Dismiss Notice
Join Physics Forums Today!
The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

What is the reason for Gravity ?

  1. Aug 2, 2004 #1
    Hi All,

    I am not very much familiar with Physics but i read some physics in my Engineering education days. Surprisingly, i am getting more & more attracted to Physics , particularly astrophysics ( relativity ,...etc ).

    I read that earth has gravity G with certain value. Per Einstein, space is curved with presence of Huge object(mass) . My question is does Earth have gravitation because of it's mass ? If so , what is the relation between Mass of a Object and Gravitation ? i mean to ask , how does a object with certain mass makes the space around it to be curved ?

    Thanks for your help
  2. jcsd
  3. Aug 2, 2004 #2


    User Avatar
    Science Advisor
    Homework Helper

    The question you're asking is slightly ambiguous, and could be interpreted to mean either:
    Describe (for example mathematically) the way that a mass bens space (spacetime really) around it.

    What is the mechanism for gravity?

    The answer to the first question is part of physics and is known quite precisely. The answer to the latter question is not something that physics deals with. That's one of the reasons that gravity is referred to as a 'fundemental' force.
  4. Aug 2, 2004 #3
    Hey , Thanks a lot for bringing clarity in my question & answering.

    I think my question was about mechanism for gravity .

    Your answer ("The answer to the latter question is not something that physics deals with") is really surprised me. When scientist discovered that Gravity is raising from Mass( particales ) then why cant the reason (Mechanism) for Gravity MUST be subject of Physics ???

    Is my understaning of Gravity raising from Mass(particales) is wrong ? I would like to know what is preventing Physics to study further about Gravity Mechanism ? also are there any other "fundamental" forces exist in universe which we say Physics cant deal with ?
  5. Aug 2, 2004 #4
    the force of geavity is defined by this equation
    g=G m1 m2/r^2
    G gravitational constant
    m1 mass of one object
    m2 mass of second object
    r distant between ther centres

    now newton gaves us gravity but really did not understand what it was and einstien formulated his relativity theory to help define it.
    now trying to explain it on this forum in a few short words would be difficult so i will cheat and recommend either a brief history of time by hawking or the elegant universe by brian greene
    please do not hesitate to say if this is not the answer you are looking for

  6. Aug 2, 2004 #5


    User Avatar
    Science Advisor
    Homework Helper

    At some point the thought was that there were four fundemental forces:
    The Weak Nuclear Force
    The Strong Nuclear Force

    Electricty and magnetism were linked by Maxwell in the 19th century.

    In the 20th century, theory was developed that linked electro-magnetism with the weak force, so that it is now referred to as the 'electro-weak' force, and, if I recall correctly, indications are that the 'electro-weak' force might also be linked with the strong force.

    Research into gravity is difficult because, among other things, it is so weak compared to the other forces.

    Physics is about predicting results rather than describing mechanisms. Technically, there may be some valid physical discussion about the mechanisms for gravity, but ultimately, they either require a new fundemental force or are effectively another description of the action rather than the mechanism of gravity.

    Although it is philosophically unsatisfying, physics is only a description of the way we believe that things are, and not a description of why we believe things are the way that they are.
  7. Aug 3, 2004 #6
    Thanks you guys for giving me excellent insight into Fundamentals of Physics.

    btw , I think i didnt understand Einstein's (special)Relativity theory very well. Can you pls suggest some good material available online to go thru for better understanding.

    Thanks so mych for your help
  8. Aug 3, 2004 #7
    If you are willing to buy a book, I suggest:

    "Fabric of the Cosmos"
    by Briane Greene
  9. Aug 4, 2004 #8


    User Avatar

    Check out the general and special relativity section of this site. There's always good book recommendations floating around.
  10. Aug 4, 2004 #9
    praveen, hey I definately if you can read EINSTEIN SIMPLY EXPLAINED, by Martin Gardener, he is a excellent writing and the book covers Special and General Relativity, every page practicly has a illustration helping with the material, I just finished it and it gave me a super grasp of the concept, the math behind is a different story, but one must understand the concept before applying math, at least my idea. Gardener doesnt stop with just relativity, he then gives you the implications of relativity, such as types of universes, etc, and where we are going in physics with this, also some other ideas from other people he presents.

    Fabric of the Cosmos, is absolutely a beautiful and elegant read for anyone no matter what they like, i couldn't put it down.

    But from what you asked I was suprised no one mentioned that physists are looking for the theorized gravitron. You asked for the mechanism, well that is supposedly the mechanism for the grav. force just as the strong, weak, and electromagn. have carriers , it is my assumption most believe that gravity should also, my phys prof hates the idea he says grav is the ultimate force lol that it is all of them put together, i don't put much stock in it, but if the gravitron is found that gr could be replaced by Quantum Physics with all of its ideas because they both explain gravity but much different, in QP there is no bending of space time.

