Gravity

Steve
Steve
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Topic 190877

This is a little self test for folks to throw rocks at and hopefully I will learn something from it.
1. This experiment isn’t about detecting gravity waves; it’s about detecting the mechanical effects of gravity interactions that approach us in wave form.
2. Gravity waves (if they exist) are the method by which the earth is held to the sun, or the force that pulls a ball back to earth when it is thrown.
3. The ripples we are searching for have ripple-lengths or frequencies that depend on their rotation speed. If it is a mountain on a neutron star we get the ripple every rotation. If it is two black holes the ripples frequency is tied to the rate as their rotation around one another.
4. We don’t currently know how gravity attracts other matter. Is it a wave? is it a particle? We can measure it and calculate the effects, but we don’t know how to detect it from a distance and this experiment is not about finding out how to detect it.
5. We know it is not instantaneous and travels very close to if not exactly the speed of light.
6. Gravity gets weaker with distance, but never entirely dissipates to nothing. It is energy and energy never disappears, it just changes form.
7. The only branch of physics that is currently trying to figure out what gravity is, is String Theory.
8. All other natural forces, “strong and week nuclear and magnetic� are electromagnetic forces.
9. Maybe gravity does propagate in a wave but we just don’t have antennae of the proper length to detect them.

So, did two weeks of reading net me a score above 50%? It has been very interesting reading up on the subject, but it makes me really regret flirting with the girls during math classes.

Any recomended books on the subject would be appreciated.

Ben Owen
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Gravity

Steve,

Welcome to the project. What have you been reading?

If you are looking for scientific background, you might start with some of the links on the Einstein@Home front page. The report on the S3 search contains useful background information as well as the specific information on the S3 search. You may also find some helpful answers in my previous posts, which you can access from my profile (click on my name at left).

This experiment is indeed about detecting gravitational waves. They are the part of the gravitational interaction that manifests itself as waves, rather than the static part that holds the Earth in orbit around the Sun and brings thrown balls back to Earth. They have the mechanical effect of stretching things a little in one direction while squeezing them in another. This effect is so small that it is almost impossible to observe anywhere except in the specially constructed arms of LIGO, where it is merely very hard.

The waves do travel at the speed of light, or at least imperceptibly close to it. They get weaker with distance, just like light waves, and never die off entirely. Unlike light, they go through everything, which is part of why they are hard to detect. The waves are usually produced by rotating things such as neutron stars with hills (that's the kind Einstein@Home is looking for) and their frequency depends on the rotation frequency of the source. The corresponding wavelength tells you what instrument can detect them. The wavelength should be longer than the arms of the instrument (4km for LIGO, much more for the proposed LISA which would be in orbit). If the wavelength is too long, corresponding to low frequency, the detector gets swamped by seismic noise (there is something like that even in space).

The quantum "Is it a wave? Is it a particle?" business doesn't really affect LIGO, since we're not looking on microscopic scales. In addition to gravity, the other three "fundamental" interactions are traditionally the electromagnetic interaction and the strong and weak nuclear interactions. The weak nuclear one has been shown to be related to electromagnetism in some sense, but the strong hasn't although people suspect it should be. Those three all have observable quantum effects and gravity doesn't (yet). Those three interact with different types of matter, but gravity interacts with all matter and can even do things on its own without matter. It's definitely the oddball.

"The only branch of physics [...] is String Theory." Funny, I've heard that statement, with and without the words I represented with dots, from some string theorists. But the truth of it is not at all apparent to me, nor to many other people.

Hope this helps,
Ben

Odysseus
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RE: This is a little self

Quote:
This is a little self test for folks to throw rocks at and hopefully I will learn something from it.
1. This experiment isn’t about detecting gravity waves; it’s about detecting the mechanical effects of gravity interactions that approach us in wave form.

I'm not clear on how you draw that distinction.

Quote:
2. Gravity waves (if they exist) are the method by which the earth is held to the sun, or the force that pulls a ball back to earth when it is thrown.

I don't think so. Note that in general-relativity theory gravity isn't treated as a force, but rather as a description of the (four-dimensional) shape of spacetime. In other words it's a pseudoforce, an illusion created by our using a frame of reference in Euclidean space—just as the "centrifugal force" you experience on a merry-go-round is an artifact of the rotating frame of reference.

