About a Star Destroyer main power output rerouted to guns...

[Mr. Oragahn]

If you fire a massless beam, you're basically minimizing the ratio of momentum per unit energy.

The higher-end Saxtonite model of an ISD is about 1e25 watts; if you fired that out the front end as lasers, you'd have (1e25 J)/(3e8 m/s) ~ 3e16 N of force. If your ship masses 30 million tons, or 3e10 kg, that would be about a million m/s^2 acceleration, or about 100,000 g.

OK, momentum of photons is something fine in my book. But F = P/c... Is that allowed when dealing with photons?
By comparison, F = m.a would look like a terrible thing to think of, if only for the idea that light is at c no matter what.
So "a" would be zero. To arrange for this, you'd be asking yourself how long does it take for some kind of matter to emit a photon and pick some average velocity between the not-yet-a-photon moment and here-it-is moment. Although ridiculously small, at least you'd be dealing with a figure different than zero I presume, but having the force figure shoot through the roof because of c being divided by this, indeed, ridiculously small value.

OTOH, if you were firing them as sublight projectiles whose energy is mostly in terms of some sort of thermal content (explosive plasma packets) then momentum is completely decoupled from energy content.

If you're talking about the sort of yields actually discussed in Wong's botched momentum calculations based on the Falcon, however, we're talking about orders of magnitude less. Wong's actual claim in that calculation was that the instantaneous output of a turbolaser is "over 215,000 TW," i.e., in the close vicinity of 2e17 watts. That's 50 million times less. (And not a sustained power level.) At that level, there isn't really a recoil problem.

You mean an insane recoil problem? :)

Also, the calculation appeared sound, but the premise was the faulty part. Assuming a near massless beam. Therefore I'm not seeing what's botched about it.

This is one of the problems the Saxtonites had; lots of them produced "lower limits" without realizing that those "lower limits" were not only not actually lower limits, but also typically bounds to within an order of magnitude for that model of the technology.

I don't follow you here.

Saxton himself, I think, had a vague idea of the recoil problem, but was (A) assuming that Star Wars ships were supermassive and (B) assumed the existence of neutrino-based recoil compensators (without carefully considering the problems involved there).

Honestly, no matter how massive the ship would be, it's only as sturdy as its weakest parts. Thinking of the moorings of those turrets, they'd be the first to pop off, champagne style.

[Jedi Master Spock]

OK, momentum of photons is something fine in my book. But F = P/c... Is that allowed when dealing with photons?

F = dp/dt, basically. Force measures the rate of change in momentum over time. Power is dE/dt.

For something with no rest mass, E = pc. So P = dE/dt = c*dp/dt = cF, i.e., F=P/c.

That relationship will only hold for things with no rest mass, but if you're actually firing lasers, the recoil force is the power divided by c.

By comparison, F = m.a would look like a terrible thing to think of, if only for the idea that light is at c no matter what.
So "a" would be zero. To arrange for this, you'd be asking yourself how long does it take for some kind of matter to emit a photon and pick some average velocity between the not-yet-a-photon moment and here-it-is moment. Although ridiculously small, at least you'd be dealing with a figure different than zero I presume, but having the force figure shoot through the roof because of c being divided by this, indeed, ridiculously small value.

OTOH, if you were firing them as sublight projectiles whose energy is mostly in terms of some sort of thermal content (explosive plasma packets) then momentum is completely decoupled from energy content.

If you're talking about the sort of yields actually discussed in Wong's botched momentum calculations based on the Falcon, however, we're talking about orders of magnitude less. Wong's actual claim in that calculation was that the instantaneous output of a turbolaser is "over 215,000 TW," i.e., in the close vicinity of 2e17 watts. That's 50 million times less. (And not a sustained power level.) At that level, there isn't really a recoil problem.

You mean an insane recoil problem? :)

Well, if you think about it, the shots in question are shorter than a tenth of a second. Wong's calculation is for energies under 1e16 J.

The battleship Yamato had 18" guns that fired shells with a momentum of about 1e6 kg m/s. (That's not factoring in the recoil from gases.) A 3e14 J laser shot would have comparable recoil. We're dealing with a gun as much as ~10x as massive, with the actual firing being possibly not quite as concentrated in time, so it's not really that impressive of a recoil problem at that point.

That is, of course, if you work with the notion that these calculations correspond to heavy-duty laser shots.

Also, the calculation appeared sound, but the premise was the faulty part. Assuming a near massless beam. Therefore I'm not seeing what's botched about it.

Faulty premises makes for a botch in my book. I believe we've also seen the particulars of the measurements and scalings called into question as well.

I don't follow you here.

If the beam was an order of magnitude more powerful, then you would see a significant difference in effect.

