luvluv

I've heard this term used a lot, usually centering around discussions about the big bang, and I'm wondering what the heck it is?

ohwilleke

Quantum gravity is a theory that a lot of people would like to discover which would explain gravity in terms of a force transferring particle (commonly called a graviton) moving between massive particles in a way that is consistent with what we already know about how gravity works.

This would be great to discover, because every other non-contact force known to science (electromagnetism, strong nuclear force (which holds atoms together), and weak nuclear force (which makes big atoms radioactive)) works in this way. You could have a few simple equations accompanied by a handful of numbers that would fully describe the fundamental laws of nature. Until you do that you have three similar sets of rules that explain a bunch of the laws of nature, and another totally different set of rules that aren't even similar that explain gravity.

The problem is that no one has ever observed a graviton and coming up with a hypothetical set of rules for how a graviton acts that is consistent with the laws of gravity we observe in the real world (originally devised by Newton and refined by Einstein in his general theory of relativity which among other things says that even light is affected by gravity despite the fact that light is pure energy with no mass) is very difficult.

It so happens that the only possible theories that could both match reality and explain gravity in the way the rest of the non-contact forces observed by science works, involve all sorts of things we have never observed in real life (ten dimensional space, scads of particles no one has ever encountered, etc.). Fortunately, for physicists seeking tenure and wanting to write books, there are possible quantum gravity theories whose predictions that have never been observed in real life are things we don't have the ability to see with current technology anyway, so these theories can't be proven true or false.

Starboy

For that matter no one has ever observed a gravity wave. There is an experiment currently being constructed that is expected to detect them. It will be interesting to see if there continues to be a negative result when it is operational.

Starboy

luvluv

Thanks a bunch ohwillke. I take it you are skeptical of the field?

I found this link to the first chapter of a book called "Three Roads To Quantum Gravity":

<a href="http://www.qgravity.org/roads/prologue/" target="_blank">http://www.qgravity.org/roads/prologue/</a>

It was good stuff. I'm probably going to get up right now and see if I can find the book at Barnes and Noble.

I'm not sure if I followed you: are you saying that electromagnetism and the strong and weak nuclear forces are "enforced" by particles or not? And what are these particles called?

Also, am I to understand that this would eliminate the Fine Tuning Argument because it would make all the anthropic coincidences go away?

What bearing does it have on the question of a quantum fluctuation being the cause of the universe. I guess a lot, since the universe started so small and ended up so big, but I can't say that I exactly follow exactly how.

Thanks a bunch! This stuff is fascinating even if I can't quite understand it! (A lot like college, in many respects )

(This is kind of funny to me, as comic book fan. When I was a kid I use to collect these books called "Handbook of the Marvel Universe" They explained how all these characters had their powers. That was the first and only place until TODAY that I had ever heard of gravitons [the comics said that the people flew because their bodies produced "anti-gravitons" which neutralized the gravitons!]. I always thought that stuff was made up until today!)

Philosoft

Indeed, they are.

<strong> They are all special types of bosons. The EM particle you might have heard of: the photon. The weak nuclear force particle is the weak-gauge boson; the strong force particle is the gluon. Quantum theory predicts they are all massless. This may or may not be true.

<strong> Well, mathematically, string theory shows that, at the very least, EM and weak nuclear can be shown to be the same force at a higher energy level. This required something called gauge symmetry in which a weak-gauge boson interacts with a Higgs boson to give mass to the weak-gauge bosons. Or something. Anyway, the resulting theory is called the electroweak theory. The prevailing wisdom is that all four forces will be shown to be low-energy manifestations of one high-energy force.

<strong> I think the key is a particular quantum fluctuation (or series of QFs) that introduced matter in a fractionally larger amount than antimatter.

<strong> If you don't already have it, Brian Greene's Elegant Universe is an outstanding intro to string theory. Also, I've begun poking around <a href="http://superstringtheory.com/index.html" target="_blank">here</a>. Lots of fun stuff there.

Nameless One

I guess i should start reading that book soon
then, i buy more books then i have time to read.

Philosoft

If you are as interested as you seem, I'd advise dropping everything you're reading right now in favor of that one. I was that impressed.

Answerer

I feel that the main difficulty about discovering quantum gravity is that no one has the slightest idea of quantizing spacetime(which is continuous in classical physics).
So far, the only closest evidence about quantum gravity is the discovery of gravity waves in 1993. Other than that, it is another dream for the physicists to accomplish.

Starboy

Was that a theoretical discovery or an experimental discovery? Could you be confusing gravity waves with gravitational waves?

Starboy

[ September 06, 2002: Message edited by: Starboy ]</p>

ohwilleke

I wouldn't say that I am skeptical of the field. It's just that I think it is important, especially when laying out the field to a lay person, as opposed to an enthusiast engrossed in the field, to go back to basics and distinguish between what is known and what is conjecture and hypothesis. The popular press and people talking about modern physics are always attracted to the unknown.

Quantum theory is fantastically accurate and well proven, as far as it goes.

We have what amounts, for all practical purposes, to a complete understanding of electromagnetism. Since electromagnetism is the force that is at work in all chemical interactions (holding, for example, the atoms of a water molecule together), in theory, we know everything we need to know to work out all of chemistry (including the chemistry of life) from first principals without doing experiments. We don't have the mathematical capacity to do it, but the theories are all there.

Our understanding of the inner workings of atoms is truly remarkable. We know, not only that gluons hold nuclei together, but that they come in three different kinds (commonly called "colors"). Hundreds of kinds of observed atomic scale particles have been boiled down to combinations of a dozen or so fundamental typical of particles that interact in very predictable ways.

Another sign of the maturity of the field is that there are lots of things which we know that we don't know. For example, there are two main competing views regarding what the fundamental structure of time-space itself is. Some people think that time-space is "continuous", others think it may be a bit like an infinite tilework of very small pieces of a length that is called for historical reasons "Plank size" (about 10^-34 meters, I believe). We also know that the nature of gravity is something that we don't fully understand.