I didn’t realize until recently that supermassives and classic black holes are formed differently.
https://en.wikipedia.org/wiki/Supermassive_black_hole
It’s also fun to remember that this event happened 26,000 years ago.
https://en.wikipedia.org/wiki/Supermassive_black_hole
It’s also fun to remember that this event happened 26,000 years ago.
This is one of my favorite (but also the most daunting) things about space. It is so awesome (and bizarre) that we can essentially observe the past. In this case, the event occurred sometime around when humans were figuring out how to make baskets[1].
[1]: https://en.wikipedia.org/wiki/Timeline_of_human_prehistory
No, the averages ignore input order. Reality is order dependent.
The past is the only thing we can observe!
The info has to take time to travel.
Would the statement stand if it were updated to say, "The (presentMoment - latency) is the only thing we can observe"?
Perhaps "experience" instead of "observe" would have fit better - although, experience by definition is only in the present, otherwise it would be a memory.
Then a counter argument might be: everything is memory, even the current moment being experienced. I suppose that sliver of "now" might be infinitesmal, approaching zero..
In situations on the universal scale, it is incredible to know that no matter what, we will only be able to see the distant past (thousands to millions of years) for most other celestial bodies.
1. You have a collection of black holes that formed from stars in a very dense cluster at the nucleus of the Galaxy early in its life that all merged to form an intermediate mass black hole, which then grew into a SMBH.
2. An intermediate mass black hole formed directly from the direct collapse of a large gas cloud and then grew into a SMBH.
But even the better known case of the formation of stellar mass black holes is still not well understood!
Based on my limited understanding of cosmology, wouldn't the gas first form a star, and not form a SMBH until the star's nuclear reaction became too weak?
Tweet from the author saying this, in case anyone else was looking for a citation: https://twitter.com/quantumpenguin/status/116070888526032896...
Wouldn't it have occurred much less than 26,000 years ago from the perspective of the black hole due to time dilation caused by the difference in gravitational potential of the blackhole compared to us?
So doesn't it make sense that it only became part of reality yesterday ? And therefore to say that it happened yesterday. The famous supernova of 1572 AD also very much didn't happen anywhere close to 1572 AD.
If we would take a snapshot of the Universe VM spacetime at the current moment at the start of the event, the "now" in time would have to correspond to the current state for every other location if you want it to be useful and not only intuitive.
But this is where paradoxes come in because the black hole, stationary from in its frame of reference, would still experience 26,000 years and not see you arrive until then. From my understanding, general relativity solves this paradox with de-acceleration causing the one who was traveling at the speed of light seeing the rest of the universe's clocks' speed up.
So technically, your clock would have not moved 26,000 years forward, but other's would have.
This makes me think of lone sad CMB photons, who accidentally stop by hitting an atom and cannot even recognize a tiny bit of the universe they remembered since. A spark moment, and they lost everything to billions of years.
Or is it just the first collision ever that stops them and they just happen to miss every last particle in outer space for 13 billion years perhaps.
Special relativity solves this fine (there is no paradox in it for that particular thought experiment), no need to involve general relativity. There is nothing paradoxical about the observer near the center of the galaxy not seeing you until you are near, because you are traveling at speed almost as great as the speed of the information carrier (the light you emit if we are talking about space, or the sound you make if we are talking about something on Earth). So you actually do not even need relativity to explain the "they do not see us coming". You do need relativity to explain the fact that to you all of this took only a blink of an eye.
In other words, you can say "26,000 years ago from Earth's perspective", but it isn't the definitive answer for everything in this galaxy, and it definitely isn't the same answer for the rest of the universe.
I am struggling to guess exactly what you mean by that. Could you clarify? Is the first part of the quote (up to "have") just a statement about coordinate charts? If not, and it helps with the clarification, I know how SO+(1,3) (and more particularly the one-parameter SO(1,1) boost subalgebra of O(1,3)) relates to the gauge symmetry of massless |helicity| >= 1 particles and flat spacetime, and I'm pretty sure you do too.
Here is the conversation I frequently have in intro physics if relativity is included (it is this particular very common misconception I was trying to point out):
student: "So, time is relative because light(information) has a finite speed. Therefore, if a star at distance 1kly explodes 1k years ago, for us it happens today?"
me: "No, it is a bit more subtle. What you have said is true both for sound and light, but we do not talk about 'sound' theory of relativity, so there must be something more to light. Time is relative because light(information) has a fixed finite speed in all inertial reference frames. The star exploding and we observing the star explode are two different events. The time between these two events is measurable, it is 1k years in our reference frame for instance." (and then I go into how the space-time-interval is the thing that is constant, how these two events are always in the same order, how only in the reference frame of the photon they happen at the same time, how in another reference frame it might be 1year or 1M years, etc).
