Empirical evidence for gravitational field of EM radiation

[ANS]

Hello,

Is there any empirical evidence that electromagnetic radiation
produces gravitational fields? I understand that as a form of energy
it is expected to induce a spacetime curvature, but has this
prediction of GR actually been tested? Is it at all subject to any
controversy?
Thanks,

Armin

 

[mathkills@ureach.com]

On Aug 7, 9:41 am, ANS <ar...@umich.edu> wrote:

> Is there any empirical evidence that electromagnetic radiation
> produces gravitational fields? I understand that as a form of energy
> it is expected to induce a spacetime curvature, but has this
> prediction of GR actually been tested? Is it at all subject to any
> controversy?

I'm not sure if there is a "classically" accepted proof of this
concept, but I do remember in a familiar GR textbook (I think Carrol,
or maybe Weinberg) they motivate this by the mass differences from a
proton and an electron compared to the hydrogen atom. The positive
proton and negative electron, when closer, have a lower
electromagnetic energy, resulting in the atom having a lower mass than
the sum of its consituents (sp?).

Like I said, I'm sure there is a better answer, but atleast this
should help.

-s

 

[DonJStevens]

Hi ANS,

The clear evidence that photons produce space curvature (a ripple in space-time) comes from the observation that photons appear to be gravitationally attracted to the sun when passing near its rim. (Ref. Eddington observations and many other later observations)

Recall that the sun cannot exert a force to alter the photon path without a matching force from the photon to move the sun. The photon must gravitationally attract the sun if the sun is to gravitationally attract the photon.

There is no controversy in physics today regarding the equivalence of mass (with its gravitational field) and energy. Energy added to any object will increase the mass of the object and will increase its gravitational field force. A spring driven watch or clock will be (a tiny bit) more massive (have more weight) when fully wound than when it is run down. The mass increase is equal to the added energy divided by (c^2).

A perfectly reflecting (mirror) box would have more weight (more gravitational field) when it contains many photons (electromagnetic energy) than it would have if the photons were allowed to escape. The mass increase is equal to electromagnrtic energy divided by (c^2).

 

[isly ilwott]

Is it possible that the minute attractions are caused by magnetism rather than gravity? After all, we do call it "electro-magnetic field".

 

[Chalky]

On Aug 8, 8:16 am, mathki...@ureach.com wrote:
> On Aug 7, 9:41 am, ANS <ar...@umich.edu> wrote:
>
> > Is there any empirical evidence that electromagnetic radiation
> > produces gravitational fields?

I don't think so.

> > I understand that as a form of energy
> > it is expected to induce a spacetime curvature, but has this
> > prediction of GR actually been tested?

Not AFAICT. Last time I checked the value of Newon's constant (G),
there is far too much uncertainty in that to permit such testing in
the observational domain.

> > Is it at all subject to any
> > controversy?
>
> I'm not sure if there is a "classically" accepted proof of this
> concept, but I do remember in a familiar GR textbook (I think Carrol,
> or maybe Weinberg) they motivate this by the mass differences from a
> proton and an electron compared to the hydrogen atom. The positive
> proton and negative electron, when closer, have a lower
> electromagnetic energy, resulting in the atom having a lower mass than
> the sum of its consituents (sp?).

That doesn't prove that EM fields gravitate, it only proves that they
result in a change in massenergy.
At face value, this argument seems to suggest that EM interactions
reduce as opposed to increase gravitational fields, but I would be
happy to get corrected on this.

> Like I said, I'm sure there is a better answer, but atleast this
> should help.

I would appreciate a better answer too.

 

[DonJStevens]

Hi Chalky,

Some quotes from the book "Geons, Black Holes, And Quantum Foam" by John Wheeler may be helpful. The first is from page 234.

"One of the pillars of physics is Newton's third law. A common way to state it is that for every action there is an equal and opposite reaction. Another way to say it is that forces come in balanced pairs."

The next quote is from page 235.

"Newton's third law played a role in my first paper on general relativity. One of the three confirmations of Einstein's theory had been the deflection of light by the Sun. While teaching the relativity course in the spring of 1953, I began to think about the fact that if light is influenced by gravity, then gravity must be influenced by light. To put it differently, light not only responds to gravity; it creates gravity. This was not itself a new idea. Einstein had showed that all energy, not just the energy locked up in mass, is a source of gravity. Since light has energy, it can be the source of gravitational force."

This is the best answer that I have seen in print.

 

[johndevers@iprimus.com.au]

I noticed this paper.
http://dx.doi.org/10.1016/S0375-9601(00)00260-7 Weak gravitational field of the electromagnetic radiation in a ring
laser

Ronald L. Mallett

Department of Physics, 2152 Hillside Road and University of
Connecticut, Storrs, CT 06269, USA

Received 19 January 2000; accepted 3 April 2000 Communicated by P.R.
Holland Available online 19 May 2000.

Abstract
The gravitational field due to the circulating flow of electromagnetic
radiation of a unidirectional ring laser is found by solving the
linearized Einstein field equations at any interior point of the laser
ring. The general relativistic spin equations are then used to study
the behavior of a massive spinning neutral particle at the center of
the ring laser. It is found that the particle exhibits the phenomenon
known as inertial frame-dragging.

