
Does Gravity Travel at the Speed of Light? (1998) - jonbaer
http://math.ucr.edu/home/baez/physics/Relativity/GR/grav_speed.html
======
mturmon
People may be interested in experiments to detect these waves. From the
Wikipedia page on LIGO:

"LIGO, which stands for the Laser Interferometer Gravitational-Wave
Observatory, is a large-scale physics experiment aiming to directly detect
gravitational waves. [...] At the cost of $365 million (in 2002 USD), it is
the largest and most ambitious project ever funded by the NSF.

Observations at LIGO began in 2002 and ended in 2010; no unambiguous
detections of gravitational waves have been reported. The original detectors
were disassembled and are currently being replaced by improved versions known
as "advanced LIGO", scheduled to be operational by 2014.

[...]

Measurable emissions of gravitational waves are expected from binary systems
(collisions and coalescences of neutron stars or black holes), supernova of
massive stars (which form neutron stars and black holes), accreting neutron
stars, rotations of neutron stars with deformed crusts, and the remnants of
gravitational radiation created by the birth of the universe."

For more:

[http://en.wikipedia.org/wiki/LIGO](http://en.wikipedia.org/wiki/LIGO)

[http://en.wikipedia.org/wiki/Gravitational-
wave_detector](http://en.wikipedia.org/wiki/Gravitational-wave_detector)

There is a fantastic lecture series by a Yale U. astronomer that touches on
this and related topics from a calculus-free perspective:

[http://oyc.yale.edu/astronomy/astr-160](http://oyc.yale.edu/astronomy/astr-160)

Also on youtube and iTunes U.

~~~
platz
I was fascinated with LIGO when I learned about it. It seems the best plans
for such devices exist in the planned space-bound installations, such as LISA
([http://lisa.nasa.gov/](http://lisa.nasa.gov/))

~~~
mturmon
NASA participation in LISA was removed a couple years ago due to budget
issues. The ESA portion went on (it's called NGO now), but I think NGO is
pretty much an acronym without funds.

In general, in fact, space interferometers have been held up and moved to
ground-based systems. (See also SIM and TPF.)

------
bfe
Weisberg and Taylor showed, with 30 years of data, that the orbital decay of
the binary pulsar PSR 1913+16 agrees with the gravitational radiation
prediction of general relativity to within 0.2%:

[http://arxiv.org/abs/astro-ph/0407149](http://arxiv.org/abs/astro-ph/0407149)

------
gtani
Baez has published some great booklists and a FAQ for self study

[http://math.ucr.edu/home/baez/physics/index.html](http://math.ucr.edu/home/baez/physics/index.html)

[http://math.ucr.edu/home/baez/books.html](http://math.ucr.edu/home/baez/books.html)

[http://math.ucr.edu/home/baez/physics/Administrivia/booklist...](http://math.ucr.edu/home/baez/physics/Administrivia/booklist.html)

------
yawgmoth
Something very interesting whenever this question is asked - what _is_ the
speed of light? It is "the speed at which data propagates through space time".
I think that is a fascinating truth to our universe.

There is a very good discussion from r/askscience on this topic:
[http://www.reddit.com/r/askscience/comments/1kjp6z/does_grav...](http://www.reddit.com/r/askscience/comments/1kjp6z/does_gravity_travel_at_the_speed_of_light_if_the/)

~~~
mahranch
I always liked the analogy from here: [http://zidbits.com/2011/04/why-cant-
anything-go-faster-than-...](http://zidbits.com/2011/04/why-cant-anything-go-
faster-than-the-speed-of-light/)

The speed of light can be thought of as infinite speed in that context. Since
time slows as you get closer and closer to the speed of light, it stops
completely at 299,792,458 m/s (hypothetically, if you gave photons a reference
frame).

Trying to go faster than light could be compared to a car slowing down for a
red light. Once at the red light, the car stops. Going faster than light would
be like asking the car to go slower than completely stopped. It just doesn't
make sense in a philosophical or mathematical sense.

------
stephengillie
It makes me think of hanging sand in Minecraft -- you disrupt 1 column and
cause it to be recalculated and fall, and this causes the columns next to it
to be recalculated and fall, dominoing on until all of the hanging sand has
fallen. This is the propagation in action.

The propagation time is a combination of the 'tick rate' and the 'size of the
granules'. In Minecraft, I'm pretty sure the tick rate is around 1/10 of a
second, and the granule size is defined as 1 cubic meter.

In our universe, the tick rate probably is 1/c, or .00000000333 seconds
(please correct my math), and the granule size is similarly infinitesimal.

