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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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Pakar Ilusi
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"Dreams aren't a matter of Chance but a matter of Choice." -DC-
balducci
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I think that was debunked a while back.
Make America Great Again! - Trump in 2020 ... "We're a capitalistic society. I go into business, I don't make it, I go bankrupt. They're not going to bail me out. I've been on welfare and food stamps. Did anyone help me? No." - Craig T. Nelson, actor.
balducci
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Make America Great Again! - Trump in 2020 ... "We're a capitalistic society. I go into business, I don't make it, I go bankrupt. They're not going to bail me out. I've been on welfare and food stamps. Did anyone help me? No." - Craig T. Nelson, actor.
MobilityBundle
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Every so often, there's an article like this. (I note, BTW, that this article is from the year 2000. So take my "every so often" for what it's worth...)

There are usually two things going on:

1. Contrary to the first sentence in the article, it is NOT the case that "[o]ne of the most sacred laws of physics is that nothing can travel than the speed of light in a vacuum." Lots of stuff travels faster than that speed. For example, a shadow can travel faster than light. Or if you imagine a giant pair of scissors closing, the point at which the blades meet can travel faster than light, if the scissors are big enough.

More precisely stated, one of the most sacred laws of physics is that information cannot "travel" (i.e., be communicated) faster than the speed of light in a vacuum. So, try as you might, you can't send a message from one person to another using a shadow (or a giant pair of scissors) faster than light.

2. Light can be thought of as a wave. Turns out, there's more than one way to describe "the" velocity of a wave. In particular, there are two common concepts: phase velocity and group velocity. Wikipedia has pretty, animated pictures:

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

The group velocity is a little bit like the velocity of the wave people do in a stadium or sports event. (Just in case there are any cultural differences, here's what I mean: http://www.youtube.com/watch?v=3NxLh-3DdaE&feature=fvst )

Long story short, turns out that one can arrange for the group velocity of a light wave to actually go faster than light. In fact, with a big enough stadium, you could even arrange for a "stadium wave" to go faster than light. But, the kicker is that one can't send information that way... information can only be sent at the phase velocity, which is (... again, long story short) usually what people think of when they think of the velocity of a wave.

In those "gee, we got light (or something else) going faster than light!" articles, they're talking about a group velocity, not a phase velocity.
gdw
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So the photons didn't breach light speed, but the wave did?
It's amazing, people will criticize you for "biting the hand that feeds you," while they're busy praising the hand that beats them.

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gdw
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Ah, I see. The stadium made it clear. Though, if the photons aren't there, how is the wave?
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 07:38, MobilityBundle wrote:
For example, a shadow can travel faster than light. Or if you imagine a giant pair of scissors closing, the point at which the blades meet can travel faster than light, if the scissors are big enough.


Umm - No.

Mike
gdw
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Quote:
On 2011-08-20 07:48, Michael Daniels wrote:
Quote:
On 2011-08-20 07:38, MobilityBundle wrote:
For example, a shadow can travel faster than light. Or if you imagine a giant pair of scissors closing, the point at which the blades meet can travel faster than light, if the scissors are big enough.


Umm - No.

Mike


I didn't think that part made sense.
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.
kcg5
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I love these arguments. One person is a professor, and so is the next guy, and the next....
Nobody expects the spanish inquisition!!!!!



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Grandillusionsmagic
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Quote:
On 2011-08-20 07:38, MobilityBundle wrote:
For example, a shadow can travel faster than light.


no a shadow is a lack of light so it would travel the same speed as light
think of the water coming out of the faucet, it you put your hand in blocking or diverting the water the water before you put your hand in will will continue to travel the same until it reaches the sink, then the "shadow" of your hand will be there.
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Magnus Eisengrim
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Smile I have a sneaking suspicion that Mobility Bundle is right about the shadow moving faster than the speed of light. He has geometry on his side.

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
acesover
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Quote:
On 2011-08-20 07:38, MobilityBundle wrote:
Every so often, there's an article like this. (I note, BTW, that this article is from the year 2000. So take my "every so often" for what it's worth...)

