Forgive me if this has been covered in one of the numerous earlier threads, but I don’t generally read them since I don’t think it’s possible (hey, I trust the experts on this one).
Anyway, here’s the “model:”
If you were in a big, big circular room–oh, let’s say about 70,000 miles across, and you had powerful enough flashlight, there would be a spot on the wall 35000 miles away.
You could easily spin the flashlight in the middle of the room in one second, in which time the spot on the wall has traveled over 200,000 miles!
Exactly. Nothing can go faster than the speed of light and NO THING is going faster than the speed of light in this scenario. The light is going outward at the speed of light. The only thing that is changing FTL is the location of where the light is striking, and there is no problem with that.
The light will always take the same amount of time to reach the wall and reflect back to you, but as your rate of spin increases, the spot will begin to ‘lag’ behind where you’d see it it, if you were not rotating (assuming that your eyes and the flashlight were always alligned). Rotate fast enough, and by the time the light reflects back to you, the beam will be out of your peripheral vision, apparantly having ‘drifted’ out of sight off to one side. If you rotate really fast, the beam should actually ‘drift’ back into view on the other side of your vision.
Imagine it as a wall ten feet away and you are spraying it with a garden hose; it’s possible for you to make a wet patch on the wall and move the hose so that the wetness encroaches dry parts of the wall at a greater rate than the outward flow from the hose, but no individual water molecule is travelling faster than it normally would and there is certainly no water moving along the fence.
What you would see in the example mentioned in the OP… the spot on the wall wouldn’t appear until about 1/5 of a second after the flashlight is turned on, and any subsequent movement of the beam on the wall would be similarly delayed.
Now, try this same trick with an incredibly strong metal pole, 1,000,000 miles long. As you spin it at one end, you’ll notice that it’ll begin to bend as the far end lags behind the near end. Eventually, as you keep spinning, you’ll wind up with a metal pole that’s shaped like a swirl… but when you stop spinning, the pole will catch up with itself and straighten out (ignoring inefficiencies in the pole itself that would make it remain slightly bent).
Make a ring of flashlights and turn them on in sequence. The spot can move as fast as you like – even infinitely fast by turning them all on at once. What moves faster than light is not a thing and cannot be used to transmit information faster than light. There are several examples of un-real things that move faster than light, but none involve real things and none can be used to transmit information faster than light.
I’ll refer you to a book that is dedicated to this topic. It’s called “Faster than Light: Superluminal loopholes in physics” by Dr. Nick Herbert. It contains several FTL propositions along the lines of the flashlight trick. Basically the resolution of this apparent paradox is that relativity doesn’t say that nothing can travel FTL, it says that no causal influence can travel FTL. The implications are kinda subtle, and I doubt my ability to do them justice in a few words. Read the book.
I’m not sure if this is a hijack…apologies if it is.
If not the flashlight, then could quantum entanglement be used to transmit information faster than light? e.g. I entangle two particles. I keep one and send the other to you, a lightyear away. When I want to “tell” you something, I cause my particle’s wave function to collapse. Can you learn of this instantly by examining your particle? I’ve read about the Bell Inequality and the EPR experiment, but either I didn’t understand it fully or there is no consensus about whether this would work - the word “spooky” seems to get used a lot.
To answer your question DarrenS, no, you can’t send information this way. In order to tell if your particle’s wavefunction has collapsed or not, you have to look at it, and if it hasn’t already collapsed, it will at that point. But how are you to know when the collapse occurred?
Similarly, if I collapse my particle’s wavefunction to a particular value in some way, even though this will also collapse the wavefunction of your particle, no information is transmitted because even though you measure, say, spin up, you have no way of knowing whether you got this due to random chance or because the wavefunction had already collapsed to spin up at this point.
What if we set up a specific time that you will collapse your particle’s wavefunction? Say I’m a light year away and I don’t want to wait a year to find out who won the Stanley Cup. The playoffs will end no later than June 17th, so on the 18th you collapse your particle (up for Wings, down for 'Canes, or whatever) and on the 19th I check my particle.
Or, I could take 20 particles instead of 1, and your job is to collapse 20 wavefunctions. If I look at my 20 particles and they’re not all the same, then I know you haven’t yet collapsed yours. If they are all the same, I have good confidence that this has not happened by chance.
Hmm… that one I’ll have to think about a bit. Offhand, I’d say in the second example that yes, if you find 20 independent and distinct particles all doing the same thing, you could have good confidence that this isn’t random, but of course you have no way of KNOWING that this isn’t random. You’ve also probably got to figure out a way of dealing with the fact that your 20 particles will interact and probably be impossible to tell apart, which might cause problems. On the other hand, if you could overcome these issues, why not make it 1000 particles? 2[sup]1000[/sup] is a big number, and if all of them are spin up, this would be virtually impossible to have happen by chance.
So both of these are good questions, and I’ll have to give it some thought to find the catch, because I’m pretty sure there ought to be one somewhere.
Correct me if I’m wrong, but I believe that you cannot “collapse your particle” in a certain way. I.e., you cannot collapse it to “down” to make the other party’s particle down as well. When you collapse your particle by viewing it, it collapses in a random way to up or down. There’s no choosing which you will see. So yes, the other party’s particle(s) will also collapse, but their’s will be random, too. So you won’t be sending 'em any information other than that their particles have collapsed, and they won’t even be able to tell if it’s because they were collapsed by you, or because they are looking at them themselves.
Oh, yes, of course. I’m losing my mind. Chalk that one up to blithering idiocy, if you please. bryanmcc is of course correct. I can measure, say, S[sub]z[/sub], but I can’t force the result to be S[sub]z[/sub] = +1/2. I could maybe do some fancy spin control tricks, but that would screw up the entire system.
You also can’t look at your particles to see if they have collapsed because of observation at the other end; looking at them at your end collapses them.
More or less irellevant. In principle, relativity doesn’t say nothing can go faster than c, only that you can’t accelerate anything past c. By the same token, there’s no way to rule out tachyons that I can think of off the top of my head (disclaimer: I am not an expert), but since they can’t be decellerated below c, which they’d have to do to interact with normal matter, the question is rather moot. Sure, maybe they’re out there and maybe they’re not, but since we can’t hope to detect them and they won’t change physics, why assume they exist?
You’re assuming that if they exist, then we can’t hope to detect them. If they do exist, I bet we’ll be able to detect them in time. Probably not in your lifetime or mine though.
Yes, I’m assuming we won’t be able to detect them, if they exist. This is because, as I understand it, either relativity would have to be wrong or else there’d have to be entirely new physical implications. Naturally, I can’t rule either of these scenarios out (lord, if there’s anything to learn about the history of science, it’s that!), but I agree, it’s not worth waiting around for.
Partially, by the way, the reason I assume that we won’t be able to detect them, ever, is that if they do have the ability to affect normal matter, our notions of causality rather go out the window (the entire relativity of simultaneity deal means that you could receive a tachyon before I sent one to you, with attendant causal paradoxes), and we’re rather strongly wedded to these notions. Which is not to say that it’s impossible that they’re wrong, just that it would shock me that we’ve done so well with them if they are. Still, I concede the point.