~~~
aphyr
Since c is a velocity, 1/c doesn't have units of time; it has units of
time/distance.

Special and general relativity (at least, as commonly formulated) also assume
that space and time are isotropic and continuous, which means there is no
"granule", no "smallest increment" in the field equations. That may or may not
be true; we're still trying to figure out if spacetime is quantized or not.

~~~
DougWebb
Does anyone know what Wolfram would say about the tickrate? I bought the huge
book and know that he believes space is quantized, at least, and I assume he
believes time is as well, as the rate the universal state machine changes
states. But I never read enough to learn what he thinks that rate might be.

~~~
aphyr
Cellular automata violate continuity and isotropy, which means they're
inconsistent with the continuous spatial formalism used in special relativity,
general relativity, classical electrodynamics, and as far as I know, quantum
electrodynamics. We have no experimental evidence inconsistent with continuous
models of space. We _also_ have evidence that Lorentz symmetry holds over
scales smaller than the Planck length, which suggests spacetime is not
quantized.

One of the problems with quantized spacetime is that the cells would have a
_size_ \--and sizes change depending on your velocity. Were spacetime to be
comprised of cells, you'd be able to establish a single, most privileged
reference frame--and that doesn't sit well with the principles of relativity.

That said, we really don't have good models for what happens at small
spacetime scales yet. Gravity is poorly understood. Theories change as new
evidence and mathematics develop. I just wouldn't trust Wolfram's opinion
without either a.) experimental evidence or b.) a model which is consistent
with (or radically reformulates) significant parts of relativity and quantum
electrodynamics.

~~~
aphyr
Ah, here we go. Aaronson 2002 shows that Wolfram's model cannot be consistent
with experimental evidence:

 _In physics, we examine Wolfram 's proposal for a deterministic model
underlying quantum mechanics, with 'long-range threads' to connect entangled
particles. We show that this proposal cannot be made compatible with both
special relativity and Bell inequality violation._

[http://arxiv.org/abs/quant-ph/0206089](http://arxiv.org/abs/quant-ph/0206089)

~~~
3rd3
For some interesting reads: Just take a look at the summaries, introductions
and conclusions in Aaronson’s publications.

------
ars
Here's another way to understand it.

The difficultly: Despite the propagation delay, gravity always points to where
the object should be, instead of where it was when the gravity was "emitted".

The explanation: Just like the object itself is moving, the gravitational
field (or electric/magnetic field) is ALSO moving! It moves at exactly the
same speed, and in the same direction as the object which created it.

It's "disconnected" from the source - so if the source suddenly changes
direction the field it emitted doesn't know about it, and continues to point
to where it expected the object to be.

More technical:

Why wormholes are impossible, and why you can not create a single magnetic
monopoles:

What about conservation of momentum? If an object is pulled to where the other
object is expected to be, not where it is, then momentum exchange between the
two of them would seem to not add up!

But it works because you can never create a gravitational or electric field
from nothing. You can only move them around, so the momentum always catches
up.

What about a magnetic field? You can create those from nothing - ah, but you
can only create magnetic dipoles, with opposing fields, so again it works.

But, you can never create a magnetic monopole because you would suddenly have
a magnetic "charge" where none existed before and the momentum would not add
up. What you can do is create two monopoles, of opposite poles. (So this
implies a conservation of magnetic pole, just like conservation of electric
charge - assuming monopoles exist.)

What about wormholes? They have the same momentum problem - an object suddenly
appears where none was before and it pulls on other objects. You could then
move it away before it gets pulled in turn by those objects, and that would
violate conservation of momentum. So you can't do that.

And, faster than light objects would have the exact same problem - they could
move out of the area before they properly shared the momentum. So I suspect
they can't exist either. (Unless there is some complicated math I didn't think
of which "fixes" it.)

~~~
Semiapies
Now, here's the thing that confuses me.

Say some advanced civilization can move around some large mass - an asteroid,
a planet, a black hole, whatever, we'll just call it "the big mass". They can,
at will, fly it back and forth between two distinct positions we'll call 0 and
1.

And let's say some distance away - a light-year, say - they have an facility
where they can measure with excruciating precision the force of gravity on a
test mass. After isolating out all other known gravity sources, they can use
the remaining vector of gravitation force to compute the current position of
the original big mass - and whether it's at position 0 or position 1.

Assuming they can drag around the big mass from one position to the other in a
short amount of time, shouldn't the people at the remote facility be able to
detect where the big mass is long before light could reveal its position?
Couldn't they use those observations to receive a low-bandwidth, but faster-
than-light message?

How does relativity prevent that?

