IM Egdall

I love your "less wrong" view of the scientific process. Excellent!

Thank you for your response. I ask one thing. Please use the terms "observable universe" and "entire universe" so I can better understand your points. I do agree with you that "if the universe seems flat here, then it still could be spherical." I think this is saying the observable universe seems flat, but the entire universe could be spherical. And I don't think the possibility that the entire universe is spherical or negatively curved is slight. Doesn't general relativity say it is very likely curved in some way. After all, having a net curvature of zero seems an extraordinary coincidence. In fact, the big question before inflation theory was why our observable universe appears flat. Inflation explained why the observable universe is flat -- because it has been expanded exponentially. But again, the entire universe would still retain some curvature even after inflation. Wouldn't it? What I am trying to say about the cosmological principle is that if the entire universe is not flat, then this principle applies only to the observable universe. And yes, I agree that anywhere in the entire universe, the observable part obeys the cosmological principle. But not necessarily the universe as a whole. Does this make sense to you?

Hubble did not see the acceleration of the expansion of the universe. He only saw that the universe is expanding (in fact, actually it was Georges Lemaitre). But anyway, it was observations of supernovae in the late 90's which showed this expansion has been accelerating for the past 5 to 7 billion years or so. How did the supernova data show this? Please see the article "Hubble, Keck, and Nobel Prize" in my science forum blog - Its Relative - for an explanation.

Am reading The Goldilocks Enigma by cosmologist Paul Davies. On page 275, he says "The limited accuracy of these (WMAP) observations cannot establish the universe is exactly flat. What they tell us is that if the universe is shaped like Einstein's hypersphere (positive curvature, not flat), then the radius of the hypersphere is exceedingly large, so that within the volume of space probed by our instruments (the observable universe), we cannot discern any curvature. Similar remarks apply to any negative curvature." In other words, like ants on a gigantic balloon, we measure the region of the overall universe we can see as flat, but the overall universe may be positively (or negatively) curved. I find physicists are very sloppy with their language and often use the word "universe' when they mean "observable universe." I claim the cosmological principle applies to the observable universe (due to inflation) and may not apply to the entire universe. I think Davies backs me up on this. Please comment -- I welcome enlightenment.

I think what Sagan was getting at is this. If you detected some kind of signal from outer space and say its strength varied over time -- and if you measured this strength you would get a series of values. If you looked at these values and they were proportional to the prime numbers, then it would most likely be an indication of some kind of message from an intelligent being, rather then the results of a natural phenomenon. (Prime numbers are those which cannot be factored into two or more numbers -- like 1, 2, 3, 5, 7, 11, 13, 17, 19 etc.)

Yes, the fabric of spacetime can be thought of as embedded in a higher dimesional space. It is called an embedding diagram. For example, see link: http://www.astrosoci...2/lockwood.html Here's how I think of it. Take two points in empty space. They are a certain distance apart. Let's place the Sun near the two points (so that a line between the two points goes through the center of the Sun -- i.e. a radial line). Now the distancee between the two points is stretched by the presence of the Sun's mass/energy. How do you depict this longer distance? One way is to draw a downward curve between the two points (or upward) representing the longer distance. This curve is extended in a hypothetical space called hyper space.

I don't think physicists know this for sure. I think the entire universe is expected to not be flat. The curvature of the entire universe could be anywhere from -1 to +1. WHy does the obervable universe happen to have the exact value of 0? Inflation theory is supposed to answer this question. Inflation makes our local observable universe flat. The analogy is: an ant looking across a football field thinks Earth is flat (this represents the observable universe). But a global view of the entire Earth shows it is curved. Inflation expanded the universe so much that it appears flat in the part we can observe, but the entire universe may not be. So the cosmological priciple holds for the observable universe but probably does not for the entire universe.

You are close. However, it is not quite correct to say "gravity warps space." It is better to say the presence of mass/energy warps space (and time). LIke the presence of the Sun warps space and time in its vicinity. And this warping or curvature of space and time (spacetime curvature) is what causes planets to orbit the Sun and holds us down to the Earth. We call this effect gravity. So spacetime warp or curvature IS gravity. Physicist John Archibald Wheeler said something like "mass/energy grips spacetime and tells it how to curve -- and curved spacetime (gravity) grips mass/energy and tells it how to move."

I think most cosmologists would say the reason why we see a "flat" observable universe is due to inflation -- the exponential expansion of the very early universe. Per Wikipedia: http://en.wikipedia....ion_(cosmology) "As a direct consequence of (inflation), all of the observable universe originated in a small causally connected region. Inflation answers the classic conundrum of the Big Bang cosmology: why does the universe appear flat, homogeneous and isotropic in accordance with the cosmological principle when one would expect, on the basis of the physics of the Big Bang, a highly curved, heterogeneous universe?" Doesn't this imply that, even though our observable universe is flat, the entire universe is most probably not?

I don't think this is right. A photon appears to move slowly through a massive substance because it is not the same photon. A photon enters the substance and is absorbed by an electron in an atom. The electron then goes to a higher energy. At some random time later, the electron drops to a lower energy, and releases a new photon. The new photon is then absorbed by another atom's electron. And another new electron is then released. Etc. So each individual photon travels at the speed of light from atom to atom in the substance, but there is a delay time from its being absorbed and a new photon released. So light appears to be going slower through the substance. I think the warping (bending) of space (and time) due to each atom's mass is so tiny as to have a negligible effect here.

