IM Egdall

Einstein's light postulate: The speed of light is independent of the speed of its source. This is a foundation of SR and validated in numerous experiments. In this situation, the MMX would give a null result (which it did). Now what if the speed of light IS NOT independent of its source (just for the sake of argument). The MMX would still give the same null result. Why? Because the source of the light and the detector in the MMX are not moving with respect to each other. THey are in the same reference frame. (See my guns on a train argument above.) So the MMX does not prove or disprove Einstein's light postulate. The fact that the perpendicular beams of light went at the same speed no matter what the orientation of the set-up or time of year the experiment was conducted just showed that their is no ether. In all these cases, the source of the light beam and the detector were at rest with respect to each other.

If there is nothing to detect the photon, it remains a wave. It only gets a definite location once it is detected.

I like to think of it like this: An electron travels like a wave but hits like a particle. Assume you have an electron gun here and a whole bunch of electron detectors over there. Once an electron is emitted from the gun, it spreads through space like a wave. This wave is a probability wave. It tells us the odds of where the electron may be found. This, in essence, is the so-called wave function. But the electron hits (is detected) only in a single location in space, like a particle. We have no idea exactly which of our many detectors will capture that electron. We can only calculate the odds of finding it at a certain detector location based on the wave function. And not just electrons, but all fundamental particles, composite particles, and even atoms behave this same way. Take a look at http://www.marksmodernphysics.com/ and click on Selected Animations and "Light through a Beam-Splitter" for picture of how wave functions work. Hope this helps.

BRian Greene's popular science book The Elegant Universe has a good introductory chapter on the concepts of Quantum Mechanics. (chapter 4 - microscopic weirdness).

So if black holes cannot form, how come there is so much compelling observational evidence for them (albeit indirect)? You know, like accretion discs, jets, extreme motion of neighboring stars, X-ray emissions, quasars, etc. Can Krauss explain what produces these phenomena if black holes don't exist?

I should have been more specific. I was talking about popular science books, not text books. I agree wtih Mike-from-the-Bronx. MMX is consistent with special relativity, but it does not ratify it. Good way of saying it.

Imagine two identical guns A and B which shoot bullets at the same speed. One is pointed perpendicular to the other. And, as expected, the speed of bullet A is the same as speed of bullet B. Now we place the two gun set-up on a uniformly moving train and repeat the experiment. Observers on the moving train find speed of bullets A and B are still the same. (Per Galieo's dictum, in a uniformly moving reference frame, its motion has no effect on phenomena within this frame). Let's place the two gun set-up on a table which can be rotated. Again place all this on the uniformly moving train. No matter how you orient the table, bullets A and B still record the same speed for observers on train. Now substitute two light sources for the two guns, photons for the bullets. and a moving Earth for the moving train. You have, in principle, the MM experiment. So the speed was the same value for two perpendicular light beams for observers on the Earth. This is just like the speed of the two perpendicular bullets is the same for observers on the train. MM only showed that there is no "ether wind" slowing down one direction of light more than another. MM tells us nothing about Einstein's light postulate. A number of books on relativity get this wrong. They either imply or state directly that MM demonstrated Einstein's light postulate. But it does not.

In GR, gravity is spacetime curvature. Take the Earth for example: - Time warp: The mass/energy of the Earth slows time down. That is time runs slower on the surface of the Earth than it does in outer space where there is no gravity. - Space warp: The radial distance between two point in space near the surface of the Earth is stretched compared to the distance between these same two points if there was no Earth present. This combination of time warp and space warp due to the presence of the Earth's mass/energy is called spacetime curvature. It is this spacetime curvature which determines the path of of everything in its neighborhood (such as the Moon around the Earth). Notice I said mass/energy. Both mass and energy warp space and time (produce spacetime curvature). So a photon which is massless but has energy also produces spacetime curvature (gravity). The so-called geodesic is the shortest possible path on a curved surface. Particles with mass and particles with zero mass like photons follow a geodesic path through curved spacetime. See link http://marksmodernphysics.com/ and click on selected animations, "What is a geodesic?" for simple example of a geodesic.

To answer the original question, the Michelson-Morley experiments did not prove Einstein's light postulate (speed of light independent of frame of reference). It just failed to find the so-called ether. The MM expeririments were conducted in a single frame of reference , the Earth. So they could not verify Einstein's light postulate one way or another. The first experiment to support Einstein's light postulate was the 1913 DeSitter binary star analysis. See link: http://en.wikipedia....star_experiment From a book I am writing on relativity for the non-expert: In the over 100 years since Einstein first proposed his light postulate, "thousands of scientific observations" have confirmed its validity.(i) For example, a vastly more accurate version of the de Sitter experiment was performed by MIT physicist Kenneth Brecher in 1977. His tests using Uhuru satellite images of X-ray pulsars verified Einstein's light postulate to one part in a billion.(ii) See link for more info: http://www.marksmodernphysics.com/ i Victor J. Stenger, Quantum Gods, Creation, Chaos, and the Search for Cosmic Consciousness, p. 74. ii Nigel Calder, Einstein's Universe, The Layperson's Guide, p. 176-178.

