# QM = Positive Mass / GR = Negative Mass **URL:** \ **Category:** Great Debates\ **Created:** [February 6, 2013, 8:28am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471 "2013-02-06T08:28:00Z")\ **Posts on this page:** 20\ **Page:** 2
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 7, 2013, 5:34pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/21 "2013-02-07T17:34:36Z")
> [@cmyk](#): > > Upon further thought, I suppose I’d like to argue here (my admittedly half-baked proposition) that space-time is “anti mass-energy” (or the equivalence that mass-energy is “anti space-time”). It’s not. Basically, the gravitational field in general relativity _is_ spacetime: it’s what determines distances in both space and time, and hence, lengths and durations. But it enters the stress-energy tensor (the generalization of mass-energy, i.e. the cause of the gravitational field) just as ordinary matter- and force-fields do. Spacetime does not ‘push’; there are solutions of Einstein’s field equation that describe an expanding universe (e.g. de Sitter space), but there also are solutions (for instance the appropriately named anti-de Sitter space) that describe a contracting space, solutions that do neither, and solutions that twist and turn in all kinds of interesting ways. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 7, 2013, 7:05pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/22 "2013-02-07T19:05:04Z")
Yep in basic GR the vacuum is no more or less spacetime than a non-vacuum region. The geometric difference between a region of spacetime representing a vacuum and one representing space filled with matter, is that a certain tensor, derived from the Ricci curvature tensor, vanishes. GR is a very complicated and very, very general theory (arguably TOO general as large classes of perfectly valid solutions to the EFEs are usually rejected as being unphysical). Gravity as an attractive force between two masses only really emerges within certain limits. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 7, 2013, 7:57pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/23 "2013-02-07T19:57:17Z")
Again, thanks. This helps steer my ontological thinking in areas I haven’t quite grasped, and get a feel for what the scientific consensus is right now in what I find the more fascinating aspects of modern physics and cosmology. If anyone has any further thought along these lines, or can point me to some interesting papers, books or abstracts, I’d love to soak it in. I’m currently reading _Why does the World Exist: An Existential Detective Story._ Interesting read on subject. That said, how does something like Twister Theory hold up to QFT, for example? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 8, 2013, 4:35am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/24 "2013-02-08T04:35:24Z")
> [@Half\_Man\_Half\_Wit](#): > > No. The gravitational field has stress-energy (and is thus a source of the gravitational field) just like the other forces do. And the expansion of space is due to it having a _positive_ energy density (via the cosmological constant), leading to a _negative_ pressure. Not to beat a dead horse, but would you (or anyone) mind elaborating a bit more about on this? I realize this is an area in physics and cosmology that’s incomplete and fervently researched on many different theoretical fronts, which is why I started the OP in GD. The standard model is a marvel, despite the arguably growing notion that it’s describing the shadows on Plato’s proverbial cave wall — and once one starts to become ever more convinced of it, it’s all the more tempting to imagine what might be casting these “shadows” because our vantage is frustratingly forced one way. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 8, 2013, 2:47pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/25 "2013-02-08T14:47:30Z")
> [@cmyk](#): > > Not to beat a dead horse, but would you (or anyone) mind elaborating a bit more about on this? Gravity doesn’t have stress-energy just like the other forces, if we’re strictly talking within the formalism of GR. However there is a non-linear aspect to GR which means that, unlike Newtonian physics, the gravitational field itself is a source of gravity and you can in fact express it’s stress-energy in terms of a pseudo-tensor (stress-energy is expressed in the form of a tensor). So far most of the Universe’s expansion has been due to it’s initial conditions/conditions very early on in the Universe, why it should have such conditions we can’t really say as we only infer them by evolving the current state of our Universe back in time. In the present era though, according to our best cosmological models, the expansion has become dominated by dark energy which is causing it to accelerate. The cosmological constant was originally a negative pressure term introduced by Einstein to counteract the expansion of the Universe (a positive pressure or zero pressure in a non-empty Universe always leads to a Universe with either expanding or contracting dynamics). One of the problems though with the introduction of the term is that it is inherently unstable: either the Universe is exactly static or you get runaway expansion or contraction. The term was re-introduced and labelled dark energy though when it was noticed the Universe’s expansion is in fact running away (i.e. it is accelerating). ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [February 8, 2013, 6:17pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/26 "2013-02-08T18:17:00Z")
