IanC

There may be gasses that travel faster than the speed of light through another meduim, but not through a vacuum.

I think, simply put, it is that nothing with positive mass can accelerate past the speed of light in a vacuum.

Ian

Ravear

*mumbles something about tachyons*

travc

The answers have already been provided, so a little bit off topic...

Ever seen the old Planet of the Apes movie? In the beginning the main char (Heston) is traveling back to Earth talking about how the mission he was on has only lasted a short time (like a year of 6 months or something) but thousands of years have passed on Earth because he has been traveling near the speed of light. Really caught me off gaurd that they got something scientifically correct in that film.

Anyways, the real brain twisters involve comparing what one sees from one vantage point with what one sees from a different point. If all the relevent observers are in the same place moving the same speed, then it is pretty simple. The supernova did happen 190,000 years ago to us slobs stuck on Earth. However, there is no way that anyone on Earth could possibly find out about that supernova before 1987. Telescopes just collect light better than eyes, but they see the same light. Even if we had an observatory sitting 1 light year from the supernova (assuming it would have survived), it would have taken at least 189,999 years to tell us on Earth about it no matter how it communicated (sent it's own EM signal, shoot off a rocket with a letter in it, whatever). So from our POV on Earth, we can say that it did "actually happen" in 1987 in a meaningful way. This is of course all assuming Relativity is actually corrert, which emperically it seem to be at least for big things.

Jesse

Nope, there is no particle known to science that can exceed the speed of light in a vacuum. Since light travels slower in a medium, things can exceed the speed of light in a medium, and also it is possible for something called the "phase velocity" of a wave to exceed the speed of light (see this page for a detailed explanation), but no actual matter or energy is moving FTL in this case and the phenomenon cannot be used to send information faster than light. As an analogy, imagine a bunch of sports fans on a bench 1 light year long, standing in sequence to do "the wave"--if each one only stands when he sees the person next to him stand, the wave cannot move faster than light, but if you arranged things so each person had a preset time they would stand at you could make the wave travel faster than light, but no particles/energy/information would be travelling FTL here so it wouldn't violate Einstein's theories.

Jesse

They didn't quite get it right though, because instead of treating it as a predictable result of relativity, the characters explained the differential aging in terms of some new theory by a made-up scientist, and acted surprised that his theory had turned out to be correct. Even more brain-twisting is if you look at observers in the same location but moving at different velocities--if each observer assumes light moves at c relative to himself, then they can disagree about how long ago the event happened! For example, suppose at the moment the light from the supernova reached the position of the earth, there was a ship moving at 0.6c passing right next to the earth, seeing the supernova at the same moment. Because of relativistic length contraction, this observer will say the distance to the supernova is only 152,000 light years from earth, and since this observer assumes light moves at 1 light year per year in his own rest frame, he will conclude that the supernova happened only 152,000 years ago rather than 190,000 years ago. This is an example of the "relativity of simultaneity", which basically says that different reference frames can disagree about whether two distant events happened at the "same time" or not (whether both events are assigned the same time-coordinate in that frame), and even if they both agree they happened at different times they will disagree about the time interval between them (they can even disagree about which event happened earlier and which happened later).

epepke

You seem fairly thoughtful, so I'm going to say some things that may make your brain hurt to give you some idea of what a wonderful universe we live in and how interesting relativity is.

Of course they named it 1987 because that's when they saw it. And it's 190,000 light years away. To say that something happened so many years ago requires a definition of "now." Here's the kicker: that "now" doesn't exist. According to relativity, the notion of simultaneity over long distances fails.

When doing special relativity, we imagine a "now" defined as a bunch of clocks distributed through space such that a clock two light-seconds away from us reads two seconds before our clock. That allows us to think in terms of time and distances in a way that's familiar to us. But it's really a kind of fiction, and it only gives the proper results when there's no acceleration or gravity (flat spacetime). This is the kind of fictional frame of reference that people use when they say that an observer measures something.

If we say that for us, now, the supernova is just a bunch of remnants, 190,000 years old, anybody moving with respect to us, as long as some of the motion is along the line from us to the supernova, would disagree.

In a very real sense that is in a kind of way more accurate, one can also say that the 1987 supernova happened here, now, and also 190,000 light years away, 190,000 years ago.

One thing you said was very interesting, and I don't know if you have thought it through:

Of course, we think of motion and speed as something that happens with respect to time. However, we can also think of moving through time, which is what happens if we just sit there. When we're sitting stationary to a clock, we're moving through time at the maximum possible rate, but we aren't moving through space at all. However, when we start moving through space (relative to the clock), we're moving through time at a slightly slower rate.

Light moves through space at the maximum possible rate, but it doesn't move through time at all. Light does not age.

DBT

I thought that tachyons were just a part of a theoretical model, and there wasn't any empirical evidence for their existence?

travc

The loss of simultanaity is the big mind f*ck of Relativity IMO. My favorite example is the polevaulter paradox since it is so basic.
http://www.phys.unsw.edu.au/einstein...le_paradox.htm

OdysseusTheInnkeeper

In the reference frame of light, both the temporal and the spatial dimensions in which the light ray is travelling collapse to zero. The universe is a 2D plane in the reference frame of a photon.

IanC

Still cant accelerate past c though, they start off greater than c (if the exist).

Ian