[Mr. Oragahn]

Okay.
Now, if we remember the last change in the database, I think it was said that concussion missiles and other proton torpedoes were not inert projectiles but very advanced projectiles made of bottled particles.
I'm not sure how this translates into momentum because we're not told anything about the densities. What if the turbolasers fired "shells" and not beams? We pretty much had, for years, to consider the projectiles to be quite more complex than mere beams: they had to be self-bottled coherent stuff that was either limited in time, or assembled in such a way that it could reproduce the behaviour of a bomb to explain what we saw (FLAK, explosions in AOTC, etc.).
In general, all the guns, from imperial scout bikes, Trade Federation tanks to large turbolaser pieces, no matter the size, seem to have a recoil that lasts about two frames out of twenty five or so.
A jacketed high tech "hard" plasma projectile would need to have a significant mass for the bores to be rocked backwards that violently.
If the projectiles were a type of plasma that could be made to explode on contact, perhaps as when the membrane is breached (a kind of contact fuse with no necessary angle of contact), then by knowing the thing's overall mass and speculating a bit on the nature of the ionized gas, perhaps we could guess theoretical boom yields?
Now, it's not that we need much of those guesses, because for many of these weapons, we do actually see the damage they do. The exercise is meant to see if there's a logical correlation between observed various gun recoils and what is expected about the nature and content of those funky projectiles. Their yields hardly are superior to conventional explosives; in fact, they largely seem thermal in effect, with little blast

[Pheonix]

Elsewhere, consensus has been that "blasters" encompassed both direct energy weapons and energy-coated projectiles because in many scenes, blaster bolts do not arc at all, and considering the very slow velocities of the bolts, the arcing should be significant if they held any mass. If these were projectiles, perhaps in some cases there is a anti-gravity component, thus allowing projectiles to travel without arcing. But if this is the case, I guess the projectiles would also need some kind of built in propulsion, to keep them moving through the atmosphere at such a steady rate with negligible mass.

If some varieties of "blaster" do fire projectiles, like some variations on the AT TE cannon (which is said to be a "projectile cannon" at SW.com, and sometimes used for indirect fire in TCW), then the projectiles kinetic energy cannot be their primary damaging component, because bolt velocity is far too slow to carry significant KE. The "projectiles" would be a type of explosive, or contain something more exotic. Or perhaps, the projectiles would simply contain blaster matter / energy (whatever), in addition to the regular bolt, but the projectile has the advantage of indirect fire capability, whereas regular raygun varieties are only capable of direct fire, at line of sight.

The blaster bolt which surrounds the projectile might act somewhat like a regular blaster bolt (through probably nowhere near as powerful, because the rest of the energy is in that projectile) and penetrate the target slightly before the projectile itself releases the rest of the energy, or "explodes".

If blasters can fire hundreds of shots before reloading, and each shot was a projectile, then the projectiles themselves would have to be tiny compared to bullets in real life, to fit so many of them into the clip. Placing the hundreds of bullets into the clip by hand would be a nightmare, and probably impractical. "Recharging" depleted clips wouldn't be possible, because you must replace all of the projectiles.

I think that the most common variety of the blaster is a type of exotic direct energy transfer weapon, or "ray gun", but that projectile varieties clearly do also exist, and can be identified by their arching shots. In my opinion, this best fits the official narrative and descriptions of weapons at SW.com and in the new EU, as well as the observed variable nature of blaster tech on screen (direct fire vs indirect fire; shell loaded guns vs cable powered guns both firing "glowing bolts").

[Pheonix]

If the projectiles were a type of plasma that could be made to explode on contact, perhaps as when the membrane is breached (a kind of contact fuse with no necessary angle of contact), then by knowing the thing's overall mass and speculating a bit on the nature of the ionized gas, perhaps we could guess theoretical boom yields?t

Interesting idea.

I recently began an article attempting to estimate lower limit yields for explosive effects in SW, extrapolating from Curtis's page on fireballs and explosions. I consider these calculations conservative, especially for ray guns. If blaster bolts were essentially dedicated explosives, then the estimates on this page would be more appropriate. Thus far only a section on blasters has been published, whereas sections on heavier tank mounted weapons are in draft.

This type of estimate yields ~133 kilojoules for a blaster at maximum power, or over 100 megajoules for a tank mounted blaster cannon at maximum power. When you consider the feats of demolition that either weapon has achieved at full power, these estimates seem a little low. If blaster bolts were dedicated explosives, then 1 MJ for small arms and 1 GJ for tank guns might be more reasonable, given their ability to blast rock or destroy buildings.

http://www.weaponsofstarwars.com/explosions

[Mr. Oragahn]

With less mass involved in the propulsion or at least the expulsion of the projectile, the recoil must be explained with something else.
A highly energized gas expanding inside the barrel and even more outside of it would fit, but would require the assumption that a lot is wasted as the projectile is propelled forward. However, we're facing the necessity of explaining why so much energy would need to be spent just for expulsion.
Conventional cannons do suffer recoil because of the massive internal combustion of chemicals needed to push quite heavy projectiles. But when dealing with relatively lightweight projectiles, whatever their composition would be, the needed power for accelerating them outside of a barrel would be far smaller.

We might cheat. For one, if the weapons are lighter, especially their barrels, they're ought to kick back more easily. Logically, with not so heavy projectiles to be fired, you wouldn't need any heavy barrel aside from whatever is used for course correction, acceleration and perhaps cooling, like for example in an advanced coil gun.

Another more complicated explanation would be that in the process of creation of the "hard light" projectile, somehow mass does appear, only to be expelled or vanish. This is barely elegant but maybe there's a hint to begin with there.

As for antigravity, we might be able to avoid this one if we assume that for observed cases of short, medium and long range shots (excluding typical ballistic-range-long yet not truly ballistic shots), all in atmosphere, they'd appear to benefit from some buoyancy effect. In other words, they don't fall because they're lighter than the atmosphere, and that for some reason, every single bolt that is created is totally tailored based on the current atmospheric conditions. So they'd actually go up, but gravity also pulls them down. This would require both gravimetric and atmospheric sensors within the weapons. Nothing too fancy, they'd probably be smaller than a small size modern chipset.

