IM Egdall

The strong Equivalence Principle (EP) guided Einstein to his theory of general relativity, but is has its limitations. It only takes into account the warping of time, but does not include the warping of space in a gravitational field. Consider the bending of starlight grazing the Sun. Time warp only per the strong EP predicts a bending of 0.875 arc seconds. (This is the same value predicted by Newton's theory.) But space warp predicts another 0.875 arc seconds for a total bending of 1.75 arc seconds. Einstein's early gravitational equations based on the Strong EP included only the warping of time (circa 1912). In his field equations of 1915, he corrected this and included both the warping of time and space; giving the full 1.75 arcsecond prediction. A number of tests confirm the larger value. In what I believe was the most accurate of these tests, a 1975 experiment measured a value of 1.75 ± 0.019 arc seconds. (K. R. Lang, Astrophysical Formulae, Vol. 1, p. 159.) So the strong EP is a historically important but incomplete model of reality. (Edited for spelling and clarity. )

People think of time travel as pure science fiction. But according to Einstein, it is something we experience in our everyday lives. It is just that the amount of time travel we undergo is so small we don't notice it. But it is real. This is not pseudo-science, but solidly grounded in Einstein's theories of relativity. Per special relativity, time slows down with relative motion. This allows us to travel into the future. Again this is a tiny amount at the speeds we experience. If one day we develop a spaceship capable of speeds which are a significant fraction of the speed of light (which is about 670 million miles an hour), then this time travel effect would be appreciable. Per general relativity, the rate at which time passes is affected by gravity. A lower clock runs slower compared to a higher clock. This means we can, in effect, travel into the future and into the past just by changing altitude. (But when we arrive back where we started, some time has always passed.) The effect here is also miniscule, because Earth's gravity is relatively weak. See my blog posts under "Its Relative" for a detailed explanation. (Edited for spelling and clarity.)

Why does light always travel at the same speed (about 670 million miles an hour) in a vacuum? Per Maxwell, light is an an electromagnetic wave. And an electromagnetic wave contains a constantly changing electric field and perpendicular constantly changing magnetic field. Each field generates the other. The key word is changing. An electromagnetic wave must move (change) to exist. There is no such thing as an electromagnetic wave at rest. So Einstein thought about this and wondered what would happen if you traveled at the same speed as the electromagnetic wave (the speed of light). If you did, wouldn't the wave appear to be standing still with respect to you? If so, then according to Maxwell, there would now be no electromagnetic wave. How can this be? Einstein concluded you can never catch up to an electromagnetic wave, no matter what speed you travel at. The electromagnetic wave always travels at the same speed (the speed of light) with respect to you. This is (roughly) Einstein's light postulate, which has since been verified in a number of experiments. Relativity also says that any particle with mass cannot reach the speed of light. And all massless particles, such as photons (carriers of electromagnetism), gluons, and the yet to be discovered gravitons always travel at the speed of light.

I found one example demonstrating length contraction for microscopic objects in the Science Forums archives: "The people who design experiments for the accelerators have to take length contraction into acount. A spherical bunch of particles coming at you looks like a flattened ellipsoid due to relativistic shortening, and the detection probabilities and expected directions of ejecta are affected. So you could say that all these experiments are also testing length contraction, in that they are designed around it, and they work." REF: SelfAdjoint Mar 9, 2004 07:08 PM http://www.physicsforums.com/archive/index.php/t-15958.html

We in fact do time travel all the time and we don't need a time machine to do it. One way is through motion per special relativity. Say your roommate is at home sick. You travel to work, school, or whatever in your car. Then you drive home. Your watch has run a tiny bit slower than your roommates watch because you have been in motion with respect to him. (And because it is you who has experienced acceleration, not your roommate, time has run slower only for you.) So when you return home, your arrive a tiny bit into the future. See link: http://www.marksmodernphysics.com/ and click on Its Relative

Chris is right. Just ask a particle physicist at CERN or any other particle accelerator facility. They find exponentially more and more energy is required to get a particle with mass to go closer and closer to the speed of light. This indicates that an infinite amount of energy would be needed to get that particle to actually reach the speed of light. This is just what relativity predicts.

I think we agree here. All I was trying to point out is that from the point-of-view of an astronaut falling into a black hole. he is at rest and he sees the black hole approaching him faster and faster. At the event horizon, it is passing by him at the speed of light. So from his point-of-view, he would have to go faster than the speed of light to escape beyond the event horizon. (Edited for clarity.)

Unfortunately, there is always a bias towards accepted knowledge, whether conscious or not. Scientists are human. It is not impossible to get an unconventional theory published, it is just harder. And history shows that the theories we now accept initially also went through a period of doubt. When a measurement is made that agrees to good accuracy with a new theory (and not the established ones), then scientists take notice.

