Question of the Week

How can light bend?

Sun, 23rd Jan 2011

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Question

Ayush, India asked:

If light is massless, how does it bend due to gravity?

Answer

We posed this question to Andrew Pontzen from the University of Cambridge...

Andrew -  Regardless of the mass of the object, the acceleration caused by gravitational pull is the same for any object.  Now, Newton came along and gave a mathematical explanation of this and the maths essentially is that mass appears on both sides of the equation which governs this behaviour.  So it actually cancels out.  A simulated Black Hole of ten solar masses as seen from a distance of 600km with the Milky Way in the background (horizontal camera opening angle: 90°)But if the mass is actually zero, then it’s no longer really mathematically valid to do that cancellation.  Nonetheless, it’s certainly true experimentally and mathematically that as you go to smaller and smaller masses, these things are still deflected in the same way by gravity. But since this sort of mathematical paradox of trying to divide by zero, that isn’t conclusive.  To get the full mathematical answer actually requires coupling a description of what we call electromagnetic waves, that's the kind of physics underlying the wave light travels, to Einstein’s theory of gravity which is general relativity.  Only then do we get rid of this paradox of dividing by zero and end up with a conclusive answer that shows that just as objects of any mass are affected by gravity.  So light which has no mass is also affected by gravity.

Diana -   So, what is it that relativity tells us about gravity that can help us solve the problem?

Andrew -   So in the end, Einstein’s description of gravity which is general relativity tells us that the effect of gravity is caused by distortions in space and time itself.  Now if you do something as fundamental as distorting space and time, and reshaping it, anything that lives inside space and time will be affected. That includes waves, and so, waves can be bent and can follow different paths if you change the geometric properties of the space they live in.

Diana -   Gravity can effectively bend space and time.  Meaning that anything in its field is also distorted and that includes light.

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Excellent question, and I will await the answer.  As I understand it, the photon only has a relativistic mass when travelling at or near the speed of light.

And if an object radiates light.  For example, as I'm sitting here, I'm radiating IR light which is the release of energy. 

While I don't feel lighter, this energy that I'm releasing is related to a mass loss of the amount E-mc2

Do arguments flow in circles like energy  CliffordK, Mon, 17th Jan 2011

This is a massive simplification - and even this is hard to visualize - but here goes....  In essence; gravity comes from mass and energy which distort the whole of space and time  - light still travels the shortest distance within this spacetime, but as spacetime is curved it now follows a geodesic not a dead-straight line. 

The geodesic is the shortest distance between two points - we call them great circles when we look at them on the surface of the earth; if you look at the flight path from London to New York, it might appear not to be the most direct route but it is the shortest path.  Space time distortion is almost impossible to explain easily - it isn't that some thing within space is curved - it is that the space itself is no longer flat.

  The mass/enery of a star warps spacetime,  and light must travel in this curved spacetime.  Anything in free motion will follow a straight world line unless it is acted on by an exterior force. Light is in free motion and thus follows the "straight world-line", the most efficient route, which in curved spacetime is the geodesic.  This is described by general relativity and it is not simple nor does it allow easy human-scale examples - but it is one of the most tested theories in science and till now has been very robust.  imatfaal, Tue, 18th Jan 2011

It's because a photon posesses acceleration. Acceleration and curvature and gravity are all ensembles of the same thing in relativity, so the acceleration of a photon couples to the gravitational curvature of spacetime. QuantumClue, Tue, 18th Jan 2011

I recently read.......all of the ancient debates concerning religion, science and philosophy  come down to to just a single mathematical question - does zero have actual existence or is it just a mathematical construct that never appears in reality?Science and religion will at last be reconciled when scientists stop waging their crazy war against zero. One day soon they will see the light - light being the quintessence of the zero domain, as Einstein proved. light is both dimensional and dimensionless it is massless and experiences no distance and no time when it travels so is everywhere all at once
how does that sound to anyone else?
knights, Thu, 20th Jan 2011

That would be like saying that understanding a human concept like zero is to understand the all and all of physics. This is not true. QuantumClue, Thu, 20th Jan 2011

between 0 and infinity lies the totality of physics:........ I read this......


