Tuesday, February 2, 2016

Brief description of general relativity

I listened to the UK BBC An Infinite Monkey's Guide to General Relativity:
It is 100 years since the publication of Einstein's great theory, and arguably one of the greatest scientific theories of all time. To mark the occasion, Brian Cox takes Robin Ince on a guided tour of General Relativity. With the help of some of the world's leading cosmologists, and a comedian or two, they explore the notions of space time, falling elevators, trampolines and bowling balls, and what was wrong with Newton's apple. It is a whistle stop tour of all you will ever need to know about gravity and how a mathematical equation written 100 years ago predicted everything from black holes to the Big Bang, to our expanding universe, long before there was any proof that these extraordinary phenomena existed.
They try to give some short simplified descriptions of general relativity, but they seem unsatisfactory to me. So I will try.

General relativity is a way of reconciling gravity with the causal structure of spacetime. Newtonian gravity involves masses exerting a force over a large distance, without any intervening mechanism. Maxwell devised a causal theory of electromagnetism, where charges generate local fields that propagate to interact with other charges. Relativity interprets this in terms of a geometry of space and time, and describes gravity as a similar curvature field that propagates to other masses.

They said that GR was developed from theory, not experiment. That is partially true, but I would not say it that way. Special relativity was driven by trying to reconcile the Michelson-Morley experiment with Maxwell's equations. The result was spacetime non-Euclidean geometry. Was that theory or experiment? Some of each.

A commenter notes that Einstein's work did not directly use the Michelson-Morley experiment. That is true, but even Einstein conceded that the experiment was crucial for the discovery of special relativity. It just was not needed for Einstein's recapitulation of Lorentz's work.

Solar system observations were the experimental data for gravitational theory. Newton and Laplace showed that gravity must be instantaneous or propagate much faster than light to explain planetary orbits. Poincare used relativity to show that gravity could propagate at the speed of light, and also to partially explain anomalies in Mercury's orbit. Einstein showed that a Ricci-flat spacetime could fully explain the Mercury anomaly, and that convinced him that he had the correct relativistic gravity theory.

Other aspects of GR were not properly tested until decades later, but experiment was still crucial for its original development.

In the last month there have been a lot of rumors about a pending announcement of the discovery of gravity waves, and the news stories always say that this would be a vindication of Einstein's theory of general relativity. As a reader points out, Einstein himself did not believe in gravity waves, and tried to publish a paper proving that they did not exist. It was Einstein's only refereed paper, and the referee said that it was wrong.

Poincare proposed a theory of relativistic gravity waves in 1905. He was decades ahead of Einstein on this point.


  1. Is there any movement possible in spacetime? No. You have already employed your time metric (badly) as a fourth spatial dimension (time is not space or geometry). Unless you wish to screw with meta-time, literally nothing ever can happen (much less move, orbit, collide, or affect one another) in spacetime. Movement is not an illusion, neither is time, though it does appear that people who think so were just born an instant ago.

    How is a model which can not account for movement, an impulse to motion, or have more than a single mass be so useful for determining anything but how dysfunctional physics has become?

    The basis of all physics is actually the study of motion in time and how those motions interact with various forces and other objects. All observation of the influence of gravity has ALWAYS been through motion, no motion no interaction, no interaction no observation of the effects of gravity.

    Spacetime is at best a purely mathematical space which can describe at most one imaginary thing that can't move or start to move or interact with anything else...which also makes it utterly useless to predict things in a universe of countless objects where everything is moving.

    Call me unimpressed.

    1. You are unimpressed because you don't listen to Roger. The map is not the territory. Zeno is what smart people moved beyond with numerical methods. The Kantian "noumenal" may never be directly described. It's just silly mathematicians that get caught in logical paradoxes because they use outdated math.

      We just tolerate the Aspies but no one thinks they are actually smart: Numerical Methods Already Unified Physics

    2. Matthew,
      I do listen to what Roger says. That does not mean I always agree with him.

      As to your map analogy, I never did say a map is the territory. I will say that a map that does not describe any territory is useless. Spacetime describes a block universe with only a single block in it where nothing moves or does anything. This does not in any way resemble our universe we live in, and I can prove it to be silly just because I can demonstrate that things do not happen at the same time just by typing these words to you on the screen. If you read them, you know that that sentences are sequential in nature, and if there is a sequence of any kind, you do not have a universe with spacetime. Math is also impossible in space time since there are no operations possible in timeless arenas. Math is a manipulation of quantities using operations. It requires process, there is no instant anything in reality or physics.

      Why you invoke Zeno is beyond me. 'Smart people' didn't actually didn't move beyond him, as plenty of 'smart people' to this day foul themselves in science and mathematics journals daily quacking about paradoxes as 'design features'.

      In fact, if a person actually was smart, they would realize Zeno was using the concept of paradox to teach a lesson: There are no paradoxes in reality, only in faulty assumptions, reasoning, logic, or math. If your initial assumptions or premises are incorrect, you get a paradox. If you notice, infinity plays a large part in all the paradoxes of Zeno (and Einstein). Think it through, the paradox is actually the infinity itself, it isn't real, it is not truly measurable though you can wrap it up in words, and is ridiculous when you try to encompass it in any kind of calculation in a set amount of time, being that doing an unending process (no matter how quickly) of any kind would take an infinite amount of time. The flaw of the assumption no one usually spots is: Zeno admits you can travel half a distance in his premise setting up his paradox. The moment you confront the fact that even moving half of a distance is possible, so movement over a distance is openly declared as possible, possible, then it follows moving the whole distance is also possible, and you have resolved the paradox: to even entertain an infinity in a finite process, you in effect contradict yourself. If the length of time is finite, so is the process. You can not intermingle the finite and the infinite logically, physically, or mathematically, as there is no ratio possible between them. Injecting an unending number of steps into any finite process effectively stops the process, and any half competent first year programming student can tell you this is a big no-no. Why mathematicians and physicists who should know better do it and think they can succeed is a lovely illustration of how stupid smart get when they confuse the memorization of countless bad ideas with understanding of which ideas are actually any good.

      Outdated math my ass. All math is reason's bitch. Calculation is utterly useless when the question is isn't even wrong.