The projectile wouldn't lose much thermal energy, it would be well sealed.

Also, to deal with cases of bolts seemingly behaving as if solid, one would have to explain why matter would fail to stop some of them.
I'm thinking of Rex' dual guns, perhaps the drilling guns of Slave-I (it's not a fact but considering the way the small asteroids clearly popped from within like, perhaps, a planted fuse-bomb or a fragmenting projectile, that's what I go with). These are rare cases though.

[Mr. Oragahn]

If the projectiles were a type of plasma that could be made to explode on contact, perhaps as when the membrane is breached (a kind of contact fuse with no necessary angle of contact), then by knowing the thing's overall mass and speculating a bit on the nature of the ionized gas, perhaps we could guess theoretical boom yields?t

Interesting idea.

I recently began an article attempting to estimate lower limit yields for explosive effects in SW, extrapolating from Curtis's page on fireballs and explosions. I consider these calculations conservative, especially for ray guns. If blaster bolts were essentially dedicated explosives, then the estimates on this page would be more appropriate. Thus far only a section on blasters has been published, whereas sections on heavier tank mounted weapons are in draft.

This type of estimate yields ~133 kilojoules for a blaster at maximum power, or over 100 megajoules for a tank mounted blaster cannon at maximum power. When you consider the feats of demolition that either weapon has achieved at full power, these estimates seem a little low. If blaster bolts were dedicated explosives, then 1 MJ for small arms and 1 GJ for tank guns might be more reasonable, given their ability to blast rock or destroy buildings.

http://www.weaponsofstarwars.com/explosions

The movies and series not being scientific demonstrations, we have to accept to exclude cases which sit outside of the norm and treat them as outliers.
I don't agree with your conclusions regarding maximum "reasonable" yields, to me they're ten times greater than what I'd go with based on observation. I don't deny the capability of weapons to do more damage than what we know of today, but there just are limits to how powerful they can be even at their maximum power. Anything at 1 MJ for infantry guns and 1 GJ for tanks is beyond unlikely.
Nor does this really mean much regarding the current problem of what the weapons are and how they can be powered.

Not to say that we're also largely drifting away from the main topic! :)

[Pheonix]

Ah well, the lower limits are an order of magnitude lower than the numbers I mention here, back on the page itself. I just note certain caveats before I get to the weapons themselves.

[Moff Tarquin]

Edited once, to add italics tags to the relevant phrase in the quoted post.

In the strange, peculiar land of Greater Saxtonia where the folks are well-dressed and have their own unique versions of Star Wars quite distinct from those Lucas made, I believe the prevailing theory of hypermatter requires that the ship be capable of adjusting its mass as desired. As such, they could drop proverbial anchor when firing.

Of course, conservation of momentum would suggest that velocity should then change, but I presume they have compensation of some kind.

After all, in Greater Saxtonia, compensation is the national obsession. Some might even suggest they overdo it.

That's what bugs me about all "inertial modification" technologies. How do they balance conservation of energy and conservation of momentum at the same time?

Here's a simple thought experiment to illustrate my problem.

There is a 1000 kg big, dumb object traveling at 100 m/s in Alice's reference frame. It has a momentum of 100,000 N*s, and a kinetic energy of 5 MJ. Alice has a mass-lightening ray, and uses it to reduce the BDO's mass from 1000 kg to 100 kg. In obedience to the law of conservation of momentum, it accelerates to a velocity of 1000 m/s. It now has a kinetic energy of 50 MJ, so Alice's mass-lightening ray must have used up at least 45 MJ.

But Bob is riding on the BDO, and has a mass-lightening ray of his own. In his reference frame (which, according to Newton and Einstein, is just as valid as Alice's), the BDO is traveling at 0 m/s. He reduces its mass to 100 kg, but since (from his perspective) it isn't moving, he doesn't need to put in any energy beyond what it takes to alter the gravitational binding energy.

But if Alice is watching while Bob does this, she sees him pull 45 MJ out of his ass!

[Mr. Oragahn]

Edited once, to add italics tags to the relevant phrase in the quoted post.

In the strange, peculiar land of Greater Saxtonia where the folks are well-dressed and have their own unique versions of Star Wars quite distinct from those Lucas made, I believe the prevailing theory of hypermatter requires that the ship be capable of adjusting its mass as desired. As such, they could drop proverbial anchor when firing.

Of course, conservation of momentum would suggest that velocity should then change, but I presume they have compensation of some kind.

After all, in Greater Saxtonia, compensation is the national obsession. Some might even suggest they overdo it.

That's what bugs me about all "inertial modification" technologies. How do they balance conservation of energy and conservation of momentum at the same time?

Here's a simple thought experiment to illustrate my problem.

There is a 1000 kg big, dumb object traveling at 100 m/s in Alice's reference frame. It has a momentum of 100,000 N*s, and a kinetic energy of 5 MJ. Alice has a mass-lightening ray, and uses it to reduce the BDO's mass from 1000 kg to 100 kg. In obedience to the law of conservation of momentum, it accelerates to a velocity of 1000 m/s. It now has a kinetic energy of 50 MJ, so Alice's mass-lightening ray must have used up at least 45 MJ.

