# The Naked Scientists Forum

### Author Topic: How are photons affected by gravity?  (Read 8855 times)

#### Gary Mason

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##### How are photons affected by gravity?
« on: 02/08/2010 14:30:02 »
Gary Mason  asked the Naked Scientists:

Hey Chris, newbielink:http://www.thenakedscientists.com/HTML/podcasts/ [nonactive] - I have all of the episodes of naked everything, lol!

I have never sent in a question, but i have one, which has been bothering me for a while.

1: because light consist of photons, which have no mass, how is it that light can be bent by the gravity of a black hole? as gravity pulls on mass and light has no mass, just energy, is it the space being bent around a black hole and light still travels straight, or does the gravity pull on the associated mass of the energy, actually bending the light?

Thanks a lot, and keep up the good work, i have learnt a lot from all the shows i have listened to over the years. i am a graduate of the naked scientist university lmao.

thanks

Gary Mason, birmingham

What do you think?
« Last Edit: 02/08/2010 14:30:02 by _system »

#### daveshorts

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##### How are photons affected by gravity?
« Reply #1 on: 03/08/2010 16:15:47 »
The general relativity answer to this is that space is bent, which means that although the light is going in a straight line locally, when you look at the path from further away it looks bent.

Eg in curved 2d space (on a sphere)  you can draw a triangle with three 90° corners with straight lines between them.

#### Pmb

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##### How are photons affected by gravity?
« Reply #2 on: 24/08/2010 01:29:36 »
...
1: because light consist of photons, which have no mass, how is it that light can be bent by the gravity of a black hole? ...

Whomever told you that photons don't have mass were choosing to use the term in a particular way, that was different than Einstein and, say, Feynman. Consider what Richard Feynman said on this subject. From Feynman Lectures Vol -I page 7-11 Section entitled Gravitation and Relativity
Quote
One feature of this new law is quite easy to understand is this: In Einstein relativity theory, anything which has energy has mass -- mass in the sense that it is attracted gravitationaly. Even light, which has energy, has a "mass". When a light beam, which has energy in it, comes past the sun there is attraction on it by the sun.
This means that photons have passive gravitational mass. You're thinking of the notion that photons have zero proper mass (aka "rest mass"). This is a common mistake which is propagated and fueled by the silly desire to have everyone use the term "mass" to refer to proper mass. A bad idea if you were to ask me. It confuses people and causes questions like this to appear nearly every day on discussion forums.

#### yor_on

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##### How are photons affected by gravity?
« Reply #3 on: 08/09/2010 02:28:48 »
Sorry, I think I will have to agree to disagree on that pmb. To me it seems a very good question. As I see it it must be what we call the momentum that gives the photon the ability to 'bend'. Like two parallel beams diverging towards each other. The question, for me at least, is what this marvelous 'momentum' would suffice of as a photon have no invariant mass (matter-less, but 'more' in this case:). Ah well.

" A 5 kg medicine ball is thrown directly at her by someone standing in front of her, and only a short distance away, so that we can take the ball’s flight to be close to horizontal.  She catches and holds it, and because of its impact begins to roll backwards.  Notice we’ve chosen her weight so that, conveniently, she plus the ball weigh just ten times what the ball weighs by itself.

What is found on doing this experiment carefully is that after the catch, she plus the ball roll backwards at just one-tenth the speed the ball was moving just before she caught it, so if the ball was thrown at 5 meters per second, she will roll backwards at one-half meter per second after the catch.  It is tempting to conclude that the “total amount of motion” is the same before and after her catching the ball, since we end up with ten times the mass moving at one-tenth the speed.

Considerations and experiments like this led Descartes to invent the concept of “momentum”, meaning “amount of motion”, and to state that for a moving body the momentum was just the product of the mass of the body and its speed.  Momentum is traditionally labeled by the letter p, so his definition was:

momentum = p = mv

for a body having mass m and moving at speed v.  It is then obvious that in the above scenario of the woman catching the medicine ball, total “momentum” is the same before and after the catch.  Initially, only the ball had momentum, an amount 5x5 = 25 in suitable units, since its mass is 5kg and its speed is 5 meters per second.

