Ten light seconds is a very long distance: 1.86 million miles (3 million Km). For comparison, the moon is about 1.3 light seconds from Earth and the shortest recorded distance between Earth and Mars is 187 light seconds.

Let us assume that an attacking battleship has the ability to lock onto a defending battleship ten light seconds away (this ability, whatever it is, does not benefit the defender). The energy beam is photonic and delivers approximately 109 joules of energy at a distance of ten light seconds. Let us also assume that the beam can be sufficiently focused that it arrives with no more than one meter of dispersion.

For the purpose of the question, let us assume the defending ship does not know the attacking ship is present.

Finally, let us assume that if the defending ship could detect the incoming beam with two seconds to spare, it could maneuver to avoid the attack. (Oh, and no shields. It's a whomping big problem for the defending ship to be hit by 109 joules of energy for any period of time.)

OK, one more thing. Ignore the length of time the energy weapon is activated. That's actually irrelevant to the question. Whether the beam was on an attosecond or all ten seconds of transit doesn't change the question.

Question: Is there anything about photons and/or space-time that we know or theorize today that would suggest it's possible for the defending ship to detect the incoming beam of energy before it strikes the ship (ideally two seconds before it hit the ship, but at all is the question)?

Please note the tag.

EDIT I apologize that I went to bed after writing the question and so didn't see any of the comments. It's obviously true that the photons, themselves, cannot be detected before impacting on the defending ship's sensors — which would suggest there's no way to detect the incoming beam. However, the Voyager space probe detected the bow of the solar magnetosphere and wind before passing into interstellar space, where it found hot plasma. It got me wondering if the passage of light might "push" something before it, like the bow of the solar magnetosphere or wind, and that incoming rush of "something" could be used to detect the incoming beam.

From this perspective it might be necessary to indicate where the attack is taking place, since the solar wind and magnetosphere are keeping most of the stuff that might be "pushed" out of the solar system, meaning it might be easier to have a successful detection in interstellar space. However, that might also simply mean it's harder inside a solar system, but not impossible. At worst, please consider both locations. If we all believe light can be used to push a space craft then it seems obvious that, so long as there is something to be pushed, that something could be used to detect the incoming beam.

  • $\begingroup$ Comments are not for extended discussion; this conversation has been moved to chat. $\endgroup$
    – L.Dutch
    Commented Jun 11, 2020 at 17:24
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    $\begingroup$ I'm assuming that you aren't interested in sideband solutions, i.e. watching for the other ship to open their weapon bay doors? $\endgroup$
    – bta
    Commented Jun 11, 2020 at 19:43
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    $\begingroup$ Where is the firing ship's beam production process on the spectrum between "push button; flock of photons instantly hurtles toward target" and "some guy in a space suit with vary long tongs has to rub two charged black holes together for a few minutes to build up enough charge to fire the weapon"? Maybe we don't detect the beam, we detect all the electromagnetic flailing during the seconds before it departs. (imagery similar to this: Consider the visible scatter from the tributary beams for the second(s) prior to the launch of the main beam from the Death Star.) $\endgroup$ Commented Jun 11, 2020 at 20:07
  • $\begingroup$ Similar? worldbuilding.stackexchange.com/questions/159685/… $\endgroup$
    – Willk
    Commented Jun 11, 2020 at 22:58
  • $\begingroup$ @Willk, that's a near duplicate. The only practical difference is that I'm interested in what characteristics of light, if any, would allow someone to detect the incoming beam of light. $\endgroup$
    – JBH
    Commented Jun 12, 2020 at 2:07

7 Answers 7


Since it is hard-scince question - the answer is NO, defending ship can do nothing to detect incoming laser attack from enemy ship it does not see.

Argumentation is obvious: speed of light is the fastest way to propagate info, so that info arrives as a gigajoule laser burst. Since by your conditions defending ship know nothing about attacker - it can not detect any preparations for the shot.

To solve that problem defender can apply exactly the same tactic as seaships were (and are at miltary exercises) using against submarines: just periodicaly change course at random.

