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There are 2 ships in space about 1 AU1 apart.

Ship A fires a deathbeam on Ship B.

The deathbeam travels at speed of light with no acceleration (or deceleration) at all and is guaranteed to hit its target in about 8.3 mins if Ship B does not take evasive actions.

The question stands, How can Ship B, detect the incoming deathbeam, to dodge it in time, Or even if it is possible to detect it ?, given that

  1. it cannot detect a lock-on before the deathbeam was fired, like the modern fighter jets can detect a lock-on.
  2. Neither Ship is FTL.
  3. It needs sometime to take evasive actions (you can alter the time according to answer)

1 One AU (astronomical unit) is equal to 149,597,870,700 meters or about 149,597,870 kilometers.

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    $\begingroup$ deathbeam comes from a source. if the source is shown charging up and pointing at, move. or just random moves. $\endgroup$ – ncalep Oct 30 at 13:11
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    $\begingroup$ The answers below can be summarized as: don't wait until the other fires; take evasive maneuvers now. $\endgroup$ – Shawn V. Wilson Oct 31 at 6:02
  • $\begingroup$ "Travels at speed of light with no acceleration" - in your universe, is it possible for anything to accelerate beyond the speed of light? Because that might change the answers. $\endgroup$ – Mark Oct 31 at 6:46
  • $\begingroup$ @Mark What I meant to signify is that, the beam emerges from Ship A, at the speed of light and does not change its speed throughout the journey. $\endgroup$ – V.Aggarwal Oct 31 at 7:38
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    $\begingroup$ Note that without any faster than light communications, this is comparable to the situation of two people of whom one is gun. There is practically no time to react to the gun being shot, so the best you can do is make it hard to shoot it (in a relevant way) $\endgroup$ – Jasper Oct 31 at 13:38

15 Answers 15

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You have two choices here.

  1. Use mechanisms like wormholes that provide a "shortcut" through space. Wormholes, or wormhole-bearing things are sent out in a screen ahead of you, and can watch the enemy ship and relay signals back to you through the wormhole, thus avoiding the lightspeed lag. This does not need to violate causality or require FTL travel or signalling, though a more thorough discussion on the nature of wormholes and what would happen with overlapping wormhole networks is out of the scope of this question.

    You could of course just fire your deathbeam through a wormhole and avoid the lightspeed lag too...

  2. Just dodge, jink and random walk continuously. There's no way of sending information faster than the deathbeam through simply-connected space, so there's no way for you to know in advance that you've been shot at. You just have to act as if you are under attack, and ensure that if a beam is fired now, there's no way they shooter could know where you'll be in a few minutes time so they can't hit you until they get close.

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    $\begingroup$ An example of the jinking: this fascinating video about WW2 bombers dodging flak: youtu.be/PIYVwqHM488?t=240 $\endgroup$ – Borgh Oct 30 at 12:49
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    $\begingroup$ As I explained in recent answer (here)[worldbuilding.stackexchange.com/a/159682/17556], if you can use wormholes to send information faster than light, you can send information back in time and will actually receive response before you even send those things through it. It still does violate causality, assuming known rules apply. $\endgroup$ – Tomáš Zato - Reinstate Monica Oct 31 at 10:08
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    $\begingroup$ @TomášZato wormholes do not necessarily imply time travel. There have been a substantial number of arguments on this matter by people much more familiar with the issues than you or I, so perhaps you should have a read up on that. $\endgroup$ – Starfish Prime Oct 31 at 10:12
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    $\begingroup$ "This does not need to violate causality" mmm... no, since information usually propagates at the speed of light, that's literally violating causality (because this is what causality means!). I believe the research you're remembering is more about how we can resolve that violation in some self-consistent way, but the violation itself seems completely unavoidable. $\endgroup$ – Lightness Races with Monica Oct 31 at 13:15
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    $\begingroup$ This is a great answer, but there's also a psychological/tactical approach. If B knows where A is, and has reason to believe that A will fire the beam, crew on B can calculate how quickly A would receive information on B's position, vector, and velocity, they can duplicate computing A's firing solution and then maneuver to avoid it. Hard to do in real time, but one of the Gap Cycle books (3, I think?) had a magnificent battle sequence under these exact conditions. $\endgroup$ – Upper_Case-Stop Harming Monica Oct 31 at 18:39
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Based on the information you provided, I will assume that the beam is straight (so it travels along the straight line connecting the two ships in three-dimensional space) and unguided (so once fired, its vector cannot change, even if the ship of origin moves out of position)

If those facts are true, and you know your enemy has this weapon, there are actions you can take. The most obvious solution is to constantly be in motion (not along the vector connecting your ship to theirs), and ensure that at any given time, your position in three-dimensional space is outside the vector that was connecting your ship to the enemy ship, X minutes ago (where X is the distance light needs to travel between your ship and theirs). With a sufficiently advanced computer, this should be enough to guarantee the enemy cannot hit you.

But, there is a big problem with this scenario, and that is the fact that information is travelling along with light. Assume ship A is cruising in empty space, and ship B is "parked" behind, say, a planet (behind meaning that the planet is between ship A and ship B). Light signalling ship A's approach will be reaching the planet, but light signalling ship B's presence will not be reaching ship A. If ship B "emerges" from behind the planet when ship A is 1 AU away, it will become immediately aware of ship A's approach (although its information will be 8.3 minutes late). Assuming a computer is handling the weapons systems (because who would really let humans get their messy hands in there at that point?), with a delay of only a few milliseconds, it calculates the trajectory of ship A and (assuming the ship is not randomly bouncing about in space, since it hasn't detected any enemy presence yet) approximates its most likely location 16.6 minutes in the future (you have to account for where the ship will be now that you saw it, and where it will be when the beam reaches its general location), then fires the death beam, which shoots forward at the speed of light, only a few centimeters behind the information containing ship B's presence. Ship A receives visual information on ship B's existence and the computer frantically orders evasive maneuvers, but it is already too late, because ship A's computer has to handle mass that cannot be accelerated to relativistic speeds, and thus ship A dies. The only hope for ship A in this scenario is, with equally fast calculations, to defend with some weapon or defense of its own that also moves at the speed of light and is capable of surviving / deflecting the incoming beam, but of course, this not only assumes the existence of such tools, but that the collision of two such weapons will not destroy the ship anyway.