After the catch, there is a total mass of 50kg moving at a speed of 0.5 meters per second, so the final momentum is 0.5x50 = 25, the total final amount is equal to the total initial amount.  We have just invented these figures, of course, but they reflect what is observed experimentally."

And there you have a historical definition :) Now assume he had thrown an awful lot of candlelight instead on that poor woman? How many candlelights would it had taken to move her the same amount? And exactly where does that 'force' come from?

Well, let's read some more.. "Massless particles are known to experience the same gravitational acceleration as other particles (which provides empirical evidence for the equivalence principle) because they do have relativistic mass, which is what acts as the gravity charge. Thus, perpendicular components of forces acting on massless particles simply change their direction of motion, the angle change in radians being GM/rc2 with gravitational lensing, a result predicted by general relativity. The component of force parallel to the motion still affects the particle, but by changing the frequency rather than the speed. This is because the momentum of a massless particle depends only on frequency and direction (compare with the momentum of low speed massive objects, which depends on mass, speed, and direction). Massless particles move in straight lines in spacetime, called geodesics, and gravitational lensing relies on spacetime curvature"

Also note this when reading 'relativistic mass' above. Many contemporary authors such as Taylor and Wheeler avoid using the concept of relativistic mass altogether:

"The concept of "relativistic mass" is subject to misunderstanding. That's why we don't use it. First, it applies the name mass - belonging to the magnitude of a 4-vector - to a very different concept, the time component of a 4-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself."

So what the heck is it?

- We seek him here, we seek him there, Those physicists seek him everywhere. Is he in heaven? - Is he in hell? That damned, elusive -

Ah well, here I stand without finding anything rhyming? But it's a good question to me :) Solve it for me and I will, ah, at least be grateful :)

#### yor_on

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##### How are photons affected by gravity?
« Reply #4 on: 08/09/2010 02:45:20 »
Maybe I should clear my confusion some more. That light bends to invariant mass like a star won't surprise me, but that two light beams will bend towards each other does. But, I've seen that statement at a lot of places, and I believe it to be a fact.

So where we otherwise can speak of "the increase of energy with velocity originates not in the object but in the geometric properties of spacetime itself." and sound reasonable enough becomes in the later case, observing two beams diverge towards each other in a empty space, slightly weird..

But we should note that it seems that the culprit for a momentum isn't any intrinsic property of that or those photons, but, as I'm starting to believe at least, a property of SpaceTimes flabbergasted reaction on finding a photon disturb it, well, sort of :)

#### Vern

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##### How are photons affected by gravity?
« Reply #5 on: 08/09/2010 13:54:50 »
There is a fact of nature that we often overlook. It was known a hundred fifty years ago and  continues to be true. It is this:

Nature behaves exactly as if its most elemental constituent must always move at the invariant speed of light in flat space time.

That fact explains all relativity phenomena, including the how and why of it.

Remember, the fact is that nature behaves that way. That is a fact. Whether or not nature is that way will never be known.
« Last Edit: 08/09/2010 15:08:14 by Vern »

#### yor_on

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##### How are photons affected by gravity?
« Reply #6 on: 11/09/2010 17:45:18 »
One way to see it is "Since light has an energy density (which is relativistically equivalent to mass) it WILL create a gravitational field (local distortion in the metric), which will cause another nearby laser beam to bend.".

So what is gravity?

It the weakest 'force' we know, about 100 000 000 000 000 000 000 000 000 000 000 000 000 000 weaker than the electromagnetic force, but with an 'infinite reach' meaning that it never stops existing. It 'propagates' with the speed of light, if you want to think of it in terms of 'gravitons'. Its strength diminish "with a force which is directly proportional to the product of their masses and inversely proportional to the square of the distance between them." in accordance with Newton's law of universal gravitation.

So is it a 'force'?

Not according to Einstein. As he saw it gravity was a result of mass distorting the SpaceTime. With the 'gravitational force' then just becoming the way space 'curved/wrinkled' around it, depending on the objects mass/energy/speed/velocity. Entering his world you will find that all gravity-induced accelerations just are the results of them following a so called geodesic (which represent the shortest line possible in a curved SpaceTime, or if you like, the most 'energy saving' path). And so it is equivalent to a free fall, just like that apple falling of the branch. So the orbits of satellites above you are in reality them being in a constant free fall, due to the way gravity curves the space around our Earth.

