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      CommentAuthorJon Wake
    • CommentTimeJul 26th 2011
    Okay, Relative speed, here I go.

    So here's the deal: Nothing can go faster than the speed of light, not even light. Imagine you're in a car going a hundred km/hr, and you throw a baseball out of the car at 50 km/h. Relative to the car, the baseball is moving at 50 km/h. Relative to the Earth, the baseball is moving at 150 km/h. It's all good here.

    Now lets say that you're in the same car, and you shine a laser at a distant object. (Lets also assume a perfect vacuum for the sake of simplicity.) The laser is moving at around 300,000 km/s relative to the car. However, to an observer on the ground, the laser is moving the exact same speed, completely ignoring the additive speed of the car.

    Any honest physicist will tell you they have no idea why this is true. It's been tested and tested again, so it is, but the reasons for it are a mystery, just like what mass is, or why particles sometimes behave like waves.
  1.  (10075.22)
    And thank you, Jon Wake.

    • CommentTimeJul 27th 2011 edited
    You know how if sharks* stop swimming, they die? Photons are basically SHARKS MADE OF LIGHT. True story.

    Point is (I mean, besides the obvious LIGHT SHARKS -- and, really, I won't be upset if that's your takeaway here): Light is moving, or it's not light. The cause of that moving is energy, and a side-effect of that moving is mass (physicists in the room will forgive this simplification because, if you can't tell from my LIGHT SHARKS metaphor, I'm using Fake Math. I do that. Stay with me.).

    So. E=mc^2: Energy equals mass times 299,792,458 metres per second squared.

    Your eyes may have just glazed over a bit there. That's totally fair. Because that 299,792,458 metres per second is actually totally recursive; a metre is based on how fast light travels in 1/299,792,458 of a second. OH YES. Because we made up all the numbers.

    The truth is, you can call the speed of light "Bob, the fastest shark in the world!" and as long as "Bob" means "how fast light goes in a vacuum" you're good.

    So. energy equals SHARKS times "Bob (how fast light goes)" squared.

    If you're allergic to math, you can go ahead and toss out that squared. I mean, really, we'll just call whatever we're left with, "but MORE" right? And we've established that Bob is entirely ridiculous and arbitrary and only exists to give us a nice constant number, so let's toss him out, too.

    And we're left with ENERGY and SHARKS.

    These are the two most important things in the faster-than-light discussion. Energy is really straightforward. Even if you can't wrap your brain around the math, you can get that it takes some amount of energy for a shark to move through the water. But then you say "okay, well, sharks have a max speed that's a lot slower than, say, launching a shark out of a cannon -- why don't we just do that?"

    And you know what? You can. You're going to have a dead shark at the end, but you absolutely can. There are also things that can move faster than Bob. No lie -- there's plenty of things that were (or could be, probably) in one spot and then they were in another spot and the time that passed was way shorter than our arbitrary 299,792,458 metres per second**. But no information moved that quickly. Essentially, we were tossing dead sharks. When we talk about faster-than-light travel, what we're really talking about is getting LIVE sharks from point A to point B.

    And sharks can only move as fast as Bob. If we shove more energy into them or fuss about with that constant, we lose the ability to carry information (real sharks go brain dead, and light no longer has wave modulation).

    Faster-than-light is not an impossibility. We're pretty sure of that. If it were, then we'd call "light speed" INFINITE SPEED! Faster-than-light-with-a-payload-intact is what we're really talking about. And that remains questionable.

    * You know what? There's some small chance that someone is going to take this beautiful metaphor and try to ruin it by being that guy that says there are some sharks that can, in fact, hold still and get oxygen into their systems by opening and closing their mouths. You know what you are? No fun. And not invited to this party. And, anyway, even most of the mouth breathers still have to keep swimming because of their low blood pressure.

