# Can a laser in outer space really appear as visible “bullets”?

Inspired by Star Wars and other sci-fi space series, we often see their ships shooting lasers as a "big needle", shot three or four times consecutively to perform a volley of laser fire, similar to our metal bullets.

I know from other threads that laser space battle is very unlikely, if not impossible to do, but this question is not about that. I'm also not interested in this laser being able to do (substantial) damage.

Using a similar, but empowered version of today's laser machinery as a weapon in the outer space, is it possible to make a "laser bullet" that is visible to our naked eye in the outer space?

It does not have to be "big". A thin line like a sniper targeting through a smoke screen is enough. A brief series of laser is what I wanted. I'm not concerned with the distance (because laser loses power on a distance). I'm more interested in making it visible and "bullet-like" in series of shots.

Update: This is not a duplicate of Could it be possible to build a Pew Pew Laser? since that the answers in that question focusing on "pew, pew" as in its sound. My question is more about making the laser visible in its trajectory as "bullets".

• Actually, when the laser cannon fires it farts a bit of smelly gas. That's why you get "pew, pew, pew". – Hot Licks Aug 26 '17 at 20:01
• – Secespitus Aug 26 '17 at 21:23
• @L.Dutch I'm even surprised that typing the title didn't even suggest to me those questions. With "pew, pew, pew" I'm actually referring to the 3 shot salvo (and a reference to one of Dota 2 heroes voice) – Vylix Aug 27 '17 at 9:05
• @LeonMeier: not quite. Blood pressure in the eye is dictated by the structure of the capillaries, so the blood wouldn't boil. The liquid on your eye would though, and the capillaries would start to burst as they're quite delicate, but you'd be able to see a few pew pews before blacking out! – Joe Bloggs Aug 27 '17 at 9:15
• @JoeBloggs In vacuum without a suit, you have 15 seconds before you are out. As soon as you hit vacuum, air escapes your lungs, if you try to hold your breath, you will cause ruptures and then lose breath. After that, vacuum will actively de-gas blood, as soon as outgassed blood reaches brain, you lose consciousness and it's over. You have about 15 seconds total. You could survive a bit more if someone else pulls you back into safety, but timetable is much much harsher than in ordinary hypoxia. – M i ech Aug 27 '17 at 15:55

In Star Wars, they shoot plasma made of energized gas (Tibanna). Plasma travels significantly slower than the speed of light, so the human eye can perceive it as a 'bullet' kind of thing.

Making a laser bullet isn't feasible with upgraded versions of modern laser weaponry, since a true laser travels at light speed - every shot would appear to be a line between target and source unless the distances involved were enormous. Even if that's okay, you still have the problem that lasers are only visible because of light scattering off of things in the way. Vacuum wouldn't have enough to make this significant, and enough scattering to make it visible would probably result in blooming, which would prevent the weapons from being very effective.

Now, if you want something a lot like the Star Wars style, you can take off your skepticism hat and look into the Marauder project. If The internet is to be believed, there exists technology (as of 1993) which can launch hyper-projectile rings of energy. If you were far enough away for the relative velocity to appear to be less than ~60 mph, then you would perceive it as a laser bullet kind of thing. (NOTE - the project seems 100% real - the skepticism is in whether or not it really achieved what it claimed.)

• All lasers do not necessarily appear as "a line unless the distances were enormous". Some of the most powerful lasers are used in pulse mode, delivering a huge amount of energy in a short time frame. For instance femtosecond lasers pulses are shorter than a um. – Benjamin T Aug 27 '17 at 10:17
• "Plasma travels significantly slower than the speed of light, so the human eye can perceive it as a 'bullet' kind of thing." That is besides the point. Plasma is hot and thus radiates light. Laser light on the other hand is only visible when scattered or hitting stuff - given that space is rather empty, even "slow" light would not be visible at all. – Chieron Aug 27 '17 at 12:57
• @Chieron definitely mentioned the light scattering stuff - see 2nd paragraph. – Jeutnarg Aug 27 '17 at 13:06
• For laser to appear as line, there must be something for it to partially disperse on, like atmosphere. Not in space. In space, only the shooter and the target can tell that a laser is being fired at all. – Agent_L Aug 28 '17 at 12:30
• @BenjaminT, even if the pulse is only one micron long, the response of your eye is so slow that (if the wavelength was visible and there was enough scattering to see it) you'd see it as a pulse around 15,000 km long (assuming 1/20 s eye response). Maybe that's small enough to see as a distinct pulse on the scale of a space battle...but it's not consistent with the kind of space dog-fight where you can actually see your adversary with the naked eye. – The Photon Aug 28 '17 at 16:36