But Bob is riding on the BDO, and has a mass-lightening ray of his own. In his reference frame (which, according to Newton and Einstein, is just as valid as Alice's), the BDO is traveling at 0 m/s. He reduces its mass to 100 kg, but since (from his perspective) it isn't moving, he doesn't need to put in any energy beyond what it takes to alter the gravitational binding energy.

But if Alice is watching while Bob does this, she sees him pull 45 MJ out of his ass!

Unless Bob is glued to the BDO, he's suddenly going to drop off the BDO as soon as he fires his beam. Not because of recoil but simply because Bob keeps his realspace rest mass while the BDO enjoys a cut.
Also, it might turn out that something needs to be ejected when mass is reduced. So if Bob is attached to the BDO and shoots his left foot with the fancy mass lightening gun, triggering an effect that spreads to the entire BDO plus Bob, maybe both leave a shadowy residue of mass, perhaps a bottled field that will take some time to be assimilated back into real space as local space averages the differences.
Or perhaps it's impossible to do so, and the affected object needs to be detached from the device for a differential to exist.
But then no ship could ever achieve that upon themselves, unless again they leave something behind as they progressively lower their overall mass.
Typically, if one system loses something, the environment registers a net proportional gain as a compensation.

[Moff Tarquin]

Edited once, to add italics tags to the relevant phrase in the quoted post.

That's what bugs me about all "inertial modification" technologies. How do they balance conservation of energy and conservation of momentum at the same time?

Here's a simple thought experiment to illustrate my problem.

There is a 1000 kg big, dumb object traveling at 100 m/s in Alice's reference frame. It has a momentum of 100,000 N*s, and a kinetic energy of 5 MJ. Alice has a mass-lightening ray, and uses it to reduce the BDO's mass from 1000 kg to 100 kg. In obedience to the law of conservation of momentum, it accelerates to a velocity of 1000 m/s. It now has a kinetic energy of 50 MJ, so Alice's mass-lightening ray must have used up at least 45 MJ.

But Bob is riding on the BDO, and has a mass-lightening ray of his own. In his reference frame (which, according to Newton and Einstein, is just as valid as Alice's), the BDO is traveling at 0 m/s. He reduces its mass to 100 kg, but since (from his perspective) it isn't moving, he doesn't need to put in any energy beyond what it takes to alter the gravitational binding energy.

But if Alice is watching while Bob does this, she sees him pull 45 MJ out of his ass!

Unless Bob is glued to the BDO, he's suddenly going to drop off the BDO as soon as he fires his beam. Not because of recoil but simply because Bob keeps his realspace rest mass while the BDO enjoys a cut.

But the whole point of the problem is that nobody has the right to say that Bob is moving and Alice is sitting still in a relativistic universe. It's just as fair to say that Alice is moving at 100 m/s as it is to say that Bob and the BDO are moving at 100 m/s.

The crux of the issue is that one value contributing to kinetic energy (the rest mass of an object) doesn't change with reference frame, where as the other (the velocity of an object) does. In a relativistic universe, it seems to be impossible to satisfy both conservation of energy and conservation of momentum while nevertheless allowing for "mass lightening." Even merely obeying conservation of momentum in all reference frames would require for it to accelerate in two different directions simultaneously!

But let's put a bit more of a point on this issue. Let's add a third person into the mix: Chris. From Chris's reference frame, the BDO is traveling at -100 m/s. When Bob uses the mass lightening device (which is still summoning 45 MJ out of the void, incidentally), the BDO ends up traveling at -1000 m/s.

Let's sum up what's going on so far (all of this depends on the fact that if A measures the velocity of B at 100 m/s, then B measures the velocity of A at -100 m/s):

From Bob's perspective:
Bob starts out near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary.
Bob ends up near the BDO. Chris and Alice continue to move at 100 m/s and -100 m/s respectively.

From Alice's perspective:
Bob and the BDO are both moving at 100 m/s. Chris is moving at 200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to 1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at 1000 m/s, Bob ends up left in its wake. Chris continues to move at 200 m/s.

From Chris's perspective:
Bob and the BDO are both moving at -100 m/s. Alice is moving at -200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to -1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at -1000 m/s, Bob ends up left in its wake. Alice continues to move at -200 m/s.

From this, we can try to piece together the BDO's perspective:
Bob begins near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary. Chris accelerates to 1000 m/s and Alice accelerates to -1000 m/s.
Bob ends up near the BDO. Chris and Alice suddenly recede into the distance for no readily apparent reason.

Of course, an absolute reference frame in space would solve the problem, and would also allow for reactionless thrusters and FTL travel without time-like loops.

Also, it might turn out that something needs to be ejected when mass is reduced. So if Bob is attached to the BDO and shoots his left foot with the fancy mass lightening gun, triggering an effect that spreads to the entire BDO plus Bob, maybe both leave a shadowy residue of mass, perhaps a bottled field that will take some time to be assimilated back into real space as local space averages the differences.

So basically you magically ditch the mass? It turns into photons or something? That's called a photon drive that you magically convinced to run using the rest mass of your spaceship instead of using a reactor.

Or perhaps it's impossible to do so, and the affected object needs to be detached from the device for a differential to exist.
But then no ship could ever achieve that upon themselves, unless again they leave something behind as they progressively lower their overall mass.

