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Thread: Gedanken Experiment

  1. #61
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    Quote Originally Posted by Doodler View Post
    Is it just me, or does MentalAvenger's OP sound like a retelling of Tired Light?
    It's just you. It doesn't sound that way to me at all.

    Tired light loses stuff over time don't it? In the OP, the speed of light is modified at emission.

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    Quote Originally Posted by hhEb09'1 View Post
    It's just you. It doesn't sound that way to me at all.

    Tired light loses stuff over time don't it? In the OP, the speed of light is modified at emission.
    Tired light states that the speed of a photon is equal to c relative to teh emission source, but can be altered by intervening conditions. Deep gravity wells, gas clouds, relative velocity, that kind of thing. Its still a matter of relative speed of light, with a few extra bells and whistles.

  3. #63
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    Quote Originally Posted by Doodler View Post
    Tired light states that the speed of a photon is equal to c relative to teh emission source,
    Ah, no, tired light is something else, I think.

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    Quote Originally Posted by hhEb09'1 View Post
    Ah, no, tired light is something else, I think.
    Hmm...some of my first posts on Bad Astronomy were questions about whether photon velocity was variable with respect to different objects moving at different velocities relative to each other, and it was compared to tired light...

    My mistake.

  5. #65
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    Quote Originally Posted by crosscountry View Post
    done, except for the "never been done before" part, what makes it any less valid than when it was done hundreds of years ago?
    I didn’t say it was any less valid if it was done hundreds of years ago.
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    Quote Originally Posted by hhEb09'1 View Post
    There is an aspect that you haven't addressed though.

    I'm not sure why you need the first sensor--if you just hooked up the timing mechanism to tell you when the gate was opened, that would be the same thing wouldn't it?
    As noted above, by using identical sensors and equal cable length, calibrated together, the reading will be the most accurate possible with existing equipment.

    Quote Originally Posted by hhEb09'1 View Post
    But that makes it more obviously like the setup in which we measure the aberration of light--in fact, it pretty much is the setup. So, you don't get an exact value for the velocity of light--you get the aberration, which depends upon the movement of the mechanism. It's not enough to just align the mechanism to the star, you also have to make sure that the mechanism is moving towards the star--that is, that there is no sideways component of velocity.
    That setup would be different. That is not what I am attempting to measure.
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    Quote Originally Posted by SMEaton View Post
    In your gedanken, what do you mean by "cables"?
    Electrical cables to carry the signal from the sensors to the Comparator.
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    BTW, for those who wonder why I made “devise a new experiment” part of the requirements, the explanation is simple. I have dozens of books which contain probably every classical experiment performed. After reading a few books, it becomes clear that they are all saying the same thing, using the same arguments, often copy/pasted from another source. Also, Google has over a million websites which address the speed of light, and again they all use the same information, present the same arguments, and provide the same experiments. In addition, some members on any discussion board are more than willing to parrot the same stuff over and over and over again with no originality at all. I don’t need any Google Fruit, I have plenty of that. I am looking for creative, novel ways of studying the situation.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

  9. #69
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    Quote Originally Posted by MentalAvenger View Post
    Electrical cables to carry the signal from the sensors to the Comparator.
    You stated that these cables would eliminate transmission time errors, correct? How so? Does the signal transmit instantaneously?

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    Quote Originally Posted by SMEaton View Post
    You stated that these cables would eliminate transmission time errors, correct? How so? Does the signal transmit instantaneously?
    I think he means that they would have the same error.



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  11. #71
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    Transmission time is not an error, it is merely part of the system. An error would be a difference in transmission time between the two cables.
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    I misspoke when I said "transmission time error". My confusion was in the logistics of your gedanken, and how the cables might compare to a lens or mirror in the same situation. Considering that cables will transmit data at variable speeds, it seemed a reasonable question.
    Replace Detectors #1 & #2 with Parabolic Mirrors #1 & #2 and eliminate the cables, with each mirror focused on the comparator, or in this case a CCD which would record a nice picture of said star. Would you expect a difference between the two setups?

  13. #73
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    You must have missed the part where we eliminate mirrors and lenses. See the portion of the discussion regarding light being absorbed by a mirror and retransmitted.

