IM Egdall

OK, assume Sam and Mary are in uniform motion with respect to each other, and they send timing signals to each other. From Sam's timing signals, Mary concludes his time is running slower. And from Mary's timing signals, Sam concludes that her time is running slower. As long as there is no change in speed or direction (no acceleration), both Sam and Mary observe the same thing -- symmetry is maintained. There are two effects to take into account here: time dilation and the Doppler Effect. Again, I recommend you please read my article on the Twins Paradox to see the details. Click on: http://www.marksmodernphysics.com/ and then click on Its Relative, then Archives, then The Twins Paradox.

I think a lot of people have this question when they start really thinking about relativity. Let me try to give you an answer. Say stationary Sam is at rest and Moving Mary is moving in uniform motion (constant speed and constant direction). You are correct: from Mary's point-of-view she is at rest and Sam is the one that is moving. So Sam sees Mary's clock running slower than hers because Mary is moving with respect to him. AND Mary sees Sam's clock running slower than hers because she sees Sam moving with respect to her! As looney as this sounds, this is just what special relativity predicts. Time effects are symmetric between Sam and Mary. THERE IS NO ABSOLUTE REFERENCE FRAME. But both watches can't both be running slow compared to the other, can they? To check this, we have to get the two watches side-by-side. And this action resolves the conundrum. Say in order to compare the two watches side-by-side, Mary decides to travel to Sam. To do this, she must change her speed and direction. So she is no longer in uniform motion (more formally no longer in an inertial reference frame). And any change in speed and/or direction is acceleration (in the physics definition.) This acceleration by Mary destroys the symmetry. So because she experiences this acceleration, it is Mary's time which runs slower than Sam's. And when Sam and Mary compare watches side-by-side, they find that yes, Mary's watch has run slower than Sam's. Now I have seen a number of ways to show this, using special or general relativity. The simplest explanation I have found uses general relativity's gravitational time dilation. When Mary slows down and turns around to get back to Sam, she is pressed against her vehicle like gravity. This gravity effect slows her time down. Sam, in this example, stays in uniform motion. So he does not experience acceleration. So there are no acceleration/gravity effects on his time. So time does not slow down for Sam, only Mary. If you want a detailed explanation per special relativity, go to my website, marksmoderrnphysics.com and click on Its Relative, Archives, and The Twins Paradox. (Here the effect is explained as a combination of time dilation and the Doppler effect.)

I don't know if this explains why light speed is constant, but from what I have read, this is how Einstein came up with this idea. Per Maxwell, light is electromagnetic radiation. It is a wave of continuously changing electric fields and continually changing magnetic fields. Again per Maxwell, the changing electric field produces a magnetic field. And the changing magnetic field produces an electric field. They produce each other. The key word here is changing. These fields must be continuously changing over time. So the light wave must be continuously moving. Eisntein imagined what it would be like if he could travel at the same speed as light. Then the light wave would appear to be standing still from his point of view. But then the electric part and magnetic part would not be changing -- they would be static. So one would not create the other . Thus the electric and magnetic fields would cease to exist. In other words, light must move to exist. There is no such thing as a light beam at rest. Form this conundrum, Einstein made a brilliant and counter-intuitive leap. If a light beam must move to exist, then it must always be moving no matter what speed you move at. So you can never catch up to a beam of light. No matter what speed you are moving at, the light beam is always traveling at the same speed relative to you. Light speed is constant! Hope this helps.

Elen Sila -- Here's a real world application of your idea you might find interesting: Global Positioning Satellites: GPS receivers compare "time signals received from a number of GPS satellites (usually 6 to 12) in its line of sight." From this, the receiver calculates "its current position and heading." The key here is time signals from the different satellites must be known to an accuracy of 20 to 30 nanoseconds. The two relativity effects which alter the timing of the satellite clocks are: Relative Motion. Atomic clocks on board the satellites run slower than clocks on Earth by about 7 thousand nanoseconds per day. This is due to their motion relative to the Earth (kinematic time dilation per special relativity). Relative Altitude. Satellite atomic clocks run faster than Earth clocks by about 45 thousand nanoseconds per day. This is due their much higher altitude. (gravitational time dilation per general relativity). The two relativity effects produce a net gain of about 45 -7 = 38 thousand nanoseconds per day in the timing of satellite clocks (compared to clocks on the ground). This net gain is huge compared to the 20-30 nanoseconds accuracy GPS needs to work. Thus, if the system ignored the effects of special and general relativity, "a navigational fix from GPS would be false after only 2 minutes. And errors in global positions would continue to accumulate at a rate of about 6 miles (10 kilometers) each day!" So the success of the GPS system in providing locations on Earth (and in airplanes) to accuracies of 5 -10 meters is a continuous verification of Einstein's predictions that time is slowed by both motion and gravity! <BR clear=all> http://www-astronomy.../Unit5/gps.html

I think Albert Einstein would agree with charles brough. But I think most physicists today would say that uncertainty per quantum mechanics is inherent in nature itself. All the experimental evidence points this way. We may not like it or even understand it, but this is the way nature is. Could some future theory go beyond quantum theory and explain the evidence we see differently, and show that quantum mechanics is incomplete and the universe is really deterministic? I'm no expert, but I doubt it.

Are you referring to the increase in pressure as the star collapses? Pressure I think is the flow of momentum and is a source of gravity in general relativity. So as the star compresses, the added pressure does contribute to the stress-energy-momentum tensor, increasing the collapse.

Actually three questions: How do I accept (or reject) comments for my latest blog article? How do I add links to my articles? What are TAGS and how do I add them to my articles? (Sorry to be so thick.)

The Big Bang theory. It ain't perfect (what is?) But it sure is impressive. It makes predictions on the origin, structure, and evolution of the universe all the way back to some 10^^-43 seconds after the Big Bang. And there are lots and lots of independent observations and measurements which agree with these predictions to remarkable accuracy. It is today's best theory on the origin of the universe. So does it tell us what happened before 10^^-43 seconds after the Big Bang? No. We can guess, postulate, conjecture all we want about time at and before the Big bang, but until someone comes up with a new theory supported by substantive evidence, we just don't know.

May I suggest you read The Elegant Universe by Brian Greene. It explains string theory and multiple dimensions in everyday language.

All fundamental particles (photons, electrons, , etc.) travel like a wave and "hit" like a particle. Photons at all frequencies, whether visible light or radio frequency or any other, work this way. Consider the double-slit experiment. Turn the light source way down so that it sends out only one photon at a time. The photon travels like a wave, goes through both slits, but is detected on the screen in only one location. Repeat the experiment with another photon, and it hits at a different location. (There is no way to predict where each photon will be detected, only the probability it will be detected at a certain location). Over time, the photon detector pattern builds up and shows interference (there are places where the photons do not show up). This same kind of interference pattern shows up for electrons in the double-slit experiment. This says that electrons too travel like a wave and hit like a particle. So are photons, electrons, etc. particles or waves? As Richard Feynman said, they are neither. They are their own quantum mechanical thingies. There is no word in English (or any other human language as far as I know) which fits the description of how subatomic particles behave.

Super. Thanks!

I messed upo my blog. A Rose is A Rose appears twice in recent blog list, but clicks on both do not work. I tried to delete them but it didn't work. How do I get them off the site, and reload a correct version? Please help.

I guess you are talking about string theory, which postulates that all fundamental particles are extremely tiny one-dimensional strings. However, unlike relativity, string theory is still hypothetical -- no substantive experimental evidence has been found to confirm (or deny) its predictions. But in either case, E=mc^^2 holds. In the case of string theory, I believe the energy of vibration of a string indicates that string's mass (using the E=mc^^2 equation). Take the atom bomb, for example. Say we weigh the bomb before it explodes. Then say we somehow collect all the mass of the bomb materials after the explosion; gases, dust, etc. and weigh that. We find the weight after is less than the weight before the explosion (by about one percent). Where is this missing mass? It has been converted to energy(radiation) according to E=mc^^2. So mass can be converted to energy. And energy can also be converted to mass. Both are common occurrences in particle accelerator experiments.

Duh. Sorry for being dense, but how do you ask Capn?

I want to change my user name from I ME to IMEgdall. How do I do that?

depending on their relative motion. Correct?

