# IM Egdall

Senior Members

591

1. ## Time as a dimension

Fisrt of all, in special relaitivy, relative motion effects time and space. 1) Time slows down with relative motion. For example, a clock actually runs slower in a moving car than it does in one at rest (by a very tiny amount). And the faster the car moves through space, the more slowly time passes inside the car. 2) Space contracts with relative motion (in the direction of motion). The car's length is actually a tiny bit less when it is in motion. And the faster the car goes through space, the more it's length shrinks. (All this as seen from a stationary observer.) But a certain formula called the spacetime interval is the same value no matter what the (uniform) motion. In other words, it is unaffected by relative motion. (The square of the spacetime interval equals the difference between the square of the time interval and the square of the space interval). This crazy formula, the spacetime interval, gives physicists something that does not change in a world of relative motion. From this, the equations of physics are written with three space and one time dimension. That's where the so-called fourth dimension of time comes from in relativity. And, as swansont says, motion through space slows down time (like in the moving car.) In fact, when you are at rest, you are moving through time (e.g aging) as fast as you can. As soon as you move through space, your time slows down. (You are now moving through time or aging more slowly.) Its a kind of trade-off. The faster you move through space, the slower you move through time. For a photon, which moves through space at the speed of light, there is zero motion through time. So a photon never ages. To a photon, time is frozen. Wild huh!
2. ## What is the reason for gravity?

OK, I think the best explanation I have read as to how gravity works is from Einstein's Universe by Nigel Caulder (pp. 82-84). It is based on Einstein's wild and crazy idea that time and space are not rigid but flexible, and change or warp in the presence of mass/energy. (This has been verified again and again by numerous experiments and observations.) This is my version of Caulder's explanation. Here goes. Why does an apple fall from a tree? According to general relativity, the Earth's mass/energy does two things: 1) Space warp - space is stretched more and more as it gets closer to the Earth (in the radial direction), and 2) Time warp - time slows down more and more as it gets closer to the Earth. (All this as seen from an observer far away.) Warping of Space - An apple. once freed from hanging on the tree moves toward the greater stretching of space, or towards the Earth. So the warping of space by the Earth gives the apple its direction; towards the ground. Just below the apple where gravity is a tiny bit stronger (closer to the Earth), space is a tiny bit longer. And just below that point, gravity is a little bit stronger still, thus space is a tiny bit longer still, and so on. The apple moves towards this continuously increased stretching of space; in other words downward towards the Earth. Wapring of Time - OK, so the apple heads towards the Earth because of the warping of space, but why it does continually gain speed (accelerate) as it falls? Time gets slower and slower as the apple approaches the Earth because the Earth's mass/energy slows down time. And the closer yit is to the Earth, the more time is slowed down. So as the apple moves closer and closer to the Earth's surface, the atoms and molecules and the particles within them (quarks and electrons) vibrate more slowly. This reduced vibration means the apple has less and less internal energy of motion (kinetic energy) as it falls. So? Well, per Conservations of Mass/Energy, the apple's total energy must be maintained. So the apple speeds up its overall motion so that its total energy remains the same! In other words, because the Earth's mass/energy slows down time more and more as the apple approaches the ground, the apple has to speed up more and more (accelerate) to maintain a constant overall energy. In summary, 1) the apple heads toward the ground due to the continuous stretching of space below it, and 2) the apple continually increases its speed (some 32 feet per second every second) so that its overall kinetic energy increase makes up for its internal loss of kinetic energy due to the continual slowing of time. This is, albeit, a bit simplified, but I think it gives some feel for the dynamics of Einstein's theory of gravity. I hope this helps.
3. ## Where the atoms came from?

From what I undertstand, our current understanding gets us all the way back to an incredible 10-43 seconds after the big bang. This is called Planck time. It is at this point that physicists generally believe that space and time began to behave as we see them today . So what about time zero? Nobody knows. From time zero to Planck time, all known science fails us. At time zero, the mathematics breaks down; that is the equations of general relativity give us infinity for answers. The so-called singularity at time zero has infinite density, infintie spacetime curvature (gravity), infinite pressure, infinite temperature. There are all kinds of new theories which propose answers to what this cosmic singularity really is, and in some cases even what caused the big bang. These new theories try to combine quantum mechanics with general relativity. String theory is the most popular of these so-called quantum gravity theories. An overwhelming number of experiments, measurements, and observations support the ephicacy of both general relativty and quantum mechanics; but there is no definitive evidence one way or the other to support any of these new theories. So we must remain skeptical for now.

