# Could gravitational lensing be used to protect a spaceship from a laser?

Suppose Ship A is being targeted by Ship B, which intends to use a laser weapon in an attempt to destroy Ship A.

Ship A has special technology that allows it to alter gravity (mainly used for generating artificial gravity for the crew and to lessen the effects of inertia when changing direction), but this device also allows it to increase the gravitational pull of the ship, as if it had more mass.

If Ship B points it’s laser weapon directly at Ship A and fires, and Ship A created a gravity well (similarly to an interdictor from Star Wars), could the gravity produced by the ship shield it from the laser by redirecting it via gravitational lensing?

And if so, would using this trick inadvertently (on the captain’s part) turn Ship A into a black hole?

• The Impeller Wedges of David Weber's Honorverse work in this fashion to protect starships, but it's way outside the realm of "science-based", and more of a genre convention to make space battle tactics work the way Weber wanted. – notovny Apr 4 '19 at 15:44
• Yes. If you have very large Black holes you can shift around your ship this is possible. Why you would fear a laser by that time is a different question – Demigan Apr 4 '19 at 16:32
• Gravitational lensing is caused by light that would have missed, but by a small amount. Increasing the gravity would pull more light toward the ship, making it easier to hit. – Mooing Duck Apr 4 '19 at 20:52
• The chief problem is that gravity inherently does not discriminate. it bends space itself. That is why light is affected, despite being massless. But it also means the ship itself is affected. – MSalters Apr 5 '19 at 8:00
• Wouldn't you be "redirecting" the laser to, well, yourself? Lensing looks impressive in photos because light that wasn't originally heading towards us has been "bent" along its path. But Ship B is going to be aiming at ya. So increasing Ship A's mass is not going to help; instantiating a gravity well somewhere between the two ships might be viable though. – Lightness Races in Orbit Apr 5 '19 at 9:21

There are several problems with this.

First of all, when someone fires a laser at you, you aren't going to know it until it hits you, so this would only work if Ship A were CONTINUOUSLY creating a gravity well in between itself and ship B. You couldn't use it reactively without letting it hit you first, although you could potentially limit the damage.

And if so, would using this trick inadvertently (on the captain’s part) turn Ship A into a black hole?

No, but in order to do this you'd have to be able to artificially CREATE a black hole, and a pretty massive one at that, in order to deflect a laser beam any meaningful distance. At that point it'd be simpler to just create the singularity right on top of Ship B and destroy it rather than mess around deflecting laser beams.

TLDR: If you can create a gravity field powerful enough to deflect a laser beam, you're so powerful you don't have to worry about deflecting laser beams.

• This is an excellent example of deep exploration of the ramification of a fictional tech/power - "Tech X does Y... But if we explore it a little more it becomes obvious that doing Z with it instead is infinitely more useful..." Something many sci-fi and fantasy writers fail to do properly. – TheLuckless Apr 4 '19 at 15:53
• What if you can create a black hole up to 10km away from your ship, but the enemy ship is firing its laser from 11km? – Display Name Apr 4 '19 at 19:33
• @DisplayName I'm not sure how much mass is needed to produce sufficient lensing over a distance of mere kilometers, but if you need at least 0.4 solar masses, the enemy is still inside the event horizon. If you need 3.8 solar masses, you are also inside the event horizon. Coincidentally, 3.8 solar masses is the mass of the smallest known black hole. – Ray Apr 4 '19 at 21:48
• Not to mention that is the device was already active, B can just shoot A around it anyway just by aiming differently. – Draco18s no longer trusts SE Apr 5 '19 at 0:58
• @DisplayName Black holes do a terrible job of scattering light. (But if you're making black holes, why aren't you doing literally anything else with this device?) – Draco18s no longer trusts SE Apr 5 '19 at 16:09

Gravity lensing works for light paths skimming the attractor. For light paths crossing the attractor the impact would not be avoided.

So, your device would simply deviate the laser passing around the ship, not the laser hitting in.

Basically, it would work on protecting the ship only if the enemy had a poor aiming.

• You're really just objecting to his use of the term 'lensing' though. His idea would still WORK, you'd just put the attractor right next to the direct path between Ship A and Ship B; you would just call it gravitational deflection instead of lensing. – Morris The Cat Apr 4 '19 at 17:01
• @MorridTheCat OP specifi ally said his device alters the gravity of the ship. The ship is the attractor. – The Square-Cube Law Apr 4 '19 at 21:15

Depends on the range and timing.

• Ship B tracks ship A with visual sensors. Ship A has this "gravity shield" running, so the apparent position of A will be distorted.
• Ship B fires a laser at the apparent position of A, and the laser beam is distorted the same way the detection is distorted. The laser hits the actual position of A.

What you describe might work if the distances are high enough to "re-focus" the "gravity shield" between firing and impact. In that case, wouldn't there be time for conventional evasive maneuvering? So your idea calls for long range combined with an inability to dodge. Even with gravity manipulation technology, does A require fuel or reaction mass? Or is it really large?

Since such a shield would need to be turned on in advance of a laser being fired, it must cover all points of the ship that might be hit, effectively wrapping the ship in a layer dense enough to exert a high enough force on a beam of light so as to effectively make it miss the target entirely. This would place enormous force on the ship itself, as it would basically be drawn to collapse outward into said layer.

Also, as pointed out in o.m.'s post, any changes to the path of an incoming laser beam would also equally change the apparent position of the ship, cancelling out the efficiency of the shield. However, an application of this that fits his parameters of the target being shot at from long distances and being extremely large could be interplanetary laser strikes, so maybe there would be a use. Of course, the ecosystem on the planet would be catastrophically altered by the addition of a black hole layer to the atmosphere.

L.Dutch is correct in his answer. Increasing the mass of the ship would be useless if the enemy has good aim. It will still be hit at the center.

You could then be smart and fire a mass effect missile. The missile itself becomes more massive and deflects the laser. But how much mass would you need?

Einstein himself calculated that the Sun works as a lens with a focal distance of 542 AU's. The formula is:

$$lensing \space angle = \frac{4GM}{rc^2}$$

Where $$M$$ s the mass of the lens, $$G$$ is a well known constant, $$r$$ is the distance from the center of mass and $$c$$ is the speed of light.

So at for one Earth mass, if the beam passes by the mass at a distance of 100 km:

$$\frac{4 \times G \times 6 \times 10^{27}}{10^5 \times (3 \times 10^8)^2}$$

That equals a whopping 2,666,666.666 x $$G$$ degrees. However, G has a 10-11 built into it, so it works out to a fraction that is closer to $$\frac{1}{100,000}$$ degrees. You'd have to go very far for that to cause a miss.

Now think about what you just did. If you are able to summon the mass of an Earth to use as defensive measure, don't use it as a lens, use it as a shield. The Earth has a radius of around 6,400 kilometers. It's more than enough to hide behind if it comes trailing at you, and massive enough that you should not be hit by the laser.

• No spaceship can "hide" behind a planetary mass - it must orbit it at a period relative to the distance at which it orbits. – Pieter Geerkens Apr 5 '19 at 10:31
• @PieterGeerkens if you're able to "summon" a planet, I think keeping yourself behind it propulsively wouldn't be an issue. – TemporalWolf Apr 5 '19 at 17:49