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Thread: Light

  1. #61
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    Quote Originally Posted by Grashtel
    Einstein's Relativity has nothing to do with it,
    Uhh, That’s why I said we are dealing with Ecludean space-time rather than Riemann space-time.

    Quote Originally Posted by Grashtel
    Camera shutters open from the bottom to the top and close in the same way so objects moving relative to the field of view of the camera appear distorted.
    It depended on the cameras. Many old-time focal plane shutters (and 35 mm fps) went from right to left. That is, unless the camera is turned sideways, then they can go from bottom to top or top to bottom, depending on how the camera is rotated. This photo was taken with the camera oriented so the slit went from the bottom to the top.

    Quote Originally Posted by Grashtel
    In that photo the car is driving to the right and the camera is panning to follow it, though not quite fast enough, so as the open part of the shutter moves up the car is moving across the field of view to the right and the background is moving to the right (from the point of view of the camera) producing the apparent slants in that picture.
    The background is moving to the left, relative to the camera rotation to the right. Remember, the camera is panning to the right, so this produces the visual effect of the background appearing to move to the left. And you are correct about the car moving to the right faster than the camera is panning to the right.

    What is your opinion about the “acceleration” issue?

  2. #62
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    Hello:
    We should not interprete theories right, we should understand them. This is how I understand it. The more I read here the more confused I become. I've actully read quite a few books on relativity, and I'm beginning to wonder if I should trust them. I set out asking for some help in seeing a concept in my head, which seems to me has turned into an argument in interpretations. Marry Christmas all.

  3. #63
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    Hello:
    OK, I have started rereading a book I read along time ago, About Time-Paul Davies. The original problem was with the Newtonian view and the electromagnetic theory, right? In Newtonian physics, the speed of light would change in relation to your speed, right? But that would change the properties of that light beam, this is what Einstein had a problem with, am I correct? "Speed is distance traveled per unit time, so the speed of light can only be constant in all reference frames if distances and intervals of time are somehow different for different observers, depending on their state of motion". This I understand.

  4. #64
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    Quote Originally Posted by Russell
    Hello:
    OK, I have started rereading a book I read along time ago, About Time-Paul Davies. The original problem was with the Newtonian view and the electromagnetic theory, right? In Newtonian physics, the speed of light would change in relation to your speed, right? But that would change the properties of that light beam, this is what Einstein had a problem with, am I correct? "Speed is distance traveled per unit time, so the speed of light can only be constant in all reference frames if distances and intervals of time are somehow different for different observers, depending on their state of motion". This I understand.
    Russell,

    Unfortunately the “mass media” books like that won’t tell you exactly what’s true and what’s not. They will give you a lot of opinions. And here is mine:

    In the original Newton theory days, and in the 19th Century, they thought one big giant “ether” controlled the speed of light in space. They thought the universe was pretty much “fixed” and not moving, and so they thought that ether was “fixed” with the universe.

    In a special experiment conducted in 1886, Michelson and Morley found no evidence of a “universe stationary” ether, so the old ether theory was thrown out by Einstein in 1905, but he sort of brought it back, somewhat, in 1911, when he developed his gravitational redshift theory. His “ether” was the local gravitational field of an astronomical body, so it was a local ether.

    But, there were complications with that theory, since it sort of depended on the rates of atomic clocks to measure the local “speed of light”.

    A strange effect of the gravitation field is that a strong one slows down atomic clocks, while a weak one speeds them up. Also, a strong field slows down light while a weak one speeds it up. So, if you measure the “speed of light” at you, while the photons are arriving at you, and you measure their speed with an atomic clock located at you, resting on the surface of the planet where you are, then that particular clock is supposed to measure those particular photons traveling at “c”, right there locally.

    But if you can measure or calculate the speed of the photons while they are passing near the sun, you will find that they slow down slightly. However, if you are “resting on the sun” with an atomic clock that is “resting on the sun”, then that atomic clock will measure those photons passing by the sun to be traveling at “c” there locally at the sun, but “faster than c” when they travel away from the such, such as at the earth.

