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Posts posted by NowThatWeKnow

  1. Using the GPS satellites to compare GR and SR corrections to clocks.


    "...Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion ...A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day..."


    Not sure of the exact #'s concerning the moon but but Earth's gravity (vs way out in space) will slow clocks by .022 seconds a year.

  2. of the episode where they make some outlandish claims about the Mayan 2012 predictions. ...implying that the sun will actually *be* the center of the Milky Way. Is this not a gross misrepresentation AND an outlandish claim?

    I did catch the "center of the Milky Way" error but I took it as just an error in words and not a claim.


    The whole story was presented as one interpretation of a legend that some may believe is a prophecy. There were no claims that any of it was true other then a few things concerning the accuracy of their calendar and knowledge of astronomy. Anyway, that is the way I took it and I didn't leave wondering if the world was coming to an end in 2012.


    A little drama added to programs to help ratings. I find some of their stuff intertaining and other stuff pretty lame. But I do not see it as "gross misrepresentation AND outlandish claims". If that is what you got from that clip then our minds work very differently.


    Enough said a bout the History channel. :P

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    ... they just treat utterly ridiculous subjects with complete seriousness, with the implication that it's something to worry about. They have on "experts" on prophecies and UFOs and stuff and let them talk without further comment, and they're treated in exactly the same way they treat actual historians....

    The History channel UFO programs are usually pretty bad. I have only seen one or two that were worth watching. Most get turned off in the first few minutes.

  3. Yeah, that's just wrong, man. They lose all credibility as a source. They could approach it as a "historically, this is what some people believe", but they really don't.


    While I do see the History channel present a different point of view often, I see it being done with interest in a factual and not misleading way. I have never heard any outlandish claims as facts and their speculation is obvious to me when I watch it. I challenged people awhile back to point out a specific gross misrepresentation of the truth, considering a layman approach, and I am still waiting. :)

  4. ...You have really good intuition and a lot of inititiative, you think vigorously (is my impression). But you seem to think "reference frame" can mean what makes sense to you for it to mean...

    Thank you, I think. :) I have no training in cosmology and most of what I know is from reading and posting in the last few months. It does seem that most examples of frames have to do with motion of one object relative to another. And if you are in the same frame you will share the same time. I would like a definition of a "frame" that I could understand but would also make sense to you.


    ...where did you hear that space was flat? As far as I can remember I have never said such a thing. We don't know. Of course it is obviously not flat locally. The sun bends light rays. All sorts of things do grav. lensing. Space is very wavy gravy. But it MIGHT be zero curvature if you take a very largescale average. We just do not know,...

    Maybe I should have said that I heard the universe was flat, not space. I know you do not think much of the History channel but they said that during measurements, the triangle totaled 180*. And on a scale our technology was capable of dealing with, the universe was flat or nearly flat from what we can tell. I guess that kind of goes along with what you are saying.

  5. ...Maybe we should have a poll to find out what people here at SFN think a "frame" is. I think we are just having a minor verbal difficulty here...

    A frame could be any number of coordinates where there was no motion relative to each other and, considering GR, they would also have to share the same gravitational potential. Is that not right?


    It wasn't long ago I heard that our current technology indicated space was flat, or so nearly so we could not determine any curve. Is there new proof that space is curved?

  6. I don't see how anybody can talk about a preferred reference frame (except limited and local, for a given observer). What am I missing?

    Didn't you say that you could be anywhere in the universe and that the age of the universe would be the same when calculated as long as you had a constant CMB temperature in all directions at each location? Wouldn't that make it a common reference frame?

  7. ...So it isn't practical to ignore GR in your scenario...


    ...Recession is not ordinary motion (it doesnt go anywhere) and therefore it is not subject to Special Rel.

    Thanks for your reply. My question has been answered. As far as ignoring GR, I just didn't want local gravity having an impact on the clocks and complicating things.

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    Thanks Martin. So we can consider all points that measure cmb isotropy to be in the same time frame, no matter how far apart, and no matter their relative velocities due to the expansion alone.


    Seems like a preferred reference frame.

    My thoughts exactly. If nothing else, it would make sense for us to choose it as a "preferred reference frame". Then make adjustments for local gravity and we would have a universal GMT (Zulu) time.

    Edit - Or master clock as swansont would prefer :D

  8. ...This is the error that I believe has been made for so long. Yes, space-twin measures the Earth-star contraction as 2.23LY. But that doesn’t make his trip 2.23LY...


    It was put this way in a different thread.


