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The Magic Cafe Forum Index » » Not very magical, still... » » Laser travels faster than speed of light! (0 Likes) Printer Friendly Version

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Jonathan Townsend
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Okay, that's likely good enough to estimate the time it would take an impulse to get to Jupiter.
When you push, pull, tug or snap a solid incompressable object (like a string) the signal moves at the speed of sound in that medium.
...to all the coins I've dropped here
gdw
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Jonathan, I can see how plucking it would only travel at the speed of sound, but if it did not stretch, and it was pulled, or if it was solid and did not compress then if pushed,

Actually, nevermind, it just clarified in my mind how it would still be the speed of sound, though you couldn't have a material that truly does not compress.
It's amazing, people will criticize you for "biting the hand that feeds you," while they're busy praising the hand that beats them.

"You may say I'm a dreamer, but I'm not the only one."

I won't forget you Robert.
tommy
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Smile
If there is a single truth about Magic, it is that nothing on earth so efficiently evades it.

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MobilityBundle
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Quote:
On 2011-08-22 19:54, pegasus wrote:
If you have a very powerful telescope, you can see the back of you looking out, i/e the universe is curved around on itself. This is caused by the even distribution of matter and dark energy which warps spacetime.


Just by way of clarification, being able to "see the back of your head" (technically, the term is a "closed universe") is a theoretical possibility, but it is far from an established fact. Moreover, even if the universe were closed, it's not significantly curved. Except for spots here and there (like near stars, black holes, or other heavy objects), the curvature of spacetime is essentially zero.
mastermindreader
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What's a tort string? A series of accident cases in civil court?
MobilityBundle
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Jonathan, you're right insofar as the conclusion that special relativity can't be right while a perfectly rigid object exists. (In fact, even "perfectly rigid" may not mean what you think it means... more on that in a bit.)

But, given the choice between these two things, I'd go with relativity. We have ample experimental evidence of special relativity, but have no good reason to suspect a perfectly rigid object exists. I mean, the idea is nice because it's simple. And surely there's some stuff that comes pretty darn [can't say dam?] close to being perfectly rigid. But still, give me relativity if we're picking.

In fact, there's a kind of perfect rigidity WITHIN special relativity. It's the same idea as what I'd call "naive" or "Newtonian" rigidity. (Let's say an object is naively rigid if it can't be deformed... so it's length, width, etc. can't change, no matter how much force is applied.)

The reason that's naive is, as we know from special relativity, ideas like length and width aren't actually physical ideas... the length of something depends on what reference frame you measure it in. Fortunately, there's a special-relativistic analogue of length called the spacetime interval.

So using that analogue, you can define a rigid object within special relativity to be one where the spacetime interval between any two of the object's points is constant. But it works kind of how you'd think: the speed of sound through such an object is the speed of light, if you tugged on one end, the "tug" would propagate through the object at light speed, etc.
critter
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Big deal. Somewhere out there is a magic blue box that's bigger on the inside. Now that's cool.
"The fool is one who doesn't know what you have just found out."
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acesover
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Quote:
On 2011-08-20 23:10, gdw wrote:
Aces, basically what he is saying is, imagine you point a laser, and then spin in a circle. The further along the beam, away from yourself, the faster it appears to be moving in a circle. The same way the out side of a spinning tire is moving much faster than a point much closer to the center.

So, assuming the laser theoretically didn't disperse at all, and you could see it at the end of the universe, and say it took you 1 second to complete your spun in a circle, then the end of the laser at the end of the universe would circle the entire universe in 1 second.


I really feel that theory is flawed. For if when the laser dot is emitted and I am at the end of the universe and lets say the end on the universe is only 10 light years in length I will not see the laser dot for ten years.
If I were to agree with you. Then we would both be wrong. As of Apr 5, 2015 10:26 pm I have 880 posts. Used to have over 1,000
MobilityBundle
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I really feel that theory is flawed. For if when the laser dot is emitted and I am at the end of the universe and lets say the end on the universe is only 10 light years in length I will not see the laser dot for ten years.


That's correct, Acesover, but it's not inconsistent with what I said.

Here's an image of a spiral, which is what the wavefront of a light beam looks like when it comes from my hypothetical laser points spun three times in a circle instead of just one time:

http://upload.wikimedia.org/wikipedia/co......iral.svg

Recall, in my hypothetical example, it takes the laser pointer 1 second to make a complete revolution. Consequently, no matter where you see the laser pointer from -- whether you're 10 light years away or 10 feet away -- you'll see them beam, then see it disappear, then see the beam again in one second.

In other words, the "gap" between turns of the spiral is equal to 1 light-second. This is true no matter how far out the spiral travels... the gap is always equal to 1 light-second.

To be sure, I'm not talking about the time it took for the light to first reach you. If you're 10 light years away, then you're right... it will take 10 years for the first red dot to reach you, but it will take only 1 more second for the second one to reach you.
acesover
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On 2011-08-23 01:20, MobilityBundle wrote:
Quote:
I really feel that theory is flawed. For if when the laser dot is emitted and I am at the end of the universe and lets say the end on the universe is only 10 light years in length I will not see the laser dot for ten years.


That's correct, Acesover, but it's not inconsistent with what I said.