 

[Chalky]

On Aug 21, 3:26 am, johndev...@iprimus.com.au wrote:
> I noticed this paper.
>
> http://dx.doi.org/10.1016/S0375-9601(00)00260-7
>
> Weak gravitational field of the electromagnetic radiation in a ring
> laser
>
> Ronald L. Mallett
>
> Department of Physics, 2152 Hillside Road and University of
> Connecticut, Storrs, CT 06269, USA
>
> Received 19 January 2000; accepted 3 April 2000 Communicated by P.R.
> Holland Available online 19 May 2000.
>
> Abstract
> The gravitational field due to the circulating flow of electromagnetic
> radiation of a unidirectional ring laser is found by solving the
> linearized Einstein field equations at any interior point of the laser
> ring. The general relativistic spin equations are then used to study
> the behavior of a massive spinning neutral particle at the center of
> the ring laser. It is found that the particle exhibits the phenomenon
> known as inertial frame-dragging.

Does inertial frame dragging have anything to do with whether or not
EM radiation generates gravitational fields?
(Beyond the obvious fact that this is a prediction of GR theory,
whereas the gravitation of EMR is an implicit assumption of the
stressenergy tensor in EFE) ?

 

[carlip-nospam@physics.ucdavis.edu]

ANS <armin@umich.edu> wrote:
> Hello,

> Is there any empirical evidence that electromagnetic radiation
> produces gravitational fields? I understand that as a form of energy
> it is expected to induce a spacetime curvature, but has this
> prediction of GR actually been tested?

There has been no direct test for radiation. There have been direct
laboratory tests showing that electrostatic energy produces a gravitational
field, with the expected gravitational mass of E/c^2 (with a precision of a
few percent). A slightly less direct test, using the motion and chemical
composition of the Moon, gives a value of E/c^2 to a precision of about
one part in 100,000,000.

It's very hard to experimentally test the same effect for electromagnetic
radiation, because gravity is such a weak force; it would take a truly
enormous amount of radiation to give a detectable signal. But it would be
very peculiar, to say the least, to find that some forms of electromagnetic
energy gravitate while others don't.

> Is it at all subject to any controversy?

No.

Steve Carlip

 

[Sue...]

On Aug 7, 3:41 pm, ANS <ar...@umich.edu> wrote:
> Hello,
>
> Is there any empirical evidence that electromagnetic radiation
> produces gravitational fields?

There sure is. Just boil a pot of water and you will
see water molecules move away from the earth's
surface as the London couplings become
less coherent.

http://en.wikipedia.org/wiki/Optical_tweezers

Emergent gravity
http://relativity.livingreviews.org/Articles/lrr-2005-12/articlesu25.html#x34-720006.3

Sakharov's induced gravity: a modern perspective
http://arxiv.org/abs/gr-qc/0204062

The Origin of Gravity
Authors: C. P. Kouropoulos
http://arxiv.org/abs/physics/0107015
http://arxiv.org/abs/physics/0107015v1

http://en.wikipedia.org/wiki/Fritz_London

> I understand that as a form of energy
> it is expected to induce a spacetime curvature, but has this
> prediction of GR actually been tested? Is it at all subject to any
> controversy?

<< Sakharov observed that many condensed matter
systems give rise to emergent phenomena which are
identical to general relativity quantitatively. >>
http://en.wikipedia.org/wiki/Induced_gravity

Sue...

> Thanks,
>
> Armin

 

[J. J. Lodder]

ANS <armin@umich.edu> wrote:

> Is there any empirical evidence that electromagnetic radiation
> produces gravitational fields? I understand that as a form of energy
> it is expected to induce a spacetime curvature, but has this
> prediction of GR actually been tested?

It is known that a light beam is deflected by the sun,
so we would be very surpised indeed
(action = -reaction, or momentum conservation)
if there were no corresponding force on the sun,
while it is doing the deflecting.

The gravitational field of the light beam
is the only thing imaginable that can do that.
(and of course the GTR predicts just that)

> Is it at all subject to any controversy?

No, why should there be?

Jan

 

[Murat Ozer]