To make a long story short, as long as both ships have computers of about equivalent power managing weapons, navigation etc, the game is one of information advantage. Did you manage to perceive the enemy ship first (and do you know it's hostile)? Then you can almost definitely both destroy it with your death beam, or evade its own death beam, and there's very very little the other ship can do to stop it. If both ships become aware of each other at around the same time (highly implausible if we assume they are not just cruising aimlessly through the endless lightyears of interstellar space, and are actually somewhere close to other things that matter), then dodging death is possible if you just constantly maneuver to avoid it.

Then, of course, if only one side has the weapon and the other has to keep dodging around erratically to avoid it, the enemy can just wait for you to run out of fuel and run you down... but this is another story.

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    $\begingroup$ In other words, "stop thinking like a battleship, start thinking like a submarine"? $\endgroup$ – Chronocidal Oct 30 at 22:30
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    $\begingroup$ Quite complex, and yet very amazing scenario. $\endgroup$ – V.Aggarwal Oct 31 at 7:40
  • $\begingroup$ I like the idea that in world with deathbeams, ships will automatically start "dancing" when unknown ship is detected even if it's not a known threat, just to be safe. $\endgroup$ – Tomáš Zato - Reinstate Monica Oct 31 at 10:12
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    $\begingroup$ tl;dr: Peeker's Advantage is an actual thing in outer space combat. $\endgroup$ – Hugo Zink Oct 31 at 15:09
  • $\begingroup$ "a few centimeters" nope, a light-millisecond is much longer than that. A light nanosecond is short enough to hold in your hand, about a foot. Admiral Grace Hopper used to like to give out lengths of wire that were a light-nanosecond long. whatis.techtarget.com/definition/Grace-Hopper-nanosecond. Other than that inconsistency, nice answer. $\endgroup$ – Peter Cordes Nov 1 at 0:26
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This is fairly easy to dodge or avoid.

Ship A fires a deathbeam on Ship B.

Let's assume the best-case scenario for Ship A:

  • Weapon takes 0 seconds to warm up or recharge.
  • Fired deathbeam targeting is sufficiently precise, with negligible error at the target range. If the beam is desired to hit an arbitrary point in space, that hopefully contains a ship, the point will be hit.

The amount of time it takes for light to travel from Ship B to Ship A is about 499 seconds. This means that when Ship A sees Ship B, they are actually seeing Ship B about 499 seconds in the past.

Upon firing from Ship A, the deathbeam takes another 499 seconds to reach Ship B.

As long as Ship B doesn't stay in one spot (or maintain its current speed and direction) for longer than 998 seconds or about 16 minutes, Ship B is safe from the deathbeam.

This is because accurate targeting at that range becomes nearly impossible. As long as Ship B continues to change its course every few minutes, it will be safe at those longer distances.

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  • $\begingroup$ I think the question is about two ships that are "at rest". They already see each other. Maybe they are communicating over the radio. Suddenly... ship A fires! A popular trope states "you can't dodge a laser". This question asks if this is true. $\endgroup$ – daveloyall Oct 31 at 21:22
  • $\begingroup$ It is confusing that you say both "targeting is 100% precise" and "targeting at that range becomes nearly impossible". $\endgroup$ – daveloyall Oct 31 at 21:22
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    $\begingroup$ @daveloyall shooting is 100% precise, but hitting is 100% impossible. Think of trying to hit a moving fly with a sniper rifle at 1 km. The rifle might be perfectly precise (ignoring wind), but you still won't hit the fly. $\endgroup$ – Morfildur Nov 1 at 10:28
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    $\begingroup$ +1, you are never firing at a ship, you are firing at where you hope it to be at 2x light speed distance. You are firing at a probability bubble. Great for ambush, not so great (except for knife fighting distance) for ongoing combat. $\endgroup$ – ShadoCat Nov 1 at 19:05
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Ship A is about to pump out an incredible amount of energy in the form of it's death beam.

It first needs to generate that energy, unless it can do so instantaneously which needs much bigger handwavium than the actual death beam.

So Ship A brings extra reactors & capacitors on line, and as they start to produce and pool energy, some of that extra energy will bleed out, and so be detectable by Ship B.

If Ship B knows Ship A has a destruct-o-beam and Ship B knows what the pre-firing energy spike looks like, then Ship B can have some foreknowledge of the beams firing, and can bob or weave accordingly.

If the death ray takes 30 seconds to charge and fire, then Ship B has 30 seconds notice of the beam arriving.