Let me try to answer you. The distance between galaxies is increasing over time. I guess you could call it the "gap" between galaxies. Evidence for this is the so-called red-shift. Light from distant galaxies is shifted in frequency towards the red or lower frequency. Now where ever we look in the sky, the further away a distant galaxy is the greater the red-shift we see. This is interpereted per general relativity as due to the expansion of the universe itself over time. This expansion stretches the frequency of the light from these distant galaxies; thus we see a lower frequency by the time the light reaches us. So it is though these galaxies are speeding away from us. And the further out we look, the greater the red-shift so the greater a galaxy appears to be speeding away. And, per general relativity, there is no limit to this speed. It can be greater than the speed of light. In fact red-shifts of very distant galaxies make them appear to be speeding away from us at greater than the speed of light. The tricky part is that this apparent motion of distant galaxies away from us is actually, per general relativty, the expansion of the space between the galaxies. The galaxies themselves show relatively little motion. It is the space between galaxies everywhere in the universe which is expanding. To quote from Wikipedia: "The metric expansion of space is the increase of distance between distant objects in the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space. In other words, the universe is not expanding "into" anything outside of itself." Hope this helps. Einstein himself famously pondered a similar scenerio (at age 16). Per Maxwell, light is made of continuously changing electric and magnetic fields. The changing electric field produces a changing magnetic field, and vice-versa. The key word here is changing. The fields have to change over time to continue to produce each other. Einstein realized that if he caught up to a light wave, it would look as though it were standing still to him (as you point out). But per Maxwell, a stationary light wave is impossible. It must move to exist. From this, Einstein realized that we can never catch up to a light beam. In fact, the speed of a light beam is always the same; it is unaffected by our (uniform) motion. By the way, a hypothetical particle called a tachyon does travel faster than the speed of light, giving some wild time effects like the ones you mention. See Causality in the link below: http://en.wikipedia.org/wiki/Tachyon

I don't think it is quite that simple. Einstein's original 1915 equations of general relativity predicted that the universe is not static-in other words it changes in size over time. Observations by Hubble in the 1920's showed that the universe is indeed expanding. Einstein's original equations were used to model this expanding universe. Note that there is no cosmological constant, no dark energy in this model. Supernova data in the 1990's showed that the universe is not only expanding, but this expansion is speeding up (accelerating). A cosmological constant was added to Einstein's equations to represent this acceleration of expansion. The cause is unknown and is dubbed dark energy. So dark energy does not account for the expansion of the universe, just the acceleration of this expansion. Per general relativity, gravity is considered to be negative. And normal matter/energy, dark matter (whatever that is) and dark energy (again a mystery) are all positive. Measurements in the visible universe show that the total negative (gravity) and total positive (everything else) are equal so cancel out, giving us as a so-called flat (zero spacetime curvature) visible universe. Any thoughts and clarifications on this are most welcome.

Huh?

We go but cannot see into the future because of the arrow of time. The arrow of time is what brings us from the present to the future. And no light from the future reaches our eyes because it has not been produced yet, so we do not see into the future. (What exactly causes the forward arrow of time is still somewhat of a mystery in physics.) We cannot go but can "see" into the past because of the finite speed of light. We see when light from an object reaches our eyes. SInce this takes a finite amount of time, we see objects as they looked in the past.

As I understand it, when we have lots of light, we get the interference pattern predicted by Maxwell. But what happens when we "dim" the light source. We find that if we dim it enough, we get single points of energy recorded one at a time on the detector screen. This is most certainly not predicted by Maxwell's theory. And over time, these single events bulld up to produce the same interference pattern produced when we had lots of light. Take a look at the simulation: http://phet.colorado...ve-interference Now set the simulation to "double-slits", the screen to "hits" the gun control to the left and fire the laser What you see is the screen filling slowly with points (photons). And there is no way to predict where an individual photon shows up on the screen. This is the probabilistic nature of light as predicted by quantum mechanics. Again, over time, we see the interference pattern. So it seems that an individual photon goes through both slits and interferes with itself! Quantum field theory says that the "photon" travels like a wave (the wave function), passes through both slits, interferes, but is detected only locally as a single point of energy. Again Maxwell's equations have no explanation for this behavior.

