It’s worth noting, re. Cecil’s excellent summary, that Cerenkov radiation can also exist outside of nuclear-radiation contexts. Those of you old enough to remember vacuum tubes (or who still use them, like me) may remember that in some of them the glass would emit a blue glow when the tube was operating. This was Cerenkov radiation, emitted when fast-moving electrons, instead of striking the plate, instead leaked out and struck the glass envelope at a speed greater than that of light in glass.
A link to the column is appreciated. If you were radioactive, would you glow? Who wrote the clock chime tune?
Actually, it’s not impossible for something to travel faster than the speed of light in a vacuum. We just don’t have a good model to describe what would happen if it did.
You would be correct in saying it would take infinite energy to accelerate a mass to the speed of light.
So, that means that you can’t cross c, right? Things moving at FTL speeds must always move at FTL speeds, and slower things must always be slow? So, there’s a “light barrier”, sort of like the sound barrier?
Well sort of. What if you were going slower than c, and you wanted to go faster than c? And you didn’t really care whether or not you ever actually went at exactly c? You map out your trip on a chart, but instead of a chart of positions, it’s a chart of speeds (more correctly of velocities, 'cause you care what direction you’re going in). And you see you have to get from here (slower than c) to there (faster than c) while skipping over or tunneling <–(BIG HINT) under c.
This is not impossible. To actually build a device to do this has not yet been done. But if you want to know how to do such a thing, you’ll have to ask Heisenberg.
Why it takes so much energy to get to c:
Special relativity says that for an object moving at a speed (v) relative to the observer, the mass (m) of that object is given as:
m = m0 / [1 - (v^2/c^2) ]
where m0 is the mass of the object and c is the speed of light (twelve million milea a minute, and that’s the fastest speed there is, according to Monty Python’s Galaxy song).
At any rate (eeewww), taking v to c, you find that the limit of v^2 / c^2 as v approaches c is 1.
So, that means that the limit of 1-(v^2/c^2) as v->c is 0.
Which means that the limit of m as v->c is infinite.
As the mass increases, it requires increasingly greater energy to accelerate it. So you never could quite get that mass to go at exactly the speed of light.
I should have said, in the last section, the mass of a “massive” object, or one with nonzero rest mass.
Clearly some objects travel at the speed of light all the time (take light for example–it pretty much always travels at the speed of light).
But light has no “rest mass”, and 0/0 is whatever you want it to be.
The idea of tunneling past the speed of light looks appealing on paper, but it wouldn’t work in practice. Tunneling only works with relatively “narrow” barriers, but that’s not what we have here. You may think that you can get arbitrarily close to c from the slow end, and then tunnel to a velocity arbitrarily close to c on the fast end, and make that barrier as narrow as needed, but one of the consequences of Special Relativity is that when you’re going at .9999999999999999 c, you’re just as far from c as when you’re “at rest”. In fact, when you’re going .9999999999999999 c, you are at rest, relative to yourself, which is the only frame of reference which would matter, for this problem. Another way of looking at it is that even though your speed, as measured in some reference frame, would have to jump only a small amount, your momentum would have to jump by an infinite amount, which is not allowed.
Now, it’s still possible (so far as we know) for a particle to exist which travels faster than c, and can never be slowed below c. However, all current theoretical models of such particles, called tachyons, suggest that they wouldn’t be able to interact with tardy matter like ourselves in any way, so practically speaking, they’re completely uninteresting.
Tachyons, of course, if they exist, have imaginary mass.
Whatever that is.
Yup. In fact, the whole damn Special Theory of Relativity is nothing but Einstein’s attempt to make theoretical sense of the following fact, which was determined by experiment as far back as 1881:
No matter how fast you’re going, light is always c faster than that.
There’s another way to glow in the dark, but it doesn’t (really) involve radiation; it involves fluorescence, as was mentioned. There’s a bit more of a chance to make people glow in the dark healthily through that than through disintegrating them.
The imaginatively named GFP (Green Fluorescent Protein), found in a certain species of jellyfish that I’m too lazy to look up, is fluorescent, which means that it has all the properties of a fluorescent molecule. Which is why some parts of the ocean glow, as in Robert Deniro’s speech to Amy Brennigan in Heat.
Normally, a fluorescent event is very, very fast, on the order of a couple of nanoseconds. When a photon of a particular wavelength of light strikes a fluorescent particle, that particle enters a higher-energy state. Most of the time, the particle immediately relaxes and releases another photon of longer wavelength, because you can’t get more energy out than in, and a longer wavelength photon has less energy than a shorter one. Fluorescein, the most commonly studied fluorescent particle, fluoresces at 488 nm and emits at 512 nm.
