Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

Newbie to this forum with a silly question

  1. Jan 21, 2008 #1
    First of all I would like to say I'm a total noob here, I don't have a single math nor physic degree (I'm a computing guy) and I only know English from high school as a foreign language so I wish to apology for any kind of nonsense I could write here :P.

    Well, there is something that bugs me for a few days.

    - As far as I understand time is a relative dimension and is affected by the velocity.
    - The speed of light, beyond which time is messed up, is impossible to reach under normal condition because at this speed our weight would be multiplier to infinity and we would need an infinite force to move.
    - However an object falling in a black hole may find itself an unlimited force pulling it in (gravity)
    - And, assuming stellar objects are pushed away from each others at an ever expanding speed, won't that reach at some point and unlimited value too (considering there is a force, proportional to the distance, pushing objects away from each others).
    - In these two contexts, matter (or what remains of it, whatever it could be) would go back to the origin of time, won't it? That would be the Big Bang right?
    - Assuming (and I would tend to believe it) that in those conditions information cannot be maintained it would be impossible to influence the big bang, avoiding therefore the universal censorship (causes cannot be modified by their consquences).

    Then, is that possible our universe is ever recreated, I mean the big bang being eternal and constantly fed from matters (or what remains of it) comming from our future? (assuming we can speak of past and future). That would imply time cannot be draw as a straight line but more as a curvature too.

    Am I speaking nonsense and should I hide in shame?
  2. jcsd
  3. Jan 21, 2008 #2


    User Avatar
    Science Advisor
    Homework Helper

    And, thinking about [itex]F = m a[/itex], you think: "why then wouldn't there be an infinite acceleration?", right?
    Basically, in relativity, that formula is modified to [itex]F \propto \gamma m a[/itex] where [itex]\gamma[/itex] depends on the velocity. For very small speeds (let's say, less than (speed of light)/2) it's nearly equal to 1 and we don't see the difference with the original law of Newton. But as the velocity comes closer to the speed of light, [itex]\gamma[/itex] goes to infinity. Therefore, infinite force does not imply infinite acceleration, and the velocity stays sub-c.

    Stellar objects are not pushed away from each other, in fact, they attract (gravitational law, [itex]F = G M_1 M_2 / r^2[/itex]). There is no "force" pushing them apart, instead, the space in between is expanding. You can consider the stellar objects as dots marked on the surface of a balloon. If you blow up the balloon, the dots get farther apart, though you are not actually doing anything with them --- you're just increasing the space in between. Because there is not really a "force", the above reasoning does not hold and they can go faster than the speed of light (in fact, they seem to be doing just that from our point of view). This is no problem though, as this cannot be used to transmit any information.

    Finally, it will not "reach an unlimited value". Consider the sum
    1 + 2 + 3 + 4 + ....
    It will never "reach" a value (say, you add the last number n and claim the result until then is the final value; then I'll add n + 1 and prove you wrong); rather, it will always keep growing. You might say that the sum "approaches infinity" but your formulation which supposes some "final state" does not apply. Same with the expansion of the universe (although there is a theory, that the expansion slows down, eventually comes to a halt and even reverses into a collapse of the universe; until the entire universe is again shrunk incredibly small and a new Big Bang can happen --- too bad we will be reduced to a heap of quarks and not able to record it :smile:).

    You should never do that; actually you asked a sensible question, too bad it doesn't work out. It's interesting stuff and worth thinking about. Centuries ago people who asked "Could the earth be more like a sphere than like a plane" were made to hide in shame...
    Last edited: Jan 21, 2008
  4. Jan 21, 2008 #3
    Well, I've got no to little knowledge in these stuff. I didn't have the opportunity to get a good education (only did casual school and a computing degree, that's all) and I'm certainly out of place here anyway. Anyway, I thought at a point black holes caused a time warp. And I thought information would be lost in. I kind of like the idea of an everrecreated universe self fed (much like Uroboros). I know of the Big Crunch theory, I heard of it when I was a teenager and liked it, however it seems to me, now, to be a too simple answer to a too complicated problem (maybe I'm just being too full of myself here though).

