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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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Magnus Eisengrim
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Quote:
On 2011-08-20 12:54, Michael Daniels wrote:
Quote:
On 2011-08-20 12:17, Magnus Eisengrim wrote:
You have an argument to back that up?


The dot is produced by photons travelling to the moon at the speed of light. If the beam moves sideways, each photon is still reaching its destination by having moved from the source at the speed of light. It can't move any faster. It is misleading to say that there is an actual dot moving sideways - the sideways movement is an illusion caused by persistence of vision - just as we perceive the separate frames of a movie to be a continuous moving image. Actually the "dot" itself is also just an illusion created by the brain.

Mike


In that case you aren't disagreeing with my statement at all.

John
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.--Yeats
Michael Daniels
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OK John, you had 'move' in quotation marks. Yes the movement is purely illusory, as is the dot. We must also realise that the photons also have to return to the earth from the moon (at the speed of light) in order that they can be registered by the retina and permit the brain to subsequently interpret (perceive) the sequence of photons as a dot.

There is never any actual dot on the moon, and never any real movement of the dot. Because the dot doesn't actually exist or move, it cannot move faster than the speed of light - that was what I was disagreeing about.

Mike
gdw
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I was thinking that actually. Coming from a film and animation back ground, each new "location" of the "dot" would only be updated at the rate at which the photons arrived.

You may as well point two lasers at two different spots on the moon, and turn one off at the same time you turn the other on, and the "dot" would "move" "instantaneously" between the two spots.

Too many " " s. I need something more literal.
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 2011-08-20 13:32, gdw wrote:
I was thinking that actually. Coming from a film and animation back ground, each new "location" of the "dot" would only be updated at the rate at which the photons arrived.

You may as well point two lasers at two different spots on the moon, and turn one off at the same time you turn the other on, and the "dot" would "move" "instantaneously" between the two spots.


Yes, good point. It's an illusion based on what psychologists call the Phi Phenomenon.

http://en.wikipedia.org/wiki/Phi_phenomenon

Mike
MobilityBundle
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To clear up some confusions about the shadow, dot from a laser point, and intersection point of the scissors examples:

It's all the same phenomenon. In fact, it's also the same phenomenon as the "group velocity" concept. There is a "thing" (whether it's a shadow, a dot from a laser pointer, or a wavefront) that exists mathematically, but in some sense isn't a physical thing. That thing can move faster than light, precisely because it's in some vague sense not physical. (And in a more precise sense, because one can't use this "thing" to transmit information.)

----

So, quick proof that the dot of a laser pointer can move faster than light:

Take a laser pointer and, while shining, rotate it 360 degrees in 1 second. (Certainly, that doesn't require any faster-than-light travel.) What does the wavefront of the light beam look like from t=0 to t=1? It's a spiral -- a smooth, continuous curve -- that is propagating radially outward. Moreover, the "gap" in the spiral is equal to 1 light-second (i.e., about 186,000 miles, if you like those units better.) By "gap," I mean the distance from one turn on the spiral to another... but we're just dealing with a single turn.

Now consider some spheres centered on the laser pointer, with some different radii. No matter the size of the radius of a given sphere, the dot from the laser pointer will take 1 second to travel around a circular arc of that sphere. That's because the location of the laser pointer "dot" on the sphere is just the intersection of the sphere and the spiral-shaped-wavefront, and the wavefront itself has a gap of 1 light-second. So the intersection of the spiral and the sphere necessarily returns to the same point (on the sphere) after one second.

Okay, so if the dot takes 1 second to complete it's journey around a sphere, REGARDLESS of the size of the sphere, just pick a sphere as large as you like, and you end up with a dot moving as fast as you like. For example, take a sphere with a 1 light-year circumference: it takes the dot a mere 1 second to "travel" a whole light-year. That's roughly 31 million times faster than light.


---

(I know that might seem like a long-winded explanation, but hopefully it's a bit more persuasive -- if less concise -- than "ummm... yes.")

But, to be sure, this scenario does not involve a single photon traveling faster than light. The spiral-shaped wavefront propagates radially outward at exactly light speed. THAT propagation speed is the analogue of the "phase velocity."

--

So, to be sure, why is this not a paradox? Because you can't use this setup to communicate that fast. Suppose you live on the north pole of a sphere with a 1 light-year diameter and I live on the south pole. According to relativity, you can't send me a message in less than one year. It might seem tantalizing to think that you can send me a message in seconds by arranging for a coded "laser pointer dot" to go from you to me.

But in order to arrange that, you FIRST have to communicate to the guy working the laser point at the center of the sphere... and that takes half a year, minimum. And then, the guy at the center of the sphere can make whatever wavefront he wants, but it only propagates outward at light speed, so it will take another half a year to reach me at the south pole.

In other words, communication is limited to the phase velocity of the wave.
acesover
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Quote:
On 2011-08-20 17:42, MobilityBundle wrote:
To clear up some confusions about the shadow, dot from a laser point, and intersection point of the scissors examples:

It's all the same phenomenon. In fact, it's also the same phenomenon as the "group velocity" concept. There is a "thing" (whether it's a shadow, a dot from a laser pointer, or a wavefront) that exists mathematically, but in some sense isn't a physical thing. That thing can move faster than light, precisely because it's in some vague sense not physical. (And in a more precise sense, because one can't use this "thing" to transmit information.)

