IM Egdall

I think Iggy hit the nail on the head when he said the arrow of time comes from the initial condition of the universe. As I understand it, the entropy of the universe was lower at its beginning and has been increasing ever since. This, I think, is at the root of the perceived forward arrow of time. But why the initial universe was at lower entropy -- I don't think anyone has an answer for this.

URAIN - I think you misunderstood what has been said. Galaxies do rotate. So from that we can see how fast stars at the edges of galaxies are moving. But the outer stars are orbiting so fast they should be flying off into outer space -- that is if nothing held them but the gravitational pull of the visible stars. So, physicists conclude, there must be more matter, thus more gravity, out there than we can see. No one nows what this additional unseen matter is -- it is called "dark matter". Dark matter is also used to explain the motions of groups of galaxies, and other astronomical phenomena. For more, see link: http://en.wikipedia....iki/Dark_matter

Why just photons? Gluons, the strong force carrier are also massless and travel at the speed of light (as supposedly do the yet to be found gravitons, the gravitational force carrier.) And the W and Z bosons, the weak force carriers, have mass. But I believe electroweak theory (the unification of electromagnetism and weak forces) says that at extremely high temperatures, these weak bosons are also massless and travel at the speed of light. All I am saying is that photons are not the only particles which are massless.

You go Morgsboi! Someday soon, you will be teaching us.

I''ve trying to read John Barrow's The Anthropic Cosmological Principle. It's pretty dense, but I am struggling through it. On page 133 he makes the same point as Airbrush. He says "the evolution of cognition . . . is the result of highly specific specific evolutionary events." With two large super-kingdoms, the prokaryote (no cell nucleus) with thousands of lines, eukarytokes (plants, animals, fungi, and protists) with tens of thousands of lines -- only one species, human, has the "kind of intellegence that would permit the development of advanced technology." He says the general concensus of evolutionists is that "the evolution of intellegent life (on par with humans) is so improbable that it is unlikely to have occurred on any other planet in the entire visible universe." Ouch! So most likely we are alone in all we can see -- no ET's, no aliens, no other beings with our information-processing abilities anywhere in the part of the universe where its light has had enough time to reach Earth. If true, it's kind of sad. It also says the fact that we exist is nothing short of extraordinary.

As I undersrtand it, the word "curve" or "curvature" is a mathematical term. I think it is equivalent to saying "warp" or more simply "change". An example. Imagine two points in space - one above the other. They are a certain distance apart. Now let's place the Sun below the two points. (The points are radial points in that a line from the center of the Sun goes through the two points.) Now because of the mass/energy of the Sun, these same two points are now farther apart (as seen from far away). This stretching of space is an example of space warp or space curvature. Oh, and photons are also a source of this curvature, like all matter and energy. You are thinking about photons as waves, but per quantum field theory, all matter and energy particles (e.g. photons, electrons, quarks, etc.) travel like waves. I hope this helps.

Per Einstein's general relativity, gravity does not cause space and time to bend -- gravity IS the bending of space and time. Or more properly, gravity is spacetime curvature. And mass/ernergy is the source or cause of gravity, of spacetime curvature. More properly mass/energy and its movement (momentum) are the source of spacetime curvature.

I think this whole "flat universe" notion is a simplification. It is based on observations of the observable universe. What about the rest of the universe, the part where objects are so far away the light has not reached us yet? Is the whole universe flat? Or is it just the part we can observe? Form what I have read, it is not at all clear that the entire universe is flat. It is more likely curved in some way. So all this talk about positive energy being canceled by negative gravity in a flat universe may not apply to the universe as a whole.

Yes, the second law of thermodynamics gives a forward arrow of time -- but it is a statistical law. It applies only to a large number of things. When we look at the behavior of a small number of particles, they show time symmetry. No arrow of time is revealed.

