User:Weorthe/anisotropic

Say there is a small dust cloud in space, called Cloud C. 2 parsecs away is Star X.  2 parsecs in the other direction is Planet d:

X-C-d

Star X goes supernova, sending X-rays in every direction at the speed of light (whatever that is). Some of these X-rays pass through Cloud C, causing it to glow. X-rays and photons from the glowing cloud arrive at the same time at Planet d.

An astronomer on Planet d, named Lisle, proclaims: "Light travels toward an observer at infinite speed.  Star X just now went supernova, and Cloud C just now began to glow!"

Another astronomer on Planet d, named Tyson, cries out: "No, light travels in every direction at c.  Star X went supernova more than 7 years ago, and Cloud C began to glow 3.5 years ago!"

Alas, these are their final thoughts, as Planet d is way too close to a supernova.

But which astronomer was correct? The answer is that neither was more or less correct than the other. By using different, arbitrary but equally valid definitions for the one way speed of light, they ended up with different but equally valid definitions of time, and even of simultineity.

Safely 100 parsecs away, situated perpendicular to the line from X to C to d, sits Planet e. An astronomer on Planet e, named Lisle, watches as Star X goes supernova, then years later Cloud C begins to glow, then years after that all life is destroyed on Planet d.  He says:  "Light travels toward an observer at infinite speed.  Seven years ago Star X went supernova, and 3.5 years ago Cloud C began to glow, and now Planet d is being destroyed.  I hope they knew Jesus."

Which astronomer named Lisle is wrong? Neither is more or less correct than the other. If the speed of light is anisotropic, then time really is dependent on (a function of) the angle at which the information (light) about an event arrives at the observer.

Another astronomer on Planet e, named Tyson, retorts: "Light travels at c in every direction.  Star X went supernova 320 years ago, and Cloud C began to glow a few years after that, and all life on Planet d ended just a few years after that."

But the natural day to day experience for humans is to assume that light really does travel from an object to an observer (oneself) instantly. When we see a rock fall to the ground, we assume we are watching it as it happens, and do not bother to consider any time delay due to a finite speed of light. Even astronomers discussing celestial bodies often speak of events happening as they are being seen, because that is a more natural way of thinking. One might say that the pulsar PSR B1257+12 rotates 9650 times a minute, even when she means that it did so a thousand years ago (and who knows if it still is?)

Can you design an experiment to prove that the one way speed of light is either c or something else? You can't, because any one way speed of light results in the same universe, as long as the round trip speed equals c.

For example, say there are two teams of rocket scientists trying to send maneuvering instructions to a probe flying by the former planet Pluto. The isotropic team has to take the following into account, among other things: Pluto is 4-7 light hours from Earth, so it is not where it appears in the sky from Earth - it is moving 10000 miles per hour, so where we see it is where it used to be. Time will slow down on the probe (relative to Pluto) while it decellerates. Pluto's small gravity will also affect the speed of time on the probe. Instructions will take 4-7 hours to reach the probe. The anisotropic team will take into account the following: Pluto is exactly where it appears in Earth's sky, but because it is distant from Earth, time happens more slowly there. Time passes more slowly on the probe also, because it is going fast, but that has nothing to do with accelleration or gravity. Instructions will take 8-14 minutes to reach the probe. After crunching their numbers, each team comes up with a set of instructions for the probe. They are identical.

Is it ever useful to assume an anisotropic speed of light? Sure. When measuring anything at non-relativistic speeds, say the speed of a baseball, or a car, it is reasonable to assume that light from the ball or car arrives at the measuring device instantly (the difference due to relativity would be insignificant). Or say you are a scientist with a sensitive piece of equipment on Eath that measures gravity, and you want to eliminate the effect of the gravitational pull of the planet Jupiter on your experiments. Gravity propogates at the speed of light. If you effectively assume that Jupiter is where it appears in the sky, and that gravity propogates at infinite speed, then you're set. But if you take into account that Jupiter is 33 light minutes away, then it is not where it appears to be, but has since moved. After figuring out where Jupiter is now, then you need to calculate what the gravitaitonal effect of Jupiter would have been on your experiment 33 minutes in the past, back when Jupiter was located where it in fact now appears to be.

An anisotripic speed of light does not make the universe look different, and certainly not any younger. It's 14 billion years old any way you look at it. Anisotropy just exchanges one kind of time dilation for another. It's not "true" or "false," rather, it's using the same yard stick, but making different assumptions about how far apart the marks are.