    "Earth says it is because he aged slower, and he says it is because the distance was shorter. Either explanation is equally valid (There is no one right explanation.)"


    I found it easier to understand the twin paradox when I started with how acceleration affected the clock speed. Then I could add other components of the story and it made more sense.

  9. ...Sidenote: Has anyone ever noticed that on internet discussions about relativity people always talk about if and whose clock goes slower and why. But seemingly no one asks how much?...

    Knowing "how much" takes specific information. We do consider "how much" when we talk about the GPS satellites or 1G rocket rides to a particular location.


    I see everything as a clock. The rate of decay of road kill measures time.

  10. You began with a comparison to two galaxies, fixed in expansion with the reasonable assumption that time between the two is synchronized in a line drawn through their centers...


    ...Have you considered that even in this simplest of universes, the point of launch cannot be isolated in time relative to the remainder of the universe?


    To make things a little less abstract I would like to ignore GR in my scenario. The variable gravitational fields only complicate things when considering if two separating galaxies can share the same time frame. My only concern is if expansion of space causes time dilation and length contraction and at this point it seems it does not.


    My twin example had both twins leaving Earth. I was just contemplating if it was possible for separating mass because of movement, not expansion, to share the same time frame. It seems it is possible but length contraction may be a different story.


    As I am sure you can tell, I am very much a layman just trying to understand it all a little better. I do appreciate all participation as it is all food for thought. :)

  11. Have you considered the possibility that since you are in time-sync between objects at fixed expansion you cannot blast off in perfectly opposite directions at the same time because to remain in sync you find yourself "in the up" side of one object and the "at the down" side of the other?


    Not sure what you are saying here but my point was that the two twins could blast off in any direction and their clocks would remain synchronized as long as their acceleration was equal. Both of their clocks would slow down, relative to Earth clocks, equally. Any course or speed change after blast off by either twin would unsyncronize their clocks.

  12. It takes time for the light to pass through the barn. Even if they are opened out of sync, if that time difference is less than L/c, light will get through.

    Could you simplify this for me considering just the barn's time frame. If you have two synchronized clocks, one at each door, what are the doors doing relative to each other.


    It seems that when you consider what you would observe due to the time it takes light to reach your eyes, the event is unnecessarily complicated.

  13. I understand the concepts of Relativity. What I said is beyond that. All speed of light measurements are made here on Earth or in the Solar System. There is some questions about the behavior of the light outside this gravity field. As we know, the speed of light will be slowed if the light has a big mass near it. That effect is known as “Shapiro delay” (http://en.wikipedia.org/wiki/Time_delay_of_light). Maybe my theory can explain the anomaly observed at the Pionner (http://en.wikipedia.org/wiki/Pioneer_anomaly).


    That "Shapiro delay" link was just what I needed to put a cap on a few thought I had. Thanks!


    Since the clocks slow down with increased gravity, light must slow down to maintain the speed of light within that frame. The speed of light is constant relative to the frame it is in.

  14. Neat video. Nice and simple. Thanks for sharing. :)


    It was gib65 that shared it with me recently and I also thought it was well done.

    asprung, you will enjoy it.

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    Take a deeep breath. :D


    Edit - Not sure why "master time" and "real time" seems to bother so many. It is clear in my mind. I think.

    At least when I look at it as a cosmology issue and not just a relativity issue.

  15. So...how does that differentiate time dilation due to expansion displacement vs movement displacement?


    Not disagreeing or agreeing there's a difference, but I don't see it in those paragraphs.

    I agree that I do not see the proof in those two paragraphs but it is possible that I do not see the relevance in the words.


    Lorentz interval specifically said


    "it is not affected by relative-velocity length contractions or time dilation."

  16. Nope. :D

    Thank you! :)

    I actually read that link before asking the question but limited terminology in physics let me pass over what I now think is the key.


    "...distance is measured using the Lorentz interval...


    I should have looked up:

    Lorentz interval

    "The Lorentz interval is a quantity that is used instead of distance when dealing with space-time geometry, because it is the only quantity that is the same to all observers regardless of how fast they are moving in spacetime, that is, in all reference frames. In other words, it is not affected by relative-velocity length contractions or time dilation."

  17. I dont undrestand.


    Someone here shared this video on time dilation with me recently and I thought it was one of the better ones. Check it out.

    If you could view Earth in real time after accelerating out of Earth's frame, you would see people age quickly. They would see you age slowly in your frame if viewing in real time was possible.

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