Several commenters have asked for the subscription hyperlink or e-mail service for my blog - It's Relative. Is there such a thing? Also how can I find how many views my blog has received? Any info is most appreciated.

i I suggest you read Brian Greene's The Fabric of the Cosmos. He points out a wild notion backed up by special relativity -- one you are alluding to. "Now" is relative. If we are moving with respect to each other, an event which is in my present for me may not be in the present for you -- we may not have the same now. Based on our relative motion, your now may be in my past or in my future! And your view of what is happening now is no more valid than mine. So, per special relativity, time is not like a flowing river going by -- time (more properly spacetime) is more like a frozen block of ice. As Greene says, "we envision all of space as being out there. We should envision all of time as being out there too! Reality encompasses all events in spacetime." "Experience of the now means something special for man, something essentially different from past to future. But this important difference does not occur within physics . . . " - A. Einstein.

So if your conjecture is science, not just philosophy, make a testable prediction based on your theory -- a precise prediction for an experiment or measurement that has yet to be performed. When and if your prediction is validated by some one else's observation, and independently verified, then I will stand up for the validity of your theory. If you have no new prediction, then you are only arguing about your opinion, not science.

Well said, Iggy

The problem is the equations of physics are time-symmetric -- that is they work just as well going in both time directions. So it is hard for physicists to come up with a physics-based explanation for the arrow of time. see for example: http://en.wikipedia....i/Arrow_of_time The so-called second law of thermodynamics, however, does imply a direction of time -- but this is associated with a probability for a large number of constituents, and does not apply to just a few particles. But I think you make a good point. Relics from the past are evidence for the arrow of time - aren't they?

Our understanding of the universe and that it has no edge is, I believe, based on general relativity and observations of the visible universe -- the part that is close enough so its light has had time to reach us. I am reading The Goldilocks Enigma by Paul Davies. It's really good and he says something I think sheds light on the subject: " . . . raises the obvious question of whether, in the real universe, the reason that no center or edge is apparent is simply that our telescopes are not powerful enough to probe that far. And that may be so. It may be that the observable universe is buried deep in an assemblage of galaxies that, viewed on a grander scale, does have an edge . . . "On the otherhand, the universe . . . may be infinite in all directions . . . " I have also read that it is possible the universe to be finite but still not have a center or edge, due to space warp. So it seems the jury is still out on whether the universe has an edge. (I have not read the 34 pages of this thread, so I apologize if I repeat something that has already been said.)

In whose reference frame do they occur simultaneously? And why is this particular reference frame "special"?

Oy! The fact that it takes 8+ minutes for light from the Sun to reach the Earth does NOT contradict the relativity of simultaneity. Why do you claim it does?

Here is a (simplified) explanation from quantum field theory (as my simple mind understands it): A particle travels like a wave and hits (is detected) like a particle. Take a single electron in the double-slit experiment for example. The electron gun emits the electron as a wave, which spreads out so that it goes through both slits. This produces a wave at slit one and another at slit two (similar to if you put a water wave through two slits). The two waves from the single electron then interfere with each other. But once this interfering wave reaches the detector screen, the electron shows up at a single location on the detector screen. Where exactly on the screen does a single electron show up? Nobody knows. Nature doesn't know. We can only predict the probability of the electron being detected at a certain location. You see, the electron wave is a probability wave (called the wave function). It tells us how likely the electron is to be detected at a certain location. When the electron gun shoots a number of identical probabilty waves at the double-slit, over time an interference pattern builds up on the detector screen. This is exactly what these probability wave predicts. So we can't tell where a single electron is detected, but can predict where a very large number of electrons will end up. It is statistical. So what happens when we "look" to see which slit the electron went through? Say we find the electron passes through slit one. This means the probability of find the electron at slit one is 100% and the probability of it being at slit two is zero. So the electron's probability wave has collapsed to 100% at slit one sand zero at slit two. So now we have only a single probability wave coming from slit one to the detector screen. So there is no interference. Very strange, but this is how nature works. And quantum field theory's predictions are in superb agreement with experiments. I found the simluation below helpful - Quantum Wave Interference: http://phet.colorado...ve-interference

Here's how I think it goes. The second law of thermodynamics is a probability law. For lots of particles, the probability is they will become more and more disordered over time. This implies a forward arrow of time. But for just a few particles, the second law does NOT hold. The laws of physics for individual particles show no arrow of time -- they work the same for forward time and backward time. Our macro world is made up of many, many particles. So this is perhaps why we see a forward arrow of time. But it is due to a statistical law. Very mysterious. And if you ask why the universe was so ordered in its original state (per big bang cosmology), it gets even more mysterious. (I got all this from reading Greene's Fabric of the Cosmos and other sources.)

