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Question on Spacetime

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  • 10-12-2009 12:24am
    #1
    roosh
    Registered Users Posts: 2,552 ✭✭✭


    Just a quick question, with regard to how time is treated with regard to General Relativity.

    How does the inclusion of time (in spacetime), affect the GR equations, with regard to expansion and contraction of the universe.

    Apologies if this isn't the best phrased question.

    I suppose what I am trying to get at, is, if time is unbundled from spacetime, would the equations still show us to be in an expanding universe?

    I understand this comes across as "crackpottery", but as a completely theoretical query, if you could indulge me, that would be much appreciated.


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    #2
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    How does the inclusion of time (in spacetime), affect the GR equations, with regard to expansion and contraction of the universe.

    ...

    I suppose what I am trying to get at, is, if time is unbundled from spacetime, would the equations still show us to be in an expanding universe?

    That's not exactly how general relativity works. In GR you have field equations which determine the structure of the spacetime. It isn't possible to separate time from space, the equations simply don't make sense.

    The expansion of the universe is simply the way the slice of spacetime (i.e. the space bit) that we perceive is increasing as we follow a timelike curve through the spacetime manifold.

    A simple example of this is to imagine spacetime as a ball. imagine a line through the ball joining two points on the surface of the ball, passing directly through the ball. If we take a 2D slice perpendicular to this line, the area increases as we move along the line until we reach the mid point. This is essentially what cosmic expansion is, although spacetime isn't really shaped like a ball.


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    #3
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Professor_Fink wrote: »
    That's not exactly how general relativity works. In GR you have field equations which determine the structure of the spacetime. It isn't possible to separate time from space, the equations simply don't make sense.

    The expansion of the universe is simply the way the slice of spacetime (i.e. the space bit) that we perceive is increasing as we follow a timelike curve through the spacetime manifold.

    A simple example of this is to imagine spacetime as a ball. imagine a line through the ball joining two points on the surface of the ball, passing directly through the ball. If we take a 2D slice perpendicular to this line, the area increases as we move along the line until we reach the mid point. This is essentially what cosmic expansion is, although spacetime isn't really shaped like a ball.

    Ok, but in the hypothetical scenario that time were found to be non-existent, what effect would this have, or would it have any at all?


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    #4
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    Ok, but in the hypothetical scenario that time were found to be non-existent, what effect would this have, or would it have any at all?

    If in the future time were to be found not to exist? The sentence doesn't even make logical sense. Of course time exists.


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    #5
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Professor_Fink wrote: »
    If in the future time were to be found not to exist? The sentence doesn't even make logical sense. Of course time exists.

    Ok, if time is found to be non-existent, what would be the implications?


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    #6
    Turtwig
    Registered Users Posts: 26,578 ✭✭✭✭Turtwig


    mangaroosh wrote: »
    Professor_Fink wrote: »
    mangaroosh wrote: »
    Ok, but in the hypothetical scenario that time were found to be non-existent, what effect would this have, or would it have any at all?
    If in the future time were to be found not to exist? The sentence doesn't even make logical sense. Of course time exists.
    Ok, if time is found to be non-existent, what would be the implications?

    An infinite loop would result perhaps..
    Sorry, couldn't resist.
    *gets coat*


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    #7
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    Ok, if time is found to be non-existent, what would be the implications?

    Mangaroosh, your question doesn't make sense. If time didn't exist, then there would be no future, so you can't discover something in the future. It's like asking what would happen if you discovered you didn't exist.


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    #8
    Kevster
    Closed Accounts Posts: 2,980 ✭✭✭Kevster


    ...well my watch has stopped and that's indication enough that time has ceased to exist :P

    mangaroosh, if time didn't exist, then the Universe would just be static, where nothing changes. It'd be like hitting the pause button, surely? Time is a fundamental requirement for the Universe. It simply cannot exist without it.


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    #9
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Professor_Fink wrote: »
    Mangaroosh, your question doesn't make sense. If time didn't exist, then there would be no future, so you can't discover something in the future. It's like asking what would happen if you discovered you didn't exist.

    Nothing can be discovered in the future. When any discovery is made, it is always the present (not the present moment in time).

    Not to get bogged down in that though. Lets just say that you make a scientific discovery now, that time doesn't exist, what would be the effect on the General Relativity equations?