If that defender ship has charachterstic size 100m, it just needs to fire it's side thrusters at random direction and interval (but no more than 10 seconds) with deltaV slightly more than 10 m/s. It means it will spend about 1 m/s of it's deltaV reserve every second. Thus garantee that attacker at 10 light seconds distance will always have an outdated aim. Information propagation speed limit works in both ways.

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    $\begingroup$ The same tactic (of periodically and semi-randomly changing direction) was also used by WWII bomber aircraft to avoid flak. $\endgroup$
    – Izzy
    Commented Jun 11, 2020 at 14:05
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    $\begingroup$ @ksbes, the interval between firing thrusters can be as long as 20 seconds. To aim the laser, the attacking ship needs to know the position, course and speed of the other ship is, and this information travels at light speed too! $\endgroup$
    – Vorbis
    Commented Jun 12, 2020 at 7:36
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    $\begingroup$ Insects on Earth use the same trick, relying on that they're much lighter than their predators. $\endgroup$ Commented Jun 12, 2020 at 13:22
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    $\begingroup$ "speed of light is the fastest way to propagate info, so that info arrives as a gigajoule laser burst." +1 just for this $\endgroup$
    – DenisS
    Commented Jun 12, 2020 at 16:20
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    $\begingroup$ Using 1m/s^2 or even 1/2 m/s^2 of deltaV isn't cheap. Delta-V is precious. You really only want to burn it up when you really need to. Instead, what about angular momentum? You spin the ship randomly, and it's vital organs' locations are unpredictable. Or reaction mass attached to tethers that can be reeled back in randomly? $\endgroup$ Commented Jun 13, 2020 at 0:52

You won't see the laser coming, but you will see it charging up to fire

Ksbes's answer is a really good idea if you know you are in a fight, but if you are unaware as the OP stipulates, chances are you are not making continuous major course corrections.

As for solving your problem, you will not be able to see the beam coming at you, but lasers that pack enough juice to cause meaningful damage use up a LOT of power. And generating power creates heat which also radiates at the speed of light. While you might not be able to see the ship from 10ls away while it is hiding and waiting to ambush you, once it is ready to act, it will need to spool up its reactors to charge the weapon. If the weapon has a 2 second charge time, and your sensors are adequately sensitive, then it would be the heat from charging the weapon that would clue you in that something out there is about to shoot at you.

This would give you the moment you need to start evasive piloting and then everything in Ksbes's answer holds true moving forward.

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    $\begingroup$ This is the best answer that meets the objective of the question. You can posit a heat/radiation/neutrino spike that peaks 2 seconds before the incoming energy beam as a result of reactors or other energy sources coming on line to power or charge the weapon. $\endgroup$ Commented Jun 12, 2020 at 13:15
  • $\begingroup$ Too minor for an edit, but you say 'ly' for lightyear but the question is in light seconds. :) $\endgroup$
    – Erik
    Commented Jun 12, 2020 at 13:28
  • $\begingroup$ @thanks, typo fixed. $\endgroup$
    – Nosajimiki
    Commented Jun 12, 2020 at 13:53
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    $\begingroup$ This is my instinct as well; hard-science dictates you have finite generator capacity, would need some sort of capacitance, and the faster you charge it the hotter it gets. The preparation to fire can provide warning, much like in submarine warfare, where flooding torpedo tubes and opening the doors can be heard by enemy submarines, even if you had been previously undetected, as long as they are listening. $\endgroup$
    – CodeShane
    Commented Jun 13, 2020 at 7:45
  • $\begingroup$ Alternatively, you might see stray light from a near miss, the next shot coming a couple seconds later. $\endgroup$ Commented Jan 2, 2021 at 5:53

The speed of light is the fastest way for information to travel. It is absolutely impossible (from a hard science PoV) for your targeted ship to know that the firing ship has fired before they are hit (or before the shot misses if it was incorrectly aimed)

