Former astronomer here! A lot of the comments here are about how this event occurred 26,000 years ago. I thought it would be useful to describe how we think about these things in astronomy.
Firstly, yes it is true that this happened 26,000 years ago! More specifically, the event happened 26,000 years ago in our reference frame. There are other reference frames in which this event occurred two minutes ago or 5 million years ago. But generally these are not useful reference frames since the Earth's reference frame is not too different from the reference frame with respect to the center of mass of the Galaxy (we are not moving all that fast compared to the speed of light).
That said, as astronomers we are not particularly interested in dating precisely when the event happened. 26,000 years is a very long time in human history, but it's not very long on astronomical timescales. Not very much has changed in the Galaxy over the past 26,000 years, so we don't gain much by dating it at the time the event occurred (with respect to the Earth's reference frame, of course). Furthermore, we wouldn't even know exactly when it happened even if we wanted to! Our clocks are very precise here on Earth, but our distance measurements to most celestial objects are very fuzzy, particularly the further away you get. Because of this, these sorts of events are always referred to with respect to the year they were observed on Earth. (Thus, the most recent nearby supernova is known as 1987A because it was the first supernova observed in 1987.)
This all changes when you are studying more distant objects though! For very distant galaxies, we are now seeing things when the universe was considerably younger and things were much different. Then it becomes important to keep the event's age in mind. For these objects we will actually refer to their redshift. The redshift can be measured relatively well, and is related to the distance and age of the event via the Hubble constant and the acceleration parameter. Measuring these parameters is tricky and is a whole subfield of their own, so we usually stick with redshift as it is more related to things we can easily measure.
A final note on the paper itself. These observations were taken at Keck on Mauna Kea. There has been a lot of controversy around the construction of the Thirty Meter Telescope, so the authors actually acknowledge indigenous Hawaiians specifically in their paper:
> The authors wish to recognize that the summit of Maunakea has always held a very significant cultural role for the indigenous Hawaiian community. We are most fortunate to have the opportunity to observe from this mountain.
> A lot of the comments here are about how this event occurred 26,000 years ago.
Which does miss the point -- whether it happened a billion years ago somewhere else or 2 seconds ago somewhere else, the effects of that event are being measured by us right now, meaning if it was gonna kill us, it would have happened now, not 26,000 years ago.
Is that true? In the way you see a bullet then hear it, can the reverse happen with space events where light reaches you then something else? I don't know anything about this other than light is the faster we can travel so assuming we can see things before the effects hit us...
Speed of light is not really just speed of light, but universal speed limit of causality. Light (electromagnetic waves just happens to be one of the few things capable of hitting that limit. Even the effect of gravity is limited to speed of light. PBS Space time has a good video on this - https://www.youtube.com/watch?v=msVuCEs8Ydo&t=5s.
So to answer your question, no it is not possible for anything to affect us any faster than the universal speed limit of causality (that light happens to be able to hit).
There is at least one known exception: neutrinos from a supernova. (Don't get too excited, this doesn't contradict Einstein, as we'll see.) In the hot, dense core of a supernova, photons will scatter around for a while before escaping. Neutrinos, which interact much less often than photons, and travel also more or less at the speed of light, get out basically right away. The difference can be as much as a few hours! Of course it's the high-energy photons that are more dangerous, so the neutrinos are really more like an early-warning system.
This is very misleading. In fact, neutrinos are not going faster than speed of light (c), they are going faster than photons that are emitted from the supernova.
Not just that, but lights when it passes through a medium slows down depending on it's refractive index (speed of light in water is about 225 000km/h while in vacuum it's 300 000km/h). This is important distinction as recently we had an uproar when it looked like neutrinos are slightly faster than c (in the end, it was a measurement error).
You can observe effects of particles going faster than speed of light in a medium if you look at photographs of Cherenkov radiation.
The correct way to say it is that the speed of light is the speed of causality only in vacuum. In any medium the speed of light is slower but the causality or other fields (for example gravity field) that are not impeded by matter can and will propagate faster than the speed of light.
That's still not an exception though. Neutrinos escape first by virtue of the fact that they don't interact with matter and don't bounce around, while light does. They aren't travelling faster than light, just travelling a shorter distance.
My (limited) understanding is that the limit is only applicable to movement through space. The expansion of the universe is not bound by the limit, as it is expansion of space itself. Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?
> Thus it seems incorrect to talk about a “universal speed limit of causality”. Am I missing an essential concept?
Probably. Why would points becoming more distant due to expansion of space be a counterexample to the speed of light being a limit on the speed of causal propagation? Spatial expansion doesn't move anything, so it can't propagate information.
Everytime i here this- it sounds like a cheesy speed limit introduced by a post-grad into a late-night hacked together simulation for which s/h/it couldnt aquire enough server power for real parallel execution.. we should make it our job, to label this project as a "F" for all to see..
I think those answers are actually wrong. When the object casting the shadow moves, the shadow remains in the same place for an observer in it until the light from the source reaches the observer inside the shadow.
I know that's a weird explanation, so consider:
t0: S~~~>O U (shadow exists)
t1: S~~~~~~> U (shadow exists)
t2: S~~~~~~~~~>U (shadow !exist)
Where "S" is a light source, "O" is an opaque object, "~" are photons traveling to the right, "U" is the observer, and "t0", "t1", and "t2" are times (increasing).
At time "t0", "U" thinks he's in the shadow.
At time "t1", "U" thinks he's in the shadow.
At time "t0", "U" thinks he isn't in the shadow, since the photons are now hitting him.
A similar calculation/thought-experiment can be done for shadows with "angular momentum", in case you think the tangential velocity of the shadow will exceed the speed of light.
Thanks for that nice diagram! What happens when the light source moves and the shadow is far bigger than the object casting it ? Wouldn't the speed of the shadow on the surface be faster than the speed of light ?
For example, when a light source is super close to an object and the shadow gets super big really far away, and the light source moves?
t0: SO U1 (U1 in shadow)
t1: S~~~~~~~~~~>U1 (shadow leaves top first)
Dotted line is the path of a fixed point on the shadow during the time S moves.
I didn't do the precise math, but I'm pretty sure the tangential velocity of the shadow along the dotted line won't be greater than the speed of light. The curvature of the "wave front" formed by the tips of the arrows ">" above will be lesser than the dotted line curvature, so the photons near the top of the diagram hit the dotted edge before the ones towards the bottom. This is because the source, S, takes time to move away from O.
Note that the wavefront formed by the photons moves radially outward from S, but ascii art is limiting.
A shadow isn’t actually a thing. It’s an image, like a mouse pointer. If I had a sufficiently large screen, and I made the mouse pointer jump (by setting its position) to the other end of the screen, I could calculate its speed and make that number higher than the speed of light. But has anything actually moved? No it hasn’t, because a mouse pointer, like a shadow, is only an illusionary image of a thing, not an actual thing.
An absence of something can only be defined relative to that something and never taken just by itself. So a shadow only exists as a function of light (no light) or a consequence of the absence of light. So it would not travel faster than light in a way that can carry additional information.
Quantum entanglement works faster than light but cannot carry any information. As such the speed of causality (and implicitly of light) is still the real limit.
from the point of view of someone in the shadow, and subsequently not in the shadow, the speed of light is still the limit. If the light source is 1LY away, it will take 1LY for me to notice that I am no longer in shadow, regardless of how fast the shadow moves. There's no way of measuring the shadow movement that isn't limited by the speed of light.
A. Earth is going to be hit by a super-mega-death laser focusing the entire power output of a star into a concentrated beam.
B. A long long time ago, a very dense moon was very far from a supernova. Unfortunately, far is relative, and so is speed in space. Specifically, relative to Earth, the moon is now traveling 5% the speed of light and is on a collision course.
In scenario A, there is no way for us to see this coming, because information also cannot travel faster than light. The light is what kills us.
In scenario B, it is theoretically possible to see this rogue-moon coming, since it is possible for light to reflect off of it and reach us before the moon does.
Most of the stuff we observe that comes from space moves at the speed of light. Light, radiative heat, gravity. There are plenty of slower-moving things, like rocks and stellar wind, but it's hard to meaningfully measure that stuff directly for distant stars.
75x... factor in the inverse cube law... yeah I'm gonna say unless it's somehow a directed effect it's still won't even be close to mattering a little bit, even if there is some sub-light "wave" of some kind coming toward us from this event.
This seems like a prudent approach: Catalog objects and events according to the things we can measure on earth right now, since our direct measurements (calendar dates, spectral lines, etc.) are probably quite accurate, but the inferred distances might change in the future as we improve our overall empirical and theoretical knowledge. With the exception of the timing of an event, we can go back and re-measure the other things if we can at least find the same object later.
Just to add to this, there is no absolute simultaneity in General Relativity, and also no absolute way of saying, "This event happened T time ago." The only concepts that are rigorously defined are future, past and spacelike-separated (points in spacetime you could only reach by traveling faster than light). So if we try to define a reference frame that allows time comparisons between Earth and the Galactic Center, we have to be very careful, and keep in mind that it is just one of infinitely many possible coordinate systems, each of which will give a different answer to the question, "How long ago did this flare happen?"