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    #10
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Kevster wrote: »
    ...well my watch has stopped and that's indication enough that time has ceased to exist :P

    mangaroosh, if time didn't exist, then the Universe would just be static, where nothing changes. It'd be like hitting the pause button, surely? Time is a fundamental requirement for the Universe. It simply cannot exist without it.

    That is a slightly different topic, and no less interesting [I find personally], but lets imagine that time ceased to exist, how would this be reflected in the General Relativity equations?


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    #11
    Kevster
    Closed Accounts Posts: 2,980 ✭✭✭Kevster


    I dont think that anyone here can answer your question. The best guy to ask is - unfortunately - the 'man' himself (i.e. Einstein). The thing about it is, however, 'time' was a fundamental parameter of those equations, and they are all based on it. If you take out time, then the equations just wouldn't make any sense.

    ...?


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    #12
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Kevster wrote: »
    I dont think that anyone here can answer your question. The best guy to ask is - unfortunately - the 'man' himself (i.e. Einstein). The thing about it is, however, 'time' was a fundamental parameter of those equations, and they are all based on it. If you take out time, then the equations just wouldn't make any sense.

    ...?


    Cheers, that does go some way to answering the question.

    I'm sure though, there must be someone who knows enough about General Relativity still living who could answer the question. At a stretch in this country (and hopefully on this board).

    Cheers for the answer though.


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    #13
    Kevster
    Closed Accounts Posts: 2,980 ✭✭✭Kevster


    You're welcome. I actually wouldn't be confident that anyone knows the Relativity equations too well thuogh; and I don't suspect that anyone ever will. Picture how hard it is to read and understand someone else's work, and this particular situation is made worse by the fact that Einstein was a messy writer!


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    #14
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Kevster wrote: »
    You're welcome. I actually wouldn't be confident that anyone knows the Relativity equations too well thuogh; and I don't suspect that anyone ever will. Picture how hard it is to read and understand someone else's work, and this particular situation is made worse by the fact that Einstein was a messy writer!

    was he yeah? :D

    there is a book by Julian Barbour called the "end of time". Someone else mentioned it to me. I was just checking it out on Google books. Only read the preface and the first few lines, but sounds like it should be interesting.

    anyway, bedtime for me, lad. Thanks again


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    #15
    Kevster
    Closed Accounts Posts: 2,980 ✭✭✭Kevster


    No problem. By the way, Hawkins' A brief history of time should be the first book to read by every newbie cosmologist (that's if you are one).

    Edit: Considerin that guy did his PhD theseis on relativity, Id' say he's one of the few who actually understands it to a high degree. That book sounds interesting too, but I don't think his ideas have anjy ounce of practical sense (i.e. in reality) about them. It's always nice to dream though...


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    #16
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    Kevster wrote: »
    I actually wouldn't be confident that anyone knows the Relativity equations too well thuogh; and I don't suspect that anyone ever will.

    Oh, come on! It's a major field of study. GR is even an undergrad course on many theoretical physics courses. It's not terribly complicated.

    The reason I haven't given a more direct answer is that the question doesn't make a whole lot of sense. Time clearly does exist, so it will never e discovered not to exist.

    If you want to ask the question, you need to phrase it in terms of a toy model, not our universe. So, can we have a 3+0 formulation of GR? Sure, but it's very boring. The metric will have signature {+,+,+}, so it will just be Euclidean space. Since there is no time, there is no proper time, and so you have to ditch all the geodesic equations. You're just left with a rather boring static solution to the Field equations.


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    #17
    eZe^
    Registered Users Posts: 7,046 ✭✭✭eZe^


    Kevster wrote: »
    You're welcome. I actually wouldn't be confident that anyone knows the Relativity equations too well thuogh; and I don't suspect that anyone ever will. Picture how hard it is to read and understand someone else's work, and this particular situation is made worse by the fact that Einstein was a messy writer!

    What are you talking about? There is a lecturer in the ucc physics department named Niall O Murchadha who is (apparently) one of the worlds experts in General Relativity. If someone I see nearly everyday can be an expert in the field, I'm sure there are plenty of them out in the big bad world.

    If you really want to ask someone who knows the topic, you could try emailing him, but I can see the question just frustrating him as it makes little sense. It's like building a structure made entirely from sand, then asking 'what if sand didn't exist', well... Then the structure wouldnt exist either..

