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playadefaro t1_j262fha wrote

Reply to comment by Aseyhe in How fast does the Milky Way spin? How far does Earth move through space in a year? by Sabre-Tooth-Monkey

Thank you for the detailed response. Do we know the fastest celestial object?

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amaurea t1_j26aodj wrote

  • Individual particles reach speeds extremely close to the speed of light, but I guess those don't count as celestial objects
  • Streams of plasma ejected as beams by quasars, or as shells of matter ejected by supernova explosions, or plasma spinning around a black hole as part of an accretion disk also move close to the speed of light, but these aren't really objects either.
  • Black holes or neutron stars that orbit each other in a pair gradually lose energy, causing the orbit to gradually shrink while speeding up. Just before they hit each other they reach speeds very close to the speed of light. With current technology we can observe these minutes to seconds before merger in gravitational wave detectors like LIGO, when they're at their fastest but most transient. But they will be moving very fast for years before the merger - we just haven't found any at this stage in their life yet. The famous Hulse-Taylor binary is 320 million years away from merger, but is already moving at 0.15% the speed of light and will only speed up from here.
  • Stars orbiting supermassive black holes, such as those orbiting the one in the center of the Milky Way, can reach very high speeds. The fastest of these with a robust speed measurement is S14, which reaches 3.83±0.06% of the speed of light.[*] Of course, there could easily be even faster stars orbiting this (or other) supermassive black holes that have not been discovered yet
  • Simulations show that collisions between rapidly spinning black holes result in asymmetric emission of gravitational waves. These carry away huge amounts of momentum, and by conservation of momentum, the merged black hole gets a substantial kick in the opposite direction. In the most extreme cases, this may result in the black hole reaching 1.3% the speed of light.
  • The fastest known free-flying star, S5-HVS1 according to Wikipedia, moves at 0.59% the speed of light compared to the galaxy.

[*]: The stars S4714 and S175 are nominally faster, at 8±3% and 4.27±0.47% of the speed of light, but given their large uncertainty they are probably slower than S14. S62 with 7.03±0.04% looks like it's the fastest one with a good measurement by a good margin, but this one turned out to be an error (which reminds me that I should get around to updating the wikipedia articles mentioning this star).

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clocks212 t1_j26hnaj wrote

I’ve searched before but never found an answer, maybe you know. Could you be close enough to merging black holes to feel/be killed by the gravitational waves without already having been killed by the black hole or its accretion disk?

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Kantrh t1_j26scg5 wrote

Possibly. According to this article https://www.forbes.com/sites/startswithabang/2020/02/15/ask-ethan-could-gravitational-waves-ever-cause-damage-on-earth.

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Onetime81 t1_j28r1du wrote

If death didn't occur until after crossing the event horizon, then this would be the best way to die, imo

As you cross you'd be able to watch the universe age and die. You'd get a conclusion to the story... Right before you cells started to unmesh themselves from your body.

Trade offs, amirite ¯⁠\⁠⁠(⁠ツ⁠)⁠⁠/⁠¯

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Theban_Prince t1_j290xqy wrote

Would you just freeze in time halfway there? Meaning even ypur brain will not be able to perceive anything.

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Onetime81 t1_j2a5z34 wrote

Depends on your perspective, naturally, with spacetime being relative and all.

From an observers pov, say comrades who couldn't catch you in time, you would freeze for forever, until you the light bouncing off you slowly redshifted out of our visual range. Which sounds awful to experience, even just watching.

On the plus side, you would allow accurate mapping of where the horizon actually is, since it's invisible, the moment you 'froze' would be the moment when you crossed over.

From you're pov its speculated that at that distance you'd essentially be outside time. And past the horizon light only goes one way, and that's in towards the black hole, which you wouldn't be facing, so you'd see all of the light from all time descending towards you. Whether that's linear, and just like a VCR on fast forward, that I can't say, and I don't know if that will or would ever be verified.

So you'd get the answers of how it all ends (heat death, big crunch, cyclical, neuvo-physics bubble, great unraveling, grey goo, thetons, who knows) but you'd never be able to share the answers... Unless each black hole IS an Einstein-Rosen Bridge.

I like to think hidden behind each horizon is a great cosmic/galactic space-truck stop full of alien yokels. That's the flavor of multiverse I want to be in :)

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TripleJeopardy3 t1_j2a52ei wrote

If that happens you should power cycle and reboot. So maybe take a nap and when you wake up your brain will work again.

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clocks212 t1_j2dyrqf wrote

Only from an outsiders perspective would you freeze. From an outsiders perspective a black hole can be thought of as “a region of space where nothing has ever happened”. From your perspective you would just fall straight through the event horizon like nothing was there until you were killed by gravitational forces or impacted whatever exists at the center of the black hole.

