What Do We Understand About Gravity?

[orgmark]

What do we know about gravity? - condensed (if possible)

 

[russ_watters]

Translation: "Hi, could you teach me a career's worth of physics in one post?"

Quite a bit is known about gravity. Too much to condense it into a post. Start with the Wik entry on gravity and go from there. http://en.wikipedia.org/wiki/Gravity

 

[Andy Resnick]

Occasionally we perform gravity tests on various pieces of equipment to make sure gravity still works- so far, 100% of the time, gravity does indeed cause things to fall, generally to the detriment of the test equipment.

 

[Landru]

You should have a forum rule that requires people to try asking Google first.

Speaking of Wikipedia, it's science articles are absolutely the worst resource for beginners. They are usualy thick with jargon and often written by people who aren't any good at explaining things. Sites like howstuffworks.com and http://www.colorado.edu/physics/2000/ are awesome.

 

[arildno]

Read Wheeler's, Misner's and Kip Thorne's booklet "Gravitation".
That's a brief, easy-going introduction to a complicated subject..

 

[Varnick]

Arildno, that's just cruel. Don't dissuade him from study of such an excellent subject!

 

[Landru]

I swear people are trying to keep this stuff a secret; just click here http://www.howstuffworks.com/question232.htm

 

[Renge Ishyo]

Read Wheeler's, Misner's and Kip Thorne's booklet "Gravitation".
That's a brief, easy-going introduction to a complicated subject..

And if it still doesn't make sense after that, this book makes for one hell of a paperweight.

 

[robphy]

What do we know about gravity? - condensed (if possible)

It's still an attractive subject of study.

 

[siena]

As indicated by the previous respondents, there is a lot we know about the subject. But there are some pretty basic things that are either not known, or not universally agreed upon:

1) Is gravity a fundamental force or a pseudo force?

2) Are gravitons required to explain gravity, or is general relativity sufficient?

Perhaps these two questions are really the same, and could be rephrased as: Is there any way to reconcile the different perspectives of the particle physicists and the general relativists?

 

[Landru]

One thing about photons, and consequently theoretical gravitons, is wouldn't you expect that over distance the space between them would widen? So that if you aranged a flat layer of dust in space far away from Earth wouldn't some of the dust be pulled closer to Earth as a graviton impacted it while the particles of dust beside it would remain in place? Or is the dispersion of gravitons infinately dense over any distance? (that would be wierd)

Same with photons, if you see a quasar a billion light years away how is it that it manages to supply such a constant, even and hardy dose of photons to a telescope right here?

 

[turbo]

As indicated by the previous respondents, there is a lot we know about the subject. But there are some pretty basic things that are either not known, or not universally agreed upon:

1) Is gravity a fundamental force or a pseudo force?

2) Are gravitons required to explain gravity, or is general relativity sufficient?

Perhaps these two questions are really the same, and could be rephrased as: Is there any way to reconcile the different perspectives of the particle physicists and the general relativists?

Actually, there are lot of things that we know about the accuracy of historical assumptions about gravity, including the Newtonian approximation that embodied the inverse-square law, and Einstein's relativistic refinement of that model. Since then, there have been increasingly accurate experimental efforts to test the accuracy of these models. Still, the mechanics behind the relativistic approximation that claims that matter warps space is unknown. That mathematical approximation works OK on Solar-system scales, though the Pioneer anomaly and the fly-by anomalies may be hints that we don't have all the answers.

As for the existence of gravitons as particles mediating gravitational attraction, there is a pretty large and perpetually ignored problem. If the gravitons mediate the forces of the gravitational field, and the Higgs bosons mediate the mass-granting function of the theoretical Higgs field, then we have to step back and ask "what do observations show us?" As deeply as we can see currently (about z~6-6.5) the universe appears to act exactly like our local universe. If the Higgs field and the gravitational field are actually fields (subject to evolution, polarization, densification, and rarification - as fields are) we might expect to see age-related differences in gravitational interactions. We don't. This is a pretty good reason to question whether the concepts of separate fields and mediating particles can be responsible for gravitational interaction. In fact, given the extreme conditions and matter densities one might expect at z~6-6.5 the normal behaviors of galaxies at that era suggest that both mass and gravitational attraction arise from matter's interaction with a single field, with no mediating particles. The "vacuum" of space is not really a vacuum, and there is a robust body of peer-reviewed work that treats space as a dynamical participant in gravitation.

 

[Landru]

One thing about photons, and consequently theoretical gravitons, is wouldn't you expect that over distance the space between them would widen? So that if you aranged a flat layer of dust in space far away from Earth wouldn't some of the dust be pulled closer to Earth as a graviton impacted it while the particles of dust beside it would remain in place? Or is the dispersion of gravitons infinately dense over any distance? (that would be wierd)

Same with photons, if you see a quasar a billion light years away how is it that it manages to supply such a constant, even and hardy dose of photons to a telescope right here?