Quote:
8. All other natural forces, “strong and week nuclear and magnetic� are electromagnetic forces.

No, the nuclear forces aren't electromagnetic, but all three are capable of description by a unified theory—gravity is the 'odd man out'. I don't know what they're called as an ensemble; effects of the first and last are sometimes referred to together as "electroweak interactions".

Mike Hewson
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0. Have no regrets of

0. Have no regrets of flirting with the girls. :-)
1. Same horse, different color. A language issue.
2. Waves occur if there is a non spherically symmetric alteration in mass distribution. There may be such waves in a given circumstance, but gravity is attractive regardless.
3. Well, the measurements we make hereabouts in time and space depend on the behaviour at source, some of which is related to the parameters you quote. Black hole mergers, a target group in detection efforts ( sensitivity permitting ), is modelled as phases - I think approach, inspiral, merger and ringdown - in what is a continuous process.
4. There are effective theories which which attempt to describe and predict phenomena. The test is experiment, and the good thoughts survive by definition. At some level of explanation a given model may please you more or less, particle or wave or whatever. Action at a distance is a concept with a long pedigree, an idea still retained but filled in with a mechanism of mediation - gravitons if you like. You chuck it and I catch. The LIGO's pretty well assume such, certainly there will be a big fuss if it doesn't confirm it!
5. Close enough. At the speed of light really.
6. Close enough. Gravitational potential energy can convert.
7. Easy to guess where that one came from. Did any source mention that string theory is currently unburdened by data........?
8. Strictly no. However there is an effort to unify our understanding of the separately labelled forces. A deep topic. Meaning that for those particles that have 'force charges' for several force types, can we come up with a single framework which predicts all the relevant experimental findings? Electricity + Magnetism = Electromagnetism occurred in the late 1800's. Electromagnetism + Weak = Electroweak occurred circa 1970's. Electroweak + Strong = Standard Model might have happened, not as completely, and certainly not everybody is happy ( a Higgs detection would help ).
9. Jury is out.
I'd put you over the 50%, but I'm by no means an expert! Definitely a shoe-in if you could sort out item zero! :-)
As for books, it depends on how many yards you want stride and and what depth! Cheers and welcome! Mike

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

Solomon
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RE: 4. We don’t currently

Quote:
4. We don’t currently know how gravity attracts other matter. Is it a wave? is it a particle?

If we believe in quantum mechanics, really the only possible answers to this are "both" or "neither." Nothing is one or the other. The only real question is whether gravity is a field theory (like Electromagnetism and the others) where we can thing of a force carrying particle - the graviton - mediating interactions between everything else, or if it's in essence a geometrical background in which all the other forces take place.

Quote:
7. The only branch of physics that is currently trying to figure out what gravity is, is String Theory.

To paraphrase Dr. S. James Gates (with whom I'm taking a class right now!), "String Theory is not physics. It looks like physics. It smells like physics." But, it's not physics until it's shown to agree with observations.

String theory is not physics, "but it's the only game in town." That is to say, it's the only construct we have right now that looks like it could be a sucessful way to unify all four forces under a single theory.

This isn't to say that no one else is looking into the question of how we can develop a quantum theory of gravity (c.f. Loop Quantum Gravity); but, there aren't any other efforts that would give a single theory describing, basically, all of physics.

tullio
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RE: As for books, it

Message 25669 in response to message 25667

Quote:

As for books, it depends on how many yards you want stride and and what depth! Cheers and welcome! Mike


A suggestion: "Subtle is the Lord..." by Abraham Pais.
It is a scientific biography of Einstein, a good textbook on relativity.
Tullio

Steve
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Thanks to everyone for the

Thanks to everyone for the rocks. I appreciate them. Tullio, I ordered that book this morning. $2.98 on Amazon.com That fits in my budget!

I seem to have really struck a nerve on the string theory mention. Last fall I read a couple books by an Author named “Greene� I think. I remember one title was “the elegant universe� I don’t recall the other. I read an article in the January 2006 issue of Astronomy magazine that that really takes a nasty shot at the whole theory. You may want to look it up, it is on page 16. Greene may be full of Bologna but he definitely has a gift with words to a lay person such as me. Prior to reading his books I had heard Time was the fourth dimension, after reading his books I could visualize in my head that Time was the fourth dimension. However, somewhere between the 11th and the 26th dimension I became as lost as a ball in tall weeds. If you were a lost ball in tall weeds, all you would see is weeds. If you were looking for a ball in tall weeds, all you would see is weeds. That’s where I was; take your pick on the point of view.

