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First speed of gravity measurement revealed
NewScientist.com ^ | 01/07/2003 | Ed Fomalont and Sergei Kopeikin

Posted on 01/07/2003 6:23:34 PM PST by forsnax5

The speed of gravity has been measured for the first time. The landmark experiment shows that it travels at the speed of light, meaning that Einstein's general theory of relativity has passed another test with flying colours.

Ed Fomalont of the National Radio Astronomy Observatory in Charlottesville, Virginia, and Sergei Kopeikin of the University of Missouri in Columbia made the measurement, with the help of the planet Jupiter.

"We became the first two people to know the speed of gravity, one of the fundamental constants of nature," the scientists say, in an article in New Scientist print edition. One important consequence of the result is that it places constraints on theories of "brane worlds", which suggest the Universe has more spatial dimensions than the familiar three.

John Baez, a physicist from the University of California at Riverside, comments: "Einstein wins yet again." He adds that any other result would have come as a shock.

You can read Fomalont and Kopeikin's account of their unique experiment in an exclusive, full-length feature in the next issue of New Scientist print edition, on sale from 9 January.

Isaac Newton thought the influence of gravity was instantaneous, but Einstein assumed it travelled at the speed of light and built this into his 1915 general theory of relativity.

Light-speed gravity means that if the Sun suddenly disappeared from the centre of the Solar System, the Earth would remain in orbit for about 8.3 minutes - the time it takes light to travel from the Sun to the Earth. Then, suddenly feeling no gravity, Earth would shoot off into space in a straight line.

But the assumption of light-speed gravity has come under pressure from brane world theories, which suggest there are extra spatial dimensions rolled up very small. Gravity could take a short cut through these extra dimensions and so appear to travel faster than the speed of light - without violating the equations of general relativity.

But how can you measure the speed of gravity? One way would be to detect gravitational waves, little ripples in space-time that propagate out from accelerating masses. But no one has yet managed to do this.

Measuring the speed of gravity

Kopeikin found another way. He reworked the equations of general relativity to express the gravitational field of a moving body in terms of its mass, velocity and the speed of gravity. If you could measure the gravitational field of Jupiter, while knowing its mass and velocity, you could work out the speed of gravity.

The opportunity to do this arose in September 2002, when Jupiter passed in front of a quasar that emits bright radio waves. Fomalont and Kopeikin combined observations from a series of radio telescopes across the Earth to measure the apparent change in the quasar's position as the gravitational field of Jupiter bent the passing radio waves.

From that they worked out that gravity does move at the same speed as light. Their actual figure was 0.95 times light speed, but with a large error margin of plus or minus 0.25.

Their result, announced on Tuesday at a meeting of the American Astronomical Society meeting in Seattle, should help narrow down the possible number of extra dimensions and their sizes.

But experts say the indirect evidence that gravity propagates at the speed of light was already overwhelming. "It would be revolutionary if gravity were measured not to propagate at the speed of light - we were virtually certain that it must," says Lawrence Krauss of Case Western Reserve University in Cleveland, Ohio.


TOPICS: News/Current Events
KEYWORDS: crevolist; gravity; podkletnov; realscience; science; stringtheory; tvf
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To: ward_of_the_state
the time it takes the bowl of spaghetti to hit the floor is directly proportional to the time it takes for the dog to arrive at the spill and clean it up and inversely proportional to the time it takes for mom to grab a towel and clean it up!

Also, there is no relationship between the amount of spaghetti that spilled and the extent of the stain. The stain will spread to a surface area of 2304 square-inches regardless. :^)

101 posted on 01/08/2003 9:45:39 AM PST by Aracelis
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To: ward_of_the_state
Can somebody please get Bill Nye the Science Guy to explain this so I can understand it?

ROFL! These guys just love to out-science each other. As long as your feet stick to the floor, don't worry about it (assuming they are not stuck in a pile of spaghetti).