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    $\begingroup$ This is the equivalent of detecting a lock-on (actual lock-on detection works by realising an increased energy output (from the radar's illuminator) from the attacker). $\endgroup$ – Logan Pickup Nov 1 at 8:03
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    $\begingroup$ @LoganPickup: Cool, that's something I did not know :) $\endgroup$ – Binary Worrier Nov 1 at 9:32
  • $\begingroup$ There's no need for handwavium. If the death beam is gamma radiation, then all it would take is mixing matter and antimatter. There would be mere milliseconds between the time that energy is being generated but is not yet lethal, and the time when the output is at its maximum wattage. Even if it is electrical, sufficient RF shielding could attenuate the signal. $\endgroup$ – forest Nov 2 at 2:32
  • $\begingroup$ @forest: How does Ship A protect it's self from the gamma radiation & neutrino storm the antimatter explosion will unleash? If you're talking "Shields", do they come on line instantly? Really? If they take time to get to full power? If they can come on line quickly enough that they can't be detected, then Ship B's shields can come up just as instantly to protect it from the worst of the gamma radiation. If the shielding is built into the ships bulkheads, why wouldn't Ship B have it built into it's bulkheads too, nullifying the purpose of the beam. $\endgroup$ – Binary Worrier Nov 2 at 14:03
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How can Ship B, detect the incoming deathbeam, to dodge it in time

It doesn't have to.

Before Ship A can shoot at Ship B, it must at least be aware of Ship B's presence. And that's not all it needs to know.

At 1 AU, even the slightest error in aiming could make Ship A's shot miss by hundreds of kilometers. Therefore, Ship A requires extremely precise information about Ship B's position.

Ship B is also unlikely to stay fixed in space. Even if it's not moving towards a specific goal, gravity is still pulling at it. Thus, Ship A also needs extremely detailed information about Ship B's heading, acceleration rate, and possible forces acting on it.

And finally, in order for Ship A to predict Ship B's location 8.3 minutes from now (assuming Ship B isn't moving towards Ship A, or away from it), the information has to be reasonably up to date.

If Ship A can have that, Ship B should have access to similar information (unless they have a spy on board).

Even if Ship B cannot know about Ship A firing until the shot hit, they can still know about Ship A's presence, guess their intent, and start flying randomly to avoid getting hit if Ship A is indeed firing at them.

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Random Walk Avoidance

The beam moves at the speed of light and nothing moves faster (without handwavium). There is no way to dodge because there is no way to detect the firing before it hits you. Others talk about using quantum entanglement but physicists theorize that it also moves at the speed of light so doesn't bypass the speed of light limits.

What you have left is random walk which is you randomly change direction and speed so the enemy can't actually tell where you will be when the shot is fired. It's like dodging a sniper. You can't see him or the bullet and you can't hear the shot fired until after the bullet has already gone past so you duck and weave, roll and jump all the way to cover. All the shooter has is peppering the area with bullets and hope to get a lucky shot

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The true answer will depend on the mechanics of firing the deathbeam:

  1. How long does it take to fire the weapon?
  2. How precisely does it require to be targeted?
  3. How does the beam work (Does it physically destroy the entire target through e.g. heating target material, neutron beam that will destroy life forms but not the structure of the ship)? What I'm asking is, if you're hit is it instadeath for the whole ship or do you maybe have time to conduct an emergency manoeuvre to evade the beam at the cost of having taken some damage? "Deathbeam" sounds to me like a marketing term for something like a laser that will take a bit of time to cut through its target.

If it's a very tight beam (fired from 1 A.U. away it probably must be in order to achieve enough intensity at the target location to destroy it) and relatively expensive to recharge and re-fire, then making frequent random course changes is a very good tactic. Even if it's a constant beam that they can switch on in your general direction and sweep about trying to hit you, once it has been fired up as long as it doesn't score an instant hit you may be able to observe the beam from scattered photons and manoeuvre to avoid the beam as it is steered about.

If you can observe the ship scanning particular sectors of space, then even if you can't detect the actual lock-on you may be able to fly tactically (adopt an ultra-low emission state etc.) to reduce the possibility of detection.

Bear in mind that if the aggressor ship is 8 light minutes away then even using passive scanning (listening for your emissions) it will take 16 minutes from your emission for the death beam to reach the aimpoint (8 minutes for your EM emissions to reach the aggressor ship and 8 minutes for the death beam to propagate at the speed of light to where it is aimed). That means the aggressor is going to have to base their fire control solution on target motion analysis, and if you can manoeuvre rapidly enough to invalidate the assumptions in their TMA algorithm then you can evade.

1 AU is an extremely long range to be targeting something from - at that range, even a spaceship a kilometre across would subtend an angle of 1.38 milliarcseconds. For comparison the Hubble Space Telescope has an angular resolution of 50 milliarcseconds. So even a colossal 1km spaceship would be a tiny fraction of a pixel in a Hubble image, and you then have to be able to form a death beam with approximately that tight a beamwidth, which is also very hard.

Why can't your protagonists detect lock-on? Is it because the targeting is passive, or is it an active tracker that your protagonists don't have the technology to detect? (There is a comparison here with submarine warfare: submarines may generate a fire control solution entirely passively just from listening on sonar and using target motion analysis, or they may ping the target with active sonar. Some navies can transmit active sonar at frequency ranges other navies are unable to detect passively.)

If a broader beam will still result in annihilation then jinking is less practicable because to effect the kind of delta-vee you need to generate any significant bearing movement within 8 (or 16) minutes at a target separation of 1 AU would require colossal amounts of energy and acceleration - you'll need your ships to have inertial dampers for a start, as well as exotic propulsion even if it's not FTL. In that scenario your best bet is probably to avoid detection in the first place - travel EMCON silent in a vantablack painted ship shaped like a stealth fighter and keeping propulsive manoeuvres to an absolute minimum.

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It is possible that more energetic wavelengths of light are slightly slower.

The speed of light in a medium of course depends on wavelength. In a vacuum light is supposed to move at the same rate regardless of wavelength. But there is a theory that space itself is a medium of sorts, which slows photons in proportion to their energy.

https://www.ucdavis.edu/news/gamma-ray-delay-may-be-sign-new-physics

The MAGIC (Major Atmospheric Gamma-ray Imaging Cherenkov) telescope found that high-energy photons of gamma radiation from a distant galaxy arrived at Earth four minutes after lower-energy photons, although they were apparently emitted at the same time. If correct, that would contradict Einstein's theory of relativity, which says that all photons (particles of light) must move at the speed of light.

