Great flick! That, OHMSS, and Thunderball are my favorite Bond films. I much prefer the classics.
You have to watch some obscure German-language movies to hear Gert Frobe's real voice. In Goldfinger, and a couple other popular English films featuring him, all his lines were dubbed in by American or British actors. In Chitty Chitty Bang Bang it was the actor that played the malfeasant Harry Mudd in the old Star Trek series.
To figure out the size of the spot on the Moon lets assume the diode is a point source to make the math a bit easier. I Googled the divergence of laser diodes and came up with an average of 1.5 mRad.
Now visualize a triangle with the base opposite us on the surface of the Moon, the distance of the Moon is adjacent to us, and we're at the corner of the triangle which has the divergence as the angle.
Tan (divergence) = opposite / adjacent
½ beam diameter = distance * tan (divergence angle)
beam diameter = 2 * distance * tan (divergence angle)
To figure out the size of the spot on the Moon lets assume the diode is a point source to make the math a bit easier.
1,153.2 kilometers
Thanks! I will have to go back and figure where I went wrong. Where ever I see mRad, I make mistakes. I recall last time I calculated I got something like 1/4 mile dot size, but didn't have confidence that I did it correctly.
Can we make a spot on the moon that we could detect from earth? I heard NASA bounced a laser off the retro reflectors left by the Apollo missions, could we do the same?
Lets see. Wiki tells me Mare Imbrium is 1123 km, about the size of our dot. So the dot would be big enough to see, just not bright enough.
A little more looking reveals a relevant xkcd (seems there is always at least one) http://what-if.xkcd.com/13/
So its not going to be naked eye visible, but could we detect the dot with a telescope?
Assuming we could get all the lasers exactly parallel, how many 21 cent lasers would it take to make laser dot we could bounce off the moon and detect from earth?
But lasers used for bouncing off the moon have optical collimators. Taking the raw divergence of any laser won't give you the actual values in practical use.
Here's another one, recalled, albeit foggily, from a rather besotted confab with some of my Physics profs:
You have a laser, whose beam you're projecting into, and sweeping through, space at one radian per second. At some very distant point in the universe, that beam will be sweeping past faster than the speed of light. How can that be, since nothing can move faster than lightspeed?
Here's another one, recalled, albeit foggily, from a rather besotted confab with some of my Physics profs:
You have a laser, whose beam you're projecting into, and sweeping through, space at one radian per second. At some very distant point in the universe, that beam will be sweeping past faster than the speed of light. How can that be, since nothing can move faster than lightspeed?
-Phil
Is there a treadmill anywhere in this situation?
I guess the beam curves as it maxes out in velocity?
Here's another one, recalled, albeit foggily, from a rather besotted confab with some of my Physics profs:
You have a laser, whose beam you're projecting into, and sweeping through, space at one radian per second. At some very distant point in the universe, that beam will be sweeping past faster than the speed of light. How can that be, since nothing can move faster than lightspeed?
-Phil
It can't be. Einstein continues to rest in peace. The beam in this question is just the expected trajectory of the photons. They will take some time to fulfill that expectation and none of them will violate any laws of physics in doing so. The photons only travel outward in a straight line (mostly), not in the arc that is created by sweeping the beam.
But lasers used for bouncing off the moon have optical collimators. Taking the raw divergence of any laser won't give you the actual values in practical use.
True, I was only trying to answer the laser pointer question.
Can we make a spot on the moon that we could detect from earth? I heard NASA bounced a laser off the retro reflectors left by the Apollo missions, could we do the same?
Even an uncollimated beam would likely result in a few photons making it back to Earth. The problem is detecting them as the what if points out.
It's my understanding that astronomers don't use their eyes to try and see the reflection. They use instruments that are sensitive enough to detect it.
With collimation the divergence can be reduced to the limit imposed by the atmosphere which is one arc second. This translates to a 2 km wide spot on the moon. You can expect about 1 in 30 million photons to make the return trip, that is significant signal attenuation to say the least.
from a rather besotted confab with some of my Physics profs:
Odd that physics professors would make this claim. Speed c is constant regardless of the source or observer. The wiki page sums it up better than I can (emphasis added): "Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer."