~~~
fragsworth
The gravitational field points at where the object would be, given its current
velocity.

This is not to say that the object will _actually end up there_. If the
velocity changes, the gravitational field changes direction according to the
new velocity. The change propagates at the speed of light.

~~~
jimmahoney
Yes.

This is a case when a picture (well, a GIF animation) is worth _lots_ of
words.

Look at
[http://jimmahoney.net/pulse_acceleration_field_lines.gif](http://jimmahoney.net/pulse_acceleration_field_lines.gif)
.

(I made this from the "Moving Charge" java physics applet at
[http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/Mo...](http://www.cco.caltech.edu/~phys1/java/phys1/MovingCharge/MovingCharge.html)
. It's a demo of the physics of electric charges, not general relativity, but
the essence of the idea is the same even if the field equations are different.
This follows directly from the central idea of relativity: the physics of
uniform constant motion is the same as no motion, if you're moving along at
the same speed.)

The point charge (mass for gravity) in the center is first moving slowly to
the right, then suddenly changes direction to move slowly to the left. (The
small red vector indicates velocity.)

The white lines represent the direction of the field, that is, the direction
an object would be pulled.

Near the point source, the field lines point to where it is. Far away, the
news hasn't arrived yet that it's motion has changed, and so they point to
where it would have been. During the brief acceleration when the source
changed direction, the field lines connecting these two regions are "kinked"
strongly - that's the radiation, which propagates outward at the speed of
light.

A second object would feel a sideways pulse as the wave (kink) passed by.

------
chockablock
Date in title should be 2011 (date of latest revision), not 1998. Article
includes a citation from after 1998.

------
AlwaysBCoding
This is the winning comment from the reddit discussion from /u/RobotRollCall

 __*

This is a far more interesting question than it might seem at first glance,
and it deserves some attention because it tells us something fundamental and
wonderful and just bloody awesome about the universe.

But I don't know how to tell the story succinctly. So I'm going to do that
thing I do. I am very, very sorry. Please feel free to move on if this strikes
you as tiresome.

Consider the Earth, and you on it. You're not floating freely, so clearly
something's going on. We call that "gravity." We can call it, in the most
generic sense, an interaction: you and the Earth are interacting somehow, and
that's what's keeping you from floating freely.

We can then ask what the speed of that interaction is by putting it in these
specific terms: How much time will elapse between your changing your position
relative to the ground and your beginning to fall?

Yes, it's the Wile E. Coyote problem. Wile E. Coyote runs off a cliff, floats
in mid-air long enough to hold up a sign that says "Help," then begins to
fall.

Clearly that's an exaggeration. But just how much time does elapse, in real
life, between stepping off a cliff and beginning to fall?

We can approach the problem naively by remembering that all propagating
phenomena in the universe are limited by the speed of light. Given that fact,
it makes sense to hypothesize that the time between the moment when Wile E.
steps off the cliff and when he begins to fall will be equal to or more than
the distance between him and the ground divided by the speed of light. It
certainly can't be less, right?

We can then construct a set of very, very precise experiments with very fine
tolerances — probably involving electromagnets and lasers or something — to
test this hypothesis.

And then we can find that we're totally goddamn wrong.

To the absolute limit of our ability to measure it — and our ability to
measure it is really good, since we used electromagnets and lasers and other
expensive science things — when an object is dropped, it begins falling
instantaneously. Not after a very small interval of time, but absolutely
instantaneously. As in zero time elapses between dropping and falling.

This is fairly earthshaking, really. Because it implies that somehow a
"signal" of some kind is getting from the ground to Wile E. faster than the
speed of light. Which is supposed to be impossible.

I'm going to skip ahead a bit here, because I don't feel like explaining the
entire theory of general relativity, and it won't be that useful in answering
the question anyway. Suffice to say that no, no time elapses between dropping
and falling, but at the same time no, no signal or interaction has to
propagate upward from the ground to Wile E. in order to make him start
falling. In fact, what's going on is that Wile E. is always falling, due to