All observers, no matter what speed they're travelling at, will always agree that a hypothetical ruler being carried at light speed will always measure zero distance. That result is built into Einstein's equations. Everything is measured with respect to the speed of light except the speed of light, which is measured with respect to itself, making it a unique reference frame. This is an example of one of the great enigmas of existence: self-reference. So often, paradoxes reduce to situations of self-reference. When a Cretan says, "All Cretans are liars," is his statement true, false, or undecidable? Gödel's famous Incompleteness Theorem is the logical consequence of the enigma of self-reference.

In a self-referential frame of reference, zero and infinity come to the fore. So, all distances between photons are zero, their mass is zero, and time passes infinitely slowly i.e. time has stopped. And consider this paradox: given that speed = distance divided by time, if time never passes then no matter what distance is covered, the time elapsed is always zero, hence we are dividing by zero, hence the speed of a photon always equals infinity, in its own frame of reference. But equally, since all distances experienced by a photon are always zero, then no matter what time elapses, the speed of a photon will always be zero. So, what is the speed of a photon in its own frame of reference? Is it zero or infinity, or somehow neither and both? Is it simultaneously all possible speeds from zero to infinity? Such is the enigma of light speed. The mystery is compounded when we add in the result employed by Einstein that the speed of light in a vacuum is invariant for all observers in any frame of reference other than that of light itself
Returning to the central point, doesn't this bizarre world of photons sound exactly like the dimensionless r = 0 domain, outside space and time? In fact, there's only one difference. Despite being so hard to define in terms of any concrete existence, photons do possess real energy in the physical world.

What is the conclusion? When dimensionless, purely mental energy is converted into dimensional energy, it appears initially as massless, sizeless, unindividuated  "particles" such as photons. The link between thought and light (electromagnetic radiation) is a close one, and the differences are exceptionally subtle.

One of the keys to establishing the reality of the r = 0 dimensionless domain to skeptical scientists is to show how it can realistically give rise to dimensional existence. We have now done so. Light is the bridge between the dimensionless and the dimensional. Light is a hybrid of the dimensionless and the dimensional, with one foot in each domain. Light, although part of dimensional reality, also exists in a twilight zone of dimensionlessness where one photon cannot be readily distinguished from another. Light might be said to be "physical thought". Photons might be said to be like brain cells in a cosmic brain, blazing with illumination. Enlightenment is therefore almost a literal expression rather than merely metaphorical.

In a black hole singularity, time doesn't pass and there is no distance between any particles within the singularity. Light can't escape from the singularity. Imagine that all of the countless black holes in the universe sucked in all of the matter around them and then came together in one vast, cosmic black hole. This Super Singularity would contain everything in the universe. It would be the reverse of the original Big Bang Singularity from which everything emerged in the first place. It would be timeless, spaceless, dimensionless. It would be the r = 0
Our universe is a bizarre hybrid of the infinite and finite, the dimensionless and the dimensional, r > 0 and r = 0. If mass is the essence of r > 0, thought is the essence of r = 0, and light is the bridge between the two domains. In the centre of black holes, light cannot escape and mass is no longer recognizable in dimensional terms. Physicists are unable to say what a black hole singularity is made of. Light, mass, dimensionality and dimensionlessness, infinity and finitude all come together in black holes, the dimensional portals to the dimensionless domain.   

But there's something else to think about. Light can't escape from a black hole, but nor can it escape from its own domain. If (from its perspective) it exists in a domain of zero dimensions, to where can it escape? It has nowhere to go. When we say that light can't escape from a black hole, we're talking about light relative to our perception of it, but, relative to itself, light is unaffected by a black hole since light is outside space and time. It is everywhere at once. 

knights, Thu, 20th Jan 2011

My gran used to say i was her ray of light.

I ESCAPED

Sorry :) ukmicky, Fri, 21st Jan 2011

Ayush, light doesn't 'bend'. But it depends on how you see it. I look at space as a 'topology', which in a way is a daft thing to do :), as I then assign a shape to a 'nothing'. And space is a 'nothing', really it is. If we assumed gravity to be something like magnetism we could speak of it 'attracting' light, but that's not the truth, there is nothing EM about gravity as far as I know.

