The Nature of Gravity: Part One

Gravity – we all feel it; it limits much that we can do or build; and we don’t understand it!

Say you drop something and it falls, as you’d expect, vertically to the ground. A routine happening! You can easily explain what happened (and where and when), but can you explain how it happened, or why that something fell vertically down and not in some other direction? You probably can not.

Gravity is associated most strongly with two physicists – Newton and Einstein. Post Newton, gravity just had something (mathematical) to do with how massive objects are and how far apart they are. There was no real explanation of why or how. Gravity was just the way it was, and Newton’s Laws of Gravity were mainly predictive, not explanatory. [At least the ancients had an explanation. Objects (air, earth, fire and water) sought their natural place in the ordered scheme of things – as if they had minds of their own obeying cosmic laws. Thus, solids were at the bottom and rocks fell down because their place was with solids; water fell down but sat atop of solids; air rose to sit above water; and fire wanted to rise above the air to be with the sun, an obvious ball of fire in the sky.]

Post Einstein, gravity was just a phenomenon that was attributed to joint interactions between mass and space-time. Gravity was a property of the geometry of space. Mass distorts the shape of space-time, and so other objects move in accordance with that warped shape just like moving objects on the surface of the Earth follow the contours – the warps.

Today, physicists are trying to absorb gravity (which is a continuous phenomenon) into the now well established realm of quantum physics (which is not continuous). So far, no dice, but it’s not for lack of imaginative trying. The basic reason for trying to merge the two is that there are several things in nature that can only be adequately explained by unifying the two – primarily the singularities at the heart of Black Holes and the Big Bang.

So what exactly is gravity? Well, at first glance, gravity is obviously a force – it forces you to fall downwards or conversely, you have to apply a force to overcome it.

Traditional physics texts list four known forces at work in the Universe – gravity, electromagnetism, and the strong and weak nuclear forces. Unfortunately, gravity is pretty much now the exclusive property of Einstein’s General Relativity Theory while the other three are based around quantum physics. Physicists have as a first Holy Grail the desire to link as one the three quantum forces, or GUTs (Grand Unified Theories), and have just about succeeded – at least in combining convincingly the weak nuclear and the electromagnetic forces – electroweak theory. The second Holy Grail is to link all four into a TOE – a Theory of Everything, or in more common physics language, finding a quantum theory of gravity or quantum gravity as mentioned above.

Alas, despite intense effort (over several generations by theoretical physicists), no such link has ever been experimentally shown. The only thing to date that has achieved this TOE is the solely mathematical theory of strings, which, alas, has no experimental runs on the board some thirty years on. Even so, it takes string theory to require some ten or eleven spatial dimensions to achieve this, again something for which there are no experimental (and no common sense) evidence. It’s proving an interesting area for nerdy thinkers, but it (string theory) remains, 30 years on, theory, Theory and more THEORY!

What if one assumes the opposite (for a refreshing change)? So perhaps it just isn’t possible to relate or link gravity (hence General Relativity) to the other three known (quantum) forces. TOE is not only elusive, it’s impossible. Gravity and the other three forces can’t be combined any more than one can turn an apple into a pear. [“Heresy, heresy” I hear you cry!]

So how is gravity different? Perhaps gravity is just so different that it not only stands alone, it must stand alone.

Gravity is different #1: Unlike the other three forces, gravity has an opposite – antigravity, or in today’s cosmology, ‘dark energy’ which is causing the Universe’s expansion rate to ever increase or accelerate. You’d be hard pressed to come up with a concept of an anti-strong or anti-weak nuclear force, or an anti-electromagnetic force! What would an anti-magnetic field be? Could you have anti-light? Such concepts only induce headaches!

Gravity is different #2: Two of the three quantum forces operate over atomic (or sub-atomic) lengths. Both gravity and electromagnetism (EM) can, in theory, extend their influences to infinity. But, while EM can be blocked (at least for a while) by placing an object or insulator in front of the EM source, gravity can’t be. No known physics can block gravity. Place a sheet of metal in front of a lamp, and you block the light. Place the same sheet between you and the Earth, and you won’t start floating upwards! 

Gravity is different #3: What can gravity do that quantum forces can’t – bend light – that’s a pretty neat trick! [See below for more details.]

Gravity is different #4: Of all the four known forces, gravity is by far and away the weakest of the weak. If gravity were on the beach, gravity would get sand kicked in its face! Now you may not think gravity is all that weak while in freefall from a 12 story building roof heading for the concrete sidewalk far below, but it is – relative to the rest. [In fact it’s the electromagnetic force that terminates your 12 story fall, and presumably you!] I mean it is easier to lift up a paperclip from your desk with the entire mass of the Earth trying to stop you, than it is to separate that same paperclip from a reasonably strong magnet. In fact you could use that magnet to pick the paperclip up in the first place. Magnet one; Earth’s gravity zero! And you certainly would have to use a lot more force trying to walk through brick walls, or other solid objects, so the electromagnetic forces acting between the atoms and molecules holding them together as a solid must be pretty strong. And don’t even think about trying to pull apart the nucleus of an atom or to separate the quarks that make up a proton or neutron. No, we may think of gravity as a giant force, but it’s still the smallest of the four giants, sort of like a gnat compared to elephants.

Gravity is different #5: If string/membrane (brane) theory is right, then gravity alone of the four known forces can ‘travel’ off our brane (actually termed a braneworld) to another braneworld (and vice versa) through what is termed ‘the bulk’. The other three forces are stuck to our braneworld, and presumably, those three forces would also be glued to another braneworld. Thus, relative to the other three forces, gravity is diluted and thus is experienced as being a weaker (the weakest) force. It also explains how two braneworlds can attract one another and collide. Such a collision results in a Big Bang for each braneworld, but a Big Bang that occurred in pre-existing space and time. [Look up ‘Ekpyrotic Universe’ for the nitty-gritty details.]

Gravity is different #5 (continued): Invoking the braneworld concept and associated forces further helps to explain ‘dark matter’, which one recalls has positive gravity, yet is invisible or ‘dark’ (apparently it has no association or interaction with electromagnetic energy). Anyway, the idea is that ‘dark matter’ is in fact just ordinary matter, but on another braneworld (or in another universe – part of the Multiverse). We feel ‘dark matter’s’ gravity cause gravity can travel through ‘the bulk’ or leak between braneworlds, but we can not see ‘dark matter’ because electromagnetic energy can not travel across ‘the bulk’ that separates braneworlds. (String theory may still be, thirty years on, only pure mathematics and theory, but it can explain some real physics phenomena! Now if only someone could figure out how to slot ‘dark energy’ into this scenario, they’d be a candidate for the Nobel Prize!)

To be continued…