Einstein's GR passes extreme test

Yep, At the Robert C. Byrd Green Bank Telescope (GBT), Green Bank, WV... my home state. One of my uncles was a technician involved in its construction... he's still alive, mid '80's.

Named after Democrat scumbag Senator Robert C. Byrd. Byrd and I share a 50th anniversary this summer... he for being elected to the Senate (and he's still there), and me celebrating my 50th high school reunion.

West Virginia shows what can happen when you get the fully involved socialist state... almost everyone on welfare, beholding to the state for their very existence.

Less population than in 1958 when I went off to MIT... anyone with any smarts flees the state.

...Jim Thompson

It is absolutely amazing that Einstein was able to conceive of all these effects almost 100 years ago. He was definitely smarter than everyone I know -- combined!

To me, however, it is still sad that people (like the author of that article) still refer to gravity as having a "tug". It is one of my goals to get people to stop thinking about gravity as a force, since if an object is affected by another's mass, and as long as that object is not touching anything in its way (geodesic), then there are no forces involved (apart from the so-called tidal forces due to spacetime curvature gradients).

Sigh...

Bob

I like the thought experiment where a beam of light is shining across an accelerating elevator car ... the beam appears to bend as if it was under the influence of the tug of gravity. Acceleration and gravity are duals, in some cases. In QM, nobody knows for sure.

One has to be very careful when using the word "acceleration" when speaking of gravity. Photons following their geodesics in "curved spacetime" are not undergoing what's normally thought of as acceleration just as someone freefalling toward the earth (excluding effects of atmosphere rubbing against you) is not undergoing acceleration.

These are the toughest concepts to swallow in General Relativity. Einstein's concepts of "equivalence" should be discussed much more than they are (imho). As Einstein's GR defines it:

Objects that are freefalling (e.g., someone floating out in deep space and someone plunging toward a planet's surface) are both equivalently in inertial frames of reference. Objects that are not freefalling (e.g., someone standing on Earth and someone in a lit rocket) are in both equivalently in accelerated frames of reference.

Newton's F=ma formula is still valid -- it just needs to be taken in the proper context. Newton would have had a hard time believing that there are no forces involved in keeping the planets orbiting the sun. Where was Einstein when Newton needed him the most?

Bob

The book I am reading is helping me to wrap my brane around some of these concepts. Much of the difficulty is our accepted ways of visualizing time and space, based on our own senses. The mass and energy equivalence is just the beginning, and then the curvature of space-time and the fact that mass and time affect each other is the basis of an entirely new way of thinking for people who grew up with Newtonian physics. I said that time and mass affect each other because I think cause and effect may be viewed in both ways.

It is difficult to think of one standing on the earth as accelerating and yet not changing in velocity. But acceleration involves terms of time, so you may gain velocity because the time element is changing, and not the spatial element, as we perceive it to be.

Paul

Paul,

When you say "it is difficult to think of one standing on the earth as accelerating and yet not changing in velocity", you need to get into bed with the first postulate of Special Relativity:

Thanks, excellent news.

In QM it appears nobody is sure of anything except possibilities.

Do you disagree with the theory that there are four fundamental forces, of which gravity is one? When considering an object falling from a buiding to the pavement below, are you saying there's a better way of thinking of gravity than as a force? When you get people to stop thinking of gravity as a force, what do you want them to think of, with respect to the above?

Ed

"ehsjr" wrote in message news:38sfk.13$df1.9@trndny02...

I do! There's only three: Gravity Nuclear Strong Nuclear Electroweak (includes E&M ;-) )

At least that's my understanding of electroweak. Which is, as the name suggests, rather weak. :-p

Tim

Is gravity a fundamental force? Not in the sense that it applies an attractive force between objects. In fact if it were, it would not be able to have any effect on (massless) photons -- but it most certainly does (look up "gravitational lensing"). The ability of gravity to affect photons (electromagnetic radiation) was predicted by Einstein. When it was finally observed, Einstein instantly became a lot smarter than he was previously. It was huge.

I don't know much about the strong nuclear force.

With the electro-weak force, it applies a force between objects regardless of the state of those two objects. For example, two charged particles will exhibit a force on each other regardless of whether those two charges are freely traveling or not.

What I've learned is that there are no forces involved, due to gravity, when an object is freely travelling in our universe (that is, as long as it's not being influenced by a strong nuclear force or an electro-weak force).

The thing that keeps our moon in its orbit around the Earth is the curvature of spacetime. This curvature is due to the mass of the Earth and our moon. There is no force that can be measured that attracts the moon to the Earth.

The only forces involved, when speaking of gravity, is when an object is kept from its freely-travelling path (its geodesic). For example, when a person is falling toward the Earth, the interruption of that person's freefall (that sudden stop at the bottom) is due to a force. The thing that keeps that person's remains (may he/she rest in pieces) on the surface of the Earth is a force upwards equal to his/her weight.

Bob

Although GR predicts that gravity would affect the trajectory of any massless particles moving at finite velocity, photons have mass. Although photons do not have rest mass, they do have finite velocity and nonzero momentum.

- Don Klipstein ( snipped-for-privacy@misty.com)

AIUI, the equivalency of mass and energy means that anything that exists can be said to have one or the other, or some of both. Or perhaps you can characterize an object in terms of either. It gets interesting when one contemplates a quantum mechanical vacuum and the spontaneous creation of a particle and an anti-particle. Yet the combination of these does not result in nothing, but releases energy in some form. I could better accept it if matter and anti-matter had mass and anti-mass, or energy and anti-energy, the sum total of which would be zero, but apparently it is not quite so simple (or elegant).

Paul

Generally photons, maybe also a pair of a neutrino and the corresponding antineutrino.

Positrons and electrons have the same mass - both positive. If one collides with the other, the amount of energy released is positive.

- Don Klipstein ( snipped-for-privacy@misty.com)

So how much mass does a proton have at 1 TeV kinetic energy? Hint: the correct answer is 938 MeV/c^2. Mass is always RME, and photons have none of it. They are able to posess momentum because they move at the speed of light; any mass would immediately restrict their velocity considerably. And we all know c is c whether 60Hz or 600THz.

Tim

No. It doesn't help at all to say photons have mass.

They have both momentum and energy but no mass. If they had a non zero mass then they could not travel at exactly c.

Photons follow geodesics in spacetime that represent the shortest time path between any two points (all other paths the photon could take roughly cancel out).

Non-zero mass is currently thought to possibly afflict neutrinos. Although I would like to see better experimental evidence for this. Intro at:

If you try pretending that photons have mass m = p/c and then work out their expected deviation in a gravitational field you get an answer that is wrong by exactly a factor of two. See for example:

ISTR Einstein also did that calculation at one point before he had completely formulated GR (although Mitchell and Laplace thought of it much earlier). It was the experimental confirmation of the deviation predicted by Einsteins new theory by Eddington during the total eclipse of 1919 that first validated General Relativity.

Regards, Martin Brown

messagenews:487d22d3$0$25503$ snipped-for-privacy@news.coretel.net...

I pretty much agree. The neutron star pair is separated by ~ 2 x Earth to Moon, so the tidal "forces" would be rather extreme compared to the Earth Moon system. Given the unknowns within the structure of n-stars, I could fudge any parameters to agree with nearly any theory. That said, by setting the parameters to agree with GR, provides insight into the structure. But then 3rd body contamination may be present. It would be quite surprising if this could really be called a "confirmation of GR", but it doesn't disprove it. Regards Ken S. Tucker

attractive force between objects. In fact if it were, it would not be able to have any effect on (massless) photons -- but it most certainly does (look up "gravitational lensing"). The ability of gravity to affect photons (electromagnetic radiation) was predicted by Einstein. When it was finally observed, Einstein instantly became a lot smarter than he was previously. It was huge.

No. It doesn't help at all to say photons have mass.

They have both momentum and energy but no mass. If they had a non zero mass then they could not travel at exactly c.

Photons follow geodesics in spacetime that represent the shortest time path between any two points (all other paths the photon could take roughly cancel out).

Non-zero mass is currently thought to possibly afflict neutrinos. Although I would like to see better experimental evidence for this. Intro at:

nonzero momentum.

If you try pretending that photons have mass m = p/c and then work out their expected deviation in a gravitational field you get an answer that is wrong by exactly a factor of two. See for example:

ISTR Einstein also did that calculation at one point before he had completely formulated GR (although Mitchell and Laplace thought of it much earlier). It was the experimental confirmation of the deviation predicted by Einsteins new theory by Eddington during the total eclipse of 1919 that first validated General Relativity.

Regards, Martin Brown

..

He didn't. At least not where neutron stars are concerned. They were only discovered by Jocelyn Bell in 1967, her supervisor Anthony Hewish got the Nobel prize for working out the physics of pulsars. To find one in a bound orbit surprised most astronomers. The birth of a pulsar is a very violent supernova event.

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The tidal forces are very brutal for a pair of pulsars in tightly bound slowly decaying orbit. The first work on the double pulsar entailed determining the rate of energy loss as a result of gravitational radiation. It matched theory.

The great thing about this system is you have extremely accurate clocks in a tightly bound orbit (star quakes excepted they are almost as good as our best atomic clocks). When the first binary pulsar was discovered the measurements revealed a systematic error in observational timing that was eventually traced to an error in the computer algebra program that was used to compute the GR path corrections in our solar system.

A paper on the oldest known one with 30 years of data summarised is online free access at:

There is a rather nice summary of gravity through the ages at:

There are enough observables in the new system to tightly constrain theoreticians.

It would have to be a special sort of third body to hang around in that neighbourhood. And if it was there and close enough to make a real difference it would prevent the orbital parameters from matching the GR expected values.

Regards, Martin Brown