Cosmology-I am very confused-who can answer this question?

Question

I watched Nova last night on PBS concerning the black hole in the center of the Milky Way.. My brain was spinning in my head. They were talking about events occurring millions of light years away. Doesn't that mean that they occurred millions of years ago? The scentists were speaking in the present tense. I could not get past this confusion and got very little out of this show. Could someone "un-confuse" me? thanks

J K M has a point. there is an element of fear. Who agrees?

Answer 1

I think John put it pretty clear; but I would like to say that some of these programs leave me pretty dam scared. I also find it hard to buy into some of the ideas. But that is just me.

Answer 2

When you talk about astronomy, you talk about your current observations in the present even though they happened in the past. If you saw a bright flash on the moon from a meter impacted, there was a 1-1/4 second delay from when it happened to when you saw it. I've observed eclipses of Jupiter's moons, that had actuallly occured several minutes earlier. The supernova that created the crab nebula was seen on Earth in 1054AD actually exploded before 5000BC at the dawn of civilization.

This is confusing. I might suggest some simple observations that use sound to make the delay. Thunder and lightning are created at the same time, but the sound travels at 1000ft/sec. If you see the flash and count the seconds until you hear the thunder, you can calculate the distance. Sit in the outfield at a baseball game. On a good hit, the ball will have travelled a fair distance before you hear the crack out the bat.

(Sorry about the examples if you're not from the US.)

Answer 3

Yes. That's correct. Events that are observed to happen light years away are not actually observed when they happen, but when the light actually does get to our observation point. So a black hole millions of light years away is being observed as it was millions of years ago.
Which means we have no way of knowing (though we can project) exactly what that black hole looks like at our present time.

Answer 4

Well, the Milky Way is about 100,000 light years across, so the center is about 40,000 light years away, max. 40,000 years (and even millions of years) are not very long on astronomical timescales, so we are fairly certain things are about the same now. Plus, we would see gravitational effects from the loss of a black hole (if it managed to go away somehow), so we just think of anything in the last million or so years as 'now'.

Answer 5

Correct. Bear in mind, though, that the core of the Milky Way is thousands, not millions of light years away. And, if we thought it was blowing itself to bits right now, we wouldn't be affected until thousands of years into the future. I agree, it is confusing, but the more you learn of cosmology the less the confusion.

Answer 6

True, we see things millions of light years away as they were millions of years ago. When they speak in the present tense, they mean as we presently see it, not as it presently is. This is even true for the Sun, which we see as it was 8 minutes ago. We see the Moon as it was one second ago. But it is convenient to just say we see them and neglect the time delay. It isn't really important to our understanding what we see anyway.