    It’s hard to keep all of this stuff straight and it’s difficult to explain, and, the worst part is that some people disagree with this 1911 Einstein theory. Another problem is that light-speed slows down and speed up such a very small amount, it’s difficult to measure the speed changes.

    Regarding the "state of motion" of the observer, that complicates the issue even more.

  5. #65
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    Are you sure Russell didn't say essentially that, Sam?

    I was about to say you're crazy for claiming light slows as it passes the sun, but after having it explained by Steve Carlip, I'd have to agree that it depends on where you're sitting (I think).
    Everyone is entitled to his own opinion, but not his own facts.

  6. #66
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    Quote Originally Posted by Cougar
    Are you sure Russell didn't say essentially that, Sam?

    I was about to say you're crazy for claiming light slows as it passes the sun, but after having it explained by Steve Carlip, I'd have to agree that it depends on where you're sitting (I think).
    Hi. Glad to know I’m not crazy.

    This is from your link:

    "Einstein went on to discover a more general theory of relativity which explained gravity in terms of curved space-time and he talked about the speed of light changing in this new theory. In the 1920 book "Relativity: the special and general theory" he wrote: . . . according to the general theory of relativity, the law of the constancy of the velocity of light in vacuo, which constitutes one of the two fundamental assumptions in the special theory of relativity . . . cannot claim any unlimited validity. A curvature of rays of light can only take place when the velocity of propagation of light varies with position."

    That’s part of what I’ve been trying to explain here. This stuff is in his 1911 paper. It is difficult to find in the paper, however, since he babbles a lot about different clocks, different kinds of clocks, atomic clocks, and “U” clocks. So, the 1911 paper says two significant things, which few people realize or understand today: 1) The internal harmonic oscillation rates of atoms resting in a gravitational field is influenced by the gravitational potential at the place where they are resting (ie atomic clocks slow down in strong gravitational fields), and 2) The speed of light waves/photons slow down when they pass through strong gravitational fields.

    The two effects = this: An atomic clock resting in a gravitational field does not notice the slowdown of light waves/photons if the speed of the photons are measured at that clock, when they reach or pass by that clock, since the speed of the photons and the rate of the clock are slowed down the same amount by the gravitational field. Then it follows that the speed of waves/photons traveling in a weak gravitational field, when measured by an atomic clock resting in a strong gravitational field, will appear to that clock to be faster than “c”, while the speed of waves/photons traveling in a strong gravitational field, when measured by an atomic clock resting in a weak gravitational field, will appear to that clock to be slower than “c”.

    If we use a distant rapidly spinning pulsar as our “clock”, then it’s rate should not be affected by our observation of it from within a gravitational field, and by use of that kind of clock, then we can determine that the speed of light does speed up and slow down as it travels through various parts of space, when it travels through areas of higher or lower gravitational potential.

    The reason light “bends” as it passes massive objects like the sun is because the light “rays” (groups of photons traveling side by side) act like plane waves of light when they enter glass or water. They not only slow down but they change direction. Thus, light rays are “refracted” by the gravitational field of massive bodies, when they pass near to those massive bodies. The light is not “changing media”, like when it goes from air to glass, but it is gradually going from a less dense medium through a more dense medium, and that single medium is the gravitational field of the astronomical body through which the light passes, and the light's local speed regulator is that medium.

    I got all of this right out of his 1911 paper, but it took me about 12 years to understand what the heck he was trying to say.

    Uhh, you might want to print out this post for future reference. You won’t find it explained any better than this.

  7. #67
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    Quote Originally Posted by Sam5
    A strange effect of the gravitation field is that a strong one slows down atomic clocks, while a weak one speeds them up.
    I think perhaps you mean this relatively, as it were. A weak gravitational field slows clocks, but less than a strong one, so a clock in a field will speed up as the field weakens. But it's still slowed relative to no field at all.