102 posted on 01/08/2003 9:51:18 AM PST by Aracelis
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To: Gary Boldwater
Not all motion is relative. Acceleration is absolute.

LOL. As I wrote my post, I wondered whether some lawyer was going to come along and say that acceleration is not relative. (That doesn't make it absolute, either, but that's a longer story.) The poster wasn't talking about acceleration, but about velocity.

103 posted on 01/08/2003 9:53:49 AM PST by Physicist
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To: forsnax5
I've always thought of gravity as a field of some kind even before learning about fields as a mathematical construct. Never worried about the speed of propagation of a change in field, still don't, but it looks like somebody ought to, so it is good to know that such changes propagate at the speed of light. Waiting for news of the gravitational blue shift.
104 posted on 01/08/2003 10:00:10 AM PST by RightWhale
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To: blam; Physicist; Dan Day
Here's a link to a response to the news item at the top. It contains some interesting revelations about the experimenter's methods:

Abstract. New findings announced today by S. Kopeikin are invalid by both experimental and theoretical standards. They do a disservice to science in general and the advancement of physics in particular because the announced findings do not represent the meaning of the actual experimental results and cannot possibly represent the physical quantity heretofore called "the speed of gravity", which has already been proved by six experiments to propagate much faster than light, perhaps billions of times faster. 


105 posted on 01/08/2003 10:29:24 AM PST by aruanan
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To: forsnax5
Interesting. Thanks for the post...
106 posted on 01/08/2003 10:30:27 AM PST by MeekOneGOP
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To: blam; Physicist; Dan Day
Whoops. Here's the DIRECT link to a response to the news item at the top. It contains some interesting revelations about the experimenter's methods:

Abstract. New findings announced today by S. Kopeikin are invalid by both experimental and theoretical standards. They do a disservice to science in general and the advancement of physics in particular because the announced findings do not represent the meaning of the actual experimental results and cannot possibly represent the physical quantity heretofore called "the speed of gravity", which has already been proved by six experiments to propagate much faster than light, perhaps billions of times faster. 


107 posted on 01/08/2003 10:31:12 AM PST by aruanan
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To: forsnax5
GRAVITY!
It's not just a good idea...
Its THE LAW!




(oooh I am glad I was first with that one)
108 posted on 01/08/2003 10:31:20 AM PST by Mr. K
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To: RightWhale
Never worried about the speed of propagation of a change in field, still don't, but it looks like somebody ought to, so it is good to know that such changes propagate at the speed of light.

See this link for a rebuttal of this paper referred to at the top.
109 posted on 01/08/2003 10:35:17 AM PST by aruanan
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To: aruanan
I'm going to agree with Aruanan on this one. Kopeikin has released a press release on something that hasn't cleared peer review yet. It's extremely frustrating when someone tries to cheat the peer review process. At least have the good sense to wait until your paper has been accepted for publication!

space.com notes that the article is still in the peer review process because of problems from the reviewer.

On the other hand, if it is shot down, it is a vindication of the peer review process, something that Aruanan claimed was part of the corrupt scientific establishment just the other day.

110 posted on 01/08/2003 10:48:18 AM PST by ThinkPlease (Tag, you're it!)
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To: ThinkPlease
part of the corrupt scientific establishment

Whoa!!!! Multimode attack!!!! Shields up!!!!

111 posted on 01/08/2003 10:52:11 AM PST by RightWhale
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To: Physicist
Sorry I'm late to the thread! I had to come all the way over from the "I-told-you-so" Department.

LOL. :D
112 posted on 01/08/2003 11:19:49 AM PST by FreeTheHostages
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To: Southack; Dan Day
ROFL. Heck, you can see that Gravity bends Light by simply holding your thumb between your eye and a light source and looking at the edges, however, this phenomenon doesn't happen when Light is traveling at slower speeds because so few Gravitons are being emitted.

Ah, c'mon, Dan, no response to this? PLEASE PLEASE I enjoy reading your responses. And if ever there were a highly inflexible, NON-bended straight line, Southhack's given you one.

Now, some will say it's a cruel sport. It's too easy. It's like hunting elephant in a zoo. But I say: Go for it!
113 posted on 01/08/2003 11:25:23 AM PST by FreeTheHostages (making the popcorn)
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To: Physicist; Southack
Looks like somebody's forgotten that all motion is relative.

Hmm, I hate to be niggling, but don't you think you presume to much in this case by using the word "forgotten"?
114 posted on 01/08/2003 11:26:42 AM PST by FreeTheHostages (making the popcorn, enjoying the show)
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To: Semper911
I was told in high school that gravity didn't exist...the world sucks. Since then, coomon sense tells me that gravity does not 'pull', it pushes. HOWZAT?
115 posted on 01/08/2003 11:50:16 AM PST by metacognative
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To: ThinkPlease
On the other hand, if it is shot down, it is a vindication of the peer review process, something that Aruanan claimed was part of the corrupt scientific establishment just the other day.

Your characterization of what I said "just the other day" is an over-generalization. The funny thing about the reception of Kopeikin's interpretation of the results of his experiment is that it ignores previously published data* already subject to peer review on a very controversial subject that present quite a different outcome.

*T. Van Flandern and J.P. Vigier (2002), “Experimental Repeal of the Speed Limit for Gravitational, Electrodynamic, and Quantum Field Interactions”, Found.Phys. 32, 1031-1068.

T. Van Flandern (1998) , “The speed of gravity – What the experiments say”, Phys.Lett.A ,/em>250, 1-11.
116 posted on 01/08/2003 12:29:34 PM PST by aruanan
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To: RightWhale
part of the corrupt scientific establishment

See what was actually said. You know, trust, but verify?
117 posted on 01/08/2003 12:43:58 PM PST by aruanan
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To: Nick Danger
"Perhaps there is a minimum distance -- a quantum of space -- and a minimum amount of time -- a quantum of time"

If there is a quantum of space and a quantum of time this could be the soucre of the uncertainties of quantum mechanics: since a particle would have to jump from one point on some invisible grid to another, there is uncertainty about it's position when it is jumping from one point to the other.

The bad news is that if we find out that space and time is quantized, it could be because we are all part of some very sophisticated "first-person shooter" game driven by a computer with a fixed clock speed (the time quanta) and a fixed resolution (the space quanta)!

Where's my missile launcher?

118 posted on 01/08/2003 1:06:33 PM PST by who_would_fardels_bear
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To: Dan Day
First, it would be *really* hard to vibrate a mass large enough to produce any non-trivial amount of gravity.

Then, how would one modulate the baseband? Or would we have to use CW?

119 posted on 01/08/2003 1:15:37 PM PST by Chemist_Geek (Better Living Through Chemistry!)
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To: Chemist_Geek
how would one modulate

Mass has to move. Perhaps a rotating double star would give off CW strong enough to detect. Perhaps Jupiter is massive enough and close enough for the instrument to pick up something.

120 posted on 01/08/2003 3:08:45 PM PST by RightWhale
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To: RightWhale
OK, here's a question I've been asking various people for over 15 years:

Einstien stated that there is no way to measurably differentiate between gravity and accelleration.

Newton invented calculus to prove that any gravitational mass can be treated as if all the gravity was coming from a single point, at the center of mass. A point source.

Let's suppose that you were in an elevator, under either accelleration or gravity, and you hung two static pendulums from the ceiling, from strings.

My question: Under gravity, wouldn't the pendulum strings angle toward each other, exactly towards Newton's common point? But under accelleration, wouldn't the strings would be perfectly parallel?
121 posted on 01/08/2003 4:41:29 PM PST by MonroeDNA
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To: MonroeDNA
Good question. Don't know right off. We'll get back to you.
122 posted on 01/08/2003 4:47:55 PM PST by RightWhale
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To: DoctorMichael
Does gravity travel through masses? I think it does. How do we account for that if it is gravitons? What is its speed through different masses?

I think I will go with the idea of a rubber sheet for now.

123 posted on 01/08/2003 5:14:32 PM PST by Cool Guy (Why do we need this one?)
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To: MonroeDNA
Technical staff has examined the situation and wants to know the size of the elevator. Is it small -- 20 passengers or so, or is it large -- the size of the moon? Makes a difference. If it's small, it will be impossible to measure a difference. If it is large, the gravity field will have measurable curvature and the acceleration will be of course uniformly flat. So it depends on whether you want to treat the gravity field as a uniformly flat, infinitesimally small segment of the spherical gravity field.
124 posted on 01/08/2003 5:16:57 PM PST by RightWhale
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To: Physicist
Now you're calling me a lawyer??!! :)
I'm of the school that acceleration is absolute, relative to only one preferred reference frame. Why doesn't SRT apply to accelerated frames? Isn't the entire cosmos a series of accelerated frames, starting on the rotating earth (in a gravitational field no less) revolving around the sun which is part of a revolving cluster of stars and so on. Even the velocity of light is referred to as local, it is not universal.
125 posted on 01/08/2003 5:19:31 PM PST by Gary Boldwater
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To: Cool Guy
Gravity is a field and does not move. The field will be non-smooth where there are masses, a bumpy set of values. The thing they are measuring the speed of is the change in intensity and direction of the field at two different points with respect to time. The speed of propagation of a change in intensity, rather than a motion of the field. The field can't move.
126 posted on 01/08/2003 5:22:14 PM PST by RightWhale
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To: Gary Boldwater
I'm of the school that acceleration is absolute, relative to only one preferred reference frame.

Consider one observer at the center of the Earth and another at the surface, in a sealed box. Is the one on the surface accelerating? According to his measurements--remember, he can't see his surroundings, so he measures his acceleration with a scale--he is (c.f. the equivalence principle). According to the observer at the center of the Earth, he is not.

Why doesn't SRT apply to accelerated frames?

I assume you mean an accelerating frame. Frames by construction don't accelerate; they refer to inertial rest. SR does apply to accelerated frames (i.e., frames at different velocities). But what you want to know is, is there a principle of relativity that relates observers under arbitrary acceleration? Yes, it's called the General Theory of Relativity.

127 posted on 01/08/2003 5:30:13 PM PST by Physicist
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To: MonroeDNA
My question: Under gravity, wouldn't the pendulum strings angle toward each other, exactly towards Newton's common point? But under accelleration, wouldn't the strings would be perfectly parallel?

They would gravitate toward each other under either circumstance (whether the elevator they were in was sitting on the ground or being lifted.) A passenger in the elevator wouldn't know which was the true situation.

128 posted on 01/08/2003 5:35:27 PM PST by PatrickHenry (If I don't respond, you're on "virtual ignore.")
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To: RightWhale
Technical staff confirmed my point!

In the right situation, with the right measuring instruments, gravity and accelleration CAN be distinguished, which is against what Einstein said.

Oh, the elevator example? It's what Einstein used as an example of how they are indistinguishable.

Assume the elevator is large enough such that the subtle angle difference can be measured. With today's technology, that would be around 100 feet wide, more or less.

129 posted on 01/08/2003 5:42:13 PM PST by MonroeDNA
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To: PatrickHenry
"They would gravitate toward each other under either circumstance..."

Technically true, but under gravity they would always be closer than under accelleration.

Think of a strong gravity directly under them.

130 posted on 01/08/2003 5:45:33 PM PST by MonroeDNA
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To: MonroeDNA
Yes, depending on geometry they can be distinguished. That, of course has little to do with the STR or the GTR; it's just Newtonian physics. Since Einstein's gravity field was uniformly flat in the elevator example, in that case they could not be distinguished.
131 posted on 01/08/2003 5:46:38 PM PST by RightWhale
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To: Physicist
See my post, #124. Comments?
132 posted on 01/08/2003 5:46:56 PM PST by MonroeDNA
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To: RightWhale
Under what circumstance can a gravity field be "flat?"
133 posted on 01/08/2003 5:48:39 PM PST by MonroeDNA
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To: MonroeDNA
I'm not sure of that. The two pendulums (pendula?) would attract one another, period. If they were in a box floating in space, it's true that the field of the earth wouldn't restrict their movement, and they'd swing closely together. But I was assuming the elevator was close enough to the earth's surface that the earth's gravity wouldn't matter either way (stationary elevator or rising elevator).
134 posted on 01/08/2003 5:50:34 PM PST by PatrickHenry (If I don't respond, you're on "virtual ignore.")
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To: RightWhale; Physicist
Not trying to be arguementative, if I come across that way.

I guess my point is that accelleration is always 2-D, while gravity is 3-D. They can ALWAYS be distinguished from each other. They are not the same, and under ANY circuimstance imagined, they can be easily distinguished.
135 posted on 01/08/2003 5:53:39 PM PST by MonroeDNA
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To: MonroeDNA
can a gravity field be "flat?"

If the mass is not a limited volume. A limited volume, if spherical, will act like a point source. Earth, for example would be such a volume, although it is lumpy, masswise, and the gravity vectors point in various directions slightly away from the center of mass.

136 posted on 01/08/2003 6:00:12 PM PST by RightWhale
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To: MonroeDNA
accelleration is always 2-D

Not at all. Spacecraft have control thrusters pointing along 3 axes and they can impart a 3-D acceleration. In the superstring model there can be more than 3 dimensions, maybe even 11 in some cases, and there could be corresponding accelerations.

137 posted on 01/08/2003 6:03:26 PM PST by RightWhale
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To: Southack; Physicist
There are really only two possibilities being discussed here:
1. That G = C^2
2. That G = C
I.e. either Gravity travels at the speed of light (C) or else it travels at the speed of light squared (C^2).

Southack, I'm no physicist, but I think you're misinterpreting "=". "G=C" doesn't mean that "gravity travels at the speed of light" but rather that "in any equation, C can be substituted for G without changing the result."

I'm pinging a real physicist to correct me if I'm wrong.

138 posted on 01/08/2003 6:07:34 PM PST by Lurking Libertarian
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To: RightWhale
Ummm....under what circuimstance can a mass be not a limited volume?
139 posted on 01/08/2003 6:10:22 PM PST by MonroeDNA
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To: RightWhale; Physicist; PatrickHenry
I phrased that wrong.

Accelleration can be defined in 1 dimension only, whereas gravity always has more than 1 dimension.

Under accelleration, it doesn't matter if you are two feet higher, two feet to the left, or two feet forward (change in X,Y, or Z location) if you are part of the object being accellerated. The force does not change, no matter how you change your location (static location!)

Under gravity, if you are two feet higher, it changes. If you are two feet to the right (not spherical, XYZ right), the force changes, and can be detected. Under gravity, a static change in location will cause a shift in the gravity vector. That's why the pendulum detection method works.

Gravity and accelleration can be distinguished, under any imaginable scenario. Even in a sealed box, or elevator, IMHO.

I've been asking this question for 15 years, and either I'm wrong (won't be the first time, for sure!), or the elevator or sealed box thing is wrong.





140 posted on 01/08/2003 6:25:23 PM PST by MonroeDNA
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To: RightWhale
Spacecraft have control thrusters pointing along 3 axes and they can impart a 3-D acceleration.

But at constant thrust, the acceleration is in one direction as the sum of the vectors.
141 posted on 01/08/2003 6:36:34 PM PST by aruanan
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To: MonroeDNA
See my post, #124. Comments?

Don't congratulate yourself too much. It's called the tidal force, and scientists and sailors are aware of it.

Einstein's example was for a uniform (i.e. divergence-free) gravitational field. In principle, you can create a uniform field by constructing a large, flat, massive sheet; near the center of the sheet, the field will be uniform. Or, you can achieve an arbitrary uniformity it by examining a sufficiently small volume of space.

The point of the equivalence principle is not that it isn't possible in practice to tell whether you're on the surface of a planet, but that accelerations and gravitational fields are physically the same phenomenon. That's a pretty radical notion.

Ice is cold and hard while steam is hot and gasseous, but that doesn't alter the fact that they are fundamentally the same stuff.

142 posted on 01/08/2003 6:48:46 PM PST by Physicist
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To: MonroeDNA
See? You can't go wrong if you stick with Einstien. Big Al is awesome.
143 posted on 01/08/2003 6:52:11 PM PST by PatrickHenry (If I don't respond, you're on "virtual ignore.")
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To: Physicist
Hey, if I sounded like I was congratulating myself, I sure didn't mean it to come accross that way. I just asked a question that has been buging me for a long, long time.

My physics books all say that the two were indistinguishable, and gave the elevator example. It always bugged my profs to no end when I pointed out that they are always distinguishable. Frowns, bad grades, "troublemaker," always followed.

I'm so scarred!!!! LOL!

Yes, of course a "uniform gravitaional field" would be equivelent. But there ain't no such thing, unless someone can give me an example (then I'll go back to the class and shut up). Never has been, never will be.

The elevator thing taught to all budding physicists should have a disclaimer:

Caution: What you are about to hear is false, even reduced to a simplification."

Or am I wrong?

144 posted on 01/08/2003 7:18:09 PM PST by MonroeDNA
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To: PatrickHenry
Trust me, I believe in Einstein. Also Lorentz, Newton, Maxwell, and Bush.

:)

145 posted on 01/08/2003 7:21:36 PM PST by MonroeDNA
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To: forsnax5
Mass, time, space, gravity.

All interrelated and subject to the constant speed of light.

Nowhere is stated the Calabi-Yao. A deficit article, imo.

146 posted on 01/08/2003 7:23:55 PM PST by Zuben Elgenubi
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To: MonroeDNA
My physics books all say that the two were indistinguishable, and gave the elevator example.

If you simplify your version of the example and assume that it's just you in the elevator (no pendulums, no instruments, just you, personally, as the solitary observer), can you tell the difference?

147 posted on 01/08/2003 7:45:53 PM PST by forsnax5
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To: Oberon
Right. Whatever you say. (brain still dripping out my EARS.)
148 posted on 01/08/2003 8:25:55 PM PST by seams2me
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To: MonroeDNA
But there ain't no such thing, unless someone can give me an example (then I'll go back to the class and shut up).

Is something wrong with the large, massive sheet?

And don't forget, for almost any conceivable gravitational field and any given level of sensitivity, there is a calculable scale below which the field is indistinguishable from a uniform field. That means that this problem is not irrelevant to the laboratory.

Or am I wrong?

You're missing the point. You might as relevantly have pointed out that the situation where someone is on a rocketship without knowing it is unlikely ever to arise. The equivalence principle is a far-from-obvious statement about the physical nature of gravity and inertia, not merely a practical limitation on figuring out whether you're travelling on a rocketship. Instead of pettifogging about the practical details of a given example, why don't you try to focus on the principle that the example was meant to illustrate?

149 posted on 01/08/2003 9:02:12 PM PST by Physicist
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To: Piltdown_Woman
No joke. It's kind of like when you shut the locked car door just as you realize that the keys are still in the ignition. The door swings slower but you still can't quite grab it fast enough.
150 posted on 01/08/2003 9:20:39 PM PST by Excuse_My_Bellicosity
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