[Mike_Cobley (OP)]

Hey all

Quick question for you. I hear all the time, 'the elusive gravity particle' . But why do we even need one? Doesn't Einstein's General theory describe gravity as mass warping space/time. So, i thought of gravity more like rivers within the sea. You get pulled 'along' but there is no particle per-se that does that pulling.

Or am i completely off base here?

Rgds

[evan_au]

why do we even need (a graviton)?

You can understand an electric current, and wire together batteries and electric motors without knowing anything about an "electric particle". In fact, Michael Faraday did just that.

But when you get down to steering electrons traveling through a vacuum (needed to build an old-fashioned TV tube) or manipulating small numbers of electrons and their precise energy levels (needed to build a transistor), you need to understand the electric particle (electron) in far more detail.

You can build things as big as a Solar System, or as small as a neutron star (which has pretty much the same mass) using Einstein's General theory of relativity. You can detect gravitational waves using the same theory.

But when you get down to the gravitational field close to the event horizon of a black hole, Einstein's General Relativity comes up with infinities that physicists have not been able to get around. So physicists (including Stephen Hawking) want to know more about what happens in this environment.

Since quantum theory has managed to resolve some other contradictions in electromagnetism, it is thought that a quantum theory might be able to resolve some contradictions in gravity.

"Graviton" is the name given to this hypothetical particle. There is even a quantum description of this hypothetical particle that would have the right properties: a spin-2 boson.

Most physicists think that a Graviton probably exists, but its energy is so low that current technologies cannot detect individual gravitons. Assuming the graviton exists, we can generate them easily enough by accelerating small masses, but they fly off in all directions at the speed of light. Today, we have nothing equivalent to a "gravitational insulator", so we are unable to bend the graviton to our will.

See: https://en.wikipedia.org/wiki/Graviton

[jeffreyH]

Photons can be reflected. What about gravitons? Can they be deflected by interactions with other particles? This is moot to a large extent since a gravitational field needs no acceleration to exist and have an effect on objects. We can think in terms of virtual gravitons for purposes of theorising but it is much more abstract than that. The field may have quanta but that will not answer the important questions.

[Mike_Cobley (OP)]

So, if I get this right. Because of the contradictions with gravity, in some situations we need to describe it as a particle to overcome these? Apologies if i miss understood.

Rgds

[evan_au]

Because of the contradictions with gravity, in some situations we need to describe it as a particle to overcome these?

Special Relativity does have contradictions near a black hole.

Theoretical Physicists working on String Theory have constructed a model of a graviton which exactly matches Special Relativity - including the contradictions near a black hole.

I am sure those researchers are now working on a variant of their theory which agrees with Special Relativity wherever we can measure it, but doesn't have contradictions near a black hole.

Unfortunately, finding out what really happens near a black hole may require us to go to a black hole - a trip I, for one would not be volunteering for!
See: https://en.wikipedia.org/wiki/String_theory#Applications_to_quantum_gravity

[guest45734]

If nothing gets out of BH, how does a graviton virtual or otherwise get out. Also if gravitons are radiated by matter away from mass why dont they push other masses away.
Is it better to view vurtual gravitons as appearing in space and being absobed by mass creating an inflow rather than outflow.

[evan_au]

If nothing gets out of BH, how does a graviton virtual or otherwise get out.

Gravitons represent energy propagating away through space at the speed of light.

However, left to itself, the gravitational field around a black hole is a static thing which radiates no energy, so there are no real gravitons.

When an object (like a neutron star) orbits a black hole, gravitational energy is radiated away through space. But you could imagine this as originating from the space between the two objects, rather than being emitted from within the black hole's event horizon.

if gravitons are radiated by matter away from mass why don't they push other masses away

The effects of a single graviton are much like the effects of a gravitational wave (only much, much tinier). The gravitational waves we have detected are effectively just very many in-phase gravitons, arriving in massive bunches.

Gravitational waves don't push matter away, since they are not actually absorbed by matter, and don't deliver any linear momentum.

Gravitational waves act to twist space in one direction, and then twist it back to the original position. They slightly accelerate time, and then decelerate it, back to the original rate. So overall, everything is left back where (and when) it was before the gravitational wave arrived.

See: https://en.wikipedia.org/wiki/Gravitational_wave#Introduction

[evan_au]

Photons can be reflected. What about gravitons?

For something to be a generally useful technology, we need methods to:
1. Manufacture/generate it
2. Manipulate/modify it
3. Monitor/detect/use it

Unfortunately:
1. It takes at least a pair of neutron stars to manufacture a significant number of gravitons. Currently out of our league.
2. Gravitons affect space and time, rather than matter, so they are currently impossible to manipulate
3. Our smallest detectors are about 4 miles long, and even then have extreme problems detecting trillions of gravitons arriving at once, in phase.

So graviton technology is not yet ready for general use by members of the public.

I expect that black holes will deflect gravitons, just like they deflect photons. So maybe one day we may be able to do graviton spectroscopy by diffracting them with an array of micro-black holes?

[jeffreyH]

@evan_au I thought I'd throw that one in for good measure. 

[yor_on]

I think you're right :)

[geordief]

Very interesting to learn that gravitational waves may be made of gravitons in phase (so like photons in that respect?

Can gravitons be understood as an excitation in a particular wave ? (maybe said gravitational wave?)

[guest45734]

However, left to itself, the gravitational field around a black hole is a static thing which radiates no energy, so there are no real gravitons.

Just to confirm, the graviton spin 2 boson, theorized to exist, https://phys.org/news/2014-03-elusive-graviton.html DOES NOT EXIST.

Are you stating the Graviton is a virtual particle which has a very short life time existing momentarily in space, like a quantum fluctuation of quantum foam.