  8. #68
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    Quote Originally Posted by swansont
    Quote Originally Posted by Sam5
    A strange effect of the gravitation field is that a strong one slows down atomic clocks, while a weak one speeds them up.
    I think perhaps you mean this relatively, as it were. A weak gravitational field slows clocks, but less than a strong one, so a clock in a field will speed up as the field weakens. But it's still slowed relative to no field at all.
    That’s basically what I said. The weaker the field the atomic clock “feels”, the faster an atomic clock will “tick”.

    But I and Einstein are talking only about “atomic clocks”, and NOT about other kinds of clocks. (See my posts on the SR thread.) For example, a pendulum clock will “speed up” in a strong gravitational field, not “slow down”. So we aren’t talking about “all of time”, we are talking specifically about the rates of atomic clocks.

  9. #69
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    Quote Originally Posted by Sam5
    But I and Einstein are talking only about “atomic clocks”, and NOT about other kinds of clocks. (See my posts on the SR thread.) For example, a pendulum clock will “speed up” in a strong gravitational field, not “slow down”. So we aren’t talking about “all of time”, we are talking specifically about the rates of atomic clocks.
    No, it applies to all clocks. Pendulum clocks have a specific dependence on gravity that dominates - the relativistic effect is about a part in 10^16 near the surface of the earth - i.e. it's small.

    Einstein couldn't have been talking about atomic clocks, as the hadn't been invented yet.

  10. #70
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    Quote Originally Posted by swansont
    No, it applies to all clocks.
    Sorry, wrong.

    It does NOT apply to “all clocks”.

    Quote Originally Posted by swansont
    Einstein couldn't have been talking about atomic clocks, as the hadn't been invented yet.
    Sorry, wrong again.

    He WAS talking about “atomic clocks” in the 1911 theory. This is why he said:

    “Let fo be the vibration-number of an elementary light-generator...”

    His “elementary light-generator” was an individual “atom” of any particular “element” that radiated light.

    In order that you can understand EXACTLY what he was talking about, you need to read Maxwell’s two 1873 books: “A Treatise on Electricity and Magnetism”, in which Maxwell said:

    ”In astronomy a year is sometimes used as a unit of time. A more universal unit of time might be found by taking the periodic time of vibration of the particular kind of light whose wave length is the unit of length.”

    See? Maxwell’s 1873 “particular kind of light” is emitted by his the atom of his particular “periodic time of vibration”, and this is Einstein’s 1911 “elementary light-generator”.

    Of course, since Einstein was right in 1911, then when the more complex “manufactured atomic clocks” were invented and manufactured, they followed Einstein’s rule of “slowing down” their “tick rate” while resting in a strong gravitational field and “speeding up” their “tick rate” while resting in a weaker gravitational field. But this “gravitational redshift” effect DOES NOT influence ALL KINDS of clocks in the same way, as Einstein noted in his 1911 theory, when he said:

    ”But from what has just been said we must use clocks of unlike construction, for measuring time at place with differing gravitational potential.”

    I.E., we must NOT use “atomic clocks”, because if we use atomic clocks, their rates will be affected by the differing gravitational potential.

    As a matter of fact, I called the atomic clock physicists – the world’s top experts – in Boulder, Colorado, to confirm this, and they told me that our world standard of “atomic clock time” is “averaged” among several atomic clocks, to "average out" all differences in “tick rate” caused by the various clocks resting at different gravitational potentials at different elevations. And their “average atomic time tick rate” is constantly compared to the revolution and rotation rates of the earth. This is why they have to add “leap seconds” to the atomic clocks every now and then.

    I think you need to read a little more about 19th Century electrodynamics and kinematics, so you can understand what Einstein was actually saying in 1911.

