[IzzieC (OP)]

Andrew asks:

I have managed to confuse myself and would like to ask some question about the graviton and space. I have been reading a bit about string theory, and understand that bosons are connected to an assumed multidimensional membrane of space, DBranes. These Bosons pass through space whilst still being connected to the membrane. The graviton is normally assumed to be a spin 2 boson / virtual particle.

Under General relativity gravity is caused by mass/energy which stretches the space around it, the force is unidirectional and directed inwards. Also nothing except hawking radiation gets out of a black hole.

If gravity is assumed to be created by the graviton, can it be viewed as an inflow of space towards the mass. Ie does the graviton represent space, and being a virtual particle should it be assumed to be destroyed by mass thereby creating a vacuum for more gravitons / space to be pulled into. In the absence of mass do gravitons/virtual particles cause the expansion of space ie dark energy.

Could dark matter be multidimensional, ie not visible in normal 4 or 5 d space time. Does dark matter and the graviton exist?

Can you help?

[jeffreyH]

Something causes galaxies to rotate in an unexpected way. This has been given the name dark matter. There is not enough detectable mass in the galaxies to explain the rotation profiles. The missing mass appears to be dark, meaning that it is hard to detect for some reason. It also appears not to interact with electromagnetism but does generate a gravitational field. There is no answer as to what constitutes dark matter at this time.

Virtual gravitons are not real gravitons. We can be fairly certain that real gravitons exist since gravitational waves have been detected. Waves usually indicate particles. In this case massless spin 2 bosons. The virtual particles really stand in for the potential at positions in the gravitational field. They can be thought of as a device that accounts for the force of the field itself.

[Bill S]

Virtual gravitons are not real gravitons.

I understand this applies to virtual particles in general.  Matt Strassler says:

"The best way to approach this concept, I believe, is to forget you ever saw the word “particle” in the term. A virtual particle is not a particle at all. It refers precisely to a disturbance in a field that is not a particle. A particle is a nice, regular ripple in a field, one that can travel smoothly and effortlessly through space, like a clear tone of a bell moving through the air.  A “virtual particle”, generally, is a disturbance in a field that will never be found on its own, but instead is something that is caused by the presence of other particles, often of other fields.”

[evan_au]

If gravity is assumed to be created by the graviton

We assume that there is a graviton; all other known forces have a quantised force carrier particle, so we assume that gravity has one too: the graviton. And a spin 2 boson would have the right properties.
 
I would express is as: gravity is created by the presence of mass.

A disturbance in the gravitational field is carried by gravitons. Such a disturbance would be caused by acceleration of large masses, like two black holes merging.

If gravity is assumed to be created by the graviton, can it be viewed as an inflow of space towards the mass.

I prefer to say that:
- a graviton travels through space
- Mass distorts spacetime

Could dark matter be multidimensional, ie not visible in normal 4 or 5 d space time.

We see the effects of Dark Matter spread through galaxies at different times, so it definitely can travel through our familiar 4 dimensional spacetime.

But the leading theories (eg "WIMP" particles like Axions or Sterile Neutrinos) suggest that these particles, while they exist in our spacetime, only interact very weakly with the familiar electromagnetic and nuclear forces.

Does dark matter and the graviton exist?

Mainstream physicists expect they do, and are running experiments that could detect Dark Matter and get a better handle on the graviton (we already have lots of experiments that can demonstrate gravitation).

[opportunity]

Andrew asks:

I have managed to confuse myself and would like to ask some question about the graviton and space. I have been reading a bit about string theory, and understand that bosons are connected to an assumed multidimensional membrane of space, DBranes. These Bosons pass through space whilst still being connected to the membrane. The graviton is normally assumed to be a spin 2 boson / virtual particle.

Under General relativity gravity is caused by mass/energy which stretches the space around it, the force is unidirectional and directed inwards. Also nothing except hawking radiation gets out of a black hole.

If gravity is assumed to be created by the graviton, can it be viewed as an inflow of space towards the mass. Ie does the graviton represent space, and being a virtual particle should it be assumed to be destroyed by mass thereby creating a vacuum for more gravitons / space to be pulled into. In the absence of mass do gravitons/virtual particles cause the expansion of space ie dark energy.