Speed is relative. From the infalling mass's reference frame, it is at rest. From this point-of-view, the event horizon is going by it in the opposite direction at the speed of light. That is why the infalling mass cannot escape the black hole.

The age of the universe is measured in "cosmic time". See link: http://ncse.com/evolution/science/age-universe-measuring-cosmic-time

I believe Chris said "From what I've read most cosmologists agree that the requiste pre-inflationary conditions are high temperature and (to resolve the "flatness problem") an almost perfectly uniform density ." I think this is not quite right. I beleive the almost perfectly uniform density of the universe is a result of inflation, not a pre-inflationary condition.

I think you will also find this same focus on measurement in the so-called Copenhagen interpretation of quantum mechanics by Neils Bohr etc. The physics of relativity and quantum mechanics works superbly. That is their predictions agree with measurements of nature to extraordinary accuracy. So these theories must be telling us something about reality. Perhaps "things" should come first, as you say, and the focus on measurement is a poor attempt to interpret what these theories are telling us about our world. It is, I think, a limit to our human perception. Or perhaps "things" are merely a human abstraction, and Bohr and Einstein are on the right track. This is a matter of philosophy. We can argue about it till the cows come home, but, unfortunately, we cannot determine whose philisophical argument is more valid through experiment. Or maybe we are like children who have discovered some remarkable models of the physical world. They work but we are unable to understand what they mean.

Here is an excerpt from a book I am writing on relativity which discusses your question. Hope it helps: For special relativity, Einstein came to embrace the notion that nothing exists beyond what we observe, what we can measure. He defined time as simply "what you read on a clock", and space as simply "the distance you measure between two points". The notion of time and space as anything beyond these "operational" definitions were, according to Einstein, simply the creation of the human mind. In other words, time is relative; how fast it passes depends on the motion of the observer. This is measurable. However, the concept of time we hold in our minds is merely an abstraction. Einstein applied a similar view to "space". But how can space contract? And how can the apparently same space contract differently for two (or more) observers in relative motion? Here Einstein is saying there is no "space" per se; only the distance between two points. The distance between the two points is measurable, albeit differently by the two observers. But "space" itself is again a mere abstraction. In summary, we must "stop thinking about 'space and time' (as) something that is 'given to us', and must instead think about 'measuring positions and times', which is something we can do," writes Morton Tavel, professor of physics at Vassar College. "Only our measurements have real existence. We build up an intuition of something called space and time, which we believe exists beyond these measurements . . . (But) Einstein's first commandment was to pay attention only to your measurements and worry later about the properties of the more abstract notion of space and time." (My italics.) Perhaps you find this approach to reality quite difficult to accept. So do I. Whether it is the years of thinking a certain way, or a particular bias of the human mind; I find it hard to accept that time and space are not real entities in their own right. But in special relativity, Einstein tells us that we must think of time and space as only the position on a clock and the markings on a ruler. Einstein was to later develop a much broader view of space and time; first in considering the spacetime physics of his mathematics teacher, Hermann Minkowski, and most significantly, in his development of general relativity. Ref: Victor J. Stenger, Quantum Gods, Creation, Chaos, and the Search for Cosmic Consciousness, p. 66. J.A. Wheeler, A Journey into Gravity and Spacetime, p. 5 Morton Tavel, Contemporary Physics and the Limits of Knowledge (Rutgers, 2002), p. 58, as cited in Am. J. Phys. 74, 891 (2006).

There is an excellent discussion of this entropy/cosmology issue in Brian Greene's book, The Fabric of the Cosmos. And it is in plain English. He says that the universe began in a highly ordered (low entropy) state, and this state was highly unlikely. The entropy (disorder) of the universe has been increasing ever since. He goes on to say this highly ordered state came from inflation. As the inflation field slid down to lower energy, it relinquished pent-up energy, producing about 10^^80 particles of matter and radiation, resulting in a huge amount of entropy. Ever since the end of inflation, entropy has continued to increase due to the effects of gravity. OK, so my question is: where did this inflation come from? And what was the entropy of the universe before inflation? And does this violate the second law of thermodynamic? Unfortunately, I don't think anyone has an answer for these questions outside of speculation. Perhaps a new quantum gravity theory will shed more light on this issue.

And we must remember that the data supporting a flat universe is only data from the visible part of the universe, that is the part of the universe we can see. "In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion." Wikipedia So what about the rest of the universe we cannot see? How can we know whether the entire universe, whether finite or infinite, is indeed flat (net zero spacetime curvature)?

Thanks for the clarification. Yes, the vacuum energy discrepancy is a great puzzle. What I was talking about is conservation of energy. Perhaps a better wording would have been that for virtual particles, conservation of energy is only temporarily violated. Per The Particle Adventure: "The kinetic energy plus mass of the initial decaying particle and the final decay products is equal."