There are still a few ways you might be able to avoid being shot by an attacker you do not know is there though

  • Randomly altering your course: lightspeed propagation of information cuts both ways. If your attacker is 10 light seconds away from you and you randomly adjust your course at least every 10 seconds (it is even better if the time between course changes is also random), it is impossible for them to know where you are with enough certainty for accurate targeting. They then have no choice but to either let you go or fire shots that are almost certain to miss
    • pros: very good protection, at pretty much whatever distance you like
    • cons: you're having to burn extra fuel for all of this dancing around the route you actually want to take. If you want protection at closer ranges, or from larger area of effect weapons, you need to change course more drastically, and more often. Random changes of course are likely quite unpleasant for anyone onboard
  • Keep a look out for some early part of the firing process: in a city fight, it's not generally possible to react to a muzzleflash before the bullet hits you, but it is possible to keep an eye out for a glint from the sniper's scope. Similarly, it may be possible to detect some early part of the attacker's firing process, for instance the targeting lock, or the weapon warming up. Provided this early part of the process occurs long enough before the weapon actually fires, your targeted ship has the opportunity to take action
    • pros: doesn't use any extra fuel and doesn't cause any discomfort to anyone onboard when not under attack
    • cons: keeping a constant lookout across the whole celestial sphere is tricky. To avoid blind spots (although admittedly, these blind spots are going to be near the ship so less relevant here) you'd need at least four lookout points. It's also going to take a fair bit of computing power to rapidly compare all this data to look for anomalies that could be due to an attacker. Depending on how long the firing process is you might have a very short amount of time in which to dodge, so it may necessitate more extreme evasive action than in the first case (e.g. if the firing process takes 2s, you need to have moved more than the width of the beam within 2s, whereas with constant dodging, you have 10s to do it)

I think the cons of randomly altering course are sufficiently high that you're unlikely to want to do it unless you think there is a very high chance of you being attacked*. Keeping a look out, and only switching to random course adjustments once the enemy is spotted is almost certainly the best option available in your hard science world

  • $\begingroup$ If you have good CMGs or reaction wheels and your ship isn't a ball and you have egregious amounts of power, you can save lots of fuel by rotating in certain axes every so often. If you picture the classic cigar-shaped Star Destroyer or something, it could just pitch down or yaw right and dodge most shots while spending relatively little fuel. The counter to this is to aim only within a certain part of the ship's center of mass... which means that your target just shrunk by some percentage. $\endgroup$
    – William
    Commented Jun 11, 2020 at 23:58
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    $\begingroup$ On "Looking for some early part of the firing process", you only need (according to the querent) 2 seconds of warning, not the full lightspeed delay. This assumes the ship is able to make sufficiently drastic course corrections in 2 seconds to not be where it would have been at the time of the laser's arrival. $\endgroup$
    – BBeast
    Commented Jun 12, 2020 at 1:25
  • $\begingroup$ @BBeast good point, I'll correct my answer $\endgroup$
    – Tristan
    Commented Jun 12, 2020 at 8:47
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    $\begingroup$ @William if your ship is convex, the attacker can always aim at the center of mass and have a guaranteed hit. A two-lobed ship can do the trick, but you'll need a lot of struts if you want to randomize your orientation in 20 seconds without becoming a pair of single-lobed ships. $\endgroup$ Commented Jun 12, 2020 at 13:29
  • $\begingroup$ I'm not sure how to apply the adjective "convex" in this context. "They can just fire at your center of mass" is already a big win for you, since it means your ship just got potentially a lot smaller in terms of targetable cross section. And of course faster rotation is better for evasive purposes and would require a lot of structural integrity, any amount of randomized rotation along the short axes of the ship is evasion that doesn't need to cost fuel. Failing that, I think the fuel cost for rotating the ship might be less than translating it. If nothing else, it's more room for randomness. $\endgroup$
    – William
    Commented Jun 12, 2020 at 22:31

No Target Lock

A "target lock" as used in modern military parlance refers to an active radar imaging a target with fairly high resolution and accuracy. A target which is 10 light seconds away cannot be "locked" in this sense of the term. It can be detected passively, by watching blackbody radiation or engine heat, or actively, by sending out EMR pulses (radar/lidar).

