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Looking back in time

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#1 Dennis_S253


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Posted 13 December 2012 - 10:53 AM

Can someone explain "looking back in time". Lets take an easy one. M45 "the seven sisters, Pleides" they say it's between 390-460 LY away. So when I look at it naked eye I'm seeing it as it looked 390-460 LYs ago. But what about when I use my telescope at different magnifications?

#2 CosmoSat



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Posted 13 December 2012 - 10:59 AM

You will be seeing the magnified image of what they looked like 390-460 light years ago... :p

Magnification is about how an object will look like from close up.. It dosnt really take u any closer physically. The amount of time is definite once the photons hve left the object. Magnification wont alter it.

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#3 tecmage



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Posted 13 December 2012 - 11:08 AM

I think of it this way- the light we see tonight from M45 took approximately 390-460 years to get here. Yes, that means we're seeing M45 the way it looked 390-460 years ago. Your telescope doesn't change the amount of time it took for the light to reach your eye. If it did, that would mean that your telescope changed the speed of light.

#4 FirstSight


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Posted 13 December 2012 - 11:31 AM

You will be seeing the magnified image of what they looked like 390-460 light years ago...

Yes, but at higher magnification, 390-460 years won't SEEM to have been as long ago as at lower magnifications. Kind of like the more fun you're having the quicker time seems to pass, the less fun, the slower...

#5 Dennis_S253


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Posted 13 December 2012 - 11:40 AM

Well, I have always wondered that. I have read articles that say, looking back in time. I just read another one about the 7 galaxys that Hubble imaged. Now in that article they said it's "effectively like looking back in time". That makes more sense.

#6 Pharquart


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Posted 13 December 2012 - 12:50 PM

Here's the interesting thought: it's possible that the stars of M45 don't exist anymore. They may have gone supernova in Galileo's time, but since the light from that event wouldn't have reached us yet, we wouldn't know it. In other words, not only are we looking back in time, but we have absolutely no way to look at the "present" in space. Even light from the sun takes 8 minutes to reach Earth, so it's possible that 6 minutes ago the sun blinked out. We'd have no way to know until the light (or darkness) reached us.


#7 Tony Flanders

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Posted 13 December 2012 - 01:09 PM

Here's the interesting thought: it's possible that the stars of M45 don't exist anymore.

Not really. Everything we know about stars indicates that the Pleiades look almost exactly the same "now" as they did 400 years ago.

Without some degree of faith in the constancy of physical laws, there can be no knowledge at all. Astronomy isn't special this way. When you look at another person in the same room, you're seeing him/her as he/she was a few nanoseconds earlier.

For that matter, how do you know the other person exists at all? Maybe the entire physical world is an illusion, as Bishop Berkeley claimed. Maybe you yourself are an illusion -- though that's a tougher nut to crack.

However, light travel time does have real and important effects in astronomy. Jupiter's moons appear to slow down as the planet moves away from us and speed up as it moves toward us.

Space probes on other planets require a certain degree of autonomy, because the round-trip time for radio signals precludes direct feedback.

The Pleiades stars are surely all essentially unchanged, but it's entirely possible that Betelegeuse has already gone supernova.

There's no doubt at all that the light from thousands of as-yet-unseen supernovas in the Andromeda Galaxy is now wending its way toward us.

For more distant galaxies -- ones still readily visible in backyard scopes -- their shape has changed significantly since the light that we're seeing left them. The spiral arms we see have burned out, and the dark lanes between have spawned new stars.

#8 MikeBOKC



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Posted 13 December 2012 - 02:10 PM

Stephan's Quintet is a classic example of what Tony is discussing. Four of the five galaxies in the group are merging or at least interacting in a major way. This grouping is something like 300 million light years away, so it is very likely that those four galaxies we see as distinct entities are already combined into a massive single elliptical galaxy which looks entirely different.

#9 Dave Mitsky

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Posted 13 December 2012 - 02:52 PM

The various cosmological distance scales in use can be rather confusing. The article at http://www.atlasofth...m/redshift.html explains them fairly well.

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#10 TL2101


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Posted 13 December 2012 - 05:52 PM

Every night we see new photons so every time we view an object aren't we seeing it for the first time? Last nights photons are gone never to return. Just a crazy thought. :grin:

#11 Danzup77


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Posted 13 December 2012 - 06:31 PM

"You cannot look out into the universe......without looking back into time." - Carl Sagan.

#12 BlueGrass



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Posted 14 December 2012 - 06:57 AM

The concept that always intrigues me is the idea that the Universe as we 'know' and 'see' it really doesn't exist now. It's a hard concept to relate to our daily lives where things happen in a perceived, orderly sequence that we can interact with. We can only observe the universe as it was, not as it is.

#13 Meadeball


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Posted 14 December 2012 - 09:37 AM

Here's something that will bring it all closer to home:

Our sun is approximately 8 light-minutes away from us, meaning the light we see from it, along with the heat and everything else, takes about 8 minutes to get to Earth.

Seven minutes ago, the sun went supernova. Mercury and Venus are both boiled into oblivion. A huge wave of expanding gas -- an astronomical tsunami, if you will -- is heading this way. But alas, we won't know about it for one more minute. And nothing will help us learn any faster: whether it's our eyes, a big mirror or a radio antenna, we're all receiving the same light, heat and radiation from the sun at the same speed. The orbiter we have currently in Venusian orbit disintegrated two minutes ago, but since radio waves also travel at the speed of light, we won't lose the signal until we're annihilated ourselves. Nope, we'll all find out about this together. (Well, the folks on the Earth's night side will have a couple of terrified seconds wondering why the sky suddenly looks so strange and the ground is shaking under their feet. And what is that rumbl-----)

Since light travels 11.18 million miles per minute (670.8 million mph) and this explosion is traveling outward at probably just under the speed of light, the last vestige of Man's existence -- our own Voyager 1 spacecraft, now roughly 8 billion miles from Earth -- will witness the supernova and then burn up in this fireball in just under 12 hours.

Have a nice day!


P.S. I wouldn't mind dying this way. Imagine -- no pain, nearly instant. Only a split-second of time, probably not even long enough to say, "What the ..."

#14 Michael Rapp

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Posted 15 December 2012 - 10:47 AM

One can a great deal of fun with verb tenses with this that will blow your mind.

Consider the oft-observed transit of a moon of Jupiter. Think of the view when the moon, let's say Io, is just about a few minutes of transitting off the planet. "In reality," the transit has already ended! Jupiter is roughly 40 light-minutes away, so if the transit of Io is twenty minutes from ending -- from our vantage point -- it already has ended!

What is time? What is space? I need to go read a Brian Greene book...

I feel small and my head hurts. :)

#15 GeneT


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Posted 15 December 2012 - 04:47 PM

If it did, that would mean that your telescope changed the speed of light.

My Portaball does. :grin:

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