Recent comments in /f/space

noncongruent t1_jdxowzx wrote

Reply to Hypothetical question by jd-sutton

The problem is that in order to see past events, you need to get the telescope out there faster than the speed of light. A photon leaving the Earth travels at the speed of light, and since no known or suspected technology allows traveling faster than light, or for that matter even at the speed of light, it means that no matter how far away you put the telescope it'll always only be able to catch photons that left Earth after the telescope did.

https://en.wikipedia.org/wiki/Light_cone

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Head_Weakness8028 t1_jdxohht wrote

Reply to Hypothetical question by jd-sutton

I definitely see where you’re going with this. Assuming we could fold space-time and position a telescope as you mentioned. Also assuming that this hypothetical telescope is far more powerful, and could extract actual details from the surface of the planet; Then yes, the telescope would “out-pace” the photons reflecting from the earth, and you could hypothetically see anything during earths history. Interesting thought experiment for sure. With our current understandings of the universe it seems the only time traveling we can do is into the future. By either traveling at velocities near the speed of light or parking close to an extreme gravity source you can slow your local resonant frequencies as compared to those on the Earth.

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AncientMarinerCVN65 t1_jdxn2x5 wrote

Reply to comment by Nerull in Are galaxies just giant accretion disks around super massive black holes? by darthvadercock

I had heard our galaxy's super massive black hole was a small percentage of the overall mass, but I didn't know it was that small. That's amazing, considering it's over 4 million times the mass of our sun.

Another way to look at it, also, is that the matter in accretion disks around black holes is slowing down and spiraling in due to friction. We ain't! We're content to just keep orbiting way out here in the galactic boondocks, at least until we collide with the Andromeda galaxy in 5 billion years.

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AncientMarinerCVN65 t1_jdxl5r4 wrote

Reply to comment by wombat5003 in Are galaxies just giant accretion disks around super massive black holes? by darthvadercock

That's interesting, I've never heard it put that way. All of the stars in our galaxy are bound to each other via gravity. And they would have similar magnetic fields if they are all rotating the same direction (adding together and making a cumulative magnetic field, similar to two waves in the ocean combining to make one big wave). But over long distances, gravity is much more powerful than electromagnetism. So the force of gravity wins out on large objects like planets and stars. But the galactic magnetic field would definitely have an effect on individual electrons or ions floating around in space, perhaps funneling them towards a certain point, the same way Earth's aurorae funnel solar wind towards the North and South poles.

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Head_Weakness8028 t1_jdxkr2h wrote

Reply to comment by Nerull in Are galaxies just giant accretion disks around super massive black holes? by darthvadercock

Excellent summation and examples. Technically, to summarize even farther for OP, I would say that the supermassive black holes at the center of galaxies are the accretions of the matter that coalesced into a galaxy. Edit: I suppose you could say that the galaxy is an “accretion disk” in the making. Edit: Which begs the question; Do stars inevitably spiral into the supermassive black hole in the center of the galaxy or does the gravity from all of the stars, orbiting the center of the galaxy, balance everything out? I’m assuming the latter, however, off to research!

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UmbralRaptor t1_jdxh24v wrote

Reply to Hypothetical question by jd-sutton

1 arcsecond angular resolution implies a linear resolution of 1 au at a distance of 1 parsec. We're deep within small angle approximation land, so can easily show what 0.1" gets you at varying distances:

distance resolution comment
1 ly 0.03 au Barring clever lightcurve reconstruction, you would not be able to tell that the moon exists
10 ly 0.3 au I'm unsure that you could find Earth. (I mean, JWST's coronagraph doesn't have the contrast for this, but I'm also concerned about the inner working angle)
100 ly 3 au This would be beyond the capabilities of even proposed telescopes like the HabEx starshade

FWIW, HST has a comparable angular resolution to JWST, as do some spy satellites. (notably the ones that took the image that Trump leaked)

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0ld_Wolf t1_jdxgp20 wrote

Reply to Hypothetical question by jd-sutton

Something you had not factored in is the Earth's rotational speed.

I read once that the Hubble could potentially resolve something the size of a textbook on a desk...but the image would be blurred/smeared so badly by the Earth's rotational speed that it would not be recognizeable.

So basically, only large structures like continents, oceans, ice caps, and clouds would be recognizeable at any distance - no matter the potential power of a telescope.

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