
Equivalence principle of general relativity holds even at gravitational extremes - okket
http://blogs.discovermagazine.com/d-brief/2018/07/04/einstein-relativity-heavy-objects-fall/
======
millstone
This was a measurement of the Nordtvedt parameter which characterizes the
difference between a gravitational field and an accelerating reference frame.
If there is no difference, as GR predicts, then the parameter is 0.

This experiment tests the idea that gravity itself gravitates: gravity imbues
very massive bodies with gravitational binding energy, and therefore more
inertia. If your inertia is increased due to gravity, do you nevertheless fall
at the same rate? i.e. does gravity affect the energy that itself imbues? The
conclusion is consistent with "yes."

[http://www.scholarpedia.org/article/Nordtvedt_effect](http://www.scholarpedia.org/article/Nordtvedt_effect)

~~~
drjesusphd
I've never thought of the equivalence principle like this. I knew
gravitational fields have their own mass-energy and can cause a runaway effect
(black holes), but I never realized this could mean more inertia too. Neat!

I'm curious about what this means as the event horizon is crossed.

~~~
raattgift
Sorry I'm a few days late to this party. There are lots of things I am tempted
to comment about other comments, but even though the OP doesn't really have
anything to do with black holes directly, I'll try an answer restricting
myself to your good, honest question:

> I'm curious about what this means as the event horizon is crossed

tl;dr it means General Relativity is how we answer this; more specifically, we
expect that if we do a Galileo-like experiment replacing the Leaning Tower and
the ground with a giant scaffold surrounding a black hole, and we carefully
drop in a feather, a planet, and a neutron star, neither object's centre of
mass wins a race to the horizon (it's a tie, even though the neutron-star is
strongly gravitationally self-bound, and the feather is not gravitationally
self-bound at all).

In practical terms, and strictly with respect to your question, the results of
Archibald et al. [2018] (the work described in the original post) mostly (more
below) vindicate the use of a post-Newtonian approximation (PNA) formalism as
a short-cut to the results we would get from the full theory of General
Relativity. At the horizon of anything but the smallest black hole[1] one
expects "no drama" for a freely-falling infaller. This is not terribly
surprising, as we could already make a model black hole's mass arbitrarily
high, bringing the curvature at the horizon down well below the curvature we
experience in laboratories here on Earth.

Clifford Will, who has written many papers on post-Newtonian methods, in
(deliberately accessible to non-specialists) Will [2011] [2] writes:

 _The reason is a remarkable property of general relativity called the Strong
Equivalence Principle (SEP). A consequence of this principle is that the
internal structure of a body is “effaced,” so that the orbital motion and
gravitational radiation emitted by a system of well separated bodies depend
only on the total mass of each body and not on its internal structure, apart
from standard tidal and spin-coupling effects. In other words, the motion of a
normal star or a neutron star or a black hole depends on the body’s total mass
and not on the strength of its internal gravitational fields. This behavior
was already implicit in the work of Einstein, Infeld, and Hoffman, where only
the exterior nearby field of each body was needed, and has been verified
theoretically to at least second post-Newtonian order by more modern methods._

(So, for example, SEP means a neutron star's strong internal curvature -- the
strongest we can access observationally with current technology -- doesn't
cause the neutron star to radiate energy-momentum away as dipole gravitational
waves.)

Archibald [2018] provides observational verification for SEP for the inner
radio pulsar of the triple system to good sub-leading post-Newtonian order,
improving on results from (among others) the Hulse-Taylor binary and
(indirectly) LIGO.

Will [2011] uses the "xPN" notation for subleading orders, where 1PN is
leading-order, 2PN is next-to-leading order, 3PN is next-to-next-to-leading
order, and so forth.

A problem raised in Archibald [2018] is that the most popular formal system
(the parameterized post-Newtonian (PPN) formalism) for comparing alternative
theories of gravitation which may distinguish inertial mass from gravitational
mass is only good to 1PN; any theory that does not match the results of
Archibald [2018] at 1PN can be excluded, but anything that differs at
subleading order needs a different comparison framework.

Again, this is not tremendously surprising; PPN was explicitly constructed to
fully contain weak-field results (mostly within our own solar system), and the
region around the central binary in Archibald [2018] is clearly not weak-
field. So while this spells trouble for several families of alternatives to
General Relativity (GR), there are several others where the results of
breaking GR's inertial mass == gravitational mass equality appear only in the
strong field limit. An extension of PPN is needed to capture the results of
Archibald [2018] into a formal comparison system that may distinguish between
such theories and General Relativity, assuming the compared theories match in
every other PPN parameter (if they don't we can exclude one on that basis).

Or, in short, General Relativity looks really sound still, and post-Newtonian
programmes are not wildly off-track.

