A very short article on MSNBC says that scientists have slowed light down to 100 meters per second at room temperature and then sped it up to beyond the standard speed of light.
How is that possible? Are they talking about going over the speed of light in air but not over the speed of light in a vacuum? Or have they managed to go over the speed of light in a vacuum?
It seems unlikely they went over the speed in a vacuum otherwise I’d expect much more hoopla. 
Thoughts?
Not much information in the article, but I suspect it’s similar to quantum-tunelling experiments that demonstrated FTL proagation, essentially bu measuring the leading edge of the probability distribution waveform of the photons, rather than the Gaussian maximum–where the photon is most likely to be.
ha ha - if the only kind of FTL communication available in the real world is Quantum Tunnelling,
all we have to do is build a solid block of tungsten between here and Alpha Centauri, and voila!
instant ansible!
SF worldbuilding at
http://www.orionsarm.com/main.html
Or just put some twins in a dryer.
‘Faster’ than light communication via quantum tunneling is more illusion than reality. Causality is still not violated and closer inspection reveal you aren’t really getting FTL anything although at first blush it appears that way. There’s thread discussing that around here somewhere with details if you’re interested (search on quantum tunneling over the past week).
Slowing down light is no biggie either. The speed of light in a vaccum is as fast as anything can go. Shine light through some other substance and it is no longer in a vacuum and the speed decreases. While it is a neat trick I don’t think it nets you much and certainly doesn’t violate causality or anything cool like that.
Be careful getting your science news from the mass media. They tend to focus on the flashy stuff and be thin on the details that put most people to sleep (if they could understand it at all). If you see something cool looking in the media research more information for yourself (such as posting here as you’ve done). When the media makes astounding claims that ‘debunk’ hard rules (faster than light, perpetual motion, energy for free) your bullshit meter should peg at the high end and you should look more closely at what they are telling you. Maybe someday such things will be possible but extraordinary claims require extraordinary proof.
Posted by Whack-a-Mole:
I don’t understand. Why would FTL communication violate causality?
My colleague has put together a diagram to show that it is possible to violate causality by sending instantaneous messages from ship to ship while travelling at speed;
http://www.orionsarm.com/intro/ftl-paradoxes.html
with a little effort you can send a message into your own past, and therefore instantaneous messaging and/or FTL travel is a short step to time travel, which would destabilise the universe.
SF worldbuilding at
http://www.orionsarm.com/main.html
Here’s all the article says about this part:
The speed of light in a vacuum is 299,792 km/s, wheres the speed of light in air is 299,706 km/s. So, it’s not clear what they’re saying.
Searching for “alexandrite” and “speed of light”, I think I found a more detailed article on this, though: http://optics.org/articles/news/9/7/8/1
If I understand it right, it’s pretty lame. Yes, it is effectively going faster than c (in fact it has negative velocity in the lab frame), and no, it does not violate relativity:
Causality has to be the same for all reference frames or viewers of two events where one event affects the second event.
Basically information cannot be sent faster than the speed of light without violating causality. For the puroses of this example I will use a box (which constitutes information) but you can concoct a paradox using any signal in any form going FTL.
Consider a box sitting on an FTL teleportation pad here on earth and its counterpart is on Mars and I’m in a spaceship orbiting the Red Planet watching all this through my telescope. The pad is activated and POOF…the box disappears on earth and instantly pops into existence on Mars. From my view aboard the spaceship I see the box appear on Mars but looking through the telescope to earth I also see it still sitting on your pad.
Assume I have an FTL communication device and now send a message back to earth telling them to NOT teleport their box. Instant paradox. Different observers of the same event are perceiving cause and effect all messed up. There’s your causality violation.
I may be stupid but why should light be diferent than sound here. The box in the above example is not ON earth when you send your FTL message saying not to send it. you are only seeing reflected lght coming from it’s old position. imagine yelling really loudly across the atlantic telling the concorde not to take off after it lands.
C.
I agree. That’s not a good example, Whack.
What about nonlocality? That seems to bust up the C speed limit pretty efficiently.
The reason Whack-a-Mole’s example doesn’t work is that Earth and Mars are in (roughly) the same frame of reference. The idea is that if you go FTL in one frame of reference, you have gone back in time in some other frame. However, if we restrict ourselves to looking at a single frame of reference, there’s no causality violation.
The problem is that the math gets a little tricky when dealing with multiple frames of reference, and you can’t really show why it’s so without a few equations. But for instance, say you and I are in two spaceships travelling past each other at close to the speed of light. Within my ship, I shoot my best buddy Benny with a bullet that travels faster than c. From your spaceship, it may look like Benny was hit with the bullet before I pulled the trigger, and this is true even when you take into account the fact that light takes time to travel.
Here’s a very brief discussion (with a minimum of equations) of FTL communication and travel.
Don’t forget the very cool Scharnhorst effect. I don’t believe it’s ever been measured, but the math pretty conclusively demonstrates the c in a vacuum is not a constant.
I don’t see this as a violation of causality, just that perceive things in the wrong order. It’s just like if you got passed by a plane flying faster than sound, and you heard the noise it made five seconds ago after you heard the noise it made one second ago. So what?
The other example that eburacum45 linked to did a good job of explaining it - sending a note to myself in my own past is definitely a violation of causality.
eburacum45, tell your colleague that that’s the best diagram I’ve seen for explaining this, even if it is restricted to ASCII. Last time I tried to explain this one, I needed something like six people, and the diagram ended up looking like a game of Pick-Up Sticks.
As for the “faster than light” experiments, most of them work something like this. Suppose I’m walking through a long tunnel, moving at the Speed of Chronos. If you measure the time I enter one side of the tunnel, and the time I leave the other side of the tunnel, and divide the length of the tunnel by the difference in those times, you’ll measure my speed. But now suppose I’m holding my arms out in front of me as I walk, and you say that “I” enter the tunnel when my body enters, but that “I” leave when my hands leave the tunnel. Clearly, you’ll measure the wrong speed, faster than the speed of Chronos. And if the tunnel is short enough, or my arms are long enough, you’ll observe me “leaving” before I “entered”. Does this mean that I can walk through tunnels instantaneously? No, of course not. You’re just making your measurements in a rather silly way.
Okay, you’re right. I shouldn’t have said, “It will look like Benny dies before I pulled the trigger.” What I should have said is, “Benny dies before I pulled the trigger.” Because in your reference frame, that’s exactly what happens, and it’s not the same as perceiving things in the wrong order.
To make it more concrete, suppose that you also have a gun with FTL bullets, and as soon as you see Benny die, you shoot me. Just like in your reference frame, my bullets go back in time, in my reference frame, your bullets will go back in time, and I will take a hit before I fire.