    ** Sort of. It's also accurate, in some cases, to say it's not moving at all, because it's in more than one place at once. Sort of. It gets ugly and math-y. We're just gonna go with what we've got. As an aside, shadows are my favorite example of this. Shadows are, in physics terms, the absence of something -- nothing. And they can move faster than light, no lie, even though you can't tell until the light catches up, moving at the slowpoke speed of Bob. But that's why I will nod but smile when folks intone "nothing can move faster than light."
    • CommentAuthorFlabyo
    • CommentTimeJul 27th 2011
    @Ariana - that's a lot clearer than any of my teachers managed at college, hehe.
    • CommentAuthorroadscum
    • CommentTimeJul 27th 2011

    Roadscum's poor little head has just burst. Again. Someone get a mop and bucket please.
      CommentAuthorAlan Tyson
    • CommentTimeJul 27th 2011
    Ariana, where were you in high school science when I needed you?

    Actually, could you (or anyone else who wants to pick up the torch [heh. get it? 'cuz we're talking about light?]) expand on what exactly "information" means, from a physics standpoint?
    • CommentTimeJul 27th 2011 edited
    > Ariana, where were you in high school science when I needed you?

    Very likely talking my way out of homework with really good metaphors. And robots. And, one time, a robot metaphor. True story.

    But, information: At the very, very, very simplest, "information" is ones and zeros. Those ones and zeros can be "A light turns on, a light turns off." They can be morse code, once it starts getting a little more complicated (duration is added in). They can be change in pitch or weight or temperature (add one, subtract one). Even transfer of mass and energy eventually can break down to those ones and zeros, when the zero is "static" and the one is "change".

    The *very* fastest we can send information, right now, is turning a light on and off in a vacuum. At 299,792,458 metres per second we can flip a light on for yes, and off for no. If we can break that barrier -- if we can find something that we can observe that moves faster than that "speed of light" -- then we have the first building block for faster-than-light travel.

    As it stands, the only things that can "move" faster than the speed of light can't be perceived until the light catches up. (Or are theoretical particles that we don't know how to look for, yet.) Like being blind in a thunderstorm: if you can't see the lightning flash, you need to rely on the sound of the thunder to get the information that lightning has occurred. At this point, in physics, we're theorizing that there's something like the lightning in that metaphor, something that can move faster than we are able to observe, but all we know for certain is the sound of the thunder.
    • CommentAuthorOddcult
    • CommentTimeJul 28th 2011
    Can't we yet send information via quantum entanglement? Would that be instantaneous? Obviously, on Earth there wouldn't be much point, but even for example, controlling a robot on Mars would benefit from the faster than light applications of communication or signalling.
  2.  (10075.29)
    @Oddcult - You are a troublemaker. From what I understand, at this point quantum entanglement is a one shot deal. The information gets sent and there is no more entanglement. And even that might be theoretical. My physics knowledge doesn't seem to be as good as Warren or Ariana's (she totally won science as far as I'm concerned), so I'll let them or someone else field that more. Or perhaps it's just a matter of time until the Chinese government explains for us how it isn't possible and we are all silly people for suggesting such things. Perhaps we should get back to enjoying their finely produced products.
    • CommentTimeJul 28th 2011
    @Oddcult, smellmeyoursoul:
    Short answer, no, you can't "send" classical information through quantum entanglement. I have no idea how that idea started floating around. You can "send" quantum information, but it can't be deciphered until some classical information has arrived by the conventional means. Your light cone remains your light cone.

    Ariana and Warren have done an excellent job explaining why ftl is impossible as far as photons go.

    But back to the original article, while the paper may be valid (I haven't read it), the conclusion that keeps being discussed is not. "Time travel is impossible" does not follow from "photons can't go ftl". Faster than light photons were a potential means. That means is gone, maybe. But the fact that I can't use a hammer to cook dinner does not mean I must eat it raw. We just need to discover fire. Or maybe not?
    • CommentAuthor256
    • CommentTimeJul 28th 2011
    Ok, going to have a stab at explaining some things. Physics BSc, don't fail me now...

    Why photons move at c:

    Imagine a ball bearing rolling on a glass table. Because there's very little friction, if you give the ball bearing a push, it will roll for a very long time before it slows to a stop.

    If there was NO friction, you could give the ball bearing a push and it would roll forever, without ever slowing down at all.

    But, even without friction, the ball bearing needs a push before it will move. This is because it has mass, and mass doesn't accelerate without a force being applied to it.