    In all cases, I'm affraid I've got little things to say now. Eventually if my mind "farts" again I'll repost here or anything else but I doubt so.
  5. Jan 21, 2008 #4


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Hi Matthieu,
    you have a kind of complicated tangle of questions here so I would like to ask you to do something.

    A. could you say, in just a few words, the simplest question you have. I mean something that is a real question, that you don't know the answer to. But really straightforward. Maybe even a Yes or No question!

    B. once you have the simplest question clearly formulated, then if you want tell us briefly one other question which you think is more complex or difficult to answer.

    Try to phrase it briefly and clearly. Actually if you just want to do A that is fine, one question is plenty for one post----asking a lot is permitted of course but it isn't necessary.

    Part of this is curiousity and laziness on my part. I am curious to know what people are wondering about but I don't like to work hard to figure out what the question is.
  6. Jan 22, 2008 #5

    Lol, I've tried to be as simple as possible but let's start again.

    As far as I understand behind a black hole's horizon our physic laws get messed up. I was thinking, although this seems wrong, that "stuff" could get back in time. By stuff I mean what remains of infalling objects after the treatment they got next to the black hole. I am one of these who believes information is not maintained within a black hole's singularity. Therefore, wouldn't that possible this "stuff" go back to the beginning of time,the "big bang"? Wouldn't that imply that this "primordial stuff" that the big bang was made was comparable to "stuff that fell in a black hole" too? That way our universe could be constantly re-creating itself and our distant future (what we will be after falling in a black hole) is our distant past too (what we originated from), that implies "de facto" information cannot be maintained within a black hole to avoid breaking causality (we can't influence our "part" by altering our "future", assuming we can talk of past and future anymore).

    I know it's very certainly wrong but you have to think of a "shape of time". I mean we talk of the "shape of the universe" (flat, closed or opened) so why not a shape of time? That if we drew a line representing time to unlimited values it would always meet its beginning.

    The babling on the expanding universe is obviously wrong, please ignore it :).
  7. Jan 22, 2008 #6
    This made me wonder something. If I draw two dots on an inflating balloons, their size (occupied space) increases proportionaly to the dilatation of the balloon and therefore the space between them. How comes such does not happen to matter occupying space-time?
  8. Jan 22, 2008 #7


    User Avatar
    Science Advisor
    Homework Helper

    I'm afraid I can't really answer that question; just offer you the warning that analogies are very useful to bring a point across, but should not be extended too far. The point of the analogy I gave obviously is to explain how one can view the expansion of interstellar space and related questions, for example why there is no preferred position (like, a center all things move away from) and why this is not violating special relativity. The dots on the balloon are to be seen as dimensionless points (in the mathematical sense). As to why space expands on intergalactic scales but not the vacuum inside matter (or at least not all at the same rate) I cannot tell you - this is actually outside my field of study as well :smile:
  9. Jan 22, 2008 #8
    ^^ Well, thank you very much :)
  10. Jan 22, 2008 #9


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    galaxies are gravitationally bound structures.
    so is our Local Group of galaxies---it is a cluster.
    a steel ruler is bound by crystal lattice bonds, forces between the atoms.
    these determine the correct spacing.
    likewise the solar system is a bound structure.

    these things do not change size along with the largescale distances

    what is called expansion of space could more accurately be called in INCREASE OF LARGESCALE DISTANCES----distances between widely separated objects which are not part of some bound structure like a cluster of galaxies.