the curvature of spacetime created by the Earth. Whenever he's standing at the
edge of the cliff, on the ground, the ground beneath his feet — paws? — is
arresting his fall by, effectively, pushing up against him. The very instant
that's removed, he starts falling.

So in that sense, gravity has no speed. Because it doesn't actually propagate
through space. One way to look at it is to say the gravitational field fills
space, so wherever you are, you're already being affected by it all the time.
Another way is to say that gravitational essentially is space, so it affects
you simply by virtue of existing. The two are essentially equivalent English
translations of the equations that actually describe the phenomenon.

But okay, that's half the problem. The gravity of a static body fills space,
or is space, and as such can't be meaningfully said to have a speed. But what
about the gravity of a changing body? Like you said, what if "suddenly a black
hole appeared?"

Well, the answer of course is that that never happens, ever. Gravitation
doesn't suddenly anything; macroscopic things don't just appear out of
nowhere, and teleportation is impossible. So we don't have to think about that
… and in fact we couldn't get meaningful answers if we tried.

But things do move. The moon's moving relative to the surface of the Earth; we
can tell, even apart from the fact that we can see it up there, because the
moon is the major contributor to the tides, and the tides rise and fall. But
what's the relationship between the moon's position in space and the tidal
acceleration on the Earth? Are the two somehow always in perfect sync, or is
there some lag? If so, how much, and in what direction?

That's actually a much harder question to answer than you might think. There
was a now-infamous paper some years ago by a fellow named Tom Van Flandern
(recently passed, God rest his soul) that asserted that the change in
gravitational acceleration in a dynamical system actually propagates many
times faster than the speed of light — at least twenty billion times faster
than the speed of light — but not instantaneously. This got a lot of attention
at the time. If the propagation speed of changes in spacetime geometry were
equal to the speed of light, that'd be fine. If it were literally
instantaneous, that'd also be fine, more or less, though our theory would need
some tweaking. But faster than c but still finite? That was really hard to
explain.

It turned out not to be a problem though. Because Van Flandern just made a
mistake in his paper. See, the relationship between motion and gravitation is
not as straightforward as it might seem. In fact — and I'm glossing over this
now, because the maths are damn complicated — whenever a gravitating object
moves inertially, the gravitational acceleration vector at a point removed
actually points at where the object actually is at a given instant, as opposed
to where the object's light is seen to be coming from at that instant. So in
that sense, we're back to gravitation being instantaneous again!

But is it really? No. Because you see, if the inertially moving object were to
come to a stop instantaneously, the acceleration vector would continue to
point toward its future position for a time, as if it were still moving
inertially, even though the object is actually somewhere else. The sum of
effects that serve to cancel out aberration when everything moves inertially
would break down, and the acceleration field would point toward empty space
for however long it takes for the change in geometry to propagate through
space at the speed of light from the gravitating object to the point in
question.

Except things don't stop moving instantaneously. Things accelerate, and
acceleration requires energy, and when you factor that in, the equations
balance out again.

(If you feel up to the challenging of following a lot of advanced mathematics,
here's the best paper I know on the subject.) - [http://arxiv.org/pdf/gr-
qc/9909087v2.pdf](http://arxiv.org/pdf/gr-qc/9909087v2.pdf)

So what does that mean? It means that the "speed of gravity" is the speed of
light … technically. Changes in the geometry of spacetime actually propagate
at the speed of light, but the apparent effects of gravitation end up being
instantaneous in all real-world dynamical systems, because things don't start
or stop moving or gain or lose mass instantaneously for no reason. Once you
factor in everything you need to in order to model a real system behaving in a
realistic manner, you find that all the aberrations you might expect because
of a finite speed of light end up canceling out, so gravity acts like it's
instantaneous, even though the underlying phenomenon is most definitely not.

The universe is pretty damn cool, if you ask me.

 __ _