    Photons, however, do not have a mass. So, even without a force being applied to them, they race off at top speed.

    Two really curious things:
    1) That there actually IS a top speed in the universe - which we call c. Why this limit exists... we don't know yet. Call it a property of the universe.
    2) That c, the constant with that value of the fastest speed possible, turns up all over the place in physics. Sometimes in things that have nothing to do with light or photons. c is really a property of the universe in general, rather than just light in particular.

    Next up: Why nothing with mass can travel at c

    (posting this bit now before I give up completely).
    • CommentAuthor256
    • CommentTimeJul 28th 2011 edited
    Why nothing with mass can travel at c:

    The problem with accelerating a mass to c is that for objects moving at really, really high speeds, the universe works differently than it does for objects at everyday speeds.

    Essentially, we find that there are a lot of things which seem linear and normal at everyday speeds, that stop being so at high speeds (when we get up to big fractions of c). This is because those equations include the Lorentz factor, gamma, the value of which depends on the speed of the mass:

    Notice that gamma is very close to 1 for all the speeds you ever see in everyday life, which is why it has no effect there. But as the speed gets close to c, gamma shoots upwards. And, as speed tends to c, gamma tends to infinity.

    This matters because the amount of energy you need to accelerate mass is multiplied by gamma. So to actually get to c, you would need an infinite amount of energy. Since you can't actually have an infinite amount of energy, speed = c is impossible.

    Don't know if this makes any more sense than the previous shark-based explanations.

    Note: If you're interested, the Lorentz factor is:
    gamma = 1/ (sqrt(1-(v2/c2))). Physics is fun!

    Edit: Apparently the greek character for gamma doesn't show up. Bastards.
    • CommentTimeJul 28th 2011
    Am I the only one who, whenever time-travel is suggested, becomes terribly worried that someone is going to go to a time before I was born and muck up me being born?

    I am perfectly alright with no time-travel.
      CommentAuthorAlan Tyson
    • CommentTimeJul 28th 2011
    See, but if they had, then you wouldn't be here typing that, DJ-

    *head 'splodes*
    • CommentAuthorOddcult
    • CommentTimeJul 29th 2011
    1) That there actually IS a top speed in the universe - which we call c. Why this limit exists... we don't know yet. Call it a property of the universe.

    So, okay, the answer to my first question is really 'mmmrrruummmidunno...shrug..'? Fair enough. I'll continue to find it amazing that a crappy AA battery can propel something to the speed of light, but anything more than that needs every star ever and then some and not worry about it too much then.

    With regards to quantum entanglement, then if you link up lots of tangly things, and then they're either present in certain positions or not present, once they're unentangled, you've got the basis of a binary communications system? Instead of changing them, you destroy them in pattern? Is that feasible?

    It would pretty much be complicated ticker tape, but workable in theory, if not yet in practice, maybe?
  3.  (10075.36)
    Since this seems to be turning into a general physics/relativity free for all, here's a no doubt pretty stupid one that's been nagging at me for years.

    E=mc^2. You plug the numbers into it, and get a result, BUT, depending on the units you use (km per hour vs cm per hour for instance), you'd get a different result out of it.

    Now I can buy the fact that you use a standard set of units, and I can see how that works in that the units are defined in terms of each other. Units of Energy for instance are defined as the amount needed to move a standard Mass over a standard Distance. What I can't get my head around is where Time comes in. C is a speed, and I can see how the units for Energy and Mass are related, and how Distance plugs into them, but where does the Time necessary to define Speed come from? Or is it just there as a property of the universe?

    Or am I just typing gibberish? :)
    • CommentTimeJul 29th 2011
    metres and seconds are the units you're looking for
  4.  (10075.38)
    Yeah, I get that - what I'm stabbing at is how do Seconds relate to Joules?

    And on actually doing some goddamn research I realise that a Joule can be based on Newtons and Metres (which I knew) or Amperes, Ohms and Seconds. Now I get it! :)
  5.  (10075.39)
    Oh, China didn't actually ban time travel stories. That was a mistranslation.
    • CommentTimeJul 29th 2011
    Oh sure - that's what that article says *now*, after they chrono-retconned it.