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Thread: Do we know how old light is?

  1. #1 Do we know how old light is? 
    Forum Freshman Gen1GT's Avatar
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    How do we know how old light is? For instance, how do we know that light from a 10 light years away is X years old when it reaches us?


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    Light travels at a consistent speed: 299,792,458 m/s. Once the distance is known, simple math will calculate the time it takes to bridge the gap.


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    So then how do we know Star X is 10 light years away or 1 million light years away?
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    Distance to stars is measured by the Cosmic Distance Ladder. For nearby stars, Stellar Parallax is used to determine distance. A star is observed, at opposite ends of Earth's orbit (6 months apart). The degree of observed difference in the star's position (parallax), using other stars and galaxies as a background, gauges it's distance. A star twice as far away will have half the parallax.

    For distances further than a few hundred parsecs, luminosity is measured. A "standard candle" or object of known luminosity is compared with the subject star. If the subject star is twice the distance from the standard candle, it should be 1/4 as bright.

    Even further out, supernovae are used as standard candles. They brighten and dim at a measurable rate, and thus can be used as comparison for nearby objects.
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    This is indeed not so easy. We have a number of tools to determine the distance of stars.

    1) Parallax
    Any star subtends an angle with the diameter of the Earth orbit around the sun. This angle can be measured. The distance can be derived from simple trigonometry and the diameter of the Earth's orbit.


    2) Spectral Classification and Absolute Magnitude
    Stars can be characterised by different means. All normal stars like the sun populate a narrow parameter space of surface temperatures and luminosity (total energy output) called the Hertzsprung-Russell diagram. The temperature can be well determined, from which the the luminosity can be derived that can be easily transformed into the absolute magnitude of the star. As soon we know the absolute magnitude and we measure the apparent magnitude (its brightness at our position), we can immediately determine its distance.

    3) Special types of stars: delta Cepheids, RR Lyrae stars
    These special types of stars can be used as tools to determine the distance of stellar clusters or close galaxies that contain them. Their properties (pulsating period) are a direct measure of their absolute magnitude. In this way, they can be used as standard candles. Measuring their apparent magnitude again results in their distance.

    4) Tully-Fisher relation
    This is an empirically derived law, for which a clear physical explanation has not yet been found. Still, it can be used to determine the distance of distant galaxies.
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    This was a response on another messageboard from a creationist/geo-centric type fella. Thoughts?

    Hmmm. Though still dependent "on the motion of the earth in its orbit around the sun", we notice that the "trigonometric parallax" is only good for "nearby stars" which, this encyclopedic report notes, is only "useful for stars within 100 parsecs". One parsec is defined as "the distance...equivalent to 206,265 times the distance from the earth to the sun, or about 3.3 light-years." That would put "100 parsecs" some 330 light-years distant from an orbiting earth, virtually at our elbows in current cosmologyese. (Work it out arithmetically and see what you get in either miles or kilometers. Then try it with the Kabbalic 15 billion light-year distance and you will begin to get a real sense of the kind of funny-farm pretext of "scientific" exactitude that is involved here. One is reminded of Darwinís annoyance with Sir Wm. Thompsonís calculation that evolution took 98 million years. Southall noted that: "Mr. Darwin represents that it must have taken three hundred and six millions six hundred and sixty two thousand four hundred years.") 3

    Now notice how the orbiting earth premise continues to be the foundation of all other techniques for measuring distances to stars that are claimed to be millions and billions of light-years away even though it is admitted that the trigonometric parallax method is limited to "nearby" stars:

    "The first stellar parallax (the trigonometric one) was measured in 1838 by Friedrich Bessel for the star 61 Cygni. Its parallax of 0.3 places it at a distance of 3.3 parsecs or about 11 light-years...." 4

    I had to read this two or three times before it sank in...and the same message is standard in all reports on the subject. What message? This blue ribbon pseudo-scientific hornswoggle that ranks up there with the best the world has to offer, thatís what! Herr Besselís "finding" (in 1838!) for the distance to 61 Cygni based on his trigonometric parallax hypothesis of parsecs has become the standard cosmological reference for distances to all the "nearby" stars! This is in spite of the fact that the method was calculated upon and was therefore deduced from the conviction in virtually all cosmological circles by that time that the earth orbited the sun and was on opposite sides of the sun every six months.. as has now become a "fact", of course. Since any accurate application of parallax to the calculation of distances is wholly dependent upon the position of the observer relevant to that which is being observed, Besselís deduction was based on faith in the heliocentric assumption that the position of the earthbound observer changed every six months by some 186,000,000 miles (the diameter of the alleged earth orbit).

