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B About time

  1. Nov 5, 2016 #1
    Hi.
    Recently I was watching youtube about time and spacetime. And I have few questions about it.

    If I well understood no one knows what is the time. Here are no formulas to describe time. If it is true how we know if other watches measure time and not our perception on time which can be subjective?

    Official definition of a second is 9,192,631,770 cycles of the radiation that gets an atom of the element called cesium to vibrate between two energy states. But this is based on assumption that cycles are constant and do not change changing physic parameters like gravity or object speed in space.

    From all videos I know that time slows down in more powerful gravity field and it slows down if objects speed in space increases. But how do we know if it's time slows down and not physics? It's like running in air and in water. Your running speed in water will be slower nor in air, because the water density greater than air. Spacetime in greater gravity and at greater speed should compress and may slow down all physics, but not time. As we don't know that time is.

    Such thinking may solve problems and discussions about time travel. Because you will not travel back in time if your speed greater nor c and wormholes will not help time travelers. As only perception of time is affected by gravity and speed not the time.
     
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  3. Nov 5, 2016 #2

    Ibix

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    Time is one direction in four-dimensional spacetime. It is different from the other three directions which are space-like directions. You have a certain amount of flexibility in which direction you call time for you, which is where a lot of the "time flows at different rates" comes from.

    I've no idea what compressed space would be. More space in the same space? It doesn't really make sense.

    Honestly, if you want to do any real thinking about relativity you are going to have to pick up a text book and study it properly. YouTube videos won't tell you anything except hand wavy pictures that are distantly related to the reality. It's like trying to learn geography from landscape paintings.
     
  4. Nov 5, 2016 #3

    Nugatory

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    What @Ibix said.
    A good start might be "Spacetime Physics" by Taylor and Wheeler.
     
  5. Nov 5, 2016 #4

    Ibix

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    Or Ben Crowell's Relativity for Poets, for a maths-free introduction. Freely downloadable from www.lightandmatter.com
     
  6. Nov 5, 2016 #5
    Yes. From watch youtube documentary movies I realized that it is very simplified and many things are missing. The same go with articles on blogs and similar. It is made so simple that if you start to think about it will start to make less and less sense.
     
  7. Nov 5, 2016 #6
    I mean this one
    space-time-gravity-11748480.jpg
    https://www.google.lt/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwjdqZHDjpLQAhVJhSwKHQmADQAQjRwIBw&url=https%3A%2F%2Fwww.dreamstime.com%2Fstock-photo-space-time-gravity-image11748480&psig=AFQjCNFrtb59mb7LFZVpXPEzWzelsIbeDw&ust=1478452364975773
     
  8. Nov 5, 2016 #7

    Dale

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    Most formulas with a "t" in them describe time.

    It sounds like you are being deceived by your pop-sci sources.
     
  9. Nov 5, 2016 #8

    Nugatory

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    That would be an especially good picture to forget about - it is seriously misleading. You'll find many threads about why if you search here for "rubber sheet", as well some much better explanations.
     
  10. Nov 5, 2016 #9

    Ibix

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    As @Nugatory says, that picture is very poor. For a start, it implies that the grid is a 2-dimensional model of spacetime, then draws a 3-dimensional Earth sitting outside spacetime.

    The picture is very pretty, but it's not even internally consistent, let alone much like the reality.
     
  11. Nov 5, 2016 #10
    Really? Rubber sheet is so popular I was thinking it is standard in thinking. Even this video uses it.


    So majority of internet sources are misleading.
     
  12. Nov 5, 2016 #11

    PeroK

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    Yes!
     
  13. Nov 5, 2016 #12

    phinds

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    I think that is MUCH too weak a statement. Many of them are misleading and the rest are just flat wrong.
     
  14. Nov 5, 2016 #13

    PeroK

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    In these days of political correctness, perhaps we should call them "geodesically challenged"?
     
  15. Nov 5, 2016 #14

    Mister T

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    No, it is not based on that assumption. It's based on that conclusion. A conclusion reached from building machines that count these cycles, and observing that machines constructed using identical or totally different constructions, agree with each other. These machines are clocks, the thing they measure is called time, and the closer they agree with each other the better they are, by definition.

