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Describing a galaxy 70 million light years away, I consider it incorrect to state for example "it has an active black-hole at its center". It did 70 million years ago, who knows what is out there now. I could understand using the present tense for objects within the solar system and even nearby (less than say 100 ly) but not for such distant objects.

Is there some convention to use present-tense for all observations, or am I being a pedant?

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    $\begingroup$ The speed of light is the speed of causality. From where we are, that is the last thing that happened. $\endgroup$
    – called2voyage
    Commented Jul 15, 2016 at 17:00
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    $\begingroup$ @RobertFrost It is at a visual distance of approximately 13.7 billion light-years--or more accurately, the end of the Planck Epoch is, since it is impossible to gain any information from before then. $\endgroup$
    – called2voyage
    Commented Jul 15, 2016 at 18:36
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    $\begingroup$ You were being a pedant, and you considered it incorrect to state 'it has an active black hole at its center'. But who knows what you are doing or think now? ;-) $\endgroup$ Commented Jul 15, 2016 at 20:17
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    $\begingroup$ I'm voting to close this question as off-topic because this is a question of usage in the English language, not astronomy. $\endgroup$
    – James K
    Commented Jul 15, 2016 at 20:56
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    $\begingroup$ @JamesKilfiger: disagree, because it's specific to astronomy jargon which of the two possible conventions to follow when referring to distant galaxies. $\endgroup$ Commented Jul 15, 2016 at 20:58

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You're being a pedant and are not necessarily correct!

The present tense could be clearly defined as what is experienced at the time of observation. To write that it had a black hole 70 million years ago would (pedantically) only be correct at one instant of time, not correct when the reader read the sentence and even ignoring that pedantry, not necessarily true for gravitationally lensed light where the light has not travelled along a straight path!

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    $\begingroup$ Like the joke about the 70000040 year-old dinosaur fossil. "The galaxy is 70 million and one light years away. I know this because the light from it has travelled 70 million light years, and it's receding at 1/70 millionth the speed of light" ;-) Allowing for the precision used, the measurement "70 million years ago" actually will be as good next week as it is now! $\endgroup$ Commented Jul 15, 2016 at 20:50
  • $\begingroup$ Yes, but the 'instant' that a galaxy exists is presumably a rather long time. And my point is that present tense applies to observation not reality when it comes to temporally displaced observations. $\endgroup$ Commented Jul 16, 2016 at 23:38
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This is a great question and observation, and in fact it is not just distant events this applies to. When I look in the road and see my car parked outside, I should be thinking "it was outside", half a picosecond ago." I think it's just out of convenience that we use the present to describe all states as we perceive them now. But in actual fact we know nothing of "now" other than our own thoughts and feelings. All external stimulus is transmitted to us via various different media and through our senses, all of which take time to reach us, so if we are to be accurate, we never know anything about the present.

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  • $\begingroup$ "All external stimulus is transmitted to us via various different media and through our senses so if we are to be accurate, we never know the present." This is true, even our own visual perception is delayed. $\endgroup$
    – called2voyage
    Commented Jul 15, 2016 at 18:39
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    $\begingroup$ You would have to use a different description depending on whether you were looking through an open or closed window... $\endgroup$
    – ProfRob
    Commented Jul 15, 2016 at 18:56
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    $\begingroup$ Actually, light only travels 0.15mm in half a picosecond. Sorry, but the question is about being unnecessarily pedantic! $\endgroup$ Commented Jul 15, 2016 at 20:55
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    $\begingroup$ @SteveJessop I have a small front garden. $\endgroup$ Commented Jul 15, 2016 at 21:15
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I wasn't surprised, after reading your title, to find that you used the word "now" in the body of your post. It's difficult to ask your question unless you believe that there exists something external to and independent of you which "now" captures. There isn't. Well, let me qualify that: as long as we're all living within a light second of each other (Mars varies between 3 and 22 light-minutes from Earth, the Moon is ~1 light-seconds away) and we're not (disparately) subject to relativistic forces, we can all share an approximate "now" among ourselves. Not only does our language and much of our thinking involve this now concept, but we're probably hard-wired to treat it as a fact. Seeing someone use some other norm is disconcerting and unintuitive. Lectures and books on relativity usually discuss space-time diagrams. These diagrams include light-cones and separate the Universe into 3 parts: the time-like part, the space-like part and the light-like (or null-interval) surface. For Modern Physics, which prefers frame invariant language, the logical meaning of "now" is the point of intersection of the forward and past light cones. This gives us an observer dependent "now", and allows us to describe an observation from a black hole 70 million light years away as being BOTH 70 million years away and 70 million light-years distant. Any claim about what that black hole is doing "now" in the popular sense, is unfalsifiable (for the next 70 million years or so) and isn't very interesting (and very much useless) to empirical science. Its (popular) narrative, however, suffers in translation from the Astronomical/Cosmological "now" to the popular one.

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  • $\begingroup$ Sorry, you lost me in all of that. Is there an answer to my question in there? $\endgroup$ Commented Jul 16, 2016 at 23:31

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