IM Egdall

quote name='Seventh cavalry' timestamp='1340042139' post='685009'] I finished the book well, because i understood the theory briefly before i read it, i can't understand the old theory that dominated the physics in 1800s the ether wind theory. So can some good people help me understand it ? In the nineteenth century, physicists believed that electromagnetic radiation (like visible light) had to propagate through some kind of medium -- which they labeled the ether. (Just like sound must travel through some kind of medium like air. In a vacuum, there is no sound.) They believed this ether permeated all of space. But in 1905, Einstein declared that there is no ether -- electromagnetic waves travel through empty space. I think the idea is this. Light always travels at the same speed, c (about 670 million miles an hour). This leads to time dilation: From the view of someone inside the moving rocket, the light beam goes from the ceiling to the floor and back in a straight vertical line. But to someone at rest outside the moving rocket, the light beam must take a diagonal path to go from the ceiling to the floor and back to the ceiling. And this diagonal path is a longer path. But the light is still traveling at c. So the light traveling at the same speed takes a longer time to go from ceiling to floor to ceiling. Thus to the person at rest outside the moving rocket, time is running slower on the rocket (than for the person inside the rocket). I hope this helps.

Question: I have read that an acceleration produces only half the bending of light of an equivalent gravitional field. For example, consider the famous accelerating elevator in zero gravity with a light beam traveling across the elevator. An observer outside the elevator sees the light bend due to the elevator's acceleration. But the amount of bending is only half what one would measure for a light beam in an elevator at rest in a gravitational field of the same magnitude. Doesn't this say the Equivalence Principle fails? I'd appreciate your thoughts. Ref: Ohanian, Einstein's Mistakes, p. 226.

Hmmm. Interesting. Recent observations say the expansion of space is accelerating. No one knows what is causing the expansion to speed up -- its been given the name dark energy. If we assume this mysterious dark energy stays the same over time (a big assumption since we don't know what it is) and we assume the universe will exist for an infinite amount of time, then I guess this says the expansion of the univere will reach an infinite rate. Have I missed something here? Comments? .

There is no need to reconcile. Special relativity says nothing (no information) can travel faster than the speed of light through space. General relativity says there is no limit to how fast space itself can expand.

Dark flow? Your words sent me on a google search. I found an article on testing with supernovas that seems to say this dark flow theory isn't right. http://arstechnica.c...k-flow-mystery/ So I think dark matter and dark energy are still the favored theories of current mainstream physics. Comments?

Ya, the "laws" of physics are time-symmetric -- they do not show a direction of time. But the 2nd law of thermodynamics is a different kind of law. It is a deduction based on probability. This is because the odds of a number of constituents being highly ordered gets dramatically lower as the number gets larger. For example, if you have three cards -- Ace, two, three -and you shuffle them, the odds of drawing them in order are one in six. (3! = 3x2x1) But if you have four cards -- Ace, two, three, four, the odds of drawing them in order grows to 24 (4! = 4x3x2x1). With 10 cards, its 3,628,800. The odds very quickly become astronomical. So the increase in entropy or disorder we see as a indicating an arrow of time is just a probability-based argument, due to the very large number of constituents in the macro world. It is not a physical "law" in the same sense as other physical laws. I don't think there is anything new in these arguments.So I think the "possible solution" to the "paradox" is interesting but nothing new.

Ya but no one knows what is causing the acceleration of the expansion of the universe. The most popular theory is the vacuum of space itself is somehow producing this outward push. And physicists model this mysterious "dark energy" by adding a cosmological constant to the right side of Einstein's field equations of general relativity. Or maybe its Einstein's general relativity itself which is wrong at cosmic distances? Maybe there is some fifth force or other idea which accounts for the acceleration of the expansion. I've heard these arguments for a number of years now. The link I gave is the first actual test I have come across which examines this question. And the test results tend to support general relativity. They point to this vacuum energy as the cause. I think this is a very big deal. And not at all boring.

Saw interesting article on testing dark energy and general relativity versus the idea that there is a fifth force affecting gravity out there. Seems like Einstein wins, at least so far: http://spaceandearthsciencearticles.blogspot.com/2012/05/is-there-fifth-force-that-alters.html?spref=tw Comments?

Ya, I knew that -- its obvious. But do you know exactly what theory other than dark matter predicts the critical density needed for a flat universe without the use of dark matter? I need specifics, please.

Say the rocket took off into outer space and returned to Earth 1000 years later, as measured on Earth. This is 1000 years Earth-time for the round-trip. If the rocket traveled at 99.98 percent the speed of light, then only 20 years will have elapsed for people on the rocket for the same trip. This is 20 years rocket-time. The rocket's speed relative to Earth is what determines how much time slows down in rocket time versus Earth time. This is given by the time dilation factor: square root ( 1 - v^2 ) where v is a percentage of the speed of light. Here v = 0.9998 which gives a time dilation factor of 0.02, and 1000 years times 0.02 equals 20 years. (Gravity effects on time are ignored, as they are a small effect here.) I hope this helps.