One fun coordinate system is Cosmic Background Radiation time. The CMB redshifts as the Universe expands, so the temperature of the CMB can serve as a clock. Anyone anywhere in the Universe can measure the average temperature of the CMB, and use that as their clock.
About Mauna Kea, I urge everyone to look into the issue in more depth before forming an opinion. There is a huge amount of disinformation being spread on social media, and the basic picture most media is painting is wrong. The Thirty Meter Telescope project has gone to great lengths to try to listen to the community, and to take religious and cultural concerns into account. These concerns had a big impact on both the location and the design of the telescope. The telescope site is father below the summit than the previous telescopes, so that it cannot be seen from most of the island. The entire optical design of the telescope was made to be super compact (F/1), which was a very risky technical decision, just so the telescope wouldn't be as tall. The internal clearances of the telescope within the dome are just 20 inches, barely allowing it to be serviced, for the same reason.
The sacredness of Mauna Kea is also much more questionable, within traditional Hawaiian religion, than activists are claiming. There's actually surprisingly little evidence that it was historically held to be anywhere near as sacred as TMT opponents claim. The Hawaiian creation story, for example, doesn't mention Mauna Kea. It's important to realize that the Hawaiian traditional religion was suppressed by the Hawaiian monarchy in 1819, which means that it is not easy to reconstruct nowadays. Many of the anti-TMT activists belong to a movement trying to bring the religion back to life, but what they claim about the religion does not necessarily reflect ancient beliefs. One thing that undermines their claims is the fact that pre-contact, native Hawaiians excavated the largest quarry in Polynesia on Mauna Kea.
There are, however, culturally significant sites on Mauna Kea that clearly should be preserved. There are ancient shrines and burial grounds, for example. The TMT site was chosen to be as far removed from such sites as possible. There are also places where modern "cultural practitioners" (ironically enabled by the observatory access road) carry out their practices, and TMT does not significantly disturb these practices.
Most of the dozen or so leaders of the anti-TMT movement have at least some native Hawaiian ancestry, but yes, a lot of non-native people have joined in, especially on social media, where the barrier to entry into activism is low.
One thing I didn't mention above that's important in all this is the Hawaiian independence movement. Many of the leaders of the protest movement view the State of Hawaii as illegitimate, and believe that the Hawaiian Kingdom is under illegal occupation by the United States. The telescope is a great symbol to rally in opposition to. If the State of Hawaii doesn't exist, what right does it have to grant permits for construction on Mauna Kea (yes, this is an argument that TMT opponents repeatedly made during the legal proceedings that eventually approved the TMT's permit)?
Yes, if you had a reference frame that was moving away from the SMBH with a velocity very close to the speed of light. It would take the light an extremely long time to catch up to the observer in that reference frame and so it would appear that the event occurred an extremely long time ago.
How would time dilation play into this? That observer would see the SMBH as further away at the time of the event, but they’d experience time at a slower rate. Would the extra time ever ‘beat’ the extra distance?
The observer moving away from the SMBH would see the event proceed more slowly. The light curve would therefore be stretched out in time --- the source would get brighter more slowly and would get dimmer more slowly. This is actually an important effect that needs to be accounted for in supernova observations in distant galaxies since time dilation is non-negligible.
Thanks, but I was actually wondering more about the apparent time of the flare (say, peak brightness since as you say the duration changes) based on the observer's speed.
Say we're both on Earth 26,000 years ago. You stay on Earth and I shoot off at C/2 in a direction away from the SMBH. You wait 26,000 years and then see the flare. How long do I wait in my own reference frame before I see the flare?
My guess intuitively is that time dilation and the extra distance cancel out and I still wait exactly 26,000 years (no matter how fast I go), but I'd have to dig out my old physics textbook to figure it out properly. Also, what happens if instead I shoot off towards the SMBH? Or at 90 degrees? I'd obviously see the flare sooner in my reference frame, but how much?
It seems like there's a general rule that, by moving, you can decrease the time until you see events (at least those communicated by light) but you can never increase it.
The Lorentz factor here is 1/sqrt(3/4), around 15% over unity. So the elapsed time for you to see the flare is 2/1.15 * 26ky, or 45 ky. In general, for any λ multiple of c, the elapsed time multiple is sqrt(1 - λ^2) / (1 - λ), which reduces to 1 + λ in the λ << 1 case. You can try plotting it for λ from -1 (c directly towards the event) to +1 (c directly away from the event) to get a feel for how it behaves.
If you (somehow) keep accelerating in one direction, there are events you will never observe. It's called a Rindler horizon. Your world-line traces a hyperbola... which will always outpace photons from certain spacetime locations.
I believe that because the speed of light is the same in any inertial frame reference.. If you were two light minutes away from the event in the SMBHs inertial frame at the event and traveling toward the earth at a speed sufficient to reach it at the same time as the light from the event.. You would arrive at the earth at the same time as the light but only 2 minutes will have passed in your frame of reference?
from the black hole's perspective, the observer would be travelling slowly in time. But from the observer's perspective, the black hole would be travelling slowly in time (since the observer sees themselves as being at rest, and the black hole as shooting away from them).
But if you existed long ago wouldn't have time moved slower since you would have been closer to the SMBH and everything else (i.e., universe would be more dense). So according to general relativity wouldn't it also be right to say it happened less than 26,000 years ago from that reference frame?
It already happened for those reference frames, they just haven't been able to observe it yet. For any such beings living there, they don't know about the event yet. But when they witness it, they will measure and calculate that the event did occur before today.
The only reference frame that could see it and then measure and calculate it as having happened after today would be one travelling faster than c relative to us.
Yes, in your own reference frame you are stationary. So by definition all observers are at the same speed in their own reference frame, namely zero. What I was trying to say is that the velocity of the Earth relative to the Milky Way's SMBH is small compared to the speed of light.
I don't move relative to my reference point, but you move relative to my reference point and vice versa. So from my point of view it makes sense to say you are moving at some fraction of the speed of light.
If we (sol / earth) were moving at a speed significantly closer to the (absolute) speed of light we would experience significant time dilation.
This is significant for observing external events and correlating phase shifts to figure out distances.
Eg. Things in the external universe may appear to happen extremely quickly.
It’s actually very important.
Regardless of how fast you appear to be moving in your own frame of reference, the speed you’re travelling in absolute terms affects what you observe externally (and specifically with regard to how fast you are moving compared to what you observing)
The thing we are at rest (ish) relative to is the cosmic microwave background. That's why we aren't moving that much relative to the rest of the universe, on average.
Time dilation is relative between you and some other reference frame. All the "stuff" in the universe nearby is (fairly) static, so we can say we are not moving fast, but it's still relative to the stuff around is.
Yes and no. The laws of physics do not pick out a preferred frame of reference, and they should be symmetrical under lorentz transformation, but the fact that we live in a cosmological universe starting with a big bang breaks that symmetry in practice.
It's similar to how the laws of physics are symmetrical under translation and rotation, but the presence of matter around us means the surface of the earth is not the same as the surface of the sun.
So, while we might use the CMB as a standard reference frame, the laws of physics should not.
Maybe, kinda. It's not obvious that it's more privileged than centering a reference frame on the Earth or the Sun. The CMB is maybe just an artifact of the Universe's history (like a Sun-centered one is an artifact of ours).
If we didn't have any CMB radiation floating around to measure, everything would probably work exactly the same. Maybe you could measure all the motion of all the matter in the visible universe and measure the motion of the "center" relative to us.
My point was that if we (the solar system) were travelling as a considerable portion of the speed of light, then we would experience time dilation, which would significantly affect our observations of the external universe.
So... it's fine.
I mean, you're basically right; speed isn't absolute, and it's wrong to refer to your speed relative to the CMBR as 'absolute'.
...but, well... I think it's fair to say 'The speed of light is constant in all frames of reference, so why [insert question here]...' is a very fair, and common question. 'Everything is relative' isn't an answer, it's just a platitude.
> More specifically, the event happened 26,000 years ago in our reference frame.
Isn't this wrong? Surely we would say that, in our reference frame, the event happened precisely when we observed it happening. That's what "frame of reference" means: it's whose clock we use to say when an event occurred.
No, your interpretation is not quite correct. You do indeed use your own clock to determine the time of events in your own frame of reference. But you still need to account for the light travel time of the event. This is why you need both the position and the time of the event in order to do a Lorentz transformation.
To see this another way, suppose you saw two events at the same time, one two light-years away, and the other one light-year away. In your interpretation you would call those events simultaneous. Simultaneous events cannot affect each other in any reference frame. But if the two events were in a line, the light from the first event could have triggered the second.
> To see this another way, suppose you saw two events at the same time, one two light-years away, and the other one light-year away. In your interpretation you would call those events simultaneous.
Yes, I would say that those events occurred at the same time in my reference point, while also recognizing that we can't say in an absolute sense that they were simultaneous. Isn't that the point of relativity of simultaneity?
Special Relativity states that the time (component of spacetime distance) to an event depends on the relative velocity (angle) of the observing reference frame. It does not state that an event occurs at the moment it is observed; that would imply light has an infinite velocity.
The whole point of relativity is that events do not occur at the exact moment they are observed, because light has a finite velocity. The fact that light's finite velocity is the same in all reference frames is what causes reference frames to tilt their spacetime angle according to their velocity.
It's the same as saying the Y component (height) of a line segment changes if you rotate it. The length of the line is the spacetime distance, the height (Y component) of the line is time, the x component is spatial distance, and the angle of the line depends on the relative velocity of the observer.
No, in our reference frame it took 26,000 years for the light to travel to us so that we could see it. We measure that the event occurred 26,000 light years away so we calculate that it occurred 26,000 years ago. There are other reference frames where it also happened 26,000 years ago but that are much closer and saw it occur earlier. There are also reference frames where it happened 26,000 years ago but they haven't observed it yet because they are much further away. And as OP mentioned, there are reference frames where it occurred 5 million years ago yet it still hasn't even been observed.
The only interval that is constant in Special Relativity is the spacetime interval. It's a distance measure. In 3D space, we normally say that distance is given by
ds^2 = dx^2 + dy^2 + dz^2.
In Special Relativity, you define a quantity called the spacetime interval:
ds^2 = -(c*dt)^2 + dx^2 + dy^2 + dz^2.
You treat time as a 4th dimension, and take it into account when calculating distances. Notice that this interval can be negative (the metric is "semi-Riemannian"). "Events" (locations in 4D spacetime) can only be causally connected (one can influence the other) if the spacetime interval separating them is negative (this is equivalent to saying that no information can ever propagate faster than light).
The key is that all non-accelerating observers agree on the spacetime interval between any two events. The spacetime interval between two events is the only solid distance you can give. The time interval by itself depends on the observer's reference frame (you've probably heard of time dilation). So too does the spatial distance (length contraction). But time dilation and length contraction behave in such a way that the spacetime interval remains constant.
The long and the short of it is that time intervals are observer-dependent.
The closer location would have received the signal 16,000 years earlier than us here on Earth. They would still say that it happened 10,000 years before they observed it, but they would still say it happened in year 24,000 BC.
Assuming these people 10000 lightyears away from the black hole are also on a planet, their reference frame is almost the same as ours - the relative speed of different planets is very small compared to the speed of light. So they'll agree on when 24000BC is.
The key in that case is that the other reference frame that from our reference frame will measure a different distance to the black hole. It might be 25000 light years away from them, but we would not be 25000 light years away (but longer I believe) in their reference frame.
This can happen for instance with extreme differences in relative velocities. Someone else can probably show an example with the math, but a key concept here is the that distances also change when you move real fast or are in a deep gravity well.
Are you saying that the only reference frame where the event happened "now" is the reference frame of the black hole itself? I'm quite familiar with relativity of simultaneity, and that would lead me to conclude that from my reference point, two things happened simultaneously: the black hole flared, and the clock read 11am on May 13th, 2019. Given this, I would think we would describe that in English as "the black hole flared at 11am on May 13th, 2019."
What do you mean by "now"? Which clock are you talking about? A clock here on Earth?
Clocks here on Earth currently read May 13th, 2019. And you can conclude that we observed the event on May 13th, 2019. We believe the black hole is approx. 26,000 light years away. Thus we know the light took 26,000 years to reach us. So we say the event took place around 23,981 BC.
We have 3 events here:
A. A clock on Earth reading May 13th, 2019
B. Light from the black hole flare reaching Earth
C. The black hole flaring
We can say that A and B occurred simultaneously in all reference frames (they occurred at the same location and at the same time). C did not occur simultaneously with A or B in any reference frame (except perhaps, debatably, that of something moving at the speed of light).
> Clocks here on Earth currently read May 13th, 2019.
Are you sure that we are in the same reference frame? I’m willing to accept that you started a fairly short journey at some appreciable fraction of C, turned around and gave just recently arrived back home, but I’m going to need some proof.
I guess I’m just struggling to see how there is any useful sense in which the event happened in 20,000 B.C. from our perspective when it would be impossible for it to have had any causal effect on us until May. I don’t think this is a disagreement in the basic understanding of relativity, but rather a disagreement in semantics.
If a baby fall in another room and you only hear them crying half an hour later, it doesn't mean the baby just got hurt. Both you and the baby still agree when it happened, you just learned about it later.
The same is true regarding the black hole flare. Except that we are not only learning about it now due to negligence, but because we, with all current knowledge in physics we have and also our technology, we don't have a way to see what's happening right now. But say an astronomer finds a wormhole and points their telescope to it. They could know about things that will only be visible by everyone else in 26,000. Time to invest in those black hole futures.
> 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.
Alternatively, the present is the weighted average of past causal influences. This event had no causal influence over the humans figuring out how to make baskets. From Earth perspective, the event just happened.
...and a gamma ray burst from some distant galaxy that will strike Earth sometime in the future causing a great catastrophy could have already happened while the human was putting on the finishing touches on the basket.