> [@cmyk](#): > > Not to beat a dead horse, but would you (or anyone) mind elaborating a bit more about on this? If you’re asking about the energy density of the vacuum/cosmological constant, [here’s a nice writeup](http://math.ucr.edu/home/baez/vacuum.html) by John Baez. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 8, 2013, 9:50pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/27 "2013-02-08T21:50:26Z")
Asymptotically fat, so why the the relabeling of Λ to Dark Energy? Just to give it a less clumsy name? And thanks for the link HMHW, I’ll read it soon. One day, you’re all coming over, and we’ll play Yahtzee, and I’m buying the pizza. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 8, 2013, 11:54pm UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/28 "2013-02-08T23:54:17Z")
> [@cmyk](#): > > Asymptotically fat, so why the the relabeling of Λ to Dark Energy? Just to give it a less clumsy name? > > And thanks for the link HMHW, I’ll read it soon. > > One day, you’re all coming over, and we’ll play Yahtzee, and I’m buying the pizza. It’s more general to view it as a form of energy than as a constant of spacetime: Einstein added it as a geometric property of spacetime which pretty much decided its form, whereas seeing it as the property of a field _within_ spacetime allows it to have different forms without fundamentally altering general relativity. Also when Einstein put in the cosmological constant he was trying to achieve a static Universe, so for example it didn’t matter if it varied as the volume of a region of space varied. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 9, 2013, 2:52am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/29 "2013-02-09T02:52:43Z")
> [@Asympotically\_fat](#): > > It’s more general to view it as a form of energy than as a constant of spacetime: Einstein added it as a geometric property of spacetime which pretty much decided its form, whereas seeing it as the property of a field _within_ spacetime allows it to have different forms without fundamentally altering general relativity. Also when Einstein put in the cosmological constant he was trying to achieve a static Universe, so for example it didn’t matter if it varied as the volume of a region of space varied. It does seem to vary though according to [data from the Baryon Oscillation Spectroscopic Surve](http://www.sciencenews.org/view/generic/id/346619/description/Glimpse_at_early_universe_finds_expansion_slowdown)y > [@](#): > > Based on the speeds of the most distant of those clouds, BOSS scientists determined the universe’s expansion rate a mere 3 billion years after the Big Bang. The team then compared its measured rate with those from more recent eras to conclude that the universe’s expansion was slowing at that time. “The universe was a very different place,” says study coauthor and University of Utah physicist Kyle Dawson. > > ``` > The BOSS finding is consistent with physicists’ theories of how the universe’s growth rate has changed. Immediately after the Big Bang, the universe ballooned rapidly in a split-second era called inflation. Expansion continued afterward, but like a coasting car, the cosmos had nothing to keep it accelerating. The gravitational attraction of all the matter in the universe was acting like rolling friction, gradually slowing down the expansion. > > ``` > > But as the universe got larger and matter got more diluted, scientists believe something began pressing the gas pedal again, causing expansion to accelerate once more. Scientists don’t know exactly what the culprit is, so they call it dark energy. Eleven billion years ago, dark energy made up less than 10 percent of the total content of the universe; today it makes up almost three-quarters. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 9, 2013, 3:54am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/30 "2013-02-09T03:54:41Z")
> [@deltasigma](#): > > It does seem to vary though according to [data from the Baryon Oscillation Spectroscopic Surve](http://www.sciencenews.org/view/generic/id/346619/description/Glimpse_at_early_universe_finds_expansion_slowdown)y That article is about the Universe’s expansion rate varies and how the density parameter of dark energy (which is sometimes confusingly called the cosmological constant because it has the same sign) varies rather than how the cosmological constant varies. AFAIK it generally assumed (though alternatives like quintessence are certainly on offer) that the effect of dark energy is the same as the effect of introducing a cosmological constant (which is constant) because of the assumption that it is vacuum energy which behaves like a cosmological constant in the dynamics of the Universe. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 9, 2013, 4:35am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/31 "2013-02-09T04:35:32Z")
It’s quite possible that I’m missing a fine distinction, but in the next paragraph the article says “The measurements so far lend support to the leading theory that dark energy is a natural property of empty space: The more the universe expands, the stronger dark energy becomes.” So that sounds to me like it is a basic property of the fabric of the universe. IDK. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 9, 2013, 4:38am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/32 "2013-02-09T04:38:14Z")
> [@deltasigma](#): > > It’s quite possible that I’m missing a fine distinction, but in the next paragraph the article says “The measurements so far lend support to the leading theory that dark energy is a natural property of empty space: The more the universe expands, the stronger dark energy becomes.” So that sounds to me like it is a basic property of the fabric of the universe. IDK. What it means is that as the Universe expands space becomes more empty so dark energy becomes more dominant. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 9, 2013, 4:57am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/33 "2013-02-09T04:57:57Z")
> [@Asympotically\_fat](#): > > What it means is that as the Universe expands space becomes more empty so dark energy becomes more dominant. Actually, I interpret it to mean that there is more space so there is more zero point energy. ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 9, 2013, 5:22am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/34 "2013-02-09T05:22:16Z")
> [@deltasigma](#): > > Actually, I interpret it to mean that there is more space so there is more zero point energy. It’s the dark energy density that is important (and in fact as in the standard cosmological model the Universe is infinite, I’m not sure that there’s any way to say there is more space), for vacuum energy that doesn’t change so you end up with the cosmological constant. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 9, 2013, 5:32am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/35 "2013-02-09T05:32:38Z")
OK, so you’re saying that the energy density of any cubic cm of space remains constant. No problem there. But isn’t the expansion of the universe normally described as an expansion of space itself? I think one of the popular examples is to imagine that you’re a raisin in a loaf of raisin bread. As the bread rises, all of the other raisins move farther away. But that’s because the bread is expanding, not because the other raisins are moving. ---
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [February 9, 2013, 5:37am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/36 "2013-02-09T05:37:29Z")
> [@FinnAgain](#): > > Everything is nothing, yes. [Is nothing sacred?](http://frumheretic.blogspot.com/2008/11/is-nothing-sacred.html) ---
**Author:** ![Asympotically\_fat](https://avatars.discourse-cdn.com/v4/letter/a/e47c2d/32.png) [@Asympotically\_fat](https://boards.straightdope.com/u/Asympotically_fat)\ **Post date:** [February 9, 2013, 7:01am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/37 "2013-02-09T07:01:44Z")
> [@deltasigma](#): > > OK, so you’re saying that the energy density of any cubic cm of space remains constant. No problem there. > > But isn’t the expansion of the universe normally described as an expansion of space itself? I think one of the popular examples is to imagine that you’re a raisin in a loaf of raisin bread. As the bread rises, all of the other raisins move farther away. But that’s because the bread is expanding, not because the other raisins are moving. Yes the expansion of the Universe is for all intents and purposes the expansion of space, but the cosmological constant doesn’t change with the expansion of space and neither if we choose to view it as dark energy then the associated energy density doesn’t change either. That’s of course not true for all dark energy models. ---
**Author:** ![deltasigma](https://avatars.discourse-cdn.com/v4/letter/d/e5b9ba/32.png) [@deltasigma](https://boards.straightdope.com/u/deltasigma)\ **Post date:** [February 9, 2013, 7:13am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/38 "2013-02-09T07:13:24Z")
OK, then I suppose I’m missing the point. If we agree that each cubic unit of space has the same energy and that as space expands you get more cubic units then the bottom line is that you have more total energy. Isn’t that the bit that matters for cosmology? ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 9, 2013, 9:01am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/39 "2013-02-09T09:01:32Z")
Is there even enough data on this matter (heh) to draw a solid consensus on the nature of dark energy/cosmological constant? Huge pieces of the puzzle are missing, and everyone from Penrose to Weinberg have several theories. ---
**Author:** ![cmyk](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/cmyk/32/3353_2.png) [@cmyk](https://boards.straightdope.com/u/cmyk)\ **Post date:** [February 9, 2013, 9:17am UTC](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471/40 "2013-02-09T09:17:59Z")
I look at a star, and the vacuum of space surrounding it, and I wonder how errant it is to think that there’s any less (or more) “stuff” in the fabric of space, than there is in the hydrogen of the star. Then I’ll wonder, is this “stuff” of space, and this “stuff” of matter, really all that fundamentally different from each other? [Previous page](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471.md?page=1) [Next page](https://boards.straightdope.com/t/qm-positive-mass-gr-negative-mass/649471.md?page=3)