IM Egdall

Spyman and Zarnaxus already gave us the answer. If you add up the densities given by Spyman, they add up to 1. To summarize, the visible universe as we currently understand it is made up of: Ordinary matter - about 4%. Per Zarnaxus, it includes: 3% Free Hydrogen and Helium, ~0.5% Stars, ~0.3% Neutrinos, ~0.2% Space Junk Dark Matter - about 23% Dark Energy - about 73% Now the density figures that Spyman gave us refer to the percentage critical density. The critical density is the overall mass/energy density value of the universe which gives an overall zero spacetime curvature, aka a "flat" universe. In metric units, this critical density is about 10^^-23 grams per cubic meter. The currently most accepted explanation for why our visible universe happens to have just the right overall density value for a flat universe is Inflation theory. See link: http://en.wikipedia....tion_(cosmology)

There are all kinds of particle decays. Heavier fundamental particles are unstable and decay (transform) into lighter particles. But I believe conservation laws do not allow a matter particle (lepton or quark) to decay completely into massless particles (guage bosons) like photons or gluons. See link: http://hyperphysics.phy-astr.gsu.edu/hbase/particles/parint.html

This is a tricky subject. But I'll try. Per E-mc^2, mass and energy are equivalent. So like mass, energy has weight. And since fields have energy, they have weight too. If you measure the rest-mass of all the fundamental particles in your body (quarks, electrons, gluons, photons), it accounts for only a small amount of your body's weight. When you weigh yourself on your bathroom scale, most of your weight is due to the strong and electromagnetic force fields between the quarks in your body. To quote: . . . 90 (to) 95 percent of the mass of matter as we know it comes from energy. We build it up out of massless gluons, (massless photons), and almost massless quarks, producing mass from pure energy. That's the deeper vision. - Frank Wilczek Frank Wilczek, E = mc2 Explained, NOVA Science Programming on Air and Online.

Cool!

I think it goes like this. Both matter and energy do not "have mass". Matter- particles (like electrons) have mass. Photons are energy particles which have zero mass. So in the above example, a zero mass (no matter) photon spontaneously transforms into two matter particles such as electron-positron. And yes both matter and energy are a source of gravity; the presence of either causes the warping or bending of spacetime.

Oy! The reason we do not see a center is because there is no center. This may violate your intuition, your assumptions, and your common sense; but this is what general relativity tells us about the universe.

Thanks for the reply. Maybe some day we'll have a new theory which gives us a deeper meaning to all this. But for now, what physicists understand is that this uncertainty is inherent in nature itself.

I'll try. An example: A photon can and does spontaniously transform into an electron and anti-electron (positron). But only if the photon has enough energy. How much? We have to use E=mc^2 to figure it out. Add up the masses of the electron and positron. Multiply this number by the speed of light squared. The resultant number is the amount of energy that the original photon must have. Any less energy, then it does not have enough to transform into the two matter-particles. And the greater a photon's energy, the greater its frequency. So here only very high frequency photons (I think gamma rays) have enough energy to transform into electron/positron pairs. Notice that the total electric charge before is zero (a photon has zero electrical charge). And the total electric charge after is -1 (electron) plus 1 (positron) equals zero. This obeys the rule that the charge before is always equal to the charge after. There are also other conservation laws governing these transformations. Hope this helps. For more stuff on particle transformation (decay), see The Particle Adventure http://www.particlea...ecay_intro.html

I think physicists care deeply about "how the game might work". There are many interpretations of quantum mechanics which try to explain the deeper meaning of the mathematics. It is a very difficult task. No one has come up with a deeper explanation which all can agree on. Using the probabilistic (statistical) approach, quantum mechanics works superbly in making accurate predictions. It tells us that there is an inherent uncertainty in nature itself. Physicists are not playing dice with nature. Nature itself is playing dice. For example, no one knows exactly what a single electron will do. We can only predict the probability of what it will do. But why nature behaves this way remains a mystery.

DrRocket says it best. Who cares what you or I or anyone else believes. General relativity makes numerous specific detailed predictions which are verified by measurements and astronomical observations. That is why we know it is deeply connected to reality. That's why when we read that "there is is a 4-dimensional Lorentzian manifold without boundary -- spacetime . . i.e. there is no edge (to the universe)," we have to take it seriously. I think it was Richard Feynam who saids something like "Nature doesn't care whether you believe her or not, that's the way She is."

I agree that coins on an expanding balloon analogy for an expanding universe has its limitations. But it has its merits too. When explaining it, I try to emphasize that the surface of the balloon represents our universe and only the surface. And the coins represent the galaxies and the coins/galaxies do not expand. I ask my students to try to imagine being on one of those coins on that expanding balloon surface with no ability to sense outside that surface. From this point of view, all the other coins appear to be moving away from you. And the further away they are, the faster you see the coins move away (Hubble's Law). And there is no center to the surface of the balloon. And if you travel along the surface, you will never come to an edge. So this is an analogy for our universe having no center or edge. As was pointed out - not a perfect analogy, but I do not know of a better one.

