Lately I've been working on a SciFi setting, primarily focusing on space combat.

Something I've been working on is an energy shield that can reliably stop hypervelocity rounds (which are common in this setting). Due to (insert technobabble here) it stops physical objects, but is ineffective against light Never mind that light is particles, which are technically physical...

Looking around on Physics.SE I came across this post, from which I learned that light travels at the same velocity regardless of the velocity of whatever is producing it.

Pondering this, I had an idea: What if you strapped a big flashlight to the front of a hypervelocity round traveling infinitely close to the speed of light? If I read that post correctly, all the light produced in-transit would arrive at once, effectively being a full-powered laser at point-blank range.

The Question: Provided you could strap a flashlight to a hypervelocity round, could this work? If not, why?


  1. This is a specialty round for a very specific application. I am aware that it is overkill for pretty much everything else.

  2. Assume that the civilization making these can (a) get enough power to launch these at the required velocity and (b) ensure that it doesn't become a 0.999...c shotgun due to acceleration.

  3. Most battles occur at ranges of several light-seconds.

  4. By "big flashlight" I mean "generic large-scale light emitter". It can be scaled as necessary (although not up to the power of a ship-to-ship laser).

  5. Yes, I know that a "regular" laser is more effective unless this is way more powerful than I thought. The general idea is for this to be a cool "Abnormal Ammo." As such, I'm primarily interested in whether or not this would be effective in its own right, not whether or not there are better alternatives (although suggestions for other Abnormal Ammo are welcome).

  6. This is different from a Star Trek photon torpedo in that the light is created during transit, not upon detonation.

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    $\begingroup$ Theoretically, yes, kind of like a sonic boom. Practically, if there is energy source capable of getting many large projectiles close enough to the speed of light for the beam compression to be meaningful (which would be very close indeed I would wager), the absolutely stupendous amount of energy would be better spent in some other way. Also, at such a velocity, the projectiles can't meaningfully maneuver either, so the projectile and its pulse is trivially dodgeable and if the beam is swiveled in flight, the compression effect is lost. $\endgroup$ Apr 2, 2021 at 13:03
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    $\begingroup$ Strike that; actually it doesn't work, because relativistic time dilation works the opposite way. Photons experience zero time. As the projectile gets closer to lightspeed, the less time it experiences relative to the target ship; at just below lightspeed, the laser projectile reaches the target effectively instantaneously in its own reference frame so the flashlight is on for effectively zero time. So there's no beam compression because there's no beam. $\endgroup$ Apr 2, 2021 at 13:46
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    $\begingroup$ To point out the "obvious": something traveling at near-c with non-negligible (and by "negligible" we're talking about atomic scales) mass requires quite some hand-wavium. Also, if you have the energy to accelerate that projectile, there are probably more useful ways you can use that energy. As The Square-Cube Law notes, if nothing else, you could just fire any old mass at near-C at your enemy... $\endgroup$
    – Matthew
    Apr 2, 2021 at 14:32
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    $\begingroup$ Yes, it will work. No, it will be INSIGNIFICANT compared to the ludicrous energy levels in the "hypervelocity round" moving "infinitely close to speed of light". The physical round, moving that fast, will have a mass approaching infinity. The sheer GRAVITY of it will collapse the universe. $\endgroup$
    – PcMan
    Apr 2, 2021 at 17:47
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    $\begingroup$ No, it won't work. Light doesn't "bunch up". Light always moves at lightspeed regardless of frame of reference. Also, the statement "light is a particle" is misleading in this context, because a) it isn't really, and b) it has no mass, so it's not like any other particle, even though it isn't really a particle, it just acts like one. $\endgroup$
    – jdunlop
    Apr 2, 2021 at 18:48

3 Answers 3


The effects would be strange, and probably not what you expect - or want.

I vaguely remember something like this performed by the USS Enterprise (I think the 1701-D) some time or other.

Imagine the emitter starts emitting in its frame of reference when it is at one light-second from the target, and firing continuously while it approaches. First problem is, the faster it goes, the less time it has to shoot. At relativistic speeds this time decreases even more, and at 99.9% of c the time of flight is just about 44 milliseconds. So, you get 44 milliseconds of emission compressed in one millisecond: the target sees the emission at 1 light-second after 1 second from its emission (obviously), and the emission at point-blank range 1.001 seconds later.

At 99% of c, the emission time is 140 milliseconds and the dwell time is 10 milliseconds. We could graph the power density and see whether it goes up or down. It should go up, but even a very high power density, if coupled with very little energy, would just maybe ablate a few millimeters of armor.

At those speeds, however, the Doppler effect ensures that the incoming radiation's wavelength shortens, so you'd be hitting the target with more and more penetrating radiation, a sort of GZK effect in reverse.

