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Cassini Experiment Confirms General Relativity
MikeZilla writes "An experiment by Italian scientists using data from NASA's Cassini spacecraft, currently en route to Saturn, confirms Einstein's theory of general relativity with a precision that is 50 times greater than previous measurements."
I don't think that's necessarily a problem. What they're showing is that he is using a consistent model for how the universe works (which, as the article states later, is not exactly the issue; they're actually after more subtle questions).
It seems that, according to scientific philosophy today (and I say this as an observer, not a scientist), you still can't really believe this is _the_ truth about something. You have to keep thinking, "it might _not_ be true". I hear how a hypothesis must be "falsifiable"--what does that mean? So if science is a search for truth, how can you find it? And how does this experiment matter? I mean, didn't people already believe that relativity was (mostly, apparently, seemingly) true?
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Presumably they did measure the distance: you can get the round-trip signal when the path to Cassini is far from the Sun. That, along with the fact that Cassini is itself far from any significant gravitational source means that it's easy to know exactly where Cassini is right now. (In this limit, Newton and Einstein are pretty much identical. You can argue that this theory is incorrect, but that's a pretty difficult argument to make, given how often it has worked quite well.)
So, no, they don't really need to use GR to get the distance.
Nice measurement, no doubt. But the article is a bit misleading. This isn't the most precise measurement of GR, just the most precise mesurement of this prediction. It sounds like they got this measurement to an error of one part per fifty thousand. If memeory serves, the measurements of the orbit on pulsar 1933+16 (the one that netted Taylor and Hulse the Nobel Prize about a decade ago) are precise to one part in something lik ten to the eleventh. And they agree with GR.
One some level it amazes me that GR passes every test we throw at it with such flying colors. On another level, I agree with Albert: the theory is too beautiful *not* to be true.
Theories frequently turn into paradoxes, because bits are missing from the description that are necessary to the theory's application to more than one set of circumstances. So the theory sits in limbo for awhile until somebody starts asking the right questions. Einstein recognised this several times, although i think he'd be spinning at the thought of what's happening with his work now.
My favourite Einstein quote got translated several times, but the best one (provided by Eistein himself, in later years) comes out to, "God's slick, but he ain't mean."
That sentence always comes to mind when stuff like this comes up.
"I'd say 'Have a good time,' but arson is still illegal.
Cassini's experiment
The researchers measured how much the Sun's gravity bent an electromagnetic beam, in this case the radio signal transmitted by the spacecraft and received by the ground stations.
1919 Eclipse source: http://www.esa.int/esaSC/SEM7I9R1VED_index_0.html
Sure it's a theory, but this "theory" was used to prove the "laws" of Newtonian mechanics were wrong and not all-encompassing--they are just good approximations to what we observe as humans during our daily routines.
That's not really true. "Law" is something of a layman's term. In mathematics and science, there are no absolute facts, just postulates and theorems. General relativity is basically identical to Newton's theory of gravity in basic situations, but it differs when you're working with high speeds and large scales. In fact, Newtonian physics is proven wrong for many planetary-scale gravitational effects.
It's a "law" because it seems immovable to us.
Measuring the distance only requires you know the speed of light in a vacuum, which is fairly easy to measure. They were repeating Eddington's 1919 measurement of the deflection by a massive body. As Eddington wrote while developing the plates in Brazil:
Oh leave the Wise our measures to collate
One thing at least is certain, light has weight
One thing is certain and the rest debate
Light rays, when near the Sun, do not go straight.
Cassini is never near the Sun. The signal sometimes passes near the Sun, but Cassini never does.
Cassini is on a well-measured orbit far from the Sun. As I said, in this limit, Newtonian gravity is all you need to get the trajectory. (Newton agrees with GR, and no one doubts the validity of either in this regime)
What a lot of people don't seem to know is that the fact that it is still called the 'theory' of relativity means that it hasn't been accepted as gospel by scientists yet.
:)
That's not a very good description of the situation.
Contrast this with the 'law' of gravity, which has.
Actually the 'law' of gravity have been proven incorrect. It has been superceded by relativity. The common usage of 'theory' and 'law' don't quite match up with the scientific usage
According to the "law of gravity" the results from this measurement should have been zero. Relativity says the value isn't zero, and the value given by relativity is at least a 99.998% match for the measured value. The remaining 0.002% doesn't indicate a problem with relativity, it is just the limit of the accuracy of devices they used to make the measurement.
Relativity has been challenged with the most stringent scientific testing ever devised in countless ways. Actually part of the "problem" is that relativity is "too good". Absolutely everything it describes it does so with unbelievable accuracy. The irony is that you can't learn anything new when every single measurements exactly matches your predictions. It leaves them without anything to grab on to to try to explore the things that relativity doesn't explain.
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That's not really true. "Law" is something of a layman's term. In mathematics and science, there are no absolute facts, just postulates and theorems.
I view it more as a probability. The more evidence that backs something, the higher its probability of being the proper theory. But, nothing is 100% certain, except may death and taxes.
In fact, Newtonian physics is proven wrong for many planetary-scale gravitational effects.
I think gravity is called a "law" because for the vast majority of things people deal with it, it works well and predicts dead-on. But astronomers may not find it as useful. By the way, the interplanetary space probe gravity anomally is indeed odd. It may trigger a revolution in gravity theory together with solving "dark matter" problems.
Table-ized A.I.
you have to factor in the fact that both objects are in accelerated motion around the sun, which of course uses Einstein's equations.
Given suitable equipment, Issac Newton himself could calculate the precise, simple, and smooth ellipse of the orbit. There is no relativity involved in this step.
As Cassini passes directly behind the sun there is a fairly rapid anomolous bump in the radio-measured distance. The apparent bump in distance is caused by the part of the path that passes very close to the sun.
By measuring the bump they measure just the breif anomoly caused by relativity.
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Given suitable equipment, Issac Newton himself could calculate the precise, simple, and smooth ellipse of the orbit.
Sure, you can calculate the elipse of the orbit. But that's not the same as calulating the round-trip distance between two accelerated objects.
Oh please. I keep telling myself I should stop reading the comments on science articles on
Scienctists do not consistently use "law" and "theory" to mean two different things.
Science doesn't have gospels. Science has testable theories/laws.
Contrast this with the 'law' of gravity, which has.
Actually Newton's universal law of gravitation has been definitively disproved. It just happens to be a very good approximation under certain conditions.
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There's no difference between a "theory", a "theorem", a "law", etc in science. They're all just synonyms for theory, to give them different names. Science deals with theories. Math deals with theorems.
I've remarked before, it's only Americans that have this idea that a "law" is better than a "theory", etc.
Can someone explain to me why that is? Is this taught in schools? Is it caused by Creationists (another US phenomenon) trying to muddy the waters by suggesting "evolution theory" hasn't made it to "law status" yet?
I believe posters are recognized by their sig. So I made one.
- The orbit of Cassini is independent of the path its signal takes to Earth.
- The influence of the curvature of space around the Sun can be separated from anything which changes the orbit of Cassini (or the Earth) by measuring the delay properties of the signal path when the signal passes nearer or further from the Sun.
We can also make very precise measurements of other signal paths, such as the timing of signals from pulsars. We can even do this at the same time as we measure the signal from Cassini....If you want to take issue with the results (and be taken seriously), you need to make an effort to understand those results and the previous work which underpins it. This is not the same as repeating buzzwords; it takes much more in the way of both effort and raw intelligence.
Scientists restrict study to entire physical universe; creationist
It's misleading to say that this experiment "confirms" General Relativity. What it does is fail to falsify GR. That's nothing to sneeze at. But it tests such a small part of GR that one really can't say that it "confirms" GR. These kinds of delays are part of many alternative theories as well. If you say that this experiment "confirms" GR, then it also "confirms" many theories that otherwise wildly disagree with GR.
Can someone explain to me why that is?
My guess would be that a lot of people think a law is absolutely correct and immutable (due to the legal system?), while they think a theory is just a vague set of guesses.
The truth, of course, is more the opposite. A law tends to be a detailed observation, while a theory is the best explanation we can come up with given the evidence we have to work with.
