does gravity bend light or space?

Does strong gravity actually bend light, or the space through which
the light passes?
I don't see why a massless particle like a photon would be affected by
gravity.

In article , Filip Houdek
writes:

Does strong gravity actually bend light, or the space through which
the light passes?

One observes the bending. How it is actually described doesn't really
matter. Note that there are two effects of the same size, which is why
GR predicts twice the bending that Newtonian gravity does: there is the
deflection which Newton also predicts in the limit of a massless
particle and also one due to the stretching of space, which essentially
changes the index of refraction, so the light is bent for the same
reason that, say, glass bends light.

I don't see why a massless particle like a photon would be affected by
gravity.

In GR it is obvious. Newtonian physics probably isn't completely well
defined here, but one gets the correct result in the limit of a massless
particle. Gravitational mass and intertial mass are equivalent, which
is something of a puzzle in Newtonian theory but natural in GR. So,
even in Newtonian theory, deflection doesn't depend on the mass of the
deflected particle (at least if it is much less massive than the
deflector) and the limit of zero mass is well defined.

Note that the bending of light was actually predicted with Newtonian
theory, but as described above the effect is only half as large as the
real deflection (predicted by GR and also observed). It so happened
that the first observations were done after GR made the correct
prediction, but it conceivably could have happened the other way around.

Filip Houdek wrote:
Does strong gravity actually bend light, or the space through which
the light passes?

A better way to put it is probably that gravity (whether strong or
weak) is a manifestation of a "bent" (we usually say "curved" or
"not flat") spacetime.


I don't see why a massless particle like a photon would be affected by
gravity.

A photon travels on a straight line through spacetime. Because
spacetime is curved, we see effects which look like the photon's
path is bent. "Bent with respect to what?" you ask? That gets a
bit more complicated....

A useful analogy for understanding what's going on here is to consider
the paths ships take from (say) Tokyo to Los Angeles, plotted in
latitude/longitude. If you plot such a path, you'll see that it's
curved (it first heads northwards, then bends to the south again),
as if the ship were being attracted to the Earth's equator by some
"equator-attracting force".

From a larger perspective, it's easy to see what's going on: the
ship's path is roughly a straight path on the Earth's surface,
[Note 1: How do we define a "straight path"? One
reasonable definition is that it's what you get
if the ship's hull is symmetrical and the ship's
rudder is not deflected either right or left.]
but the Earth's surface is curved.
[Note 2: I.e., Euclid's axioms of plane geometry
don't hold on the Earth's surface. For example, the
sum of the interior angles in a triangle doesn't
equal 180 degrees.]

This analogy is (I think) generally a good guide to thinking about
general relativity as a theory of curved spacetime. But I should
note two ways in which the analogy is a bit misleading:
* Gravity is actually a manifestation of (4-dimensional) *spacetime*
being curved, rather than (just) 3-dimensional *space* being curved.
As Phillip Helbig noted in another post in this thread, for light
passing near to the Sun, the curvature of space accounts for half
of the observed bending; adding in time is needed to get the other
half.
* The curvature of the (2-dimensional) Earth's surface is due to that
surface being embedded in a 3-dimensional space. But in contrast,
we *don't* think that the curvature of (4-dimensional) spacetime
is due to it being embedded in a higher-dimensional "thing". Rather,
we view spacetime curvature as being intrinsic to spacetime itself.

in this spirit, observe that both of my comments "Note 1" and "Note 2"
above were phrased entirely in terms of things defined within the
curved surface itself. Working this way, we don't *need* any
"embedding in a higher-dimensional 'thing'" to define curvature,
to measure it, or to figure out its effects.

For more detailed discussions, you might want to consult a good book
(or two or more) on the subject. Some highly-regarded classics (whose
authors are all experts in relativity) are

Roberg Geroch,
"General Relativity from A to B"
(U of Chicago Press, paperback ISBN 0-226-28864-1)

Kip S. Thorne
"Black Holes and Time Warps: Einstein's Outrageous Legacy"
(W. W. Norton, New York, 1994)

Robert M. Wald,
"Space, Time, and Gravity: the Theory of the Big Bang and Black Holes"
(University of Chicago Press, 1977)

Bernard F. Schutz
"Gravity from the Ground Up"
(Cambridge University Press, 2003, ISBN 0-521-45506-5)

--
-- "Jonathan Thornburg [remove -animal to reply]"
Dept of Astronomy & IUCSS, Indiana University, Bloomington, Indiana, USA
"Washing one's hands of the conflict between the powerful and the
powerless means to side with the powerful, not to be neutral."
-- quote by Freire / poster by Oxfam

On 2011/12/04 10:19, Filip Houdek wrote:
Does strong gravity actually bend light, or the space through which
the light passes?
I don't see why a massless particle like a photon would be affected by
gravity.