This theory that space is not empty is called Quantum Foam theory.

Doing some division, I figured that comparing fast and slow gamma photons as per the article (4 minutes over 500,000,000 years), at the distance of 1 light year (to make math easier) gives your target ship a 48 nanosecond warning before the energetic photons hit. That is not enough to move the ship but might be enough to put shields up. If there are even less energetic photons you might get an even longer warning time.


Skeptics - this quantum foam theory is not proven - it is a theory proposed by people who do that sort of thing for a living, not by fiction writers. I am not claiming this is for sure how the world works, just that it might be suitable for a fiction.

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  • $\begingroup$ Instandly powering up the shield requires handwavium technology, because electric energy takes nanoseconds just to switch and move along the wires, and usually on the scale of milliseconds to build up significant power. $\endgroup$ – toolforger Oct 31 at 5:41
  • $\begingroup$ The effect of a shield can never travel faster than the speed of light, and neither can your signals to the shield generator. Ignoring sensor response times, shield strength charge times, and signal-to-generator times, your shield could cover an area with radius 14.3(and a bit) meters around the shield generator IF your shield grows with the speed of light. That's not that big an area for a beam fired a year ago, and this assumes perfect hardware. (and the existence of shields). Not moving predictably sounds like a superior solution to me. $\endgroup$ – Gloweye Oct 31 at 9:21
  • $\begingroup$ I totally agree the shucking and jiving is the better solution. This is my humble effort to find a different solution. $\endgroup$ – Willk Oct 31 at 20:38
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You avoid getting creamed by the death laser by giving the guy who has the death laser something more important to worry about.

Fire off some homing missiles that can detect the enemy and chase him, following a randomized path so that they're harder to hit.

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1 AU is probably too far to hit a target

So the basic idea is that you have distance/c time between the point the enemy fires and you get hit.

What more, you have 2*distance/c time between the time the enemy saw where you where and the point where the beam passes your location.

If you are willing to spend x m/s^2 of acceleration to "dodge" the beam, and we assume your acceleration is small relative to your distance, and you know where the enemy is, you can position yourself uniformly at random within a disk of size 1/2 (x m/s^2)(2*c/distance)^2.

Assuming 0.1 G acceleration used to dodge at 1 AU range your position can be anywhere within a 500 km disk.

Supposing a death-beam radius of 10 m and a ship-radius (along the direction of fire) of 100 m, this means the enemy has a (105 m)^2/(495 km)^2 = 0.000000045 chance of hitting you.

If you instead devoted 100% of your thrust to dodging, you'd be 100x less likely to be hit. If you devote 1% of your thrust, 100x more likely to be hit.

Increasing your range reduces your chance of being hit, but requires thrust. A bit of calculus could calculate the ideal amount of thrust put into dodging depending on enemy fire rate and acceleration and survival goals.

Your chance of being hit grows quadratic-hyperbolically with distance. Halve the distance, 4 times the chance of being hit.

For 0.1 G on a 100 m ship against a 10 m beam to have a 1% chance of being hit, you need to be sqrt(0.01/0.000000045) about 500x closer than 1 AU.

Your chance of being hit grows quadradically with beam-diameter or ship-radius.

So if you are 10x larger (1 km radius) and the beam kill-radius is 100x larger (also 1 km radius), you have a 0.000000045 * 100 * 10000 = 4.5% chance of being killed at 0.1 G dodge and 1 AU range (this assumes even a grazing hit will take you out).

(The math above implicitly assumes the kill-radius of the beam is small relative to the dodge-radius of the ship; so as the chance approaches 100% it will have errors.)

In short, the problem isn't dodging at 1 AU, it is hitting with a mere speed of light weapon.

Generally there will be a range, depending on fire rate and enemy thrust abilities, beyond which it is pointless to shoot. And there will be a relatively small transition zone where you have a low, but reasonable chance of taking the target out. Finally, there will be a short range where dodging is nearly or completely impossible.

A rule of thumb to estimate "you are screwed" range is when (kill-radius)^2 / dodge-radius^2 =~ 0.5, where kill radius is the sum of beam-radius and target-radius.

dodge-radius is 1/2 (x * 10 m/s^2) * (2*distance/c)^2

This happens when distance = sqrt(kill radius * c^2 / enemy thrust).

For a 1 G enemy drive and a 100 m enemy ship, this is sqrt(100 m * c^2 / (10 m/s^2)) = 1 million km.

If you can fire N times, this "kill range" increases by sqrt(N).

Kill Range

The Kill Range of a beam attack is

$$\sqrt{ \frac{N R c^2}{A} }$$

where $N$ is the number of shots you shoot, $A$ is the acceleration of the enemy ship and $R$ is the radius of the "kill spot" on the enemy ship. If your beam has a significant radius itself, you can add it to the kill radius of the shot.

At that range, your chance of scoring a kill is on the order of 50-50. Significantly closer, and a kill is nearly certain.

At 10x that range, you have on the order of a 1% chance of scoring a kill (if the enemy burns all acceleration to avoid your attacks), and it keeps on dropping off quadradically.

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Advanced AI-based Prediction

The best way to avoid something traveling at the speed of light is to "see" it before it arrives. While it isn't technically possible to receive information faster than the speed of light, powerful & highly accurate prediction is just as good.

Since we're dealing with advanced tech here, it's obvious to suppose that AI is fairly advanced. In fact, we are now (in 2019) closer to inventing highly accurate AI-based prediction than we are to inventing starships and deathrays.