Could dark matter be multidimensional, ie not visible in normal 4 or 5 d space time. Does dark matter and the graviton exist?

Can you help?

It reads like you're separating the idea of a graviton and mass? Maybe you're suggesting something like the Higgs Boson involves a field-process to be effective?

"Dark matter" is a cosmological issue, which technically hasn't been married up with the atomic scale......yet.

[jeffreyH]

The idea of space and time curving when combined into spacetime is quite valid when considering time. An inertial observer can calculate the time dilation in other frames travelling relative to themselves. A length that they measure in their own frame would be measured differently by all frames travelling relative to the inertial observer. The length cannot be changing for all the different observes since a photon travelling along the same this distance has to always have a constant speed in vacuum. So that, while time dilation is very real, length contraction is a visual consequence of the slowing of perception with increasing velocity, when observed from a separate inertial frame.

[Bill S]

The idea of space and time curving when combined into spacetime is quite valid when considering time

I have no problem with the idea that spacetime curvature is a valuable aid to calculation, and even to understanding how gravity works, but I continue to have reservations when it comes to trying to construct a physical interpretation. 

Does spacetime curve?  If so; relative to what?  Can a part of spacetime curve relative to the rest of spacetime?

[jeffreyH]

What does curvature mean? Does it relate to acceleration? Read on.
https://en.m.wikipedia.org/wiki/Curvature

[guest45734]

If gravity is assumed to be created by the graviton, can it be viewed as an inflow of space towards the mass. Ie does the graviton represent space, and being a virtual particle should it be assumed to be destroyed by mass thereby creating a vacuum for more gravitons / space to be pulled into. In the absence of mass do gravitons/virtual particles cause the expansion of space ie dark energy.

Could dark matter be multidimensional, ie not visible in normal 4 or 5 d space time. Does dark matter and the graviton exist?

Virtual particles of all types exist throughout all of space. The virtual graviton is a virtual particle and for gravity to be a force with infinite range this virtual graviton must exist also throughout all of space. Space is expanding between galaxies, all of this new space must also be full of virtual particles/gravitons possibly known as dark energy. Space only exists if virtual particles/gravitons pop into and out of existence continually. Without virtual particles there is no space. Mass appears to absorb virtual particles/gravitons/space causing space time curvature. As for other dimensions, the jury is out, but string theory would suggest their are more spacial dimensions than meet the eye. As for Dark matter if it exists it is exotic and could  exist in another spacial dimension or not at all if MOND is correct. There are many theories that suggest DM exists and others that suggest it doesnt, again the jury is out. When the jury is out, you can believe what the hell you like until there is conclusive evidence, fanatics will claim otherwise. If DM does not exist then Einsteins ideas need some modification or perhaps MOND or some other theory will prevail such as a Holographic universe etc etc. Einsteins theories have held up pretty well but are being seriously questioned in the absence of dark matter ie 80% of the matter in the universe might not exist if einstein is wrong etc

[Bill S]

If DM does not exist then Einsteins ideas need some modification or perhaps MOND or some other theory will prevail such as a Holographic universe etc etc.

https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics

MOND and its generalisations do not adequately account for observed properties of galaxy clusters, and no satisfactory cosmological model has been constructed from the theory.

Does DM account for the observed properties of galaxy clusters?

[jeffreyH]

We will only know when somebody finds some dark matter.

[Bill S]

I should have said: Does the theory of DM offer an account of the observed properties of galaxy clusters?

An interesting quote from Tom A.

Thomas Aquinas, commenting on Aristotle's objection, wrote "Instants are not parts of time, for time is not made up of instants any more than a magnitude is made of points, as we have already proved. Hence it does not follow that a thing is not in motion in a given time, just because it is not in motion in any instant of that time.

[guest45734]

We will only know when somebody finds some dark matter.

I agree we will only know IF somebody finds some dark matter. The curvature of space time may be caused by the graviton or quantum foam. The graviton has not been detected, we will only know for sure IF someone finds some gravitons. 

[guest45734]

Under General relativity gravity is caused by mass/energy which stretches the space around it,

Gravity is more correctly described by the curvature of space.