Think of it this way. The energy in "empty" space is zero, right? Physicists used to think this was true until quantum mechanics, The Uncertainty Principle of quantum mechanics says that the more accurately we can know the energy, the less accurately we know the time. And vice versa. So the shorter the time period over which we make our measurement, the more uncertain is the value of the energy. This explains the existence of virtual particles. They can spontaneioulsly appear out of empty space, but over only a limited amount of time. For per Heisenberg's formula, a virtual electron and virtual positron, each of mass 9.11x10^^-31 Kg, can pop up and remain in existence for no longer than 3.22x10^^-22 seconds. One virtual particle has positive energy and one has negative energy, so the average energy remains zero. So, yes, in this sense, something (virtual particles) can come from nothing (empty space). And physicists have speculated that the big bang was a quantum fluvtuation, effectively creating our universe. See links: http://en.wikipedia.org/wiki/Virtual_particle http://en.wikipedia.org/wiki/Big_Bang

Special relativity says that nothing can travel through space faster than the speed of light. That is, anything with mass must travel through space at speeds less than the speed of light. And particles with zero mass must travel through space at the speed of light. But general relativity says that space itself can expand faster than the speed of light, and it does. And per Quantum Electrodynamics(QED), photons do not travel at only the speed of light through space. Per Richard Feynman's QED, The Strange Theory of Light and Matter,(p. 89): "There is a (probability) amplitude for light to go faster or slower than the conventional speed of light. However, these amplitudes are very small and tend to cancel out over large distances."

The universe has not been expanding at a constant rate throughout its lifetime. In fact, at this very moment the expansion is speeding up: The big bang/ inflation theory holds that the universe first expanded enormously in an extremely brief moment of time, then continued to expand at a much much slower uniform rate for billions of years, and then this expansion began to speed up (I think some 5 to 7 billion years ago) due to an unknown entity called dark energy. The reason this scenerio is considered our best model on the origin and evolution of the universe is because of all the evidence found which supports it, like the comsic microwave background. Suggest you look at link below on how big bang theory predicts the cosmic microwave background: http://www.astro.ubc.ca/people/scott/cmb_intro.html

The article refers to "ordinary" barionic matter, not dark matter.

One of the virtual particles has positive energy and one has negative energy. So when they collide, no energy (such as gamma rays) is produced.

A terrific discussion on the nature of time is in Brian Greene's classic The Fabric of the Cosmos. In chapter 5, he discusses the meaning of "now." To quote: "Observers moving relative to each other have different conception of what exists at a given moment, and hence they have different conceptions of reality." So one observer's list of what is happening "now" is not necessarily the same as another's. Now is relative! Relativity reveals a universe that is very strange. An enormous amount of evidence confirms that this strange universe is indeed our universe.

I like to think of the Uncertainty Principle in quantum mechanics as a kind of see-saw. One side goes up when the other goes down. For example, the more accurately I know the location of a particle, the less accurately I can know its momentum (mass times velocity). And vice-versa. Say I know to great accuracy where a particle is. This means I can only know its speed and direction (velocity) to very poor accuracy. So since I do not know exactly where the particle is going or how fast, I cannot predict where it will be in the future (or where it was in the past).

DrRocket - Excellent summary on time travel issues. I edited to add these thoughts: Special Relativity - You travel from Earth to outer space and back in a rocket at say 87% the speed of light. Say to you on the rocket the round trip takes 5 years. But per special relativity, to us on Earth 10 years have gone by. This is because time runs faster on the Earth than it does on the rocket. Say you leave in the year 2020. Per your rocket calendar, when you arrive back on Earth it is the year 2025. But calendars on Earth say it is the year 2030. So you arrive back on Earth 5 years into the future! An everyday example: You go to work or school but your roommate stays at home. Your motion to and from work or school means your time runs a tiny bit slower than time for your stationary roommate. So you arrive back at home at the end of the day a tiny bit into the future. We are all time travelers. General Relativity - Here we travel into the future and into the past. Say you go to the top of a mountain. While there, your time runs (a little bit) slower than time at sea level. At some later time, I go to the mountain top. Since time runs faster on the mountain top, I find more time has gone by on the mountain top than at sea-level. So I have traveled into the future. But what if I stay at sea-level and at some later time, you travel from the mountain top to sea-level. Because time runs slower on the surface than on the mountain top, you effectively travel into the past! But you cannot return to the surface to a time that is before when you left - casuality is maintained. (For simplicity,we ignore the effects on time of motion up and down the mountain here.) So whenever we are in motion or change altitude, we are time travelling! And this is not a possible outcome of relativity, but a definite outcome based on experimentally verified time effects.

The following link gives a picture of how everything travels at the speed of light through spacetime: