What is spatial mass?

[Who is like God arbour]

In the novelization of »A New Hope«, it is described, that the destruction of Alderaan has caused a reduction in spatial mass.

• • Star Wars: Episode IV: A New Hope
    page 161:
    • • Vader stared at the motley array of stars displayed on the conference-room map while Tarkin and Admiral Motti conferred nearby. Interestingly, the first use of the most powerful destructive machine ever constructed had seemingly had no influence at all on that map, which in itself represented only a tiny fraction of this section of one modest-sized galaxy.
        
        It would take a microbreakdown of a portion of this map to reveal a slight reduction in spatial mass, caused by the disappearance of Alderaan. Alderaan, with its many cities, farms, factories, and towns - and traitors, Vader reminded himself.

What does that mean?

What is »spatial mass«?

If Alderaan is destroyed with a conventional weapon, it's mass would not disappear. The matter would still be there, even if only as asteroids and gas.

And to that time, the mass of Alderaan haven't yet spread out to such an extent, that it could have significant gravitational effects on other astronomical objects.

• • Star Wars: Episode IV: A New Hope
    page 155 f.:
    • • “Steady... stand by, Chewie.” Solo adjusted several manual compensators. “Ready to go sublight... ready... cut us in, Chewie.”
        
        The Wookiee turned something on the console before him. At the same time Solo pulled back on a comparatively large lever. Abruptly the long streaks of Doppler-distorted starlight slowed to hyphen shapes, then finally to familiar bolts of fire. A gauge on the console registered zero.
        
        Gigantic chunks of glowing stone appeared out of the nothingness, barely shunted aside by the ship’s deflectors. The strain caused the Millennium Falcon to begin shuddering violently.
        
        “What the -?” a thoroughly startled Solo muttered. Next to him, Chewbacca offered no comment of his own as he flipped off several controls and activated others. Only the fact that the cautious Solo always emerged from supralight travel with his deflectors up - just in case any of many unfriendly folks might be waiting for him - had saved the freighter from instant destruction.
        
        Luke fought to keep his balance as he made his way into the cockpit. “What’s going on?”
        
        “We’re back in normal space,” Solo informed him, “but we’ve come out in the middle of the worst asteroid storm I’ve ever seen. It’s not on any of our charts.” He peered hard at several indicators. “According to the galactic atlas, our position is correct. Only one thing is missing: Alderaan.”
        “Missing? But - that’s crazy!”
        
        “I won’t argue with you,” the Corellian replied grimly, “but look for yourself.” He gestured out the port. “I’ve triple - checked the coordinates, and there’s nothing wrong with the nav ’puter.
        
        We ought to be standing out one planetary diameter from the surface. The planet’s glow should be filling the cockpit, but - There’s nothing out there. Nothing but debris.” He paused. “Judging from the level of wild energy outside and the amount of solid waste, I’d guess that Alderaan’s been... blown away. Totally.”

There was enough debris in the space, where Alderaan was supposed to be, to be described as the worst asteroid storm.

That also means, that this debris would cause gravitation and would pull back other debris and either form an asteroid field or - sooner or later - would form a new planet.

But how could one speak of disappearing spatial mass?

[2046]

Having dealt with that line previously as part of the Superlaser Effect research, I took it to mean simply that there was a slight reduction of the actual mass (read: matter) in that area of space.

Nothing more complicated than that. The novel has several slightly-overstated remarks about things, so I never thought anything more of it.

[ILikeDeathNote]

Sounds like a case of needless technobabble.

[Mr. Oragahn]

Yes, I think it's purely related to space, density in a given volume.

[Praeothmin]

Well, since the DS novel has specifically pointed out that the DS "shunted" the planet in Hyperspace, then it probably means that most of the planet's mass has been transferred from realspace to Hyperspace...