But since that happens at the nanosecond time scale, another thing is actually happening to make things glow in the dark, since the dark is coming a few hours after the light, as in these jellyfish or a glow-in-the-dark-watch. That’s phosphorescence. What happens there is that the fluorescent particle is struck by a photon, but instead of releasing another photon immediately, it enters a triplet state, and releases another wavelength of light much, much later. These events occur statistically (meaning that they are more or less random, but over a long enough time course, you get a distribution that’s very predictable, and, with enough particles, one you can see with the naked eye).
In science that uses fluorescence, phosphorescence is called ‘bleaching’ and is something to be avoided at all costs, because a particle in the triplet state cannot release light on that nanosecond time scale. A great deal of research has gone into making fluorescent dyes and proteins that do not bleach as easily as those that happen in nature. In nature, since what the jellyfish probably want to glow to scare away prey, then they would need a mechanism that ‘charges’ and releases that charge later. (Ok, I have no idea what evolutionary advantage glowing confers, but this seems as good a guess as any)
What does this have to do with people glowing in the dark? Well, there’s been some research into making living things glow in the dark by transfecting them with GFP (ie, inserting the ability to make GFP into the genome of the critter). So you spend the day sunning yourself and ‘charging’, then go to the club and light up the room (literally). It hasn’t worked yet, otherwise the next christmas gift would be glow-in-the-dark rabbits. But don’t underestimate the power of science and commercialism.
Especailly since there’s many other, man-made forms of the GFP now, such as CFP (Cyan), YFP (Yellow), etc, the possibility of being Rainbow Brite for Halloween seems a bit less remote.
That seems like a pretty bold statement. How can this be if c is a constant? c would have to be a variable for that to hold true it seems…
Actually, if I am moving it would mean the speed of light would exceed c. What you are saying is “light speed = my speed + c” sooo…
Where am I wrong?
Fascinating thread by the way.
As the saying goes, no matter where you are, there you are. The puzzlement that went to figuring out relativity was that the physicists around the turn of the century thought that there was the “ether,” kind of like the air, through which anything moved.
If you’re in a car, and another car is moving past you at 60 miles an hour, which one is “really moving?” Duh. If you roll down the window, and there’s no air moving, you’re not driving. (Pretend for a second that wind doesn’t exist.)
They figured out that there’s no such thing as the ether. Each driver is “parked,” from her/his own perspective. The speed of light looks just about the same, no matter how fast the other drivers seem to be going.
As to bremsstrahlung - that’s a cool story. The physics boys in the late 1800’s 
thought that they knew everything, and that physics from now on was going to be dull and boring, once Maxwell did his gig on light. 
Here’s bremsstrahlung in a nutshell. When you accelerate any charged particle, it emits electromagnetic radiation.
They got it figured that atoms are electrons orbiting around protons.
Uh-oh.:rolleyes:
#1) electrons are charged particles.
#2) and they’re orbiting.
#3) orbiting particles are undergoing constant acceleration toward their center.
#4) Therefore, they’re constantly emitting light (NOT! :mad: )
#5) and they’re constantly spiraling into the nucleus (NOT!!:o)
#6) Therefore, atoms can only last less than a nano-second before they explode into pure energy. :eek: :eek: :eek: :eek: :eek: :eek: :eek: :eek:
#7) The universe seems not to be detonating, actually.
#8) Therefore, we aren’t nearly as smart as we thought we were, we physicists - we’re purty stupid.:wally:
Hence, the birth, and some of the afterbirth, of Quantum Mechanics.
That’s what physicists kept saying to each other between 1881, when the Awful Fact was discovered, and 1905, when Einstein found an answer, the Special Theory of Relativity, but it was an answer depending on the premise that space and time just don’t work the way we instinctively think they do – just as the world looks flat, but isn’t. Essentially, the whole idea, “If A is moving at speed a and B is moving past A at speed b, B’s total speed must be a+b,” is a reasonable guess, but dead wrong.
For almost a hundred years now, every new experiment has shown that Einstein was right.
The exact explanation requires too much math to explain here. But there are plenty of books that go into it.
(In 1919, Einstein produced a General Theory of Relativity that covered more complex cases than the Special Theory, which only covers objects moving at constant speeds in straight lines. This one, scientists aren’t quite so certain of. It’s at least almost right, and seems more right than anything else that’s been proposed, but there are places in it where there is still some doubt.)
What Einstein uncovered was that pure velocity doesn’t exist; things only have velocity from the perspective of some observer, an observer which may itself be moving from some other perspective. The speed of light is constant for ALL observers. For someone moving at 60 k/s, the speed of light is c, not c+60, because from his perspective he is stationary. From your perspective, you are stationary, he is moving at 60 k/s and light SHOULD be moving at c+60. But its not, because c is constant for all observers. Which means that almost nothing else is.
–John
An article on radioactive lumnious wristwatch dials.
(Note: The author of the above article got the decay modes for Radium-226 and Radium-228 backwards. Radium-226 really emits 4.8 MeV alpha particles, and Radium-228 really emits 46 KeV beta particles, not the other way around. The half-lives listed in the article are accurate, though.)