    By contrast, Geocentric parallax is a technique which uses the diameter of the earth as a baseline. Because of this small baseline (8000 miles) this parallax is useful only for close objects such as the Moon and perhaps a few planets. Thus, if the earth and its observers are not orbiting the sun, the standard for distance measurements that is arbitrarily set at 206,265 x 93,000,000 miles based on an 0.3 parallax taken from an observation point 186,000,000 miles away from the previous observation point...those measurements would produce a distance that is 23,250 times larger than a distance calculated from a stationary earth! (8000 x 23250=186,000,000)

    The importance of what has happened here relevant to true measurement of distances in space can not be overstated! The only reason any parallax at all showed up for Besselís star 61 Cygni (0.3) was because the baseline for the observer was assumed to be One-Hundred and Eighty Six Million Miles further away than the baseline of the observer six months earlier...thus giving a baseline on a triangle of 186,000,000 miles instead of 8000 miles!

    The proof that calculations of "nearby" star distances are dependent upon the heliocentric assumption of a rotating, orbiting Earth couldnít be more clear. What we have here is mathematical tautology, pure and simple, viz.,: The earth orbits the sun and provides a parallax figure which pushes even the close stars out 23,250 times further than they would be if a non-moving earth parallax were used. Then the light-year distances derived from this slight of hand are used to "prove" that the earth cannot be stationary because the stars are too far away to get around nightly!

    Some epithets come to mind...but I forbear. Using stationary earth parallax, how far away is Besselís star 61 Cygni really? Based on an orbiting earth, he put it at 3.3 parsecs (about 11 light) years distant from earth. Thatís about 64 trillion, 660 billion miles. If the earth is not moving, his figure is 23,250 times too big and the parallax distance would be c. 2 billion, 800 million miles (the alleged Earth to Neptune distance). Thatís more like it, but stationary earth parallax calculations are too small to triangulate even at that distance.
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    Quote Originally Posted by Gen1GT
    This was a response on another messageboard from a creationist/geo-centric type fella. Thoughts?

    Hmmm. Though still dependent...
    I usually would simply ignore such statements, mostly because it is mostly wrong, and even worse, often even based on false assumptions and a lack of scientific method. But since you asked for some replies, well here is one.

    The main thing the unknown poster (UP) - deliberately - misunderstands is the difference between a method and a measurement. It may well be that Bessel used the parallax method to determine the distance to a star, but it is not true that this measurement is the reference for all further distance estimates. However, the method is still widely used and refined. There has been a satellite mission labelled Hipparcos that used this method to determine the distance to millions of stars very accurately. The main difficulty in employing this method is measuring the parallax angle. With tools Bessel had, the precision was obviously not as good as has been possible with the modern space based instruments.

    But it is true that the distances that can be reached by the parallax method is limited. Yet, it serves as the key calibration to the other methods that are commonly used. If you are able e.g. to determine the distance to a number of Cepheid variables via the parallax method, you can use this to calibrate the Cepheids themselves as a distance measure for more extended objects like local galaxies. This is commonly referred to as the distance ladder.

    The second main point in the UPs post is totally invalid, because he basically concludes that the inaccuracy of a measurement means that the phenomenon on what is based does not exist. This is obviously wrong, because the earth parallax during its orbit around the sun can be observed, even though its accurate value might be difficult to measure. Even with very crude technologies, it can be verified at least qualitatively. If his belief were true, we should not see any parallax (based on the Earth orbit around the sun) phenomenon at all.