A laser beam is made by a bunch of photons travelling with the same phase and direction.

Thus, the only way to see it (when you are not the target) is that the beam is scattered along its path.

In open air this is possible, since dust is everywhere together with other particles, giving the usual appearance in laser labs

But when you are in space you hardly have any particle around to scatter photons.

So you won't see any laser beam when it's fired.

• Well ... in principle, at least, no matter how thin the atmosphere a sufficiently powerful laser will be visible. It's just a question of whether the power required is too high to be plausible. – Harry Johnston Aug 27 '17 at 6:54
• @HarryJohnston yes, however even if you had such a ridiculously powerful laser that it would significantly scatter from the particles in a space-physics plasma, the effect would be very different from a normal laser scattering from air. Such a plasma is extremely hot (> 100 000 K, more typically many million K) – the few particles there are in it already have much more thermic energy than a normal laser can ever pump into a molecule from the air. I really don't know what the result would look like, but pretty sure it wouldn't be a sharp monochromatic ray. – leftaroundabout Aug 27 '17 at 11:00
• @HarryJohnston This is not true. For one, in intergalactic space the pressures can reach $10^{-17}$ torr, and the number of particles per cubic centimeter then drops to single digit numbers. Regardless, even with a finite number of particles, there's a limit to how fast the particles can scatter photons, related to the saturation intensity of the particles' transitions. – aquirdturtle Aug 27 '17 at 19:30
• @aquirdturtle, I didn't interpret the question as talking about intergalactic space, but even then, if your laser beam is big enough (say a light-year across) I'm sure it would be visible. :-) – Harry Johnston Aug 27 '17 at 21:19
• @paracetamol, I can assure you that once I got a pulsed Nd-YAG on my foreharm, and I was able to feel it before seeing it ;) – L.Dutch - Reinstate Monica Aug 28 '17 at 16:23

If you want lasers to be visible they need to hit something. Space is notable for the lack of things to hit.

For a pew-pew 'laser bullet' to be visible you want them moving less than the speed of light and radiating light. So something physical and hot, like plasma, a rocket or a kinetic weapon.

• That second sentence really ought to read "Space is notable[citation needed] for the lack of things to hit." Where's the Skeptics question? :-) – a CVn Aug 27 '17 at 10:24
• ... only at the beginning of a battle. After an extended battle, there could be plenty of particulate crap floating around to scatter light. – Eric Towers Aug 28 '17 at 6:57
• Relevant: youtube.com/watch?v=kPW7ffUr81g – user42036 Aug 29 '17 at 6:23

Well... kindof...

When a laser is on, it is continuously streaming coherent light. The only way to get "bursts" is to pulse the beam. But, remember the speed of light (c = 299,792,458 ms). A clever answer suggests that humans can generally see a 20-meter long F-14 flying by, which suggests a pulse-on duration of 50 ms, which gives us a pulse length of 14,990 km.

Everybody's different, but I'm going to assume we need the same "off" duration to easily distinguish between pulses. In the electronics world we call that a 50% duty cycle.

OK, laser on = 50 ms, laser off = 50 ms.

Note that this is horribly inefficient and no space captain in his right mind would waste so much off time not firing a laser... unless...

It was honking huge and the optics or emission source couldn't handle a long "on" time and needed the "off" time to cool down. Or maybe we need to charge some batteries. Or maybe we need some taunt time between shots. Let's roll with this.

