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## If gravity bends light, how do we know we're not looking at the same old light going round and round in circles?

I've been musing over this for years but never thought of putting it on here. Happily, I came across Jah Kable's post suggesting the same thing. His conversation was closed though, but I don't think the question was answered. The question is; if light can be refracted by a strong gravitational field, black hole, sun, etc. then given enough objects in its path, could it not be slingshot all over the universe? If this is true, all the stars we can see at night might be (have once been) in a radically different position to where we see them now. We might even be seeing light from the same star in two places, or more. Imagine light is a ribbon strewn around the universe, twisted round gravitational objects. If that ribbon happened to pass by Earth more than once, we'd count it more than once in our sky, and each time it would look a different age and as if it were coming from a completely different direction.
There must be someone who can calculate whether this is possible or not! Trouble is, how would we know which objects are (were) real and therefore capable of bending the light and which are the resultant image of light that's already done umpteen laps of the cosmos?! It's bending my head, I know that much!

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Jun 27 2013: Dear Darren,
I am having trouble understanding your question. What exactly do you mean by 'same old light going round and round in circles'?
When you see an object, be it something on earth or galactic, one thing is for certain - a photon from the object (either emitted or reflected) has just ended its journey on your retina. It cannot go past your retina, move through the rest of your brain and head (like x-ray) and continue its journey. So for all practical purposes light (considered a stream of photons) cannot go round in circle and be seen at the same time. Your observation stops its journey.
You are right to say that there is no guarantee that the path of this photon coming to your eye was perfectly straight (if perfectly straight has any meaning at all in relativistic spacetime).
Could it slingshot? Yes it could and it does.
The stars we can see at night might be (have once been) in a radically different position to where we see them now. We might even be seeing light from the same star in two places, or more. - yes they can be but not at different earth times rather simultaneously.
http://en.wikipedia.org/wiki/File:Einstein_cross.jpg
Additionally gravitational lensing would distort the shape of the object. This could make a galaxy appear like a ring.
http://en.wikipedia.org/wiki/File:A_Horseshoe_Einstein_Ring_from_Hubble.JPG
http://en.wikipedia.org/wiki/File:Gravitational_lens-full.jpg

I lost you when you say: If that ribbon happened to pass by Earth more than once, we'd count it more than once in our sky, and each time it would look a different age and as if it were coming from a completely different direction.
• #### Paul Lillebo

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Jul 26 2013: You raise an interesting point. No doubt astronomers and cosmologists are well aware of this issue, but I doubt if a computer program has been constructed yet that could back-calculate "original" locations of all known stars, taking into account the complexities of large numbers of gravitational interactions.

When we look out of our galaxy, more or less perpendicularly, I suspect there is minimal gravitational distortion between us and other nearby galaxies, such as the Andromeda, since there are relatively few stars in the way. The rather neat shape we see of the Andromeda and other near-by galaxies we also see repeated in very distant galaxies. I think this could give some confidence that the distortion in our view is not extreme. Still, it's an interesting question. It could be that the vastness of "empty" space between stars dominates the relatively tiny volume of star-occupied space to the degree that the great majority of photons get through undisturbed. Though 10 billion years is a long trip ...

Good question. Paul L.
• #### Tify Ndanoboi

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Jul 8 2013: Hang on a minute, since light travels along a path unless acted upon by an external force (in this case gravity) then it literally moves away from the point of origin, ie the emitter. Now unless one can prove that the whole path is enclosed, ie the universe is contained within and inside a sphere, then how would you ever be able to assume that light would return to either the point of origin or the observers frame of reference. Now trying string theory might help proving this, but I believe it assumes that we are all on the surface of a sphere, not contained within. The only possible way I could envisage this if one was trapped between two black holes, then you might get a Lorenz attractor scenario where one mass of one black hole would sling shot light in a path that would be inversely effected by the other. Since it's believed that there is a possibility of a black hole at the center of our galaxy, it would mean for us, that another galaxy would have to be currently colliding with ours for us to observe this. I suppose we should be grateful it isn't.
• #### A wal