Recent Scientific American has good article on dark matter. Says it is most likely a new form of neutrinos.

In his book The Elegant Universe, Brian Greene has figures showing what some strings are predicted to look like. They are one-dimensional.

36grit - all this dimensions talk. In physics, there has to be a reason to chose the number of dimensions. And it has to be backed up by the mathematics. The core reason why physicists speak of four dimensions of spacetime is because of the so-called spacetime interval. Take two events. Each takes place at a certain point in space and certain moment in time. Per special relativity, the separation in time or time interval between the two events is not the same for two people moving with respect to each other (called time dilation). The same is true for the separation in space or space interval (distance) between the two events (called length contraction). But, and here's the key, a certain combination of the time interval and space interval called the spacetime interval is the same value for both people. In fact, the spacetime interval between two given events is the same for all people, no matter how they are (uniformly) moving. So that's why physicists use four dimensions in relativity (one for time and three for space). Time is relative. Space is relative. But the spacetime interval is absolute. This is a great simplifying feature in a world where everything is moving with respect to everything else. By the way, the formula is: The square of the spacetime interval equals the difference between the square of the time interval and the square of the space interval. So if you want to propose a different number of dimensions, you need a physics reason and the mathematics to back it up.

I believe the rule is: If you can in principle tell which path the electron took, there is no interference. Say you use UV light and a UV detector. Human eye cannot see the UV light, but UV detector can. So no interference.

The big bang theory is based on general relativity. And general relativity blows up (gives infinite answers) at time zero of the big bang. Thus our current best theory of gravity cannot describe the exact moment when the universe began. So what was our universe like at its very beginning? Nobody knows. We need a new theory. Physicists are working on a number of approaches to try and combine quantum mechanics with general relativty into a new theory of "quantum gravity". String theory is one example. Hopefully, once one of these theories is validated by experimental evidence, we may know more about the exact moment when the universe began. Until then, all new theories, proposals, ideas are speculation.

If our Sun were to suddenly disappear, the Earth would continue in its orbit around where the Sun was for about 8.3 minutes. Then the Earth would fly out of orbit into space in a straight line (ignoring the gravity from the other planets). Gravitational disturbances travel at the speed of light, some 670 million miles an hour. At this speed, it takes about 8.3 minutes to travel the roughly 93 million miles from the Sun to the Earth. Oh, and sunlight would also continue to be pouring down on the Earth for about 8.3 minutes after the Sun dissapears because light also travels at the speed of light.

Just some of the evidence supporting Enstein's theories of relativity: - Tests from atomic clocks on airplanes, rockets, and satellites, from the measured lifetimes of subatomic particles, and from numerous laboratory experiments that Einstein was right. Time is relative; time does slow down with motion; and in just the amount his formula predicts. -DeSitter binary star experiment in 1913 verified Einstein's Light Postulate; the speed of light is unaffected by the motion of its source. A vastly more accurate version Brecher at MIT in 1977 using Uhuru satellite images of X-ray pulsars verified Einstein's light postulate to one part in a billion! - In 1992, experiments at Colorado State University on muon lifetimes verified Einstein's time dilation to an accuracy of under 3 parts per million. - In 2005, researchers at NIST and MIT confirmed E = mc2 to an accuracy of better than one part in a million! - A laboratory experiment in 2010 by Miller, Peters, and Chu confirmed Einstein's gravitational time dilation to 7 parts in a billion. - GPS has to take into account both time dilation due to motion (special relativity) and gravitational time dilation (general relativity) in order to work. It is a continuous verification of Einstein's predictions. - In 2005 astrophysicist Kopeikin of the University of Missouri and his colleagues used the Very Long Baseline Array (VLBA) and four distant quasars to confirm Einstein's bending of light prediction to within an accuracy of three-thousandths of one percent. - Numerous telescopic observations of Einstein's gravitational lensing prediction. - Weisberg and Taylor found a steady decrease in the distance between the binary pulsars over time; giving indirect evidence for Einstein's gravity waves to better than a third of a percent. - Indirect astronomical evidence for black holes predicted by Einstein's general relativity, including motion of stars around unseen source at center of our Milky Way galaxy, accretion discs and jets, tremendous high-energy radiation from centers of many many galaxies (Quasars). In 2007, a suite of space-based and ground-based telescopes recorded X-ray evidence for over a thousand black holes in a narrow region of our sky about 40 times the size of the Moon. - Numerous independent observations and measurements supporting the expansion of the universe and the big bang; phenomena based on Einstein's theory of general relativity. This is far from a complete list. But the evidence supporting Einstein's theories is simply overwhelming. This is not to say that it is the last word. Science is a work in progress. There is still much physicists do not know (e.g. what dark energy is). There may someday be a new theory which explains things beyond Einstein's theories. But we must remain sceptical. Until a new theory gives us new predictions which are independently verified by observation, measurement, and test; it is still just speculation. To date, no other theory can compare with the outstanding predictive success of Einstein's theories of relativity.

