What are currently the most credible proposed theories explaining gravity?
I know the laws governing its behaviour - that’s not what I’m looking for. Rather, I am looking for a relatively brief and succinct explanation of how we think it might work (ie. particle interaction, energy transfer, gravitational waves, etc.)
Basic undergrad physics and calculus here, so I would appreciate responses kept at or below that level for the purpose of discussion.
Gravity unlike the other potentials affects all objects and particles, even things that we think of as massless like light (we see this from special relativity), if we look at spacetime in special relativity we see that the way that gravity affects objects moving through space-time is exactly equivalent to if space-time was curved, so we look on gravity as the curvature of space time.
In quantum field theory, particles can be thought of as changes in a field and changes in a field can be thought of as particles, a static field can be thought of in terms of virtual particles. Gravity though it can be thouht of as the curvature of space-time is still a field, therefore it can be thought of in terms of particles which have been named ‘gravitons’ (which can be mostly thought of in virtual terms as most large graviational fields are static). Currently there is no fully-functional quantum field theory of gravity, but it is probable that gravitons exist and certain properties that they must posess are well-known.
Well in the way that question is framed, I could answer “gravity!” and for a deeper answer I could speculate on the various theories of quantum gravity, string theories or the Higgs field (though it would most defintely be speculation as I only have a smidgen of technical knowledge in these areas). The thing is though you can keep on asking ‘why?’ ad infinitum as every theory must include axioms.
I’ve always been uncomfortable with the “gravity causes space to curve” thing. Maybe that’s why I got a “D” in freshman physics. (Nah, it was playing guitar all night and sleeping through class; that’s what did it.) Since no one ever describes what space-time is made of, or even suggests that it is an actual thing, I have always wondered what it is that is curved.
Now gravitons, undiscovered though they are, I can get behind. A particle is emitted here, travels to there and causes an effect. That’s something even a guy with my limited intellect can understand.
Imagine two points on a piece of paper, if we then draw a rectangle with these two points as diagonal points on this rectangle the shortest distance between them (ds) will be the diagonal of this rectangle and from Phythagoras’s theorum we know that:
ds[sup]2[/sup] = x[sup]2[/sup] + y[sup]2[/sup]
Where x and y are the lenght of two sides of the rectangle that form a corner.
If we extend the idea to find the shortest distance between two points in 3 dimensions we find:
Spacetime is a sort of 4-dimensional space (well it’s not a space it’s a spacetime), but with one of the axes proportional to time time (ct), we can then find the ‘distance’ (in spacetime this is called an interval) between two ‘points’ (in spacetime these are called events). Time isn’t a spatial dimension however so we can’t simply think of it as an extra dimesnion of space and if we think in terms of light cones and which events can influence each other we see that space and time can’t have the same signs in our metric, we therefore define the interval between two events in space time as:
I = (ct)[sup]2[/sup] - x[sup]2[/sup] - y[sup]2[/sup] - z[sup]2[/sup]
Now going right back to the pieace of paer what if we weren’t writing on a flat surface, if we were writing on a sphere for example? we then find that the equation:
ds[sup]2[/sup] = x[sup]2[/sup] + y[sup]2[/sup]
Is no longer true as the shortest path between points is no longer a straight line but a curve.
Simlairly if we look at curved spacetime, the shortest interval in curved space-time between two events no longer obeys our equation and we have to formulate a new equation to take into account the curvature.
That’s absolutely correct, but in fairness, saying that gravity is because of a curve in space-time isn’t a very satisfying answer since it doesn’t really advance the explanation of what mediates between massive objects.
Why do you subtract all the spatial terms? I don’t see any logical reason. In fact, let’s say ct=0 (two events are simultaneous). According to that equation, the interval between them is less than zero, which is extremely counter-intuitive, at least to me. Did I miss something, or should those really be plus signs?
One of the popular ways to illustrate gravity, the ‘rubber sheet’ analogy, can be misleading - it likens spacetime to a thin sheet of rubber, stretched tight, objects with mass sink into it, causing dimples which other objects will roll into.
The problem is that the analogy itself requires gravity to work - if there was nothing pulling the objects downwards, there would be no dimples in the rubber sheet and no reason for other objects to roll into them.
It is better in some ways to think of the rubber sheet as having been ‘puckered’ or ‘gathered’ (like this by objects with mass, so that an object travelling in what would otherwise be a straight line actually follows a curved path.
The most important thing about the interval is that it should be invariant under a Lorentz transformation, so that two observers no matter what their rest frames will always agree about the length of an interval between two events.
Remember though in relativity there is no such thing as ‘simulataneous’, two different observers will not necessarily agree with each other over the order of two events. A negative interval will correspond to what is called a space-like seperation, it is impossible for two events with a space-like seperation to affect each other. A postive interval will correspond to a time-like seperation and the two events may influence each other and an interval of zero can correspond to two events which are seperated in space and time, but whose seperation in time will be equal to the amount of time it takes to light to travel between there spatial co-ordinates.