A standard-issue ship AI is probably capable of predicting many or most human actions with very high accuracy. It uses huge swaths of data to statistically analyze details that no human would pay attention to. It might even be able to tell you what you will do before you even decide to do it.

What's that? Ship A just vented "imaginarium" from its 74-E exhaust port? That increases the probability of Ship A attacking in 31.849 seconds by 0.0834%. What's that? Ship A just had a barely perceptible output fluctuation in its Delta-4 thrust engine? That increases the probability of Ship A attacking in 922.492 seconds by 0.0297%. Etc. x 1,000,000,000,000.

An AI might also possibly be capable of predicting the actions of other AI, in the right circumstances, although this would be something like a cyber-prediction arms race.

Depending on how long it will take Ship B to take evasive maneuvers (including considering the diameter of the beam and the diameter of Ship B), you may not need to predict that far into the future. Obviously a small, light, and fast Ship B would fare far better.

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Here is another answer, since some have objected to my first answer.

The firing officer on Ship A probably wouldn't fire the deathbeam unless absolutely certain that it will strike Ship B and destroy it.

The OP says that the deathbeam will destroy Ship B if it hits Ship B, even if fired at a distance of one Astronomical Unit, (1 AU).

That is a very impressive and deadly deathbeam.

An AU is defined as being 149,597,870,700 meters or 149,597,870.7 kilometers or 92,955,807 miles.

The deathbeam is defined as traveling at the speed of light, so it should consist of electromagnetic radiation.

And any possible beam of electromagnetic radiation, even a laser, will gradually spread out over distance, and as the cross sectional area of the beam increases, so the density of photons in the beam with decrease, making it less intense.

Suppose that the diameter of the deathbeam doubles every 14,959,787.07 kilometers. That means that its cross sectional area will be four times as large and the density of photons will be one quarter its original value.

After the deathbeam has traveled 29,919,574.14 kilometers its diameter will increase four times from the original value, the cross sectional area will be sixteen times as great, and the density of photons will be one sixteenth of the original value.

And so on.

After the deathbeam has traveled the full AU or 149,597,870.7 kilometers to hit its target, it will have doubled in diameter 10 times, and now have a diameter 2 to the 10th power as large as originally, or 1,024 times the original diameter. Thus it will have a cross sectional area 1,024 X 1,024 times as large as originally, or 1,048,576 times the original area. And so the density of photons will be one divided by 1,048,576 times what it originally was. And yet the OP says the deathbeam will still have a density of photons sufficient to destroy Ship B.

That is a mighty impressive deathbeam.

But would the deathbeam actually increase in diameter so slowly that it doubles in diameter only 10 times while traveling a distance of 1 AU?

As it happens, Apollo astronauts placed reflectors on the Moon. And astronomers on Earth have shined intense laser beams at those reflectors and measured how long it takes for light reflected from those reflectors to be detected by telescopes on Earth. Thus they measure how long it takes for light to reach the Moon, and calculate the slow increase in the Moon's distance from Earth. And of course they also measure the intensity of the reflected light and can calculate how much a laser beam spreads out with distance.

So I am certain there are laser experts who could estimate how many times the deathbeam would double in diameter over a distance of 1 AU and thus how much weaker it would be at a distance of 1 AU, and thus the total energy in the Deathbeam if it was still intense enough to destroy ship B at a distance of 1 AU.

I get the impression that the deathbeam would be so powerful and use so much energy in a single firing, that the situation would not resemble ordinary patrol ship A firing at ordinary patrol ship B. Instead it would be more like Deathstar A firing at Deathstar B, or The Skylark of Valeron firing at Skylark DuQuesne - the ultimate super weapon of one society firing at the ultimate super weapon of another society.

And ultimate super weapons tend to take hours, days, weeks, months, years, or decades to be recharged or refueled, if they don't destroy themselves and have to be replaced by totally new ultimate super weapons each time that they fire.

So you have to make every shot count when firing an ultimate super weapon.

The firing officer on Ship A would know how far away Ship B is. He would know that the deathbeam would strike the position where Ship B was 16.6 minutes (which is 2 times the 8.3 minutes required for light to travel 1 AU) before the deathbeam arrives, and that he could only count on the deathbeam destroying ship B if the deathbeam would be wider than the greatest distance ship B could get from its observed position in 16.6 minutes. And he would probably know the capabilities of engines of Ship B and how far it could travel from it's observed position in 16.6 minutes.

Therefore the firing officer should not fire the deathbeam at the specific distance of 1 AU, instead of some other specific distance, unless he knew that:

1) the deathbeam would spread out enough at a distance of 1 AU that it's radius would be wider than the greatest possible distance that Ship B could travel in 16.6 minutes, and thus that Ship be would be certain to be hit by a part of the deathbeam.

and:

2) the deathbeam would still be so intense, even after spreading out so much, that Ship B would be destroyed by being struck by part of the deathbeam.

If it is possible that Ship B could travel thousands or millions of kilometers from its observed position in 16.6 minutes, the radius of the deathbeam after gradually expanding over a distance of 1 AU would have to be thousands or millions of kilometers to be certain that Ship B would be hit by part of the deathbeam.

Thus the deathbeam might have expanded to millions or billions of times its original radius by the time it reached a distance of 1 AU, and the density of photons at a distance of 1 AU might be only a trillionth or a quadrillionth or a quintillionth of the original density, and yet still be dense enough to destroy Ship B.

I am beginning to get the impression that it might be a good idea to hold a seance and get the spirit of E.E. Smith to describe the awesome intensity of the deathbeam in sufficiently purple prose.