  11. #71
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    Quote Originally Posted by Sam5
    Quote Originally Posted by swansont
    No, it applies to all clocks.
    Sorry, wrong.

    It does NOT apply to “all clocks”.
    No, really, it does. I'm not kidding. It'll work on quartz oscillators and spring-driven flywheels.

    Quote Originally Posted by Sam5
    As a matter of fact, I called the atomic clock physicists – the world’s top experts – in Boulder, Colorado, to confirm this, and they told me that our world standard of “atomic clock time” is “averaged” among several atomic clocks, to "average out" all differences in “tick rate” caused by the various clocks resting at different gravitational potentials at different elevations.
    With whom did you speak at NIST? Because he/she is wrong, or your interpretation of it is wrong. And why didn't you call an expert at the US Naval Observatory, where they have even more atomic clocks than at NIST? (S)He would have told (and in fact is now telling) you that the elevations are factored in. The averaging happens because no two clocks are identical, and never independently keep synchronous time (even clocks that are nominally at the same frequency). Both the stability and stationarity of a clock ensemble improves with more clocks. You need to have multiple clocks to do pair-wise comparisons and discern any biases in individual clocks. And you use different kinds of clocks (e.g. H masers, cesium beam, fountains, ion trap clocks) so you aren't prone to a common systematic error.

  12. #72
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    Quote Originally Posted by swansont
    Quote Originally Posted by Sam5
    Quote Originally Posted by swansont
    No, it applies to all clocks.
    Sorry, wrong.

    It does NOT apply to “all clocks”.
    No, really, it does. I'm not kidding. It'll work on quartz oscillators and spring-driven flywheels.
    No, not just due to “relative motion” alone, since no physical “force” is placed on the oscillating mechanism of the clocks due only to “relative motion”. Your atomic clock rate changes due to elevation are GR, not SR changes.

    Anyway, a greater gravitational potential slows down atomic clocks, while it speeds up pendulum clocks. Synchronize one of your atomic and a pendulum clock in DC, make them run as synchronously as possible, then take both of them to Denver and set them up. The atomic clock will run fast, and the pendulum clock will run slow. If you happen to ever find a pendulum clock that “runs fast” in Denver and “slow” in DC, please let me know.

    Quote Originally Posted by Sam5
    As a matter of fact, I called the atomic clock physicists – the world’s top experts – in Boulder, Colorado, to confirm this, and they told me that our world standard of “atomic clock time” is “averaged” among several atomic clocks, to "average out" all differences in “tick rate” caused by the various clocks resting at different gravitational potentials at different elevations.
    Quote Originally Posted by swansont
    With whom did you speak at NIST? Because he/she is wrong, or your interpretation of it is wrong. And why didn't you call an expert at the US Naval Observatory, where they have even more atomic clocks than at NIST? (S)He would have told (and in fact is now telling) you that the elevations are factored in.
    Hey! You work at the US Naval observatory??

    Do you know if they “average” the rate to “sea-level” elevation or not?

    Quote Originally Posted by swansont
    The averaging happens because no two clocks are identical, and never independently keep synchronous time (even clocks that are nominally at the same frequency). Both the stability and stationarity of a clock ensemble improves with more clocks. You need to have multiple clocks to do pair-wise comparisons and discern any biases in individual clocks. And you use different kinds of clocks (e.g. H masers, cesium beam, fountains, ion trap clocks) so you aren't prone to a common systematic error.
    [see added note at bottom of post]

    Perhaps I did not explain it thoroughly, since they told me there are other reasons for the averaging too. I’ve said in other posts that they told me that the “averaging” is also done to eliminate some of the glitches in individual clocks that often occur, “biases” and “drifts”, such as sudden “leaps” ahead or back, but the averaging also factors in the different elevations, since if they only used DC atomic clocks for the averaging, the “world standard” would be running too slow (or too fast for sea-level time), and if they used only Boulder clocks, the “world standard” would be running too fast.