But what if you want to get your mass back? Or increase your mass?

There are some technologies that result in major modifications to physics (eg, the introduction of a privileged reference frame). This one just seems to break physics.

[Mr. Oragahn]

Unless Bob is glued to the BDO, he's suddenly going to drop off the BDO as soon as he fires his beam. Not because of recoil but simply because Bob keeps his realspace rest mass while the BDO enjoys a cut.

But the whole point of the problem is that nobody has the right to say that Bob is moving and Alice is sitting still in a relativistic universe. It's just as fair to say that Alice is moving at 100 m/s as it is to say that Bob and the BDO are moving at 100 m/s.

The crux of the issue is that one value contributing to kinetic energy (the rest mass of an object) doesn't change with reference frame, where as the other (the velocity of an object) does.

Well, depending on the reference frame, you will be considered to weigh more if you begin to approach relativistic speeds.

In a relativistic universe, it seems to be impossible to satisfy both conservation of energy and conservation of momentum while nevertheless allowing for "mass lightening." Even merely obeying conservation of momentum in all reference frames would require for it to accelerate in two different directions simultaneously!

But Bob's gun is going to produce recoil. No matter what, there is going to be a difference between the moment the gun kicks back and the moment the beam of magic it shoots, which will have mass I suspect, hits the BDO Bob is on.
Even if it's not a gun, no matter what the mass lightener device does, it's a machine that has to transmit something tangible first, strongly implying a change in motion.
Although, yes, that doesn't solve the problem with the values.
But isn't the problem that we're using rather newtonian equations for what might very well involve seriously advanced physics?
Even if the speeds involved there are rather small, the process of mass lightening literally infringes on the relation between light and matter.
As far as we've gone in physics, the more (rest) mass is lost, the more is turned into almost massless particles with great energy. A point I'll return to later on.

But let's put a bit more of a point on this issue. Let's add a third person into the mix: Chris. From Chris's reference frame, the BDO is traveling at -100 m/s. When Bob uses the mass lightening device (which is still summoning 45 MJ out of the void, incidentally), the BDO ends up traveling at -1000 m/s.

Let's sum up what's going on so far (all of this depends on the fact that if A measures the velocity of B at 100 m/s, then B measures the velocity of A at -100 m/s):

From Bob's perspective:
Bob starts out near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary.
Bob ends up near the BDO. Chris and Alice continue to move at 100 m/s and -100 m/s respectively.

From Alice's perspective:
Bob and the BDO are both moving at 100 m/s. Chris is moving at 200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to 1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at 1000 m/s, Bob ends up left in its wake. Chris continues to move at 200 m/s.

From Chris's perspective:
Bob and the BDO are both moving at -100 m/s. Alice is moving at -200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to -1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at -1000 m/s, Bob ends up left in its wake. Alice continues to move at -200 m/s.

From this, we can try to piece together the BDO's perspective:
Bob begins near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary. Chris accelerates to 1000 m/s and Alice accelerates to -1000 m/s.
Bob ends up near the BDO. Chris and Alice suddenly recede into the distance for no readily apparent reason.

Of course, an absolute reference frame in space would solve the problem, and would also allow for reactionless thrusters and FTL travel without time-like loops.

There's perhaps a faulty assumption, that the kinetic energy is directly related to energy expenditure of the device.
Returning to my former point, we're most likely dealing with relativistic physics. At which point calculations become weird.
Up to there be no point using the traditional Ek=0.5*m*v² formula, because they simply cannot combine.
Now, for the sake of this thread, should we really bother considering doing such math, when we don't even know what phenomenon would be going on for mass lightening to ever happen?
In universe, it works, and I think the safest assumption is to consider that the energy that's needed is directly related to the difference between rest mass and lightened mass that's being seeked, and that our little kinetic energy problem might be solved at a higher level of physics.

Also, it might turn out that something needs to be ejected when mass is reduced. So if Bob is attached to the BDO and shoots his left foot with the fancy mass lightening gun, triggering an effect that spreads to the entire BDO plus Bob, maybe both leave a shadowy residue of mass, perhaps a bottled field that will take some time to be assimilated back into real space as local space averages the differences.

So basically you magically ditch the mass? It turns into photons or something? That's called a photon drive that you magically convinced to run using the rest mass of your spaceship instead of using a reactor.

I think it's going to be fairly more complicated than that and I have no intent to come up with a Nobel-worthy theoretical model for a science fiction trope. :)
The entire system depends on manipulating the very essence of mass, something which as of today cannot even be properly defined because we're dealing with a concept at a low fabric-of-the-universe level here.
It's quite interesting to consider what might happen though. Obviously, all elements become less massive, yet all known interactions between particles, atoms and whatever still work the same way.
In other words, the natural "solid" structure automatically occuring thanks to the universe's laws is replaced with a temporary simile, an artificial construction that, perhaps, binds light to behave like mass, but still abides by the same laws, obviously, although in a more arcane way.
In other words, mass lightning involves generating solid and totally autarchic partial holograms (a bit like our plasma friend from the other thread, not needing anything else but itself), that is, partially turning your ship into something which we might call hard light. 60% still solid, 40% woo-woo.

Or else. If a shadowy mass isn't ditched, or tugged in another dimension, at least it would need to be stored, perhaps within the object itself.
And this might result in a bigger kaboom if something goes wrong.