\- --

[1] The region near (but outside) the horizon of a small black hole is going
to be dramatic for other reasons, ranging from astrophysical ones like the
virtual certainty of a hot accretion structure to theoretical ones like the
increasing heat of Hawking radiation as one takes the black hole mass to zero.
An object like a space capsule with a person inside would be vapourized by the
hot matter outside a small black hole well before reaching the horizon. As we
make the mass of a black hole larger, the matter just outside the horizon --
at least on average -- is a lot cooler and sparser, so a space capsule could
easily freely fall through the horizon.

[2] Proceedings of the National Academy of Sciences of the United States of
America, April 12, 2011. 108 (15) 5938-5945;
[https://doi.org/10.1073/pnas.1103127108](https://doi.org/10.1073/pnas.1103127108)
Thankfully PNAS makes it freely available at
[http://www.pnas.org/content/108/15/5938](http://www.pnas.org/content/108/15/5938)

------
mar77i
I've heard of the new atomic clocks underway to make this ridiculously precise
measurement even more precise:

[https://www.nist.gov/news-events/news/2013/08/nist-
ytterbium...](https://www.nist.gov/news-events/news/2013/08/nist-ytterbium-
atomic-clocks-set-record-stability)

~~~
mchahn
Although that news is actually five years old. I guess in some time-frames
that could be considered new.

------
phyzome
> Galileo famously (and likely apocryphally) demonstrated the principal by
> dropping lead balls of different weights off the Leaning Tower of Pisa and
> observing them hit the ground at the same time.

This never made sense to me as something you'd need to test, once the question
had been considered.

Imagine 1000 cannonballs all connected to each other with threads. The result
is a single object with 1000x the mass of a cannonball. Would you expect it to
fall faster?

~~~
SketchySeaBeast
We shouldn't write off the difficulty of the problem, just because it feels
self-evident now. It's much easier to come up with a thought experiment once
you have the answer. It's completely intuitive that "0" is a thing, I mean,
you can just go "whats 2-2?" but it wasn't always the case. It's very hard to
get into a frame of mind that includes only those items the contemporary
discoverer would have known.

~~~
dboreham
I'm pretty sure that "0" as in "2-2" has been known since before we became the
current species.

What was new and interesting was the use of "0" as a place holder to allow
decimal number representation.

------
acqq
The new paper claims to add to the confirmation of "the strong equivalence
principle of general relativity."(0)

Reading Wikipedia to find which theories are (or were) the alternatives which
assumed that principle not to hold:

"The strong equivalence principle": "The first part is a version of the weak
equivalence principle" "The second part is the Einstein equivalence principle
(with the same definition of "local"), restated to allow gravitational
experiments and self-gravitating bodies."

"This is the only form of the equivalence principle that applies to self-
gravitating objects (such as stars), which have substantial internal
gravitational interactions. It requires that the gravitational constant be the
same everywhere in the universe and is incompatible with a fifth force. It is
much more restrictive than the Einstein equivalence principle."

"Einstein's theory of general relativity (including the cosmological constant)
is thought to be the only theory of gravity that satisfies the strong
equivalence principle. A number of alternative theories, such as Brans–Dicke
theory, satisfy only the Einstein equivalence principle." (1)

So it seems that "Brans–Dicke theory" and similar are ruled out by this
observation, that is that the following line from one Wikipedia page can't
remain:

"At present, both Brans–Dicke theory and general relativity are generally held
to be in agreement with observation."(2)

But note that the authors of (0) don't claim that the alternative is
completely impossible, but that they have measured even bigger space where the
"strong equivalence principle" holds, compared to all measured up to now:

"our limit on the strong-field Nordtvedt parameter, which measures violation
of the universality of free fall, is a _factor of ten_ smaller than that
obtained from (weak-field) Solar System tests"(0)

That is, of course, a success, as every scientific and measured increase of
"known" is. Note that other "Solar System" tests mentioned in (0) are also
very new, from this year. Also: the parameter is "a factor of almost a
thousand smaller than that obtained from other strong-field tests"!

The article from Nature, of course more scientific that the OP on HN
(currently: from discovermagazine.com), ends with:

"Although the [scalar–tensor] theories are not completely quashed, their hopes
for validity have been made that much fainter."(4)

0)
[https://www.nature.com/articles/s41586-018-0265-1](https://www.nature.com/articles/s41586-018-0265-1)

1)
[https://en.wikipedia.org/wiki/Equivalence_principle#The_stro...](https://en.wikipedia.org/wiki/Equivalence_principle#The_strong_equivalence_principle)

2)
[https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory](https://en.wikipedia.org/wiki/Brans%E2%80%93Dicke_theory)

3)
[https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.34...](https://journals.aps.org/prd/abstract/10.1103/PhysRevD.48.3436)

4)
[https://www.nature.com/articles/d41586-018-05549-4](https://www.nature.com/articles/d41586-018-05549-4)

~~~
hymen0ptera

      A gravitational constant that is the same 
      everywhere in the universe and is incompatible 
      with a fifth force.
    