See link for explanation of proper time and coordinate time: http://en.wikipedia....Coordinate_time I think you are correct on proper time. It is, in fact, the spacetime interval between two events in units of time. (The square of the spacetime interval equals the difference between the square of the time interval and the square of the space interval). And the spacetime interval is absolute, in that it is unaffected by relative uniform motion. Consider two events that happen at different times and at different locations in space. Say you are present at both events. For example Event 1: you leave the Earth in your rocket. Event 2: You arrive on Mars. From your point-of-view (reference frame), assuming uniform motion, you are at rest and Mars comes to you. So in your reference frame, the space interval you measure is zero! Thus since there is no space interval to subtract, the spacetime interval is simply equal to the time interval between the two events. This for you is the time on your wristwatch from event 1 to event 2. So proper time is also called wristwatch time. Co-ordinate time is what someone who stays on Earth would measure for the time interval between your leaving Earth (event 1) and arriving on Mars (event 2). This observer is not present at both events (only at event 1). Co-ordinate time is not absolute. It is affected by relative uniform motion.

I believe we can apply the currently understood laws of physics all the way to t = 10^^-43 seconds or "Planck time". At this time, space and time begin to behave as we see them today. Before this time, all known science fails us. To quote link below: "In the era around one Planck time, 10-43 seconds, it is projected by present modeling of the fundamental forces that the gravity force begins to differentiate from the other three forces. This is the first of the spontaneous symmetry breaks which lead to the four observed types of interactions in the present universe. Looking backward, the general idea is that back beyond 1 Planck time we can make no meaningful observations within the framework of classical gravitation." In other words, general relativity (classical gravitation) theory breaks down below this Planck time. We need a so-called theory of quantum gravity to go beyond Planck time to time zero. No such theory has been substantiated by empirical evidence. See link: http://hyperphysics.phy-astr.gsu.edu/hbase/astro/planck.html

Question: if there is a sea of dark matter engulfing our solar system, why doesn't it coelesce due to its gravity into dark matter clumps and form dark matter planets, etc.?

I think it goes something like this: In 1911 Einstein used the EP and a uniformly accelerating elevator with a beam of light traveling parallel to the direction of acceleration to predict gravitational redshift. This is equivalent to gravitational time dilation or the warping of time. Notice how there is no mention of the effect of space warp in this thought experiment. Einstein also used the same thought experiment but with beam of light traveling perpendicular to the direction of acceleration to predict the bending of light in a gravitational field. But the value he got for the bending is only half the value predicted by his 1915 theory of general relativity. THis is becuase the EP only considers the warping of time, while general relativity considers both time and space warp. I learned this from Hans Ohanian in Einstein's Mistakes: "(Einstein's) 1911 calculation of the bending of rays of light, which was based on the Equivalence Principle, yielded a result half as large as the new calculation (in 1915) based on his new theory of gravititation. Einstein understood that this the reason for this discrepency was the new calculation included an extra deflection coming from the warping of space, whereas the 1911 calculation had effectively included only the warping of time . . . For the physicist enclosed in a box . . . the bending of a ray of light in an accelerated box is half as large as the bending in a box at rest in a gravitational field." Einstein's Equivalence Principle is fully compatible with Newton's gravitational theory; and the elevator in free-fall thought experiment takes into account time warp only; ignoring the effects of the warping of space. But if we evaluate Einstein's gravitational field equations for time warp only and ignore the effects of space warp, we get solutions for gravitational fields which are identical to Newton's. Assume a uniform density, non-rotating, uncharged spherical stellar object (ala Schwarzschild geometry). If we then compute the global change in the spacetime interval in the neighborhood of this stellar object including the curvature of time only, we get the following equation from both Newton's and Einstein's gravitational theories: Spacetime Interval in Gravitational Field – Time warp only (Newton and Einstein) ds2 = (dx2 + dy2 + dz2) - (1 – 2GM/r) (dt)2 where G is the gravitational constant, M is the mass of a Newtonian star, and r is the distance from the star to a particular point in space. If we include the curvature of time and space, we get the full solution of Einstein's field equations, which no longer agrees with Newton's theory: Spacetime Interval in Gravitational Field – Time and space warp (Einstein only) ds2 = (1+2GM/r) (dx2 + dy2 + dz2 ) - (1 – 2GM/r) (dt2) (Derivations for these equations can be found in Schutz, Gravity from the Ground Up, p. 226.