A 6 kW laser will punch through 1.0" stainless steel, but that's for cutting purposes. The hole is itty-bitty, about 320 μm. But King Goombah's ship is a half-mile long! We need something that will punch a 10-meter hole! So we need 187.5 megawatts. Except, that watts are joules-per-second and our cutter is a continuous-on system. So, to deliver the same whomping impact that one second of cutting would give us we need yet another 20× the power.

3.75 Gigawatts. Take that Doc Brown! But further still, that only cuts through one inch of stainless steel. We need to cut through at least 20 meters of ship to make this worth our while, which brings us up to 147.6 gigawatts. Now we're cooking with gas! And, we're finally at a power level where I could believe the need for a short burst and a long cool-down.

But, for the last part of your question, could you see it?

(A) Your first problem is distance. Remember, that pulse of light is humming along at 300,000 kms. We've already established that the pulse is 14,990 km long. That again for the off-time. Maybe once more so you can enjoy how the pulse looks. Your ships are separated by 44,970 km. That's about 10× the width of the United States or ⅛th the distance from the earth to the moon. You could watch it leave and see it coming, but not see it impact without a telescope.

(B) You need something to burn. Now, there is something there. Hydrogent atoms are about 1-per-cubic centimeter and cosmic dust is 1/1000th that. But we're talking about a 10 meter diameter beam 14,990 km long travelling across 44,970 km of space. It won't be Hollywood spectactular, but it's believable there's something ghosty to see. Our beam encompasses π·r2·h = 1.18×109 cubic meters and will traverse 3 "beam lengths" between the two ships for a total of 3.53×109 cubic meters. That's burning through 35 quadrillion hydrogen atoms and 35 trillion particles of cosmic dust (it sounds like a lot, but it isn't). But I'd like to suspend my disbelief and suggest there'd be something to see, if the lights on your bridge weren't too bright and you weren't fighting for air at the moment.

• +1 I think it's just a matter of scale, if you're far enough away and the beam is big enough, you should see something (especially if you're a side observer & not the one being shot at). – Xen2050 Aug 26 '17 at 23:15
• even if all you saw was an afterglow afffect as the heated atoms/particles cooled off, that would be cool in a story. The battle between two ships would almost look like a purplish glow. – JBH Aug 26 '17 at 23:34
• Not entirely true. Some kinds of lasers create continuous beams, others inherently produce pulses. The pulses can be very short, on the order of femtoseconds. See e.g. "pulsed laser" or "femtosecond laser". – jamesqf Aug 27 '17 at 4:41
• The "off" time allows the strongly absorbing plasma at the surface of the target to dissipate so that the subsequent beam is ablating fresh material, not just making previously ablated material glow. – Eric Towers Aug 28 '17 at 6:59
• @EricTowers, that is a Great point! – JBH Aug 28 '17 at 14:38

Assuming the fighters are far enough away (a light second or so) there should be enough time to see discrete laser "bursts", but the environment of the fighters may have a HUGE impact on the visibility of lasers, even at closer ranges quick shots could be visible (like in a laser tag game).

There are more than 20,000 pieces of debris larger than a softball orbiting the Earth.

There are 500,000 pieces of debris the size of a marble or larger.

There are many millions of pieces of debris that are so small they can’t be tracked.

China's 2007 anti-satellite test, which used a missile to destroy an old weather satellite, added more than 3,000 pieces to the debris problem.

There haven't even been any space battles around Earth (right?) except for the anti-satellite test (and I think it only added 3,000 trackable pieces larger than a marble, probably a lot more tiny untrackable pieces) so these millions of small less-than-a-marble-sized bits are just from 6 decades or so of occasional "local" space flight.

If your battles take place around a planet (or any object) that's got centuries of space flight, and especially has had battles there before, there should be a lot more junk, maybe even a fine "mist" of old debris (like proto-rings). Even if the ships used there have magnetic or gravity "deflectors," a laser beam doesn't.

• All these particles would essentially fill in the emptiness of space, like dust in a room, that would get hit by any laser beams and "light them up," so you could see them just like you can see the beams in a laser tag game, and if you're far enough away and the beams are short or quick enough they would look like discrete separate "shots."