Here's an image of a spiral, which is what the wavefront of a light beam looks like when it comes from my hypothetical laser points spun three times in a circle instead of just one time:

http://upload.wikimedia.org/wikipedia/co......iral.svg

Recall, in my hypothetical example, it takes the laser pointer 1 second to make a complete revolution. Consequently, no matter where you see the laser pointer from -- whether you're 10 light years away or 10 feet away -- you'll see them beam, then see it disappear, then see the beam again in one second.

In other words, the "gap" between turns of the spiral is equal to 1 light-second. This is true no matter how far out the spiral travels... the gap is always equal to 1 light-second.

To be sure, I'm not talking about the time it took for the light to first reach you. If you're 10 light years away, then you're right... it will take 10 years for the first red dot to reach you, but it will take only 1 more second for the second one to reach you.


Yes that is true but what you are seeing is nothing more than a blinking light not a light that traveled around the universe. Your light source is actually stationary and has not moved at all much less moved around the universe.
If I were to agree with you. Then we would both be wrong. As of Apr 5, 2015 10:26 pm I have 880 posts. Used to have over 1,000
MobilityBundle
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Yes that is true but what you are seeing is nothing more than a blinking light not a light that traveled around the universe. Your light source is actually stationary and has not moved at all much less moved around the universe.


Also true. But recall, my claim isn't that the light itself is going faster than light. My claim is that the "little red dot" -- the intersection point of the beam from the laser with the ground -- is traveling faster than light. In other words, you can be watching the ground one day, and this mysterious little red dot shows up (from a source far, far away). Then the little red dot moves around for a while. And then the little red dot starts moving around faster than light. All without violating physics.
gdw
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Yes, aces, you are correct, and that was part of what he was attempting to demonstrate.
It's amazing, people will criticize you for "biting the hand that feeds you," while they're busy praising the hand that beats them.

"You may say I'm a dreamer, but I'm not the only one."

I won't forget you Robert.
Michael Daniels
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Quote:
On 2011-08-23 00:00, mastermindreader wrote:
What's a tort string? A series of accident cases in civil court?


LOL Smile

Mike
tommy
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Ye knows it is only the nose that matters don't ye?
If there is a single truth about Magic, it is that nothing on earth so efficiently evades it.

Tommy
Jonathan Townsend
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Quote:
On 2011-08-22 20:43, Jonathan Townsend wrote:
...
When you push, pull, tug or snap a solid incompressable object (like a string) the signal moves at the speed of sound in that medium.


Maybe I should have written that "an impulse like a push, tug or pluck on a string, moves at the speed of sound in that object."
No relatives impugned so no torts served. Smile
...to all the coins I've dropped here
stoneunhinged
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Googling "tort string" puts a link to this thread up on the first page of results. The Magic Café will soon be invaded by physicists world wide. Then they will figure out all our secrets and expose them on their blogs. And then "magic" will be gone.

It's a sad, sad day. Smile




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pegasus
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Tachyons travel faster than the speed of light. In fact, if you used tachyons as bullets, in a gun, you would see them hit the target before you pulled the trigger.
landmark
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Sounds useful. S2000, is DARPA on this?
acesover
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OK. I am staring at the ground and suddenly a little red dot appears that was generated 10 years ago. Now this dot begins to move in a circle that has a radius of 1 mile. Wow do you have any idea how infintesible a movemenet this has to be from the original source of the laser? I do not know if it can be caculated because of the minute angle involved. I cannot even imagine how small a movement would have to be to move the laser dot around the circle. Remember we are in more than one dimension and not on a sheet of paper from poiint A to Point B. Point A being the light source and Point B, the ground from which the light is reflected, would not be on the paper. So a movement of say 1 inch with a straight line extending out for 10 light years would have to be just about incalcuable (by me anyway).

The dot you first saw is not the dot that traveled around the circle. Take a hose and spray it on a wall creating a circle. The place where the water first wets the wall is not the same water that hits the wall at a later time. Just as the dot you first see on the ground is not the same dot you see later...both are from a continuous source. In the case of the dot whenever you see it, it is 10 years old. When you see it one second later it is not the same dot that is ten years old and one second. What you are seeing is the laser that was generated 10 years ago and just reached you now.

This is definitely interesting.
If I were to agree with you. Then we would both be wrong. As of Apr 5, 2015 10:26 pm I have 880 posts. Used to have over 1,000
MobilityBundle
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Acesover, again, I think I agree with everything you're saying. To add a bit of clarity, you say that the "dot you first see on the ground is not the same dot you see later." In fact, I claim that more is true: that dot is not really a physical thing, at least in the sense that its location is constrained by physics.

To be sure, as you rightly note, that dot is not an object. It's not like I cut a red dot out of a piece of paper and put it on the ground. Your hose analogy is completely on point... except instead of water, the hose (laser) is spouting light particles. Physically, the red dot is just the point where those light particles are "splashing" against the floor. Mathematically, the red dot -- or at least the location of the red dot (an important distinction) -- is the intersection of the line coming from the laser and the floor.

Great, so the location of the dot is just some mathematical intersection of two other things. The thing themselves are constrained by physics. After all, light can't go faster than light, nor can the floor. But the intersection of two things isn't a physical object, and it's not limited by relativity. So the intersection point can zip around all you want... faster than light, what have you.

Separately, yeah, controlling the laser pointer so as to meaningfully control the red dot 10 light years away is a nearly unfathomable engineering problem. I haven't calculated it, but even thermal vibrations are probably significant sources of error at that distance. But you don't need to go that far out. The moon, for example, is a little more than one light-second away from Earth, on average. Even at that distance, the effects are significant.
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