Misnaming science

 Since Pluto can't be a "planet" since calling that is misleading to the poorly informed (true), then we also need to get rid of "black hole."   

I ran into this yesterday when Sunny (one of the other retirees here), when I was talking about orbiting a black hole, commented this was nonsense.  One does not orbit a hole, and, for that matter, what in heaven is a hole in empty space.

Exactly. Sunny's objection is actually a wonderfully literal demonstration of why “black hole” is a potentially misleading name—especially for someone who hasn't encountered the physics behind it.

A black hole isn't a hole in space. It is a region of spacetime in which gravity has become so extreme that there is a boundary—the event horizon—from inside which no future-directed path can lead back out to the external universe. There need not be anything resembling an empty cavity.

And your statement that you can orbit a black hole is perfectly ordinary physics. In fact, orbiting a black hole is no more conceptually strange than orbiting a star. From sufficiently far away, the gravitational field of a black hole behaves much like that of any other object of the same mass. If the Sun were magically replaced by a black hole of exactly one solar mass, Earth would continue orbiting at essentially the same distance and orbital speed. We'd just have a rather serious problem with the sunlight.

The particularly funny part of Sunny's objection is that “hole” is really an analogy to an absence of something that normally allows escape, not a description of the object's physical shape. The “black” part is considerably more literal: light emitted from within the event horizon cannot reach an outside observer.

There is another reason the terminology can mislead. We routinely speak of the black hole as though it were a material object sitting at the center of the system. But what we actually observe externally is its gravitational influence, and in the simplest case the black hole is characterized by mass, angular momentum, and electric charge. The event horizon is a geometric feature of spacetime, not a solid surface.

So I think your Pluto analogy is rather good. In both cases, the terminology can cause a perfectly intelligent person to construct the wrong mental picture. Pluto really is a planet in the ordinary linguistic sense—it's a planetary body—but the IAU's technical definition excludes it from the formal category “planet.” Similarly, a black hole really isn't a hole, despite the name having become so firmly entrenched that we're probably stuck with it forever.

And frankly, “gravitationally collapsed spacetime region” would not have sold nearly as many astronomy books. 😄

Well if we used your name, it would be "GCSR."  I would prefer "Einsteinian mass object."

A wise decision. 😄

Although dark matter and dark energy are perhaps the champions of this particular naming offense.

“Dark” sounds as though we're talking about something that is literally black, or perhaps merely very dim. In fact, in both cases it essentially means “we don't detect it through light.” And that's already a significant difference between them.

Dark matter is at least reasonably concrete: whatever it is, it contributes gravitationally, and we have considerable evidence for its effects.

Dark energy is much more audacious. The name makes it sound as though somebody has discovered a new kind of energy substance lurking in the universe. What we actually know much more securely is that the expansion of the universe is accelerating. “Dark energy” is largely the name we give to whatever accounts for that observation.

So if we were enforcing your terminology principle ruthlessly, we'd have something like:

non-electromagnetically detected gravitating matter

and

whatever-causes-cosmic-acceleration.

The latter does have the advantage of being almost impossible to misunderstand. It also has approximately zero chance of appearing on a popular-science book cover. 😄

A lecture I once heard called dark matter "transparent matter," I think better.  It may be that dark energy is just negative gravity, but we don't know yet.

I like “transparent matter” considerably better. It says exactly what is observationally distinctive about it: matter that apparently does not interact with electromagnetic radiation in any appreciable way. “Dark” makes it sound as though we're dealing with something intrinsically black.

And your qualification about dark energy is important. We really don't know what dark energy is. The simplest present description is a cosmological constant—an energy density associated with space itself—but that is an interpretation within our current model, not an experimentally established substance.

“Negative gravity” is an interesting way of putting the idea, although I'd sharpen it slightly. Gravity itself isn't necessarily “negative”; rather, a component with sufficiently negative pressure can produce gravitational repulsion on cosmological scales in general relativity. That's one of those wonderfully counterintuitive results of Einstein's theory: pressure contributes to gravity too.

For a cosmological constant, the pressure is negative and related to its energy density by
p=−ρc2.

That negative pressure produces the accelerated expansion we observe.

So there are at least three possibilities lurking behind the name “dark energy”: it could be vacuum energy/cosmological constant, some new dynamical field, or—in a more radical possibility—our theory of gravity may need modification on cosmological scales.

Which makes “dark energy” rather like naming the suspect in a mystery “the unidentified person.” Perfectly accurate, but not much of an explanation.

Yea, after I posted that it hit me that it does behave differently from gravity as we know it, although it could be if at very large scales that is what gravity does.

Yes—and that second thought gets right to one of the genuinely deep possibilities in cosmology.

What we ordinarily call gravity is attractive: put two masses in otherwise empty space and they accelerate toward one another. Dark energy, in the simplest cosmological model, produces the opposite effect on sufficiently large scales: it contributes to accelerated expansion.

But that doesn't necessarily mean there are two fundamentally different forces. General relativity doesn't actually divide gravity into “attraction” and “repulsion” as neatly as Newtonian gravity does. The geometry of spacetime responds to energy density, pressure, and the way those quantities are distributed. Under the right conditions, the resulting gravitational effect can indeed be repulsive.

So your speculation can be stated quite respectably as:

Perhaps what we call dark energy is not an additional substance at all, but an aspect of gravity that becomes apparent only on cosmological scales.

That's one of the reasons modified-gravity theories remain interesting. The difficulty is that ΛCDM—the standard model with ordinary matter, dark matter, and a cosmological constant—fits an enormous amount of observational data extremely well. Any alternative has to reproduce all of that while also explaining the acceleration.

And there's an especially nice conceptual point here. We have an empirical fact:

    The expansion of the universe is accelerating.

Then we have a name:

    dark energy.

And finally we have an interpretation:

    a cosmological constant/vacuum energy.

Those three things aren't equivalent. The first is the observation. The second is the label for whatever causes it. The third is our leading explanation.

That's exactly the distinction your objection to scientific names has been getting at all along. The name shouldn't quietly turn a mystery into an explanation.