h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

So gravity is a distortion of space time caused by mass?

OK...but why on earth would that exert a force?

The common analogy is the old spacetime-as-rubber sheet sceanrio. In this, a heavy object on the sheet causes a dip. This dip *is* gravity – the force is the pull exerted by the dips, as things roll down towards the original object. Is that about right so far?

Well...

As far as I can see, the analogy only works in an environment where gravity already exists. In a zero-gravity environment, the heavy object would rest lightly wherever you put it, because it would have no weight. So there would be no dips.

But I don't want to split hairs, so I'll go with that. I'm curious why a body should cause this distortion (anyone?), but in the meantime I'll accept that a body of mass causes a stretching of spacetime, for reason or reasons unknown (to me). So let the dips stand.

But on this analogous rubber sheet, the *only* reason for things to roll down into the dips is that the whole experiment is happening in a gravity environment. Again, if you did the rubber sheet thing in space (hey - whatever turns you on), nothing would roll down the dips. There would be no reason to. At all.

I can't see anything intrinsic in the distortions of space time that would exert a force on things.

Can anyone explain?

Thanks

The rubber sheet thing may be a misleading illustration, IMO.

I always think of forces as the result of a potential gradient, or of something that is not in equilibrium. If something distorts space-time (and whatever) in one direction (read: no equilibrium) then it has to cause a force. Or in other words, if you bent the path of a moving thing you had to use 'the force' to do it. Gravity bends all kinds of trajectories of moving things.

Hmm...

HELL

Thanks Doc, I feel better now

That's a more digestible explanation. So to summarise:
- Matter distorts spacetime
- This distortion creates disequilibrium (?)
- To correct this disequilibrium, everything in the vicinity (including other matter, light and time) are pulled towards the matter you first thought of
- And that's gravity

So does the disequilibrium actually distort space? Time, yes, but space? Is the dimension itself, rather than just the things which inhabit it, distorted? And could that ever be measured? And can you separate time from space in that way anyway?

Actually, you may have fired some of my neurons re. the rubber sheet...

Any given chunk of matter is only a brief visitor to the universe anyway, until it's broken up, turned into energy or transformed into something else. So perhaps the analogy is more like a bouncing (rather than static) object on a rubber sheet.

Or given that matter moves from one part of spacetime to the next, as it transforms, coagulates, disintegrates etc... perhaps it's a perpetually bouncing object in a rubber bag, going from one side to the other. Self-mobile particles in a stretchy enclosure. Angry wasps in an inflated balloon. So the matter, or wasps, keep bouncing into different parts of the membrane of the spacetime balloon, and each time they hit, the membrane stretches and everything else on that part of the membrane is pulled in toward the impact point. Then the matter-wasp bounces off again, splits into atoms (possibly not traditional wasp behaviour, but they aren’t traditional wasps, as such), collects into a swarm, and bounces into the balloon again, causing another gravity-impact-stretch event.

Strangely, that seems to make better sense to me than the original. And actually the universe feels more like a load of angry wasps in a balloon than some billiard balls on a rubber sheet. But that probably says more about me than about the universe...

Anyway, thanks for that.

Food for thought there, eh?

In my experience all of this 'force', 'gravity' and 'quantum mechanics' stuff is merely there as a convenient way to explain (or to make predictions about) things.

Equilibrium: No force
Disequilibrium: Force

... which is something Master Yoda could've said.

Dr.H

That’s funny! But new questions appear....

In fact, why does space – time “bend”, or rather, go around mass? And why does it create a potential gradient and “disequilibrium”?

Why space – time does not pass straight through mass, with no bending at all? And just not “noticing” mass?

Or, it may turn out, that space – time has a structure similarity with mass ?... And flows just like water around a stone ?

If I knew the answers, a Nobel prize could be mine....

does inertial mass due to aceleration of matter create its own gravity distortion in space time. if it does then why is the universe expanding.

If gravity is due to mass, then we have a problem because there is rest mass, inertial mass due to acceleration (f=mi.a) and mass due to relativity (gama factor or beta = velocity/light speed). But what about rotational forces, does an accelerating rotation of a mass cause additional mass, and hence extra gravity. The obvious question being instead of a big bang, was there a big rotation speed of the early universe which is getting slower with time as the universe expansion rate reduces.. might explain why the Hubble constant is only known approximately.

What effect would a spinning universe have on the appearance of the cosmic microwave background (cmb), would it appear smudged or smeared untill almost the same in all directions, assuming our part of the visible universe is just a small part of the whole universe.

Yes, and if it is rotating is it rotating clockwise or anti-clockwise? ()

Dr. H

Good point, not being a cosmologist I admit that I don't know the answer to the question of spin direction, although the big bang cosmolgy theory seems to sugest a wave of spherical expansion from a big bang point, followed by a possible contraction or big crunch, leading to spherical spin(rotation/reflection/bounce) from yet another big bang point. New cosmologies are popping into existance all the time, if you don't believe me, then have a look at some recent science magazines. Is there a noun for a collector of cosmology theories? Are there 2 big G's one for newton and another for large cosmic distances(as seen in same recent edition of science magazine)?

Looked up the units used for the gravity constant "G", meter cubed per second per second per kg. or units of "G" are m^3.s^-2.kg^-1 This implies that rate of change of change of volume depends on mass. Also G times mass= volume per second per second. For a particle E=mc^2 =hf joules, so particle mass=h.f.c^-2 and G.h.f.c^-2=d^3.t^-2 Hence "G" assumes that the particle(electron etc.)not only has volume,but that this volume vibrates at frequency "f" hz. So why are particles said to be points in space-time with no volume? also if acceleration adds mass, while slowing takes it away. and most particles have mass due to internally stored energy. this energy should be due to changes of volume, or particle size,by changing radius at frequency "f" as above.

sorry, see above,re; G x mass= G (h f)/c^2 = d^3/t^2 where h= planks constant,in kg meters squared per second, d,is in meters, t,is in seconds, f, is in hertz, and G is in meters cubed per second squared per kg... As the sub atomic particle accelerates its frequency is doppler shifted upwards (or downwards as it is slowed).The same particles amplitude(sinusoidal change in spherical volume) stretches and contracts space-time(with its associated charge density),creating further outward spherical space-time layers(like an onion), the amplitue of each sucessive layer diminishes with distance from the particle centre... Does this imply that if a positron and an electron collide (both having a different frequency) their 180 degree oposite phase holds them together, while their difference in frequencies get equalised by emitting a photon, the electron and positron might join up to form a quark, ie,not be destroyed?

oops,h= plancks constant.. thinking about the space-time volume changes of a sub-atomic particle, which vary with it's frequency "f", it's not the space-time volume that's realy important, but what it contains, see Wiki,page on space-time energy density. An apparently empty volume of space-time is in energy equilibrium (entrophy)? While a particle is in dis-equilibrium because it's volume and energy density are varying. Energy density (j.m^-3= kg.m^(2-3)s^-2) uses the same units as pressure (pascals,kg.m^-1.s^-2).. implying that a particle emits pressure waves, into open space, does the particle loose energy this way? Is the particle energy loss made good by incomming pressure waves from other particles? Are the frequency and phase differences between different spherical pressure waves sources the cause of gravity? Sorry I'm not a scientist and, I don't know if my previous ideas are in any way correct.

If Mass is due frequency dependant energy density waves,and space-time is full of energy density pressure waves. Then two separated masses will be in each others energy shadows. So the closser each object gets, the more pressure there is pushing from behind. While the two facing sides receive less and less pressure from the space-time that's around them. As the shadow removes more and more energy density from between the masses, the greater the gravity effect. Thus Gravity is Just the amount of shade one mass puts on another mass from the energy density of their space-time sky.

Just found on Wiki, Le Sage theorem of gravity, which uses shadow or shade effect to postulate how gravity is caused.. See previous posting, positive energy density pressure, due to cosmic inflation +9x10^-10 joules per meter cubed of the free space vacuum, alows the shade effect to create gravitational forces.

force in Newtons = (G.Mass1.Mass2)/r^2 = energy density (j/m^3).r^2 = Pressure (pascals).r^2 If the vacuum energy density is approx.10^-9 and G is approx.10^-10 the electron mass 9.10^-31kg.its mass squared is approx 81.10^-62 using r^4 = G.M ^2/energy density 81.10^(-10-62+9 =63).. so roughly, r = 3.10^-15 meters.. nearly the clasical radius of an electron r, = charge squared/ 4.pie.electrical permitivity.electron mass.c squared = 2.8179.10^-15 meters. Which I think is quite a coincidence, still it's not my job to challenge relativity theory,my maths just isn't up to it.

here are some web sites, that offer an alternative view of what gravity force may be.. www.rbduncan.com/shortTOE repositioning theory,by Denys Lepinard, also the wave theory, by Gabriel Lafreniere, and the Phi-Wave Aether, by Caroline H Thompson.

Just found a web site for daily measurements of the solar wind at, www.secnoaa.gov/SWN/sw_dials.html at the moment its pressure is 2.4 nano pascals. This changes all the time, and presumeably would add or detract from the centrifugal force which is holding the Earth away from the Sun(by varying amounts).Except that the energy density of space expansion just happens to be around 9x10^-10 pa or 0.9 nano pascal.Do they almost cancell each other out most of the time? Is there a climate effect?

Remember, a rubber sheet is also used to prevent pee from getting onto a matress!!!!!!

Taking the earth radius as 6.4 x10^6 meters, average rho for planet earth as 5515 and acceleration g, at the earths surface as 1.64 x 10^-24 for a 1kg mass pulling the earth, then it's pressure = 4/3 pie x radius x rho x g (in pascals). or 2.3 x 10^-13 pascals. This implies that the pressure between two masses is reduced below that of the energy density of the free space vacuum, 1 x 10^-9 pascals or j/m^3.. Or to put it another way, the energy density ( or rubber sheet ) flows like air pressure from higher density or inflation to the lower density or deflation between the two masses.