Looking back in time.

[astroboy3k]

Ok, they say that looking at stars you look back in time... If I'm looking at a star that is 1000 light years away, I'm looking at it 1000 years ago. Forgive me for being ignorrant but am I the only one that see a fault in that statement. They say that the light from that star I am seeing just got here after traveling for 1000 years. I don't get it. When I go outside I smoke a cigarrette and look up, I just see a star (present time), no thousand, millions, or billions years ago, I see it now. And how do people know that light travels over 180,000 miles per second? Is there anyway to test this?

 

[henryhallam]

The speed of light has been measured in many different ways, both directly and indirectly.<br />As an example of one early technique:<br />Find two hilltops a few miles apart. Set up a mirror on one of them, and then some apparatus on the other. The apparatus consists of a lamp which produces a focused beam of light, and a slotted disc which can be rotated at a high speed. The light shines out through a slot in the disc, bounces off the mirror on the other hill and comes back to the disc. This happens extremely quickly but it does take a definite amount of time, t=2d/c where d is the distance between the hills and c is the speed of light (which we are trying to measure).<br />Now, in the time t that the light took to travel that distance, the disc has rotated through a certain angle. The technique is to adjust the speed at which the disc rotates so that when the light gets back, the disc has rotated enough that the light can pass through the next slot so the observer behind the disc can see it. If you know the speed at which the disc is rotating, you can figure out the time between one slot and the next, and since you know the distance to the hill you can calculate c.<br /><br />There are many other "classical" ways to do it, including tricks with timing the passage of Jupiter's moons (which I thought was a particularly neat way to do it). And there are more modern techniques; you can measure certain other physical constants to do with electrostatics and electromagnetism, and then calculate c from the values of those. The following link shows you how to measure c using nothing more than a microwave and some chocolate, using the fact that c=frequency * wavelength for any electromagnetic radiation (including light, radio waves and microwaves)<br />http://physics.about.com/cs/opticsexperiments/a/290903.htm<br /><br />So since we know that light takes a definite time to travel a certa

 

[vogon13]

It turns out the speed of dark is the same as the speed of light.<br /><br /><img src="/images/icons/wink.gif" /><br /><br /><br />(I've got a million of 'em)<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[vogon13]

Joke, anything said or done to cause amusement.<br /><br />{courtesy of Randon House Webster's Dictionary, 4th edition}<br /><br />'nother Vulcan on Uplink today.<br /><br /><img src="/images/icons/wink.gif" /><br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[dick_in_mi]

>>When I go outside I smoke a cigarrette and look up, I just see a star (present time)<<<br /><br />First off, quit smoking.<br /><br />Second, you don't see a star....you see light from that star. Now you might think "what's the difference?" There is a difference.<br /><br />You're not seeing the star...in some cases the star might no longer exist. You're seeing the light that was transmitted from that star long ago.<br /><br />Here's an analogy....someone takes a picture of a child, puts the photo in an envelope and mails it to you. But the Post Office loses the letter and it doesn't get delivered until 50 years later. OK, you look at the photo and say "this is what the child looks like!" No, this is what the child looked like 50 years ago. Today the child looks like a 54 year old man (and being a 54 year old many myself, I can tell you that I've changed a bit in 50 years). Or the child in the photo is now dead. Either way, what you're seeing represents a snapshot in time, and that time was very, very long ago. Same as when you look up and see a star.

 

[rodrunner79]

<blockquote><font class="small">In reply to:</font><hr /><p>You're seeing the light that was transmitted from that star long ago. <p><hr /></p></p></blockquote><br /><br />So it's like saying there's a vessel (light) that left that star long ago and it's just now getting to us and we see that vessel with naked eye many days which with probability might not exist anymore.<br /><br />Ok first of all, that's a long ass vessel (beam of light) to see it for many days. Second, if you where a big giant creature looking at earth on the left and the star on the right. Can you actually see that beam of light between earth and that star. It doesn't seem like it when i see space photos, but that's just me.

 

[Saiph]

First, the beam isn't a "single" object. It sends a stream of data. It's pretty easy to see if you think of it as a bunch of photons heading in the same direction. Each one holds a piece of the information, each one was released at a different time. <br /><br />Lets look at the sun, ~8 light minutes away. The photons you see, now, were emitted 8 minutes ago, and are just now getting to you. The ones you see now (a second or so later) were emitted 8 minutes ago. The ones you missed the first time, have now traveled for 8 minutes, 1 second, are are a bit further. The one's you'll see in the next second, have traveled for 7 minutes and 59 seconds.<br /><br />Now, say the sun blows up, right this instant. The light from the explosion leaves, and takes 8 minutes to reach you. In that time period, the light already en-route is still reaching you...so the sun seems normal. After 8 minutes, the light from the explosion finally reaches you...and the sun's gone.<br /><br />As for seeing light between a Star and a planet...no can do. If you see it, it wasn't heading from the star to the planet anymore. In order for you to see that light, it has to be redirected. Take a dusty room, and you see a "beam" of light come in from the window. The light that hits the floor is <i>not</i> the light you see. the light you see is the light that hit the dust, and reflected to you (and thus never made it to the floor!). <div class="Discussion_UserSignature"> <p align="center"><font color="#c0c0c0"><br /></font></p><p align="center"><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">--------</font></em></font><font color="#999999"><em><font size="1">----</font></em></font><font color="#666699">SaiphMOD@gmail.com </font><font color="#999999"><em><font size="1">-------------------</font></em></font></p><p><font color="#999999"><em><font size="1">"This is my Timey Wimey Detector.  Goes "bing" when there's stuff.  It also fries eggs at 30 paces, wether you want it to or not actually.  I've learned to stay away from hens: It's not pretty when they blow" -- </font></em></font><font size="1" color="#999999">The Tenth Doctor, "Blink"</font></p> </div>