Now, regardless of active or passive, whatever positioning information is received is necessarily 10 seconds old! As every other answer observes, a lot can happen in 10s. The time between a positioning ping and beam-on-target is 20s!

Passive Targeting

Unless the ship is backlit by the sun, or actively thrusting away from the attacker, it will be very difficult to detect it precisely and passively. If the defender has very efficient engines that don't leak thrustable energy out the sides, the only EMR you will see is whatever blackbody glow the defender is unable to hide. At 10 light seconds, this glow will be very faint. And it will also not be sharply resolved, because it will be coming off every part of the ship, going in every direction. It will be blurry, and moving. So, the best the attacker can do is build a predictive model of where the ship will be in 10s, and hope it does not change course in that time. That isn't a "lock". That is quite literally a shot in the dark.

Active Targeting

This is much more dangerous, because any space navy worth its salt will know which frequencies are typically used for targeting and will have passive sensors to pick up any scanning attempts. Active targeting pretty much announces both your hostile intentions as well as your direction, giving the target 10s to maneuver out of your "lock". Because you can throw a lot of energy at your imaging target, you can get a much sharper picture of it. A coherent, collimated beam with a lot of photons can resolve your target's location and velocity to as much precision as you are willing to spend energy on. But the more energy you spend on imaging, the more information the defender has about your location, too! So, it's really a kind of bidirectional targeting system!


What a defender wants to do is frustrate the attacker's ability to determine their position and direction. The best way to do that is via stealth and misdirection: i.e., decoys. You want to make sure that your ship doesn't have nice sharp angles which reflect back the most common targeting frequencies to the attacker. You want EM baffles that bounce the energy around and reflect it at oblique angles that are away from the attacker (this is roughly how stealth paint coatings work today). You can take this a step further by maintaining a fleet of drones that escort your ship at a distance. They can have "tunable reflection". That is, they can have a bright, shiny side designed to look like your ship, and a dull, stealth side that is designed to hide. By manipulating these drones, combined with stealth characteristics of your ship, you can possibly cause an active targeting radar to misjudge your position and velocity.

Also, once you receive a targeting ping, you can actively launch decoys with decent sideways velocity to actively mislead the targeting radar in a different trajectory. The trick is for the decoys to present a compelling radar signature that could plausibly be your ship, without overdoing it and announcing that they are just decoys. I'm sure there would be an arms race surrounding this technology.


To overcome the "bidirectional lock" problem, as well as mitigate the effects of ECM, attackers will deploy counter-counter measures, such as their own targeting decoys. These will be drones that are designed to fly a trajectory which masks the attacking ship, and also provides a much wider effective radar aperture to catch stray bounces of the targeting radar. This is how current anti-stealth technology works today (multiple ground stations pick up spurious radar bounces to assemble a virtual picture).

The end result will be huge spheres of possible ship positions, with the actual attacker and defender maneuvering somewhere inside those spheres. And unless they are severely outmatched, it is ambiguous to say that one ship is the attacker and the other the defender.

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    $\begingroup$ While the points on the challenges of precise targeting and detection are good, I'd contest the target being "very faint" at 10 lightseconds. Figures in projectrho.com/public_html/rocket/spacewardetect.php#nostealth indicate that for reasonable parameters a spaceship simply coasting with its life support and reactors running is visible up to distances on the order of 100 lightseconds. This is a ~300K object against a 4K backdrop, after all. But I agree that this object will be a tiny dot, but you could use laser optics as telescope optics since your laser can focus that far. $\endgroup$
    – BBeast
    Commented Jun 12, 2020 at 1:55

If you want to stick to hard science, there is effectively nothing you can do, as any radiation to indicate they are firing (visible or otherwise) will be travelling at the same speed as the actual pulse that will damage the ship.

