So around 2013-14 a massive gas cloud called G2 passed very close to the super massive black hole at the centre of our galaxy but did not get sucked in.
So it was observed in 2013-14.
But when did it actually take place?
Few thousand years ago as thats how long it takes for the light from the event to reach us? Thats what they say about the stars. The light we see now left any star a few thousand years ago!
A recent BBC programme called Stargazing live indicated that the event in 2013 was seen in real time by earth. Surely that cant be right.
Actually, it depends on one’s interpretation, and from one important physical perspective, the event really did occur in 2013 and was observed in real time.
The perspective that something occurred at time = {present minus light transit time} actually has some value when observing very distant events occurring billions of light years away because it gives us insights into the earlier stages of the universe. However it’s also very wrong if you define time – and the concept of “now” – in terms of causality, which if you stop to think about it, is actually how we define time: a chronology of events – the ticking of a clock, the flowing of sand through an hourglass, the aging of ourselves. In those terms, it turns out that our intuitive idea that there is such a thing as “now” at all points in space is completely wrong.
This article describing the relativistic concept of the light cone is a good read, as is the diagram at the top of the article. “Now” and the whole idea of simultaneity is purely a local concept – simultaneity is defined as the outer edges of the light cone which represents the leading edge of a flash of light traveling through spacetime. The points along such a line are all referred to as having null separation in terms of causality. “Now” exists only where the future and past light cones meet at one point in space. Anything separated by the spacelike interval is also causally separated by the timelike interval.
When you’re observing the events around the black hole at the center of the galaxy, in terms of that diagram you’re observing them with null separation along the lightlike interval of the past light cone from the center of the galaxy. They are happening “now” in terms of the limits of causality. If you could travel to that point any faster than implied by that lightlike interval – any faster than the speed of light – you would arrive there “earlier” than 28,000 years ago and be able to affect future events as observed from earth. You’d be changing things that have already been observed here, thus rather seriously violating causality. What you see when you look at it from earth is in one sense 28,000 years ago, and in another important sense it’s “now” because anything “more recent” is in your future, every bit as much as next Tuesday.
While it’s true that “simultaneous” isn’t well-defined in relativity, for any events that are separated by any distance, the word is not ever actually used for events with lightlike separation. Nobody in physics would ever say that an event that we’re seeing now is happening now, except in an informal sense. We would always say that it happened X lightyears away, X years ago. Now, two observers in different reference frames might disagree on the value of X, but they’ll still each have some consistent nonzero value for it.
Simultaneity in relativity is linked with global spacetime coordinate systems.
Special relativity automatically defines a nice set of global spacetime coordinate systems called inertial frames, so for inertial observers there is a well-defined concept of simultaneity that is observer-dependent.
In accelerated frames in special relativity the concept of simultaneity is more problematic. Some observers like Rindler observers have a fairly easy to define concept of simultaneity, but that still runs into coordinate artifacts like Rindler horizons. Often times though naively trying to define simultaneity for accelerated observers can lead to bad coordinate systems (e.g. coordinate systems where a single event is given more than one time coordinate), such a thing often happens when trying to look at simultaneity when analyzing the twin paradox. More generally though for accelerated frames simultaneity is a much less useful idea than it is in inertial frames.
In general relativity the concept of simultaneity can fail completely, but for most spacetimes representing physically-realistic situations simultaneity can be defined, though most of the time even for a single observer the exact definition becomes arbitrary. For cosmological spacetime the spacetime provides something akin to a preferred frame of reference and so you have cosmological time which gives you a nice universal concept of simultaneity.
Oh well they mean that you can’t press fast forward nor rewind - so you have to wait for it , and if you aren’t recording what you need, you can’t then go back and get it. They were just briefly mentioning that it requires patience and dedication and good predictions.
The issue the ABC version * (See below) said that the cloud was not seen to enter the black hole. Then the telescopes went back to other work, as there was no prediction of seeing anything after that.
Except a NASA telescope thing recorded two blips two months LATE. They said this probably was the cloud dropping into the blackhole, but no one can be sure. This implied that no one knows where the cloud is now (now - corrected time - as in the time line that we can observe now - yes it all occurred 22000 years ago… but we can only observe this 22,000 year delayed version of the story. )
The NASA blips may be something else - but the behaviour of the cloud and what was wrong with the predicted timing of the cloud eating is yet to be explained… it could go either way… does the correct prediction put the eating two months later, or does the correct prediction put the cloud surviving ?
In 2017, following on from the seventh series of BBC Stargazing Live in Australia, ABC TV plan to air an Australia-focused series on 4-6 April 2017, headed up by Brian Cox and Australian television presenter Julia Zemiro. The proposed format and content is different from the BBC …