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StandardSudden1283 t1_j26ilsf wrote

I want to add on to this question - would the answer to the above be different if the black hole had no accretion disk?

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Lurker_IV t1_j29j0d0 wrote

If you get too close to the wrong black hole then you become its accretion disk...

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tomrlutong t1_j29gfae wrote

I did the math a while back- you have to be crazy close, like 100s of km, to a merger for the gravity waves to affect you directly. /u/StandardSudden1283 Even a 'cold' pair of BHs (no accretion disk) would kill you from tidal forces at a much greater distance.

The article/u/kanrith links to suggests the waves could cause earthquakes or something on the planet you're on, and so indirectly hurt you, but its not verry convincing.

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duroo t1_j26y2rb wrote

Where does the speed of star collapse just before a supernova fit into this?

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MichiganBeerBruh t1_j28ajxp wrote

What is the speed of two of the furthest known points of the universe, relative to each other, with the rate of expansion of the universe?

And how does that compare to the speed of light?

Sorry there is probably many better ways to word that

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amaurea t1_j28gqoz wrote

In an expanding universe things like distance and speed become ambiguous at large distances, with several sensible definitions that all give the same results under normal circumstances suddenly disagreeing. When it comes to distance, this is due to the expansion of space changing the scale of the universe while light is traveling towards us, so effectively changing things in the middle of our measurement. When it comes to speed, it is due to the difference between things moving apart because of their own motion, or things moving apart because new space appeared between them.

As a rough analogy for the former, imagine two ants separated by a piece of string they can walk along, but they're currently standing still. Now someone cuts the string and splices in a much longer piece of string between the ants. The ants didn't move, but now the distance between them (along the string = through space, in this analogy) is much longer. Does that mean the ants had a huge relative velocity when the splicing took place?

It's up to you, really, but I think most of us would prefer to factor out the expansion part and only include the moving part in the definition of velocity. In cosmology, this definition of speed is called peculiar velocity, and would not be particularly lage for two objects on opposite sides of the observable universe.

All of these complications go away if you only look at nearby objects. It's relative speed between two objects close to each other that's limited to 299792 km/s.

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RIPphonebattery t1_j293unq wrote

S4714 at 8+/-3% c seems like its lower bound (5% c) would be higher than the 3.83% c from S14. What am I missing?

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amaurea t1_j2a89xt wrote

Firstly, 8±3 doesn't mean "it's definitely between 5 and 11". It just means "it's 68% likely that it's in that range", and then it's 95% likely that it's in the range [2:14] and 99.7% likely that it's in the range [-1:17] (really [0:17] in this case, since it can't be negative).

Secondly, why do I say that it's likely to be lower than 8% rather than higher, given that the error bars go both up and down? That's because we're looking at extreme value statistics. Put simply, we're not looking at a random data point, we're looking at the data point with the highest value. That means we're much more likely to see a positive noise fluctuation than a negative one, because a positive noise fluctuation makes a data point more likely to be the highest one while a negative noise fluctuation does the opposite.

Here's a concrete example. Let's say we have two sets of numbers, set A and set B, each with 1000 numbers in them. In set A, each number has a true value of 5 but ±1 in errors. In set B, each number has a true value of 0, but ±10 in errors. So in reality the numbers in set A are much bigger than in set B. But now look at what happens when we take errors into account. In set A, it's unlikely that we will see any numbers higher than around 8, since a +3 error has a likelihood of only 0.15%. Meanwhile, in set B we're almost guaranteed to see numbers higher than 20. So if we're not careful we will incorrectly conclude that B is really bigger than A.

Sorry, that didn't come out as clearly as I had hoped. If you know some simple programming, then I recommend just writing a 5-line program to test it out yourself.

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RIPphonebattery t1_j2ah81c wrote

This makes perfect sense to me! I didn't realize speed was generally expressed with 1-sigma error bars, I'm most used to seeing 95% CI's.

Thanks for taking the time to reply

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ivanparas t1_j263wj6 wrote

Not a single object, but the material jetted out from quasars moves at near the speed of light.

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drummerandrew t1_j282th4 wrote

Woah. This got my brain going. Solar, galactic, super group orbit timed just right could move pretty fast.

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garbageemail222 t1_j289edi wrote

Physics gets weird when talking about aligning speeds approaching the speed of light. They're not additive.

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playadefaro t1_j29kc8j wrote

This is the most mind bendy thing ever! Is it because of some reason we know or is it because, well, it is!!?!!

Also, at what speed does the non-additive-ness start?

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Harnellas t1_j29r5bc wrote

I still don't understand why .6c plus .6c cannot equal 1.2c, I only know that it can't.

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