    Also his denouncing remark on Darwin clearly shows that his thinking is based on prejudice and a bad sense of science. Drawing analogies between apples and oranges is a common habit among those crackpots - even more so because the apparent facts on which those analogies are based are often wrong, or skewed at best.
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  9. #8  
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    Some points>

    By contrast, Geocentric parallax is a technique which uses the diameter of the earth as a baseline. Because of this small baseline (8000 miles) this parallax is useful only for close objects such as the Moon and perhaps a few planets. Thus, if the earth and its observers are not orbiting the sun, the standard for distance measurements that is arbitrarily set at 206,265 x 93,000,000 miles based on an 0.3 parallax taken from an observation point 186,000,000 miles away from the previous observation point...those measurements would produce a distance that is 23,250 times larger than a distance calculated from a stationary earth! (8000 x 23250=186,000,000)
    So he is trying argue that a earth-width based parallax could be mistaken for an orbit based one. This simply is not true. For a earth-width based parallax you have to take your measurements when the star is either rising or setting. This so that you are sighting from across the width of the Earth.

    With orbit-based parallax you only have to worry about the sight line with respect to the orbit; it doesn't matter where the star is in the night sky relative to the horizon.

    Objects near the horizon are more heavily affected by atmospheric distortion that messes up the measurement.

    With an earth based parallax this can not be avoided. However, with an orbit based parallax it can. Since the star's position with respect to the horizon doesn't matter, the measurements are made when the star is at its highest point in the sky. This means that when the two measurements are made, the width of the Earth is not involved. Thus you either get a parallax due to the Earth's movement around the Sun, or you get none at all. We measure a parallax which can only be due to a shift in the Earth's position and in no way could be confused with a parallax due to the width of the Earth.

    The proof that calculations of "nearby" star distances are dependent upon the heliocentric assumption of a rotating, orbiting Earth couldnít be more clear. What we have here is mathematical tautology, pure and simple, viz.,: The earth orbits the sun and provides a parallax figure which pushes even the close stars out 23,250 times further than they would be if a non-moving earth parallax were used. Then the light-year distances derived from this slight of hand are used to "prove" that the earth cannot be stationary because the stars are too far away to get around nightly!
    This ignores other independent evidence that the Earth moves around the Sun besides that of parallax. For instance, Doppler Shift. The Earth's velocity around the Sun is large enough to produce quote a measurable effect on the light we get from a star. Since at different points of it orbit the Earth can be moving away from or towards the same star, its spectrum will shift over the course of a year. Since all the stars in a given part of the sky show the same shift, this can only be due to a relative circular motion between the stars the the Earth.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Thanks for all the info guys. Your info is actually ammunition against individuals at the Hot Stock Market Messageboard's Politics and Relgion section. In an Atheism vs Religion thread, a guy is arguing now that the geocentric universe model is true. His argument is based Einstein's Theory of Relativity.

    Why do I even bother???? Haha...
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    Another independent evidence for the Earth's revolution around the Sun is the aberration of light. Similar to the parallax, the apparent position of a star in the sky is slightly shifted depending on the velocity (value and direction) of the Earth relative to that star. Different to the parallax, there is no similar phenomenon that could be attributed to the rotation of the Earth. No telescope on the Earth would be able to accurately locate any astronomical object, if it would not correct for this effect.
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    Quote Originally Posted by Dishmaster
    Another independent evidence for the Earth's revolution around the Sun is the aberration of light. Similar to the parallax, the apparent position of a star in the sky is slightly shifted depending on the velocity (value and direction) of the Earth relative to that star. Different to the parallax, there is no similar phenomenon that could be attributed to the rotation of the Earth. No telescope on the Earth would be able to accurately locate any astronomical object, if it would not correct for this effect.
    Dishmaster, for reference purposes, what is your educational background. I like to point out to armchair, pseudo-scientists that their leisurely review of science to suit their argument doesn't hold a candle against real scientists.
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    Quote Originally Posted by Gen1GT
    Dishmaster, for reference purposes, what is your educational background. I like to point out to armchair, pseudo-scientists that their leisurely review of science to suit their argument doesn't hold a candle against real scientists.
    Gen1GT is sort of referring to me with the above and has graciously invited me to come here and defend my position among real scientists.