As to the rocket, once the earth is put in place near the two points, both space and time are affected (warped.) The rocket would then follow the shortest path in the warped spacetime or what is called the geodesic. As to why mass/energy causes the warping of spacetime -- I think we need new physics to answer that. Maybe the combining of general relativity and quantum mechanics in a new theory of so-called "quantum gravity" will someday tell us why.

We are talking here about the OBSERVABLE universe here. This does not necessarily mean the entire universe, including what we can see and what we cannot see, is also "flat".

At the risk of oversimplifying, I will try to answer your question with no mathematics. The presence of mass (and energy) causes space and time to warp. This is at the heart of general relativity. Let's look at examples of each: Space warp: Imagine two points in space where all objects are so far away that gravity is virtually zero. Now imagine placing the Earth near these two points. As seen from far away, the two points will now be a different distance apart. This in essence is space warp or space curvature. Time warp: Imagine a clock also in space where all objects are so far away that gravity is virtually zero. Now again imagine placing the Earth near this clock. The clock now runs slower. due to the Earth's presence. This is called time warp or curvature of time. Together time warp and space warp are called spacetime curvature. And this spacetime curvature determines the path of the Moon around the Earth, and all the planets around the Sun etc., and holds us to the surface of the Earth. Spacetime curvature IS gravity. I hope this helps.

One way to see why the person who experiences acceleration is the one who ages more slowly is with the twins paradox. Swanson is correct. Both A and B see the other's time running more slowly. But the acceleration breaks this symmetry. This can be explained using time dilation and the doppler effect. See my write-up: Click on http://marksmodernphysics.com/ then on Its Relative, Archives, The Twins paradox. Hope it helps. (For some reason the direct link below does not work) http://marksmodernphysics.com/Mark's%20Modern%20Physics/Musings/index.html

Per general relativity, gravity is the warping of space and time (spacetime) in the presence of mass/energy -- or so-called spacetime curvature. It is not a "force". Any energy and/or mass in the universe produces spacetime curvature. (And gravity itself has energy.) At the big bang singularity, general relativity breaks down. So I think most physicists would say this singularity did not really exist and will be explained away by some future theory which combines general relativity and quantum mechanics. The universe began in a low entropy state (as compared to its state going forward in time). As far as I have read, no one has explained why this is so (using theories verified by empiricial evidence). Galaxies were formed as matter began to collect in the more dense regions of space -- due to gravity (spacetime curvature). What is the origin of gravity? Since there was energy at the very beginning of the universe, it produced spacetime curvature or gravity. Where did all the energy at the beginning of the universe come from? I don't think any one really knows. Some suggest it may be a quantum fluctuation. Virtual particles appear and annihilate each other in "empty" space. This is well-established from quantum theory and backed by experimental evidence (e.g. Lamb shift). So maybe there was a quantum fluctuation that occurred out of nothing which produced our universe. I hope this helps.

If A and B meet and compare watches. In one example, imagine person A remains in uniform motion but person B accelerates --that is she/he changes speed and /or direction so as to arrive where A is. They meet, compare watches, and find B's watch has run slower than A's. And B has aged less than A. Why? Because only B has experienced acceleration.

And how do I eliminate all the spam I get on my blog? I get tons of it.

"Dark" photon and antiphotons are not the same thing. Antiphotons and photons are the same particle. See link: http://van.physics.illinois.edu/qa/listing.php?id=1153 I have no idea what "dark" photons are.

Nice video. But Marcy basically said he has no idea what the probability of human-like intellegence is on other planets in our galaxy. He throws up a number "a million to one" with no justification. Maybe its a billion to one. Or some other number. He admits earlier that this is all pure speculation. So I remain speptical on the existence of "technological ET's" in our galactic neighborhood. (But I hope I am wrong.)

Super picture of Moon's far side! All the craters makes me think how vulnerable we are to an astoriod collisions, One's gonna hit us eventually. We really do need to develop a program to deflect astoroids.

Both photons and gravitons have zero mass. This is why they both go at the speed of light. All massless particles do (like gluons).

Yes intelligence can be observed in a number of life forms. And some even make tools. But the point is that, with all the myriad species on this planet, and some 5 billion years of its existence, one and only one species has the intelligence-level to create and comment in this forum. This is an emperical fact. Your extrapolation of chimps to becoming technological is a supposition. Right now, I have to say I am more inclined to believe Barrow and the evolution experts.

Its relative. Time stops at the event horizon of a black hole as seen from far away. For an obserer passing through the event horizon, his/her time is running normally.

Why light does not escape a black hole -- Recall gravitational red-shift: Say a beam of light in zero gravity has a certain frequency. This same beam of light will have a lower frequency when it is in a gravitational field. In other words, its frequency is stretched towards the red end of the spectrum in the presence of mass/energy. And this so- called gravitational red-shift is relative; the light's frequency is as seen as lower by an observer far away (in zero gravity). So what about a black hole? This ultimate source of gravity produces the ultimate red-shift. Light's frequency inside the event horizon is stretched to zero.