On Aug 26, 9:43 am, carlip-nos...@physics.ucdavis.edu wrote:
> ANS <ar...@umich.edu> wrote:
> > Hello,
> > Is there any empirical evidence that electromagnetic radiation
> > produces gravitational fields? I understand that as a form of energy
> > it is expected to induce a spacetime curvature, but has this
> > prediction of GR actually been tested?
>
> There has been no direct test for radiation.  There have been direct
> laboratory tests showing that electrostatic energy produces a gravitational
> field, with the expected gravitational mass of E/c^2 (with a precision of a
> few percent).  There is a smell of semantics in this statement. For simplicity, let
us consider a single free atom. The sequence of events are as follows:
(1) The electrostatic energy inside the atom undergoes metamorphosis
according to E=mc^2. (2) Accordingly, the mass of the atom increases
by m=E/c^2 and the total mass of the atom becomes M=M*+m, where M* is
the mass of the atom before this metamorphosis. (3) What interacts
with the external gravitational fields is the gravitational field
produced by the mass M of the atom. Therefore, it is incorrect to say
that the electrostatic energy produces a gravitational field.
Actually, the electrostatic energy produces a tiny contribution to the
total mass of the atom; and it is this mass that produces the
gravitational field. That this is so can be seen as follows: A
hypothetical electric field (or equivalently electric energy)
detector capable of detecting the smallest field values would detect
no electric field outside the atom. But a similar gravitational field
detector would detect a gravitational field outside the atom. It is
this field that interacts with the other external gravitational
fields. Most relativists cite the Eotvos experiment as a proof that
the electrostatic energy inside the atom interacts with the external
gravitational fields. This is nothing but semantics. They say
"electrostatic energy" when they are supposed to say the " mass due to
the electrostatic energy". The Eotvos experiment proves that the
electrostatic energy, along with the other atomic energy components,
contributes in precisely equal amounts to the gravitational mass and
inertial mass of the atom. In the above, the electrostatic energy
considered was not free, namely it was metamorphosed. On the other
hand, the electrostatic energy outside a charged sphere of total net
charge Q exists freely. AFAIK there is no experiment performed to
prove whether or not this kind of free electrostatic energy interacts
with the gravitational fields outside the charged sphere.

Murat Ozer

>A slightly less direct test, using the motion and chemical
> composition of the Moon, gives a value of E/c^2 to a precision of about
> one part in 100,000,000.
>
> It's very hard to experimentally test the same effect for electromagnetic
> radiation, because gravity is such a weak force; it would take a truly
> enormous amount of radiation to give a detectable signal.  But it would be
> very peculiar, to say the least, to find that some forms of electromagnetic
> energy gravitate while others don't.
>
> > Is it at all subject to any controversy?
>
> No.
>
> Steve Carlip

 

[ANS]

Thanks everyone for their replies. I do have some comments below.

John,
Thank you very much for the link to the paper. I am a little
disappointed that it apparently refers only to theoretical predictions
as opposed to actual experiments carried out, although it states that
the technology has sufficiently advanced to test them. I wonder
whether this has been done already?

Steve Carlip,
Yes, I did not mean to ask whether it was controversial that some EM
radiation induces spacetime curvature and not others, but whether it
was controversial that any EM radiation induces spacetime curvature at
all (see below for my view).

Sue,
Thank you for the links. I am most interested in Einstein’s theory of
gravity, however, because it is the relativistic theory of gravity
with the greatest empirical support.

Jan,
Thank you for your comments. I think you put your finger directly on
one of the main issues to which I will respond in a future post. For
now, I just would like to explain my main motivation for asking the
question.
We know that the classical picture of EM radiation is an approximation
to the quantum mechanical picture in the limit of a large number of
photons. It appears to me that if EM radiation does indeed produce
spacetime curvature, then one could, in principle, determine the
trajectories of photons (or groups of them) in space (To do so,
observe the direction in which some appropriately placed small test
masses accelerate, under the assumption that the photon inducing the
curvature is much much more energetic than those used to observe the
test masses) . Yet this would be clearly in conflict with quantum
theory.
This leads me to question whether EM radiation really produces
gravitational curvature. This is the reason why I asked whether this
subject was at all controversial.
Any comments?

Armin
I

On Aug 30, 7:44 pm, nos...@de-ster.demon.nl (J. J. Lodder) wrote:
> ANS <ar...@umich.edu> wrote:
> > Is there any empirical evidence that electromagneticradiation
> > producesgravitationalfields? I understand that as a form of energy
> > it is expected to induce a spacetime curvature, but has this
> > prediction of GR actually been tested?
>
> It is known that a light beam is deflected by the sun,
> so we would be very surpised indeed
> (action = -reaction, or momentum conservation)
> if there were no corresponding force on the sun,
> while it is doing the deflecting.
>
> Thegravitationalfieldof the light beam
> is the only thing imaginable that can do that.
> (and of course the GTR predicts just that)
>
> > Is it at all subject to any controversy?
>
> No, why should there be?
>
> Jan

 

[yoron]

To late by far (as usual), but still. Murat have it beautifully straight. That's the post answering this question, imagining gravity to act as a radiation is not working. Imaging particles to add 'energy' to themselves though, even though we have no isolated measurement of 'pure isolated energy', will work and will add a very slight invariant mass. Check out the examples with compressed springs.

How that 'mass' comes to be is another question and, if you like, may be seen as a proof for our magical 'energy' existing. But 'energy', as far as I know, is only defined through 'interactions'. Potential energy may be a very useful concept in Physics. But if you look at a spaceship :) Uniformly moving near light speed, relative earth, its 'atoms' won't radiate any more. It will not 'glow', telling you that even though you can define a greater 'relative mass', or 'momentum', or 'potential energy' to it, it's above all a way of defining what we expect to happen in that later 'interaction'. To me only compression seems to be able to prove the concept?