Then start with simply rescaling the coordinates on the timelike axis. Nothing changes but the labels. The same thing if we simply rescale the spacelike axis. We can measure in parsecs, lightyears, lightseconds, metres, and so on; on the time axis we can use seconds or fortnights or kiloyears and the but for the coordinate labels the diagram does not change. Then demonstrate a translation by sliding the timelike axis down so that the timelike origin is last week, last year, next year, and so on; demonstrate a spacelike translation by sliding around the origin on the spacelike axis. Again, invariance is manifest. We can do both types of translation at once, and put the origin on the exploding star instead of on the Earth. All we're changing is the labels associated with the events (and the labels for points along the intervals).
One can also show that a rotation of the coordinates also only changes labels, not relative locations -- this is where a pair of transparencies, or one paper one transparent layer works well. It might be handy to have the rescalings handy on transparencies so you can show that you can freely mix coordinate rescalings and translations and rotations, and the (literal!) underlying picture remains invariant in the face of all of these.
This opens up at least four interesting types of discussion: (1) if we calculate out the light's interval it's 0 (thus "null"). Any segment of the null path from exploded star to detector is also itself lightlike between the two ends of the segment, so dS^2 is still 0. What gives? (Answer: using a set of coordinates lets us deal in coordinate-based distances; and for extra credit we could talk about arbitrary parametrization, affine-parameter style); (2) why are the slopes of the different explosion products different, or why is the interval timelike for the non-light material?; (3) what do boosts do to coordinates?; (4) coordinates representing a moving observer with the origin always fixed on itself leave the underlying picture invariant but suggests kinematics and dynamics; and (5) a contrast between a "conventional" observer that remains at spatial zero but which "evolves" along the timelike axis compared to the same observer against a different set of coordinates.
For extra credit, explicitly distinguish between types of Lagrangian and Eulerian observers that differ only in their choices of systems of coordinates as in (4) and (5), even as the underlying picture of events remains identical for all observers.
This leads us into a block universe picture, where we have (classical or quantum) fields filling the whole of spacetime -- completely everywhere -- with arbitrary sets of coordinates imposed on portions of the spacetime used to describe field-values as (a) "objects" like worldtubes and instantaneous snapshots giving us something like a classical extended object; (b) dynamical laws that in a block universe picture simply predict the field-values (or "objects") at at a coordinate-location in spacetime given some set of field-values at another location in spacetime [1]; (c) the "conventional" splitting of the block universe into 3+1 spacetime, but showing that the split is as arbitrary as any choice of system of coordinates because we can e.g. rotate a set of Cartesian coordinates differently for relatively boosted "conventional" observers.
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[1] Essentially, this is pulling in the idea that initial values need not be in the relative past for a spacetime-filling IVP solution to obtain! Indeed, undermine the idea that we are evolving initial values in time, rather than extending some set of values outwards from a kernel. Any thought of time evolution can be cured by remembering that one should as a matter of course at least briefly consider all sorts of messed up coordinate charts on top of the underlying block picture. One is simply preferring to work with one static chart for purely personal reasons (which may include the ease with which an intended audience may follow the path to a set of results).
> 1year or 1M years, etc.
Here I would like to believe that "M" means "Martian". Or perhaps "Mercury". :D But those would correspond with a coordinate rescaling rather than a boost.
A reference frame is a coordinate system. Usually with 3 space-like coordinates and a time-like coordinate we call "time". That last coordinate is what gives meaning to "now" between two distant places. But this meaning entirely depends on the reference frame chosen.
In Special Relativity, there are special reference frames called "inertial reference frames". From these we can easily calculate elapsed time for any particular path through space-time.
In General Relativity it is harder - there are generally no inertial reference frames to be found. But we can still do the same calculation using tensor math and something called "the metric". You can actually do the same thing with special relativity, however the metric is dramatically simpler in an inertial reference frame so people usually take the easier approach.
The various measures of "curvature of space" are measurements of how quickly any attempt to describe space-time an inertial reference frame will be wrong. Just as the curvature of a ball makes it impossible to properly describe its surface as a flat plane. (Mathematically this description is very precise.)
Now in a low gravity situation, we can find inertial reference frames that are approximately a match. The almost inertial reference frame where the distant galaxies are moving straight away from us is the one we usually implicitly think of. In that reference frame, this light was emitted 26,000 years ago. But you can pick a reference frame that gives literally any answer that you want.
But that reference frame is a bad description of the experience of timelines for fast moving objects, or what is happening in deep gravity wells. However that is fine, since from that reference frame we can predict exactly what the experience of those timelines will be, and our predictions turn out to be correct.