Here's an image from earthobservatory.nasa.gov showing just the objects around Earth bigger than 10cm, the scale of the dots is definitely off, but considering the millions of smaller objects it may not be too bad.

• NASA.gov Frequently Asked Questions: Orbital Debris - The number of particles smaller than .4 inches (1 cm) probably exceeds tens of millions.
• Space Debris Elimination (SpaDE) - remove debris from orbit by firing focused pulses of atmospheric gases into the path of targeted debris. [Sounds like they want to knock the Earth's atmosphere into space, to slow down the space junk, I wonder if the atmosphere would come back too.]
• Why the downvote? This makes lasers visible in a battlefield, exactly "like a sniper targeting through a smoke screen is enough" and you don't even have to fiddle with the laser... – Xen2050 Aug 27 '17 at 0:36
• Not my downvote, but I do want to point out that space junk remains in its original orbit. Sure, the impulse from whatever caused a single large object to split into many smaller ones would most likely impart an initial orbit (and possibly orbital plane/inclination) change, but absent any further propulsive maneuvers or drag, it will remain in the new orbit. This was a major problem on Apollo 13: after the oxygen tank explosion, they had to maneuver to get away from the debris cloud, but maneuvering in space when your spacecraft's attitude is dodgy is problematic at best, making it a catch 22. – a CVn Aug 27 '17 at 10:34
• Even with many millions of small pieces of debris in low Earth orbit, space is not dusty. Space is way large, and there is just one piece of debris per a lot of cubic meters or kilometers. "Many millions" is not near the number of dust particles in the atmosphere: "many millions" is about the number of birds in the whole atmosphere, and the atmosphere is very smaller than the space in low Earth orbit. – Pere Aug 27 '17 at 18:47
• @Pere there are not many millions of birds in the whole atmosphere. There are hundreds of billions of birds in the whole atmosphere – Lyndon White Aug 28 '17 at 3:11
• @LyndonWhite That just adds to my point: "many millions of pieces of debris" in LEO is so tiny that space is very far from dusty. – Pere Aug 28 '17 at 12:35

For "laser bullets" to be visible would require:

• Visible frequency - either visible wavelength laser (as opposed to IR/UV/Xray/other wavelengths), or some kind of scattering or surrounding medium that absorbs and re-emits at a visible frequency (due to heating or otherwise) a small part of the beam (but not much of it).

If the beam is immensely powerful then a tiny proportion being scattered or re-radiated could be enough. We can imagine the beam powerful enough to make "a small proportion" as powerful as is needed to light up in the surrounding medium, which helps.

• Duration which has two aspects: how long the beam actually fires, and the duration/persistence of the visible effect. As the question is mainly about visible effect, I'll do that one first, although really they are closely related.

(a) Visible effect / "persistence of vision" (technically: afterimage) - a laser 'bullet' (visual effect) would have to last long enough to create an image or afterimage. That's a problem, because the human eye needs the visible flash to last some number of milliseconds for a perceptible or substantial image or afterimage to be seen, but a realistic laser weapon beam wouldn't last for a fraction of long (Much better and easier to aim for a tiny area and the shortest power delivery time, in order to get the greatest TW/mm^2, and therefore to fire a 100TW laser for 200 nanoseconds than a 1TW laser for 20 microseconds). A further problem is that many of the most powerful lasers today are pulsed lasers (higher power levels by far, cooling between pulses, even if the pulses are nano or microseconds apart), and there's no current reason to think that'll change.

But "pulse" here doesn't mean pulses like you see in movies. In films the pulses are like bullets, a beam fires for say 0.05-0.1 sec which is a duration easily seen by the eye, and then refires again after say 0.1-0.4 sec depending on the film. In reality a pulsed laser's duration is more likely to be measured in pico- to milliseconds, and depending on the design can be a "one-off", or can sometimes be (probably will be) so close as to seem continual for the purposes of visible afterimages, which allows power to be accumulated (technically: energy levels pumped) and then released in a short time.