    So yes, I think we can say either that the elevations are “factored in” or we can say that the different clock-elevation rate “averaging” is done to “average” the different rates due to the different elevations. And as I mentioned, I suspect that the “world standard” time rate is probably “sea-level” atomic clock time, but you should know the answer to that.

    They also told me that no two atomic clocks can be manufactured to run at exactly the same rate. In other words, when being manufactured side by side, every one of the clocks will have a slightly different rate.

    Would you agree with this or not?

    Added to post:

    Ok, I’ve thought about it some more, and I think your term “factored in” is better than my term “averaged out”, because my term could be misconstrued to mean that if 5 clocks in Boulder are “averaged out” with 5 clocks in DC, then the “average” would represent the rate of an ideal atomic clock placed at the elevation of about 2,600 feet, and that is not what I intended to imply. I should have said “factored in”.

  13. #73
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    Quote Originally Posted by Sam5
    No, not just due to “relative motion” alone, since no physical “force” is placed on the oscillating mechanism of the clocks due only to “relative motion”. Your atomic clock rate changes due to elevation are GR, not SR changes.

    Anyway, a greater gravitational potential slows down atomic clocks, while it speeds up pendulum clocks. Synchronize one of your atomic and a pendulum clock in DC, make them run as synchronously as possible, then take both of them to Denver and set them up. The atomic clock will run fast, and the pendulum clock will run slow. If you happen to ever find a pendulum clock that “runs fast” in Denver and “slow” in DC, please let me know.
    If you go back to what I wrote, I think you'll see that I never implied otherwise. Gravitation is a GR effect on all clocks, and there is another, far larger dependence that affects pendulum clocks.

    Quote Originally Posted by Sam5
    Hey! You work at the US Naval observatory??

    Do you know if they “average” the rate to “sea-level” elevation or not?
    It's corrected to the geoid, which is essentially the idealized sea level.

    Quote Originally Posted by Sam5
    [see added note at bottom of post]

    Perhaps I did not explain it thoroughly, since they told me there are other reasons for the averaging too. I’ve said in other posts that they told me that the “averaging” is also done to eliminate some of the glitches in individual clocks that often occur, “biases” and “drifts”, such as sudden “leaps” ahead or back, but the averaging also factors in the different elevations, since if they only used DC atomic clocks for the averaging, the “world standard” would be running too slow (or too fast for sea-level time), and if they used only Boulder clocks, the “world standard” would be running too fast.

    So yes, I think we can say either that the elevations are “factored in” or we can say that the different clock-elevation rate “averaging” is done to “average” the different rates due to the different elevations. And as I mentioned, I suspect that the “world standard” time rate is probably “sea-level” atomic clock time, but you should know the answer to that.

    They also told me that no two atomic clocks can be manufactured to run at exactly the same rate. In other words, when being manufactured side by side, every one of the clocks will have a slightly different rate.

    Would you agree with this or not?

    Added to post:

    Ok, I’ve thought about it some more, and I think your term “factored in” is better than my term “averaged out”, because my term could be misconstrued to mean that if 5 clocks in Boulder are “averaged out” with 5 clocks in DC, then the “average” would represent the rate of an ideal atomic clock placed at the elevation of about 2,600 feet, and that is not what I intended to imply. I should have said “factored in”.
    Averaging corrects for biases, drifts (which are measured and removed) and the range of frequencies, but really not for "glitches" - if a clock's frequency changes in a non-statistical way, it will be removed from the mean until it can be re-characterized. The weird thing is that once you are convinced a clock has been working correctly, you can re-do all of your averaging as if the clock were in tyhe mean the whole time. You do calculations that are not real-time, as it were, so if you decide that your ensemble is off by e.g. a nanosecond (after you've added the clock back in), you can slowly steer the clock output to correct for that, and do it such that it can't be seen by any user.

