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Thread: Time a long time ago and in empty spaces

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    Time a long time ago and in empty spaces

    Time moves (relatively) slower in strong gravitational fields. On that basis, would it be safe to say that time ticked slower during the initial expansion of the universe when it was small and dense compared to how time ticks today ? Does that have any implications wrt the speed of light then and now ?

    On a similar note, does time tick slower in empty intergalactic space compared to how it ticks inside galaxies ?

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    There's no way to compare the "speed of time" in the early Universe to the "speed of time" today--we can't send a clock there for a while and then bring it back again. The cosmic time against which Big Bang events (like the age of the Universe) are calibrated is the time measured by a theoretical clock moving with the expansion of the Universe and embedded in an average-density part of the Universe. And that time is just time, elapsing at one second per second.
    In reply to your other question, yes, a pair of extremely accurate clocks, one in intergalactic space and one inside a galaxy, would run at vey slightly different rates, which we could in principle detect by bringing them together and comparing readings. The galactic clock would show more elapsed time than the intergalactic clock.

    Grant Hutchison

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    Thanks, Grant.

    1) Understood that a direct comparison isn't possible but in principle wouldn't the number of ticks per tocks be different or am I asking a pointless question ?

    2) Wrt second question, over the life time of the universe, how much would two clocks differ today ?

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    Quote Originally Posted by hush36 View Post
    Thanks, Grant.

    1) Understood that a direct comparison isn't possible but in principle wouldn't the number of ticks per tocks be different or am I asking a pointless question ?
    If I understand your question, it is pointless. You need different points in space to do the comparison. If we were all on a spaceship deep in a black hole gravity well we wouldn’t notice any difference in time running for us. We would only notice a difference if we compared to someone distant from the black hole.

    If the entire universe was denser than now, there would be no other place to make a comparison against. Any two clocks (or things acting like clocks, such as long lived radioactive elements) surviving from the early universe would show no difference to each other due to differences in density affecting the entire universe.

    2) Wrt second question, over the life time of the universe, how much would two clocks differ today ?
    I don’t understand the question. What two clocks? Or does my response above answer your question?

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    Quote Originally Posted by Van Rijn View Post
    I donít understand the question. What two clocks? Or does my response above answer your question?

    I was referring to the second question in my opening post or "does time tick slower in empty intergalactic space compared to how it ticks inside galaxies ? " the answer to which is yes.

    Thank you both.

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    Quote Originally Posted by Van Rijn View Post
    I don’t understand the question. What two clocks? Or does my response above answer your question?
    The clock in intergalactic space and the clock embedded in a galaxy. Given that galaxies are not gravitationally homogeneous entities, and the difference between clock readings will be very small, I suspect the estimated answer will vary over orders of magnitude.
    I've only ever seen the calculation done for the difference between the centre and surface of the Earth--the centre having aged a couple of years less than the surface, over 4.5 billion years.

    Grant Hutchison

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    Quote Originally Posted by Van Rijn View Post
    If the entire universe was denser than now, there would be no other place to make a comparison against. Any two clocks (or things acting like clocks, such as long lived radioactive elements) surviving from the early universe would show no difference to each other due to differences in density affecting the entire universe. ?
    Forgive my persistant ignorance but the universe was denser after the BB than it is today and so, relative to how time ticks today, shouldn't time have ticked relatively slower ? no direct comparison can be made but isn't there a theory where all of space-time exists (past, present and future) and therefore, in a sense, if you were able to look at that entirety you would be able to determine time ticking differently then compared to now ?
    Again, forgive me for failing to use the right words/concepts to express this question.

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    Quote Originally Posted by hush36 View Post
    I was referring to the second question in my opening post or "does time tick slower in empty intergalactic space compared to how it ticks inside galaxies ? " the answer to which is yes.
    Ah, thanks. I thought the second question was “Does that have any implications wrt the speed of light then and now ?”

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    Are there any implications wrt speed of light ?

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    Quote Originally Posted by hush36 View Post
    Are there any implications wrt speed of light ?
    No, the speed of light is a constant. Mind you, I believe there have been experiments to test if the speed of light has changed over time anyway (scientists don’t like to assume things), with negative results.

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    Quote Originally Posted by Van Rijn View Post
    No, the speed of light is a constant. Mind you, I believe there have been experiments to test if the speed of light has changed over time anyway (scientists donít like to assume things), with negative results.
    How would one test that ?

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    Quote Originally Posted by hush36 View Post
    Forgive my persistant ignorance but the universe was denser after the BB than it is today and so, relative to how time ticks today, shouldn't time have ticked relatively slower ?
    The problem remains, "slower than what"? If you plot the history of your block universe, you can only plot it against elapsed time as measured within the Universe. You have no external standard of comparison.

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    The problem remains, "slower than what"? If you plot the history of your block universe, you can only plot it against elapsed time as measured within the Universe. You have no external standard of comparison.

    Grant Hutchison
    and thats why I'm not a physicist.. I imagine you could simply plot it against elapsed time as measured at different times within the universe, i.e. ticks per tock then Vs ticks per tock now. Thanks again.

