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Laser shockwave accelerator

 Punch a hole, then ride the collapse
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Use a high power laser light pulse to rapidly heat a straight path in the air. Probably arrange several laser beams in a circle, with the object in the centre, initially accelerated by an electro-magnetic pulse rail. Time it so that when the expanding air finally contracts it squeezes the object at the bottom, and propels it along the remaining column of rarified air. The object would need to be quite long to take advantage of this. Not sure how to get the lower part of the column to collapse before the upper part...maybe no need since the change in air pressure might do this. Coriolis effect may be overcome by the channeling effect of the column; effectively bouncing the object off the walls.

I think the main advantages would be that the object would fly through a vacuum, and accelerate over a longer period. Oh, and it would be loud.

Of course, the main disadvantage would be that it might not work.

—Ling, Mar 29 2015

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       I think it won't work. Consider a lightning bolt's effect on air --the air expands away from the bolt, creating a near- vacuum, and very very quickly fills that vacuum when the bolt is done (causing thunder). There won't be time to make significant use of that vacuum corridor.
—Vernon, Mar 29 2015
   

       Let's suppose a typical air molecule at STP moves at a few hundred metres per second (I read that somewhere, it might not be right).   

       Then you have a few hundredths of a second to take advantage of a 1 metre wide (that's a lot of laser) rarified-air column before it collapses.   

       That's the easy part.   

       The hard part is that the bottom-pinch effect would proceed along the length of the column at around light-speed. (That's how fast the tail end of your laser light-beam recedes from you into the distance, right?)   

       Therefore, to make said bottom-pinch work for you, not against you, your projectile has to be moving at light speed already, pre-pinch.   

       Apart from that, er, yeah, eminently practical.
—pertinax, Mar 29 2015
   

       STP fortunately only applies when the temperature and pressure are standard. So the speed of sound, and the collapse of the column will be slower at altitude?   

       Edit: seems that the speed of sound depends on temperature, not pressure. It's just that in our local environment it usually gets colder as we go up.
—Ling, Mar 29 2015
   

       If Vernon and Ling are both right one would predict that lightning could shoot materials up the low pressure path: a jet of dust propelled up above the level of the bolt.
—bungston, Mar 29 2015
   

       I think I need a diagram to understand what you're proposing here.
—notexactly, Mar 29 2015
   

       //Laser shockwave accelerator//   

       Sounds like a new computer graphics card.
—21 Quest, Mar 29 2015
   

       On second reading, I think I understand it. But I don't see why contracting gas will push on something; isn't it usually expanding gas that does that?
—notexactly, Oct 02 2017
   

       Sounds like Rudolph's red nose. //eminently practical//
—wjt, Oct 04 2017
   


 

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