There are usually two things going on:

1. Contrary to the first sentence in the article, it is NOT the case that "[o]ne of the most sacred laws of physics is that nothing can travel than the speed of light in a vacuum." Lots of stuff travels faster than that speed. For example, a shadow can travel faster than light. Or if you imagine a giant pair of scissors closing, the point at which the blades meet can travel faster than light, if the scissors are big enough.

More precisely stated, one of the most sacred laws of physics is that information cannot "travel" (i.e., be communicated) faster than the speed of light in a vacuum. So, try as you might, you can't send a message from one person to another using a shadow (or a giant pair of scissors) faster than light.

2. Light can be thought of as a wave. Turns out, there's more than one way to describe "the" velocity of a wave. In particular, there are two common concepts: phase velocity and group velocity. Wikipedia has pretty, animated pictures:

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

The group velocity is a little bit like the velocity of the wave people do in a stadium or sports event. (Just in case there are any cultural differences, here's what I mean: http://www.youtube.com/watch?v=3NxLh-3DdaE&feature=fvst )

Long story short, turns out that one can arrange for the group velocity of a light wave to actually go faster than light. In fact, with a big enough stadium, you could even arrange for a "stadium wave" to go faster than light. But, the kicker is that one can't send information that way... information can only be sent at the phase velocity, which is (... again, long story short) usually what people think of when they think of the velocity of a wave.

In those "gee, we got light (or something else) going faster than light!" articles, they're talking about a group velocity, not a phase velocity.



Where can I a get a pair of these scissors? You said a lot of things travel faster than the speed of light so in your statement you are saying that these actually exist. Does Walmart cary them? Smile

Shadow at best can only travel at the same speed as light as it is the lack of light from a light source that is blocked. So it does not exist without the light.

Also your scissors theory is flawed as they would have to keep getting larager in size in order to be capable of doing what you say. Many say that the universe is expanding at the speed of light so your scissors could not exist because they could never be large enough to fit in the universe.

My response makes about as much sense as your post. This is way beyond my limited knowledge. Just think it through. I feel there is nothng that travels faster than light. If so it could not be seen coming or going.

Wait! I hear a cutting noise...it is the fabric of the universe being cut with a big pair of scissors but I missed it I was not quick enough. Maybe next time.

We will just have to ask Dr. Who.
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
balducci
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Don't know if this link will work, but if it does you can read here Martin Gardner's take (Relativity simply explained By Martin Gardner) on the scissors closing example:

Martin Gardner
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Jonathan Townsend
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So when you point a laser at something and move the beam you can claim the line is moving faster than the speed of light? The model is not the physical thing.
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critter
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But a laser is light, soooo wouldn't whatever speed it's moving at then literally be the speed of light? It can't move faster than the speed of light, 'cause it is light.
"The fool is one who doesn't know what you have just found out."
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Magnus Eisengrim
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It is tempting to think of the dot created by a laser, the edge of a shadow or the point of contact of two blades as a single object, and that's where the confusion is coming in.

Imagine a laser shot at the moon, making a round dot on the surface. If you move the laser a couple of mm on earth, the dot will move quite a bit further on the moon. But what is moving? Different photons are being emitted and they are taking different paths. What we observe is the dot made by photons hitting the moon. The dot can "move" faster than the speed of light without any of its constituent photons doing so.

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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Quote:
On 2011-08-20 12:06, Magnus Eisengrim wrote:
Imagine a laser shot at the moon, making a round dot on the surface. If you move the laser a couple of mm on earth, the dot will move quite a bit further on the moon. But what is moving? Different photons are being emitted and they are taking different paths. What we observe is the dot made by photons hitting the moon. The dot can "move" faster than the speed of light without any of its constituent photons doing so.


Ummm - No.

Mike
Pakar Ilusi
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My head hurts... Smile
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Magnus Eisengrim
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Quote:
On 2011-08-20 12:12, Michael Daniels wrote:
Quote:
On 2011-08-20 12:06, Magnus Eisengrim wrote:
Imagine a laser shot at the moon, making a round dot on the surface. If you move the laser a couple of mm on earth, the dot will move quite a bit further on the moon. But what is moving? Different photons are being emitted and they are taking different paths. What we observe is the dot made by photons hitting the moon. The dot can "move" faster than the speed of light without any of its constituent photons doing so.


Ummm - No.

Mike


You have an argument to back that up?
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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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
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