# Could laser collisions be used as an interstellar dust deflector?

What inspired this question is this : I was once reading that a real effort is underway to dream up a 0.2c spaceship (0.2c means one fifth the speed of light). Another story I read tells that interstellar space is so dense with dust that hitting too much of it at such a speed would destroy the ship.

What I don't know is whether crossing two lasers of any type creates loose material, stray "lastertrons" or something. So if a ship was traveling with a laser array of these focused far ahead, the particles formed by the collision of these lasers would generate a shield of sorts around the physical ship and either ram interstellar dust or incinerate it before it posed a physical problem.

• Light cannot collide with light. Commented Dec 7, 2023 at 21:25
• Just so you know, the common notation for the speed of light is "c". "e" usually means the exponential constant which is closely related to the exponential function. You know, the function that grows as fast as it gets bigger (ie. ultra-fast ^^). Commented Dec 7, 2023 at 22:05
• I think this Q is based on the false idea that two lasers intersecting creates something like a solid material barrier to deflect dust (where did you get this idea anyway?). You could take one laser away and ask the question "Can a laser be used as an interstellar dust deflector?" Which would be a pretty interesting question.
– BMF
Commented Dec 7, 2023 at 22:32
• What do you expect to happen to the matter you've heated with light? It'll still be there, just hotter. Commented Dec 7, 2023 at 22:47
• @Escapeddentalpatient. It'll be hotter and faster and indeed won't be there. That's the point.
– BMF
Commented Dec 7, 2023 at 23:49

To expand on @AlexP's comment, light cannot collide with light. The problem is that photons have zero mass. They are objects of energy. When two photons "collide" what you get is, well... two photons.

BUT!!! There's something we can do that's kinda along the lines of what you're thinking about

The idea that matter particles, an electron-positron pair, can be created when light particles or photons collide is central to modern physics. The concept illustrates that energy and mass are two sides of the same coin, immortalized in Einstein’s famous equation E=mc2. However, direct observation of the phenomenon involving just photons has proved elusive, since the colliding photons need to have extremely high energy, still well beyond the current technology. (Source)

But...

Researchers at Brookhaven National Laboratory recently provided the first clear evidence of a direct, single-step particle interaction where electron-positron (matter-antimatter) pairs were formed as a result of a photon-photon collision, as originally predicted by Breit and Wheeler. (Ibid.)

In other words, physicists are investigating the possibility that colliding photons could be used to create an electron-positron pair. And what's another name for an electron-positron pair?

• a matter-antimatter pair.

Now, I'm not going to even try to pretend that I know enough bout electron-positron pairs to speak authoritatively about them... but I have no problem at all using them for a science fiction solution!

And that's useful for solution #1, which is for your incoming dust to interract with the antimatter portion of that pair and disappear in a poof of energy, which you could harvest to supplement the energy you're using to create those two beams of light (they needn't be lasers).

But let's also look at solution #2. Electron-positron pairs have cool equilibrium state magnetic properties that have been described as an electromagnetic spring that lets the electron and positron bounce back and forth.

Right up until the incoming dust upsets the equilibrium.

At which point you have two electrically (and oppositely) charged particles winging off into space creating magnetic fields that can deflect mass.

Is this real science?

Some of it is. The part about using it to rationalize a deflector shield for your ship isn't... but does that matter? I was willing to believe this would work just with the utterance of the phrase "electron-positron pair."

interstellar space is so dense with dust that hitting too much of it at such a speed would destroy the ship

Obviously, hitting too much of anything at speed is bad news, but space is big and there's not a lot of stuff in it. In the Local Interstellar Cloud, you get .3 atoms per cubic centimeter, and that's the figure I'll use in my calculations below as a sort of "bad case" baseline, but in the immediate neighbourhood of the Sun you find the Local Bubble which has less than a tenth of that density of material. You can scale the figures below appropriately, depending on how vacuous the vacuum in your setting is.

A spacecraft with a cross-sectional area of 100 m2 travelling 10 lightyears though the Local Interstellar Cloud will sweep out a volume of ~9.418 m3 containing ~2.8 x 1024 atoms. This will be ~91% hydrogen and 9% helium, and everything else is basically a rounding error.

That's about 6 grams of stuff hitting you at .2c, which is slow enough that you can get away with using the regular kinetic energy equation, which gives 1013 J, equivalent to ~2.6 kilotonnes of TNT. That's obviously a bit of a bang were it all to arrive at once, but spread out over 50 years that's an average flux of about 6.8 kW, and you might have electrical heaters in your house that are more powerful than that.

Being hit by one of those hydrogen atoms is like being hit by a proton, because the electrons aren't held on very hard compared to the energy of the impact. At .2 c, a proton has a kinetic energy of ~19 MeV. You can use the NIST PSTARS tool to see that these particles have a CSDA range of ~0.426 g/cm3 in water. Given the density of liquid water, you can see that they won't penetrate more than half a centimeter... you don't need a massive thick shield. Water at its melting point, with a surface area of 100 m2, has an emissivity of about .96 so you can rearrange the Stefan-Boltzmann law to see that it has an equilibrium temperature under that dust and gas bombardment of ~188K... that's a nice chilly layer of ice that is in no danger of melting or boiling during your journey. Carry enough in a nice aluminium dish to last you the whole trip without worrying about loss from atoms or ions being knocked overboard... the water shield only weighs a tonne per centimeter of thickness which is small beans by the scale of a serious interstellar spacecraft.

You need to be a little more careful about larger objects... a 1 g fleck of dust packs a punch like 430 tonnes of TNT which will wreck a thin shield and potentially the ship behind it, but such objects will be vanishingly rare, and can be dealt with via thin sheets of armor with very wide spacing between them, Whipple shield style.

the particles formed by the collision of these lasers would generate a shield of sorts around the physical ship and either ram interstellar dust or incinerate it before it posed a physical problem

As JBH has already pointed out, pair-production from a vacuum is very much a thing, and it requires the interaction of two photons, so there is some physics underlying your idea.

In order to do it though, you need a laser emitting photons with the energy of a rest mass of an electron, about 511 KeV. That requires a laser with an 8 picometer wavelength, which is very much hard gamma rays and generating such things is remarkably difficult, and once you've generated them there's no practical means of focussing or reflecting them because the wavelength is so much smaller than individual atoms that stuff made of regular matter is just too lumpy to do a good job.

But as we've already discovered, there's not actually that much stuff out there, and it isn't actually hitting you very hard!

There are two better things to do with your lasers.

The first is to simply use light pressure to help deflect or at least reduce the relative speed of gas and dust ahead of the ship. Just warm up your laser and shine it ahead of your ship. If you had a powerful pulse laser you can strobe it from time to time and maybe even deliver enough energy to break up the big dust grains that present the biggest threat to your spacecraft. This does require quite a powerful laser array, but this doesn't seem to bother you too much

The second and simpler option is to use your laser to push lightsails ahead of your ship. Each lightsail is a very very thin and light (<5g per square meter even with today's technology) sheet of metal or plastic so that you don't need an overwhelmingly powerful beam to shove it. Just push it gently ahead of you so it reaches a higher velocity than the ship, and watch it drift away. Impacts with gas and dust will destroy it eventually, but it weighs so little you can carry a bunch and shove a load ahead of you... they'll drift away into the far distance, thousands or millions of kilometers away depending on how straight you can shoot. You can then watch carefully for the telltale flashes of dust grains hitting the sails, and use your lasers in a focussed defense mode to mop up those troublesome bits of grit, should you ever be unlucky enough to meet one.