# Light has Gravity?

## Main Question or Discussion Point

Howdy,
Does light exert gravity? last i heard, eletro-magnetic radiation doesn't have mass, and thus no gravity, yet it is apparently effected by gravity.
How can something which has no mass be effected by gravity?
does that mean that gravity is a pattern in space itself?

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does that mean that gravity is a pattern in space itself?
That's an excellent way of thinking about it.

I'd like to leave it at that, but I'd feel hypocritical if I let you draw the correct conclusion from compromised reasoning. Light does exert it's own gravity, and whether it has mass is largely just a question of semantics (it certainly has momentum).

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How can something which has no mass be effected by gravity?
does that mean that gravity is a pattern in space itself?
Light certainly is affected by gravity.And the reason can be understood by the equivalence principle.

Imagine you are in an elevator, and suddenly someone from outside shines light into the elevator (let the elevator be transparent!).If you start accelerating upwards,you will see the light bending towards the floor.

The same happens in a gravitational field as the equivalence principle is valid locally

Light does exert it's own gravity, and whether it has mass is largely just a question of semantics (it certainly has momentum).
hrmm, interesting.
i will try to find the studies which show that light has gravity. I'll probably use a search engine- can you point me to any particular experiments? Are you telling me that a "wavicle" which travels at a constant speed (c) has momentum? Are you saying that because electrons jump shells when photons impact them? I don't personally think its the same phenomena as momentum if thats the reason.

cheers,

Oh yes the wave travelling at c(it has energy) has momentum.

And the reason isnt because of the electron jump,it is because mass and energy both curve spacetime.

Also,an interesting fact:You could create a black hole if u focussed enough light to curve spactime very very much(high energy density)

Cheers!

HallsofIvy
Homework Helper
last i heard, eletro-magnetic radiation doesn't have mass, and thus no gravity
This is an invalid step. In relativity, gravity does not necessarily depend on mass.

xantox
Gold Member
Howdy,
Does light exert gravity? last i heard, eletro-magnetic radiation doesn't have mass, and thus no gravity, yet it is apparently effected by gravity.
How can something which has no mass be effected by gravity?
does that mean that gravity is a pattern in space itself?
Light is affected by gravity : in general relativity, gravity is understood in terms of curved space-time. So that a photon is always moving in a straight line, but it bends because space-time itself is bent.

Light exerts gravity : in general relativity, gravity is coupled to energy density and momentum flow, not only mass like in newtonian gravity. An electromagnetic wave will exert its own gravity, though extremely weak and not currently measurable. Gravity exerted by massive bodies is much higher because of their huge energy content (see the c squared term in Einstein formula).

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Light certainly is affected by gravity.And the reason can be understood by the equivalence principle.Imagine you are in an elevator, and suddenly someone from outside shines light into the elevator (let the elevator be transparent!).If you start accelerating upwards,you will see the light bending towards the floor.The same happens in a gravitational field as the equivalence principle is valid locally
So the effect on light in a moving frame of reference is the same effect that gravity has on it within a stagnant reference? Thats really interesting, even though i don't understand why it is equivalent... Light appears to be bent from a moving frame of reference, but is it really bent the same as in a gravity field?
thanks,

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Oh yes the wave travelling at c(it has energy) has momentum. And the reason isnt because of the electron jump,it is because mass and energy both curve spacetime. Also,an interesting fact:You could create a black hole if u focussed enough light to curve spactime very very much(high energy density) Cheers!
Wow, that is amazing! I'm still having difficulty with the momentum concept... ie momentum= mass X velocity... c is the constant velocity, but whence cometh the mass? do you just substitute mass for the equivalence in pure energy by Einstein's conversion? Also, a black hole is a singularity... wouldn't you have to focus the light on a massive object to create a black hole? IN other words. Without something to stop it, where would the black hole form- near the source?

So it has been proven that light curves spacetime around itself?! Wow, i really have been out of the loop!
peace and grace,

This is an invalid step. In relativity, gravity does not necessarily depend on mass.
Ah, thanks for the correction. What does gravity depend on? Heres a silly question:
The other fellow says its energy density, so, hypothetically, if i were in deep space traveling by a laser beam with the focused energy of a million suns, would i be sucked into it? (me being the only substantial matter around)
cheers,

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Light is affected by gravity : in general relativity, gravity is understood in terms of curved space-time. So that a photon is always moving in a straight line, but it bends because space-time itself is bent.
Light exerts gravity : in general relativity, gravity is coupled to energy density and momentum flow, not only mass like in newtonian gravity. An electromagnetic wave will exert its own gravity, though extremely weak and not currently measurable. Gravity exerted by massive bodies is much higher because of their huge energy content (see the c squared term in Einstein formula).
If the gravity of light cannot currently be measured, then how do you know it's there? Light fills the entire universe, which means that if it does exert gravity, theres alot of it, but no focal point of attraction. Since light appears to be moving constantly, where would the focal point of gravity be?
forgive my ignorance,

xantox
Gold Member
What does gravity depend on?
On energy density and momentum flow.

