Thread: Can GR produce an illusion of light travelling superluminally

Yes. The speed of light remains constant, but not because time is relative, but because space is discrete.

As I responded in a different thread, theory must be consistent with observations. Quantum-Geometry Dynamics is not only consistent with observations made at the fundamental level of reality, it is consistent with observations made at the cosmological level of reality and everything in between. I was even able to derive a new universal equation for gravity that shows Newton's equation as a special case and introduces a correction to truly makes Newton's law universal. I show that Newton was much closer to the truth than Einstein was. With the introduction of the small correction, Newton's law applies at the fundamental level of reality, to particles that approach the speed of light, for example.

Two thoughts

1) There an effect in astrophysics called "superluminal jets" where physical particles that are ejected from neutron stars and other things can appear to move faster than the speed of light. In this case, the illusion has to do with the fact that the object is getting much closer to you ever time a photon is emitted, and the time delay is changing.

2) the intersection of a beam of light with a beam of light with any material object can appear to travel faster than the speed of light (even though the light does not move faster). For an exposition of this idea, look here.

3) Cherenkov radiation only appears when a material object is travelling "super-luminally" inside a medium, where the speed of light is less than that of vacuum (due to far off resonance scattering, usually). For instance if you accelerated an electron to above 71% of the speed of light (= accelerating through a potential difference of something like 219 kilovolts), and shot it into a tank of water, the electron would be moving faster than the speed of light in the water, and would transfer its excess kinetic energy into photons emitted in the same direction as it is moving. That is Cherenkov radiation, exactly. It is stronger for charged particles, since they couple to the EM field quite strongly and perturb normal matter more.

There MIGHT be some kind of argument that a particle would interact with the vacuum field of the universe in a similar manner, but how could you possibly shoot a particle into the universe from a place where the speed of light is faster?

It's my understanding that within a gravitational well, time runs slower. So, for instance at the bottom of a valley, time runs slower, and at the top of a mountain, time runs faster.

So if you are on the surface of a heavy planet, looking up, then yes. Because you are experiencing time in slow-motion, it will appear to you that the far-away light is traveling "superluminally."

Yes, but in such an environment the perception of distance is distorted to exactly compensate.

The only way to make something appear "superluminal" withing the framework of GR is for it to be an illusion, a non-physical object (i.e. the trace of a laser beam on the surface of the moon, or the scissor example above), of for there to be a region of negative curvature.

The last forms the basis of several faster-than-light means of travel that are consistent with General relativity. One of the oldest known solutions is the Krasnikov Tube, and a more realistic metric was proposed by Alcubierre. It should also be possible to form kinds of optical illusions using negative curvature, but I have never heard of them.

The illusion of the OP ... was meant to be from a changing state of gravitation. From deep in a gravitational well to outside of that well. If you release the photons from inside the well ... then wait a little while then leave the well the distance that the photons have traveled away from you is greater than the speed of light would allow if you have never been in the gravitational well. The trick is that at any one point the speed of light is c. So regardless of if you are deep within the well OR if you are outside the well speed of light is c ... the illusion comes from the change in metric that happens after you release the photon. Does that make sense?

Originally Posted by bgbirdsey

Yes, but in such an environment the perception of distance is distorted to exactly compensate.

The only way to make something appear "superluminal" withing the framework of GR is for it to be an illusion, a non-physical object (i.e. the trace of a laser beam on the surface of the moon, or the scissor example above), of for there to be a region of negative curvature.

The last forms the basis of several faster-than-light means of travel that are consistent with General relativity. One of the oldest known solutions is the Krasnikov Tube, and a more realistic metric was proposed by Alcubierre. It should also be possible to form kinds of optical illusions using negative curvature, but I have never heard of them.

Originally Posted by bgbirdsey

Yes, but in such an environment the perception of distance is distorted to exactly compensate.

There is really no way to put it more succinctly than this. In GR (assuming just the weak energy condition), one cannot produce any effect which appears to materially violate covariance. So, no physical object can appear to surpass the fixed speed of light and no or beam of light or graviton pulse can appear to deviate from the speed of light.

As in the example of the superluminal jets, above, is that there is false assumption about the path of the jet that makes one interpret it as moving superluminally. This is an illusion.

To answer the OP, if you want to talk about a GR-consistent labelling of coordinates, realize that every time you move relative to the metric you have to compute a new set of "rods and clocks." At every point in time, for every observer, these rods and clocks will make the light beams appear to travel at the constant speed C.

