Gravitons

[BlameTheEx]

linda

Its very simple. Everything that has energy has mass. A photon "at rest", if such a thing has any meaning, has no energy so it is massless. From this point of view, matter has mass BECAUSE it has energy. It only becomes complicated if you think of gravity as a property of matter, rather than energy.

Sadly, I am as confused by Simhony's explanation as you.

There is a somewhat simpler way to explain the particle/wave duality. A photon actually exists as a particle only at two events. The event where it is transmitted, and the event where it interacts at it's destination. Between those two points it doesn't exist except as a wave covering the entire field of it's possible directions of travel. It doesn't travel as a particle. The proof is that a single photon can suffer from interference effects.

[lindagarrette]

It seems the only time something could be at rest would be if abruptly changing direction say, by colliding with something else. What happens when atoms collide? Energy fields run into each other not particles, right? The interference effect still has to be a wave event. (Am I doing any better?)

[Aki]

How much energy does a photon or graviton have anyways?

[lindagarrette]

If light is not a wave, it would then consist of little particles, or quanta, called photons, each with an energy of Planck's constant times its frequency.

[BlameTheEx]

Linda

"It seems the only time something could be at rest would be if abruptly changing direction say, by colliding with something else. What happens when atoms collide? Energy fields run into each other not particles, right? The interference effect still has to be a wave event. (Am I doing any better?)"

Much better. NOTHING, and that includes atoms exist as particles EXCEPT when they interact, At all other times they exist only as a field of posible positions. Interactions can't happen unless there is motion, and moving objects are waves, so interference effects are inevitable.

One use of particles is the election microscope. Beamed electrons can be focused very well indeed, but there are still limits. The limits are due to the wavelength of the elections. The faster they move, the shorter their wavelength. A practical proof that moving objects exist as waves. If larger particles were used, say protons, the waves would be shorter, and the microscopes definition better. Of course one could wonder if the sample being examined would survive such a battering. Probably not.

Admittedly something as large as, say, a thrown brick will have a wavelength that is too short for measurement. The uncertainty in it's position is not measurable ether. In theory the probability of its interactions are dependent on the whole universe, and it could be anywhere. In practice, it is better to assume that it will go just where it is thrown, and duck.

Aki

Regarding Gravitons.

They are as yet purely theoretical particles. They should follow Planck's constant too, but who knows for sure? I think neutrinos may yet prove to be gravitons. Does anybody know of evidence that they are not?

[Tormod]

Originally posted by: BlameTheEx
Regarding Gravitons.

They are as yet purely theoretical particles. They should follow Planck's constant too, but who knows for sure? I think neutrinos may yet prove to be gravitons. Does anybody know of evidence that they are not?

I know of no evidence that they *are* photons.

Here is the wikipedia antry on gravitons:
http://en.wikipedia.org/wiki/Graviton

Note the comment that some theories do not need gravitons at all, for example the loop quantum gravity theory.

Neutrinos have been found to have mass, thus they could not be photons if this is correct.

Here is one source (altough it is almost 5 years old now):
Neutrinos Have Mass: Experimental Evidence for Neutrino Oscillations
(University of Wisconsin)
http://www.pheno.info/hottopics/neutrinoshavemass/

[BlameTheEx]

Tormod

Yet again my thanks.

The evidence for neutrino mass seems rather indirect, with experimental evidence only setting an upper value of 3ev. Still, is very much a blow against them being gravitons.

[maddog]

To answer Aki first.

Gravitons are the Bosons thought to carry the force of gravity. The conventional wisdom as
part of the Standard Model is that all massless Boson (force carriers) travel at the same
speed as light. However, no experiment has yet verified the existence of such Graviton
particles (or Gravitational Waves for that matter). Like the Dual theory nature of Light,
analogously it is thought Gravity should also have this same nature and can be described
both as particles (Bosons) interacting with Fermions (that have mass) or as waves. Yes, it
is true that this expression according to Einsteins General Theory of Relativity has that space
is bent in the process. As an aside, it was the first success in String Theory that predicted
that spin of a Graviton would be 2 and not 1 like the photon. It was this that have people
thinking Gravitons have no mass.

Lindagarette comment that gravity can extend into the other branes is what some Physicists
are thinking. This may be another approach to look for dark matter.

In string theory (Supersymmetric or otherwise) all particles can be composed of strings
(open or closed).

Another way of thinking about photons as they do have a "sense of time". Photons can be
viewed interacting with matter in a time independant fashion. This eliminates the concern or rest mass. Instead is the notion of the vacuum potential or the Potential Energy of nothing. Now as for the Higgs Boson, this is a particle I desire to understand more fully
myself. It is covered more thoroughly in Quantum Field Theory (a course I have yet to take).
From what little I do understand on the Higgs Field (the expression of the particle) is that
particles in a Higgs Field "feel" there mass. Brian Greene talks about it a bit in the Elegant
Universe. He also has a new book out (I haven't read yet) called "Fabric of the Universe".

From Special Relativity a Photon travels at the speed of Light in all reference frames. As I said earlier were a Graviton to be massless then this would be true for it as well. String
Theory even implies this.

Mass is a measure of how much something is with respect to its resistance to inertia. That
is the resistence to being put in motion by a force. A photon does Not have mass. It does
have energy and momentum (mass times velocity). The article cited is very interesting. I
think I would have to read it in its entirety.

Aki: The energy of a photon (assumed the same with gravitons) is inversely proportional to
its wavelength.

Linda: Light is both a wave and a particle (that dual theory again), just not at the same time.
Still confused ? It turns (from Quantum Mechanics) the matter can be expressed as waves
as well (De Broglie wave mentioned in the article).

I think I have said too much now. So I will rest. Have a good weekend. Later.

Maddog

[Aki]

And Greene says that gravitons probably are made up of closed strings (loop), and thus, they are able to escape to other universes, unlike fermions or other bosons, where they are open-ended strings that can attach to the brane.
Greene's new book is actually called the Fabric of the Cosmos. I recently got a copy, but haven't had time to read it yet.

[maddog]

Yeah I have seen that book by Greene. I have yet buy it, I have waiting for a paperback.
I made an erroneous statement earlier by mistakenly implying that photons have a "sense of
time". There is a "not" missing in that statement. Basically the notion of time does not exist
for a photon. This is what forces a photon to behave differently wrt Heisenbergs eqaution for
energy. Oops!

Also about photons, they don't have rest mass as they never rest. Period. Created/destroyed
and move at the speed of c throughout their whole life.

Neutrinos (of any flavor) could not be gravitons as they are leptons (a kind of fermion) having
half integral spin. A graviton (Boson) has integral spin of 2. Very different critters. It is now
thought that if Neutrinos were very light yet a mass more than 0 might account for their being
non-interacting as they are.

Just a few of the flawed comments made in this thread. Cheers.

Maddog