Wow, that is amazing! I'm still having difficulty with the momentum concept... ie momentum= mass X velocity... c is the constant velocity, but whence cometh the mass?
From the general energy formula $$E=\sqrt{p^2c^2+(m_0c^2)^2}$$ you can set $$m_0=0$$ and find that the momentum of the photon is $$p=E/c$$. E is the photon energy, equal to its frequency times Planck's constant.

wouldn't you have to focus the light on a massive object to create a black hole? IN other words. Without something to stop it, where would the black hole form- near the source?
It should form in any volume of space happening to contain enough photons with enough energy.

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pervect
Staff Emeritus
This is an invalid step. In relativity, gravity does not necessarily depend on mass.
Yes. To amplify this point a bit, in GR, one can say that energy, momentum, and pressure (and not just mass) causes gravity. Specifically, the density of energy and momentum and also pressure are components of an entity calledthe "stress-energy tensor" that appears on the right hand side of Einstein's equation. This "stress energy tensor" can be regarded as the "source" of gravity in General relativity.

Since light has energy and momentum, it causes gravity. The idea that only "mass" causes gravity is a carryover from Newtonian theory, things are different in GR.

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On energy density and momentum flow.
From the general energy formula $$E=\sqrt{p^2c^2+(m_0c^2)^2}$$ you can set $$m_0=0$$ and find that the momentum of the photon is $$p=E/c$$. E is the photon energy, equal to its frequency times Planck's constant.It should form in any volume of space happening to contain enough photons with enough energy.
Ah, i see. the momentum of a massive object is its mass X velocity, but the momentum of a wavicle is its energy divided by its velocity. Hrmm that means the momentum is very small... so this has been measured and not just theorized by manipulating equations?
Also, has the gravity of a beam of light been measured in a lab? Can u reference me?
thanks, you have been most helpful,

gravity exerted by a light beam is extremely small.I dont know of any instance where it has been measured though!

Yes. To amplify this point a bit, in GR, one can say that energy, momentum, and pressure (and not just mass) causes gravity. Specifically, the density of energy and momentum and also pressure are components of an entity called the "stress-energy tensor" that appears on the right hand side of Einstein's equation. This "stress energy tensor" can be regarded as the "source" of gravity in General relativity.Since light has energy and momentum, it causes gravity. The idea that only "mass" causes gravity is a carryover from Newtonian theory, things are different in GR.
interesting. here are a bunch of related questions, hope you don't mind!
so light exerts pressure and momentum? If light doesn't have mass, yet exerts gravity, then mass isn't really a source of gravity... its just extreme energy density. Yet, mass particles seem distinct from photons... but maybe i'm off. What do they both have in common, such that both exert gravity? Does a photon occupy space?
hrm so theres energy density, momentum, and pressure- but the latter 2 depend on the former, which means that gravity depends on energy, right? Is electromagnetic energy the only kind which exerts gravity?
Pressure is caused by particle density and energy- does the same go for light? Could you knock me off a chair with a beam of light? Light is thought of as an oscillating magnetic/electric field. Does either one of those exert gravity or have momentum? Why should they when combined?
sry, don't mean to pummel with questions,
i'm just trying to figure things out,
cheers,

gravity exerted by a light beam is extremely small.I dont know of any instance where it has been measured though!
wait... you are telling me that nobody has ever measure the gravity of light, yet its a huge tenant of modern physics theory? How does that equate? It seems easy enough to make a big laser beam and measure its gravity...
Isn't there more light in the universe than anything else- like the cbr?! You'd think it would all add up to quite a bit of gravity...
cheers,

pervect
Staff Emeritus
Mass is the source of gravity in Newtonian theory, but not in GR.

Any sort of energy can be said to "gravitate" in GR. Massive particles are different from photons, but as they both contain energy, they both contribute to gravity, since it is energy that causes gravity (along with the other things I mentioned).