How about the idea that the (observable) universe contains roughly 5% ordinary matter, 25% dark matter, and 70% dark energy. This sum gives the so-called critical mass/energy density needed for a flat universe (zero overall spacetime curvature). And observations show the observable universe is indeed flat. Analysis of the Cosmic Microwave Background is one example. So if there is no dark matter (which I doubt), then the total matter/energy in the observable universe would be too little for the flat universe we see, wouldn't it?. It would imply an open universe of negative overall spacetime curvature. Does anyone know if MOND or any other theory has an answer for this? Or am I missing something here?

In order to get the attention of the scientific community with your new theory of gravity, make a specific, detailed measureable prediction -- one which is different from general relativity (GR). If it explains current observations which disagree with GR, it should generate some interest. If it explains an observation which has yet to be made; once that observation is made and it agrees with your theory and not with GR, I think then it will really draw attention to your theory. This is typically how any new theory gets recognized -- through measurements which verify its unique predictions.

Good article explaining the latest evidence putting dark matter into question: http://www.newscientist.com/article/dn21732-nearby-darkmatterfree-zone-poses-cosmic-conundrum.html

I am not an expert in quantum mechanics, so please feel free to educate me here. I thought there are two basic ways to model in quantum mechanics. In the first way, quantum field theory, a wave function travels from place to place. When an interaction occurs, the wave function collapses to a local wave packet of energy we can refer to as a particle. The wave function gives the probability of detecting this particle at a certain place and time. The second method, Quantum Electrodynamics (QED), models particles as single points. The sum of all paths method is used to determine the probability that the particle is detected at a given place at a certain time. The probability amplitudes for each possible path the particle could take are summed, then squared to compute this probability. There is no wave in QED. (Summing amplitudes from Feynman diagrams of interaction events and squaring is also a way to determine probability.) The first method uses waves and wave mechanics. The second uses particles and the sum of all paths. I think they both give equivalent results.

When a star like our Sun runs out of nuclear fuel, its core collapses to a white dwarf. The gravitational collapse is stopped by electron Fermi pressure. For a star I think 4 to 8 times larger, the core collapse overcomes electron Fermi pressure to become a super-dense neutron star. The collapse is stopped by neutron Fermi pressure. For even bigger stars, we know of nothing which can stop the collapse. So a black hole is formed. For an idealized non-rotating black hole, all the mass of the black hole star's core is crushed into an infinitessimally small point. It may seem ridiculous to you, but this is our current best understanding. This "singularity" is a place-holder until a new theory combining general relativity and quantum mechanics gives us a better explanation.

You statement is how I always understood it. Maybe this is quibbling with the words, but "energy in the form of mass" does not sound exactly like "an object of mass m contains simultaneously an energy E".

So when an electron and positron interact, they can annihilate each other and give off energy in the form of photons. The mass of the electron and positron are converted to photons per E=mc2. Is this too simplistic? If we are to think of the electron and positron as having mass and energy simultaneusly, then the mass is destroyed in the interaction and the energy released. Is this a correct interpretation of the physics?

Well put.

Thanks for the info. Wikipedia or for that matter, other sites on the internet are not always reliable. I appreciate the heads up. In any event, it was Einstein and only Einstein who came up with the mass/energy equivalence under the relativity construct. And that is what is most important (and most impressive).

Spayman: If by "assured possibility", you mean guarantee of time travel in physics, I agree. But as I said, physics hasn't ruled it out completely. For example, see interesting time travel possibility using general relativity and quantum mechanics: http://physics.aps.org/story/v23/st18 But like the article says, we won't really know until we have a theory of quantum gravity.

Hmmm. It seems there may be something to this De Pretto and E=mc2. see link: http://en.wikipedia.org/wiki/Olinto_De_Pretto

I read that an problem with an oscillator caused a problem in the opposite direction. Does anybody know by how much?

We aren't going to get a convincing argument for what happened before the big bang, because no one knows what happened before the big bang. Our current theory, general relativity, breaks down at time zero. One possibility is that time and space began at the big bang, so there is no "before" before the big bang. This is pure speculation, but its fun to think about (though it boggles the mind.) Some say there had to be a "before" before the big bang. Why? How do we know the true essence of space and time. It remains a deep mystery which, perhaps with new physics someday, we will have a better answer.

I donno. I thought the equations of general relativity do not rule out time travel. And quantum mechanics says particles can travel backwards in time.

Who invented E=mc2? Found an interesting link. Apparently a German scientist named Fritz Hasenöhrl went part of the way. (This is like Lorentz and Poincare, who sent part of the way with other relativity ideas). But only Einstein took the full leap to relativity. See link: http://physicsworld.com/cws/article/news/2011/aug/23/did-einstein-discover-e-mc2