Or perhaps it's impossible to do so, and the affected object needs to be detached from the device for a differential to exist.
But then no ship could ever achieve that upon themselves, unless again they leave something behind as they progressively lower their overall mass.

But what if you want to get your mass back? Or increase your mass?

You turn a knob ! I've seen McKay do it in a puddle jumper. Piece o' cake.

There are some technologies that result in major modifications to physics (eg, the introduction of a privileged reference frame). This one just seems to break physics.

LOL. Science fiction anyone?

[Moff Tarquin]

But the whole point of the problem is that nobody has the right to say that Bob is moving and Alice is sitting still in a relativistic universe. It's just as fair to say that Alice is moving at 100 m/s as it is to say that Bob and the BDO are moving at 100 m/s.

The crux of the issue is that one value contributing to kinetic energy (the rest mass of an object) doesn't change with reference frame, where as the other (the velocity of an object) does.

Well, depending on the reference frame, you will be considered to weigh more if you begin to approach relativistic speeds.

Yeah, but in this case, we're (for the sake of argument) dealing with a device that only effects the rest mass of the object. At relative speeds of 100 m/s and 1000 m/s, nobody's going to see any "kinetic mass" showing up.

In a relativistic universe, it seems to be impossible to satisfy both conservation of energy and conservation of momentum while nevertheless allowing for "mass lightening." Even merely obeying conservation of momentum in all reference frames would require for it to accelerate in two different directions simultaneously!

But Bob's gun is going to produce recoil. No matter what, there is going to be a difference between the moment the gun kicks back and the moment the beam of magic it shoots, which will have mass I suspect, hits the BDO Bob is on.
Even if it's not a gun, no matter what the mass lightener device does, it's a machine that has to transmit something tangible first, strongly implying a change in motion.
Although, yes, that doesn't solve the problem with the values.

The problem with the values is, in my opinion, the biggest one.

For the sake of argument, we will assume that the mass-lightening ray fires massless particles that are absorbed by the massive object, transferring momentum equal to that which would be imparted by a laser beam of equal energy. Everybody will agree (or agree as much as we can hope them to in a relativistic universe) on the transfer of momentum.

But isn't the problem that we're using rather newtonian equations for what might very well involve seriously advanced physics?

Yeah, there's a part of me that worries that this whole discussion is equivalent to somebody trying to use the Twin Paradox as a reductio ad absurdum of Special Relativity. But at the same time, there's a reason why High-School level courses still use Newtonian Mechanics: it works almost all the time. It's only when you're dealing with ridiculously small or ridiculously large amounts of energy that it breaks down, and 50 MJ is a fairly reasonable amount of energy to be dealing with. The black box of the mass-lightener aside, the Newtonian approximation should work here.

Even if the speeds involved there are rather small, the process of mass lightening literally infringes on the relation between light and matter.
As far as we've gone in physics, the more (rest) mass is lost, the more is turned into almost massless particles with great energy. A point I'll return to later on.

But let's put a bit more of a point on this issue. Let's add a third person into the mix: Chris. From Chris's reference frame, the BDO is traveling at -100 m/s. When Bob uses the mass lightening device (which is still summoning 45 MJ out of the void, incidentally), the BDO ends up traveling at -1000 m/s.

Let's sum up what's going on so far (all of this depends on the fact that if A measures the velocity of B at 100 m/s, then B measures the velocity of A at -100 m/s):

From Bob's perspective:
Bob starts out near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary.
Bob ends up near the BDO. Chris and Alice continue to move at 100 m/s and -100 m/s respectively.

From Alice's perspective:
Bob and the BDO are both moving at 100 m/s. Chris is moving at 200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to 1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at 1000 m/s, Bob ends up left in its wake. Chris continues to move at 200 m/s.

From Chris's perspective:
Bob and the BDO are both moving at -100 m/s. Alice is moving at -200 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy.
The BDO's mass decreases, and it accelerates to -1000 m/s, gaining 45 MJ of kinetic energy from nowhere.
The BDO ends up moving at -1000 m/s, Bob ends up left in its wake. Alice continues to move at -200 m/s.

From this, we can try to piece together the BDO's perspective:
Bob begins near the BDO. Chris is moving at 100 m/s, and Alice is moving at -100 m/s.
Bob turns on the mass lightener, which uses up a negligible amount of energy to counteract the BDO's gravitational binding energy.
The BDO's mass decreases, and it remains stationary. Chris accelerates to 1000 m/s and Alice accelerates to -1000 m/s.
Bob ends up near the BDO. Chris and Alice suddenly recede into the distance for no readily apparent reason.

Of course, an absolute reference frame in space would solve the problem, and would also allow for reactionless thrusters and FTL travel without time-like loops.

There's perhaps a faulty assumption, that the kinetic energy is directly related to energy expenditure of the device.

Perhaps, but if the energy doesn't come from the device, then where does it come from?