----

So, quick proof that the dot of a laser pointer can move faster than light:

Take a laser pointer and, while shining, rotate it 360 degrees in 1 second. (Certainly, that doesn't require any faster-than-light travel.) What does the wavefront of the light beam look like from t=0 to t=1? It's a spiral -- a smooth, continuous curve -- that is propagating radially outward. Moreover, the "gap" in the spiral is equal to 1 light-second (i.e., about 186,000 miles, if you like those units better.) By "gap," I mean the distance from one turn on the spiral to another... but we're just dealing with a single turn.

Now consider some spheres centered on the laser pointer, with some different radii. No matter the size of the radius of a given sphere, the dot from the laser pointer will take 1 second to travel around a circular arc of that sphere. That's because the location of the laser pointer "dot" on the sphere is just the intersection of the sphere and the spiral-shaped-wavefront, and the wavefront itself has a gap of 1 light-second. So the intersection of the spiral and the sphere necessarily returns to the same point (on the sphere) after one second.

Okay, so if the dot takes 1 second to complete it's journey around a sphere, REGARDLESS of the size of the sphere, just pick a sphere as large as you like, and you end up with a dot moving as fast as you like. For example, take a sphere with a 1 light-year circumference: it takes the dot a mere 1 second to "travel" a whole light-year. That's roughly 31 million times faster than light.


---

(I know that might seem like a long-winded explanation, but hopefully it's a bit more persuasive -- if less concise -- than "ummm... yes.")

But, to be sure, this scenario does not involve a single photon traveling faster than light. The spiral-shaped wavefront propagates radially outward at exactly light speed. THAT propagation speed is the analogue of the "phase velocity."

--

So, to be sure, why is this not a paradox? Because you can't use this setup to communicate that fast. Suppose you live on the north pole of a sphere with a 1 light-year diameter and I live on the south pole. According to relativity, you can't send me a message in less than one year. It might seem tantalizing to think that you can send me a message in seconds by arranging for a coded "laser pointer dot" to go from you to me.

But in order to arrange that, you FIRST have to communicate to the guy working the laser point at the center of the sphere... and that takes half a year, minimum. And then, the guy at the center of the sphere can make whatever wavefront he wants, but it only propagates outward at light speed, so it will take another half a year to reach me at the south pole.

In other words, communication is limited to the phase velocity of the wave.


I am not sure I follow what you mean. I feel that it all depends as to where the source of the laser is located and where you viewing it or am I missing something?

I definitely admit that much of this is beyond me. But isn't it like looking at the stars? When we see them that is not where they really are because we are seeing light that has been traveling several years so by the time it reaches us the source (star) is in a diferent location and we are only seeing the light not the star. In fact isn't it possible that we are seeing what we think is a star and what we are really seeing is the light from a star that could have burned out long ago?
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
Jonathan Townsend
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How fast does an electron spin? Smile
...to all the coins I've dropped here
gdw
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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.
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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Does the universe have an end?
If there is a single truth about Magic, it is that nothing on earth so efficiently evades it.

Tommy
S2000magician
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Quote:
On 2011-08-20 23:17, tommy wrote:
Does the universe have an end?

The end of the universe is "e".
Magnus Eisengrim
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Quote:
On 2011-08-20 23:17, tommy wrote:
Does the universe have an end?

I hear that you can see it from Devil's Island.
The blood-dimmed tide is loosed, and everywhere
The ceremony of innocence is drowned;
The best lack all conviction, while the worst
Are full of passionate intensity.--Yeats
critter
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The universe has at least a few billion ends. Most with heads jammed up them.
"The fool is one who doesn't know what you have just found out."
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Pakar Ilusi
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Quote:
On 2011-08-21 02:05, critter wrote:
The universe has at least a few billion ends. Most with heads jammed up them.


Smile

Smile

Smile
"Dreams aren't a matter of Chance but a matter of Choice." -DC-
Michael Daniels
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Quote:
On 2011-08-20 17:42, MobilityBundle wrote:
<snip>


As far as I understand these concepts of group velocity and phase velocity it seems that although, in purely theoretical and mathematical terms, faster than light speeds can be conceived, this isn't possible in any physical sense. In no circumstances can information, energy, or anything having mass, be transmitted faster than light.

Mike
magicfish
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Quote:
On 2011-08-20 23:17, tommy wrote:
Does the universe have an end?

Yes, according to Stephen Hawking.
Pakar Ilusi
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I thought the Universe's end was Uranus? Smile
"Dreams aren't a matter of Chance but a matter of Choice." -DC-
pegasus
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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.
pegasus
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Take a long length of tort string that stretches from earth to Jupiter and a person at each end. Now, if one end is tugged then surely the person on the other end will feel this immediately, thus breaking the light barrier.
Jonathan Townsend
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What's the speed of sound in a tort string?
...to all the coins I've dropped here
pegasus
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Not sure but in fresh water its about 1,482 metres/sec
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