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Q & A: Why Gravity?

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Most recent answer: 09/30/2012
Q:
Why is gravity produced from a mass of something?
- nick (age 16)
sixth form college, uk
A:
Nick -

We don't yet know any deeper reason why gravity, as expressed by the laws of general relativity, has to exist. maybe someday the string theorists or someone will have a deeper set of laws, but that will only push the problem of "why" back one step.

Mike W.

(published on 10/22/2007)

Follow-Up #1: gravity and gravitons

Q:
How can there be gravitons and gravity waves when Gravity Probe-B has put to rest the question re. frame dragging and geodetic effect? Seems to me it's either/or. Is there still a place for gravitons?
- Warren Updike (age 72)
Towson, MD, United States
A:
Warren- I've modified the previous answer, which mixed up the type of evidence needed to demonstrate gravity waves with the type needed to demonstrate their quantized version (gravitons). Although we thought we had cleared out all the mistaken answers from earlier versions, another one crops up every now and then.

On to your question: It's not either/or.  GR implies the existence of gravity waves. The slowing down of binary pulsars provides strong evidence for their existence. Experiments (LIGO and VIRGO) are running which may find more direct evidence.
Gravitons would be the quantum version of these gravity waves, just as photons are the quantum version of electromagnetic waves. The existence of photons makes no problems for the classical EM theory of say radio waves under ordinary circumstances. The existence of gravitons would, unfortunately, pose no problem for the GR theory of gravity waves under any circumstances we can hope to observe. So that means we'll probably never find direct evidence of gravitons.

We nevertheless believe that gravitons will be present in any quantum theory of gravity, since all other large-scale waves are quantized. In fact, any non-quantized waves would lead to severe logical problems, since they would provide a way to unravel the uncertainty relations, which are a direct implication of the quantum formalism.

Mike W.



(published on 09/30/2012)

Follow-Up #2: gravity: force or spacetime effect?

Q:
I'm not sure you answered my question. Most likely I don't understand enough to understand your answer. Here is another try: If gravity is a force carried by gravitons, is that not in opposition to the GR concept of the geodetic effect which describes gravity as effect upon space-time by virtue of a local mass? Force or effect?
- Warren Updike (age 66)
Towson, MD, United States
A:
Ah, now I get what you're asking. Unfortunately it's slightly over my head. Still, I can pass on the words that are usually said.

At least on a large scale, say more than 10-44 s and 10-33 cm, on which our usual concepts of space and time are legitimate, gravity is most accurately described as a form of spacetime curvature, rather than as a force within a Euclid-Newton spacetime. That does not mean that these spacetime ripples are purely classical, however. They should in principle also be describable more accurately as quantized effects. There are notorious problems, however, in following such a description to its logical conclusions. It turns out that those problems are alleviated by string theories in higher dimensions (9 or 10 spatial dimensions, depending on the type of theory). Thus spacetime itself would emerge as the large-scale behavior of some sort of quantum theory, if that effort works out.

Mike W.



(published on 09/30/2012)

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