How do we really know how far away stars are?

I have read a couple of books on science so now I'm dangerous. Einstein tells us that gravity bends light and as a result light follows a curved path. I've also read that spacetime is curved.If that is the case how can we really know what the actual distances to the various stars are and how do we actually know how many there are?Using our planet for example. I am standing on the North Pole. I have an omni directional radio antennae and I send off a signal. My buddy is down at the South Pole. He has a direction finder on his receiver and he also knows to the nanosecond the time the signal was sent.With his direction finder he finds that he is receiving the signal from a number of different directions, (even though they are all North ), as no matter along which line of longitude the signal travels it is the same distance.By knowing the exact time it took the signal to travel that distance he can now calculate the distance to the North Pole.Let's review the findings. What is the direction to the North Pole? The signal was omni directional so we actually received many different signals at the South Pole even though they all originated from the same place.The distance that we have measured only gives us the distance that the signal travelled on its curved track around the circumference of the planet. In actuality there was a much shorter route right through the center of the Earth. In other words we measured an arc in instead of the direct route.Stay with me here. I'm doing the best I can. Honest.Now then let's apply this to a star. If light curves as it is pulled by gravitational forces how do we know that what we are measuring is really a straight line. Maybe for instance if light were to travel in a straight line the star we are now seeing in Canada would instead be seen from Australia.Also of course because light is travelling in an arc the direct distance to the star might be considerably closer than what we measure.My last point is that because light is emitted omni directionally who is to say that we are not seeing the same star in a number of different locations depending upon the route that the light took to get here.Man I hope that all makes sense. This message has been edited by GDR, 06-17-2005 08:55 PM

Thread moved here from the Proposed New Topics forum.This is a great question.This message has been edited by AdminSylas, 06-18-2005 12:39 AM

Well we do know that this happens. While individual stars lack the neccessary gravitation to lense images to the point of duplication the effect can be seen for galaxies - Link.There are further links from that page to other pages with examples of gravitational lensing. One interesting example produces three images of the same Quasar with the three images corresponding to different times in the Quasar's existence.TTFN,WK

In your message you actually ask about the gravitational bending of light and radio waves, but this example involves a completely different phenomena. The gravity of the earth is far too tiny to bend light waves around its circumference.Radio waves, especially short wave radio waves, reflect and refract off the upper atmosphere, and this can carry them nearly around the world. If the atmosphere did not exist radio waves would radiate out in a straight line off into space. Some frequencies of radio waves are unaffected by the atmosphere, and these *do* radiate off into space. And our atmosphere is not a perfect reflector/refractor, and so some portion of most radio waves do radiate out into space.I found an explanation of how radio waves propagate around the curvature of the earth at Forbidden, give it a read if you're interested. This one gets a little technical in places, if you poke around the Internet you can probably find some simpler explanations.Anyway, the real point is that you can't use your example of atmospheric bending of radio waves to ask a question about gravitational bending of radio waves. They are two different phenomena. But the question you're asking is nonetheless clear. If light traveling through space is bent as it passes through the gravitational fields of stars and galaxies and black holes and such, how can we have any idea of where the light is really coming from?Most of space is empty, and most light that arrives here on Earth has traveled through empty space. Gravitational fields weaken by the square of the distance, and so the net gravitational field in most of space is very weak and affects light to an indetectable or at least very tiny degree. It is only when light follows a path that takes it very close to an object with a large gravitational field that it is bent to any measureable extent.When you see two objects in the sky very, very close to one another, and one is closer to us than the other, then the chances are the direction of the light from the more distant object has been changed by a tiny but measureable amount. Gravitational lenses, called such because the light is bent by gravity rather than by refraction from passing through a boundary between materials of different densities such as happens between air and glass, can by sheer chance magnify a distant object, or cause multiple images of the object, or even bring into view an object that would otherwise be obscured because it was behind. I'm sure our instruments our sensitive enough to measure the amount of bending the Earth performs on the path of light, but earth's gravity is so weak that the bending is not apparent to the human eye, not even close. For most intents and purposes, light traveling through the earth's gravitational field travels in a straight line, but it *does* fall. Light traveling parallel to the surface of the earth falls just as fast as all other objects, about 10 m/sec², but since light travels a kilometer in about 1/300,000th of a second, it only falls about a billionth of a meter over that distance. Humans can't discern a billionth of a meter right in front of their nose, let alone from a distance of a kilometer.Light arriving from space is affected by similar miniscule amounts (it's not traveling parallel to the earth's surface, so the effects combine both changes in direction and frequency). The light arriving from space is also affected uniformly by the earth's gravitational field, so all light is affected by the same amount in the same direction (on the order of a billionth of a meter) and so even when measured very precisely by special instruments, the objects in space will still appear arrayed the same way no matter where viewed from on earth.--Percy

Thanks Percy. I understand that radio waves operate differently than light waves but I used radio waves to describe what I was getting at. As I understand it though, what we view as matter is only about 5% of the universe. As I understand it all energy and mass exert a gravitational pull so doesn't that include dark energy and dark mass. How do we really know what gravitational forces are working on the light that has travelled millions of light years.I also wasn't thinking of the gravitational pull of the Earth bending the light. What I was suggesting that possibly there is a star that would be seen from Canada if light travelled in a straight line but would now be seen from Australia because of the light having been affected by gravitational forces long before it ever came close to the Earth.This message has been edited by GDR, 06-18-2005 07:40 AM

Looking at your Canada vs Australia example. Consider the base used to observe the star and just draw out the triangles involved roughly. Use Canada as one observation point. Now imagine the earth as it revolves around the sun and look again when the earth is 180^o away in its orbit. Compare the distance between Canada and Australia to the distance between Canada at point 1 in the orbit and Canada at point 2 in the orbit.Would you get the results you describe?