    The cables will not transmit data at variable speed. I didn’t say that anywhere. Properly set up, they should transmit the data at exactly the same rate.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

  14. #74
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    Quote Originally Posted by MentalAvenger View Post
    As noted above, by using identical sensors and equal cable length, calibrated together, the reading will be the most accurate possible with existing equipment.
    This is a thought experiment though, so you can calibrate by carrying your cable around
    That setup would be different. That is not what I am attempting to measure.
    I don't mean to measure the aberration, I mean you have to take it into account in your experimental setup

  15. #75
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    Quote Originally Posted by MentalAvenger View Post
    As noted above, by using identical sensors and equal cable length, calibrated together, the reading will be the most accurate possible with existing equipment.
    This is a thought experiment though, so you can calibrate by carrying your cable around
    That setup would be different. That is not what I am attempting to measure.
    I don't mean to measure the aberration, I mean you have to take it into account in your experimental setup

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    How accurately do you think you could measure c, with this setup?

    What sources of error do you think you would need to a) identify, b) characterise?

    How would you characterise the errors (= estimate them, especially any systematic ones)?

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    Quote Originally Posted by Nereid View Post
    How accurately do you think you could measure c, with this setup?
    I think that the accuracy would be very high. I cannot provide a % because I do not know the resolution available in such instruments as the sensors and Comparator.

    Quote Originally Posted by Nereid View Post
    What sources of error do you think you would need to a) identify, b) characterise?
    The main source of error is the catch 22 inherent in the design, which I am surprised no one has yet questioned. That is that the further apart the sensors are, the more accurate the measurement, but the more difficult it is to accurately measure the distance between them. Obviously it would be impractical to use platinum iridium bars to measure the distance, and they wouldn’t be that accurate anyhow. One solution is to use light to measure the distance between the sensors using the same setup, but using a light source that is fixed relative to the sensors, instead of starlight. Another solution is to use radar signals and triangulation to set the distance.

    Any suggestions in this area are welcome. I was hoping one of the new experiments devised by participants might offer a creative solution to this problem.

    Quote Originally Posted by Nereid View Post
    How would you characterise the errors (= estimate them, especially any systematic ones)?
    Having the cables calibrated to each other, including the sensors and Comparator circuitry, should reduce any systemic errors to within the resolution of the equipment. I cannot give any estimates without specifications on the equipment, but any errors should be very small, and by definition, not measurable. Give a week to settle down in the cold and darkness of the Lunar night, the entire system should become very stable. Tests would have to be separated by several hours to let any heat generated by the reception and transmitting of a signal to normalize back to ambient temperature.
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  18. #78
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    Quote Originally Posted by MentalAvenger View Post
    [...] Give a week to settle down in the cold and darkness of the Lunar night, the entire system should become very stable. Tests would have to be separated by several hours to let any heat generated by the reception and transmitting of a signal to normalize back to ambient temperature.
    I'm so confused. And lazy -- I haven't studied the whole thread. Where's the gedanken in this gedanken experiment? To me it appears MA has designed an actual experiment, not a gedanken experiment, one that just hasn't been implemented yet.

    Or, is it a gedanken experiment that just hasn't been "gedanked" about yet? If it is, when can we expect the logical analysis, the scenario results, to be announced? Or, were they? Did I mention I am lazy?

    Thanks.
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  19. #79
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    Quote Originally Posted by MentalAvenger View Post
    Quote Originally Posted by Nereid
    How accurately do you think you could measure c, with this setup?
    I think that the accuracy would be very high. I cannot provide a % because I do not know the resolution available in such instruments as the sensors and Comparator.
    This assumes that their measurement accuracy - both precision ("number of decimal points") and stability ("get the same answer every time I repeat") of these is the limiting source of accuracy. As you state, in answers to my other question, I suspect this may not be the case.

    Anyway, assume (for now) that sensors and comparator are infinitely accurate (both precise and stable) - how accurately do you think you could measure c?
    What sources of error do you think you would need to a) identify, b) characterise?
    The main source of error is the catch 22 inherent in the design, which I am surprised no one has yet questioned. That is that the further apart the sensors are, the more accurate the measurement, but the more difficult it is to accurately measure the distance between them. Obviously it would be impractical to use platinum iridium bars to measure the distance, and they wouldn’t be that accurate anyhow. One solution is to use light to measure the distance between the sensors using the same setup, but using a light source that is fixed relative to the sensors, instead of starlight. Another solution is to use radar signals and triangulation to set the distance.
    Using light to measure light, eh? How would avoid all systematic errors if you went down that path? How could you estimate the systematic errors involved if you used light to measure distance?