Ship A gets more and more like a Deathstar the more I think about it. And the more that Ship A resembles an ultimate super weapon, the slower its firing rate should be, and the more important it should be to never waste a shot and only fire when absolutely certain the target will be destroyed.

Since the deathbeam will be traveling at the speed of light which seems to be the fastest possible speed in this setting, information about when the deathbeam is fired and where it is aimed out will travel from Ship A to Ship B at the speed of light, just like the deathbeam will travel from ship A to Ship B at the speed of light.

So Ship B should detect the deathbeam being fired at the exact same time it detects the deathbeam hitting Ship B. It is possible that the crew of Ship B will be vaporized before they realize what is happening, and certain they won't be able to detect the deathbeam in time to dodge it.

But Ship B might fire a deathbeam of their own at Ship A and doom Ship A to certain destruction sometime before the deathbeam from Ship A hits ship B.

So far I have discussed a space battle between Ship A and Ship B.

But what about an assassination attempt? Maybe Ship A is sent to destroy Ship B to kill someone travelling on Ship B. If the plannet trajectory of Ship B is known to the plotters, Ship A can take a position 1 AU from a position that Ship be will travel through at a known time. And Ship A can fire the deathbeam at that position 8.3 minutes before when ship B is calculated to be at it, possibly without directly detecting Ship B and knowing if it is following the planned course.

And possibly there are human spies or spy computer programs on Ship A which send a message to Ship B warning of the plans of Ship A. And maybe there are human spies or spy computer programs on Ship B that send messages to Ship A of what Ship B does to avoid destruction.

Thus there could be a tense duel of wits between Ship A and Ship B, each ship taking steps based on information that is 8.3 minutes old.

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Ship B could maintain a set of remote drones around itself at all times spaced out at a distance of less than 1 AU. If the drones were equipped with communicators that were quantum entangled to ship B they could relay information on any firings that they observed instantaneously to Ship B, which could then take evasive action. However, the drones themselves would be vulnerable to the death beam, and this strategy would only work if: ((the distance between the drone and Ship A) - (the distance between Ship A and Ship B))/(speed of light) = (an ammount of time that was reasonable for Ship B to reposition). E.g. if Ship A was 0.5 AU away from Ship B Ship B would receive notification from the drone as soon as it was hit.

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    $\begingroup$ Quantum entanglement doesn't work like this. Particle A receives no information from particle B at any range, they simply have the same state applied to both at their origin, and by observing one of the particles, you can know that the other particle must have the same state. $\endgroup$ – Monty Wild Oct 31 at 3:57
  • $\begingroup$ @MontyWild Thanks, I've been hearing the SF version of entanglement for years, nice to have someone using actual real-world physics :) $\endgroup$ – Corey Nov 5 at 5:32
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Similar to another answer, The quantum computer that runs ship A could have been compromised by an agent of ship B, having on Qbit set aside for entanglement. Ship B would then know any time the weapon is being fired, and if the rotation of that Qbit denotes where it is being fired, they would know that too. Furthermore, because of the nature of entanglement, they wouldn't have to dodge. They could manipulate their Qbit to cause a miss.

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SHORT ANSWER:

YOU ARE THE WRITER AND THE CREATOR GOD OF YOUR FICTIONAL UNIVERSE, SO YOU CAN ARRANGE VARIOUS FACTORS TO MAKE IT MORE OR LESS PROBABLE

LONG ANSWER:

PART ONE OF TWO: WITH A FASTER THAN LIGHT TYPE OF SPACE RADAR TO DETECT THE INCOMING ENERGY BEAM

If no form of faster than light energy technology is available in this fictional setting, go to Part Two.

If Ship B uses some hypothetical fictional type of faster than light (FTL) radiation for some type of FTL space radar it can detect the beam being fired at it and thus decide to move out to the way of the beam.

If Ship B could travel at exactly the speed of light perpendicular to the direction to Ship A it could travel 1 AU before the ray beam reaches its former position. So unless the ray beam can expand at a 45 degree angle and still be deadly at a distance of 1 AU, the target spaceship would be out of the danger zone and the ray beam would pass harmlessly past it without hitting it.

So if technology in this era has advanced to use the FTL radiation for FTL space radar but not yet advanced enough to use the FTL radiation for FTL death rays, the target spaceship, Ship B, can detect and dodge incoming death rays coming at the speed of light.

And when one side manages to use FTL radiation for its death rays before the other side does, it should be able to destroy enemy ships because they won't be able to dodge in time.

Of course in this example Ship B was able to dodge the death ray because it could travel perpendicular to the death beam at the speed of light.

But if Ship B uses any reasonably plausible form of rocket drive or advanced anti gravity space drive, it will not be able to instantly accelerate at the speed of light. In the 8.3 minutes until the death beam reaches the former position of Ship B, that target space ship could reach only a tiny fraction of the speed of light and thus travel only a tiny fraction of 1 AU out of the way.

So the question should be can Ship B travel at a tiny fraction of the speed of light far enough out of the way of the energy beam that it won't be harmed by the energy beam.

So if Ship B can instantly detect the incoming energy beam by using some sort of FTL space radar, how wide will the energy beam spread as it travels a distance of 1 AU while still remaining intense enough to destroy a spaceship that it hits? That will determine the sideways distance Ship B will have to travel in 8.3 minutes in order to be safe. And can Ship B travel fast enough and far enough to get out of the danger zone in 8.3 minutes?

TV Tropes has a trope called: https://tvtropes.org/pmwiki/pmwiki.php/Main/ScifiWritersHaveNoSenseOfScale1

And another trope called: https://tvtropes.org/pmwiki/pmwiki.php/Main/WritersCannotDoMath2

And I personally hate those tropes and i encourage all science fiction, Sci-fi, fantasy, Horror, etc., etc, writers to get a sense of scale and also to do the math, and to be exceptions to those all too common tropes.

