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Gravitons

Gravitons - Physics Forum

Gravitons - Physics Forum. Discuss and ask physics questions, kinematics and other physics problems.


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  #1  
Old 10-27-2005, 06:40 PM
John
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Default Gravitons



I'm led to understand that the EM 'force' is mediated by particles
(photons), and I presume this idea extends to other 'forces' too (e.g. that
nuclear 'force' is mediated by mesons, etc.).

I'm also led to understand that the idea of gravity as a 'force' was
superceded in GR by the idea of gravity as a curvature of spacetime.

How am I to understand the concept of 'graviton' then?
Is this the particle that mediates the gravitational 'force'?
Or is this just a quantization phenomenon in disturbances of the spacetime
curvature?



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  #2  
Old 10-27-2005, 09:20 PM
PD
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John wrote:

There is a form of understanding of the EM force that it represents
curvature in a different dimension attached to our 4-space -- a notion
that is sometimes described mathematically as a fiber bundle.

Gravity seems to express itself in the curvature of our 4-space, as
well as possibly in other dimensions.

In the case of electromagnetism, the interaction can also be understood
in terms of quantized fields, and this notion has been developed into a
self-consistent and remarkably accurate theory: QED.

In the case of gravity, the same kind of recasting is presumed to be
possible, but no one has yet been able to get it into a theory that is
both self-consistent and accurately predictive. Yet.

PD

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  #3  
Old 10-28-2005, 12:17 AM
N:dlzc D:aol T:com \(dlzc\)
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Dear John:

"John" <a.b@c> wrote in message
news:ja98f.4761$[Only registered users see links. ].net...

Quantum mechanics requires a (virtual?) moderator particle for
all interactions. If quantum mechanics were to describe gravity
as a force, then gravitons would be required.


I think it it the former, and not the latter. Just my opinion.

David A. Smith


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  #4  
Old 10-31-2005, 12:43 PM
tadchem
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John wrote:

Correct.


Also correct.


General Relativity (GR) and Quantum Mechanics (QM) were developed
simultaneously to describe phenomena at opposite ends of the scale. GR
describes the interactions of very large masses and interactions over
great distances (where the time required for light to travel is
significant). QM describes the interactions of the very smallest
masses and interactions over very small distances.

In trying to develop a single 'theory of everything' (aka ToE,
analogous to Russell and Whitehead's early 20th century effort to
derive all mathematics from first principles in Principia Mathematica),
it was soon realized that GR and QM are in certain areas
incommensurable.

Gravity is simply described in GR as a conserved scalar field
associated with all massive particles that happens to be distributed
throughout geometric space as a 'curvature' (implying a certain type of
variation with distance).

One effort to synthesize GR and QM was to postulate a QM analogue to
the quantized nature of electromagnetic phenomena in the atom. This is
where the idea of 'gravitons' came from. Unfortunately, the requisite
properties of a 'graviton' are such that they are extremely difficult
to observe, and in fact they have never been observed, despite the best
efforts of particle physicists.

The alternative to quantizing GR in this way is adding GR to the
equations of quantum mechanics. Unfortunately, even something as
simple as solving the Scroedinger equation, when attempted in curved
space, exceeds the capacity of our current mathematics.

We know that gravitation exists, and that GR describes it well, and
that QM cannot describe it well. OTOH, We know that there are things
that QM describes superbly well (electron orbitals with unit angular
momenta) that simply fall outside the scope of GR. Both theories are
verifiably correct as far as we have been able to test them. Neither
theory is complete.

What is the answer? My best guess (based on a partially historical
persective) is that we will need to develop a mathematics that is more
general than either QM or GR, and which will 'simplify' to either under
the appropriate limiting conditions.

The situation appears to be similar to that of the photon, which can be
'seen' as either a particle or as a wave. When treated as a
(Lorentzian)four-tensor that satisfies Maxwell's (Unified) Equations in
(Minkowski) space-time, the photon reduces to either a wave or a
particle depending on the operation(s) performed to observe it.

Many physical 'models' - actually mathematical constructs that *mimic*
the operation of 'real-world' objects, processes, and events - suffer
from being too similar to other more ordinary objects. processes, and
events. This similarity is often so strong that the word 'similar' is
replaced in the minds of people with the word 'identical,' so that
people tend to think (for example) that photons *are* waves or that
they *are* particles.

The most honest description is simply a summary of the properties of
something: "we know it has these attributes, and that it interacts with
these specific things in these specific ways."

Short answer to your question: the 'graviton' is simply a hypothetical
particle conceived of in an effort to find a common theory that unifies
both GR and QM. It was developed by particle physicists who were
trying to apply their particle interaction models to gravitation. It
*remains* hypothetical, in spite of the best effots of those who
developed the concept to demonstrate its validity empirically.

"When the only tool you have is a hammer, then every problem begins to
look like a nail." - Abraham Maslow

Tom Davidson
Richmond, VA

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  #5  
Old 10-31-2005, 05:31 PM
Mark Fergerson
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John wrote:

Yep. Note you meant "virtual particles" which are somewhat
different from "free particles".


Sorta, because the background interacts in a way that doesn't
happen in EM.


Yes, in the "exchanged virtual particle that produces attraction
between the objects exchanging gravitons" sense, like photons do in EM.


Go right ahead if you want to, but be aware that unlike EM
photons, gravitons bend spacetime just like static masses do. This
is because in relativity the "no mass or gravitons" case seems to
assume that spacetime is flat (not curved at all), but that
situation can't obtain in reality; the presence of mass _anywhere_
means spacetime can't really be flat.

Trying to use EM as a basis to model gravitation on is fairly
common but there are problems; in EM the background's state of
curvature doesn't come into play at all, but in gravitation it's
intrinsic. Also electric charge is easily quantized but mass is not.

You can look at static gravitational fields as producing the
familiar dents-in-the-rubber-sheet, and gravitons as ripples that
propagate along the sheet.

The only problem with resolving this picture with observable
reality is that we can detect the dents (with gravity gradiometers
frinst) but not the ripples (plenty of experimental non-data from
Weber etc., though Forward appeared to generate and detect them with
his rotating gradiometers but AFAIK nobody's reproduced his results).

I think part of the problem stems from the fact that we don't
have a convenient way to talk about "near field" and "far field" in
gravitation without infinities swallowing everything, as opposed to
the EM case.

That probably didn't help at all.


Mark L. Fergerson

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  #6  
Old 10-31-2005, 10:11 PM
John
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"Mark Fergerson" wrote:
<snip>

Quite wrong -- all these replies have been very helpful.
Thanks.


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