Make a specific and testable prediction based on this new theory. That is predicting the results of a measurement that has never been performed before. And your prediction is diffferent than those from establsihed theory. Then if scientists independently do an experiment which agrees with your prediction and not the other theories, scientists will stand up and take notice regarding your new theory. This approach has been used to test the predictions of quantum mechanics, special, and general relativity. These theories have passed to extraordinary accuracy again and again and again. Yes, there are still questions. Physicists are far from knowing everything. But until your new theory passes the "prediction" test. it will be regarded only as speculation.

Time runs at a different rate depending on 1) relative motion and 2) gravity. Experiments verify that a clock on a moving airplane or rocket or satellite runs more slowly than an identical clocks on Earth (due to its motion). And they show that a clock on the ground runs more slowly than a clock at higher altitude (where gravity is weaker). So consider the universe with all kinds of motions and gravitational fields. Einstein is right. There is no universal rate at which all time passes. So what time is it?

And for blackbody radiation, the charged particles which are undergoing acceleration are the molecules and atoms of the object doing the radiating. Even though each molecule and atom is electrically neutral overall (number of electrons matches number of protons). It is primarily the acceleration of the protons inside the nucleii which produces this radiation. And this acceleration is the vibration of these molecules and atoms. We measure the average overall vibration as the temperature of the object. And that is why the blackbody frequency spectrum we see is determined soley by the temperature of the object radiating. Do I have this right?

In Feynam's QED approach, inside an atom photons are constantly being sent back and forth between electrons and protons. This photon messenger particle is what keeps the probability of finding an electron highest outside the nucleus. (Feynman's classic QED,The Strange Theory of Light and Matter explains this.)

The key here is energy. The frequency of the electromagnetic (light) wave is proportional to its energy. (E = hf where E is energy, h is Planck's constant and f is frequency.) So light has energy. And the presence of energy produces spacetime curvature (gravity). But gravity is additive. So the light emitted by the Sun, for example, adds to the gravity in its neighborhood.

True, but Newtonian predictions are not the same as Einstein's. Newton's law of gravity gives the same prediction as Einstein's gravitational time dilation. But when we include the warping of space, Einstein gives a different total prediction. For example, for starlight grazing ithe surface of the Sun, Newton predicts a bending of 0.875 arcseconds, but Einstein's general relativity (which includes the warping of time and space) predicts a value of 1.75 arcseconds. Observations confirm Einstein's prediction to great accuracy.

No. Space interval is the distance between two events. Time interval is the elapsed time between two events Both the space interval and time interval are affected by uniform motion (if you are moving with respect to me, you measure a different space interval and different time interval between the two events than I do.) The spacetime interval squared equals the difference between the square of the space interval and the square of c times the time interval. The spacetime interval is NOT affected by uniform motion. (For example, if you are moving with respect to me, you calculate the same spacetime interval between two events that I do). Time-like: The time interval squared is greater than the space interval squared. We write the spacetime interval as S^2 = (ct)^2 - (x^2 +y^2 + z^2) Space-like: The space interval squared is greater than the time interval squared Here we write the spacetime interval as S^2 = (x^2 + y^2 +z^2) - (ct)^2 Light-like: The time interval and space interval are equal, so the spacetime interval is zero.

Fisrt you say "If I am in a flat space (zero gravity) . . . the cuved path will be longer than it appears to someone in that space." Later you say " . . . the heght is contracted by about 9 nm . . ." Don't these two statements contradict each other. Did I miss something? As I understand it, a radial distance on the surface of the Earth is stretched not contracted by the Earth's mass/energy (as seen from flat space). So your first statement agrees with this, but not the second. Please clarify.

Because x^2 +y^2 +z^2 - (ct)^2 is invariant with uniform motion. That is for any two events, the space interval squared (x^2 +y^2 + z^2) MINUS the square of c times the time interval (ct^2) is the same value for all observers in uniform motion. We call this the spacetime interval (S^2) Mathematically, this is demostrated by the Lorentz transform for uniform motion. Does this help?