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    Quote Originally Posted by hush36 View Post
    and thats why I'm not a physicist.. I imagine you could simply plot it against elapsed time as measured at different times within the universe, i.e. ticks per tock then Vs ticks per tock now.
    I don't understand what you mean by "tocks per tick". Clocks go tick-tock. One tick for every tock.

    Grant Hutchison

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    Quote Originally Posted by hush36 View Post
    How would one test that ?
    This gets into areas where I have little expertise. Hereís one webpage from a university that appears to be for a course that may help. It discusses testing variations in the speed of light using measurements regarding the fine structure constant:

    http://newt.phys.unsw.edu.au/einstei...6_constant.htm

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    Quote Originally Posted by grant hutchison View Post
    I don't understand what you mean by "tocks per tick". Clocks go tick-tock. One tick for every tock.

    Grant Hutchison
    I'm awful at expressing myself, sorry.

    Heres another attempt : time here on Earth ticks at a certain rate (i.e. ticks per tock). In a strong gravitional field, time clicks relatively slower hence more ticks for every tock. Does that make any sense ?

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    I'll check that out, thanks

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    Quote Originally Posted by hush36 View Post
    I'm awful at expressing myself, sorry.

    Heres another attempt : time here on Earth ticks at a certain rate (i.e. ticks per tock). In a strong gravitional field, time clicks relatively slower hence more ticks for every tock. Does that make any sense ?
    Not really. I'd suggest abandoning the whole "ticks per tock" thing--I doubt if anyone could reliably guess what you mean by it.
    The rate at which time passes is always relative--you need to compare your clock to another clock. Then and only then (if they are both accurate clocks) can you say that time in one place is running slower than time in another. So the clocks (measuring time) are separated in space. You can't compare the rates of clocks separated in time. Would it make the slightest sense if I told you that time yesterday ran twice as fast as time today? How could we ever know that?

    Grant Hutchison

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    Quote Originally Posted by grant hutchison View Post
    Would it make the slightest sense if I told you that time yesterday ran twice as fast as time today? How could we ever know that?

    Grant Hutchison
    er...by measuring the changing distance light can travel in an allotted amount of time, over time ?

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    Quote Originally Posted by hush36 View Post
    er...by measuring the changing distance light can travel in an allotted amount of time, over time ?
    In your reference frame the speed of light is constant. So in one unit of your proper time light will always travel the same distance. It doesn't matter if relative to someone else your proper time appears to change rate.

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    Quote Originally Posted by hush36 View Post
    and thats why I'm not a physicist.. I imagine you could simply plot it against elapsed time as measured at different times within the universe, i.e. ticks per tock then Vs ticks per tock now. Thanks again.
    I think the problem is that both the "ticks" and the "tocks" vary by the same degree, so the "ticks for tocks" will always be the same everywhere. It is only when you are looking at a different place where you notice that the "tocks" there are different. But the "ticks per tocks" will still be the same. You will see a clock go slower, and a person age more slowly, at the same rate.
    As above, so below

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    Thanks, I'll think about it some more.

    Another quick question : to what accuracy have we measured the speed of light ?

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    Quote Originally Posted by hush36 View Post
    Thanks, I'll think about it some more.

    Another quick question : to what accuracy have we measured the speed of light ?
    See my answer in the new thread you started on this topic.

    Grant Hutchison

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    Quote Originally Posted by hush36 View Post
    er...by measuring the changing distance light can travel in an allotted amount of time, over time ?
    As Shaula says, that doesn't work. Everyone always measures their local speed of light to be c. Someone deep in a gravity well will produce the same result as someone doing the same experiment in intergalactic space. That's how relativity works. Indeed, that's more or less the point of relativity.

    Grant Hutchison

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    Quote Originally Posted by hush36 View Post
    Time moves (relatively) slower in strong gravitational fields. On that basis, would it be safe to say that time ticked slower during the initial expansion of the universe when it was small and dense compared to how time ticks today ? Does that have any implications wrt the speed of light then and now ?
    The question as posed can't be answered. Time passing can only be measured from the "inside" of time passing. There's no objective time frame.
    "I'm planning to live forever. So far, that's working perfectly." Steven Wright

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    There is also the issue of "empty spaces". let a computer use a random number generator, and examine a cubic meter of randomly chosen Minkowski spacetime. Remove any plasma. Remove any gases. Remove any liquids. Cool it to a millikelvin with an insulated refrigerator, to remove the CMB, the microwave radiation left over from the Big Bang. It still holds two things. #1 the zero point radiation due to the Heisenberg Uncertainty Principle, and a relatively isotropic flow of neutrinos streaming through it as they are generated by the billions of stars in the millions of galaxies found per square degree in the Hubble Deep Field images from the Hubble Space Telescope.
    There is no known barrier that prevent those neutrinos from streaming through your randomly chosen cubic meter of Minkowski spacetime. One cannot talk of "empty space", if it cannot be found in the universe. It goes right in there with unobtanium, and inflationary spacetime. I'd like a cubic meter of that to put in my truck's gas tank, so that it will expand in the cylinders, carrying along all the gas/air mixture, without cooling as it expands, so that I might get several trillion miles per cubic centimeter.

    See:http://spacetelescope.org/images/opo9601c/
    Last edited by trinitree88; 2020-Aug-01 at 12:41 AM. Reason: Link to Deep Field Image

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