You might want to turn it around instead, and say that what that 'topology' really consists of, is all those 'thingies' we have and observe. They all follow certain 'lanes' in space, created by themselves and their interaction with all other 'thingies' existing, like planets and suns, giving us the impression of a 'topology' existing in a nothing. That topology is 'propagated' by light-speed from all objects of matter, and under very specific condition even light seems to be able to create a 'topology'.

yor_on, Fri, 21st Jan 2011

Haven't read other answers but I'll have a go anyway.

What we feel as gravity is really a warping of space. When a photon which is indeed mass-less encounters this wrapped space it follows the warp.

This analogy helped me with this concept: Imagine a river in hilly country. The water wants to flow down hill as quickly as possible but the ground is covered in hills (it's warped) so as if follows a course it encounters a place that starts going up a hill so it must turn around the hill. In the same way masses (like planets, stars, and even entire galaxies) warp space.

Newton didn't really describe gravity, only what it did. He did a very good job too. Using Newton's equations you can launch a probe billions of miles and hit a target only a few feet across (it's been done with several space probes). It was Einstein who actually told us what gravity WAS. We still don't know why mass or energy warps space, or why it's so incredibly weak. mountaineirc1969, Mon, 24th Jan 2011

QuantumClue stated that a photon possesses acceleration.  Perhaps I am misunderstanding the point, but I thought that a photon travels at the speed of light so how does it accelerate?  Also, acceleration is inversely proportional to mass, so calculating a massless particle's acceleration would require division by zero. Ron Maxwell, Mon, 24th Jan 2011



No, no... That is from the standard force equation yes? No, we move to differentials to explain acceleration such as dv/dt=a. QuantumClue, Tue, 25th Jan 2011

Think of the photons as being accelerated by gravity. That was easier for me./ QuantumClue, Tue, 25th Jan 2011

you have to think of space as a plane and gravity exerted by any heavy object as a bowling ball being dropped on a plane. it drops but doesn't break. now light when passing next to that heavy mass will be deviated from its path because space itself is warped Lethalwolf, Tue, 25th Jan 2011

A photon do not 'accelerate'. Instead it will 'blue shift' from the observer inside a 'gravity well'. and when leaving it 'red shift' according to an observer outside as well as inside that gravity well. Light have only one 'speed', not several. But where 'speed' fails energy comes in. Both the blue shift and the red shift will express itself as more, or less, 'work done' when measured by the observers described.
yor_on, Fri, 28th Jan 2011

Perhaps that space-bending anecdote given by Einstein has the answer...
I am, obviously, still an amateur in the field of physics, and the sciences overall, de facto, so this isn't a legitimately thought out claim. Locke, Wed, 2nd Feb 2011

Space near mass is denser, and gravity is actually refraction. Faye_Kane, Thu, 24th Feb 2011

If light is massless, how does it bend due to gravity?


Hmm,,massless,,,cannot be massless,,,means also matterless,,,


Light,,light-source is matter-think and made,,,borned light,,so light must be matter-think,,and therefore all matter-think has own mass,,,

Other question is what is the weight of that mass,,?,,one light-particle,,i mean,,?

Heikki Rinnemaa, Fri, 4th Mar 2011

Heikki

a little less punctuation, a little more reaction perhaps imatfaal, Fri, 4th Mar 2011

http://www.bbc.co.uk/news/science-environment-12620560  huh? CZARCAR, Fri, 4th Mar 2011

Gravity is a geometry. The geometry express itself everywhere, inside us and outside. Quantum physics hopes to find evidence for it becoming 'particles' of some kind, like the Higgs boson/field or gravitons. But, as far as I understand it, Einstein didn't see gravity as 'particles'. Geometry is shapes, everything radiation shows us has a 'shape' of some kind, except 'space'.

Space is a classical (as in GR) negation of everything, it's not a 'medium' like air, and it do not contain anything we can observe classically other than invariant matter and, possibly, radiation. The only thing that can define this space is 'distance'. There is no way for us to mark out 'space' in certain measures, we have to use heavenly objects like stars (and radiation) for defining the size of it.