And as Rich points out, the light is traveling in the same direction it started, and simply spreading out in diverging straight lines (black holes notwithstanding). ALL light going through the universe is from rotating objects, some of it spinning around several times a second. I don't know the what the most distant pulsar is, but light from it is still traveling at 186K mps.
Phil's question is a very old thing. It's based on some weird idea that a beam of light, like that from a laser, is some kind of stick that you wave around in space and the end of the stick moves just as yo direct it. Perhaps we can blame StarWars and the lightsabers for this nonsense.
Even if the light from your beam crosses the face of a planet in the far distance at ten times the speed of light, according some simple view, does not mean that anything actually travelled faster than light.
As a first approximation think about the situation as the same as strafing around with a machine gun.
You can expect about 1 in 30 million photons to make the return trip, that is significant signal attenuation to say the least.
I was watching a documentary the other day, Do We Really Need the Moon? where they demonstrated measuring the distance from earth to moon. Used a very bright green laser that could be pulsed, so it was probably a frequency pumped diode laser of some type. The number of photons returned from the retroflectors on the moon and received by the detector was on the order of 1 (per pulse). As in, One is the Loneliest Number. Obviously not your typical receiving sensor!
On the contrary I would say that the last thing that light does is travel in any straight line.
If that were so diffraction would not happen.
True, but perhaps a bit of an over-simplification. Diffraction, reflection, refraction, etc. may change the direction of light, but it's still involves straight lines. An exception to this, a true bending of light through spacetime, would require the influence of gravity. In the spirit of Rich's comment, the FTL laser beam thing would require bending that somehow accelerates the photons, which we know doesn't happen.
True, but perhaps a bit of an over-simplification.
I would not call Richard Feynman's Quatum Electrodynamics an over simplification. Do check out his book "QED: The Strange Theory of Light and Matter" for a simple introduction to the ideas.
If we view light as waves we have to believe that travelling in straight lines is not what it does. Waves will slurp around in all directions all the time. It is that very interaction, interference between waves travelling in all directions that gives rise to what looks like the straight lines of the laser beam. Basically the wave front has to explore all directions in space in order to arrive at a constructive interference it the direction ahead.
Tricks like the Young's slits experiment expose how this works. But it is going on all the time anyway.
No, I'm not even thinking of the bending of space time or gravity at this point.
I meant the over-simplification was made by you, not Feynman. Rich already qualified his mention of light traveling in straight lines ("mostly"), and you even included that in your quote -- you felt it necessary to "correct" him anyway, despite the clear context of his comments. There are nearly always exceptions, especially at the atomic and quantum level, but these don't apply to some wide arc in space produced by waving a laser.
Comments
Where are those X-Ray Glasses when you need 'em!
-Tor
Thanks. It's only a loss of sixty cents, so it's no big deal. However, I contacted the seller and they offered me partial refund, so I'm good.
Just as bright though
"Do you expect me to talk?"
"No Mr. Bond. I expect you to die!
When you can have that conversation, THEN you know you have enough lasers.
Had to refresh my memory...
You have to watch some obscure German-language movies to hear Gert Frobe's real voice. In Goldfinger, and a couple other popular English films featuring him, all his lines were dubbed in by American or British actors. In Chitty Chitty Bang Bang it was the actor that played the malfeasant Harry Mudd in the old Star Trek series.
[video=youtube_share;ODKxMlovi4o]
Spectrum, smectrum. At this price just add more lasers! Eventually one could cook anything. How many watts per millimeter would it take to cook a bug?
would it be 500 meters or 500 kilometers?
Now visualize a triangle with the base opposite us on the surface of the Moon, the distance of the Moon is adjacent to us, and we're at the corner of the triangle which has the divergence as the angle.
Tan (divergence) = opposite / adjacent
½ beam diameter = distance * tan (divergence angle)
beam diameter = 2 * distance * tan (divergence angle)
beam diameter = 2 * 384,400 km * tan(0.001.5)
1,153.2 kilometers
Thanks! I will have to go back and figure where I went wrong. Where ever I see mRad, I make mistakes. I recall last time I calculated I got something like 1/4 mile dot size, but didn't have confidence that I did it correctly.