(b) Actual firing pattern - The tiny time intervals of actual high power lasers are probably a non-issue. That's because critical mechanisms wouldn't need a big hole to do immense damage and deep damage repair may be harder to put right quickly (think in terms of a laser hitting any cable, wiring, microcircuit, display panel, crew or equipment vacuum barrier or enclosure), so penetration probably matters more than width/area. You can probably also sync a lateral beam movement to "slice" a bit, even in a very short time, more so if you know where to aim that will do damage even in a tiny but penetrating 'hit'.

The tiny amount converted to visible light might involve invisible (xray/UV?) lasing that heats the medium, and not only re-radiates as visible light but also re-radiates for a significant fraction of a second until it cools again. So we can probably get round the issue of beam visibility that way even for a beam duration too short for human vision to see.

• Cross-sectional dimensions - I couldn't find info on this, but intuitively, the narrower the beam, the less of the medium it heats or scatters from, or the less visible to the eye; presumably there comes a point where it is too narrow to see. But as visibility is probably due to heating or scattering, we can get around that with a more powerful beam, as above.

• "Moving bullet" effect - you won't get the "moving bullet" effect, just a lit up beam effect (where the medium permits), because on any realistic battle scale the beam will light up along the entire length of its path simultaneously, for any practical purposes.

• In an environment that uses lasers in combat, armor would be a series of sandwiched materials designed to ablate into the most laser absorbent plasmas. So deep penetration probably won't happen. – Zan Lynx Aug 28 '17 at 19:03
• Sneaky - and probably correct. Darn! Fortunately that attacks the question premise not the answer: whatever the defence, a beam that was focussed to a narrower area and output the same energy in a shorter time is probably better than the opposite, so the answer would stand. It just makes the viability of such means of battle less sure. – Stilez Aug 28 '17 at 19:47

Yes, ships weapons fire can be visible during combat, but only through the 3-dimensional immersive-VR tactical display on each ship's command deck. Defensive computers on each combatant's ship analyze sensor data in real time and render enemy offensive fire as distinct glowing laser pulses to assist the captain in making navigational and strategic decisions. Similarly, as the computer alerts the ship crew about important events, it produces the well-known legacy "pew-pew" sound for shots, or the roaring throttle sound for active space objects of different speed and mass that pass near the ship at a dangerously close distance.

The use of the distinct laser pulse imagery and the particular sound effects is just a nostalgic nod to the great space opera movies of the home world's early technological age, and might take origins from even earlier warfare in the atmospheric conditions (where ambient sounds are possible) which more of the populace might have experienced or can intuitively relate to. More modern captains prefer more data-rich line renderings where plasma density is depicted as line thickness and energy level by color. Still enough of the older captains still use the laser pulse setting on their HUD systems that is not uncommon to see battle reenactments rendered in this style.

Keep in mind that no one has ever seen a space ship battle and lived to tell about it. We have only seen computer rendered depictions of space ship battles, depicted either in real time or later for strategic review and training. With all the invisible energies racing around during even the smallest of space ship conflicts, and with the inimical dangers of direct exposure to space vacuum and hard radiation, the last thing any of us would ever want is to be a ring-side seat observer as depicted in those old time movies.

• I like how this allows keeping the fancy, completely impossible images we love yet be serious about physics in the universe. And it does make sense, most of what would happen in a space battle would kill any observer, so generated imagery. Then it doesn't have to be physically accurate, but tactically accurate. Full +1 to this! – spectras Aug 28 '17 at 23:49

As other answers have correctly pointed, a laser beam can't be seen unless there is dust around there, and space is empty - even low Earth orbit is too empty. However, battles tend to create large amounts of smoke and dust.

In space battles we can't expect dust from cavalry galloping around or from gunpowder cannons, as was common in battles a few centuries ago, and smoke disperse in space faster than in naval battles of the battlecruiser era - since there is no air to slow the smoke - but anyway an space battle like with dogfights and imperial destroyers firing at point blank like those in Star Wars yield a lot of large explosions and fires in a relatively small volume. Therefore, in such a battle I would expect an ambient dusty enough for a powerful laser beam to be seen - provided that the pulse is long enough and that its frequency is in the visible range.