You, as the writer of your story, and the creator god of your fictional universe, can set up various factors such as the distance that energy beams are deadly at, and how wide they spread at various distances while still being intense enough to be deadly, and how fast your space warships can accelerate, to make whatever situation you desire in your story.

So you can arrange those factors so that a spaceship with some type of FTL space radar can always detect an energy beam travelling at the speed of slight and dodge out of the way of that energy beam in time to avoid it.

Or you can arrange those factors so that even a spaceship with some type of FTL space radar can never detect an energy beam travelling at the speed of slight and dodge out of the way of that energy beam in time to avoid it.

Or you can arrange those factors so that a spaceship with some type of FTL space radar can sometimes detect an energy beam travelling at the speed of slight and dodge out of the way of that energy beam in time to avoid it. Whether or not a particular spaceship such as Ship B with some type of FTL space radar detects an specific energy beam traveling at the speed of light fired by a specific enemy space ship at a specific distance (1 AU in your example) in time to dodge it safely will depend on the exact values of some specific variables in the circumstances which you, the writer and the creator god, can dictate for that specific situation.

But you face some limitations since there may be other parts of your story where the properties of FTL space radar, light speed energy beams, and spaceship acceleration rates, may also be important. And possibly you might find that the values necessary for one story situation to have the result you desire may be different from the values necessary for another story situation to have the result you desire.

PART TWO: NO FTL SPACE RADAR

If There is no technology using some hypothetical fictional type of faster than light (FTL) radiation for a type of FTL space radar equivalent, there is absolutely no way for the target spaceship, Ship B, to detect the incoming energy beam when it is fired or when it is on the way. Ship B will not know that Ship A is firing upon it until the energy beam strikes Ship B.

Then, there might be time to react to being hit by the energy beam. If the energy beam has to be on the target for 20 seconds in order to destroy it, and Ship B manages to get out of the beam in only 10 seconds, Ship B should survive, though possibly the crew might have lost some years off of their lives due to being in the deadly energy beam for 10 seconds.

On the other hand, the energy beam might destroy the target if it hits the target for a full 0.1 of a second. The human crew of Ship B would not be able to react fast enough to being hit to more Ship B out of the way in 0.1 second. Computers could possibly make the decision fast enough. But how far could Ship B travel in 0.1 second with an acceleration gentle enough for the crew to survive? And could it travel far enough, perpendicular to the energy beam, in 0.1 second to get out of the energy beam?

So if the crew of Ship B can't detect when Ship A fires at them, and can't detect which way the beam is fired, they can only make guesses about those matters and dodge according to their best guesses. Or they can dodge randomly, making evasive maneuvers.

So if ship B knows where Ship A is, Ship B can move at a randomly selected direction that is perpendicular to the direction to Ship A, and move in that randomly selected direction for a randomly selected period of time until turning to another randomly selected direction for another randomly selected amount of time. And so on and son on. Each randomly selected direction would have be perpendicular to the direction to ship A, of course.

Thus shop B would hope to make it harder for ship A to compute their future position when aiming at Ship B. And also hope to possibly, by chance, move out of the way of the energy beam fired by Ship A.

Consider the other ship, Ship A, deciding when to fire the energy beam at Ship B, and deciding to do so when the two ships are separated by a distance of 1 AU, which is equal to 149,597,870.7 kilometers or 92,956,000 miles.

As you say, light takes about 8.3 minutes to travel 1 AU. So the energy beam on Ship A will be aimed at the direction to Ship B 8.3 minutes ago. And the energy beam from Ship A which reach the former position of Ship B after traveling for 8.3 minutes, and thus will hit where Ship B was 16.6 minutes earlier.

So the person who pulls the trigger on Ship A will do so knowing that the energy beam will hit where Ship B was 16.6 minutes before the beam hits. So why should they fire at where the target was 8.3 minutes before firing, and where the target was 16.6 minutes before the beam will reach that position? They shouldn't fire at all, unless they know that Ship B can not possibly get out of the way of the energy beam in a mere 16.6 minutes.

Do the space ships in this setting have rocket engines and not some hypothetical super advanced form of space drive?

If Ship B will use rockets to get out of the way, what is the maximum acceleration that the crew of Ship B can survive for 16.6 seconds? That maximum acceleration will determine the maximum distance that ship A can travel in a straight line perpendicular to the direction to ship A in 16.6 minutes. And that in turn will enable someone to calculate the total distance that Ship B can possible travel from where the energy beam is fired at during a time of 16.63 seconds, and thus whether it is possible for Ship B to get out of the cone of destruction of the energy beam in 16.6 minutes.

What if Ship B doesn't have rocket engines but some type of hypothetical space drive that uses anti gravity or something to accelerate much faster than a rocket can, without the crew feeling or being crushed to death by that acceleration?

In that case, Ship B could travel much farther in 16.6 minutes than if it had only rocket engines, and thus it could be much farther from its original position when the energy beam arrives at its original position.

And again, it should be fairly easy from someone who knows about the engines on Ship B to calculate how far it could possibly get from one position in 16.6 minutes. Thus it should be routinely simple to calculate whether Ship B can possibly get out of the way of an energy beam in 16.6 minutes.

The firing officer on Ship A will know how long it will take the energy beam to reach where ship B was 8.3 minutes before firing. And he will know that his energy beam, no matter how concentrated, will gradually spread out over time and distance until eventually it will be too thin to be deadly. And the firing officer will also know how wide the energy beam will spread, and how deadly the energy beam will be, at any specific distance, such as the 1 AU in your example.

