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4th Dimension

  1. Jan 5, 2012 #1
    Greetings! I'm new coming to this page but I have a question whose answer goes beyond my understanding of physics...really,even the question goes beyond my understanding but I feel comfortable enough to ask the question, just forgive me if I use the wrong wording...

    I've read and heard about Einstein's labeling the 4th dimension as space-time but I've had a hard time conprehending the concept. I've also read and heard about 'entropy' and understand that entropy is what will ultimately become of the universe...

    Taking those 2 concepts to mind, is it not possible to view 'time' as the entropic result of 'space'? Space is in existence and to exist means to occupy space, meaning space, and therefore time, is finite...

    Also, if I understand what is proposed to happen to time in a blackhole, since nothing escapes a black hole and if time is the result of the entropy of space then time would stand still in a black hole...

    Where would time go? As is suggested of matter and energy in black holes, it would be jettisoned through a 'white hole' into a parallel but opposite universe. Since space would be ejected into that opposite universe, so too would time...

    What do you guys think?
  2. jcsd
  3. Jan 5, 2012 #2

    Simon Bridge

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    Welcome to LQ.

    ... which is odd because he did no such thing. "Space-time" is what physicists call a particular model of what journalists and SF writers like to call "the fabric of the universe". It just means that we treat the time axis as if it were just another space dimension (it requires a few tweaks).

    ... also odd since "entropy is what will ultimately become of the universe" makes no sense ... it's like saying that "height is what will become of a growing child" or "age is what will become of all of us".

    Considering your earlier statements, this is quite remarkable because you are correct - it _is_ possible. We know that entropy and time are quite closely related but it is usually considered more subtle than that ... consider, you know the direction of time because you have memories of the past and not the future, you memories are the result of electro-chemical reactions in your brain ... these reactions are entropic (otherwise they would be reversible in time and you'd remember the furtur 50% of the <ahem> time) so entropy lays down our perception of time. But it could be deeper than that ... it an ongoing field of research, and you should be careful of simple-sounding conclusions.

    Now a black-hole inhabits a region of extremely curved space and time. You can google for the "geometry of space-time inside a black hole". What happens is the dimensions get all warped and twisted around...

    Jim Haldenwang has a decent description - you'll have to scroll through some math.

    We have lots of problems like being sure what we mean by "distance" when time is just another space dimension. Current models suggest that time and space swap roles.

    White holes are really out on the edges of speculation.
    Jims page will give you an idea of how far you have to go :)
  4. Jan 5, 2012 #3
    Einstein thought of time as linear
    meaning it constantly goes forward but never back meaning at some point there will be an end to time... right? so why not an end to time-space or space in general?
    time would not stand still in a black hole more like slowed dramatically if a black hole can pull then there is movement
  5. Jan 5, 2012 #4

    Simon Bridge

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    Hello ZenZero - welcome to LQ.
    Einstein did not think of time as linear - he thought of it as a dimension of space that could be curved and looped and ... well, pretty much anything that space can do.

    Remember that time is relative - time proceeds at the same rate as always for the observer falling into the black-hole and past the event horizon (though how the clock handles the shift to a different time axis...) A distant observer sees the falling clock slow down (and turn red) ... so there is no sense that "time gets slowed down" at all.

    If you are interested in the endpoints for time, have a look at Stephen Hawkin's lecture series as well as Jim's page in the link.
  6. Jan 5, 2012 #5
    Time as was proposed by Einstein in SR following on to GR is fourth dimension , this among the rest of the three spatial dimensions form 'space-time' now according to him time is relative i.e it's imaginary.In other words no one clock is better than the rest, each are right in respect to their reference frames.For every change in (x,y,z) co-ordinate we irrevocably shift the time dimension. I don't think Einstein believed in linearity of time or space he believed in something accelerating the universe that was coined as "cosmological constant" by him , which now has been proved to be dark energy.
  7. Jan 5, 2012 #6

    Simon Bridge

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    Not "imaginary" ... just not absolute.
    All the space dimensions are relative too - would you call them "imaginary"?
    ... that's better.
    ... ahhh, this makes no sense in terms of space time - we feel constantly and involuntarily in motion in the time direction even though we can control movement in the other three. That what you are trying to say?
    The concept of the UNiverse accelerating makes no sense - what is it accelerating into ... Einstein proposed the cosmological constant as a tweak to GR which adjust the rate of expansion predicted in the theory when it was applied to the Universe. He felt that the Universe was stationary - i.e. it had zero expansion.
    ... do you have a reference for this startling statement? I know I missed some conferences...

    "Dark Energy" is part of a model which hopes to explain the observed Hubble-shift in terms of a large amount of unseen matter. The cosmological constant is a mathematical tweak whose value allows GR predictions in cosmology to match what is observed. The two ideas are related of course. I don't think we can confidently say that it has been proved that the measured cosmological constant is a result of dark energy ... there are other models.