6. ## conservation of energy through a high-velocity entity

Per Einstein's light postulate, electromagnetic radiation (light) always travels at the same speed c (about 670 million miles an hour) through space, independent of the speed of the source of that radiation or the speed of the observer. So it does not matter how fast you or the "ship" or anything else is traveling relative to each other, all observers will still measure the radiation's speed as c. (We assume a vacuum and uniform motion here).
7. ## Apparent Inconsistency in General Relativity

So-called spacetime curvature is transmitted at the same speed as light. The mass/energy of the Sun, for example, curves (distorts) spacetime just above its surface. This spacetime curvature distorts spacetime just a little further out. And so on. Eventually, this distortion reaches where the Earth is, and holds the Earth in orbit around the Sun. This step-by-step distortion takes time, alittle over 8 minutes travelling at the speed as light from the Sun to the Earth. (REF: John Archibald Wheeler, A Journey into Gravity and Spacetime, p. 84) As an analogy, think of a trampoline which is perfectly flat. Then a jumper strikes the center of the trampoline. The fabric at the center where the jumper's feet hit is distorted. This distortion then affects the trampoline fabric a little further out. This in turn distorts the fabric even more further out. And so on. Soon the entire fabric is distorted. But this distortion takes time to propagate from the center to the edges of the trampoline. Similarly, the Sun is continually distorting spacetime (the fabric of the universe). Imagine that the immortal Q from Star Trek made our Sun suddenly disappear. What would happen? Spacetime just adjacent to the Sun would become undistorted (flat). This change would propagate out step-by-step at the speed of light till it reached the Earth. Then and only then would the Earth fly out of its orbit around the Sun. So if the Sun were to suddenly disappear, the Earth would continue to orbit around where the Sun was for a little over 8 minutes; because this is how long it would take (at the speed of light) for the change in spacetime from curved to flat to travel the approximately 93 million miles from the Sun to the Earth.
8. ## Does Gravity reduce Entropy ?

As I understand it, the net effects of gravity increase entropy. Say a bunch of encycopedias are all over your room. Then you come in and straigtens out, putting the encycopedias in proper alphabetical order in the bookcase. The "after" encylopedias are more ordered than the "before" encyopedias, so a decrease in entropy. But this is more than compensated for by the fat burned and heat generated by you in cleaning up the room. The net result is an increase in entropy. Similarly say a duffuse cloud of stellar gas in space condenses due to gravity to form a star. The entropy decrease from the intially diffuse cloud of gas to the more orderly star is more than compensated by the heat generated as the gas compresses, and ultimately by the enormous amount of heat and light produced when nuclear processes begin to take place within the star's core. The net result is an increase in entropy due to the effects of gravity. (REF: Brian Greene, The Fabric of the Cosmos, pp. 172,173)
9. ## I don't understand Theory of Relativity: A)I am not clever B)Nothing to understand

I think it is possible for the interested layperson to understand the basic concepts of relativity. But you have to have an open mind and be willing to suspend your "common sense", which is based on the limitations of our living in a world of relatively weak gravity and slow speeds relative to light speed. I teach non-credit courses in moderrn physics for non-experts at Lifelong Learning Institutes. We use Brian Greene's classic The Elegant Universe; his chapter 2 on special relativity and chapter 3 on general relativity. It is concept-based with no mathematics; and most of my students find it quite helpful.
10. ## Expansion of Universe - Red Shift

This is my first post. I hope it helps. As I understand it, if every single thing in the universe is expanding together, including the space between galaxies, galaxies themselves, and everything inside these galaxies, then we couldn't tell that there is an expansion! Think about it. If you went to sleep tonight, and woke up in the morning with everything in the universe double in size, how could you tell? However, it is only the space between galaxies which is expanding. The galaxies themselves (and things inside the galaxies) are not expanding. In the famous balloon analogy, we are told to imagine a bunch of coins attached to the surface of a balloon. (Here the surface of the balloon represents space and the balloons represent galaxies.) As the balloon expands, the space between the coins increases, but the coins remain the same size. Imagine you are a pixie on one of the coins. To you, as the balloon expands, it appears that all the other coins are moving away from you. In fact, if you calculate how much, you find that the coins further away appear to be moving away from you at faster speeds. The speed of each coin is proportional to its distance from you. Similarly, a galaxy twice as far away from us appears to be moving away twice as fast. A galaxy three times as far away from us appears to be moving away at three times the speed, etc. This is the essence of Hubble's Law (non-relativistic). Now say there is another pixie on one of the other coins. To that pixie, it appears that she is stationary and all the other coins are moving away from her. The point here is that no matter what galaxy you are in, it appears to you that your galaxy is stationary and the rest of the universe is expanding around you. To be more precise, the expansion of the universe does have a slight effect on things inside a galaxy. but the effect is overcome and stopped by gravity. So all things inside our galaxy are very slightly further apart due to the expansion of space, but are held at these distances by gravity. It is a kind of equilibrium state. Oh, and it now appears that the expansion rate of our universe is increasing. and that this rate of expansion has been increasing for the last 5 to 7 billion years. What causes this increased rate of expansion? No one knows. Physcists generally call this unknown form of repulsive energy "dark energy". And if this increase in the expansion rate continues, at some time way in the future, the force of this expansion will become strong enough to overcome gravity, and eventually overcome atomic and nuclear forces as well. Galaxies will be ripped apart, then stars and planets, then all molecules and atoms, then the nuclei of atoms. I think I got all this right. If you have any issues, please let me know
×