# Gravity and speed of light

what is the gravity experienced when an object travells at the speed of light ?
(is it zero gravity)

Drakkith
Staff Emeritus
Massless objects, such as light, experience gravity just the same as we do.

Chronos
Gold Member
Just to elaborate, massless particles do not slow under the influence of gravity. They redshift. The effect is negligible except for particles emitted by a strong gravitational field - like a neutron star.

Massless objects, such as light, experience gravity just the same as we do.

Forgive my ignorance, but am I correct in assuming that, because mass and energy are interchangeable, that the photon also exerts a fantastically minute gravitational force on the massive object? If so, I have trouble visualizing how this force plays out since the photon is traveling at light speed along with gravity. If this assumption is incorrect, then how is it that the photon can experience gravity without producing it?

Drakkith
Staff Emeritus
Forgive my ignorance, but am I correct in assuming that, because mass and energy are interchangeable, that the photon also exerts a fantastically minute gravitational force on the massive object? If so, I have trouble visualizing how this force plays out since the photon is traveling at light speed along with gravity. If this assumption is incorrect, then how is it that the photon can experience gravity without producing it?

I know it produces gravity, but I can't help you visualize how the force plays out. I believe that it ends up always pointing towards the photon, but I'm not sure.

Matterwave
Gold Member
The gravity of a single beam of light is a pretty hairy problem. It's a particular null-electro-vacuum solution of the Einstein-Maxwell Field equations (meaning, one must solve simultaneously the EFE's and the Source-free Maxwell equations for the case in which the stress-energy associated with the Faraday tensor is null, i.e. ##T^{ab}=\Phi k^a k^b## for null vectors ##k^a##) . The most pertinent solution that I could find is here: http://en.wikipedia.org/wiki/Monochromatic_electromagnetic_plane_wave

I don't know if I can provide you with any intuition into this solution though...=/

Here's an odd thought that just hit me...

Since gravity and light both travel at the same speed, wouldn't photons create something like a shock front of gravity?

$v = v_{s} = c$

Drakkith
Staff Emeritus
No. What you are depicting are gravitational waves. They are created by accelerating objects with mass. They wouldn't be created in this circumstance.

No. What you are depicting are gravitational waves. They are created by accelerating objects with mass. They wouldn't be created in this circumstance.

But how could photons create gravity if not in a manner that propagates outwards at the speed of light in all directions?

Light is effected by gravity in the same why we are. As proof of this, take Black Holes for example. The gravity of the black hole is so strong it can bend light and pull it in, as we all know. So clearly, light is effected by gravity.

Sir Arthur Eddington
Light is effected by gravity in the same why we are. As proof of this, take Black Holes for example. ...
Yes, but black holes are notoriously hard to work with. In fact, I've never even seen one.

A better proof was Sir Arthur Eddington's observation in the solar eclipse of May 29, 1919.

Light is effected by gravity in the same why we are. As proof of this, take Black Holes for example. The gravity of the black hole is so strong it can bend light and pull it in, as we all know. So clearly, light is effected by gravity.

Are you saying that gravity actually moves faster than the speed of light? And this may be why physicists have not been able to find the graviton particle?

what is the gravity experienced when an object travells at the speed of light ?
(is it zero gravity)
Gravity is caused due to a wave emitted by substance of atomic mass(object) and the wave is nearly of the same speed of light. Experience of gravity when the object is at a directional motion is as explained by Doppler Effect and yes it is zero gravity but only from the back of the moving object.

Drakkith
Staff Emeritus
Are you saying that gravity actually moves faster than the speed of light? And this may be why physicists have not been able to find the graviton particle?

No. A static gravitational field does not move. It is only changes in the field that move, and these changes propagate outwards at the speed of light as gravitational waves. This is not unique to gravity. Electric and magnetic fields behave the same way. The field merely describes the direction and magnitude of the force at any point in space. The 'force itself' does not travel, as it is not an object or a disturbance.

Drakkith
Staff Emeritus
Gravity is caused due to a wave emitted by substance of atomic mass(object) and the wave is nearly of the same speed of light. Experience of gravity when the object is

This is not correct. Gravity is caused by the presence of mass, energy, or stress, and is not 'emitted' by anything in the way that a wave is. This doesn't prevent an object from emitting gravitational waves, but that requires that the object be accelerated. A gravitational wave moves at c, the speed of light.

A gravitational wave moves at c, the speed of light.

Please forgive my ignorance, but if a photon moves at c, and a gravitational wave moves at c, could the photon be the particle and gravity its wave?

Drakkith
Staff Emeritus
Please forgive my ignorance, but if a photon moves at c, and a gravitational wave moves at c, could the photon be the particle and gravity its wave?

Nope. Not a chance. The two are very, very different. Gravitational waves are produced by accelerating objects with mass. Very strong gravitational waves are produced from in-spiraling binary neutron stars and black holes. Only very massive objects can produce high-amplitude gravitational waves, and there will be zero correlation between the gravitational waves and any photons found with them. Two in-spiraling black holes, without accretion disks, will release large amplitude gravitational waves, but zero EM radiation (photons).

This may seem irrelevant, but if we somehow found a way in which to accelerate an object with mass to the speed of light, would it have infinite gravity? Because we all know that mass increases with speed and that mass becomes infinitely large when travelling at the speed of light...

Drakkith
Staff Emeritus
This may seem irrelevant, but if we somehow found a way in which to accelerate an object with mass to the speed of light, would it have infinite gravity? Because we all know that mass increases with speed and that mass becomes infinitely large when travelling at the speed of light...

If you ask what happens when the laws of physics are violated, then the laws of physics cannot answer your question.

If you ask what happens when the laws of physics are violated, then the laws of physics cannot answer your question.
I think he meant to ask what the limit of gravity is as speed approaches the speed of light.

PeterDonis
Mentor
2020 Award
we all know that mass increases with speed

Relativistic mass increases with speed; but gravity does not depend on relativistic mass. It depends on the stress-energy tensor. Energy (aka relativistic mass) is one component of the SET, but not the only one. The overall result is that no, gravity does not increase with speed in the way you would think if you just used relativistic mass.

PeroK
PeterDonis
Mentor
2020 Award