    How accurate could Pt/Ir bars be, in terms of measuring distance? Would the accuracy scale linearly with distance? inversely?

    With perfectly accurate sensors, does it matter how far apart they are?

    Which highlights the fact that there are no such sensors, so their accuracy needs to be estimated.

    I think you've left off several, potentially big, sources of error:

    * the shutter - how fast can it be? plotted against time, what does the 'open shutter' intensity graph look like? How repeatable is that? How does it vary with distance from the shutter?

    * sensors - what is the threshhold? how sharp is that threshhold? this is important when combined with the 'open shutter' function; I expect that the accuracy of your experiment, in this regard, will involve some analysis of the shutter+sensor as a system (not as two separate devices)

    * cables - they would be made of physical materials whose properties will depend upon environmental factors - temperature, stress/strain, magnetic fields, pressure, ... how to conduct the experiment so as to minimise the influence of these factors and also estimate the inevitable errors they will introduce?

    * geometry - how important is the proper alignment, to the accuracy of the experiment? How accurately could that be determined and controlled?

    * comparator - same issues as with the sensors, only this time the system is sensor+cable+comparator. Also, the environment of the comparator itself is important - the accuracy of the timing differences will almost certainly be affected by environmental variables.

    * lenses and mirrors: how to build a suitably accurate sensor, sensor-cable interface, cable-comparator interface without any lenses and mirrors?
    Any suggestions in this area are welcome. I was hoping one of the new experiments devised by participants might offer a creative solution to this problem.
    I suspect you'll find that, in the end, this experimental setup cannot be more accurate than at least one method that's been used historically.
    How would you characterise the errors (= estimate them, especially any systematic ones)?
    Having the cables calibrated to each other, including the sensors and Comparator circuitry, should reduce any systemic errors to within the resolution of the equipment. I cannot give any estimates without specifications on the equipment, but any errors should be very small, and by definition, not measurable. Give a week to settle down in the cold and darkness of the Lunar night, the entire system should become very stable. Tests would have to be separated by several hours to let any heat generated by the reception and transmitting of a signal to normalize back to ambient temperature.
    Surely the key is to be able to a) identify, and b) characterise all potential sources of error?

    If you haven't done that, you would have no basis for estimating the accuracy of whatever result you obtained ... or would you?

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    Quote Originally Posted by Nereid View Post
    I suspect you'll find that, in the end, this experimental setup cannot be more accurate than at least one method that's been used historically.
    I think his goal is not to get a more accurate value for c, but to try to avoid a possible source of mis-measurement--the "re-setting" of the empty space velocity to the ambient space velocity, by making the length of the instrument primarily empty space.

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    Quote Originally Posted by hhEb09'1 View Post
    I think his goal is not to get a more accurate value for c, but to try to avoid a possible source of mis-measurement--the "re-setting" of the empty space velocity to the ambient space velocity, by making the length of the instrument primarily empty space.
    Ah, I see! Thanks for the clarification.

    Then why not just do some of the classical experiments in space (whether on the Moon's surface, for example, or in Earth orbit, or on the milk train to Neptune)?

  22. #82
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    Quote Originally Posted by Nereid View Post
    Ah, I see! Thanks for the clarification.

    Then why not just do some of the classical experiments in space (whether on the Moon's surface, for example, or in Earth orbit, or on the milk train to Neptune)?
    Just two reasons, near as I can tell.

    First, he wants to avoid the classical experiments that involve transmission times through glass, or reflection--that still leaves experiments like the one I suggested, or crosscountry, but (Second) his intent was to find new ones.

  23. #83
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    Quote Originally Posted by hhEb09'1 View Post
    Just two reasons, near as I can tell.

    First, he wants to avoid the classical experiments that involve transmission times through glass, or reflection--that still leaves experiments like the one I suggested, or crosscountry, but (Second) his intent was to find new ones.
    New, OK; address transmission times through glass/reflection - why not simply build multiple instances of any such into the experiment?:

    version 1, with just one lens

    version 2, essentially identical to version 1, but with an extra lens (or three)

    The effect of the lenses can be deduced by any differences between version 1 and version 2 (not a perfect solution, of course, but at least it gives a handle on the impact of lenses).