It's not really very accurate to say that pressure is caused by particle density and energy density, but I'm not sure how much detail we want to get into at this point on the topic. Not only would we have to review the basic physical defintions of pressure, but to be totally accurate we'd have to explain some of the differences between the GR defintions and the usual engineering defintions - differences that don't matter, unless one is considering moving objects.

Note that you can get a "push" from light (see for instance interstellar "light sail" propulsion). I tend to think of this as being due to the momentum of the light, but I suppose one could technically call it due to light pressure (there is a pressure term in the region where light travels in both directions).

Unless you have a very good mirror, though, you will tend to be "fried" (or at least have have a hole drilled) by an intense laser beam rather than "pushed".

On the cosmological scale (i.e the scale of the entire universe) the gravity of radiation (i.e. photons) is not important at the current time. Early in the history of the universe this was not the case (at least in theory)- General relativity predicts a "radiation dominated" era in the first 400,000 years or so after the "big bang" as per the Wikipedia stub http://en.wikipedia.org/wiki/Radiation-Dominated_Era. You might also try looking for articles about the timeline of the Big Bang, they should also mention the radition dominated era.

The biggest contributor to the radiation density in the universe is the cosmic microwave background radiation. While it is very faint, it exists everywhere. In our particular local region, things like starlight and especially sunglight are more important, but on the average, it is the CMB that is the biggest contributor.

I'm a bit unclear on what actual experimental justification (if any) we have for the existence of a radiation dominated era in cosmology, but this is something that is predicted to have happened if GR is correct.

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Gravity is a mutual attraction, so in the classical sense the measurement of light bending around a star or galaxy is a measurement of light's gravity. But classical gravity and special relativity led to a bending around the Sun that was only half of what was measured. It is better to go with the purely GR approach and describe light paths as geodesics.

xantox
Gold Member
Ah, i see. the momentum of a massive object is its mass X velocity, but the momentum of a wavicle is its energy divided by its velocity. Hrmm that means the momentum is very small... so this has been measured and not just theorized by manipulating equations?
Also, has the gravity of a beam of light been measured in a lab? Can u reference me?
thanks, you have been most helpful,
As said above,

An electromagnetic wave will exert its own gravity, though extremely weak and not currently measurable. Gravity exerted by massive bodies is much higher because of their huge energy content (see the c squared term in Einstein formula).
which means that gravity depends on energy, right?
As said above, it depends on energy density and momentum flow.

Is electromagnetic energy the only kind which exerts gravity?
No, it is any kind of energy. In GR even the energy of gravitational waves exerts gravity, and even the energy of vacuum.

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wait... you are telling me that nobody has ever measure the gravity of light, yet its a huge tenant of modern physics theory? How does that equate? It seems easy enough to make a big laser beam and measure its gravity...
Isn't there more light in the universe than anything else- like the cbr?! You'd think it would all add up to quite a bit of gravity...
cheers,
Even if you have a lottttttt of energy from light it would be pretty difficult to measure the gravitational force.This is as each photon has some energy.Now,You can by no means change the energy of a photon unless you alter its frequency.Also when you bring a laser,the energy density is still the same.Think of it like this:

If you take a bucket of water and take out a glass of water or add some water into the bucket,would the density of the water change?No .

Therefore,energy over a very small region can exert more gravity than maybe a lottttttt of energy spread around a HUGE area.I hope my views are clear

Cheers!

Gravity is a mutual attraction, [..] better to go with the purely GR approach and describe light paths as geodesics.
But geodesics are only valid in the limit where you ignore the "mutual" part. Any sort of energy can be said to "gravitate" in GR. Massive particles are different from photons, but as they both contain energy, they both contribute to gravity, since it is energy that causes gravity (along with the other things I mentioned).
does heat energy exert gravity?
another fellow said the energy of a vacuum does. that would be more like potential energy, no?

As said above...
sorry for the redundancy. just trying to nail the ideas into my skull.

As said above, it depends on energy density and momentum flow.
So energy density depends on the frequency and what else? Momentum flow... that is a propagating pattern in an infinite causation, no?
No, it is any kind of energy. In GR even the energy of gravitational waves exerts gravity, and even the energy of vacuum.
i thought gravitational waves from collapsing stars hasn't been proven yet... but yes i can kind of "see" how that fits.
now, the "energy of vacuum" is something you will have to explain in more detail for me! Vacuum is void, relative nothing. Something always tends towards it- thus entropy, diffusion, osmosis etc. How can the tendency of all things to fill non-thing- a tendency, exert gravity? That is fascinating!

pervect
Staff Emeritus