    I hate being pedantic - it's just that there is a lot of messy language around relativity and extra dimensions and so on which confuse the unwary. It must be especially difficult when English is your second language.
  8. Jan 6, 2012 #7
    Einstein added the cosmological constant to GET RID OF the acceleration of the universe that manifests in the equations of general relativity. However, this discussion seems to be a whole lot of discussion of Einstein. Einstein was brilliant but "Einstein" and "Our current understanding of GR" are very different things. Much of the results of GR that we know and love were not found by Einstein and much of what Einstein did find in general proved to be incorrect. In pop science there's this irritating tendency to lump all of modern physics as "the stuff Einstein did" including enormous swaths of knowledge that he didn't have the slightest bit to do with. What "Einstein thought" is irrelevant, he died in the 50's and by then he'd really already been out of the game for a while anyway.

    This pop sci perception that physics from 1905 to his death in the 50's was the "Einstein show" is simply completely false, in many ways people like Wheeler and Thorne understood GR better than Einstein ever did and Einstein bowed out of the development of all of Quantum Mechanics in the early years and now you're asking about open problems that only emerged in the last decade, over half a century after he died.

    So be careful, "what Einstein thought" is a historical question, "what does GR say" is a physics question.

    Also, GR does not in any way place an arrow on time (in "Einstein's thinking" or otherwise). At the end of the day in GR you get an equation which tells you the motion of a particle, this equation has a variable t in it. One does not "advance in time" from some initial time, one simply picks the value of t one wants and plugs it in, there's complete freedom to evaluate the equations for t's past or future (where the present means the t corresponding to the system being in the state it's in right now).

    This is a general property of essentially all the equations of physics, time (t) is just a variable that can take any value, nothing about it "advances" in time.
  9. Jan 6, 2012 #8
    Sorry for my ill use of certain terms, I should have been more concise , to the point. Perhaps my ppost came as dramatic to some but let me clarify that I had no intention of spreading misconceptions.

    I am a first year undergraduate just as any.other users am here to expand my understanding.

    I recall having.read an article which spoke about the existence of hypothesized dark energy, as far as relating it to cosmological constant goes it was one of the users on here who had posted that some time ago. I should refrain from posting over issues that I don't have a good understanding of ( in this case it was partially the reason).
    Last edited: Jan 6, 2012
  10. Jan 6, 2012 #9


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    I'd like to comment on the entropy vs time issue. The current understanding is that entropy is not a property of a microscopical physical system, but rather a property of the macroscopic *description* of that system:

    Here "microscopical" does not mean that the system is small, it just means that you are looking into it in all details which are to know about that system. E.g., If you describe ten cubic meters of an ideal gas as the time evolution of 6e24 individual atomic positions and velocities (and orientations if polyatomic), that is a microscopic system and this description does not have entropy. If describing the same system as "p = 1 bar, T = 273 K, V = 10 m^3" that is an macroscopic description, because there are countless possible ways of realizing that macroscopic state (thermodynamic variables) as microscopic states (positions and velocities of all particles), all of which behave identically on the macroscopic scale. And you do not know (or care) which of those microscopic states is actually realized. In practice, thus, the macroscopic state is effectively described as an ensemble of all microscopic states which are equivalent, and the lack of information about the concrete microscopic realization is encoded as entropy.
    But this is an *description*, an approximation, so to say. Entropy is thus property of the macroscopic description of the system, not of the system itself (a concrete blob of gas *does* have positions and velocities of all particles, even if all you know about it is p = 1 bar, T = 273 K, V = 10 m^3).

    There is little reason to believe that the universe as a whole is not in a microscopic state, even though that state would of couse be intrinsically unknowable (and let's not start with quantum mechanics and general relativity and stuff). In that case, speaking about the "entropy of the universe" would be just wrong. More concretely, that hypothesis is that, like in a gas, at some point the universe would become a uniform blob of particles uniformly distributed and with uniform energy distribution. (Because that is what happens in a gas if you leave it long enough!). That hypothesis, unfortunatelly, forgets to mention that not all mechanical systems do that. There are certain prerequisites for that to happen, and it is not clear whether the universe as a whole has them. If I remember correctly, pure Coulombic systems made of point particles (one could view planets, suns etc as such to a first approximation) do not necessarily lead to spatial equilibration.
  11. Jan 6, 2012 #10

    Simon Bridge

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    Not at all ... if people only spoke up when they felt they had a good understanding ot the topic, there would be no scientists and a lot more wars.

    A critical mindset is important in science with the drawback that these text-only discussions can look a lot like people are ganging up on you telling you not to be stupid. The way to read them is in a kind-of matter-of-fact voice - nobody is being critical of you.

    It does not actually matter how well you understand your subject in science, your ideas will be treated with this kind of critical analysis. You can never escape this. The alternative is to pursue some art where people are routinely nice to each other.

    But I don't think you've done anything wrong - you even phrased your ideas as questions: which is the right thing to do. You'll never run out of questions, and any worthwhile idea in science is "able to be questioned"... if you can't question it, you can't investigate it.

    Now you've learned a bit about how your use of some terms will be interpreted - it made you look like you had some common misconceptions. It is likely you do have some, there is no shame in this - you are new. The only shame would be in not learning.

    You've also learned a bit more about how the concepts you are interested in are viewed and how they do and do not relate to each other.

    What you do now is have a think about what has been said and try to relate it to other ideas that you encounter. I'd also recommend finding a course, maybe just online, on "critical analysis" or "critical inquiry" ... everyone in science has some training in this, and it will help you to understand the responses you get.
  12. Jan 7, 2012 #11
    Thanks for the wise words. I will definitely look into that.
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