A photon with frequency nu has energy E = h*nu, where h is Planck's
constant, so by the relation E = m*c^2, it has a mass m = h*nu/c^2. Its
so called rest mass m_0 is zero, but since it always travels at speed c,
it is a purely theoretical concept:

For a particle, one has E^2 = p^2*c^2 + m_0^2*c^4, where p is its
momentum. For the photon, p = h*nu/c, giving m_0 = 0. For a particle
traveling at speed v, p = m*v, which for v c gives the well known
formula m = m_0/sqrt(1 - (v/c)^2), with the prediction that for
particles with non-zero rest mass m_0, as v - c, E - infinity.

Hans

On Mon, 05 Dec 11, Hans Aberg wrote:
On 2011/12/04 10:19, Filip Houdek wrote: I don't see why a
massless particle like a photon would be affected by gravity.

A photon with frequency nu has energy E = h*nu, where h is Planck's
constant, so by the relation E = m*c^2, it has a mass m = h*nu/c^2. Its
so called rest mass m_0 is zero, but since it always travels at speed c,
it is a purely theoretical concept:

And so it would be if the photon travelled classically, that is, with
continuous motion from A to B. However, Wheeler's delayed choice
experiment shows that it does not, and that in fact it is the
"travelling photon" which is the "purely theoretical concept".
Quantum weirdness rules at C -- the photon is MIA between emission and
impact.

Eric Flesch wrote in news:mt2.0-26778-1323109791
@hydra.herts.ac.uk:

On Mon, 05 Dec 11, Hans Aberg wrote:
On 2011/12/04 10:19, Filip Houdek wrote: I don't see why a
massless particle like a photon would be affected by gravity.

A photon with frequency nu has energy E = h*nu, where h is Planck's
constant, so by the relation E = m*c^2, it has a mass m = h*nu/c^2. Its
so called rest mass m_0 is zero, but since it always travels at speed c,
it is a purely theoretical concept:

And so it would be if the photon travelled classically, that is, with
continuous motion from A to B. However, Wheeler's delayed choice
experiment shows that it does not, and that in fact it is the
"travelling photon" which is the "purely theoretical concept".
Quantum weirdness rules at C -- the photon is MIA between emission and
impact.


Unless you consider concepts like gravitational lensing or the Ahranov-Bohm
effect. Photons exist while in transit.

On 2011/12/05 19:29, Eric Flesch wrote:
On Mon, 05 Dec 11, Hans wrote:
On 2011/12/04 10:19, Filip Houdek wrote: I don't see why a
massless particle like a photon would be affected by gravity.

A photon with frequency nu has energy E = h*nu, where h is Planck's
constant, so by the relation E = m*c^2, it has a mass m = h*nu/c^2. Its
so called rest mass m_0 is zero, but since it always travels at speed c,
it is a purely theoretical concept:

And so it would be if the photon travelled classically, that is, with
continuous motion from A to B. However, Wheeler's delayed choice
experiment shows that it does not, and that in fact it is the
"travelling photon" which is the "purely theoretical concept".
Quantum weirdness rules at C -- the photon is MIA between emission and
impact.

Relativity does not include QM: how gravity acts on quantum fields.

So if a single photon wave-particle takes more than one way around a
massive object (i.e., its QM field is split as in the double slit
experiment), it would be nice to know what the gravitational effect is
on that, compared to that of photons taking only one way.

Hans

On Mon, 05 Dec 11, Hans Aberg wrote:
So if a single photon wave-particle takes more than one way around a
massive object (i.e., its QM field is split as in the double slit
experiment), it would be nice to know what the gravitational effect is
on that, compared to that of photons taking only one way.

Brief points:

(1) A photon is physically just a particle. (refer Feynman)

(2) The photon's wave function is just a map of its dynamic options --
a,k,a, its probabilistic path, but it is never classically present in
its own path -- which bars the term "exists" as we know it. This has
been experimentally shown (by Wheeler, etc), although Bohr understood
it from first principles.

(3) Another poster (EG) mentions topological (brane-tension) phenomena
which are irrelevant to the photon's nature.

Eric Flesch