Question about looking at distant objects...

[tygerxg2]

I am only 11 years old, so please excuse the poor grammatical structures in this post.<br /><br />Suppose I have a powerful telescope that can see the surface of the moon. Meaning I can see rocks as small as those cute little furry rabbits.<br /><br />Anyways, since light takes around 1.5 seconds (I am sure this is not accurate but I hope it's close) to reach the earth...<br /><br />My question is:<br /><br />If there are people on the moon, and I look at them with my powerful telescope. Would I see them 1.5 second in the past...<br /><br />Meaning if a little 11 year old boy is hitting a baseball, I am actually seeing something that is 1.5 seconds in the past in details?<br /><br />Can this same question apply to something that is light years away. If a 11 year old hits a baseball 11 light years away, we're actually seing in details what has already happened 11 years ago?<br /><br />I hope this is not a stupid question. If it is, I will be embarrassed, depressed, and would consider dropping out of elementry school.

 

[SpeedFreek]

Hello there and welcome to the forum. <img src="/images/icons/smile.gif" /><br /><br />First let me assure you that it is not a stupid question! The time light takes to travel from the Moon to the Earth is approximately 1.25 seconds.<br /><br />So you are correct in your assumption that if you are looking through a telescope at the Moon, what you are seeing is 1.25 seconds out of date. You are looking at the Moon as it was 1.25 seconds in the past.<br /><br />The same applies for the Sun. The light takes around 8.3 minutes to travel from the Sun to the Earth.<br /><br />This principle applies at all distances, so if you are viewing something 11 light years away, the light you are seeing is from 11 years in the past.<br /><br />Current estimates for the most distant objects our most sensitive telescopes have viewed put them around 13 billion light years away, which means we are seeing them as they looked 13 billion years ago.<br /><br />I would advise you not to drop out of elementary school, although I cannot promise everything you will learn there will be as interesting as this stuff is! <img src="/images/icons/wink.gif" /> <div class="Discussion_UserSignature"> <p><font color="#ff0000">_______________________________________________<br /></font><font size="2"><em>SpeedFreek</em></font> </p> </div>

 

[heyscottie]

And as speedfreek said, it applies to ALL distances. When you look at your hand in front of your face, you are seeing it as it was about 1 nanosecond in the past. When you read this post, you are seeing how your monitor looked about 2 nanoseconds in the past...<br /><br />(Light goes about a foot a nanosecond.)

 

[tygerxg2]

This is hot... So Aliens who are looking at us from 65 light years away are actually seing WWII playing out here on earth in their telescope!<br /><br />That's hot!

 

[heyscottie]

Theoretically, yes. Or more likely, they are receiving radio and television broadcasts from that time...

 

[tygerxg2]

Wait... So what do they see when they zoom in to the Earth's surface? <br /><br />Nothing?<br /><br />That's totally disappointing.

 

[heyscottie]

shuttle_guy is right. I was only saying that it is a LOT easier for these hypothetical aliens to receive one of our radio or television broadcasts than it is for them to have built a telescope capable of resolving details on earth.<br /><br />From 65 light years away, to have, say 1 meter resolution on the surface of the earth, would require resolution of 2*atan(.5 m / 65 ltyr) = 9.3E-17 degrees, or 3.3E-13 arcseconds.<br /><br />Dawes limit states that you can resolve 4.56 arc seconds with 1 inch aperture. Using this, we would need a telescope with an aperture of 3.3E13 inches, or about 210 million miles.<br /><br />By the way, it actually is possible to build a telescope with that aperture. If we had two telescopes flying at antipodal points on the path of earth's orbit, they would be separated by about 186 million miles. So an alien civilization could theoretically build an interferometric telescope with the resolving power needed.<br /><br />But it's easier for them to receive our broadcasts, which would give them pictures from the ground as we sent them out which they could presumably decode and watch.<br />

 

[R1]

so what we need is 2 Hubble telescopes, one far up north of the ecliptic, and one far south,<br />to better explore the exo-solar planets? <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

The one we have is in orbit near the equator, so over time can cover all the sky.<br /><br />Realistically, there will never be another one paid for by NASA (The Taxpayers). If someone else can come up with the scratch, there may be another one. But it's a LOT of money <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[R1]

are the telescopes that heyscottie is talking about at the antipodes, are they radio telescopes or <br />telescopes like Hubble? <div class="Discussion_UserSignature"> </div>

 

[MeteorWayne]

