Laser Recoil

[AlexanderSen]

If scientists can create solar light sails to propel spacecraft then doesn't that man that there is some sort of force created by the emission of light? So then, would powerful lasers create a recoil from the laser firing if it is powerful enough?

 

[Mirannan]

The answer to that is yes. However, the problem is that to make the effect noticeable you'd have to use a heck of a big laser. And to make the effect noticeable, the mass to weight ratio would have to be very favourable as well.

And finally on that subject - remember the Kzinti Lesson!

 

[Tower75]

I was under the impression: no. A laser is just light, even the super-science-y lasers we're using nowadays in industry and the military are light. The US Navy has fitted a destroyer with a laser that can engage and destroy inbound missiles and small attack craft, (the Royal Navy had one in the Falklands ) however, these are just beams of light. Quite happy to be told otherwise but I was under the impression that it doesn't matter the "size" or "calibre" of the laser it's still just a beam of recoilless light.

 

[Dave]

I think the confusion stems from whether or not a photon has rest mass. In any respect, the mass is extremely small and so the effect is almost insignificant in comparison to other forces such as gravity and friction. However, there is a very small force and given time in a frictionless environment it would be noticeable.

 

[Ray McCarthy]

Ground based lasers can levitate objects by ablation / heat.
Light (and radio) does have a radiation pressure. You need a very big sail for sunlight, but then there is significant pressure from solar wind, a solar sail may depend more on the solar wind than light?.
A laser on a Space craft would just be used for direct near light speed (very low acceleration) and would never be used on a sail. Ion drive would be used to get up to the limiting speed of it.

So yes, there is a very tiny recoil on a pulse laser, but it may be nothing to do with photon mass (which is zero), but some other property of Electromagnetic waves. Microwaves (Radio is same stuff as light) are being investigated for propulsion too, as we can make them with better energy efficiency than light from a laser.

 

[Mirannan]

Although photons don't have a rest mass, they do have momentum; and this means lasers have recoil by Newton's Third. However, the effect is tiny.

Regarding photon mass, there are two ways of looking at it once of which is more useful than the other. First, a zero rest mass with an infinite tau factor (from relativity) leads to an indeterminate result. 0x∞ = ?

Second, and also from relativity, energy and mass are interchangeable. Since a photon has definite energy (at least somewhat definite; quantum mechanics and Heisenberg uncertainty chime in here) that means it has a definite mass while it's in flight. It turns out that photon momentum is equal to h divided by the wavelength, where h is Planck's constant

 

[Ray McCarthy]

@Mirannan
You explained it perfect.
Since UV is shorter wavelength than Infrared, the same over all energy is same heating, but UV causes more surface damage to complex organic compounds as each photon has more energy, also not as many photons are needed for same power.
Also LEDs and Lasers etc need higher band gap electron "dropping" in level at UV than IR. So IR LED might be 1.1V, red 1.2V, green 2V and Blue/Violet more.
"White" LEDs are blue / violet / Near UV end of spectrum with phosphors, so can't run off a single cell. They need at least two Alkaline buttons cells, or even three if ordinary AAA or AA types. A lithium Coin is more than twice volts of an ordinary battery cell, more even than a Lead Acid cell, so can power a low current blue / violet / UV or "White" LED.

Despite Einstein's dislike of Quantum Mechanics he got a Nobel prize on how Quantum Mechanics (and indirectly h/(wavelength) of course) is related to the Photo Electric effect (direct conversion of photons into electricity on a surface).

 

[RCynic]

I think the most impressive demonstration I've seen of the "physical" nature of light is manipulations using laser tweezers. I'm not able to paste things here for some reason but if you google "membrane fluidity laser tweezer", you can see some very cool videos of this.

 

[Ursa major]

Thanks for mentioning this, RCynic. And it isn't just the laser tweezers that are of interest in this video (with sound):