Laws are ultimately useless, because they don't really give us anything to work with. A theory, on the other hand, can be incrementally improved or completely disproved and replaced, and the process of improving theories allows us to gain a better understanding of the universe.
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I've remarked before, it's only Americans that have this idea that a "law" is better than a "theory", etc.
Can someone explain to me why that is? Is this taught in schools? Is it caused by Creationists...?
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They don't have laws, they have commandments!
YAW.
Your head of state is a corrupt weasel, I hope you're happy.
calulating the round-trip distance between two accelerated objects.
(1) There is zero relativisic effect on the probe at these low speeds and at the probe's distance from the sun. The probe's path is purely classical.
(2) There is no need to calculate the actual distance. The earth is following a perfectly smooth orbit. The probe is following a perfectly smooth orbit. The distance to the probe must follow a perfectly smooth curve. You don't need to know the distance to know the signal timing should follow a smooth curve. Scientists can simply watch and draw that curve as it happens. As the probe passes behind the sun the curve will get an anomolous bump on it. The curve is pure classical and the bump is relativity. Just draw in the smooth curve under the bump and measure the size of the bump.
Have you taken calculus? It's a lot like that, you don't know the +C term of a function, but you can work with the exact derivative.
Many science experiments use this sort of method - you ignore the actual value of something, but you make precise measurements of the size of changes in that value.
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Good to see the word "confirms" used as opposed to "proves".
Remember, a theory can never be proved, only disproved/discounted.
I've remarked before, it's only Americans that have this idea that a "law" is better than a "theory", etc.
Can someone explain to me why that is? Is this taught in schools?
Yes. I was tought this in middle school. I was told the steps of the Scientific Method were:
1. Observe
2. Hypothosize
3. Experiment
4. Theory
5. More experiments
6. Law
Not only that but I was told that in order for something to become a law it had to hold up 100% of the time!
I was quite suprised when I got to college and learned that this was not true.
Have you tried Linux yet?
Actually, I doubt you need to factor in GR for that kind of accuracy. GR rapidly goes to Netowian mechanics as you move away from the Sun. Look at Mercury: the anomalous perihelion shift is 43 seconds of arc per *century*. Mercury itself will have orbited though more than 10 million times that angle in only one century. The GR correct, even there, is minute. Out between Jupiter and Saturn it isn't worth taking into account. NASA has guided several spacecraft out to that part of the solar system with very high accuracy without using GR.
Given all of that, I think it's up to you to show that they need to use GR to get this position. You're welcomed to show me the math and convince me that it matters. But until you do, you're just guessing, and guessing against the evidence at that.
Given all of that, I think it's up to you to show that they need to use GR to get this position.
I'm not sure it's possible to show that they must use GR to get this position. There's always a possibility that they found some way around it, I just don't see what it is.
But that's what you are asserting: that they need to take GR into account. You'd better believe that you can show that that is the case (or not the case, either way).
All *you* have to do (or them, to prove that they don't need GR) is to work out the GR correction from the Newtonian ephemeris for Cassini. You would have to show that it affects the position enough to significantly affect the measurement in question. They just need to show that the affect is too small to be important at their level of precision.
But that's what you are asserting: that they need to take GR into account.
No, I never made that assertion.
"You must also precisely measure the distance, and you can't measure that without using Einstein's equations."
Sounds like you are asserting that to me.
In any event, this has become futile. You either don't understand, don't want to understand, or are simply trolling. In any case, I'm out of here.
"General relativity remains iffy,"
Given that GR has been more precisely tested (passing all tests) than any other theory, including QM, I'm not sure that "iffy" is really a good way to describe it. Especially since the incompatability could point to flaws in either theory (or both). Frankly, and I admit to being something of a heratic here, I'm more expecting QM to fall down before GR.
The precession of the apogee/Perigee of Mercury's orbit does not agree well with Newton or Einstein. There is yet a discrepancy that is currently unexplained.
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So this didn't not un-de-falsify the "theory" of "relativity!"
(This post has been rewritten to conform to the Slashdot Scientific Grammar Police Code.)
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