    Astronomers when they talk about the balloon analogy do not usually say to draw galaxies on the balloon. What i hear them saying is GLUE PENNIES on. Then as the balloon expands, the pennies get farther apart, but they stay the same size.
    the pennies are analogs of whatever bound structures, like clusters of galaxies or lone galaxies, or whatever.

    part of the message of GR was that you should not expect distances to stay the same.
    our experience is of distance on earth which are bolted firmly to crystal lattice bonds of rock and metal, so we get the wrong impression. outside of our local environment it is natural for distance to change----the metric is dynamic---that's curve spacetime for you
  11. Jan 22, 2008 #10
    Yes, it's the image of the spots drawn on the balloon that confused me. Thanks for clarifying it :). However I kind of gave up on the idea that expansion could be triggered by a force. I would like to hear your opinion on the black holes thing please :).

  12. Jan 22, 2008 #11


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Matthieu, you asked for some response to what you said about the black hole making a new spacetime region or a new edition of the universe.

    that is a fairly common idea. Smolin has a book about it called the Life of the Cosmos,
    and then it came up again in a book last year called Universe or Multiverse?
    and it will come out again in 2008 in a book called Beyond the Big Bang

    you can find these books on Amazon searching by title, except the one that is not out yet

    if you cant find them for some reason, ask me

    as well as books there are a lot of articles written about the idea

    but you have the idea mixed up with some concept of time looping back and typically when the idea comes up it just has time continuing forward but branching so that there is a new region of spacetime---a new line of development for the universe

    be cautious too, because all that stuff is very speculative.

    the only reason Smolin's idea has gotten the attention it has is that it addresses some recognized problems in physics and offers a possible solution----and a way to TEST the idea and reject it if empirical observation shows it wrong.

    this is better than just pure speculation

    if you have an idea, it is advisable to formulate it in some way that it can be put to the test of observation or experiment
  13. Jan 22, 2008 #12

    Thank you very much, I'll look for these books by the end of the month (paycheck, we all have to live).

    Unfortunately, I am totaly unable to make a mathematic formula of this. I just don't have the skills for. Someday I will try it eventually (or maybe not).
  14. Jan 22, 2008 #13


    User Avatar
    Science Advisor
    Gold Member
    2015 Award
    Dearly Missed

    Wait! don't BUY the books. I just wanted you to know that something like what you are talking about is semi-acceptable. Gets published in scholarly books and all that.

    If you actually want to READ some about this, a lot is available FREE.

    Academic research writing is largely free online at least in physics and cosmology.

    The Smolin book you might have to buy or get at the library, but he also has free online technical articles.

    Let me know if you can read sort of dry technical stuff and if you want I will post some links.
  15. Jan 23, 2008 #14
    It depends on what you call "dry" honestly. I can always try at least.

    Anyway, what I was implying wasn't exactly on multiverse. I'll try to devellop what I meant with a short story, "the story of some stuff".

    - We start at the big bang. From this thing that forms the big bang we see the universe being given birth. Within the universe there is something we will call "some stuff". What it, planet, star or whatever it could be and become, is not so important (it's just an example).

    - The poor stuff at some point get close to a black hole and is sunk inside. It's information is totaly lost so would that stuff be conscious or motivated by some will is irrelevant.

    - The poor little stuff is crushed/more than atomised/whatever you want and now let's imagine it returned to its primordial status. What was the big bang formed of, and through time, distortion beyond the event horizon, returned to the "moment of the big bang".

    - Back to 1, the same events happen again in the exact same order with the same incidence. That wouldn't be another universe but our again.

    It's not exactly a recuring universe thing I was implying but and ever-recreating one. The concept of recuring universe implies "past and future" in that the big crunch/recuring universe model means the contraction happens after the dilatation to resume the dilatation again. Here, there is no future nor past, what forms our past is actually our future and vice-versa if you talk in human concepts (which would be as relevant here than up and down in the universe).

    That means that if we could draw a line representing time, it would not only never end but join itself back at its begining at some time.
  16. Jan 23, 2008 #15
    I just have one question though, when we say "dark energy is pushing stellar elements apart" we talk of that expanding balloon model with fixed dot?
Know someone interested in this topic? Share this thread via Reddit, Google+, Twitter, or Facebook

Have something to add?

Similar Discussions: Newbie to this forum with a silly question
  1. Newbie to the forum (Replies: 4)