    Also, even if two clocks did have the same frequency at some point, the best you can realistically hope for is that there will be white noise in the system. Time is the integral of frequency, and the integral of white frequency noise is a random walk in time - so two clocks, synchronized and at the same frequency, will still exhibit a random walk and diverge from each other.

  14. #74
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    Quote Originally Posted by swansont
    If you go back to what I wrote, I think you'll see that I never implied otherwise. Gravitation is a GR effect on all clocks, and there is another, far larger dependence that affects pendulum clocks.
    I think we are both a little mixed up about what the other one means about this. I was originally talking to Russell and others about the SR and about “relative motion” NOT changing the rates of clocks at all. Gravitation will slow down some kinds of clocks, but not all at the same rates, and not all for the same reasons. In fact, many mechanical clocks will slow down only because of the “friction” on their bearings caused by the gravitational field potential pulling down on the gears and the balance wheel. And some kinds of electronic clocks can be speeded up more by slight heating than they are slowed down by gravitational effects.

    As far as I’m concerned, “GR” refers specifically to atomic clocks or the “harmonic oscillation rates of atoms”, as per the 1911 theory. Gravitation will speed up pendulum clocks, and that was discovered 500 years ago and it’s not part of GR theory, it’s part of Newtonian relativity theory. In the 19th Century some guys used to measure elevation levels by how much a pendulum clock slows down at higher elevations.

    Quote Originally Posted by swansont
    Averaging corrects for biases, drifts (which are measured and removed) and the range of frequencies, but really not for "glitches" - if a clock's frequency changes in a non-statistical way, it will be removed from the mean until it can be re-characterized. The weird thing is that once you are convinced a clock has been working correctly, you can re-do all of your averaging as if the clock were in tyhe mean the whole time. You do calculations that are not real-time, as it were, so if you decide that your ensemble is off by e.g. a nanosecond (after you've added the clock back in), you can slowly steer the clock output to correct for that, and do it such that it can't be seen by any user.

    Also, even if two clocks did have the same frequency at some point, the best you can realistically hope for is that there will be white noise in the system. Time is the integral of frequency, and the integral of white frequency noise is a random walk in time - so two clocks, synchronized and at the same frequency, will still exhibit a random walk and diverge from each other.
    Very interesting stuff. I’d like to read an entire book by some person who works with this stuff. Most of what we members of the general public read in books is a mix of real stuff and SR and GR theory. Tell us more about atomic clocks and what they do and what you know about them. Tell us more about this "random walk" and "white noise".

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    Hello:
    When I think of white noise, I think acoustic white noise. White noise combines equal parts of all audio frequencies, which is analogous to white light, which combines equal parts of the visible light spectrum.

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    Quote Originally Posted by Russell
    Hello:
    When I think of white noise, I think acoustic white noise. White noise combines equal parts of all audio frequencies, which is analogous to white light, which combines equal parts of the visible light spectrum.
    Me too. That's why I want to know what the "white noise" is in relation to atomic clocks.

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    I was referring to a gaussian white noise, i.e. noise in a measurement. Please forgive the imprecision. ("White noise" isn't really noise from a measurement standpoint - it's a uniform frequency distribution)

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    Quote Originally Posted by Sam5
    Tell us more about atomic clocks and what they do and what you know about them.
    The biggest revelation for me was the timescale calculations. There is no perfect clock, so how you decide what the right time is has a small degree of arbitrariness. The fact that the calculations are done after the fact - a month goes by between some of the data reported to the international bureau of weights and mesures (BIPM), so you are trying to figure out what time it was and predict what time it will be, when all the numbers are crunched. Plus the ability to decide that a clock was good/bad, and add/drop it from the calculation dating back for months, and decide that your currrent time needs to be adjusted.

    Here is what I've been working on. Also this, but the web page is rather sparse since it's lower priority than the actual design and construction.

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    Quote Originally Posted by swansont
    Here is what I've been working on. Also this, but the web page is rather sparse since it's lower priority than the actual design and construction.
    Very interesting, thanks!