And I see no reason for the firing officer to push the button or pull the trigger unless they know:

1) That the beam will be intense enough at a distance of 1 AU to destroy Ship B.

and also:

2) That the beam will spread out far enough travelling 1 AU that Ship B can not possible get out of the energy beam even at maximum acceleration for 16.6 seconds.

I suppose that some more optimistic firing officers might fire if they believe that it was merely probable that both factors applied, especially if the beam weapon could recharge and shoot again rapidly.

But I strongly doubt that an energy beam powerful enough to destroy a space ship at a distance of 1 AU, despite spreading out and weakening countless millions of times over the distances, could be recharged in seconds or minutes.

If ship A is firing at Ship B at a distance of 1 AU, and if both ships probably come from planets in the same star system, since they don't have a faster than light drive, it doesn't seem like ordinary patrol ship A firing at ordinary patrol ship B to me.

Instead it seems more like Deathstar A firing at Deathstar B, or The Skylark of Valeron firing at Skylark DuQuesne (minus the interstellar setting of those stories, of course). Each ship should be the supreme ultimate weapon of its planet.

And I can believe that the supreme ultimate weapon of an advanced society would probably take hours, days, weeks, months, years, or decades to be recharged or refueled each time it is fired, if it doesn't destroy itself as well as the target the first time it fires anyway. I find it really hard to believe that the supreme ultimate weapon in a space war could fire as rapidly as every few seconds or every few minutes.

So I have to believe it is extremely probable that the firing officer on Ship A won't fire unless absolutely certain that the energy beam will hit and destroy Ship B.

So if Ship B can not use some short of FTL space radar to detect the energy beam fired by Ship A and get out of the way, Ship B should be doomed. It should be impossible for Ship B to take any type of evasive action sufficient to evade the energy beam from Ship A.

Ship A should never fire its super powerful energy beam at Ship B until it is impossible for Ship B to avoid being hit by the energy beam.

The relatively good news for ship B is, if it has a similar super powerful energy beam, it might fire its own beam at Ship A sometime before being hit and destroyed. Thus ship A might possibly also be destroyed in the conflict.

One possible variation on this scenario might be an assassination attempt instead of a regular space battle.

Someone important enough for others to want to kill them is traveling on a spaceship, Ship B, from one place to another using a more or less easily predictable trajectory. Their enemies have calculated that trajectory, and they position Ship A at a position 1 AU away from a spot where ship B will be at a specific calculated time. And 8.3 minutes before Ship B will be at that spot, Ship A fires the beam weapon at the location where ship B will arrive in 8.3 minutes.

This is the perfect murder, in so far as it being impossible for the victim to do anything to escape assassination, or even to know about it before they die.

But what if living or cybernetic spies aboard Ship A report their plans to Ship B using secret transmitters? Then Ship B can try to change its course to avoid being blasted by the energy beam, and possibly also try to shoot at ship A.

And if there are spies aboard Ship B, they might report the attempted evasion maneuvers to Ship A. And with messages taking 8.3 minutes to arrive from one sip to another, and thus possibly being 8.3 minutes out of date, the game of cat and mouse might continue for some time.

TV Tropes has a trope called: https://tvtropes.org/pmwiki/pmwiki.php/Main/ScifiWritersHaveNoSenseOfScale1

And another trope called: https://tvtropes.org/pmwiki/pmwiki.php/Main/WritersCannotDoMath2

And I personally hate those tropes and i encourage all science fiction, Sci-fi, fantasy, Horror, etc., etc, writers to get a sense of scale and also to do the math, and to be exceptions to those all too common tropes.

You, as the writer of your story, and the creator god of your fictional universe, can set up various factors such as the distance that energy beams are deadly at, and how wide they spread at various distances while still being intense enough to be deadly, and how fast your space warships can accelerate, to make whatever situation you desire in your story.

You can arrange those factors in your setting to make the story happen as you wish.

And if Ship B doesn't have any sort of FTL space radar, it can only be warned of the attack by spies aboard ship A sending a message ahead of time about what Ship A plans to do. And since Ship A can change its plans between the message being sent and actually firing the weapon, the information sent by the spies could be out of date and misleading.

As the writer you face some limitations since there may be other parts of your story where the properties of FTL space radar (if any in your story), light speed energy beams, and spaceship acceleration rates, may also be important. And possibly you might find that the values necessary for one story situation to have the result you desire may be different from the values necessary for another story situation to have the result you desire.

But it is up to you, the writer of the story, and the creator god of your fictional universe, to try to create a story that is as interesting, and a fictional universe that is as consistent believable, as you can.

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    $\begingroup$ -1 for shouting at the start of the answer. I skipped the rest. $\endgroup$ – EvilSnack Oct 31 at 2:22
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    $\begingroup$ @EvilSnack I read some of it and skimmed through some of it, and it actually seems like a decent answer other than the capital letters. Way too long, though. $\endgroup$ – overlord - Reinstate Monica Oct 31 at 4:48
  • $\begingroup$ I didn't read it all either, but there are actually whole duplicate paragraphs; and even without the shouting, the "short answer" doesn't really answer the question. $\endgroup$ – Logan Pickup Nov 1 at 8:19
  • $\begingroup$ @EvilSnack The purpose of up/down voting is to help determine whether an answer was useful or not. Downvoting because you didn't like his use of capitalization, which isn't necessarily shouting but rather to draw attention, is a misuse of the system. It is clear you could not have made a fair assessment if you "skipped the rest". $\endgroup$ – Vinny Scalon Nov 1 at 14:32
  • $\begingroup$ @Evilsnack I created a new answer which you might possibly like better. $\endgroup$ – M. A. Golding Nov 1 at 18:23

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