Well, to be incredibly pedantic, it takes time for the light being reflected off of things to reach your eyes and be processed by your brain, so you are always observing the past even if it isn’t remotely perceivable :)
The Wikipedia article says this, but for those who just read the comments, the formation of supermassive black holes (SMBHs) is very much an open problem! There are two main theories here:
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!
Not if the gas cloud is large enough. Small gas clouds (white dwarfs, neutron 'star's) produce enough presure from particle degeneracy to support themselves against gravity, medium clouds (stars) need active heating (from fusion) to keep gas particles moving fast enough to remain far enough apart to avoid forming a black hole, and for sufficiently large clouds even having the gas particles moving at the speed of light wouldn't be enough to avoid forming a black hole.
Put enough mass in the right radius and fusion can't do jack about it. Over a large enough area, IIRC it doesn't actually take a very high density, so it never has to reach a point where it can fuse before the event horizon forms.
I recall someone calculating that a sphere of gas with a radius equal to Neptune's orbit, with the same density of air at sea level would mass 4 million solar masses. That means if we filled thw Solar system with air, it would form a SMBH.
I wonder if there exists a sci-fiction that talks about how human beings detect something disastrous happened to Sgr A* and realize that happened 26,000 years ago then try to leave the Milky Way in panic.
Strictly speaking, it happened _now_ in our own timeline, whether it happened 26K years in the past for the blackhole is a mystery as everyone gets their own timeline and we have no idea (read: can only reasonably conjecture) what the future timeline for that black hole is until it gets to us.
You have to think reference frames here, not timelines.
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.
The problem is that "26,000 years ago" depends on perspective. For example, if space aliens were on a rocket that took off thousands of years ago but traveled at nearly the speed of light, they might say now (where "now" is approximately equal to our "now") that this actually happened 21,000 years ago.
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 mean you’re kind of mixing up two concepts. Distances and the passage of time do depend on frame of reference. But that’s not the same thing as saying something we observe today happened today in our “time frame”. In our frame of reference, this event happened 26,000 light years away 26,000 years ago.
To add to the other comments that are attempting to correct you (although the downvotes seem unwarranted): the misconception you are presenting is one of the most common ones I see in students when teaching relativity. The event did happen long time ago and the fact that time and space are relative have nothing to do with the fact that it takes us some time to see/hear that an event has happened.
> that time and space are relative have nothing to do with the fact that it takes us some time to see/hear that an event has happened
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.
Yeah, sorry, trying to be brief made my statement rather unclear.
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).
Depending on how "intro" the conversation is, I'd start "events first" and explain how one can describe a set of events differently. I'd do it with a couple of transparencies, or a sheet of paper (for events and intervals) and one or more transparencies (for systems of coordinates). With your example, draw a Minkowski spacetime diagram with the spatial origin always at Earth, the events of light, neutrinos, and charged particles arriving at the Earth at different times (0 for the light, later for the heavier matter), and the exploded star with the simplification that all the particle occupy exactly same point in spacetime somewhere near the stellar explosion (this is literally a coincidence, in Einsteinian language). Draw in the intervals connecting that point with the points representing the terrestrial detections.
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 ; (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.
 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.
I don't get how that works. 26,000 years ago it would have been FAR outside of our "light cone". In other words it was completely beyond anything that affected our world. Only recently that changed.
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.
So let's say you travel at the speed of light towards that event. When you arrive, 26,000 years later, would you then say the event happened 26,000 or 52,000 years ago? If the former, have you just travelled 26,000 years into the past, or did time stand still for 26,000 years from your perspective? I'd argue your clock has still moved forward 26,000 years.
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.
From my understanding, if you travel at light speed, space is compacted to very tiny distances and time stops. If you take the math of relativity to its extreme, time for you has stopped and your trip took no time. It actually takes no time because there is actually no distance between you and the black hole.
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.
> 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.
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.
My bad I meant to talk about the fact that there is no universal reference frame, so in your frame you would believe you aren't moving, but the black hole is moving towards you. In in a scenario where you are traveling at .99c, from your frame of reference you are not traveling at all and the black hole is traveling to you, so you see the black hole experiencing incredibly slow time passage. However, that is not what the black hole sees. Hence a paradox.
Not a physicist, but afaik you are right. If you go with the speed of light (or close enough), your travel time goes to zero. It is common misconception that to “reach the stars” you have to spend a couple of lives. In fact, you can get there before you finish your sandwich.
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.
>whether it happened 26K years in the past for the blackhole is a mystery
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?
Wouldn't time dialation around the black hole be very extreme? As in, from the perspective of just outside the black hole where the flare presumably started, it would have lasted only a very brief amount of time.
