# Massless Gravity

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Do massless particles (such as photons) create a gravitational field? And if so, how?

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6 minutes ago, elchapetas said:

Do massless particles (such as photons) create a gravitational field? And if so, how?

Massless particles, [photons] follow geodesics in spacetime, but also ever so slightly curve/warp spacetime due to their momentum, thereby creating a tiny infinitesamal amount of gravity. Gravity is spacetime albeit curved or warped.

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8 minutes ago, elchapetas said:

Do massless particles (such as photons) create a gravitational field? And if so, how?

Yes, because both mass and energy cause gravity (and photons have energy).

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I think a easy way to explain this is Elementary particles have spin. A 3 degree freedom function.

1. charge

2. Spin

3. Orbital

momentum of a massless particle can in theory create a gravitational field

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How do you have momentum without mass?  Do all massless particles have the same energy?  Will a gamma ray photon have greater gravitational effect than visible light photon?

If gravity propagates at the speed of light, from a photon travelling at the speed of light, how does this effect the gravitational field created by the massless excitation in the electromagnetic field?

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5 hours ago, AbstractDreamer said:

How do you have momentum without mass?  Do all massless particles have the same energy?  Will a gamma ray photon have greater gravitational effect than visible light photon?

If gravity propagates at the speed of light, from a photon travelling at the speed of light, how does this effect the gravitational field created by the massless excitation in the electromagnetic field?

In classical mechanics of course this doesn't work. But luxons (Wikipedia: Massless particles) (particles that move at light speed) can't have mass, so they only have momentum.

The coupling of Energy to Gravity is extremely weak. I suspect that it is possible that, if Spactime is granular (Wikipedia) in nature, that Energies of the scale of TeV are required to create a Spacetime change, and create their own gravity well. This is way outside of my expertise, and there might be other, better explanations that I am not thinking of now. But the consequence of this being at least a little accurate and applicable would be that there is nothing to consider because there is no gravitational field being created by the photon

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If gravity is quantum, then infinitesimal might not be enough excitation to create a single quantum of gravity that is interactable or observable.

If gravity is not quantum, then infinitesimal is still a gravitational field.

According to wiki, quantum gravity operates over distances of plank length, which is pretty infinitesimal.

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8 hours ago, AbstractDreamer said:

How do you have momentum without mass?  Do all massless particles have the same energy?  Will a gamma ray photon have greater gravitational effect than visible light photon?

The energy of a photon depends on the wavelength. So gamma ray photons have more energy than visible light photons, and they will have a correspondingly greater gravitational effect.

27 minutes ago, AbstractDreamer said:

According to wiki, quantum gravity operates over distances of plank length, which is pretty infinitesimal.

That sounds wrong. Gravity will still work over infinite distance. I would guess what they meant was that a quantum theory of gravity is required at the Planck scale.

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"One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10−35meter"

maybe my interpretation was wrong.

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4 minutes ago, AbstractDreamer said:

"One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10−35meter"

maybe my interpretation was wrong.

I think so. (Note: it is generally a good idea to provide a reference to anything you quote.)

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In my imagination of the matter, if spacetime is quantized, there would have to be a minimum energy for an accumulation of energy to contract spacetime by one granule. It's hard (damned near impossible) to give a specific value or even order of magnitude for this minimum energy because the granular size of spacetime isn't necessarily a (planck time)*(planck length)^3.

The question is whether it is at all practical to think this way. There's not really a chance for Photons to influence eachother gravitationally because interference effects are much stronger - Photon-Photon gravitational effects should be smaller than any principle method of measurement according to Heisenberg uncertainty. Photon-electron electromagnetic interactions would similarly overpower photon-electron gravitational effects beyond them being recognizeable

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Well yes when the reference is hard to find.  But this is wiki.  https://en.wikipedia.org/wiki/Quantum_gravity

If gravitational effects only appear at length scales near the Planck scale, it didn't sound wrong to say quantum gravity operates over distances of Planck length.

43 minutes ago, Strange said:

Gravity will still work over infinite distance.

If gravity works over infinite distance as you say, and its strength is proportional to inverse square law, then it cannot be quantum.

if gravity is quantum, then there must be some distance where it falls below the threshold for a measurable observation, even if it is beyond the distance of the cosmic event horizon for that observer.

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1 hour ago, AbstractDreamer said:

"One of the difficulties of formulating a quantum gravity theory is that quantum gravitational effects only appear at length scales near the Planck scale, around 10−35meter"

maybe my interpretation was wrong.

Tis means that the quantum effects are noticeable on that scale. Much like quantum effects in mechanics are noticed at small scales, but classical physics is fine at large scales. The quantum effects don't stop, it's that they give the same answer as the classical prediction at larger scales.

10 hours ago, AbstractDreamer said:

How do you have momentum without mass?

p = E/c for massless particles.

It's even true classically, for electromagnetic waves.

10 hours ago, AbstractDreamer said:

Do all massless particles have the same energy?

No. It depends on their frequency

10 hours ago, AbstractDreamer said:

Will a gamma ray photon have greater gravitational effect than visible light photon?

It has more energy, so yes.

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1 hour ago, AbstractDreamer said:

If gravity works over infinite distance as you say, and its strength is proportional to inverse square law, then it cannot be quantum.

Why not? This is true of electromagnetic forces.

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On 5/25/2018 at 10:53 PM, elchapetas said:

Do massless particles (such as photons) create a gravitational field? And if so, how?