At any rate, thanks for the replies. Give me a couple of weeks and I will post my new and improved list of misconceptions. I am not less confused than I was prior to my last post, but I am now confused at a higher level.

Mike Hewson
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RE: I seem to have

Message 25671 in response to message 25670

Quote:
I seem to have really....the point of view.


Greene writes well doesn't he? Strings are fine theory, it's that people are waiting for strings in practice. It is simply a true shame that not one testable number has emerged yet. Historically theories have moved quicker to that, but since strings are quite a paradigm shift ( from wave or point particle models ) then there is a greater handicap distance to come up to the starting block. They don't have experimental feedback to guide alas! More kindness and patience perhaps.

Quote:
...but I am now confused at a higher level.


Ha! Beautifully put, and you're not alone ...... :-)

I have made this letter longer than usual because I lack the time to make it shorter. Blaise Pascal

gravywavy
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Waves and

Waves and attraction/repulsion are different features of a field.

For example in the more familiar electromagnetic fields, an electron and a proton attract each other, whether they are moving or not.

If you wiggle the electron around then it transmits energy in the form of an EM wave.

If you make very small changes in the energy, then you find that there is a minimum size of energy change that can be transmitted away, and this depends on the frequency of the wave. Thus the particle properties of the photon are also hidden in the field, and the field needs a quantum theory to handle what happens at low enrgies.

With gravity, all we have directly seen/felt/measured so far is the steady attraction. We conjecture (that is a word that means we expect sometihing, but have not yet seen any direct evidence to support the expectation) we conjecture that gravity is a field like the EM field. If so, then we'd expect gravtiy waves and gravitons (gravity perticles) to exist.

Unfortunately, the best description we have of gravity is given by General Relativity: this is compatible with gravity waves, but incompatible with gravitons.

Which will be thrown out? GR or gravitons? Watch the string theorists, brane theorists, etc. But at present all of that is just conjecture.

The one feature common to all the conjectures is the existence of gravity waves.

~~gravywavy

gravywavy
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There are four forces of

There are four forces of nature, as far as we know.

They are gravity, the weak force, the electromagnetic force, and the strong force.

The weakest of them is gravity. Think of a fridge magnet - it clings onto your fridge interacting with maby a few cc of iron nearby, and that interection is more powerful than the fact that the magnet is interacting gravitationally with the whole of the planet. (Thanks to NewScientist for this example).

The only reason gravity seems stronger than the EM force in everyday life is that almost all the electrical charges around us are paired off and cancel out.

Gravity and the EM force are long distance forces - they attenuate at an inverse square law, whereas the strong force attenuates as an exponential times an inverse square. That is why the strong force is negligible at everyday distances, the exponential fall off makes it almost zero if you are more than a few proton widths away.

Interestingly, everything you ever experience is down to the EM force.

Touch - the atoms in the object interact with the atoms in your finger by the effects of their respective electrons on each other, EM force.

Taste/smell - again, chemistry, depending on the behaviour of electrons in orbitals and ulitmately down to the EM force

Sound - pressure waves in air, mediated between atoms in the air by interaction of the electrons: again EM force; same when the wave impinges on the eardrum

Maybe you think you can feel gravity? Well the feeling of weight you get when you stand up is the force of the ground pushing back through your shoes - more interaction of electrons. What you feel is the force that opposes the gravity, and that force is again the EM force.

Sense of balance - same again, but with liquids and particles in the inner ear.

So even as far as our inherent human senses are concerned what you said initially is true about gravitational attraction as much as about the waves: we never experience gravity, just its mechanical effects on us, and these mechanical effects are mediated at the microscopic level by EM forces.

But that is just another way of saying that on the scale of a human body, the strong force is too small to matter because the scal is too big, and gravity is too small to matter because the scale is too small. All that evolution/god/etc can work with to design a sense organ on our scale is the EM force.

gravywavy~~

~~gravywavy

Chipper Q
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Interesting to note that

Interesting to note that acceleration is a feeling we're all quite familiar with, some of us preferring it as a 'thrill' more so than others... :)

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