~~~
diminoten
The linked article directly disagrees with this post (except the last
paragraph, which itself disagrees with the rest of the post), however:

> The fact that gravitational damping is measured at all is a strong
> indication that the propagation speed of gravity is not infinite. If the
> calculational framework of general relativity is accepted, the damping can
> be used to calculate the speed, and the actual measurement confirms that the
> speed of gravity is equal to the speed of light to within 1%. (Measurements
> of at least one other binary pulsar system, PSR B1534+12, confirm this
> result, although so far with less precision.)

This was always my problem with RobotRollCall on Reddit - no one could
challenge anything she wrote lest they be downvoted into oblivion.
Furthermore, she never took criticism very well, and would assert
authoritative dominance over the topic of knowledge (how dare you disagree
with me, I read research papers!).

Her explanations were good, but they weren't ever very complete and while they
were well written, they weren't always accurate.

~~~
prawn
I think she pulled up stumps and left at some point over issues with how some
of the community was responding to her? Something like that. Real shame
because she inspired strong interest in so many who read her explanations.

~~~
InclinedPlane
She probably just got tired of being treated the way highly visible women
often are on the internet. This is not always visible to everyone else since
there are private messages on reddit. It's possible things are better now,
it's hard to say. I tried to perform an experiment on reddit a while ago by
posting under a handle that was obviously feminine but I had to cancel the
experiment because it didn't really work due to my posting style being too
abnormal.

~~~
prawn
I never noticed gender come into it on the public side, at least. A good
number of people wouldn't have even been aware she was female. (Think of the
Groklaw thread the other day and many assuming PJ is male.)

There was an abrasive way RRC responded to some that fired up discussions and
I got the impression that this ultimately frustrated her into ending her
participation. I've seen that abrasive style/response feedback loop develop
with males just the same.

------
ssivark
It's a fairly short and clear article, so no need for a Tl;DR. Note that this
is NOT yet another "revolutionary" announcement, but just contemplative
reasoning from a respected expert in gravitational physics.

~~~
peterwwillis
Yeah, well. I don't get it. Could somebody summarize it in terms a 9 year old
like myself can understand?

~~~
aidenn0
Gravity travels at the speed of light, but it pulls you towards where the
object is going to be when it left the object, not where it was when it left
the object.

~~~
acchow
Thanks for this. Tweaking it slightly:

> Gravity travels at the speed of light, but it pulls you towards where the
> object would be if the object kept moving at constant speed, not where it
> was when it left the object

~~~
aidenn0
It's been 10 years since I took a physics class, but isn't it constant
acceleration, not constant speed?

------
hcarvalhoalves
That's mind bending, never thought of gravity that way (being a force like EM,
and subject to a propagation speed).

In my mind, gravity was just some sort of "vacuum" the mass leaves as it
moves, and other bodies get "pulled in" because that's the space where the
"energy" (mass) levels are smaller, and the system tends to a stable state.
But not something like a wave that propagates, just a side effect of the
global system, not tied to speed in any way.

~~~
pachydermic
Obligatory: [http://xkcd.com/895/](http://xkcd.com/895/)

~~~
hcarvalhoalves
Haha good one!

------
RuggeroAltair
Steve Carlip used to be my advisor in grad school and his style of writing
conveys completely his beautiful way of explaining (and thinking about) deep
questions in physics.

One think that I believe is important to point out is that this short essay is
focusing on the experimental aspect of the measurement of the speed of
gravity. It gives an explanation on why this is even an intelligent question
to answer and why the answer we commonly accept is that gravity moves at the
speed of light. I will never have better words than him so I won't add
anything to what he said.

What I would like to point out, based on some comments I read here, is that
this essay is not talking about whether or not theoretically would be possible
to have a speed of gravity faster than the speed of light.

General relativity and special relativity have been tested on several aspects,
and they pretty much are in agreement with all experimental constraint. There
is no other theory that can explain everything we see (some theories tend to
explain a few things, but not all of them, or they are too vague).

A theory in which the speed of gravity is faster than the speed of light isn't
unconceivable per se, but no one has been able to write a completely
consistent one yet, mostly because it would have large consequences on what
special and general relativity imply for cosmology and particle physics (and
causality). If gravity moved faster than the speed of light then we'd have to
'fix' a lot of problems coming down to paradoxes and stuff like that.

So take this essay as a way to appreciate how elegant and fine some
experimental questions on gravity (and physics) are.

------
juliangamble
In general relativity gravity propagates at c. The rate of orbital decay of
binary pulsars is, among other factors, dependent on the speed of gravity. The
in-spiral rate of one binary pulsar system has been measured and found to
agree with the rate predicted by general relativity to within a 0.2% margin of
error.

Gravitational waves haven't been directly measured yet though, so there's no
direct confirmation. With multiple detectors currently in operation if a
signal is detected and able to be tied to a specific location in space the
timing delays between when its received between the two primary LIGO detectors
(Livingston, LA, USA and Hanford, WA, USA) and the VIRGO detector (PISA Italy)
should allow for estimating a propagation speed.

Advanced LIGO, expected to begin collecting data in 2014 is expected to be
able to detect a number of signals so hopefully the question will be settled
in a few years. However, there might not be any published results for a while
after it goes active. The search for pulsar spindown signals with LIGO data is
done via the Einstien@Home distributed computing project; and in prior runs
several years passed between when the first part of the data set was collected
and when papers on it were finally published.
[http://physics.stackexchange.com/a/26743/9521](http://physics.stackexchange.com/a/26743/9521)

------
spdegabrielle
Don't 'TL;DR' this is worth reading.

