In a previous article we discussed the use of relativistic kill weapons: impactors travelling at almost the speed of light delivering enormous amounts of kinetic energy to demolish their targets. But why go with a weapon that travels at almost the speed of light when you can use light itself as a weapon? Then the enemy will have no warning at all.

The United States has been developing laser weapons for years but current systems still require several seconds of target-lock before the thermal effects kick in. In a future warfare scenario we’re not so limited: let’s consider stellar-class beam weapons.

The sun radiates a total power of 3.8 x 1026 watts – enough energy to melt a bridge of ice 2 miles wide, 1 mile thick and extending the entire way from the Earth to the Sun — in one second. Using Einstein’s mass-energy conversion, E = mc2, this amount of one-second energy weighs about four million tons. To deploy such a powerful weapon we would need access to the power of a star: at the very least a vast amount of antimatter – but much less if we pulse the beam.

Suppose that we had a weapon system which could focus this amount of energy into a beam, perhaps a metre in diameter. We point the beam at the enemy and they’re thermally annihilated. But perhaps they got smart and developed a super-reflective mirror coating. This was, after all, the classic ballistic missile defense against ‘Star Wars’ laser weaponry.

It would be difficult to find any material which could reflect well enough to avoid total destruction, but even if it did there’s another weapon effect we can exploit: momentum. Light can push things out of the way – think of the light-sail propulsion concept. What would it feel like to be hit by a stellar-class beam weapon?

When a stellar-class beam hits an alien spacecraft and bounces back from its super-mirrored surface, the force the aliens experience is equivalent to 38,000 times the weight of Mount Everest piled on top of it.

I think it would notice.

One of the strange effects of general relativity is that light beams (as energy) can gravitationally attract one another, but only if travelling in opposite directions. As the beam bounces back from that mirrored alien spaceship, would we expect any gravitational interaction between the two beams? Those four million tons of beam mass-energy are spread over 300 million metres, the distance light travels in a second. This gives a mass of 15 kilograms per metre (10 pounds per foot). Imagine a line of small people laid out end-to-end … the gravitational effects of stellar-class beam weapons are clearly negligible.

There are other limitations to unboundedly powerful lasers though. At extremely high energy-densities non-linear quantum effects kick in as the beam photons gouge electrons and positrons from the quantum vacuum. These kinds of effects scatter and degrade the beam.

Very powerful beam weapons are clearly lethal when they make contact with the enemy but there is a fundamental problem. Consider a scenario where hostile enemy spacecraft have intruded into near-earth space and are hiding behind the moon, emerging to face our laser weapons situated in low-earth orbit. The alien ships are 150 foot (50 metre) diameter spheres which can accelerate in any direction.

Our radars and telescopes spot a ship emerging above the lunar horizon and we fire the beam weapon. The moon is 380,000 km away (1.27 light seconds) so when we spotted the craft, we were already out-of-date by that amount of time. We go ahead and fire the beam, which gets to the moon at the point where the craft was two and a half seconds ago. If the alien can move ten of its radii (250 metres) in 2.5 seconds in a random direction, our chances of hitting any part of it are just 1%. To do this it only needs a maximum acceleration of 8g.

I think any decent alien fighter could manage that.

So there’s the problem with beam weapons used in space warfare. The speed of light is too slow; it inflicts time delays which cripple the targeting of the weapon. The solution is to forgo the advantages of light speed in weapon delivery for smartness in terminal guidance, but that’s for a future article.