Does a laser experience recoil due to photon momentum?

[Frame Dragger]

lol!

I think some of those old glow in the dark stuff actually had Uranium in it...although, it wouldn't be weapons grade uranium but still...maybe the kid was real smart and found a way to enrich it XD. The fusion-able material needed (Deuterium - .015% abundance, and tritium - unstable) is easier to get than the fissile material...not easy by any means, but if the kid was able to enrich the uranium from glow in the dark watch hands, maybe he had something up his sleeve!

Anyways, @Lost Conjugate, the "kick" should be a force (which is measured in Newtons, not Joules!) and not some energy/time deal you have going on. What you are saying though is that a 1 Watt laser will get a 1kg block to 1 m/s in half a second (K=.5mv^2)...which is obviously not true. If you hold this view, my 30mW (.03W) laser should be able to accelerate a paperclip (1g) to something like 8m/s in 1 second! I've never witnessed my laser pointer moving anything like that haha. Although having such a laser pointer would be the epitome of cool...coolness does not equal fact.

Obviously, not all of the energy of the laser will go into kinetic energy of the object. Almost all of the energy is scattered off. What the laser DOES instill into the object is momentum... and the momentum of light is precisely p=E/c.

Actually I think the joke was in reference to the tritium that is used on high end watches (still used), not to mention ACOG and other high-end gun sights. Of course, mickey mouse watches do NOT use tritium. The idea was that he was going to make a real mutliple stage weapon a la Teller-Ulam! Good stuff. The tritium would fuse with deuterium... of course now this role is filled with the single Lithium-Deuteride. *shrug* a mushroom cloud is a mushroom cloud.

As for the laser pointer, it would be a coherent BEAM! So... if you swept it across a room it should act like a giant bat in that scenario. I think LostConjugate might have been thinking of the machines the Ghostbusters use.

 

[Frame Dragger]

However if you measured the energy from a light beam over 1 second and it came out to be 1 Watt, all that energy would be measured in force.

Also p =E/c but the light is traveling at c, you not measuring the force of a single photon, your measuring over a crap load of photons traveling at c for 1 second.

This problem isn't as simple as you seem to believe it is.

EDIT: I take that back, it IS simple, but you're adding needless complications with this magical kinetic quanta.

 

[LostConjugate]

well I might be wrong, but I that was just my opinion seems like a laser measured at 1Watt would give a return kick of 1Watt unless the energy was somehow dispersed as heat which I don't think it is.

 

[Matterwave]

Actually I think the joke was in reference to the tritium that is used on high end watches (still used), not to mention ACOG and other high-end gun sights. Of course, mickey mouse watches do NOT use tritium. The idea was that he was going to make a real mutliple stage weapon a la Teller-Ulam! Good stuff. The tritium would fuse with deuterium... of course now this role is filled with the single Lithium-Deuteride. *shrug* a mushroom cloud is a mushroom cloud.

As for the laser pointer, it would be a coherent BEAM! So... if you swept it across a room it should act like a giant bat in that scenario. I think LostConjugate might have been thinking of the machines the Ghostbusters use.

Ah...I didn't know some watches used tritium. I just know that old school glow in the dark used to be Uranium. My chem teacher once brought in some plates with special paint and moved a Geiger counter over them and showed us they were radioactive. XD

@LostConjugate:
Force is measured in Newtons, Energy is measured in Joules, and Power is measured in Watts which is Joules/second. You can't express force in units of Power!

Indeed most of the energy of the laser beam is scattered off, and what isn't scattered is absorbed as heat (heating up the material!). Only a tiny fraction of the energy goes to movement of the material itself (rather than movement of constituent electrons) This is why some lasers are used to cut stuff by melting through them (a la Goldfinger and that infamous moment "No, Mr. Bond, I expect you to DIE!" - one of my faves)

 

[Frame Dragger]

well I might be wrong, but I that was just my opinion seems like a laser measured at 1Watt would give a return kick of 1Watt unless the energy was somehow dispersed as heat which I don't think it is.

This isn't really something that's open to opinion. We're trying to tell you that if you were right, industrial and laboratory lasers would have phenomenal recoil.