Antimatter is produced naturally in certain ordinary matter collisions. For example, steer electrons (at very speed) to collide with large atomic number atoms. See quote: "The main idea in making antimatter is just getting enough energy in a collision to allow the particles to be made. If you get electrons going fast enough and throw them at a piece of material called a target, preferably made out of atoms that have a large atomic number, you will have a shower of electrons, positrons (anti-electrons) and photons" More details in link : http://van.physics.illinois.edu/qa/listing.php?id=1172

Feynman's Quantum Electrodynamics (QED) has made predictions on the behavior of subatomic particles which have been verified by numerous experiments to extraordinary accuracy. Based on all this empirical evidence, QED is a triumph, a superb theory. What can be confusing is that there are different approaches to quantum theory, and they give different interpretations as to what is happening. For example QED represents electrons as particles. But these particles have a "phase" and the probability of a single electron arriving at a detector is calculated by considering the sum of all possible paths of that electron (and the phase for each path). Weird, huh! And in Quantum Field Theory (QFT) an electron is considered a probability wave as it travels from place to place - the so-called wave function. This wave funcvtion then collapses once the electron is detected. So the electron travels like a wave and is detected like a particle. But exactly where the electron is detected cannot be predicted; only the probability of finding the electron at a given location. Both models, QED and QFT, are extraordinarily accurate, and give identical predictions. Both apply to all subatomic particles, atoms, and molecules (and the objects they make up.) They both tell us that a single electron or any other particle travels through both slits in the double-slit experiment and interferes with itself. And this is just what physicists see when they do the actual experiment.

Try reading The Fabric of the Cosmos by Brian Greene. I think you may like it. He talks about the nature of time and physics. For one thing, he points out that per special relativity, time is not like a flowing river but more like a block of ice. It has to do with the fact that time is not the same for everyone - it is affected by relative motion. Events which happen "now" for you may have already happened or haven't happened yet for another observer in motion with respect to you. So in this sense all of spacetime exists (like a frozen block of ice). As Greene says, we think of all of space as really being out there- we should think of all of time as really being out there, really existing too. It is fascinating stuff, and gives a whole new view of what time is.

Universe is "almost" flat? Is there a link to this info?

Physics is so much more than a "guessing game". Just because there are certain aspects of reality we do not yet understand does not mean we understand nothing. There is a wealth of knowledge gained from quantum mechanics and general relativity. Detailed predictions of both theories have been verified by numerous experiments and observations over the past century. Quantum mechanics gives the theoretical basis for the great technology revolution of the 20th century. And general relativity is the basis for the first theory of the creation and evolution of the universe supported by scientific evidence - the Big Bang.

Fisrt of all, no one has yet been able to directly detect gravity waves. It is a very difficult measurement. But say some day (hopefully soon) they are detected. An optical image works by taking light rays from a point in an object and making them come together (focus) to make a point on the detector or film. Each adjacent point in the object also has to be focused to be in the corresponding adjacent position on the film to produce an image. A lens does this job. I don't know of any way to focus gravity waves to produce an image. Perhaps an after the fact post processing might help here.

When you feel the "solid" tabletop with your hand, you are really sensing the electromagnetic forces which hold the molecules at the surface of the table together. Your hand's molecules are also being held together by electromagnetic forces. And the so-called "particles' which make up atoms and molecules are really matter fields spread out like waves. They don't act as individual particles until they are detected. So "solid" is an illusion, albeit a very convincing one.

But physics has a surprisingly difficult time explaining why there is an arrow of time. The equations of physics work the same in the plus time and negative time direction. Brian Greene has a great discussion about this in The Fabric of the Cosmos . Apparently, the only way to define an arrow of time with the laws of physics is to apply "entropy" or the disorder in a system. For example, your egg is more disordered once it is scrambled -- so a forward arrow of time. But this is a statistical rule involving probabilities and only works for lots of constituents (e.g lots of atoms and molecules in the egg). So the arrow of time is still somewhat of a mystery. See link: http://en.wikipedia.org/wiki/Arrow_of_time

Just started reading Brian Greene's new book The Hidden Reality. He says that according to inflation theory, there are likely other inflation-caused universes beyond ours. And we are just one bubble universe in a number of these universes. Plus from our perspective inside our bubble universe, it appears "as endless space". But someone outside our bubble universe sees what "appears to be an endless time." (He explains this on pp. 66-71) . And unlike string theory and other quantum gravity approaches, inflation does have substantial supporting evidence . But it is still a work in progress. I'm still trying to digest all this, so all I can say is WOW!

Ah, a non-rotating black hole has a point singularity. If you now rotate this point around a non-zero radius, you get a ring with zero thickness, not a disc. Is it that simple? And I think these so-called singularities are suspected to not exist for real black holes. But physicists need a theory of quantum gravity (combining quantum mechanics and general relativity) to make a definitive prediction. No such theory has yet been validated by experiment.

Do you have a link to this report?

History is full of examples of new scientific ideas fighting for recognition. Einstein's relativity and his proposal that light behaves like a particle as well as a wave were not accepted until there was supporting evidence. His general relativity was almost completely ignored at first. Gamow's work on the big bang theory was pretty much forgetten until the discovery of the cosmic microwave background that he predicted. Zwicky was ignored when he first proposed "dark matter". And so on. I don't know if it's better or worse now than in the past, but new theories are generally suspect until evidence is found which supports their unique predictions.

Take a look at page 2-9 to 2-10 of the link below. It gives an example of space being stretched by the presence of mass/energy: http://www.eftaylor.com/pub/chapter2.pdf

Assuming this wormhole can really go from one universe to the other (a huge assumption) and assuming this other universe, whatever this is, obeys the same physical laws as ours, then I think the rate at which time passes in this other universe would depend on how much mass and energy there is in this universe.