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A few questions =] (time and big bang)

  1. Feb 13, 2008 #1
    I've read a few things about time, and some of them seem to contradict so I would like to sort out what is true and not.

    1. Does time have to coexist with matter, or can time exist without the presence of matter?-[finished]

    2. "If we take the smallest length and divide it by the fastest speed, we get the time it takes for the fastest thing to travel the shortest distance. (light in plank's length)" Is this true? Does this not take into account the expansion of the universe, as some things recede faster (although this may not be the right word) then light, or does the expansion of the universe not happen in time itself and is independent of time (if this is true then the word would not be speed because speed is d/t and if it is independent of t and you can't have 0 then it is awhole other term all together)-[finished]

    3. Is time a dimension or just a way of thinking to compare events?-[finished]

    4. Is there a smallest unit of time (#2)-[finished]

    5. Does matter have to exist in time (is matter dependent of time)-[finished]

    6. (In #2) Does the expansion of space happen in time?-[need to reword]

    Ok onto the big bang

    1. What are the current hypothesis on what was before the big bang?

    2. What are the current hypothesis on what started the big bang?

    3. I understand space is not matter so how could it be described (although I think it is what is is, nothing but a displacement, but just clarification)

    4. How does gravity repel ( I heard that under extreme density conditions it can)
    Last edited: Feb 13, 2008
  2. jcsd
  3. Feb 13, 2008 #2


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    There are lots of qualified people who can answer your questions, including some working astronomers like Wallace. I will just respond to these four bigbang questions
    Logically, #3 comes first. What is space? Space is nothing but its geometry---the web of geometric relationships between events. To say that space expands means only that on average distances are increasing. The classic theory of gravity (1915 GR) is a theory of the metric: the distance function which tells us lengths, areas, volumes, angles, curvature etc.
    A quantum theory of gravity---in other words a quantum theory of spacetime geometry---must turn these things into quantum observables. The area of something must be an observable. Also the spatial dimensionity near something (the relation of radius to volume) must be an observable and may depend on scale. The sum of the interior angles of a certain size triangle must be an observable.

    Classical space is classical geometry---epitomized by the 1915 General Theory.
    A more fundamental description of space will be a quantum geometry. Therein all geometric measurements will be quantum observables and subject to the usual uncertainty relations that govern all observables.

    This means that at very small scale the geometric structure of space will be highly uncertain and chaotic----the dimensionality may vary with scale or magnification---it may resemble a foam or a fractal. The more precisely you try to pin down a location the more violently the curvature may vary. And so on.

    But at larger scale all that will smooth out. This is typical of how quantum theories work.

    #4 How does gravity repel? Basically all I can say is that when various experts quantify gravity in the simplest most straightforward way they know how they get some quantum correction terms in the Hamiltonian.
    This typically happens when you quantize anything. You get quantum correction terms. Under some conditions these become large, and then there are quantum effects.
    In the case of quantum cosmology, they keep finding that these effects become dominant at a density of about 80 percent Planck.

    You know in routine quantum mechanics a particle resists being pinned down. If you try to pin down the location, the momentum goes wild and becomes very uncertain. Well apparently GEOMETRY resists being pinned down. At a certain degree of concentration it kicks back. The result, in these cosmological models, is a bounce.

    #1 and #2, the big bang just means the start of the current expansion, so you are asking about what various models have prior to the start of expansion and what they have as causing start of expansion

    Basically that is the main focus of the research area called Quantum Cosmology and you can get a good overview of the most significant recent work in QC if you just use the Stanford Spires search engine. Set the sort preference to sorting by Approximate Citation Count so that what comes up first are the papers that are the most cited by other scholars. These will normally be the papers with the most impact. I would say stick to RECENT work, say date > 2005. A lot of the earlier stuff is obsolete.
    Go here
    Type in
    Select "approx. citation count" for the sort, and press search.
    You should probably do it yourself to get practice, but this is the result:

    This is a keyword search. If you look at the highly cited quantum cosmology papers that Spires gives you, typically what you find is the cosmological singularity replaced by a bounce, occurring when the density gets around 80 percent Planck. In other words, expansion is preceded by collapse. A prior contracting phase.

    But if you look around at some of the less cited, or older papers you can also find considerable variety like clashing branes and eternal inflation bubbles and so on.
    Last edited: Feb 13, 2008
  4. Feb 13, 2008 #3


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    I'll have a quick stab, but a word of advice. You suggest you could have made separate threads, but thought better of it. In fact it is always better to keep a thread on a single topic rather than 9. Maybe space them out a bit so you can discuss each topic.

    The alternative (and even better) approach is to use the search function. Most of your questions have been discussed already at length in previous threads. Don't worry, you're not alone in neglecting this but you might find some of the old threads interesting.

    Anyway, here's my brief answers:

    There is no way to measure the passing of time without some kind of device or thing. Generally if you can't measure it Science can't say meaningful things about it. Therefore this is a kind of 'tree falling in the woods' question.