It doesn't sound like you're asking in good faith, but I'll answer anyway.
It was "a couple hours", so without doing too much more math than that, it was probably the same duration here and there within "a couple minutes". I'm more than comfortable calling that the "same" duration in conversational terms, but I would expect it's not the same duration within nanoseconds, as we're accelerating relative to Sgr.A* (rotationally).
I was asking of curiosity – thank you for sating it!
I'm not familiar with spacetime, and was curious for a sense of scale here. A couple of minutes difference sounds quite significant to me (though I agree I'd conversationally call it "about the same").
But yes, space is way more enormous than most people (including myself) can wrap their head around. Spacetime warping is also hard to really get, I understand it conceptually but I can't really picture it.
Things moving less than about 15% of the speed of light experience less than 1% time dilation, and for comparison, we're moving around the center of the galaxy at 0.2% of the speed of light.
One of the wildest things to me is that GPS satellites have to compensate for time moving slightly slower because they're moving so fast around the earth, and also time moving slightly faster because they're further from the gravity of earth.
Over the years of asking these kinds of questions myself, I've come to realize that this line of questioning betrays a fundamental lack of understanding of spacetime. Time is relative. From our perspective it might have happened a thousand years ago, but from alpha cen's perspective it happened at a slightly different time. There is no universal clock. Even the order in which events take place depends on your reference frame.
So, it just happened, and the light took about 26,700 years to reach us, assuming nothing happened to it on the way (which is a bad assumption, even just because there's a lot of stuff between here and there, but especially considering it originated from a severely warped spacetime).
I think a better comparison is to the size of the region in which this event occurred. For the event to appear to last only 2 hours to us, it can't have occurred in a region more than 2 light hours across. That's only 14 astronomical units, less than the diameter of the orbit of Saturn. There are stars bigger than that.
There is a neat physical argument that was made back when quasars and the like were just being discovered. Basically if the flash of brightness only lasts an hour, then you can argue convincingly that the source of the flash must be less than a light-hour across. The faster the flash, the smaller the bound on the size of the phenomena. This is what lead people to realize that quasars are wildly energetic but necessarily very small
I'm also interested. And adding to the question: Is it possible that this particular flare-up may be a forward echo of such an event? I.e. have we observed this happen before in other parts of the Universe?
I'm more concerned that the flare implies some instability that could presage a much more energetic event. I have extremely limited knowledge of space and astronomical phenomena, but super massive black holes seem like they may have enough mass/energy to cause problems for us in a worst possible case.
X-class flares from our sun are 10E32 or so ergs. The black hole flares were roughly 10E35 ergs, about a thousand times more energetic but they are about a billion times farther away than our sun. Apply N^2 law and . . .
The amount of radiation you received from the Sagittarius flares is almost indescribably tiny. Many, many orders of magnitude less than the cosmic ray background radiation that we already live with. If you didn't have a sensor pointed right at the BH, you wouldn't be able to distinguish it from the noise.
Not an astronomer but my understanding is that at any moment life on earth could be totally destroyed by things from space.
* An undetected high velocity object
* Radation bursts
The thing is that since space is so big, we would have to be extremely unlikely to get hit. If a radation burst was directional it would still be like blindly throwing a dart and hitting a bullseye 24,000 light years away in this case.
I believe it's possible for our Sun to emit a solar flare large enough to kill us too (this is far more likely then radiation from a blackhole killing us because its magnitudes closer).
> it would still be like blindly throwing a dart and hitting a bullseye 24,000 light years away in this case.
I liked this analogy, so I felt inspired to calculated the odds. Particularly: the percentage of the "sky" (celestial sphere) that is covered by the Earth from Sagittarius A* 's perspective. That's equal to the two dimensional projection of the Earth onto a 26000 ly sphere around Sagittarius A*.
cover% = π x (6371km)^2 / (4 x π x (26000 x 9.46 x 10^12 km)^2) x 100%
= 1.677 x 10^-26 %
I tried to put that number into "human terms", but the best I could come up with is it's the same as the width of a hair at Saturn's orbit, viewed from Earth.
From a NASA website: Saturn’s rings are incredibly thin. The main rings are generally only about 30 feet (10 meters) thick, though parts of the main and other rings can be more than a mile, or several kilometers, thick.
> Based on the concentration of Fe-60 in the crust, Knie estimated that the supernova exploded at least 100 light-years from Earth—three times the distance at which it could’ve obliterated the ozone layer—but close enough to potentially alter cloud formation, and thus, climate. While no mass-extinction events happened 2.8 million years ago, some drastic climate changes did take place. 