  24. #84
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    Quote Originally Posted by Nereid View Post
    why not simply build multiple instances of any such into the experiment?:
    I think the specific thing that he is trying to avoid is "contamination"--in the idea, which I think is admitted to be an objection in theory only, any time light enters a medium, it's speed is reset. In other words, light might be retarded at emission, and its speed then be actually a fraction of c throughout its travel through space, but as soon as it touches any of our local medium, it becomes c (or whatever the speed is in that medium).

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    Quote Originally Posted by 01101001 View Post
    I'm so confused. And lazy -- I haven't studied the whole thread. Where's the gedanken in this gedanken experiment? To me it appears MA has designed an actual experiment, not a gedanken experiment, one that just hasn't been implemented yet.
    Perhaps it is a two part Gedanken. The first part is designing a valid experiment that we cannot carry out at this time due to lack of access to the Moon. Being able to ferret out all the possible problems without being able to actually set it up is a challenge in itself. The second is to see if people can override their first impulse to jump in with the same examples and arguments, and try to look at the situation from a slightly different perspective.
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    Quote Originally Posted by MentalAvenger View Post
    Perhaps it is a two part Gedanken. The first part is designing a valid experiment that we cannot carry out at this time due to lack of access to the Moon. Being able to ferret out all the possible problems without being able to actually set it up is a challenge in itself. The second is to see if people can override their first impulse to jump in with the same examples and arguments, and try to look at the situation from a slightly different perspective.
    Perhaps. OK. But, may I then say that this doesn't sound in any way like an actual thought experiment, which poses a "what-if" (a scenario, the experimental setup) and logically (the thought, gedanken) proceeds to a "therefore" (the results, an understanding). Hence, my confusion with the use of the term, the title.

    To my understanding, a gedanken experiment isn't a challenging puzzle that may require thought. It isn't designing an actual experiment that cannot yet be implemented. It isn't a psychological experiment to see if people can actually override impulses.

    But, carry on. Have fun with whatever it is.

    Thanks for the explanation.
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    Skepticism enables us to distinguish fancy from fact, to test our speculations. --Carl Sagan

  27. #87
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    Quote Originally Posted by Nereid View Post
    This assumes that their measurement accuracy - both precision ("number of decimal points") and stability ("get the same answer every time I repeat") of these is the limiting source of accuracy. As you state, in answers to my other question, I suspect this may not be the case.
    Quote Originally Posted by Nereid View Post
    Anyway, assume (for now) that sensors and comparator are infinitely accurate (both precise and stable) - how accurately do you think you could measure c?Using light to measure light, eh? How would avoid all systematic errors if you went down that path? How could you estimate the systematic errors involved if you used light to measure distance?
    If you care to enlighten me on what you think those systemic errors might be, I will address your concern.

    Quote Originally Posted by Nereid View Post
    How accurate could Pt/Ir bars be, in terms of measuring distance? Would the accuracy scale linearly with distance? inversely?
    That was a joke, of course. That would be extremely innacurate.

    Quote Originally Posted by Nereid View Post
    With perfectly accurate sensors, does it matter how far apart they are?
    Yes it does. If you are trying to get an accurate reading, the further apart they are, the lower the percentage of the total difference between the readings from the two sensors any errors would be.

    Quote Originally Posted by Nereid View Post
    Which highlights the fact that there are no such sensors, so their accuracy needs to be estimated.
    Not sure what you mean by accuracy of the sensors. The purpose of calibrating the two sensor assemblies with cables against each other is to eliminate any small differences there may be in the materials or construction. Once a consistent reading can be obtained using a single source to trigger both sensors at the same time, any systemic errors will cancel each other out. The accuracy is only limited by the precision of the Comparator.

    Quote Originally Posted by Nereid View Post
    I think you've left off several, potentially big, sources of error:
    Such as……….

    Quote Originally Posted by Nereid View Post
    * the shutter - how fast can it be? plotted against time, what does the 'open shutter' intensity graph look like? How repeatable is that? How does it vary with distance from the shutter?
    Not a critical factor. The light from a distant star would be, for the purposes of this experiment, a point source. As long as both sensors are located in the same plane as the shutter, they should each see the light at the same time. That is one reason to have the shutter so far from the sensors.