    The Planck length is tiny (muuuuuuch less than a millimeter) whereas you need to be Billions of kilometres from another object in order to see the Universal expansion. Plus space does not expand in the 'real' sense that you are referring to here, so it wouldn't be a problem anyway. See previous threads for discussions of the misconceptions surrounding expanding space.

    It is a dimension, different in nature to a spatial dimension but none the less a dimension.

    Yes, the Planck time, it is made in a similar way to the Planck length and it extremely short.

    Again you couldn't measure the existence of matter if there was no time, so again this question is outside the bounds of empirical science. None of our theories work without time so we could give no sensible answer to the question.

    Again, check the threads on expanding space or ask for some more info. Space does not *really* expand.

    Okay, I'm done for now. You really really should consider editing your post and splitting this into more threads, otherwise this will get very messy.
  5. Feb 13, 2008 #4
    To answer the first question:

    Time can exist without matter. Why? because time measures change, and you can measure change in things that are non material in nature such a photos. The alternative question to ask would be, 'Can time exist without matter or energy' in which case my answer would be no. There would be nothing to measure, and so the concept of time, which is how we perceive change would not exist.

    [and yes, I agree with the above post, if you split things up, it's more managable!]
    Last edited: Feb 13, 2008
  6. Feb 13, 2008 #5
    Thanks for the replies. The questions with meaningless answers were just there to clarify for me that they were indeed meaningless, if that makes sense =].

    With the search button, I looked for a search button last night but couldn't find one, so I suppose I need to look a bit harder, but I did read back quite a few pages but I because worried of the accuracy of the ideas because of the age of the threads, so I restated alot of questions so as to make certain of the accuracy, I ought to have checked the first page though, but I wrote this in 5 minutes before going to the gym =P.

    I will try and clear out some of tstuff to clean things up, I've been told of in other forums for having 2 threads at once, so I wasn't sure as to this =]

    Ok I think big bang stuff is just left

    Thanks for the explanations and I will be sure to read some of those papers marcus, but I need to think about alot of the stuff for a bit as I know hardly owt about this sort of thing, especially quantum stuff, but that's why I'm here I suppose =D
    Last edited: Feb 13, 2008
  7. Feb 16, 2008 #6
    Ok so I think I get a few sort of ideas of this. Wallace, you said space does not expand in the sense I am thinking. I wasn't thinking as an explosion or anything, but I was thinking of it as, not matter, but a something that could physically expand. Is what is meant by expansion the distance between things expanding due to the point at which there was the repulsion of gravity? Is that about right, I'm still having trouble exactly getting my head around the repulsion bit. So when things were repealed, did they continue to be repel or was that it and then an incredible momentum continued to allow things to expand. I'll stop here in the case that I am going in completely the wrong direction.

    Also marcus, you said 80% plank, what exactly is meant by plank, I have heard of things like planks constant, or planks law, but not just plank.

    Thanks for all your help, and hopefully I'll finally get it =]
  8. Feb 16, 2008 #7


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    Planck units are some natural units that are often used in quantum gravity because they are not so artificial as the metric system units and because it can make the equations simpler.
    Like when you set c = 1, or adjust the units so that c has numerical value 1 in those units.
    Or you adjust the units so that c, G, and hbar all have values equal to one.

    Im falling asleep, got to hit the sack. Wikipedia has stuff on planck units
    or they call them natural units. So does the US gov site (national inst. standards and technology NIST) which you get if you google "fundamental constants"

    Planck unit density is one planck mass per planck unit of volume.
    it is around 1093 times the density of water, if I remember correctly.
  9. Feb 16, 2008 #8
    Oh cool thats a pretty simple concept, thanks.

    Forget what I said in my last post, I was being stupid, doesnt make sense atall. Farst off there would be no red shift =\
  10. Feb 18, 2008 #9
    Ok I am back to being confused at how distances between things expand. It is due to the big bang, which was due to the repulsion of gravity. I've got that so far, but I'm still confused about how distances still expand to make the red shift work. Right now in my mind I've sort of got the idea stuck in my head of what I posted a few posts up, but I really don't think it's right, so I would really appreciate if you could help push it back in the right direction.
  11. Feb 19, 2008 #10


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    The simplest answer as to why the Universe is expanding is simply that it did so in the past and therefore does do today. For some reason, that we don't yet fully understand, the early Universe underwent a rapid period of inflation in which some energy field or perhaps some strange effect of gravity in extremely dense environments caused everything in the Universe to accelerate away from everything else.

    For some reason, which again we don't understand yet, this inflationary period stopped after a fraction of a second however the 'push' that it gave the expansion of the Universe caused the rest of the expansion to occur. Just like a ball thrown in the air rises due to the motion imparted by the act of throwing, the expansion of the Universe is simply due to the initial expansion started by inflation. To extend the ball analogy, the gravitational pull of the Earth slows the speed of it down after it is thrown and in the same way the gravitational effect of all the material in the Universe acts to slow down the rate of expansion.
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