Q: If nothing can escape a black hole’s gravity, then how does the gravity itself escape?

Physicist: A black hole is usually described as a singularity, where all the mass is (or isn’t?), which is surrounded by an “event horizon”.  The event horizon is the “altitude” at which the escape velocity is the speed of light, so nothing can escape.  But if gravity is “emitted” by black holes, then how does that “gravity signal” get out?  The short answer is that gravity isn’t “emitted” by matter.  Instead, it’s a property of the spacetime near matter and energy.

It’s worth stepping back and considering where our understanding of black holes, and where all of our predictions about their behavior, comes from.  Ultimately our understanding of black holes, as well as all of our predictions for their bizarre behavior, stems from the math we use to describe them.  The extremely short answer to this question is: the math says nothing can escape, and that the gravity doesn’t “escape” so much as it “persists”.  No problem.

Einstein’s whole thing was considering the results of experiments at face value.  When test after test always showed the speed of light was exactly the same, regardless of how the experiment was moving, Einstein said “hey, what if the speed of light is always the same regardless of how you’re moving?”.  Genius.  There’s special relativity.

It also turns out that no experiment can tell the difference between floating motionless in deep space and accelerating under the pull of gravity (when you fall you’re weightless).  Einstein’s stunning insight (paraphrased) was “dudes!  What if there’s no difference between falling and floating?”.  Amazing stuff.

Sarcasm aside, what was genuinely impressive was the effort it took to turn those singsong statements into useful math.  After a decade of work, and buckets of differential geometry (needed to deal with messed up coordinate systems like the surface of Earth, or worse, curved spacetime) the “Einstein Field Equations” were eventually derived, and presumably named after Einstein’s inspiration: the infamous Professor Field.

This is technically 16 equations, however there are tricks to get that down to a more sedate 6 equations.

This is technically 16 equations (μ and ν are indices that take on 4 values each), however there are tricks to get that down to a more sedate 6 equations.

The left side of this horrible mess describes the shape of spacetime and relates it to the right side, which describes the amount of matter and energy (doesn’t particularly matter which) present.  This equation is based on two principles: “matter and energy make gravity… somehow” and “when you don’t feel a push or pull in any direction, then you’re moving in a straight line”.  That push or pull is defined as what an accelerometer would measure.  So satellites are not accelerating because they’re always in free-fall, whereas you are accelerating right now because if you hold an accelerometer it will read 9.8m/s2 (1 standard Earth gravity).  Isn’t that weird?  The path of a freely falling object (even an orbiting object) is a straight line through a non-flat spacetime.

Moving past the mind-bending weirdness; this equation, and all of the mathematical mechanisms of relativity, work perfectly for every prediction that we’ve been able to test.  So experimental investigation has given General Relativity a ringing endorsement.  It’s not used/taught/believed merely because it’s pretty, but because it works.

Importantly, the curvature described isn’t merely dependent on the presence of “stuff”, but on the curvature of the spacetime nearby.  Instead of being emitted from some distant source, gravity is a property of the space you inhabit right now, right where you are.  This is the important point that the “bowling ball on a sheet” demonstration is trying to get across.

The Einstein Field Equations

The Einstein Field Equations describe the stretching of spacetime as being caused both by the presence of matter and also by the curvature of nearby spacetime.  Gravity doesn’t “reach out” any more than the metal ball in the middle is.

So here’s the point.  Gravity is just a question of the “shape” of spacetime.  That’s affected by matter and energy, but it’s also affected by the shape of spacetime nearby.  If you’re far away from a star (or anything else really) the gravity you experience doesn’t come directly that star, but from the patch of space you’re sitting in.  It turns out that if that star gets smaller and keeps the same mass, that the shape of the space you’re in stays about the same (as long as you stay the same distance away, the density of an object isn’t relevant to its gravity).  Even if that object collapses into a black hole, the gravity field around it stays about the same; the shape of the spacetime is stable and perfectly happy to stay the way it is, even when the matter that originally gave rise to it is doing goofy stuff like being a black hole.

This stuff is really difficult / nigh impossible to grok directly.  All we’ve really got are the experiments and observations, which led to a couple simple statements, which led to some nasty math, which led to some surprising predictions (including those concerning black holes), which so far have held up to all of the observations of known black holes that we can do (which is difficult because they’re dark, tiny, and the closest is around 8,000 light years away, which is not walking-distance).  That said: the math comes before understanding, and the math doesn’t come easy.

Funny because it's true.

It’s funny because it’s true.

Here’s the bad news.  In physics we’ve got lots of math, which is nice, but no math should really be trusted to predict reality without lots of tests and verification and experiment (ultimately that’s where physics comes from in the first place).  Unfortunately no information ever escapes from beyond the event horizon.  So while we’ve got lots of tests that can check the nature of gravity outside of the horizon (the gravity here on Earth behaves in the same way that gravity well above the horizon behaves), we have no way even in theory to investigate the interior of the event horizon.  The existence of singularities, and what’s going on in those extreme scenarios in general, may be a mystery forever.  Maybe.