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Jul 8 2013: Just imagine that straight lines eventually lead back to their starting point if you travel far enough. If it's spherical then it's the same distance in every direction until you arrive back at the starting point, including in time if it's a four dimensional sphere (hypersphere).
• #### Tify Ndanoboi

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Jul 9 2013: Yes but that does not state, nor prove, in anyway shape or form that the universe is of that nature. Which is why you'd need to analyses the theories of super-string theory in relation to astrophysics probably and in conjunction with quantum dynamics. As co-joined, you could then potentially theorize state that there is only 1 photon in the whole universe.
• #### A wal

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Jul 9 2013: "Yes but that does not state, nor prove, in anyway shape or form that the universe is of that nature."

No it doesn’t. You were saying you were having trouble visualising it. String theory isn’t needed for the universe to be spherical. That’s the only way it can make sense imo. I don’t believe in infinity in any real sense and I don’t see the universe having edges somehow, so that’s the only way I think it can work. Relativity doesn’t prove it but it definitely suggests it.
• #### Tify Ndanoboi

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Jul 9 2013: I think you misread what i said by you saying..."You were saying you were having trouble visualising it"

I don’t believe in infinity in any real sense ... that may well be a problem specifically regarding the mathematics of all the disciplines involved. Specifically if one requires to have any kind of proof, rather than a supposition.
• #### A wal

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Jul 22 2013: What? Mathematics isn't proof of anything. It's just a way to tighten a description because maths is totally unambiguous and isn't open to interpretation.

Are you talking about the mathematics of a hypersphere? That's already known. If you're talking about the difference between the mathematics of describing the universe in this way vs the maths of describing it beginning from a gravitational singularity then there's no difference because from our perspective the universe does shrink into a singularity as we look back into the past, because it's a sphere. It's not just what it looks like from our perspective, it's what actually happened from our perspective, but that's not what it would look like if we were there. It would look just like this.

It makes so much more sense than thinking the universe is actually smaller in the past. As we look across the curved surface objects get more red-shifted as we're looking across more of a curved surface. If we look far enough in any dimension then everything funnels into a singularity. Look up dark flow.
• #### Tify Ndanoboi

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Jul 24 2013: Mathematics isn't proof of anything.......... You need to go back to school.
• #### A wal

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Jul 24 2013: How rude!

You have some very deep misconceptions about mathematics if you think it can be used as evidence of anything. It's a description of a model, nothing more. It can be used to make predictions, but even that's not real proof. It's impossible to prove a theory right. It can only be proved wrong. All you can do is say that the data matches the model. To do that the model needs to be unambiguous. That's what mathematics is for.

You need to forget schooling because it obviously didn't work properly and learn to think for yourself. Good luck.
• #### Brendan Olson

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Jul 7 2013: It would not matter either way anyways. When we look at the sky we look millions of years into the past. The nearest black hole to us isn't exactly close enough to bend our light. It's like a telescope, we are looking out from the inside of our galaxy, so things are clear. If we were looking inside from elsewhere, things may look distorted. I wouldn't be too concerned about this.
• #### A wal

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Jul 7 2013: Yes, we’re seeing the stars where they were and not where they are now and the further away in space we look the further back in time we see.

There are scientists who are looking for patterns in the light we see from Earth. They’ve made pictures of what patterns would be formed if the universe were various shapes and we were seeing the same light multiple times so that if we ever find a repeating pattern we’ll know what shape the universe is.

Personally I think this is doomed to failure because I think the universe is spherical (in time as well as space) and looking across a curved surface makes the light red-shifted which is why the further away we look, the more red-shifted objects tend to be. It’s not because the universe is expanding imo. Light couldn’t go all the way round and back to us because it would be infinitely red-shifted at the furthest point from us; the opposite side of the universe. I think that explains dark flow as well.
• #### Darren Moss