~~~
einhverfr
See, I was going to make a joke like:

TL;DR: Relativistic models of gravity rely on retarded positions.

------
mladenkovacevic
I wonder if the recent nova event visible with the naked eye can be used to do
an experimental measurement... or perhaps it has to be a supernova as the
article suggests.

~~~
codezero
I am guessing it has to be a supernova on account of there being many novae
discovered/observed each year.

------
rossjudson
I remember perplexing a high school physics teacher with this one, many years
ago. ;) His answer was that he _thought_ it would propagate at the speed of
light, but he wasn't sure. I was just being a smart-ass, trying to figure out
if _something_ could go faster than the speed of light.

------
spenrose
So I think this article says:

    
    
      - the earth revolves around where the sun is almost precisely
    

and rules out:

    
    
      - the earth revolves around where the sun was ~8 minutes ago
    

(where 8 minutes is about the travel time at c). Can anyone confirm?

~~~
vehementi
Yeah that's what it's saying. But it's not saying we get updates as to where
the sun is instantly due to FTL gravity. It's saying objects rotate around a
projection. If the linear path of the sun were to change, those changes would
take a while to get to us to update our path, but also due to how that can't
"just happen", things don't end up getting messed up.

~~~
ajuc
But isn't sun rotating around galaxy center? So the velocity of sun isn't
line, but an orbit, so it isn't constant (it's always changing direction), and
we (Earth) do orbit the place that sun would be if not for the acceleration of
sun caused by combined gravity of galaxy?

------
jheriko
c is a conversion factor between space and time coordinates when we project
them into a space that we can intuitively grasp.

the obsession with the 'speed of light' always seems a bit senseless in this
context - its taking a concept which is known to break down in some limit and
then applying it right at that limit. this is bound to be confusing.

really the speed of light is like infinity in the geometry of our universe...
it just happens unfortunately that the geometry is not as simple as an
infinite euclidean space with an independent time coordinate so we end up
attributing a bad interpretation to the 'speed' of light.

------
ck2
Gravity is a "well" in space-time itself, it's not a wave you can benchmark?

That is why it can be "faster than light" when it's not faster at all, it's
just changing the path of light.

------
yeureka
I always wondered about this. If changes in gravity propagated faster than
light, could you build a transmitter by modulating a gravity field by
colliding matter with anti-matter?

~~~
jswhitten
How would that modulate a gravity field?

If you're thinking that the matter and antimatter have gravity and that
gravity disappears when they annihilate because their mass is gone, that's not
the case. All energy and momentum produces gravity, not just mass.

~~~
stcredzero
You could only modulate gravity as fast as you could move energy close to you
or far away from you. An implausibly powerful gamma ray flashlight flicking on
and off would let you do this. Get the beam powerful enough, and it will have
a gravitational field.

~~~
gliese1337
Get the beam powerful enough and it will have _meaurable_ gravitational field.
All beams of any intensity technically do contribute some, albeit
infinitesimal, component to the gravitational field.

It's a fun exercise to calculate just how strong a laser beam you'd need to
have to make a black hole entirely out of photons.

~~~
stcredzero
A black hole can actually cause an additional singularity due to the
blueshifting of infalling light. IIRC, one researcher studied this phenomenon
and called it a "blue sheet."

------
chiph
So, the FTL gravity pulses used in David Weber's Honorverse series (sci-fi
space opera) aren't possible. :(

~~~
negativity
But maybe a "warp drive" is?

[https://news.ycombinator.com/item?id=6238297](https://news.ycombinator.com/item?id=6238297)

...so then we still get to achieve an effective result of reaching a
destination faster than light.

...and since the gravity warps space, and the warp drive exploits warped
space, does that mean that the quantum mechanism behind the warp drive is
somehow directly related to gravity?

    
    
      the ring would contain exotic matter called 
      negative vacuum energy, a consequence of quantum 
      mechanics. The presence of this toroidal ring of 
      negative vacuum energy is what's required from 
      the math and physics to be able to use the warp trick.
    

It doesn't explicitly mention "gravity", but wouldn't this "negative vacuum
energy" have some kind of important relationship with the behavior of gravity?

~~~
pdonis
_wouldn 't this "negative vacuum energy" have some kind of important
relationship with the behavior of gravity?_

It seems like it would, yes, but I haven't really seen this discussed in
papers on wormholes.