Maybe some of the mass lost gets turned into kinetic energy. But what we'd get from 900 kg being turned into energy is some 8.1e19 J. On such a solution, we have two extremes: 1) all of that energy becomes kinetic energy. But in that case, Alice and Chris would say that the BDO suddenly accelerates to relativistic velocities (Alice saying it went one way, Chris saying it went the other way) breaking the law of conservation of momentum. And God alone would know what Bob would say happened, since from his reference frame, the BDO has no kinetic energy to begin with! Or 2) 45 MJ of that becomes kinetic energy and the rest is emitted somehow from the BDO. On the simplest such option, the energy would be radiated omnidirectionally from the BDO, vaporizing Alice, Bob, Chris, and the Mass Lightener. And Alice's Ghost would say that the BDO went in one direction, Chris's ghost would say that it went in the other direction, and God alone would know what Bob's ghost would say (actually, unless there's a medium out there who isn't a complete fraud, God alone would know what any of these ghosts would say, but that's neither here nor there). The rather more complicated idea of a cloud of phantom energy is interesting, and we'll return to it in a moment or two.

Returning to my former point, we're most likely dealing with relativistic physics. At which point calculations become weird.
Up to there be no point using the traditional Ek=0.5*m*v² formula, because they simply cannot combine.
Now, for the sake of this thread, should we really bother considering doing such math, when we don't even know what phenomenon would be going on for mass lightening to ever happen?
In universe, it works, and I think the safest assumption is to consider that the energy that's needed is directly related to the difference between rest mass and lightened mass that's being seeked, and that our little kinetic energy problem might be solved at a higher level of physics.

Okay, so perhaps the kinetic energy issue isn't so cut and dry. But we'd still have to determine which direction (if any) a mass lightened object accelerates - and it has to either accelerate in order to obey conservation of momentum.

Also, it might turn out that something needs to be ejected when mass is reduced. So if Bob is attached to the BDO and shoots his left foot with the fancy mass lightening gun, triggering an effect that spreads to the entire BDO plus Bob, maybe both leave a shadowy residue of mass, perhaps a bottled field that will take some time to be assimilated back into real space as local space averages the differences.

So basically you magically ditch the mass? It turns into photons or something? That's called a photon drive that you magically convinced to run using the rest mass of your spaceship instead of using a reactor.

I think it's going to be fairly more complicated than that and I have no intent to come up with a Nobel-worthy theoretical model for a science fiction trope. :)

Oh, of course. The "magic" is some uber complicated piece of equipment that, to us, must be regarded as a black box. I don't require that you open the black box, I'm willing to accept it and its functions for the sake of argument. All I want to know is how it fits in with the things that aren't black boxes - big dumb objects, Alice, Bob, and Chris.

The entire system depends on manipulating the very essence of mass, something which as of today cannot even be properly defined because we're dealing with a concept at a low fabric-of-the-universe level here.
It's quite interesting to consider what might happen though. Obviously, all elements become less massive, yet all known interactions between particles, atoms and whatever still work the same way.
In other words, the natural "solid" structure automatically occuring thanks to the universe's laws is replaced with a temporary simile, an artificial construction that, perhaps, binds light to behave like mass, but still abides by the same laws, obviously, although in a more arcane way.
In other words, mass lightning involves generating solid and totally autarchic partial holograms (a bit like our plasma friend from the other thread, not needing anything else but itself), that is, partially turning your ship into something which we might call hard light. 60% still solid, 40% woo-woo.

Or else. If a shadowy mass isn't ditched, or tugged in another dimension, at least it would need to be stored, perhaps within the object itself.
And this might result in a bigger kaboom if something goes wrong.

Okay, if I thought I understood what you're postulating, I'd break it into chunks and address your suggestions in detail. As it stands, however, I think I need a better holistic understanding of what you're trying to say.

Are you suggesting that a sort of "phantom copy" of the object is left behind, and that copy is what contains the extra mass? Or are you saying that a mass-lightened object becomes its own phantom copy, with the lightened mass either "masked" somehow or left behind altogether?

Of course, I don't expect you to defend either view as being what "actually happens." I recognize that these are intended as mere possibilities, alternatives to the seemingly paradoxical road my assumptions lead down. I just want to know which (if either) option is what you are suggesting.

Or perhaps it's impossible to do so, and the affected object needs to be detached from the device for a differential to exist.
But then no ship could ever achieve that upon themselves, unless again they leave something behind as they progressively lower their overall mass.

But what if you want to get your mass back? Or increase your mass?

You turn a knob ! I've seen McKay do it in a puddle jumper. Piece o' cake.

I literally laughed out loud when I read that. Literally.

There are some technologies that result in major modifications to physics (eg, the introduction of a privileged reference frame). This one just seems to break physics.

LOL. Science fiction anyone?

Well, I'm willing to accept that time travel occurs (in a relativistic universe where FTL travel is possible, it's practically impossible to avoid), there's any number of ways to deal with it - alternative universes, mutable timelines, etc. - that don't involve completely overhauling physics. Same thing with FTL that doesn't allow for time travel - all it takes is a privileged reference frame. Most of science fiction doesn't require us to do away with what we have discovered, it just needs us to wiggle it around a bit so that it fits in a larger picture - which is exactly what real science does. It's a little notion called a classical limit: any new theory that we come up with that explains exotic conditions makes predictions that agree with the old theories once it comes back to normal conditions.

But this mass lightener? It seems to require us to say that a macroscopic object ends up doing multiple mutually exclusive things at once (jet off in one direction, jet off in the opposite direction, and not jet anywhere) when all reference frames involved have relative motions of less than 0.001% of the speed of light. Which quite simply flies in the face of everything we do know about macroscopic objects and slow reference frames. The classical world that we have very well mapped out isn't being eased into a new position in a larger tapestry, it's being chucked out the window. And that isn't just contrary to scientific methodology - it's contrary to all reason.