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Jun 27 2013: Hi Pabitra! Yes, you understand what I mean! Light does bend all over the place, deflected by gravitational bodies! Okay, so the photon that hits us in the face will stop, but a light source emits a lot of photons (as pointed out by Edward), so some of these photons that we don't see will continue past Earth (possibly being bent a bit again by Earth) and onward through space. Given the size (infinite?) of the universe and the number of gravitational bodies available, it stands to reason that the photons will pass by and be slingshot by more gravitational bodies. To paraphrase Einstein, given an infinite universe and and infinite number of gravitational bodies (Ok I know the mass is finite but there's alot of it!), one photon could eventually do a full 360 degree loop and smack me in the face from another angle. Of course, the additional time that photon takes to travel to my retina means that the image perceived will be that of a much younger star (light source). It's occurring to me though, that these photons will be scattered more every time they are slingshot as photons nearer to the gravitational object will be more effected than those further away, therfore the "image" will be spead out (unfocused), so multiple slingshots will reduce the image to nothing and individual photons will be strewn out all over the universe (?).

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Jul 1 2013: Since you made a fresh post, I didn't get an alert and missed your question entirely. That explains the late in my reply.
I think in all probability a single photon's journey from a distant stellar object may be exactly what you are saying. However a photon is a quantum particle and its path between point A (the source) and B(retina of your eye) has infinite number of histories and it's observable trajectory is the sum over histories of all those paths. So with or without gravitational lensing a single photon's journey is almost infinitely indeterminate - which means it will take an infinitely long time before another photon from the same source smacks on your face, You will not be there to observe it by then.
So, it will be rather practical to talk about a stream of photons separated by very small time interval, such that we can say it is a ray of light. Such a ray or its path is described under classical theories and your question regarding gravitational lensing becomes relevant.
Unfortunately chances are nil that you will receive two rays of light from a same stellar object at different time on account of gravitational lensing simply because the gravitational lensing effect is possible only when light from same stellar object seem to come to you from two different locations SIMULTANEOUSLY.
Gravity can bend a light ray to go past you and make a 360 degrees sweep and reach you - but you will see the object for the first time and make a wrong estimate about its distance and location in all likeness.
However please remember that the physical universe is expanding and the objects at the edge of it are receding almost at the speed of light. Your solitary ray of light may get lost in an insurmountable distance.
I hope that you know however you wish you cannot see the sun right at this moment? That you will always see it 8 minutes younger than it is now?
• #### A wal

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Jul 7 2013: "(infinite?)"

NO! You know there isn’t an infinite amount of matter and space-time is what’s in between, so it can’t be infinite. It’s a closed system, it has to be.
• #### Darren Moss

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Jun 26 2013: Hi Edward, thanks for your reply! What I was thinking about was Einstein's now proven theory of light deflection, leading to phenomena such as gravitational lensing. Ok, so light is radiated spherically from the source, but we only see the bit of it that hits us. That bit travelled in a straight line. But if that "straight line" passes by a large mass, or black hole, it is deflected slightly, like rolling a ball into one edge of a dip, it drops into the dip and follows the geodesic, until it pops back out again in a new direction. Or like the bat-mobile harpooning a lamp-post to go round a corner. Or is it? This is what I understand. Or does the light enter the space-time curvature, follow the geodesic, and pop out on the same path as before?
• #### edward long

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Jun 27 2013: You describe the path of a single photon which is not what illuminates our world. That doesn't seem to fit the question. Also, if the bending force is from a spherical celestial body in what direction is the light bent? Doesn't it depend on which side of the bending body the light traversed? It seems unlikely that all of the bent light would be focused into a pinpoint (Doomsday) beam. I may have just falsified the old saying that there are no stupid questions. :-)
• #### edward long

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Jun 26 2013: RE: "If gravity bends light." It seems like a bad idea to challenge Einstein about Physics, but what the heck, here goes. Is the light from an electric lamp a focused flashlight-like beam? No. Unless focused by a lens light never travels in the form of a beam. Light travels omnidirectionally thus the lamp fills the room with light. How do you "bend" something which is omnipresent like the light from the lamp?
• #### A wal

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Jul 7 2013: Yes it expands in a sphere in flat space-time and in other shapes if the space-time is curved due to gravitational acceleration.
• #### edward long