    Do you ever compare the clock rates with the earth’s rotation rate, to be sure all the clocks haven’t drifted in the same direction for some reason?

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    Quote Originally Posted by Sam5
    Do you ever compare the clock rates with the earth’s rotation rate, to be sure all the clocks haven’t drifted in the same direction for some reason?
    Well, that's what you do to determine whether leap seconds have to be added (or subtracted). You use different types of clocks to try to make sure there is no common mode systematic error like that, but then, I don't know if you'd notice. The earth rotation is variable, and some of the fluctuations can't be predicted, plus the fact that the rotation rate changes are much larger than clock drifts, so you'd have trouble using that as a measure. Earth rotation isn't a very good clock, compared to state-of-the-art, and that statement has been true for decades now.

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    Quote Originally Posted by swansont
    Quote Originally Posted by Sam5
    Do you ever compare the clock rates with the earth’s rotation rate, to be sure all the clocks haven’t drifted in the same direction for some reason?
    Well, that's what you do to determine whether leap seconds have to be added (or subtracted). You use different types of clocks to try to make sure there is no common mode systematic error like that, but then, I don't know if you'd notice. The earth rotation is variable, and some of the fluctuations can't be predicted, plus the fact that the rotation rate changes are much larger than clock drifts, so you'd have trouble using that as a measure. Earth rotation isn't a very good clock, compared to state-of-the-art, and that statement has been true for decades now.
    Are the leap seconds put into the system so that we won’t eventually have darkness at atomic-clock “noon” and the sun high overhead at “midnight”?

    In other words, while the atomic clock average is very steady on an hour to hour basis, doesn’t the whole world basically still go by astronomical time on a day to day and a year to year basis, with the atomic clocks ticking out nearly perfect “seconds” during each day and hour of the year, with the "leap seconds" being added to keep up with long-term astronomical time.

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    Quote Originally Posted by Sam5
    Are the leap seconds put into the system so that we won’t eventually have darkness at atomic-clock “noon” and the sun high overhead at “midnight”?

    In other words, while the atomic clock average is very steady on an hour to hour basis, doesn’t the whole world basically still go by astronomical time on a day to day and a year to year basis, with the atomic clocks ticking out nearly perfect “seconds” during each day and hour of the year, with the "leap seconds" being added to keep up with long-term astronomical time.
    Yes, but there is a little more to it than that. The earth doesn't rotate or revolve at a constant rate. So even if we tried, we couldn't re-define a second in such a way as to make every day exactly 24 hours long.

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    Quote Originally Posted by Sam5
    Are the leap seconds put into the system so that we won’t eventually have darkness at atomic-clock “noon” and the sun high overhead at “midnight”?
    Long-term, the answer is yes. But near-term, since not many people care if noon is off by a few seconds, the most vocal group for having leap seconds seems to be astronomers, so they don't have make time corrections for observing (which would be a royal pain)


    As I understand it, there have been some heated discussions about whether to abolish leap seconds and go to something like leap minutes, amongst a select group of people who give a hoot.

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    Quote Originally Posted by swansont
    Quote Originally Posted by Sam5
    Are the leap seconds put into the system so that we won’t eventually have darkness at atomic-clock “noon” and the sun high overhead at “midnight”?
    Long-term, the answer is yes. But near-term, since not many people care if noon is off by a few seconds, the most vocal group for having leap seconds seems to be astronomers, so they don't have make time corrections for observing (which would be a royal pain)


    As I understand it, there have been some heated discussions about whether to abolish leap seconds and go to something like leap minutes, amongst a select group of people who give a hoot.
    Interesting, thanks!

    How much of a bother is it to add the leap seconds?