    Hope this helped
    If you want any other book ideas I read a book a day almost over this stuff, i have a massive library of phys theoretical and applied, so just name what you wanna know and ill take a stab at it.
  11. Aug 4, 2004 #10


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    We should be able to think of better books for you, praveen.
    the books so-far suggested are not mathematical.
    but you have had college calculus (as normal part of engineer education)
    so a good approach for you could have some formulas in it.

    http://math.ucr.edu/home/baez/relativity.html [Broken]
    http://math.ucr.edu/home/baez/RelWWW/tutorial.html [Broken]

    the key equation for you to understand is the einstein equation which relates curvature in a region to the density of mass and pressure in that region.

    the curvature is on the lefthand side of the equation and the
    "stress-energy" tensor is on the righthand side.

    the righthand side is usually dominated by a mass density
    but mass can be thought of as a form of energy so (just multiplying
    mass by c2) the righthand side can be all put into form
    of energy density (joules per cubic meter)

    a very small role is played by pressure (which has the same units as energy density----joules per cubic meter is same as newton per sq.meter)

    It is actually true that if material is squeeze so it has more pressure inside this contributes a tiny tiny tiny extra gravity----almost always negligible.
    amazingly einstein included even this extremely small effect of pressure.
    so it is not just mass-energy on the righthand side.

    this equation is very simple and elegant, and it would be fortunate if we could find a book which would present it to you in a way that pleased you
    and made it easy for you to understand.

    John Baez has a tutorial on the einstein equation at his website. this is an introductory simplified approach to the main equation of Gen Rel. and it also does not cost money. I already gave the link. I will give it again:

    It is called "the meaning of the einstein equation"

    It is very simplified but maybe it would be suitable as a beginning.

    this stuff has equations and expects you to know simple college calculus,
    but they are not HARD equations and there are not so many.
    So nobody who has had engineer education should be worried!
    he also has a collection of links to Gen Rel resources on the WW Web.
    he also recommends BOOKS
    Like Ian Kenyon 1991 Oxford Univ. Press text called "General Relativity"
    and also he suggests popular books (but not by Stephen Hawking as it happens)

    Now I will have my say and tell you a little about the UNITS of the einstein equation. at one point I will use a letter K for curvature instead of the usual letter G (which is confusing because same letter as the newton gravitational constant G) otherwise this is pretty standard.

    the units of curvature is inverse distance square------that is 1/sq meter.
    (time must be multiplied by c and measured as distance---cdt---to have all units same)
    that is because curvature is a second derivative----d2/dx2

    so think about the units of the Einsteinequation. On the lefthhand side it must be curvature---inverse square distance---- meters-2

    and on the righthand it must be energy density----joule meters-3------which is same units as pressure-----newton meter -2

    Guess what do you multiply the RHS by to get the LHS? the answer is 8pi G/c4
    where c is speed of light and G is the (newtonian) universal grav. const. which has units


    Amazingly, G/c4 has units 1/newton!
    this is just what is needed so you can multiply by it newton meter-2 and get the right thing, namely meter-2!

    so einstein eqn. (main eqn. of Gen Rel) says simply
    K = 8pi G/c4 T
    where K is curvature on LHS and T is energy density (officially "stress-energy tensor") on RHS.

    (careful, people use various different letters in their notation, often use G for curvature, confusing since also have letter G for grav. const!)

    Now I suggest that you go to John Baez website and try to understand this equation more deeply, with his examples.

    After that there will turn up some respectable book, like Ian Kenyon or the classic difficult Misner Thorne Wheeler or something----see what other PF people say---also see if John Baez has a recommendation

    Also BTW I thought everything NateTG said was good advice---but he did not recommend any book. I think he said do to PF "General Relativity" forum and ask. That is a good suggestion also.

    You say you had an engineer education and then later in life got interested in fundamental things like the universe and why there are forces and matter and so on. this is a good transition and i sympathize.

    I hope that in the next few years the actual REASONS why concentration of energy in a region causes curvature will begin to emerge. I think I see signs of this in Quantum Gravity research. But in any case we must first understand the Einstein Equation because that describes the way it goes even tho it cannot explain the mechanism connecting them

    matter tells spacetime how to bend
    spacetime shape tells matter how to flow
    have fun learning

    Last edited by a moderator: May 1, 2017
  12. Aug 4, 2004 #11
    Marcus, that was masterful, you have a way with words.
  13. Aug 4, 2004 #12


    User Avatar
    Science Advisor
    Gold Member
    Dearly Missed

    thanks for your kind ones woodysooner,
    BTW you have convinced me to have a look at Martin Gardner's
    book Einstein Made Simple
    From other things of his, I think of Gardner as an unusally good writer: makes it interesting and readable---you say it has a lot of good illustrations as well.
    Actually several people have said they liked that book
    so i will check it out. Might be a good one for Praveen to look at too.

    (I dont buy many books---too expensive and there are several
    good libraries here)

    bedtime. Im on the west coast :zzz:
  14. Aug 4, 2004 #13
    your welcome you and all the others on here deserve it, it is you great minds that make this forum amazing.

    About the book, it is illustrated by anothy ravielli and that was why i bought it because every other page was a picture, and normally i wouldn't like that but when it comes to relativity it helps, one i tried to read Relativity , by einstein himself, whoa, insane wording all it was this, take apt A, B, and C if light travels form a to c to b to a to b to c to b to a, get my picture, I was so confused, it probably is insightful other than that but i put it down and got Gardener and I feel i learned alot, but its odd if you already have such a grasp of GR why u would get it, just for the pleasure I assume.

    I now need some technical books of GR to somehow learn the math, i can't ever figure out what a stinking tensor is and how to get it, how to work with it, nothing. But maybe sooner or later with enough questions to you smart ones lol I'll get it. I envy you for your knowledge.

    Thanx alot
  15. Aug 4, 2004 #14
    I have ordered for 'Fabric of the Cosmos" at www.overstock.com ( chepest price --around 16$) and book is on the way.

    You know , I am more interested to know what is triggered einstein to comeup with special relativity theory . when I read in some sites , it is said that , during einstein time , Newton laws and Maxwell's equations were incompatible that lead einstein to think for new theory. then I went ahead searching what that "Incompatabilty" in newton laws and max equations. in this search , I bought a Book "Einstein's Cosmos" . I am still going thru it just to understand the history behind relativity and ofcourse relativity as well.

    can you guys pls suggest a Book that explains VERY CLEARLY the Incompatability of Newton Laws and Maxwell's equations. Also I read that Einstein's relativity and quantum theory are not compatible meaning both can not be right !!! I want to study this area too . May be "Elegant Universe" book may address this quest.

    btw , Thank you all of you for giving excellent help in my search. I am going go-thru each and every link/help/suggestion given above. Thanks again for ur support
  16. Aug 4, 2004 #15
    No prob and best of luck.

    You'll enjoy learning about the inconstincies of GR and quantum, that what I'm trying to learn now and it's hard mentally because you see how elegant GR and for einsteins sake you like no quantum is wrong, then you hear quantums side and your like yup Gr is wrong, so have fun with it and best of luck.
  17. Aug 11, 2004 #16
    to get a clear view ofspecial relativity u can go through resnick's bookintroduction to special relativity .this has very elementary treatment. if u want to go a little further,there r gems of book written by Bergmann,P.G. or u may choose Griffith,D.J. also.
    but since your question is on gravity,i suggest u to go through Sean Carroll .it is free downloadable from net and a really good book.
  18. Aug 11, 2004 #17


    User Avatar
    Staff Emeritus
    Science Advisor
    Gold Member

    The discrepancy between Classical Physics and Electromagnetism is based on how velocities add. Classical Physics used the Galilean velocity Transforms. The velocity of an object was the sum of the objects velocity + the velocity of the body from which the object originated. Maxwell found that light did not obey this law. The velocity of Light was found to be based on universal constants, thus, must be the same for all observers.
  19. Aug 13, 2004 #18
    Thanks rudra.
  20. Aug 13, 2004 #19
    Hi Praveen,

    Lets see if we can answer these questions one by one here.

    Yes, Earth has a gravitational field associated with it because of its mass. This is true from both a relativistic and Newtonian standpoint. In fact, any mass will have a gravitational field associated with it in both relativistic and Newtonian physics

    From a Newtonian standpoint, gravitation is a basic property of mass. All masses regardless of how large or small have a gravitational field associated with them just because they have mass. This gravitational field,usually referred to as g, at a distance r from any object is given by g = -GM/r(squared) Where G is the universal gravitational constant and M is the mass of the object.

    From a relativistic standpoint, a mass causes space-time to deviate from a perfectly flat condition (i.e. it makes space-time curve). This causes an object moving nearby to deviate from a straight line trajectory, appearing to be attracted to the other mass.

    If you're asking for an explanation of a mechanism by which mass causes the set of locations we know as space-time to curve, I can't give you that unfortunately!
  21. Aug 17, 2004 #20
    Does quantum theory gives any explanation/hint/clue about science behind Gravitational property of mass ?
Share this great discussion with others via Reddit, Google+, Twitter, or Facebook