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Jul 7 2013: Say what? How can there be a sphere in flat space?
• #### A wal

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Jul 7 2013: If the space-time is flat then the light will expand in all directions at the same rate, which makes an expanding sphere with the light source at the centre. If the space-time is curved then the light will move at the same speed in all directions but not at the same rate in all directions from a non-local perspective because time dilation and length contraction will cause distance in that direction to decrease (that's what curved space-time means), so you get a warped bubble of expanding light.
• #### edward long

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Jul 7 2013: RE: "If the space-time is flat then. . . ". Say what? How can there be a sphere (3D) in flat (2D) space?
• #### A wal

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Jul 7 2013: No, space-time is four dimensional (at least), whether it's flat or not. You're misunderstanding slightly what flat and curved means in this context. Flat simply means free of gravitaional acceleration in gr and curved means it isn't. Space-time is curved towards mass, which is another way of saying that gravity is a shortening of the distances in space and time (length contraction and time dilation) and the effect falls off as a inverse square of the distance. In zero dimensions it would be infinite, in one it wouldn't weaken with distance, in two if you double the distance it would be halved, and in three spactial dimension it's an inverse square (if you double the distance it's devided by four). It's simply the way it spreads out and becomes more difuse as it covers a larger volume.
• #### edward long

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Jul 7 2013: RE: "No, space-time is four dimensional. . . ". Do you deny that Cartesian Coordinates apply to this matter? A point in 2-D space can be located with an X (Left/Right) and a Y (Up/Down) coordinate. A Z (Above/Below the 2-D Plane) coordinate becomes necessary if locating a point in 3-D space.
• #### A wal

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Jul 8 2013: No of course I don't deny that. What does it have to do with this?

The point is that in flat space-time all observers will agree on the distances between objects and in curved space-time they won't. If an observer draws a grid of squares in one flat pain and then does the same at a right angle to it they would get a three dimensional grid of cubes and in flat space-time all observers would agree. They'd all say they were all cubes, but that's not true in curved space-time. If an object close to a gravitational source did this then an object further away from it would see the same grid as warped because the distance in space-time is shortened for the object closer the mass, so they'd also be moving through time at a slower rate compared to the further object, but the closer one wouldn't notice anything unusual because it's relative.

Distances aren't absolute. Observers measure different distances in the same space and time depending on the frame of reference. Space and time aren't a fixed background. They're an interconnected and dynamic background that are defined by the objects in it and measured differently by different observers depending on their point of view, and no-one is more or less right than anyone else. They're all right from their own perspective. Pretty isn't it.
• #### edward long

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Jul 8 2013: RE: "No of course I don't deny that. . ." Sorry, A wal, it's like we are speaking two different languages. I do not understand what you are saying about 3-D entities existing in 2-D space. But you say we all right in our own perspective, so I see no reason to debate. We are all right! Thanks for your time. Keep learning.
• #### A wal

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Jul 8 2013: I'm going to try explain it more clearly. Feel completely free to not reply to this post if you've had enough, I'm doing it for myself and for anyone reading this thread who might be interested. Please don't think I'm being condisending. I know that it can hard to get your head round this if you're not used to thinking in those terms and I'm trying to simplify and condense something that is unlike anythink we're used to in every day life, but it is actually very simple. I'm not trying to show off or anything by saying that. It really is simple, but it's sometimes very difficult for us to rap our heads around a concept that our normal lives haven't conditioned us to understand.

I never said in two dimensional space. I think the problem is still that you're thinking flat means two dimensional. Flat in this context is how people normally think of space and time, ie every observer agrees on the distances between objects and the passage of time moves at the same rate for everyone. Curved means that there's a mass and observers closer to that mass measure distances between the same objects as shorter than those who are further away from the mass and also observers closer to the mass measure time moving at a slower rate, so those closer to a mass will appear to be moving through time at a slower faster rate from the perspective of a more distant observer and those further away will appear to be moving through time at a faster rate from the perspective of an observer closer to the mass. One very pretty way of describing this is objects following straight paths through curved space-time. I hope that helps a bit.