    I should point out that he's sorta melded two people into one, however. I'm no scientist, I fully admit that. Nor do I even consider myself an armchair, pseudo-scientist.

    Nor am I the person whom Gen1GT quoted from above. I didn't write that, nor do I even claim to understand the argument.

    So, what is my position? Basically, this:

    We once believed that the Earth was the center of the Universe, this was back in the days of Ptolemy. We then learned from Copernicus that the Sun was the center of our solar system (Perhaps not the universe). Finally, Einstein came along with his theory of relativity and said it's both right and it's both wrong, depending on your frame of reference.

    Now, i'm not going to pretend like I can argue the science here. Nor will I accept the label of a Geocentrist that Gen1GT might want to put on me. I could care less whether the Sun moves around the Earth or the Earth moves around the Sun. What matters to me is the truth, not dogma.

    So, was Einstein wrong? If so, how?
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    Quote Originally Posted by Gen1GT
    Quote Originally Posted by Dishmaster
    Another independent evidence for the Earth's revolution around the Sun is the aberration of light. Similar to the parallax, the apparent position of a star in the sky is slightly shifted depending on the velocity (value and direction) of the Earth relative to that star. Different to the parallax, there is no similar phenomenon that could be attributed to the rotation of the Earth. No telescope on the Earth would be able to accurately locate any astronomical object, if it would not correct for this effect.
    Dishmaster, for reference purposes, what is your educational background. I like to point out to armchair, pseudo-scientists that their leisurely review of science to suit their argument doesn't hold a candle against real scientists.
    Just investigate my profile, and you will know. I am not here to brag - only to help.
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    Quote Originally Posted by Baggi
    We once believed that the Earth was the center of the Universe, this was back in the days of Ptolemy. We then learned from Copernicus that the Sun was the center of our solar system (Perhaps not the universe). Finally, Einstein came along with his theory of relativity and said it's both right and it's both wrong, depending on your frame of reference.

    Now, i'm not going to pretend like I can argue the science here. Nor will I accept the label of a Geocentrist that Gen1GT might want to put on me. I could care less whether the Sun moves around the Earth or the Earth moves around the Sun. What matters to me is the truth, not dogma.

    So, was Einstein wrong? If so, how?
    You don't need Einstein for this. Except for some cases like Mercury, where Newtonian gravitational theory fails, the entire solar system can be described very well without using General Relativity. But that does not mean that Einstein was wrong, only that Newton's theory is a specialisation for certain cases.

    It is just mathematics that allows you to switch between a geocentric and a heliocentric view of the solar system in order to describe the movements of the planets, comets, asteroids, TNOs etc. So, in some cases it is more convenient to use the geocentric representation; in other cases, the heliocentric view is more useful. But when it comes to what is the "correct way to look at it", there is no way doubting that Kepler was right (Copernicus was still wrong with the idea that planetary orbits were perfect circles. He wasn't even the first for that matter.). All the phenomena described in this thread prove it.

    So, in order to answer your questions: No, Einstein was not wrong, at least when it comes to Special and General Relativity. (He wasn't the pope, you know. He also got a few things wrong - like e.g. the denial of Quantum Mechanics.) But it does not matter for the point you are raising here.
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    Or put another way, while it is possible with GR to formulate a frame in which the Earth is motionless, it is equally possible to do so for any body in space or for that matter, any randomly picked point in space with any random velocity with respect to the Earth.
    Every point has an equal claim to being the motionless center of the Universe, meaning that there is no unique center, which is the same as saying there is no center.
    "Men are apt to mistake the strength of their feelings for the strength of their argument.
    The heated mind resents the chill touch & relentless scrutiny of logic"-W.E. Gladstone


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    Or put another way, while it is possible with GR to formulate a frame in which the Earth is motionless, it is equally possible to do so for any body in space or for that matter, any randomly picked point in space with any random velocity with respect to the Earth.
    Every point has an equal claim to being the motionless center of the Universe, meaning that there is no unique center, which is the same as saying there is no center.
    Exactly my point. So why is Dishmaster stating that there is a "correct way to look at it"?

    Einstein once said:

    The two sentences, 'the sun is at rest and the earth moves,' or 'the sun moves and the earth is at rest,' would simply mean two different conventions concerning two different CS.
    My position is equal to this one.