Think of boxes 'filled' with nothing. That nothing is a vacuum, aka 'space. Make the boxes differently large. Can you now say that you have more 'vacuum' in the larger box? Why? There's nothing there. Its not the vacuum that is 'larger' there, but the shape of the box, its geometry. So the most interesting question with space is how something not really there can get a 'distance'? And that one has irritated me a long time but maybe, just maybe? Recently Ive started to wonder about what 'gravity' really can do? Can it give a 'nothing' a distance? If it can, it really is a geometry. And a geometry that not only creates distance, but also will create the directions for both radiation and invariant mass. There are two things I know that can warp/distort SpaceTime, invariant mass and energy. So what is a 'distance'? And what is a 'speed'? yor_on, Sat, 2nd Apr 2011

I think you might be havering Yoron.

Light follows a straight line through space, but matter distorts space, so a straight line might actually be curved when viewed from a different perspective. Geezer, Sat, 2nd Apr 2011

Sorry, what's the argument Geezer?
Not sure how you think there?

That gravity distort space is clear.
That we have a three dimensional reality is also very clear.
That we live inside the 'arrow of time' having one direction is also indisputable, at least macroscopically.

So what the he* are you talking about yor_on, Sat, 2nd Apr 2011

Maybe you visualize light as being visible streaks through 'space'? And then it is 'convoluted' and so if we only could straight it out we would get more 'distance' from it? And so we could argue that 'space' exist?

Light only exist in a interaction. There is no way to see a 'photon' more than once, if you don't want to argue that a entanglement are the exact same. But then the other photon also should be annihilated. And it ain't, both live a 'life' of their own even if entangled.

So that space is a nothing. Gravity may create it and also 'distort' it. Or are you thinking of Newtons 'forces'? Everything having a 'vector' (magnitude and direction). Meaning that everything should be a 'flat' surface from the beginning? Maybe? Don't know, you can use all kinds of premises for describing SpaceTime. To assume that we 'know' how it should behave have been repeatedly wrong in the past. And most of the things we trust in today will 'somewhere' have to be changed before we get any closer to 'reality'.

It's a little presumptuous assuming that we 'know' how to build a 'SpaceTime' because we just 'discovered' alternatively 'invented' Quantum mechanics. Only *** believe that their own little corner contain the truth, nothing but the truth, and the whole truth, so help me **. yor_on, Sat, 2nd Apr 2011

If you won't agree with me that distance is a discussable concept then show me where and how it can't be disputed please. To me it seems as if both particles and space builds on the same concept, 'distance'. Then we have some things that seems to make it without. Light. yor_on, Sat, 2nd Apr 2011

When light bends to gravity then that's because it plays by the 'rules'. The 'rules' we have state that everything obeys 'gravity'. Space and matter seems very alike in that they both contain this property, 'distance'. Then we have some things that don't. Light is what comes to my mind then. Particles are also defined by their need to take up a '3D-room' wherever they are, sort of reserving a 'space'. That's also why we expect them to be there tomorrow. Light on the other side just 'is'. That you can find it everywhere tells you nothing about a speed or a 'distance'. Light only exist in a interaction, that we use our ideas of a '3D room geometry in times arrow' to measure a 'speed' is a direct result from where we live. No guarantee of this being anything else than a defect in something else, getting 'locally' (SpaceTime) some very weird properties enabling linearity inside non-linearity inside linearity inside ...

I prefer looking at light speed in a vacuum as a constant nowadays. Not that makes it any more understandable, just that light do some very weird things that? I don't know, I think of it as a 'constant'. And those 'Constants' is what we need to find. yor_on, Sat, 2nd Apr 2011

light does not bend (but it still can be considered as bending) the space and time around light is bended making it seem like light is bending hence light is bending and not bending at the same time
spook1456, Sun, 17th Apr 2011

even if light were not affected by gravity it would still bend due to gravitational time dilation. granpa, Sun, 17th Apr 2011

My understanding is that about 10 years ago the weight of photons was measured. My other question is; is the fourth dimension dark matter? David, Sat, 12th Jul 2014

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