If you're insistent on this being necessary for your story there are two options I can see.

The first option is based on estimating when your enemy will fire, based on the image you see of the enemy ship 15 seconds before they fire (for example), such as turrets moving, weapons powering up etc. which will arrive 5 seconds before you are hit by their pulse. If you know roughly the rate of fire their weapon can achieve from then on, you can infer when you need to start taking evasive actions again.

This would only work if the enemy only has one main battery. If they have 4, as WW2 battleships often would, then they could simply stagger the firing of each battery so you are constantly forced to be moving evasively.

However,this is a perfectly sensible behaviour for a ship to be doing in combat, as the enemy ship also only knows where you were 10 seconds ago, and needs to predict where you'll be. By constantly changing course while closing distance or launching guided munitions, it will be just as difficult for the enemy to hit the ship as it is to guess when the enemy ship will fire (assuming the ship can accelerate at reasonably relative to its size- provided that 1/2*acceleration*time of flight^2 is greater than the length of your ship you should be fine).

The second option, which is substantially more handwavey and not hard science by any stretch is creating very small wormholes in the vicinity of the enemy ship. By observing the ship through that wormhole, they can gain roughly a 10 second lead (ignoring the time to process the data once you receive it). This would give you ample time to prepare once they fire.

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    $\begingroup$ If you can use wormholes, why not fire the laser through the hole after you check to see if the enemy is still there? Besides, the hard science tag is in effect. $\endgroup$
    – NomadMaker
    Commented Jun 11, 2020 at 14:42
  • $\begingroup$ @NomadMaker, we are, of course, beyond the realm of hard science at this point, so any rationale is pure hand-waving. That disclaimer being said, maybe the wormholes destabilize when a certain amount of energy passes through them. The threshold could be high enough for observations, but too low to pass through enough energy to do more than slightly warm the enemy's paint job. $\endgroup$
    – Matthew
    Commented Jun 11, 2020 at 15:09
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    $\begingroup$ You're not going to get imaging of a spaceship at 10 light seconds range without a telescope the size of a small planet. You may be able to know it's there by optical/radio etc. means but that's all, unless of course the attacking ship itself emits something like a targeting beam or the pump is somehow detectable as suggested elsewhere $\endgroup$
    – Chris H
    Commented Jun 11, 2020 at 15:56
  • $\begingroup$ @ChrisH, you're not going to hit a spaceship at 10 light-seconds with a laser unless you're firing through a telescope the size of a small planet. Diffraction limits work both ways, after all. $\endgroup$
    – Mark
    Commented Jun 12, 2020 at 0:37
  • $\begingroup$ @Mark good point - so both ships would need such a telescope. Except that a Bessel beam could deliver power but not imaging, and might get you down to a telescope the size of "that's no moon". I'm now avoiding thinking about gravitational lensing with the ships either side of a neutron star $\endgroup$
    – Chris H
    Commented Jun 12, 2020 at 9:03

As other answers say, the speed of light is a hard speed limit — within the understanding of present physics, there’s no way to get around that.

However, this doesn’t mean there’s no hope. This just means that the ten light-second distance is essentially irrelevant. The possibilities are exactly the same as they would be if our defender was right next to the attacker, or a whole light-year away.

So what this means is: the defender must be able to anticipate the firing somehow. This could happen various ways. One idea: the firing itself is a non-instantaneous process, involving a few seconds charging for the laser before the main pulse is release, and this charging is detectable through some subtle emissions, if the defender has a sensor tuned to listen for them. (This doesn’t require the defender to know of the attacker’s presence: think of them as a wary zebra, always alert for the scent of lions.)


In response to the edit of the question, it does not help to try and use the ability of light to give something a push. When light does that, it just gives it a bump in the given direction, but never enough to keep the thing ahead of the wavefront. In some sense, the "No" answer to your question is just a vivid restatement of the principle that nothing can travel faster than light. A photon never sends a warning ahead of itself.


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