The time scale is set by the orbital period of the innermost stable circular orbit (ISCO https://en.wikipedia.org/wiki/Innermost_stable_circular_orbi...). That period depends on the spin of both the black hole and the accretion disk, but the possible orbital periods range from 4 to 54 minutes for our galaxy's supermassive black hole. The period is proportional to the black hole mass, so for a really big black hole like the one in M87, the period is weeks long. That's part of the reason why the EHT imaged the M87 black hole, but has not produced a Sgr A* image - it takes many hours for the EHT to gather the data to produce an image, and Sgr A* is not stable on that time scale. It's like taking a long exposure of a foot race. But M87's black hole can't change much in a few hours.
To the best of my knowledge, stars go nova because they have finished their fuel, and they can no longer sustain fusion once they begin fusing iron. It seems unlikely that being hit by a shockwave would trigger that. If it stripped away a lot of fuel, it would probably cause the star to go the route of a white dwarf rather than explode, right?
Remember when Beowulf Shaeffer took the Long Shot to the galactic core and discovered that it was exploding and would wipe out life in Known Space, so the Puppeteers moved their planets to parts unknown?
Could this flare be a foreshock of that? It'd be nice to have some time to settle my affairs.
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> Maybe we're just looking at an event that happened on Earth 2 * the distance in light years ago.
A very interesting idea!
Although I don't think that the black hole cared much of those few photons from earth.
Expanding it, how long until my hand waving could cause a strong enough "butterfly effect" for an observable cosmic phenomenon to occur beyond our solar system?
A million years? 100 million?
Edit: Thank you for your comment. Just keep it! Sometimes people here are just a bit quick to judge. It's amazing to think humans could have affected such a monster at all, no matter how little. Even for a handful of photons worth.
Expanding it, how long until my hand waving could cause a strong enough "butterfly effect" for an observable cosmic phenomenon to occur beyond our solar system?
My rough estimate says around 9 years, and 3 months.
The essence of chaos is that effects grow exponentially with time. For example with weather, errors in measurement lead to roughly an order of magnitude in errors in measurement every 3 days. Going from the scale of a single atom to the scale of a star is about a range of 10^60. At that rate of exponential growth means that in 180 days, which is 0.5 years a single atom out of place leads to a different outcome for things like solar flares.
Therefore a photon from your hand moves one atom on Alpha Centauri about 4.367 years later. A half-year later it causes the difference between a solar flare being there or not. Then 4.367 years later it comes back here. And the result is that in roughly 9 years and 3 months, one photon from your hand could have caused a visible change in another star.
OP is hypothesizing that an event happened on earth which emitted something traveling at the speed of light, traveled some distance, then interacted with some object which emitted something else which traveled back to be observed on Earth. Hence, double the distance between Earth and the black hole, because it's traveling twice.
Atlantis is usually put at "just before history" -- Atlantis was destroyed and then their survivors did all the impressive stuff around the world like building the pyramids and Stonehenge, so around ~5000 years ago.
You're looking at something more like 50,000 years ago, so something like the Toba population bottleneck (which probably didn't happen 70,000 years ago).
The cleansing of saidin in the previous turning of the Wheel, might work.
If a black hole is of brightness 0 (by definition), then 75 times that is still 0. They are probably referring to something happening just outside the event horizon and thus something that is not actually the black hole and just a phenomenon caused by the black hole.
No light comes from the black hole itself (excluding Hawking radiation, which is unimaginably weak from a supermassive black hole) but the region around the black hole, the accretion disk where material accumulates before shedding angular momentum and falling into the hole, can be spectacularly bright (because it can be heated to billions of degrees). The accretion disk and jet formation region can be brighter than all the stars in a galaxy. That's what a quasar is.
Nobody's observed Hawking radiation, so we don't know for sure.
Anyway, it's not obvious to me that the edge of a black hole is the event horizon. The event horizon is mostly empty space too. Maybe it's the ergosphere, which can be further out. Since a black hole is just a region of space, the boundary is sort of arbitrary.
There won't be any radiation from a supermassive black hole, or a stellar mass black hole. The bigger a black hole is, the colder it is, and any black hole bigger than a thimble is colder than the Cosmic Microwave Background. Its brightness isn't technically zero, but it's still darker than the background, and it's absorbing heat (and getting slightly bigger) rather than giving it off.
Because of Hawking Radiation, black holes are actually not entirely dark, and they have a temperature. Just because it's in the name doesn't mean it's part of the definition. Think of it as "black" as in "black-body radiation".
This is true, but Hawking radiation has nothing to do with the observed event. Hawking radiation is incredibly weak, orders of magnitude dimmer than even the cosmic microwave background, never mind visible starlight. The observed event is as the parent poster said, originating from just outside the hole's event horizon.