    So who is right, Dishmaster or Einstein and I?

    Because clearly the statement above cannot logically be reconciled with Dishmasters statement that there is a "correct way to look at it".

    It appears to me he wants to have it both ways though. Yes, Einstein is right but only mathematically speaking. As if we could figure these things out without using math.
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    I do not claim to possess the final truth. So, if I am wrong, please let me know where. I was just saying that you don't need Einsteins GR - I am not that experienced with it that I could say I know every consequence of its application. But it is a fact that you change perspective also by applying classical Newtonian theory (Galilean transformation). However, I am not sure whether those phenomena like Doppler shift, annual parallax and aberration could be explained from a geocentric perspective. They clearly show that the Earth moves at least relative to the objects that are observed together with those phenomena. If somone shows me a way to do this without, I will accept it.

    I assume that the quote was made in order to explain the equality of all reference frames, when only those two bodies are involved. I also want to mention that the Earth is constantly accelerating (changing state of velocity) on its orbit around the sun. This violates the prerequisites for being an inertial reference frame.
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    Could one equally state:

    "I also want to mention that the Sun is constantly accelerating (changing state of velocity) on its orbit around the Earth."??

    Also, since you brought up Wikipedia I found this discussion/question under Modern Geocentrism:

    So, within the framework of general relativity, is it or is it not true, that all frames of reference, including those which do wierd things relative to each other like accelerating and rotating, are equivalent in the sense of predicting the same kinematic evolution? I realize there is a preferred set of "inertial" frames, the ones in which the mass of the universe on average is not rotating. I realize the choice of the geocentric frame causes headaches (does not comply with Occam's razor). I realize that keeping the Earth stationary in the presence of tides, weather, and earthquakes requires some new physics, extreme luck, or God. But what is this evidence that is supposed to prove that geocentrism is not just ugly, but wrong? If you please, start with an operational definition of absolute center, absolute motion, and/or absolute rotation. If you answer by making reference to the mean rest frame of matter, I will want to know whether this choice is in any way required, because if it is not, you are begging the question.
    Now the guy asking the question, Art Carlson, claims to be a physicist, and surely would be better equipped to evaluate the answers to those questions he asks.

    So unless someone else here can join the discussion, i'll probably be forced to take your answers directly to Art Carlson and wait for a response.

    But I do find his question to be a good scientific starting point and as this is a science website, probably a good place for us to start.

    Specifically, this part of the above:

    But what is this evidence that is supposed to prove that geocentrism is not just ugly, but wrong? If you please, start with an operational definition of absolute center, absolute motion, and/or absolute rotation.
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  20. #19  
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    Quote Originally Posted by Baggi
    Could one equally state:

    "I also want to mention that the Sun is constantly accelerating (changing state of velocity) on its orbit around the Earth."??
    To be exact, both revolve around their mutual barycenter. However, the orbit of the Earth is larger than the orbit of the Sun.
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    So I think we're in agreement.

    For most calculations, we find it much easier to behave as though the Earth moves and we therefore use the Heliocentric model.

    However, if someone wanted to complicate their life and use a Geocentric model (Let's say someone traveling to mars) then the science works either way.

    Is that accurate?
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    Wow Baggi, I'm impressed that you actually came here to share your thoughts. But still, arguing a geocentric position only puts you in a position using either an abstract model or an overly simplified observers-point-of-view.

    If you traced planetary movements in our solar system with Earth at its centre, it would look like a spirograph, and we plainly know this isn't how it works.
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    Quote Originally Posted by Baggi
    So I think we're in agreement.

    For most calculations, we find it much easier to behave as though the Earth moves and we therefore use the Heliocentric model.

    However, if someone wanted to complicate their life and use a Geocentric model (Let's say someone traveling to mars) then the science works either way.

    Is that accurate?
    It depends on what the purpose of this change of perspective is. I don't think that you can make the phenomena described in this thread proving a revolving earth disappear, if you take a geocentric position. We are doing this every day, when we observe the sky, but annual parallax, aberration and Doppler shift do still exist.
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