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B How well is Time understood i. Space time?

  1. Jun 4, 2016 #1
    I have been doing some reading and from what i inderstand of relativity(wich isnt much granted) it states that space and time can be viewed as a single entity that mass bends warps and affects in various ways.

    We have a pretty good understanding of how mass affects space in 3 dimensions. For example orbital mechanics. But my question , is it known at all as to how mass affects time?

    Also perhaps a less tangible question, what is time? How does it work. Surely if we have valid theories that describe mass interactions with spacetime we xan draw conclusions about time, as we can about the space
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  3. Jun 4, 2016 #2


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    We have a good understanding of how mass, energy, and momentum behave and interact with space-time. It is called general relativity.

    Please note that putting an A label on your thread indicates that you have knowledge of the subject at the level of a graduate student or higher and expect an answer at that level. Your post makes it clear that this is not the case and I have relabeled it for you.
  4. Jun 4, 2016 #3


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    Before Einstein we thought of time and space as completely separate things. A DVD is a good analogy - space is what's on the screen and time is what the DVD player's timer shows. You could turn your TV up on one end, and "up" would become "left" , "left" would become "down" and so on. But whatever you do to the TV the DVD clock ticks as before. And whatever you do to the clock doesn't make the picture rotate or anything - there's no way to make "up" on the screen become "later in the movie".

    Minkowski pointed out in 1908 that Einstein's maths could be interpreted as a description of a 4d universe where time is one more dimension like (but slightly different from) the three spatial ones. It turns out that there is a sense in which you can turn the future into sideways, and this is where time dilation, length contraction, and the relativity of simultaneity come from. However, the unique properties of the time dimension means that you can never make effect precede cause the way you can make up become down. There is some flexibility to decide which thing happens first if two things are far enough apart that light can't get from one event to the other, so they cannot cause each other, but there are limits.

    That's Special Relativity. Unfortunately it is completely incompatible with Newton's theory of gravity. Eventually Einstein developed General Relativity, in which mass and energy produce curvature in spacetime and the effects of this curvature are gravity. It can only make sense if the curvature affects the time dimension as well as the spatial ones.

    The most famous effect of the curvature of spacetime is that the GPS satellite clocks have to be adjusted by a very slight amount in order for them to tick at the same rate as Earthbound clocks. The Pound-Rebka experiment can be interpreted this way, and Hafele-Keating included effects on time due to altitude.

    Does that help?
  5. Jun 4, 2016 #4


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    These are pretty broad question, to which I can give a pretty broad answer. Operationally, space is what we measure with a ruler, and time is what we measure with a clock. So that answers the "what".

    As far as space-time goes, one might think that rulers and clocks measure two different and unrelated things. The constancy of the speed of light for all observers, though, shows a very close relationship. In fact, the most precise operational definition of distance we have is how far light travels in a certain amount of time. One can see this, in, for example, the SI defintion of the meter, see for instance http://physics.nist.gov/cuu/Units/meter.html

    This doesn't show that space-and time ARE related, but suggests that the COULD be related. But there is more reason to think they are related than the above.

    One good description of why we unify space and time into "space-time" is given in the parable of the surveyor, which one can find in chapter one of "Space-time Physics". This is a standard SR textbook, the older first edition is available online at the author's website, http://www.eftaylor.com/download.html.

    The point of the parable is this: We first ask "why are north-south, and east-west, considered both to be parts of the same thing, rather than something completely different?"

    Then an analogy is made, in which it is shown that space and time have much the same relationship as north-south and east-west do. Rather than go through the details of the analogy, I'll encourage you to read the source I mentioned. I could give a brief description of it, but the briefeness would come at the expense of using technical language.

    If you are interested enough to read the source, and still have questions, I might say more.
  6. Jun 5, 2016 #5
    There are lots of theories about time in philosophy, check out https://en.wikipedia.org/wiki/Eternalism_(philosophy_of_time) and https://en.wikipedia.org/wiki/Philosophical_presentism for some options. Special relativity changes our notion of synchronous events, those are events that are viewed to take place at the same time for a particular observer. This Brian Greene video clip gives a good idea of what its about . This leads some people to believe that the past, present and future somehow co-exist in what's called a block universe. This is eternalism in philosophy.
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