    Here's his website anyway if you want to give him a buzz. Don't tell him I recommended you though. Hahaha.

    http://www.physics.ucc.ie/people/nom.htm


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    #18
    Kevster
    Closed Accounts Posts: 2,980 ✭✭✭Kevster


    Sorry guys, I guess I just felt obliged to respond to the OP while no-one else was; and that meant basing my answer on what little knowledge I have in this field. My background is biology, which explains things!


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    #19
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Kevster wrote: »
    No problem. By the way, Hawkins' A brief history of time should be the first book to read by every newbie cosmologist (that's if you are one)./quote]

    Nope, I'm just a lay person who has a burgeoning interest in a number of areas, including the sciences.

    I saw the documentary of the same name, haven't read the book though.
    Kevster wrote: »
    Edit: Considerin that guy did his PhD theseis on relativity, Id' say he's one of the few who actually understands it to a high degree. That book sounds interesting too, but I don't think his ideas have anjy ounce of practical sense (i.e. in reality) about them. It's always nice to dream though...

    when you have "time" at all, some logical investigation into the hypothesis, could yield some interesting insight.


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    #20
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    eZe^ wrote: »
    What are you talking about? There is a lecturer in the ucc physics department named Niall O Murchadha who is (apparently) one of the worlds experts in General Relativity. If someone I see nearly everyday can be an expert in the field, I'm sure there are plenty of them out in the big bad world.

    If you really want to ask someone who knows the topic, you could try emailing him, but I can see the question just frustrating him as it makes little sense. It's like building a structure made entirely from sand, then asking 'what if sand didn't exist', well... Then the structure wouldnt exist either..

    Here's his website anyway if you want to give him a buzz. Don't tell him I recommended you though. Hahaha.

    http://www.physics.ucc.ie/people/nom.htm

    cheers for that.

    might give him a shout for the craic.

    Julian Barbour's website shows that Barbour is actually working with Niall O Murchadha, at the moment.
    Barbour's website wrote:
    During the last few years, I have collaborated with Edward Anderson, Brendan Z. Foster, Bryan Kelleher and Niall Ó Murchadha. I describe our current research programme here . We receive offers of collaboration with interest and can provide some support and supervision to young researchers.


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    #21
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Just a further question. What evidence is there for the existence of time?


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    #22
    Turtwig
    Registered Users Posts: 26,578 ✭✭✭✭Turtwig


    mangaroosh wrote: »
    Just a further question. What evidence is there for the existence of time?

    Change.
    Entropy seems to imply it only has one direction, I think.


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    #23
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Malty_T wrote: »
    Change.
    Entropy seems to imply it only has one direction, I think.

    That would be one of the things that should be questioned.

    Ultimately, the question boils down to whether time is something that exists and is measureable, or whether time is a system of measurement derived from measuring something else.

    In what way though would you say that change is evidence for the existence of time?


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    #24
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    Just a further question. What evidence is there for the existence of time?

    You mean other than the fact that things change? That things move? That our perception of the universe has had more than one state?

    Of course time is measurable. That's what clocks do.


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    #25
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Professor_Fink wrote: »
    You mean other than the fact that things change? That things move? That our perception of the universe has had more than one state?

    Of course time is measurable. That's what clocks do.

    Just because we perceive time, it does not necessarily mean that it exists. The earth was perceived as being flat, by some, at one stage, and it was also perceived that the sun revolved around the earth.

    I would question how the idea that things change and move is evidence for the existence of time, but one of the main issues, and indeed a primary assumption that needs to be questioned is whether or not a clock actually measures time.


    Closer examination would suggest they don't. For example, what the original 24hr clock were measuring was not actually time, rather the degree of rotation of the earth. The degree of rotation was assigned a value of seconds, minutes, hours and a full rotation was called a day.

    So there, it is not time that is actually measured, rather the degree of rotation of the earth.


    With the current atomic clocks, again it is not time that is measured. What the measurement device actually measures is the microwave emissions of changing electrons.

    This is then taken as a unit of measurement, what we call time. However, it is not time that has actually been measured.


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    #26
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    Just because we perceive time, it does not necessarily mean that it exists. The earth was perceived as being flat, by some, at one stage, and it was also perceived that the sun revolved around the earth.

    Many of the physical definitions of time simply amount to "Taht which clocks measure". Given that clocks measure something, hence time exists.
    mangaroosh wrote: »
    Closer examination would suggest they don't. For example, what the original 24hr clock were measuring was not actually time, rather the degree of rotation of the earth. The degree of rotation was assigned a value of seconds, minutes, hours and a full rotation was called a day.