The fallout of this ramification is that, gravity is some sort of ambient side
effect of material presence, sort of like a shadow cast, more than an emission
radiated.

As a constant, that means that its invariance is significant, in the same way
the speed of light is significant. There is some externality pegging the
phenomena we notice, at the value we observe. Perhaps some sort of Planck-
level absolute fact, which is irreducible in the same way that the concept of
color doesn't exist very much beneath 400 nanometers (violet/near-
ultraviolet), since color is only an abstraction of our eyeballs and the
language we use to describe our sensations.

~~~
contact_fusion
I wouldn't say that general relativity suggests that gravity is an "ambient
side effect" of material presence. It means precisely that the geometry of
spacetime is determined by the matter-energy content within that spacetime;
and that matter moves on geodesics dictated by spacetime. I suppose that I am
forced to accept that whether you think gravity is more like a shadow of
matter than an active participant in dynamics is somewhat up to you, provided
that you get the physics correct. If you don't, then your perspective is
wrong.

Glancing at some of the other things you've said in the thread, you seem to be
convinced that some waves, like sound waves, are somehow illusory. Let me
assure you, there is nothing illusory about wave phenomena, both in general
and with gravitational waves in particular. Anything that produces wave-like
phenomena can be said to radiate emissions, to paraphrase, and in many cases,
there is nothing really more "fundamental" than the wave.

As for the concept of color, I suppose it depends on what you mean by color.
Color as defined by the wavelength/frequency of light is perfectly well
defined outside of the visible spectrum. As a mental concept, I don't see why
it is "irreducible." I suppose you are attempting to say that the universal
speed of light is somehow fundamental... but in the same way that a "redder"
red is impossible? This is a dubious analogy. It confuses the limitations of
the mind with fundamental physics.

~~~
hymen0ptera
Sound waves are changes in the distribution of particles within a volume over
time. The sound wave itself is a byproduct of the particles compressing closer
together or stretching farther apart.

You might be in love with the idea of describing an equation that frames the
gradient of distribution, and the nature of it's propagation through a medium,
but the sound wave is the manner in which the gaseous molecular constituents
of the air are set in motion relative to one another. They get closer, they
move apart, the changes occur at different places in the medium, at different
times, and do so at a certain velocity, in sequence as interactions are forced
upon the medium.

Indeed, the reason sound waves travel at the speeds we observe, is because
that's how fast the very molecules themselves, comprising the air, are moving
at the temperature and pressure of the environment.

Meanwhile, what color is a beam with a wavelength of one nanometer? Would you
characterize the color as "soft x-ray"?

~~~
contact_fusion
The sound wave is not a "by-product." It is precisely the phenomenon you are
describing. And while we are discussing equations and distribution functions,
you might as well get the equation right: wave phenomena arise when the
equations of motion are hyperbolic PDEs. Such systems involve the Laplacian,
not just the gradient. Indeed, the physics of such systems are typically
studied as a whole, in terms of... wave phenomena.

Further, not all waves require a medium. Light and gravitational waves are
prominent examples. There is nothing to reduce these phenomena to, except for
the fields themselves, whose form is dictated by... a wavelike solution.

While we are at it, let me disabuse you of your explanation of the sound
speed. Turns out that the sound speed is a thermodynamic quantity; it is the
speed at which small wavelike perturbations propagate. To properly derive the
sound speed, one must linearize the Euler equations, and then adopt a
thermodynamic equation of state, from which the sound speed is derived. It is,
emphatically, not the speed at which molecules move.

Finally, it is obvious that you did not understand what I was driving at with
respect to color. I agree wholeheartedly that we have no true color, within
our minds, with which to perceive, say, soft x-rays. But this issue has to do
with our own neurobiology, not fundamental physics. Comparing the two is what
is problematic. There is little reason to suspect that our mental limitations
have anything to do with anything but evolutionary necessity. Such limitations
are categorically different than, say, the speed of light.

------
Santosh83
Just a layman question: does this have any implications for theories of
quantum gravity, like LQG or String theory?

~~~
RedOrGreen
Yes, the authors discuss the implications for scalar-tensor theories, and it
doesn't look great for many of them. The Nature News and Views article by
Clifford Will [1] offers a decent overview.

[1]
[https://www.nature.com/articles/d41586-018-05549-4](https://www.nature.com/articles/d41586-018-05549-4)

------
lawlessone
>with a margin of error of only 30 meters, according to a

Amazing they can measure it that accurately

~~~
RedOrGreen
We can do even better some of the time - for example, we can get to ~few
nanosecond timing precision for pulsars like J1909-3744 [1], which is
equivalent to measuring (changes in) the path length to that pulsar to a few
meters or better.

[1] [https://arxiv.org/abs/1406.4716](https://arxiv.org/abs/1406.4716)

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zerostar07
why do they call it Principal?

~~~
okket
Double spelling error (first two occurrences), from the third on the article
got it right.

------
egjerlow
*Equivalence principle