In fact, for a non-refractive surface it IS dissipated as heat, and likewise during the diffraction process. You can melt your mirrors, but you won't shatter them from an impact.

 

[Frame Dragger]

Ah...I didn't know some watches used tritium. I just know that old school glow in the dark used to be Uranium. My chem teacher once brought in some plates with special paint and moved a Geiger counter over them and showed us they were radioactive. XD

@LostConjugate:
Force is measured in Newtons, Energy is measured in Joules, and Power is measured in Watts which is Joules/second. You can't express force in units of Power!

Indeed most of the energy of the laser beam is scattered off, and what isn't scattered is absorbed as heat (heating up the material!). Only a tiny fraction of the energy goes to movement of the material itself (rather than movement of constituent electrons) This is why some lasers are used to cut stuff by melting through them (a la Goldfinger and that infamous moment "No, Mr. Bond, I expect you to DIE!" - one of my faves)

Any watch that doesn't require exposure to a light source (or a battery) to 'glow' is lousy (well, fractionally imbued) with Tritium!

I think the half-life would expire before you could scrape enough together! Better just to throw the watches at people you don't like.

 

[SpectraCat]

If you shine the laser at a mirror without any angle it reflects perfect. I don't care about the speed of light. There is 1 Joule of force per second coming from the light, traveling at speed c, hence, 1 Watt Laser.

Again with the non-sensical units ... force is not measured in Joules per second. Also, perfect reflection = elastic collision ... how much energy/momentum is transferred to the target in an elastic collision?

However if you measured the energy from a light beam over 1 second and it came out to be 1 Watt, all that energy would be measured in force.

So now energy is measured in watts? Or is it "measured in force"? Please take the time to get your dimensions correct, so that you can communicate effectively. You are making zero sense with this stuff.

Also p =E/c but the light is traveling at c, you not measuring the force of a single photon, your measuring over a crap load of photons traveling at c for 1 second.

You have utterly no clue what you are talking about here ... please do a little reading on photon energy vs. momentum. I guarantee you that we are steering you correctly on this issue. Consider this ... kinetic energy is carried by massive particles ... what is the mass of a photon?

 

[LostConjugate]

You have utterly no clue what you are talking about here ... please do a little reading on photon energy vs. momentum. I guarantee you that we are steering you correctly on this issue. Consider this ... kinetic energy is carried by massive particles ... what is the mass of a photon?

The mass of a photon is only zero at rest.

 

[SpectraCat]

The mass of a photon is only zero at rest.

A random, irrelevant and unconnected fact ... which does not answer my question .. I did not ask "when is the mass of a photon zero". Do you really think you are correct here, and that all of the rest of us are wrong? Can you please cite some sources that back up your statements? This is getting tiresome ... for crying out loud, pick up a laser pointer and turn it on .. the power there is about 5mW ... do you really feel a "kick" equivalent to 5 grams of matter being ejected at 1 m/s? That is basically what you have been arguing you should feel ...

 

[Frame Dragger]

Again with the non-sensical units ... force is not measured in Joules per second. Also, perfect reflection = elastic collision ... how much energy/momentum is transferred to the target in an elastic collision?



So now energy is measured in watts? Or is it "measured in force"? Please take the time to get your dimensions correct, so that you can communicate effectively. You are making zero sense with this stuff.



You have utterly no clue what you are talking about here ... please do a little reading on photon energy vs. momentum. I guarantee you that we are steering you correctly on this issue. Consider this ... kinetic energy is carried by massive particles ... what is the mass of a photon?

I love the units... "ah yes, gravity acts on a falling body with 6.023x10^23 Pentacles per Snooker." Anything can be true if you just ignore facts, schooling, and what everyone who has no interest in lying is telling you.

So Cat, let's watch and see as someone deals with the following cognitive dissonance: "I am a genius and right, however I'm pretty sure lasers don't smash things. I like apples!" ... and yes, some poetic license has been taken with that.

EDIT: @LostConjugate: Ok... think of this. I go online to Amazon.com and buy a 60 WATT (not milliWatt) laser engraver. Turn it on... is the laser assembly blowing up? No? Ok... now think about the lasers in cd/dvd/blu-ray devices... how precise they need to be to read information. That must be tough with all of that RECOIL! *SARCARSM*. Come on, we're not playing with you, but you're talking absolute nonsense here!