[2046]

I just assumed they tied it in to the Heisenberg compensators and divided by zero.

[Mr. Oragahn]

Yeah, there's a part of me that worries that this whole discussion is equivalent to somebody trying to use the Twin Paradox as a reductio ad absurdum of Special Relativity. But at the same time, there's a reason why High-School level courses still use Newtonian Mechanics: it works almost all the time. It's only when you're dealing with ridiculously small or ridiculously large amounts of energy that it breaks down, and 50 MJ is a fairly reasonable amount of energy to be dealing with. The black box of the mass-lightener aside, the Newtonian approximation should work here.

The thing is, this exotic process manipulates the very nature of mass. It may not annihilate mass, but it might very well do something still involving extremely high amounts of energy for a relatively small gain. Plus, technically, if light is the lightest "pseudo" matter there is, granting it this fantastic property of the fastest thing ever, lightening a subject might involve a paradox of trying to get an object closer to a luminous state (for a lack of better words) without ever actually becoming real light, not even a percent of it. If it were the case, perhaps we might be dealing with a simpler case, well, a more traditional one of mass conversion and we'd be back to the idea of storing light but not at the expense of losing structural integrity. The overall structure of a subject, say a ship, would need to be maintained, projected and emulated at a lower level of rest mass. Honestly, I'm thinking this is seriously bound to step out of Newton's sandbox.

Perhaps, but if the energy doesn't come from the device, then where does it come from?

Maybe some of the mass lost gets turned into kinetic energy. But what we'd get from 900 kg being turned into energy is some 8.1e19 J. On such a solution, we have two extremes: 1) all of that energy becomes kinetic energy. But in that case, Alice and Chris would say that the BDO suddenly accelerates to relativistic velocities (Alice saying it went one way, Chris saying it went the other way) breaking the law of conservation of momentum. And God alone would know what Bob would say happened, since from his reference frame, the BDO has no kinetic energy to begin with! Or 2) 45 MJ of that becomes kinetic energy and the rest is emitted somehow from the BDO. On the simplest such option, the energy would be radiated omnidirectionally from the BDO, vaporizing Alice, Bob, Chris, and the Mass Lightener. And Alice's Ghost would say that the BDO went in one direction, Chris's ghost would say that it went in the other direction, and God alone would know what Bob's ghost would say (actually, unless there's a medium out there who isn't a complete fraud, God alone would know what any of these ghosts would say, but that's neither here nor there). The rather more complicated idea of a cloud of phantom energy is interesting, and we'll return to it in a moment or two.

The energy is stored into tons of particles and kept spinning within some fancy cyclotron? :P
Themselves, of course, having their mass lightened and... it's like a hall of mirrors.
Or more like a fractal. A four dimensional fiber optic maze.

Oh, of course. The "magic" is some uber complicated piece of equipment that, to us, must be regarded as a black box. I don't require that you open the black box, I'm willing to accept it and its functions for the sake of argument. All I want to know is how it fits in with the things that aren't black boxes - big dumb objects, Alice, Bob, and Chris.

This horrible little pesky problem would be so quickly solved if we could pinpoint a universal center of coordinates for the system we live in, upon which all motions would be measured.
So we would get to know each people's real, absolute speed.

Are you suggesting that a sort of "phantom copy" of the object is left behind, and that copy is what contains the extra mass?

A copy, or even a partial copy, that is, a trace of whatever is changed, duly noted by our dear universe?
For starters, if something were to be left behind, it would have to be found elsewhere, collected, scooped, assembled, you name it. Only then the process could be reverted.
So yes, perhaps it might be interesting that as one subject alters its mass, it would also need to do something inversely similar to an exact equal amount of expandable mass, perhaps belonging to something else, or at least a mass that's stored in a tank and precisely used for this sole purpose. But in such a system, the ejected waste would have its mass increased.
What if no matter where subject X stands in the universe, the moment the mass lightening effect is shut down, the waste returns to rest mass too? It would be reminiscent of that property of photons to be twined and yet be at two different places.
If you increase A, you decrease B.
Okay, that's not what I was supposed to say. I digress.

Or are you saying that a mass-lightened object becomes its own phantom copy, with the lightened mass either "masked" somehow or left behind altogether?

So, a lightened ship being some kind of new-state construct, with the real one left somewhere else, hidden?
I didn't think about that, but in another SF setting, one author could indeed decide of such a thing, with the real rest-mass-identity of a subject being imprinted upon a piece of random matter... anything, as long as it contains at least as much mass as the object that's about to be lightened.
When I said that ships would leave something behind, I didn't really think it thoroughly. But the idea of some kind of waste has some weight.
What I know is that I imagined something a bit otherwordly. It wouldn't be tangible, but would need to be expelled anyway, proportionally to what you're trying to shave off from your own ship's rest mass.
But that implies the existence of some alt-dimensional substance or else. I think it would be more realistic to go with expandable "fuel" being expelled once charged with the inverse effect of what you try to get in your ship.
That's complicated and not really simplifying anything.

I will also have to leave this stranger topic there, if you don't mind. :)

I just assumed they tied it in to the Heisenberg compensators and divided by zero.

That.