You might be able to create some illusion if you forced yourself to mis-interpret the time and distance to some distant event (i.e. the lasers scattering from an asteroid), which would be the equivalent of the superluminal jet example. However, if you MEASURED the distance (for instance, by using two cameras to record the event, and using your knowledge about your local metric, triangulated the apparent distance to the) and measured the round trip time, you woud find that the laser beam never appeared to vary from the universal speed of light.

Mis-calculating the speed based on poor experimental procedure is not the Universe's fault!

Originally Posted by bgbirdsey

Yes, but in such an environment the perception of distance is distorted to exactly compensate.

The only way to make something appear "superluminal" withing the framework of GR is for it to be an illusion, a non-physical object (i.e. the trace of a laser beam on the surface of the moon, or the scissor example above), of for there to be a region of negative curvature.

The last forms the basis of several faster-than-light means of travel that are consistent with General relativity. One of the oldest known solutions is the Krasnikov Tube, and a more realistic metric was proposed by Alcubierre. It should also be possible to form kinds of optical illusions using negative curvature, but I have never heard of them.

According to the Wikipedia article "The Krasnikov Tube allows for a return trip that takes you back to the time right after you left." It is obvious that two such tubes would cause causality problems. Set up two Krasnikov tubes on two planets 1 light year away from each other, and start planning a yearly exchange. You'll soon have information going back in time, and if the travelers switch ships they could go back in time as well. It is less obvious, but also true, that ANY faster-than-light travel would run into such difficulties.

But back to the original question, of GR producing the illusion of light traveling superluminally, I suggested that if you are within a gravitational well, the light a long distance away should appear to be traveling faster than the local light. bgbirdseye suggests that "in such an environment the perception of distance is distorted to exactly compensate." I've thought on this for the last couple weeks, and now I want to address this idea.

I think we do have reasonably good data on the fact that time runs slower deeper in a gravitational well, and so if you are in this slow-motion region, then the far-away light would indeed seem to be going faster than light. I don't think this really qualifies as an "illusion" because everyone would agree about the specifics of what is going on. People out in space see the person on the ground moving slower. People on the ground see the person out in space moving faster.

On the other hand, I don't think that gravitation actually warps "space" at all. It slows down time, and causes light to curve toward it, and squashes objects down to the surface, but all that curving and squashing, and time slowing happens in a "space" that can be determined in a nice evenly spaced Cartesian coordinate system.

You might choose instead to define the space in a spherical coordinate system (with its origin at the center of the planet.) where (to distort the perception of distance) you artificially modify the radial component in such a way to keep the speed of light constant, but then what are you choosing to let vary? You're trying to keep the time and light speed constant, and letting the distance vary? So the light-nano-second (1 LNS approx 1 foot) distance will be shorter closer to the surface of the earth, and longer out into space.

But after you've done all this work, to redefine your coordinates so the speed of light is constant everywhere, you still have the problem that in reality, the speed of light is simply NOT constant. It slows down and bends around gravitational bodies. The speed of time really isn't constant either. It's slower in the gravitational wells--in, if I'm not mistaken, exact proportion to the slowing of the speed of light.

So rather than insisting on an "everywhere" constancy of the speed of light, we should accept a "local" constancy of the speed of light, and a slowing of time in gravitational wells.

We can continue to use a Cartesian Coordinate system in which every event in the universe has a non-ambiguous coordinate location, as it should. Suggesting that it is the speed of light which is constant, and the shape of space somehow bends around it is simply not what we experience in day-to-day life.

Regards,
Jonathan Doolin

Light can be slowed, google slowing light. And there are things in this universe that goes faster than light.

Originally Posted by krueger1

Light travels the same speed in ALL reference frames, and in ALL media.

Anyone who has studied physics, knows that the speed of light is not the same in all media. It varies depending on the refractive index of the material. In glass, with a refractive index of ~1.5, light travels at v=c/ri or roughly 200,000km/s. This is the cause of Cherenkov radiation, when particles moving at close to c enter a medium where the speed of light is lower than c.
But don't take my word for it, look it up!

I agree, scientist are slowing, bending, and beating light more and more. Why does light not get mixed up and scrambled. If subject A sees object X, and at some angle to them subject B sees object Y, why at the intersection point of the two beams does light not get mixed so that subjects A and B see some mess of Objects X and Y? Why does an exploding nova sent out light in all directions, one path coming directly to my eye, the other path taking a longer route around some giant mass and than reaching my eye later, Why is there now only one apparent event, One event seams to be canceled out. Am I suppose to believe that the photon hitting my eye is the same photon that left that super nova a million light years ago? If energy can not be created or destroyed, should there not be a large pile of mass at the back of my eye from all them photons that I have accumulated, or shouldn't my head be over heating, or does our bodies run on photon energy? Just some thoughts about how crazy the world of light is.