    Quote Originally Posted by Nereid View Post
    * sensors - what is the threshhold? how sharp is that threshhold? this is important when combined with the 'open shutter' function; I expect that the accuracy of your experiment, in this regard, will involve some analysis of the shutter+sensor as a system (not as two separate devices)
    As long as both sensors have the same response time, it doesn’t matter. That is the point of having two identical sensors calibrated to each other.

    Quote Originally Posted by Nereid View Post
    * cables - they would be made of physical materials whose properties will depend upon environmental factors - temperature, stress/strain, magnetic fields, pressure, ... how to conduct the experiment so as to minimise the influence of these factors and also estimate the inevitable errors they will introduce?
    That is why I suggested conducting the experiment during the lunar night, and allowing plenty of time for the entire system to normalize.

    Quote Originally Posted by Nereid View Post
    * geometry - how important is the proper alignment, to the accuracy of the experiment? How accurately could that be determined and controlled?
    Alignment is critical, but not a serious obstacle. The most important thing is to get the two sensors as close to the same line as possible without one eclipsing the other. Having then each off 1mm, one to one side and one to the other, from the centerline of the shutter/star alignment, over a distance of 1 km should be sufficient.

    Quote Originally Posted by Nereid View Post
    * comparator - same issues as with the sensors, only this time the system is sensor+cable+comparator. Also, the environment of the comparator itself is important - the accuracy of the timing differences will almost certainly be affected by environmental variables.
    Again, Lunar nighttime. After a week in darkness, up on a mountain top, there should be no environmental variations that would create any problems. As stated earlier, the alignment procedure would include both sensors, both cables, and the Comparator circuitry.

    Quote Originally Posted by Nereid View Post
    * lenses and mirrors: how to build a suitably accurate sensor, sensor-cable interface, cable-comparator interface without any lenses and mirrors?I suspect you'll find that, in the end, this experimental setup cannot be more accurate than at least one method that's been used historically.Surely the key is to be able to a) identify, and b) characterise all potential sources of error?
    Remember, no lenses or mirrors in this setup.

    Quote Originally Posted by Nereid View Post
    If you haven't done that, you would have no basis for estimating the accuracy of whatever result you obtained ... or would you?
    I think my test procedure, including the unique all-system alignment, eliminates virtually all potential sources of errors.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

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    Quote Originally Posted by hhEb09'1 View Post
    I think his goal is not to get a more accurate value for c, but to try to avoid a possible source of mis-measurement--the "re-setting" of the empty space velocity to the ambient space velocity, by making the length of the instrument primarily empty space.
    Partly correct. I do want to avoid the possible “re-setting” of the velocity by other factors. I also want to get an independent reading of the light from stars at varying radial velocities from our Solar System. While I am at it, I’d like to also get an accurate measurement of the velocity of light from a star that, as far as we know, has no radial velocity from our Solar System.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

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    Quote Originally Posted by Nereid View Post
    New, OK; address transmission times through glass/reflection - why not simply build multiple instances of any such into the experiment?:

    version 1, with just one lens

    version 2, essentially identical to version 1, but with an extra lens (or three)

    The effect of the lenses can be deduced by any differences between version 1 and version 2 (not a perfect solution, of course, but at least it gives a handle on the impact of lenses).
    That brings us back to the problem of light being absorbed by glass and reemitted. Assume for a moment that it is possible for light to travel at a velocity different than classical c relative to us, the observer. For instance, a star moving away at from us at .1 c would transmit light towards us at c relative to itself, but at .9c relative to us. Light coming into the test apparatus strikes it at .9c, is absorbed, and reemitted on the other side at c. This is all tied in with the evidence from the Bose-Einstein experiments. You can see how any mirrors or lenses could add an unacceptable element into the experiment.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

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    Quote Originally Posted by hhEb09'1 View Post
    I think the specific thing that he is trying to avoid is "contamination"--in the idea, which I think is admitted to be an objection in theory only, any time light enters a medium, it's speed is reset. In other words, light might be retarded at emission, and its speed then be actually a fraction of c throughout its travel through space, but as soon as it touches any of our local medium, it becomes c (or whatever the speed is in that medium).
    Exactly.
    Speed of light = 1.802,265,898 MegaFurlongs / MicroFortnight

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