 

[hamster143]

How about this.

One watt laser emits one joule of photons per second.

One joule of photons weighs 1/c^2 = 1/(9*10^16) kg.

These photons travel at the velocity c and therefore their momentum is 1/c = 1/(3*10^8) N*s, and the recoil of the laser is 1/(3*10^8) N*s in the opposite direction.

Therefore the force felt by the laser is 1/(3*10^8) N.

 

[SpectraCat]

Ah...I didn't know some watches used tritium. I just know that old school glow in the dark used to be Uranium. My chem teacher once brought in some plates with special paint and moved a Geiger counter over them and showed us they were radioactive. XD

Actually, that was radium, not uranium, that was painted on old watch hands. Actually, you used to be able to buy water that had trace amounts of radium in it .. it was marketed that way, because people actually thought that ingesting(!) radioactive material was healthy ... or at least advertising exectutives thought that TELLING people ingesting radioactive material improved health would help them sell more product ... some things never change.

 

[Frame Dragger]

Actually, that was radium, not uranium, that was painted on old watch hands. Actually, you used to be able to buy water that had trace amounts of radium in it .. it was marketed that way, because people actually thought that ingesting(!) radioactive material was healthy ... or at least advertising exectutives thought that TELLING people ingesting radioactive material improved health would help them sell more product ... some things never change.

Hmmm... very famous case of a man who lost his jaw to radium water. That kind of put the Kibosh on radium as a healthful substance. Then again, we should keep in mind how such august presences as the Curies died...

A boxer might not be really crisp when he's 60, and a researcher into the unknown or potentially hazardous might not see 60. These things happen.

 

[Matterwave]

Actually, that was radium, not uranium, that was painted on old watch hands. Actually, you used to be able to buy water that had trace amounts of radium in it .. it was marketed that way, because people actually thought that ingesting(!) radioactive material was healthy ... or at least advertising exectutives thought that TELLING people ingesting radioactive material improved health would help them sell more product ... some things never change.

Hmm, I seem to remember uranium...but it's been a while since I've heard this "fact" so I very well may be wrong.

 

[SpectraCat]

Hmm, I seem to remember uranium...but it's been a while since I've heard this "fact" so I very well may be wrong.

Perhaps both were used ... I just know for sure that at some point radium was used ... check out this link. http://en.wikipedia.org/wiki/Radium_Girls

One thing .. I do not believe uranium is luminescent naturally, nor is it radioactive enough to active phosphosence in its un-enriched form. Radium is WAY more radioactive than uranium, with a half-life of ~1600 years, as opposed to uranium, which has half-lives in the 10⁸-10⁹ year range for the most abundant isotopes.

 

[LostConjugate]

How about this.

One watt laser emits one joule of photons per second.

One joule of photons weighs 1/c^2 = 1/(9*10^16) kg.

These photons travel at the velocity c and therefore their momentum is 1/c = 1/(3*10^8) N*s, and the recoil of the laser is 1/(3*10^8) N*s in the opposite direction.

Therefore the force felt by the laser is 1/(3*10^8) N.

Good point. Can't argue with that.

Cat, Its not that I don't agree when I am wrong, I just didn't know what anyone was talking about. I figured the energy of a laser is equal to the force as 1 Joule is equal to 1 Newton Meter.

 

[Frame Dragger]

Good point. Can't argue with that.

Cat, Its not that I don't agree when I am wrong, I just didn't know what anyone was talking about. I figured the energy of a laser is equal to the force as 1 Joule is equal to 1 Newton Meter.

Fair enough LC, but if that's the case TELL US that! No one is here to prove to you that they're smart, but rather to answer your question or correct a misapprehension. If it happens in a confrontational manner... sometimes intellectual rigor is confrontational.

From our perspective, what you were saying was so patently silly (laser beams like wrecking balls), that it was hard not to simply dismiss. It may also be that you should spend some more time lining up the basic concepts of Newtonian mechanics vs Quantum Mechanics.