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luc

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Posts posted by luc

  1. Do you have any evidence for this, or is this just pointless unscientific speculation dressed up as fact?

     

     

    You should agree with me that GR, and consequently Standard Big Bang Theory, gives an incomplete picture of the Universe, because it fails miserably at the initial singularity. A group of scientists is working in a quantum gravity theory called "Loop Quantum Gravity", that cures the disease of the singularity, and permits that the universe has not a beginning. I'm not a scientist at all, so It would be difficult that I could convince you about the virtues of this theory, but the paper that i think more accurately shares my point of view about the universe is this

    http://arxiv.org/abs/gr-qc/0406042

    "Oscillatory Universes in Loop Quantum Cosmology and initial conditions for inflation"

     

    Notice that in this oscillatory theory, our Universe is supposed to be closed (positive curvature). So you could say: whoaa, it's very clear that our universe is flat, but then you should consider the value measured by WMAP for the Omega of the Universe: 1.02(+-0.02), so the margin of error leaves room for the possibility of a flat Universe, but the main cipher, 1.02, corresponds to a closed Universe. So you wanted evidence, ok, I have none, but I find this theory very attractive

  2. 'kay, but I think that 4-momentum is the same as the energy-momentum 4-vector (at least is Special Relativity is that way). So energy-momentum is the magnitude of the energy-momentum 4-vector, is that what you're saying?

  3. Right now I'm reading Hawking's "The large scale-structure of space time", and he uses constantly a quantity called Energy-momentum, though I can't see it defined anywhere. I've also read in some forum that in GR, total energy is not conserved, but energy-momentum is conserved. I think that perhaps energy-momentum is (Total energy + magnitude of 4-momentum), but I'm not sure. What exactly is energy-momentum?

  4. You would write down the Hamiltonian for all the atoms in the body.

     

    Now for the real question: Why on Earth would you want to?

     

     

    Mmm, to know the wavefunction of a human body for example?

    I think that can be interesting if someday quantum teleportation of a human body is tried

  5. I possess varius textbooks about QM, and in them they always show the Hamiltonian of simple things, like electrons or harmonic oscillators, but I wonder which is the hamiltonian of a more complex thing, for example a human body

  6. If space is part of the Universe' date='then it must be continuously being creating.

    But there is no observable proof and measurement for this theory.They would have to see where the end of space is to see what is happening.[/quote']

     

    Have you considered the possibility that space is not continuously created, but stretched instead? Take like an example a plum cake, while is in the oven: it grows in volume and grows...

  7. im wondering (for a pro evoulution argument) how many planets are in teh universe (obvously a very rough estimate) do we have any idea do we have a minumum number?

     

    Two possibilities: an infinite number or a finite number. If it's a finite number I suspect that is a very HUGE number

  8. I guess that you say that I have to do the partial derivative of each of the 16 entries of the metric tensor. Doing the partial derivative with respect to the 3 spatial coordinates and with respect to time gives 4 results for each entry of the metric tensor, so 16*4=64 are the entries of the resulting 3-tensor field. Is that right?

  9. I'm trying to weave my way through General Relativity, but I find difficult to learn the concept of Christoffel symbols of the second kind. They are defined as

    [tex]

    \Gamma^l_{ki} = \frac{1}{2} g^{lj} (\partial_k g_{ij} + \partial_i g_{jk} - \partial_j g_{ki})

    [/tex]

     

     

    However I really don't know how to do the partial derivative of the metric tensor; I do know how to do partial derivarives of 0-rank tensor fields and 1-rank tensor fields, but I'd be very happy if someone could explain me how to do partial derivatives of 2-rank tensor fields. A link to a page explaining how to do this kind of derivative would also be welcome

     

     

    Edit:It seems that latex is down, but I hope you know what I'm talking about...

  10. There is only one possibility' date=' and it is the actuality. It is not the case that there are multiple possibilities here.

     

    Space is three dimensional Euclidean.

     

    It is mathematically necessary.