This is the kind of question you can't find an answer for in physics textbook, and where you might expect to find it there is nothing. It seems that people here readily respond to all the stupid homework questions with easy answers but if you go any further and ask actual questions you get ... nothing. It's sad that here on the "World Wide Web" a site called "Physics Forum" sees so little participation. I'm not suggestig my question is compelling or insightful, but it is actual curiosity and not a homework question. I get the sad feeling that our best days are behind us.

 

[Defennder]

You need to learn to be patient. This forum isn't run by people full time. There are genuine qualified physics doctorate holders who are more than willing to answer your question if they can take the time off to visit the forum and reply to your question. It's only been a day since you last posted your question, and you're complaining no one has answered it, therefore there can't possibly be experts here? Who do you think you are?

 

[Renge Ishyo]

I think people answered the query well enough. A lot of joke responses appeared to be sure, but I think that is due to the humorous nature of the original question. The original post is way too broad and asks for way too much. If someone came on here and said "Can someone summarize the subject of Thermodynamics for me? Oh, and please be brief" there would probably be some laughing there too (its just not that simple, you see...).

 

[ZapperZ]

Part of this thread has been split off to the Philosophy forum.

Zz.

 

[Gear300]

What do we know about gravity? - condensed (if possible)

enough to say "not enough"

 

[Renge Ishyo]

Part of this thread has been split off to the Philosophy forum.

This thread won't qualify as philosophy until somebody asks whether or not we "can" know anything about gravity.

 

[Landru]

You need to learn to be patient. This forum isn't run by people full time. There are genuine qualified physics doctorate holders who are more than willing to answer your question if they can take the time off to visit the forum and reply to your question.

I'd be willing to pay tax dollars for some government program to have physicists just answer questions on a physics forum. A lot of people are trying to learn things on their own and just need a few gaps filled in. If we have to apply for a four year degree every time we want to know something we'll never have collectively diverse knowledge. Such a thing would be good for humanity. Companies like Microsoft are complaining that their aren't enough smart people to hire.

 

[Defennder]

You can always contribute to PF if you're willing to pay. However, you may also want to note that it is often difficult to explain current physics theories (superstring, QFT) to a layman, if that is what you're asking. You ought to check out a popular science book on this before asking your question because your question might not even make sense. Lastly, if you want someone to answer your question, you might want to try PMing a physics mentor.

 

[Renge Ishyo]

I'd be willing to pay tax dollars for some government program to have physicists just answer questions on a physics forum. A lot of people are trying to learn things on their own and just need a few gaps filled in. If we have to apply for a four year degree every time we want to know something we'll never have collectively diverse knowledge. Such a thing would be good for humanity. Companies like Microsoft are complaining that their aren't enough smart people to hire.

The trouble with Physics is that it seems to take a very large amount of time and effort invested on the part of the student before the apparently "disconnected" phenomena come together cohesively and things start to make sense. I remember when I was first studying the subject I spent large amounts of time struggling horribly with certain concepts and it took years and years of hard work for the pieces to even start falling into place. I am admittedly not the brightest bulb on the tree, but still I just can't see a few quick replies to a physics question replacing that whole process.

Having said that, if you do not want to major in physics and want the subject explained in the clearest way possible, I can recommend the books by Issac Asimov on the subject (specifically "Understanding Physics" and "Atom"). These are by far the most easy to read descriptions of the subject of Physics that I have read to date.

 

[madmike159]

I'd be willing to pay tax dollars for some government program to have physicists just answer questions on a physics forum. A lot of people are trying to learn things on their own and just need a few gaps filled in. If we have to apply for a four year degree every time we want to know something we'll never have collectively diverse knowledge. Such a thing would be good for humanity. Companies like Microsoft are complaining that their aren't enough smart people to hire.

I understand where he's coming from. Every where on the internet seems to just explain everything using maths. The best place to start is to have a basic understanding of maths and read a book on current theories (Steven Hawking, Brian Greene, Kip Thorn etc.) Some things however are very hard to explain in words and most people here don't have time to think up good ways to say things. I would recommend reading The Elegant Universe by Bruan Greene, it explains every thing from string theory to QM very well. It also has notes in the back relating to the chapters which sometimes include a bit of maths to help you understand further.

 

[Defennder]

There's actually a book by Prof Roger Penrose which really attempts to teach modern physics to laypesons. It's called The Road to Reality: A Complete Guide to the Laws of the Universe and it has 1100+ pages(!). I haven't read it but it seems to cover just about much of what you need to learn in order to understand current theories. The maths required to understand the book, according to Wikipedia, is covered within its first 350 pages. You might want to check that book out.

 

[Chronos]

Quantum field theory gives the most definitive description of what we know [and don't know] about gravity. There really is no satisfactory lay explanation I know of that doesn't confuse as much as enlighten. One[?] problem with theories of gravity is they are devilishly hard to test. Rebka-Pound, Hulse-Taylor, and gravity probe B are among a handful of successful attempts. LIGO is the most promising experiment currently underway. There have also been some unintended experiments that have been confounding [e.g., pioneer anomaly]. I suggest reading up on these studies.