Quote: Originally Posted by s_lone
What do you think of this?
I think the news media are terrible at reporting science, and scientists are terrible at explaining science to the media, with a few notable exceptions like Carl Sagan, and what probably happened there was a failure to clearly distinguish among phase velocity, group velocity, and signal velocity. Unfortunately, I don't think I know how to explain those very well to a non-mathematical crowd. Loosely speaking, any signal is composed of a whole lot of waves of different frequencies and phases and they travel at different velocities through different media. I'm sure that makes it clear as mud. If you feel up to dealing with some relatively simple differential equations, have a look at --
The key points of relativity are that the speed of light is the same for all observers, a consequence that actually follows from Maxwell's equations (known since the 1860s) where it appears as a numerical constant defined by the ratio of electrical to magnetic forces, and that the laws of physics must be the same for all observers. Not all motion is relative in relativity, only motion that's uniform in magnitude and direction. Accelerated motion is not relative, but the laws of physics appear different in accelerated systems so it's possible to distinguish between accelerated and uniformly moving systems. If the earth was alone in the cosmos, we'd still know it's rotating simply by observing that high and low pressure systems in the atmosphere rotate in opposite directions. They're a consequence of the Coriolis effect, which appears only in rotating systems. Rotation is an acceleration, which is simply a change in velocity, and velocity has both a speed and a direction, speed and velocity are not synonyms. A change in speed and a change in direction are both accelerations.
The laws of physics being the same for all observers just means that measurements of things like time and distance and velocity and mass will always have the same relationships among each other, but different observers will not in general measure the same values for them when observing the same events, unless they happen to be unmoving with respect to each other.
I think that answers most of the questions raised here, at least in a general way. I'll have a look back in the thread later to make sure, I'm a bit rushed this evening despite the apparent long-windedness of this, but if anyone's got any other questions, lay 'em out and I'll try to answer. No guarantees on timing though...
Two other points:
1. SJP's example of an observer seeing him get a beer before getting up off the couch is a reversal of causality, and for that to happen the observer would have to be going faster than light with respect to SJP.
2. light travels at different speeds through different media, fastest in a vacuum and slower in everything else, and a medium's index of refraction is a measure of that, more or less. Actually different frequencies travel at different speeds and the index of refraction is different for each of them, which is how a prism breaks up white light into a rainbow. It is, however, possible to get particles traveling through a medium other than a vacuum faster than light goes through it. In that event, a little cone of radiation spreads out behind each particle. It's called Cerenkov radiation, and accounts for the glow you may have seen in photos of radioactive stuff, like nuclear reactors and spent fuel bundles, sitting in water.