     

    Regards

     

    PS: When you use the term 'shape', in the context of the question, "What is the shape of the universe?" do you mean the structure of space, or do you mean something else?[/quote']

    Johnny I agree that space is Euclidean, but an Euclidean 3-manifold can have 18 different topologies, please read the paper

  11. Help me to understand this. If the universe was created from a big bang' date=' how could it be other than spherical? I don't understand how it could be flat.

     

    Bettina[/quote']

     

    if the universe has an infinite volume, then it started with an infinite volume, so it can't be an sphere, because has always been infinite, and always will be

     

    Bettina, you may want to take a look to this paper. It gives a review of ten of the 18 possible topologies that our universe can have

     

    http://www.dushkin.com/text-data/articles/31943/body.pdf

    "The shape of the universe: Ten possibilities"

  12. Actually there's a theory that the universe was expanding faster[/i'] then the speed of light at the big bang, the inflation theory.

    Actually is also expanding faster than the speed of light. Any point behind our Hubble sphere is expanding faster than the speed of light

  13. In General relativity, I don't know of any model in which the Universe has an edge

     

    Actually, the most popular model is the Concordance model, an Universe flat and with infinite volume, so there's no edge. There're other possibilities though, for example that the Universe could have the topology of a hypertorus, but in this case there's no edge either

     

     

    In fact, for a flat universe, that is curvature k=0, there are 18 possible topologies, one of them is the infinite case, other the hypertorus, and 16 more. I don't have the complete list of the 18 topologies, but according to this page

    http://www.williams.edu/go/sciencecenter/center/RS01html/RepSci2001-ASTRONOM.html

    a woman called Joey Shapiro made a thesis classifying these 18 topologies. I'd be very grateful to anybody that can provide me that thesis

  14. To complete the info given in the page that Janus posted, is interesting to know that recent news suggest that Voyager 1 has reached yet the zone known as Termination shock, or is very close to reach it

    VgrHeliosphere.jpg

     

    anyway, it will continue its journey and will cross the heliopause probably around 2020. Is expected that will continue to operate and send back valuable data until at least 2020

  15. There was an urgency to find a reason for the discrepancies of Neptune's orbit, so there was very much buzz when Pluto was discovered, because it could be the explanation. Then it was found that Pluto's mass was not enough to explain Neptune's behavior, so the searches continued. But then the Voyager 2 probe discovered that Neptune's mass had been miscalculated, and with the new mass the presence of a planet X was not necessary

  16. I personally think that the Universe hadn't a beginning, and that the Big Bang simply marked a transitional point from a previous phase. However, how can I know? Maybe I'm in an error? Is there any way to know what exactly happened? I hope so, but we need more observations of the cosmos, and know more thinks about freaks such dark energy and black holes.

    The Ekpyrotic bulk brane scenario postulates that the Big Bang was the result of the collision of two branes (the proper name of this collision is "ekpyrosis")

    http://arxiv.org/abs/hep-th/0502136

  17. 65' date='000,000 years ago, the atmospheric oxygen levels were around 35%, today, they're about 21%. What phenomena caused the drop? Was it simply a huge volcanic release of nitrogen?

     

    Here's the main question. At what percentage would oxygen levels become detrimental to life (think animals and plants) as we know it? 40%? 50%? 60%? At what point would plants be starved of CO2? At what levels would high O2 levels begin to make nitrogen fixation difficult for plants?

     

    Okay, let me narrow it down. Does anyone have a ballpark idea about what the highest O2 levels possible could be and still maintain a healthy ecological equilibrium?[/quote']

    Supposedly, the mere fact of surpassing the barrier of 21% would be very damaging, according to this muslim site

    http://albalagh.net/general/earth_planet.shtml

    "If the oxygen percentage in the atmosphere were greater than 21%, the cells in our body would soon start to suffer great damages. The vegetation and hydrocarbon molecules needed for life would also be destroyed"

    Again, I don't claim any expertise in this field, so will be you who decides if the info in that page is accurate or not

    Regards

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