Can you confirm is it the earth/mass/energy that sucks or is it the graviton/virtual particle that sucks

[guest45734]

3. Our smallest detectors are about 4 miles long, and even then have extreme problems detecting trillions of gravitons arriving at once, in phase.

Do you view the gravitional waves detected by ligo as an reduced density of gravitons followed by an an increased density of gravitons creating a contraction and expansion of space , OR do you view it as a stretch of space time to view the graviton as being the cause of the apparent curvature of space.

Could the Graviton and Dark Energy be the same thing?

[Janus]

Very interesting to learn that gravitational waves may be made of gravitons in phase (so like photons in that respect?

Can gravitons be understood as an excitation in a particular wave ? (maybe said gravitational wave?)

Gravitons in a quantum gravity theory would fulfill the same role as photons do in Quantum Electrodynamics.
So in the same way as electromagnetic radiation (light, radio waves, x-rays etc) are made up of photons of particular discreet energies depending on the frequency of the electromagnetic wave,   Gravitational waves would be made up of gravitons of discreet energies.
And in the same way as the electromagnetic field is mediated by virtual photons in Quantum Electrodynamics, virtual gravitons would mediate the gravitational field.

[geordief]

Very interesting to learn that gravitational waves may be made of gravitons in phase (so like photons in that respect?

Can gravitons be understood as an excitation in a particular wave ? (maybe said gravitational wave?)

Gravitons in a quantum gravity theory would fulfill the same role as photons do in Quantum Electrodynamics.
So in the same way as electromagnetic radiation (light, radio waves, x-rays etc) are made up of photons of particular discreet energies depending on the frequency of the electromagnetic wave,   Gravitational waves would be made up of gravitons of discreet energies.
And in the same way as the electromagnetic field is mediated by virtual photons in Quantum Electrodynamics, virtual gravitons would mediate the gravitational field.

So there could possibly be more  or less  energetic  varieties of gravitons   ?

And anti gravitons  might be included in the particle soup?

[Janus]

3. Our smallest detectors are about 4 miles long, and even then have extreme problems detecting trillions of gravitons arriving at once, in phase.

Do you view the gravitional waves detected by ligo as an reduced density of gravitons followed by an an increased density of gravitons creating a contraction and expansion of space ,

No.  That would be the equivalent of saying that an electromagnetic wave is caused by a varying density of photons.( such a varying density would actually be the result of an additional modulation of the electromagnetic waves, such as we use in AM radio, where we modulate the strength of the carrier wave to encode information onto it.)

OR do you view it as a stretch of space time to view the graviton as being the cause of the apparent curvature of space.

the frequency/wavelength would be due to the energy of the discreet gravtions, while the magnitude would be due to number of gravitons per sec.( The number of gravitons arriving per sec can be constant and you would still measure a sequential compression and stretching of spacetime with a frequency equal to that of the individual gravitons)

Could the Graviton and Dark Energy be the same thing?

No.

[Janus]

Very interesting to learn that gravitational waves may be made of gravitons in phase (so like photons in that respect?

Can gravitons be understood as an excitation in a particular wave ? (maybe said gravitational wave?)

Gravitons in a quantum gravity theory would fulfill the same role as photons do in Quantum Electrodynamics.
So in the same way as electromagnetic radiation (light, radio waves, x-rays etc) are made up of photons of particular discreet energies depending on the frequency of the electromagnetic wave,   Gravitational waves would be made up of gravitons of discreet energies.
And in the same way as the electromagnetic field is mediated by virtual photons in Quantum Electrodynamics, virtual gravitons would mediate the gravitational field.

So there could possibly be more  or less  energetic  varieties of gravitons   ?

in a quantum gravity theory, this would be a requirement.   Look at the LIGO detection of gravitational waves, and you will see a correlation between wave strength and frequency.   This is because as the BHs orbited closer and closer to each other, the frequency of the Gravitation waves (which was tied to the Orbital period of the BHs) became higher and higher along with the strain measured by LIGO.  Higher frequency Gravitational waves are more energetic in the same way that higher frequency electromagnetic waves are. In a quantum gravity theory, this would mean that higher frequency gravitons are more energetic than lower frequency ones

And anti gravitons  might be included in the particle soup?

No, because, like photons, gravitons would be their own anti-particle.

[jeffreyH]

If antimatter interacts with gravity in the same way as matter does, and it's my guess that it does, then gravity was instrumental in bringing both types together to annihilate in the early stages of the big bang. That there is an asymmetry between matter and antimatter is a mystery. If there were less of an effect of gravity on antimatter by even a tiny amount, then that would be interesting. Very unlikely though.

[evan_au]

Are you stating the Graviton is a virtual particle which has a very short life time existing momentarily in space, like a quantum fluctuation of quantum foam.

All virtual particles (including virtual photons and virtual gravitons) have a short lifetime and limited range, limited by the Heisenberg Uncertainty Principle.

However, real gravitons propagate away to infinity, as oscillations on the gravitational field. This is what LIGO detected. (And real photons propagate away to infinity, as oscillations on the electromagnetic field. This is what telescopes detect.)
https://en.wikipedia.org/wiki/Virtual_particle

Look at the LIGO detection of gravitational waves, and you will see a correlation between wave strength and frequency

But correlation does not prove causation.

In this case, both the amplitude increase and frequency are also correlated with a third variable: distance.
- As the black holes spiral closer together, their distance decreases
- When they are closer, their orbital period decreases (ie the frequency increases)
- When they are closer, much more power is emitted as gravitational waves

But the trend is certainly valid - gravitational waves of higher frequency (ie gravitons of shorter wavelength) will carry more momentum. The same relationship is true of photons.

[jeffreyH]

At the moment I am investigating wavelength as it relates to the time dilation of the source of the radiation. So a wave strength and frequency correlation may be explainable.