Did you read the link? Like Capt'n said, time dilation has to do with relative speed. A muon has, on average, a certain lifetime before it decays into other particles. Muons in relative motion have a longer lifetime on average than muons at rest. Why? Because they are moving. Time runs slower for the moving muons, as compared to time at rest. At CERN, muons traveling at 99.7 percent the speed of lights showed a 12X increase in their lifetime. This as measured by laboratory clocks (laboratory reference frame). The muons last longer because they were moving at 99.7 % c . And Einstein's time dilation formula says this speed produces a slowing of the muon time of 12x. REF: C. M. Will, Was Einstein Right? Putting General Relativity to the Test, p. 255.

To OWL: You ask what curves in spacetime curvature. The word curvature is a mathematical term. I believe in this context it means warp or change. Per general relativity, time and space are warped or changed in the presence of mass/energy. For example, a clock runs slower the closer it is to an object. (Here on Earth, a clock at lower altitude runs a tiny bit slower than a clock at higher altitude.) This is time warp. And the distance between two points is stetched in the presence of an object. (Imagine two points separated in space, one above the other. Now place the Earth just below these two points. The distance between the points as seen from far away are now a tiny bit longer.) This is space warp. This time and space warp has been verified in a number of solar eclipse experiments, where starlight grazing the Sun is bent by the Sun's mass/energy. General relativty predicts 0.875 acrseconds of bending due to time warp and another 0.875 arcseconds due to space warp -- for a total bending of 1.75 arcseconds. In 1975, physicists got a value of 1.75+- 0.019 arc seconds. (REF: K. R. Lang, Astrophysical Formulae, Vol. 1, p. 159.) So spacetime curvature is not a philisophical term, it the real warping or changing of space and time in the presence of mass/energy.

Here is what I have read on length contraction of macro objects. In 1959, James Terrell of Los Alamos Scientific Laboratory explained that though length contraction is the actual effect, it is not at all what we observe. He predicted that for astronomical observations (where the incoming light is essentially parallel), objects actually appear rotated due to relativity effects. Why? Because we do not see all the light rays from the object at the same instant in time, thus the object appears rotated. See link: http://faraday.physics.utoronto.ca/PVB/Harrison/SpecRel/Flash/ContractInvisible.html

I want to add my opinion here. I hope it helps. I think Einstein's comment that past, present, and future are an illusion comes from his consideration of simultaneous events. Say you are on a uniformly moving train car. A light is at the back of the car, and another at the front. Say you arrange it so both lights flash at the same time. So from your point-of-view on the train, the two light flashes are simultaneous. Now what do people on the ground see as the train moves by them? Per Einstein, they do not see the two flashes occurring at the same time. In fact, they see the light at the back of the train car flash before the light at the front of the train flashes. This is the relativity of simultaneity -- and it comes directly from Einstein's light postulate (the speed of light is absolute). On the train - For you on the train, let's define "now" as when the two lights flash. So to you, the past is before the two lights flashed. And the future is after the two lights flashed. On the ground - What about the people on the ground? Let's define their "now" as midpoint between the times of the two flashes. So to them, the past is when the light at the back of the car flashed. And the future is when the light at the front of the car flashed. So past, present, and future are relative. They are seen as different for observers in motion with respect to each other.

http://en.wikipedia.org/wiki/Pair_production http://electrons.wikidot.com/pair-production-and-annihilation http://en.wikipedia.org/wiki/Annihilation