or the energy of the photon hitting the screen.

The reason polarization is used to carry information in quantum computers is
that you _can_ control it, so you _can_ use it to transfer information. But,
as I said, a single photon can only be used to read out one bit in this way.

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credit_guy
So, maybe someone can explain in more concrete terms. The distance from us to
the supermassive black hole in the center of the galaxy is 25k years. A photon
traveling from us to the black hole, that happens to go around the black hole
and come back would reach us in 50k years, or in 50k years plus a few million
years due to the local time-space distortion around the black hole? If the
photon goes around the black hole once and escapes and comes back, will it be
back in 50k years, or in 1 billion years?

~~~
lihaciudaniel
I would gladly answer but I'm not a theoretical physicist. Here's a video
explaining it better:
[https://youtu.be/UjgGdGzDFiM](https://youtu.be/UjgGdGzDFiM)

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femto
Can VLBI be done with a single moving telescope, with a very precise clock?

I'm thinking of a single frequency point source in space. The received signal
will be periodic. Thus we can move the receiver to a new point in space and
still know what the received waveform would be at old position based on its
periodicity (if we have an accurate clock). From there we can do
interferometry?

A Fourier transform allows the same reasoning to be extended to a multi-
frequency source. We can also extend the source spatially, in the same way an
arbitrary hologram can be considered to be a collection of "zone plates" due
to a set of point sources?

If the above is true, can we use a single Earth based telescope (with clock)
to form a telescope with a diameter of the Earth's orbit over the course of a
year?

Maybe the flaw is that it assumes that the scene being observed is static over
the period of observation? How much change can be tolerated before a scene is
no longer static?

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michaelcampbell
Do black holes appear the same no matter what direction you see them from?
Every one of the theoretical views (and indeed the "real" one) are the same. I
get the layman's description of why and how spacetime curvature causes it, but
do they look the same if you view them from the plane of the accretion disk vs
through the "poles" because of this?