Can we make a spot on the moon that we could detect from earth? I heard NASA bounced a laser off the retro reflectors left by the Apollo missions, could we do the same?
Lets see. Wiki tells me Mare Imbrium is 1123 km, about the size of our dot. So the dot would be big enough to see, just not bright enough.
A little more looking reveals a relevant xkcd (seems there is always at least one) http://what-if.xkcd.com/13/
So its not going to be naked eye visible, but could we detect the dot with a telescope?
I was thinking audio over laser.
http://www.instructables.com/id/Send-Music-over-a-Laser-Beam/
Doable or nuts? Both?
Assuming we could get all the lasers exactly parallel, how many 21 cent lasers would it take to make laser dot we could bounce off the moon and detect from earth?
-Phil
Is there a treadmill anywhere in this situation?
I guess the beam curves as it maxes out in velocity?
It can't be. Einstein continues to rest in peace. The beam in this question is just the expected trajectory of the photons. They will take some time to fulfill that expectation and none of them will violate any laws of physics in doing so. The photons only travel outward in a straight line (mostly), not in the arc that is created by sweeping the beam.
True, I was only trying to answer the laser pointer question.
I'll refer you to an XKCD what if on the topic: http://what-if.xkcd.com/13/
Even an uncollimated beam would likely result in a few photons making it back to Earth. The problem is detecting them as the what if points out.
It's my understanding that astronomers don't use their eyes to try and see the reflection. They use instruments that are sensitive enough to detect it.
I don't think we'll be able to do this with our lasers.
With collimation the divergence can be reduced to the limit imposed by the atmosphere which is one arc second. This translates to a 2 km wide spot on the moon. You can expect about 1 in 30 million photons to make the return trip, that is significant signal attenuation to say the least.
Odd that physics professors would make this claim. Speed c is constant regardless of the source or observer. The wiki page sums it up better than I can (emphasis added): "Such particles and waves travel at c regardless of the motion of the source or the inertial frame of reference of the observer."
And as Rich points out, the light is traveling in the same direction it started, and simply spreading out in diverging straight lines (black holes notwithstanding). ALL light going through the universe is from rotating objects, some of it spinning around several times a second. I don't know the what the most distant pulsar is, but light from it is still traveling at 186K mps.
I take issue with this statement: On the contrary I would say that the last thing that light does is travel in any straight line.
If that were so diffraction would not happen.
For a bit of an insight on this check out Richard Feynman's "Photons -- Corpuscles of Light" lecture here: https://www.youtube.com/watch?v=eLQ2atfqk2c&list=PL8590A6E18255B3F4 and many other such Feynman videos.
Even if the light from your beam crosses the face of a planet in the far distance at ten times the speed of light, according some simple view, does not mean that anything actually travelled faster than light.
As a first approximation think about the situation as the same as strafing around with a machine gun.
I was watching a documentary the other day, Do We Really Need the Moon? where they demonstrated measuring the distance from earth to moon. Used a very bright green laser that could be pulsed, so it was probably a frequency pumped diode laser of some type. The number of photons returned from the retroflectors on the moon and received by the detector was on the order of 1 (per pulse). As in, One is the Loneliest Number. Obviously not your typical receiving sensor!
[video=youtube_share;TeA1nvD4rjE]
Go to about 32m 35s in.
True, but perhaps a bit of an over-simplification. Diffraction, reflection, refraction, etc. may change the direction of light, but it's still involves straight lines. An exception to this, a true bending of light through spacetime, would require the influence of gravity. In the spirit of Rich's comment, the FTL laser beam thing would require bending that somehow accelerates the photons, which we know doesn't happen.
If we view light as waves we have to believe that travelling in straight lines is not what it does. Waves will slurp around in all directions all the time. It is that very interaction, interference between waves travelling in all directions that gives rise to what looks like the straight lines of the laser beam. Basically the wave front has to explore all directions in space in order to arrive at a constructive interference it the direction ahead.
Tricks like the Young's slits experiment expose how this works. But it is going on all the time anyway.
No, I'm not even thinking of the bending of space time or gravity at this point.