I;m afraid you'll have to wait for his answer to that <img src="/images/icons/smile.gif" /> <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[MeteorWayne]

I don't think antipode is the right word since that term applies to the earth's surface only. It's the point on the other side of the surface exactly opposite in latitude and longitude from a given location. I'm not sure what the correct term is. He seems to be suggesting one scope on the earth's surface, the other following the earth's orbit 180 degrees out of phase with the orbit. <div class="Discussion_UserSignature"> <p><font color="#000080"><em><font color="#000000">But the Krell forgot one thing John. Monsters. Monsters from the Id.</font></em> </font></p><p><font color="#000080">I really, really, really, really miss the "first unread post" function</font><font color="#000080"> </font></p> </div>

 

[heyscottie]

Generally speaking, they would be some larger wavelength detectors, such as radio telescopes. When doing interferometry, you need to know the relative positions of your detectors within a small fraction of the wavelength of the light you are detecting. For radio frequencies, this is possible. For visible light, which has wavelengths around 600 nm, this becomes hard. Locating precisely relative distances between two items 186 million miles apart to an accuracy much better than 600 nm? Tricky.<br /><br />Scott

 

[heyscottie]

But then again, they are the aliens, and they are very smart...

 

[R1]

oh ok.<br />I thought it was something we could very simply build here on earth.<br /><br />it's been mentioned in the SETI forums something about needing a very large antenna to pick up weak signals from afar,<br /> so large, that I understood it to mean impractical<br /><br /><br /><br /> <div class="Discussion_UserSignature"> </div>

 

[heyscottie]

Now there's a difference between being able to pick up weak signals and getting high resolution.<br /><br />To pick up weak signals, you basically need a large signal gathering area. A very large dish, or many smaller dishes are required. To get high resolution, you need a very long baseline. A very large dish, or a couple of widely separated dishes are required.<br /><br />Weak signals require lots of area. High resolution requires a long baseline.

 

[R1]

if we try to receive a tv or radio signal from so far away that that the signal is so redshifted,<br />what should we listen for? for example would a redshifted tv signal appear to us as radio?<br /><br />If a civilization millions of light years away plays AM or FM radio on their moon, and it gets redshifted a lot,<br />what kind of signal would it be when we detect it here on earth? <div class="Discussion_UserSignature"> </div>

 

[nexium]

A Uhf TV signal on about 600 megahetz red shifted 99% would be at about 6 megahertz: Typical shortwave radio frequencies. The 6 megahertz band pass would be compressed to about 0.06 megahertz = 60 kilohertz, so it would now be slow scan 0.6 frames per second.<br />We could perhaps record an hour of the red shifted audio and play it back 100 times faster in about 0.6 minutes = 36 seconds, otherwise most of the audio would be at less than 30 hertz which is about the lowest frequency that humans can hear. Neil

 

[R1]

well this is really interesting.<br /><br />1) and what if the frequency is redshifted only a value in the range of 5<z<20?<br /><br />2) and how big should be build an antenna on the moon maybe?<br /><br />3) should it be saucer shaped built on a crater 200 miles wide? or should it be like<br /> a metal rod sticking a mile and a half up into space? or both? or would it help to build a uhv/vhf tv<br /> antenna with all kinds of antenna elements on it, in orbit, about 3 miles in size?<br /><br />4) what size would the antenna need to be for us to detect a weak signal similar to ours<br /> that comes from million of lightyears away?<br /><br />5) most of these questions might be for seti people (so answering is optional)<br /><br /> and do we recompress signals we hear from space on a routine basis to different <br />recompression levels to see if it's anything <br /> that might have meaning on a tv or ham radio or any radio, or satellite tuner?<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[heyscottie]

Generally speaking, when we are talking about searching for alien signals, or sending out some of our own for aliens to intercept, we are talking about communicating with other worlds in our own galaxy. That is a hard enough of a problem, but when you start talking about even other galaxies in our own local group, it starts getting ridiculously hard.<br /><br />Within our own galaxy, or even within the local group, there is no redshift due to cosmological expansion, because we are all bound by gravity, so redshift is not an issue.<br /><br />Another reason we generally don't think about detecting signals from distant redshifted objects is that when we are looking so far away, we are looking much further back in time. We are seeing a lot of first generation stars, and much fewer metal rich stars that are necessary for planetary formation. Simply put, we are looking at a much younger universe that hasn't had time for civilizations to form yet!

 

[R1]

interesting. <br /><br /><br />thanks heyscottie<br /> <div class="Discussion_UserSignature"> </div>