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.
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).
See e.g.: https://en.wikipedia.org/wiki/SN_1987A
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.
How does a particle or its environment measure the particle's traversal of a truly empty space?
300 000 km per second
"Its exact value is 299,792,458 metres per second (approximately 300,000 km/s"
I read that as: Can something come along and snuff us out after we've observed the light wave component of the event, such as a shock wave of some sort?
That is the only way it can happen.
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.
Not that it matters, because the speed of light is still the limit: if they could influence us when they were closer, that would have happened by now.
It certainly moves matter-energy in different regions of space with respect to one another.
https://physics.stackexchange.com/questions/335537/can-a-sha...
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)
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.
t0: SO U1 (U1 in shadow)
t1: S~~~~~~~~~~>U1 (shadow leaves top first)
O~~~~~~> .
~> .
.
.
'
U2
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.
* https://en.wikipedia.org/wiki/Lighthouse_paradox
There are similar things that appear to exceed the speed of light:
* https://en.wikipedia.org/wiki/Faster-than-light
See "group" and "phase" velocities for similar things to the lighthouse.
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.
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.
[EDIT] inverse square, that is. My bad.
It seems like the effect of that event could be felt only millions of years after the flash of light, no?
I have written a few things about dynamics on my blog, though (that was my subfield): https://joe-antognini.github.io/
Anyway, it's appreciated and this really was a great answer.
You’re right, that of almost any PhD I know (admittedly not that many) they are both able to explain what they have studied or written about clearer than any of the others I know.
The family loves when they both get talking about this theory and that, it makes for great conversation the rare times everyone is together.
Do we have other data like that? How far back does our record of Sgr. A[star]'s light curve extend?
Also, I gather that Sgr. A[star]'s rotational axis isn't normal to the mean galactic plane. And I presume that the axis precesses. Does it ever point near enough to the Sun to cause damage?
Edit: typo
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.
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)?
For me, that was a surprising bit in your comment. I don't understand how it can be true (5 million years in the future, sure, but the past?). Can you give an example or explain?
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.
Only 2 minutes would have passed for the SMBH in your frame of reference (but 52k years would have passed for earth in your frame of reference)
It hasn't happened yet in reference frames around the Large Magellanic Cloud, right?
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.
From what I understand, no one ever moves (fast, slow, whatever) compared to the speed of light from their own reference point.
The speed of light is always the same for all observers from their own reference point, so how do you reconcile those statements?
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)
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.
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.
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.
There is no such thing. You always have to compare your speed to something else. Speed is calculated relative to something else. Relativity.
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.
We're not.
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.
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.
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.
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?
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.
The event occurred is 26,000 light years away, we date it as having occurred 26,000 years ago.
How is it that a location that's closer, say 10,000 light years, would have dated the event to 26,000 ago rather than 10,000 light years?
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.
I can't imagine that they would say that!
I'm sure that you mean the equivalent of 24,000 BC in our reference frame, after Lorentz transformation. Not that they would likely even know about us, or manage the transformation.
This is bloody confusing.
Or know when ours was supposedly born?
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.
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).
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.
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.
This is a Lorenz geometry definition of reference frame, not an everyday sense of “frame of reference”
If I call my friend at 7:30am and tell him, does that mean the person was murdered at 7:30am?
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.