A volley of lasers isn't possible in the sense you are talking about. As previously stated you'd need the light to scatter off of particles to make the laser visible, but even if it was visible I don't think a laser is what you're going for. Something that is contained within burst, something like a 'shell' if you will.

The shell could be electromagnetic that contains some form of plasma inside of it that would erupt when hitting a target. You could even possibly have a metal kinetic projectile that's highly magnetic to hold the plasma to it. Maybe if you want to take a leap of faith and make some for of anti matter or exotic particle that has it's own properties you could investigate/invent with some research into particle physics that would help you achieve the desired effect.

Due to effects of lightwave interference, another light source passing through the laser COULD make it visible, but barely so (and certainly not as a line).

A laser-like weapon such as superexcited matter is a far more likely candidate.

• Wow, this might be interesting as another question: how that two laser interference look like? And what do you mean by "superexcited matter" when you mention laser-like weapon? – Vylix Aug 28 '17 at 10:35
• @Vylix With superexcited matter I mostly mean things like plasma projectiles that move much slower than light. And I did not necessarily just mean two-laser interference (requires immense precision), but even starlight-laser interference. – Weckar E. Aug 28 '17 at 10:40

It is difficult to come up with a believable scenario that would create a visible raygun (I'm using a broader term on purpose) beam in space.

In the vacuum of space, a laser beam will of course be invisible, just as a laser pointer beam in non-smoky air is invisible.

As @Jeutnarg said, in Star Wars the blaster bolts are said to be made of plasma. That sounds to me like an after-the-fact justification to explain their perceived speed and the fact that you can see them at all. But it is difficult to envision a mechanism for containing plasma in a rod or beam shape. Presumably some kind of magnetic bottle would be required, but magnetic fields aren't self-sustaining. They come from moving electrons. The plasma contains a lot of those, but the behavior of a plasma is pretty chaotic. To control it into a tube, you need very strong externally generated magnetic fields. But, if the field generators are in the gun itself, then the field strengths near the transmitter would have to be immense, to give good field strengths to contain the plasma that is farther away.

In my judgment, visible "pew-pew" style blaster bolts or raygun beams require fantasy physics. But, then again, so does FTL travel, so it's not so bad.

Well you could see the laser as a bullet or at least a short ray If you slow it down really really REALLY sloooooow. The thing is light travels so fast that we cannot see lasers as the slow moving things that we see in movies (like star wars) . And yeah if it is a laser in the visible light spectrum with help of some kind of precipitate to bounce off the laser a bit you can see it . And voila you've got a laser bullet.

• Welcome to worldbuilding! I agree that massively slowing down a laser is one of things that's important to make it visible, but you might want to edit your answer to show how the precipitate would be able to 'bounce of the laser' – Mithrandir24601 Aug 27 '17 at 15:52
• Mithrandir24601 . I have no Idea! – BLACKBURNER Aug 27 '17 at 19:13

As many have said, the "laser" would have to be in actuality non-photon particles, probably plasma, anti-matter, anti-matter in plasma state...

Now for a proper "pew" the details get more complicated. Space is empty so there are no particle vibrations (sound) to receive. The "laser" being fired could give off a blast of additional particles--think like gunpowder residue, only now it is what they use to energize the particles and fire--and that go out from the source of the firing as a perceptible sound. The problem is that would be the only source of the sound. If you were right next to the "laser" as it passed by you would hear no sound. Now if the projectile particle collection were unstable, maybe radiating some of its own material out as it went on, then there could be a sound from it too (this would also mean that it would get weaker with distance, it would lose energy from shedding those particles in addition to the energy lost if it is emitting light from its own energy). But it wouldn't have a Doppler Effect as it passed by, since it would be emitting its own particles. So if it passed by your ear it would have a constant pitch.

I suppose you could remedy this shortcoming by a. Make the "pew" a combination of the the initial firing and followed by the projectile's shedding of its own material b. Include in the mechanics of the projectile itself a reason for the pitch change, e.g. it goes high from low in pitch because the highest-energy particles in it are shed first.