  25. #85
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    Ok, this is a “light” thread, and I’ve got a question. Does anyone see anything wrong with the standard model or illustration of how light colors immediately diverge from a white beam that goes into a prism, with the blue part of the beam immediately becoming a thin blue line at the bottom of the beam inside the glass, and the red part immediately becoming a thin red line at the top of the beam inside the glass, with the blue and red lines diverging inside the glass and diverging more when they exit the glass?

    LIKE IN THIS ILLUSTRATION

  26. #86
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    Ok, so here’s the question: How does a white light beam know to split up into blue and red beams immediately upon entering a prism, with the red beam diverging toward the top and the blue beam diverging toward the bottom of the expanding rainbow beam inside the glass, when the white beam doesn’t know if its entering a prism or a pane of window glass.

    LIKE IN THIS ILLUSTRATION

    This illustration also suggests that the red and blue beams somehow know that if the glass through which they are traveling is window glass, with parallel sides, then the diverging of the red and blue beams stops at a slanted angle in the middle of the glass, and the red and blue beams began to converge again, and they emerge on the other side all united as a single white beam again. And, if so, how do the red and blue beams know where the “middle” of the window glass is?

    Maybe there’s something very simple that I’m just overlooking.

  27. #87
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    Let me get this straight. You don't even understand simple refraction, but have spent 30+ pages 'splaining relativity to us? #-o

    It is really, really, really, very, very, very simple. When light hits the interface between media of different indices of refraction (at an angle other than perpendicular) it begins to diverge. When it hits the second interface (back into the original medium) the divergence will change again (assuming the ray is not perpendicular to the interface). In the case of a prism, it will diverge even more. In the case of parallel interfaces (a window pane, for example) the ray will coverge back into a parallel ray. The ray will have a slight chromaticity, but it is usually too slight to be noticed. You can prove this to yourself with a pinhole and a piece of glass held at an angle to the beam.

  28. #88
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    Quote Originally Posted by Kaptain K
    It is really, really, really, very, very, very simple. When light hits the interface between media of different indices of refraction (at an angle other than perpendicular) it begins to diverge.
    I’m asking you specifically about the standard illustration like the one I linked to. It shows a thick white beam going into the glass, and it shows red emerging from the top of the white beam and blue emerging from the bottom of it. Are you saying that in a beam of white light, all the red photons travel at the top of the beam and all the blue ones travel at the bottom, as in this illustration? You don’t really believe that, do you?

    I understand that a prism separates the red from the blue, but it doesn’t do it in the way shown in the standard illustrations. So, can you provide us with an accurate illustration showing what happens inside the glass, and exactly how the two colors diverge?

  29. #89
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    Quote Originally Posted by Kaptain K
    Let me get this straight.
    If the blue and the red photons immediately begin to diverge when they enter the glass, then all the blue photons from the bottom to the top of the beam should be bent toward the bottom of the prism, and all the red ones from the top to the bottom of the beam should be bent toward the bottom of the prism too, but at less of an angle than the blue photons, so the colors should diverge, but not in the way they are shown in the standard illustrations.

    The standard illustrations show all the red photons emerging from top of the white beam and bending a little, with all the blue photons emerging from the bottom of the white beam and bending more. I am reasonably sure that white beams of light do not have all the blue photons traveling at the bottom of the beam and all the red ones traveling at the top. If they worked that way, we wouldn’t need a prism to see the separation of the colors.

  30. #90
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    I think a better illustration would show a much thinner white beam going into the prism, with all the blue photons, top to bottom of the white beam, diverging slightly more toward the bottom of the prism than the red ones. The blue ones diverging in the glass from the top of the white beam would actually cross over the red ones diverging in the glass from the bottom of the white beam.

    When a thicker white beam is used, what emerges from the other side of the prism is not a pure rainbow, but a blob of light that is white in the center and that has different colored fringes on the outside edges of the white area.

    But in standard illustrations, when a thick white beam is shown going into the prism, the blue is often shown emerging only from the bottom of the thick white beam, and the red is shown as emerging from the top of the thick white beam, and this is not correct.

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