    Metrologists don't assume clocks are good at measuring time, they demonstrate it.
     
  16. Nov 5, 2016 #15
    This is exactly what I had in mind. If you make ruler in 20°C environment and measure size of object in 100 °C environment will it show size of object correctly? All materials have different expansion properties. So ruler size (length) will differ in 20°C environment and in 100 °C environment. Measured object size also varies in different temperatures and if it is made from different material it's coefficient of expansion differ from ruler. So will ruler show good measurements. If we make ruler in 20°C environment and always measure object in 20°C environment it will show correct measurements. Like clocks made in equal gravity or speed. But we put one clock on earth and other in rocket and let it go to Mars and back. Will both clocks show the same time? And why?
     
  17. Nov 5, 2016 #16

    Ibix

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    No. Google for "twin paradox".
     
  18. Nov 5, 2016 #17

    PeroK

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  19. Nov 5, 2016 #18

    phinds

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    If we put an odometer in a car that goes from Florida to Maine by way of New York it will show a milage. If we put one in a car that goes from Florida to Maine by way of California will it show the same number of miles? Why?
     
  20. Nov 5, 2016 #19

    A.T.

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    More volume within the same boundary.
     
  21. Nov 5, 2016 #20

    Grinkle

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    Not all discussions that use this rubber sheet imagery as an aid are misleading or wrong.

    You are being advised to look at a complete introductory discussion of the topic - you won't be able to infer the very much about relativity from one persons rubber sheet imagery discussion, even if everything they said was correct in the context within which they said it. Whether rubber sheet is right or wrong is a red herring for you. Forget about rubber sheet until you know enough to decide for yourself if it makes sense.
     
  22. Nov 5, 2016 #21

    Nugatory

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    And when you google for "twin paradox", pay particular attention to this search result: http://math.ucr.edu/home/baez/physics/Relativity/SR/TwinParadox/twin_paradox.html

    It is important to understand that the two clocks will show different times, but neither of them is wrong and both are accurately measuring the time that has passed. Suppose the twins are both growing beards at the rate of one centimeter per week, and suppose their hearts are both beating 75 times per minute. They're also both carrying samples of radioactive material that decays at a fixed rate, and they're both carrying potted plants that grow at some fixed rate. If when they compare clocks on their return the travelling twin's clock shows half as much time has elapsed..... The travelling twin's beard will only be half as long as stay-at-home's, stay-at-home's heart will have beaten twice as often as traveller's, stay-at-home will have only half as much of his radioactive material left, traveller's plants will be only half as tall as stay-at-home's. By far the easiest interpretation of these facts is that both clocks have accurately measured the time that passed for each twin, and it's not the same.

    It is also important to understand that this result is something completely different than time dilation, which you've also probably read about.
     
  23. Nov 5, 2016 #22

    Mister T

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    But it's the opposite of what you said!

    The only way to establish that is to have a length standard that doesn't change with temperature!

    We have such length standards so that's why we know that thermal expansion is real.

    Likewise, we have precise enough clocks to know that time dilation is real. Things live longer when they travel. That's a fact of everyday life for any engineer working with a particle beam.
     
  24. Nov 6, 2016 #23

    Orodruin

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    Just going to nitpick a few things.

    Technically, it will have beaten twice as many times. Whether this means twice as often depends on how you define "often".

    Are you familiar with the exponential function? :rolleyes::wink:
    Also consider the case when the mean life of the material is much longer than the trip takes.
     
  25. Nov 6, 2016 #24
    Hi,
    I have read Hafele-Keating experiment. Don't know if I understood it right. So I will try to explain how I understood it.

    Clock on earth surface have speed v in space sum of earth rotation about it's axis, rotation about sun, sun rotation and so on.
    Clock on flight going eastward will have greater speed in space compared to clock on earth surface and so it will have slower time.
    Clock on flight going westward will have slower speed in space compared to clock on earth surface and so it will have faster time.

    Is it right?
     
  26. Nov 6, 2016 #25

    PeroK

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    Those are the velocity-based variations in what the clocks read; there were also the effects of the difference in height, hence gravitational potential.
     
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