The way I would look at it is that if E=mc2 then m=E/c2 so a massless particle has a mass equivalent, but it's obviously going to be absolutely miniscule.

If you are used to visualising a huge amount of energy tied up in a small amount of mass, then this is just the reverse of that.

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37 minutes ago, Strange said:

Why not? This is true of electromagnetic forces.

Well correct me if I'm wrong.  Because if gravity is quantum, then there must be some distance where it falls below the threshold for a measurable observation, even if it is beyond the distance of the cosmic event horizon for that observer.  And at that distance where no measurable observation of the gravity field of an object can be made, it can no longer gravitationally interact with anything.  If such a limitation to gravitational range exists, how can it also work over infinite distance unless you are talking about distances in dimensions above the 3 spatial dimensions.

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4 minutes ago, AbstractDreamer said:

Well correct me if I'm wrong.  Because if gravity is quantum, then there must be some distance where it falls below the threshold for a measurable observation, even if it is beyond the distance of the cosmic event horizon for that observer.  And at that distance where no measurable observation of the gravity field of an object can be made, it can no longer gravitationally interact with anything.  If such a limitation to gravitational range exists, how can it also work over infinite distance unless you are talking about distances in dimensions above the 3 spatial dimensions.

Forces are mediated by virtual photons. This means (as I understand it) that there is no minimum value or range limit on the force.

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11 minutes ago, Strange said:

Forces are mediated by virtual photons. This means (as I understand it) that there is no minimum value or range limit on the force.

EM certainly is Photons. Strong is Gluons, and they pretty much stop at the edge of the nucleus - though that is probably due to the special relationship of quarks and gluons, not so much of the Gluons' properties (Color Confinement). If Gravity is quantum, the Graviton would have much in common with the Photon. I'm using conditional for Graviton because we have yet to observe one in a lab. W and Z Bosons are the weak interaction transmitters. https://en.m.wikipedia.org/wiki/Boson

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And EM still works at the quantum level, e.g. in atoms, where a 1/r^2 interaction is used.

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6 hours ago, AbstractDreamer said:

Well correct me if I'm wrong.  Because if gravity is quantum, then there must be some distance where it falls below the threshold for a measurable observation, even if it is beyond the distance of the cosmic event horizon for that observer.  And at that distance where no measurable observation of the gravity field of an object can be made, it can no longer gravitationally interact with anything.  If such a limitation to gravitational range exists, how can it also work over infinite distance unless you are talking about distances in dimensions above the 3 spatial dimensions.

It doesn't work that way.   For example, electromagnetic radiation is divisible into Quanta (photons).  A Photon is a quantum of EMR.  But there is no minimum energy level for photons in general, only for photons of a given frequency.    Gamma rays can only be made up of photons of a certain  minimum energy,  but radio waves can be made up of photons with a much smaller minimum energy.    The longer the wavelength of the EMR, the smaller the energy of its individual Photons.  There is no lower limit on how energetic a photon you can have.  With quantum gravity you would be dealing with gravitons (virtual gravitons to mediate the force), and the same rules apply.

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A 1 kg will attract the Earth as much as the Earth attracts the 1 kg mass.
That is how gravitational interactions work.
If a star interacts with a photon so as to constrain it along a geodesic, the photon is equally interacting with the star.
So, yes, a photon does have a gravitational field.

The reason quantizing classical gravity theories will not work is because renormalization fails.
Gravity is self coupling, that means it interacts with itself. At very close ranges a quantum gravity particle ( graviton ) would be surrounded by a 'halo' of virtual gravitons.
this also happens in QED with photons and electrons, but these fields are not self coupling, and the infinities created by the surrounding virtual particles can be cancelled away.
In quantized  gravity theory, the renormalization trick fails because the infinities don't cancel out.

Most people have given up on quantization of classical gravity and are taking new approaches such as String/Superstring theories and Loop Quantum Gravity.

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• 1 month later...
On 5/25/2018 at 5:53 PM, elchapetas said:

Do massless particles (such as photons) create a gravitational field? And if so, how?

The only mass that may fit within the smallest unit of space are photons. The curvature generated may directly interact with another piece of space of the same measure even with a dilated quantum measure of space between them. Stating that the smallest units of space may “appearance wise” exists in a higher dimension of time as isolated units/measures.

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10 minutes ago, Higg’s White Hole said:

The only mass that may fit within the smallest unit of space are photons

There is no smallest unit of space.

Photons have no mass.

Why do you think photons are smaller than any other fundamental particle?

12 minutes ago, Higg’s White Hole said:

The curvature generated may directly interact with another piece of space of the same measure even with a dilated quantum measure of space between them. Stating that the smallest units of space may “appearance wise” exists in a higher dimension of time as isolated units/measures.

Gibberish.

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43 minutes ago, Higg’s White Hole said:
On 5/26/2018 at 7:53 AM, elchapetas said:

Do massless particles (such as photons) create a gravitational field? And if so, how?

Photons/Light have zero  rest mass, but they do have momentum, and yes, as a consequence, light does by a very very tiny infinitesamal amount, warp the spacetime in which it is traversing.

Quote

Higg’s White Hole

Stating that the smallest units of space may “appearance wise” exists in a higher dimension of time as isolated units/measures.

We do not as yet have a quantized theory of spacetime/gravity

Edited by beecee

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