=

A NASA image depicting gravity's variation on Earth.

A low spot can be seen just off the coast of India, while a relative high occurs in the South Pacific Ocean.
Scientists hypothesize that Earth's deep underground structures are creating the distortions, related to Earth's past.
=

There are those wanting it to be a singular 'force', expecting that it somehow will resemble the electromagnetic force, but, as for me, I find Einsteins definition the better one.

So, knowing this, can light bend light?

Well, maybe? I don't doubt light have a gravitational 'impact' on the space surrounding itself, and if we assume that the two laser beams are parallel and extremely close to each other, the space will 'dip' around each on making it possible, possibly, for them to converge towards each other, maybe? :) But as LeeE pointed out to me, there are no actual experiments that I, or him, know of testing it? Also one have to remember that they do not 'bend', not if you believe Einstein at least. They just take the straightest/shortest path there is inside SpaceTime.

For one, I can't even decide how to think of how that 'space bending' will express itself around a beam? And considering that light is a 'boson', having no size, it is in theory possible to collect all 'photons' there are, and then superimpose them upon each other. Making them into just 'one' photon, without that one taking any place either. But if we tried to do that one, I would definitely expect space to become extremely bent around that 'particle/photon'.

You can of course assume some sort of geometrical center to each part of the beam, as we all know it is 'there' after all, visible for us or at least our instruments. Then you can count on the momentum expected from that amount of energy and define it as the equivalence of a certain amount of invariant mass (matter) being defined at each part of your beam. But as light always will need an interaction to be proved, meaning that you either destroy it by measuring, or measure its interactions through inferring it from other processes taking place around it, it's a very slippery subject to me.

The simplest way is naturally to assume that light 'travel' inside our space, but I expect it possible to consider it not traveling too, just becoming 'interactions'. That makes for a very different way of looking at it naturally. But what may imply it, is the way time-dilation and Lorenz-contraction works, redefining space and time when comparing 'frames of reference' depending on acceleration, motion and mass/energy. And in that case, without 'moving parts', there will be other rules defining space's behavior, although, equivalent to what we call lights momentum. And if that is correct it seems that 'fixed distances' are a somewhat 'artificial' definition, open for redefinitions depending on 'frames of reference'.
===

Take a look Gravity probe B, from 2007.

And "The accuracy of the GP-B experimental results has improved seventeen-fold since our preliminary results announcement  at the American Physical Society annual meeting in April 2007. At that time, only the larger, geodetic effect was clearly visible in the data. Over the past two and one half years, we have made extraordinary progress in understanding, modeling and removing three Newtonian sources of error—all due to patch potentials on the gyroscope rotor and housing surfaces. The latest results, based upon treatment of 1) damped polhode motion, 2) misalignment torques and 3) roll-polhode resonance torques, now clearly show both frame-dragging and geodetic precession in all four gyroscopes. The combined four-gyro result in the figure gives a statistical uncertainty of 14% (~5 marcsec/yr) for the frame-dragging (EW). The gyroscope-to-gyroscope variation gives a measure of the current systematic uncertainty. The standard deviation of this variation for all four gyroscopes is 10% (~4 marcsec/yr) of the frame-dragging effect, suggesting that the systematic uncertainty is similar in size (or smaller) than the statistical uncertainty. " From their homepage at http://einstein.stanford.edu/highlights/status1.html.

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« Last Edit: 11/09/2010 20:00:43 by yor_on »

#### abacus9900

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##### How are photons affected by gravity?
« Reply #7 on: 14/09/2010 13:08:16 »
Photons simply follow what appears to be the shortest route between A and B, however, as space is actually curved (although you can't see it), photons actually follow curved spacetime. This is why light cannot escape the gravitational pull of a black hole - because a black hole bends space to such a degree that light is compelled to follow it.

#### The Naked Scientists Forum

##### How are photons affected by gravity?
« Reply #7 on: 14/09/2010 13:08:16 »