---

Aslan is not a Tame Lion

Nearby stars are measured with parallax, which involves two measurements of the star's exact position in the sky. The readings are taken on opposite sides of the earth's orbit, six months apart. From this triangulation (or surveying) method, the star distance is determined.The parallax technique works for stars out to a distance of several thousand light-years. (ly). The satellite Hipparchus, launched in 1997, has been used for many of these measurements. Many thousands of stars fall within parallax range, including Arcturus (37 ly), Sirius (8.6 ly), and Spica (220 ly).For the longer star distances, indirect methods are used. The Cepheid variable technique is useful out to millions of light-years. Cepheids are a category of stars whose actual brightness is well known. If a Cepheid appears dim its distance thus can be estimated. The method is similar to judging the distance to an oncoming car by observing its headlight in the distance. Cepheids are very bright stars, so they can be identified in faraway galaxies. The Cepheid method gives the distance to the Magellanic Clouds (180,000 ly) and also to the Andromeda Galaxy (2.9 million ly).Measuring stick. Illustration copyrighted. The final yardstick for stellar distances that will be mentioned here is the redshift of starlight (see Is the universe expanding?). The shift results from the overall expansion of the universe. A star's light is altered as the star is carried outward by the expansion of space. The effect is somewhat similar to water waves stretched out behind a speedboat. The term redshift arises because red is the color of visible light with the largest wavelength. Entire galaxies appear to be receding from the earth, as measured by their redshift. The faster they are traveling, the farther away they seem to be. There is considerable guesswork in determining actual distance from the redshift, and there are also other possible explanations besides motion. If it is correct, this method gives distances for galaxies out to 15 billion light-years or more. Astronomers also use several other distance techniques for stars. Although the parallax distances are accurate to within about 10 percent, all other methods are more uncertain. Actual star distances could be somewhat smaller or larger than current estimates.

Fascinating. It does show that it does happen. I couldn't find the page you refer to where you get the images from different times. Wouldn't that indicate that the light was considerably bent and travelling by a completely different route?

There was a great picture of a gravitaional lens released a few years ago. You can see it here.

---

Aslan is not a Tame Lion

I'm not the sharpest knife in the drawer and so I'm having trouble figuring that out. The light that arrives here on Earth always appears to us to have travelled in a straight line but how do we really know just how straight that line is? Wouldn't we always get the same result no matter where we are in the orbit? The light just gets here and we can only surmise the route.

Thanks jarThat is certainly impressive and it does answer part of the question. It shows the multiple images but what if all the light from each of the images were all being bent by roughly the same amount as they pass by other galaxies on their way here?I'm out for a few hours.

Let me try again. We have a long baseline to work from. The diameter of our orbit is about 180,000,000 miles. So if we make an observation from one point in our orbit and again 6 months later it is the same as if we took two readings standing that far apart. That can give us triangulation out quite a ways.

---

Aslan is not a Tame Lion

Ok jar. I think I follow you now. If we are going to use triangulation we need straight lines. We know that that we can get a straight line between either 2 point s on the Earth or even from the same point on Earth at the extremes of our orbit but how do we know how straight the other two lines are. Don't we know that they are curved to some degree, but how can we really know how curved they are?Isn't your triangulation assuming that the light is basically travelling in a straight line?

What we are measuring is the height of a triangle formed by the star and our two bases. It doesn't matter what path the light took to get here.Remember, to see the star as a single object, light does have to get here. In the case of gravitaional lensing we see multiple objects. The bending is caused by something between us and the source. As another example, here is the Einstein Cross.Light from the source radiates in all directions but we would not see the effect of mass on any light that does not get to us. Lensing only is seen if a beam of light from the object is bent on its path to us. In that case if we follow the path backwards past the massive object we see multiple images of a single object.AbE:You really need to be asking Sylas or someone who knows what they are talking about instead of me. I'm nothing more than an old man who saw a connevction between gravitaional wells and frog giggin.This message has been edited by jar, 06-18-2005 03:49 PM

---

Aslan is not a Tame Lion

I guess it goes back to my original example. I can stand at the south pole with my direction finder and have it sense the signal coming from one direction and the turn 180 degrees and find that same signal coming from a totally different direction. If I didn't know ahead of time I would likely think that I was receiving 2 different signals.Could it be like that with stars that emit light in all directions? Is it possible that there are far fewer stars than we see because we keep seeing the same stars over and over again in different locations?The thing this hinges on I guess, is how much does light bend. It seems to me that if light is affected by the gravitational pull of visible matter, dark matter and dark energy then maybe light bends enough to give us the results that I mentioned in the last paragraph.Incidentally, I'm not trying to make any point about the age of the Earth or anything here, it was just something that occurred to me from reading Hawking and Greene.