In the end, I suspect it matters little, because the one thing your handwavium shield cannot do is violate conservation of energy. When hit by a near-luminal projectile, that energy has to go somewhere, and the most likely form it would take would be a significant radiation flash (Asimov once described such a shield in his first Lucky Starr, Space Ranger novel). At that point, the additional energy of the onboard laser emitter would be a very thin icing on a very large cake.

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    $\begingroup$ From the emitter's frame of reference, your math is right, but from the receiver's frame of reference, the emitter still has the full second to emit photons. $\endgroup$ Apr 2, 2021 at 22:16
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    $\begingroup$ @MooingDuck: but the flashlight LED's beam power / photons-per-second rate is constant in its own frame of reference. So if you want to know how many photons got emitted, you have to work in the emitter's frame of reference. (Each photon's energy from the receiver's PoV is multiplied by the same Lorentz factor as the time-compression. I think the total energy of the beam over a given travel distance is about constant for speeds close to c, such that the travel time is near constant as observed by the target.) $\endgroup$ Apr 3, 2021 at 2:24
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    $\begingroup$ @MooingDuck um, no? In the target's frame of reference, the emitter starts emitting 1.001s before impact, and the light from this event is received 1.000s later. So, the whole bang arrives within times -0.001 and 0.000. This is somewhat akin to the "superluminal jet" phenomenon ( en.wikipedia.org/wiki/Superluminal_motion ). $\endgroup$
    – LSerni
    Apr 3, 2021 at 12:13

What if you strapped a big flashlight to the front of a hypervelocity round traveling infinitely close to the speed of light? If I read that post correctly, all the light produced in-transit would arrive at once, effectively being a full-powered laser at point-blank range.

No, you wouldn't have a "sonic boom of light". Rather, the light from the flashlight will arrive with much the same energy as if you weren't moving relative to the target. However, it will be doppler shifted towards blue. If you are incoming at close to $c$, you might doppler shift the light from the flashlight all the way to X or gamma rays, so at least you deal damage through ionizing radiation.

If you are able to move close to $c$, you have a much more powerful wepon in your hands. The very first XKCD What If article is about a baseball mobing at close to light speed in a baseball field. The resulting scenario is a thermonuclear explosion, because mass moving at that speed in an atmosphere will cause the atoms in the atmosphere to break and/or fuse. Simply toss the flashlight at close to $c$ and overwhelm the shields with the power of nukes, as God intended. No atmosphere needed, just have them hit the shields if all battles are in space.

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    $\begingroup$ Note that the flight time is infinitesimally small in the flashlight's frame of reference so it won't have time to emit many photons. (time dilation effects). But the energy of each photon is multiplied by the same gamma (Lorentz factor), I think. So the difference between 99% of c vs. 99.999% of c is I think about the same energy but concentrated over a shorter interval (higher power), into fewer higher-energy photons. (For speeds close to c so outside-observer travel time is near-constant; slower and it's just a flashlight beam on longer). $\endgroup$ Apr 3, 2021 at 2:15
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    $\begingroup$ Of course this is totally dwarfed by the relativistic kinetic energy of the flashlight itself, as you say, but perhaps an interesting point of physics regardless. $\endgroup$ Apr 3, 2021 at 2:16

What if you strapped a big flashlight to the front of a hypervelocity round traveling infinitely close to the speed of light? If I read that post correctly, all the light produced in-transit would arrive at once, effectively being a full-powered laser at point-blank range.

This is called the relativistic doppler effect. Effectively the light you emit gets squeezed, shortening the wavelength proportionally to your Lorentz factor.

You don't quite get a "sonic boom of light", because your projectile is still travelling at sublight speeds and the light it is emitting is obviously not, but what you will get is a lot of nasty hard gamma rays even if the flashlight on the projectile was just spitting out regular visible light. These gamma rays will obviously have more energy than the un-shifted wavelength, and so the light will be compressed and more destructive.

There's no such thing as a free lunch, of course, because in order to develop the huge Lorentz factors you seem to want here (maybe a million? that's 0.9999999999995 c. Maybe more?) you will basically need magic, because there's no way you're going to accelerate macroscopic devices capable of doing useful work to that sort of speed in a sensible length of time over the space of a few light-seconds.

You have to put an absolutely obscene amount of energy into the projectile which, according to your description of your magic shields, is going to be almost completely wasted anyway, somehow. If you can manipulate those frankly implausible energy levels, you should just be making gamma ray lasers instead.

Something with a mere Lorentz factor of 1000000 has a kinetic energy density of 1023 joules per kilo. The sort of power source you need to drive a gun that can shoot bullets like that is an entire star... Wolf 359 radiates about that much energy per second.


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