    So there, it is not time that is actually measured, rather the degree of rotation of the earth.

    The degree of rotation of the earth? First, no, this is not what most clocks do. Second, rotation implies motion which implies the existence of time.

    mangaroosh wrote: »
    With the current atomic clocks, again it is not time that is measured. What the measurement device actually measures is the microwave emissions of changing electrons.

    This is then taken as a unit of measurement, what we call time. However, it is not time that has actually been measured.

    Emission again implies change with respect to time, and hence the existence of time.

    There is no 'time' observable, so you need to measure time via some change in another operator, but it is nonsense to claim that time doesn't exist. It's implicit in the very language we are using to have the conversation.


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    #27
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    Just wanted to link to a similar discussion on the existence of time in the philosophy forum.


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    #28
    Professor_Fink
    Registered Users Posts: 861 ✭✭✭Professor_Fink


    mangaroosh wrote: »
    Just wanted to link to a similar discussion on the existence of time in the philosophy forum.

    Seriously, this is a ridiculous conversation to be having. I suspect, and hope that you mean something a little deeper than what you are actually saying, but simply proclaiming time not to exist is ridiculous, since it effectively is defined in such a way that it must exist if we exist. Asking whether we exist may be philosophically interesting, but is completely non-constructive and has no place on a physics forum.


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    #29
    Fringe
    Registered Users Posts: 1,082 ✭✭✭Fringe


    mangaroosh wrote: »
    Just wanted to link to a similar discussion on the existence of time in the philosophy forum.

    Why are you trying to complicate something so much? Time clearly exists because we can perceive change. You're trying to speculate that it doesn't exist but there's no evidence that points towards this. That thread has to much speculation and nothing definite. (like that guy talking about time being a form of energy) Time is so fundamental to everything that it has to exist.


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    #30
    roosh
    Registered Users Posts: 2,552 ✭✭✭roosh


    the question is ultimately:

    does time exist as "something" that can be measured, or is it a system of measurement, created by man?


    it appears that time is treated as something which can be measured, and it is asserted that clocks measure time.

    However, closer examination shows that the concept of time only arises as an attempt to rationalise our observation of change. It ultimately involves a comparison of non-reality with reality.


    If we take an atomic clock for example. What we have is a measurement device that measures the microwave emissions of changing electrons.

    Therefore, what is being measured is the microwave emissions of changing electrons. It is not "time" that is being measured, rather the phenomenon that is being measured is designated as a unit of the measurement sysytem called time.


    The measurement system is similar in nature, to the metric system. It is an invention of mankind and does not exist in reality.



    So far such things as motion, life and change have been proposed as evidence for the existence of time, however, it is theses subjective interpretation of these phenomena, that actually give rise to time, or that have lead man to create the concept of time.


    Is there an explanation for how motion and change are evidence of time?


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    #31
    Morbert
    Registered Users Posts: 3,457 ✭✭✭Morbert


    mangaroosh wrote: »
    the question is ultimately:

    does time exist as "something" that can be measured, or is it a system of measurement, created by man?

    it appears that time is treated as something which can be measured, and it is asserted that clocks measure time.

    However, closer examination shows that the concept of time only arises as an attempt to rationalise our observation of change. It ultimately involves a comparison of non-reality with reality.

    If we take an atomic clock for example. What we have is a measurement device that measures the microwave emissions of changing electrons.

    Therefore, what is being measured is the microwave emissions of changing electrons. It is not "time" that is being measured, rather the phenomenon that is being measured is designated as a unit of the measurement sysytem called time.

    The measurement system is similar in nature, to the metric system. It is an invention of mankind and does not exist in reality.

    So far such things as motion, life and change have been proposed as evidence for the existence of time, however, it is theses subjective interpretation of these phenomena, that actually give rise to time, or that have lead man to create the concept of time.

    Is there an explanation for how motion and change are evidence of time?

    What would be the difference between a universe with 'actual' time, and a universe where time merely represents change? How would, say, simultaneity effects be different?

    Either way, it doesn't really matter with General Relativity. General Relativity is not a theory of what time is. Instead, it's a theory of how it (among other things) behaves. So, whether or not time is intrinsic, or a convenient representation of change, time still obeys the rules of Relativity.


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