Main Question or Discussion Point
Does a laser have a recoil since photons have momentum. Would a moving laser be slowed down (even a fraction) if was locked in the on position and then thrown at the ground
I think the only time I've really seen the concept of laser recoil explored outside of curiosity, was by Larry Niven who used the concept of humans employing stardrives that are essentially lasers (probably in practice ion drives, but that's sci fi for yah!). Even in that framework they were devestatingly powerful to create thrust with low accelleration.The laser does indeed have recoil; however, the momentum of the particles p=h/lambda isn't very much (h is a very small number).
The laser would be slowed a very small fraction. (very small!)
Of course, in that perfect frictionless space the 'drive plume' would be a MASSIVELY powerful and destrutive laser with a scary range. "Pardon me, you seem to have been roasted alive by my exhaust! How shockingly impolite." Not the best way to make an impression on any potential aliens or distant (Differentially Aged) ancestors.I mean, we can work out some math.
This laser I have at home has <30 miliwatts of power. So, each second, my laser's momentum flow is only about 1*10^-10 kgm/s. Which isn't enough to make much of a difference. To get me, a 60kg person, moving at 10 m/s, I would have to be in perfectly frictionless space and be pushed by that laser for 190,000 years.
Now, there are much larger lasers in the world. I am only talking about those hand held lasers.
Just to be totally clear, this is science fiction I was talking about, and not hard scifi either. I was just appreciating how I viewed the universe at the time of reading that novel, and now.By my calculations, it would take a 14.7 trillion watt laser to accelerate a 5 ton payload at 1g...
At 1g it would take about half a year to go .5c (I didn't want to mess with relativity, so I keep gamma small.)
Ahhhh, well in the storyline of these novels, at this time the starships in question were sublight, but could be subject to Differential Aging and such... I may be forgetting a primary form of thruster, after which a photon drive would take over.Thrust to power ratio is inversely proportional to exhaust speed. Any rocket scientist would tell you that. Lasers have huge exhaust speed (300,000 m/s), therefore very low thrust per unit of energy. Turboprops have modest exhaust speeds, therefore they are quite useful for propulsion. The real reason to go for lasers appears when you're already moving extremely fast to begin with, say, if you want to accelerate beyond 0.5c ... then lasers, and, more generally, photon engines are the way to go.
So... by your estimation a 50 Watt laser pointer experinces a 'kick' of about 5 Kg per second? I'm just throwing out a guess that such can't be correct, for reasons discussed earlier in the thread.
Ummm ... not sure how you got there.
I could have said 'CO2 laser in a lab', or 'CNC Machine', but I didn't think that someone who imagined a laser that could toss you across a room (imagine lasers at the NIF, or MW Chemical Lasers! Why that Boeing would explode! :rofl: ) was going to appreciate that.WOW ... where can I buy one of those?
Most pen lasers and diode lasers for use in read/write applications perhaps. If you want engraving you're talking about 60-100 Watts, and a good CO2 laser definitely can pump 100 Watts.Which turns out to be ~(1/3*10^-8 Newtons). A 1 watt laser is also quite strong...most conventional lasers are in the milliwatts range.
30mW POINTER?! Heh... You have good toys. So... has it ever had any noticable 'kick' when you use it to perform home LASIK surgery? :rofl:Yes, I'm talking about those laser pointers. Sorry if "conventional" wasn't the correct word to use. I have a 30mW laser, and it will point (at night of course) pretty much across town...
In order to go from Watts to N/s one would have to divide by meters...there is to reason to divide the power by 1 meter tho...
Did you just not read SpectraCat's post?:Ok I guess the correct term is 1 Watt is 1 Newton Meters per second.
So if you pointed a 1-Watt laser at a mirror the mirror should experience a force of 1 Newton Meter per second, so by conservation of energy the kick in the laser would be the same.
Or how about this. 1 Watt laser would be 1 Joule per second kick
Then, you probably just didn't read Matterwave's post where he actually GIVES THE UNITS in NEWTONS. Here, I'll refresh your memory.SpectraCat said:Ummm ... not sure how you got there.LostConjugate said:
1) *IF* the total energy in the beam were carried as kinetic energy by massive bodies, then Newton's third law would apply and you could analyze the problem in such terms. But as has already been posted, the momentum carried by a photon is given by p=h/lambda, while the energy is given by E=pc = h x frequency. So the beam carries a lot of energy per unit momentum.
2) How did you arrive at those units? Power is energy/time, energy (work) is force x distance ... how does the distance factor in to your estimate?
You're (LostConjugate) just throwing out words and concepts with no regard at all, and then you don't bother to read the answers.Matterwave said: