Are Distant Events Happening "Now"?

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Jim Leff
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Are Distant Events Happening "Now"?

Post by Jim Leff » Thu Jan 19, 2012 1:47 am

At some point in the past few months, there's been discussion of (and great explanation given for) the propriety of using the term "now" to describe celestial views of light that's traveled many light years to get to Earth. Not just a shoulder shrug re: the lack of any objective "now". The reasoning went deeper, and involved Einstein.

Can anyone help me find it?

My application is this: I want to write about the plan to network radio telescopes for a "real time" view of the event horizon around the black hole at the Milky Way's core (http://cosmiclog.msnbc.msn.com/_news/20 ... black-hole. And I want to defend the term "real time" (again, at a deeper level than simply dismissing outright the term's meaningfulness).

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Ann
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Re: Are Distant Events Happening "Now"?

Post by Ann » Thu Jan 19, 2012 2:18 am

You should ask someone else about this, but just consider the red supergiant Betelgeuse. Betelgeuse is about 500 light-years away. For all we know, Betelgeuse may "already" have gone supernova. That is, if you were able to magically transport yourself to a place in spacetime within a few light-years of where Betelgeuse ought to be, you might find that the star is gone. (Of course, if it had already exploded as a supernova, a supernova remnant would remain.)

As far as we know, a star that is about to explode as a supernova makes no outward signs of the upcoming event. It just suddenly explodes. That's why you can't say that a particular star is obviously going to explode next year. If that was the case you could carefully monitor the star and just wait for the tremendous fireworks. But it doesn't work like that, and Betelgeuse, for example, might go tomorrow or in a million years.

What do we mean if we say that Betelgeuse might go supernova tomorrow? We probably just mean that we will see the fantastic supernova in the sky tomorrow. But if we see the supernova tomorrow, then the explosion really happened 500 years ago. The star was obliterated 500 years ago, but the news about the star's demise - the brilliant light of the explosion - will reach us only 500 years later, after traveling for 500 years in our direction at the speed of light.

Keeping Betelgeuse under extremely close scrutiny to catch the supernova early is no use, since the actual explosion may not happen yet in a million years. That's because a star that goes supernova just pops, without any previous warning.

When it comes to the cloud which is about to plunge into the black hole at the center of our galaxy, the event has of course already happened some 26,000 years ago. But the "information about the event" - that is, any light flashes, radio waves, X-rays or the like that resulted from the dust cloud's death plunge - has not reached us yet. But we can see that we are going to have that information soon. We can't predict a supernova, but we can predict the demise of the dust cloud near the black hole. We can see, judging by the information that has already reached us - although it got here "26,000 years too late" - that a cloud is going to fall into the central black hole. We can say, with confidence, that if we keep monitoring the black hole, then information of any fireworks that resulted from that event that happened 26,000 years ago will undoubtedly reach us in the pretty near future.

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Chris Peterson
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Re: Are Distant Events Happening "Now"?

Post by Chris Peterson » Thu Jan 19, 2012 4:51 am

Jim Leff wrote:At some point in the past few months, there's been discussion of (and great explanation given for) the propriety of using the term "now" to describe celestial views of light that's traveled many light years to get to Earth. Not just a shoulder shrug re: the lack of any objective "now". The reasoning went deeper, and involved Einstein.

Can anyone help me find it?
I've discussed the matter a number of times over the years (if we had a technical FAQ, the matter of distance and time would need to be included). I think my most technical answer was this one.

The real issue is that it depends on how "now" is defined, and that definition depends on context and intent. To a GR theorist, "now" is almost always defined by the time of observation. You can never see back in time, and you can never see an event occur faster than the information can be conveyed at c. A cosmologist tends to consider the view of a distant object as a view backwards in time... and in a loose sense that is true. But it's still "now"!

In many cases, we use "now" for nothing more than practicality. So when we observe a supernova, we date it from its observation. We do that because it is of absolutely no practical value to try and figure out "when" it occurred in the past from its distance- because that number will be a moving target, given that we never know the distance all that accurately, and it changes with measurement technique. Every old paper would suddenly be wrong (or at the least, confusing) if dates changed based on subsequent measurements.
Chris

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Re: Are Distant Events Happening "Now"?

Post by Guest » Thu Jan 19, 2012 5:15 am

Thanks, Ann! Not the answer I was looking for, but highly interesting, thanks!

Jim Leff
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Re: Are Distant Events Happening "Now"?

Post by Jim Leff » Thu Jan 19, 2012 5:23 am

Woops, that last was me. I forgot to log in!

And Chris, that was exactly the post I'd been searching for. This is such an interesting topic....and so counter-counter-intuitive. That is, for a complete newbie, it's amazing that when we look at the sky, we're seeing into the past. But to those who've been educated on that, the notion that it's also still "now" just bends you mind over backwards the other way.

I used to run a food web site. Rank newbies would ask what's good in NYC's Little Italy. They would receive huffy, condescending replies about how NOTHING is good in Little Italy, where everything's purely touristic. But one day, someone posted and said, ahem, has anyone actually TRIED those restaurants lately? He had, and we'd apparently missed some good dishes.

So, as with "now", the hip answer is no, but the REALLY hip answer is yes!

A little knowledge, etc.... ;)