This probably doesn’t need to be mentioned, but the comic is from xkcd.

This entry was posted in -- By the Physicist, Astronomy, Physics, Relativity. Bookmark the permalink.

64 Responses to Q: If nothing can escape a black hole’s gravity, then how does the gravity itself escape?

  1. Robert Lucien Howe says:

    Angel :
    I did not put it very well. My theory does not destroy general relativity it modifies it very slightly, which makes it compatible as part of an FTL flat frame theory. Your description is not wrong general relativity is not wrong it is more that it was left half finished.
    The FTL is very difficult to understand using standard mathematics, my model uses a self complete mathematics – which is based on the idea of scalar windows and non-finite sets. I also discovered how to calculate the roots of imaginary numbers by adding superposition to numbers. It is ultimately based on the idea that mathematics is never pure but is largely based on the observation of physics and not the other way around.

    All this resolves to a completely new interpretation of quantum mechanics based on the FTL predicate. The model doesn’t use renormalisation but computes values directly. This means that quantum mechanics and the whole of physics can become completely deterministic. The problem of course being that it is only deterministic as an FTL system. The result is that all quantum systems are FTL systems. Now for this to work the speed of light in these quantum systems has to be near zero, but once this is set the whole thing begins to fit together and other new pieces also begin to fit. The result is that it forms the beginnings of a complete GUT FTL field theory of matter where almost all physics begins to resolve down to just a single predicate -curved space time. All massed STL objects resolve to gravitational singularities stabilized by local time dilation. There is a zero point – come non-accelerating frame but it is separated from us by an FTL barrier. In this model the quantum limit itself is a form of event horizon..
    The question of course is how can the speed of light be equal to zero? the answer is that the speed of light is defined by the resonance of space time – local to each location and to each context. The mass equivalence equation E=m.c^2 must also remain valid, which adds even more complexity. One explanation is that : on classic scales space is three dimensional and dimensionally stable – but at quantum scales the metric of space runs out of energy and forms a four dimensional baulk and this reduces its local speed of light to virtually zero. This is only one of a whole family of different possibilities though – and the quantum part of the model is still very far from finished. The FTL predicate itself sets the speed of light as a minima so for FTL coherent objects local C tends to 0.
    The primary FTL region is really pretty simple, but creating a complete FTL quantum theory is anything but simple.

  2. Robert Lucien Howe says:

    Correction : ‘The result is that all quantum systems are FTL systems.’ Should have read : ‘The result is that all quantum systems can be interpreted as FTL systems.’

  3. Angel says:


    That makes a lot more sense. Although, for the imaginary numbers, you can already calculate their roots without extending the set. By using polar coordinates you can get an infinite number of n-roots of any complex number. Although you may be working with quaternions, which simply add many more roots and take away commutative multiplication.

  4. Jussi says:

    SO does the rate of re-shaping space-time follow the universal speed-limit?
    Or could messages be sent instantly to massive distances if there was a way to “master” gravity?

    If gravity works through shaping space-time and there are no particles or waves included I see no reason why gravity wouldn’t affect all of space-time the second it emerged (or moved)

  5. Robert Lucien Howe says:

    Angel :
    My method was massively simpler than either polar coordinates or quaternions. I simply reverse engineered the algorithm of root(-1) itself. The operation is -1 = 1 x -1 so the only simple explanation is that the multiplicand switches between the two values inside the power operation. This would require a number with a basic internal superposition of 2 so; i = (1,-1)SP2 , in = (n,-n)SP2. It can also map standard reals as n = (n,n)SP2 or (n)SP1, and complex numbers as m + in = (m + n, m – n)SP2. Other things in mathematics like the Pythagorean circle operator and the factorisation formula also seem to fit well with this, but there is still a lot more unexplored than explored.
    The really important bit is that i sums to zero, this sets a basic net mass of all tachyons as zero.
    An obvious thing is that with a net mass of zero photons fit very well as tachyons. The photons oscillation stands in as a time differencing superposition, and its direction switching produces a positive negative energy gradient. This produces the greater generalisation that wave like behaviour is FTL behaviour and particle like behaviour is STL behaviour. That also fits well with experiments like Young’s slits where the wave interference behaviour becomes FTL interaction. The only time that light really interacts as STL particles is at emission and absorption, and maybe reflection. Most of the rest of lights behaviour, such as travelling in straight lines, comes from FTL region physics.

  6. Doug Lockman says:

    How are Einsteins equations effected by the black hole that recently burped out part of a sun being swallowed by said black hole? Did the mass of the sun increase the pressure (gravity) present in the black hole, and cause a momentary pole reversal of the magnetic field from gravity to anti-gravity, or is there a warbling effect (like a drop of water in a pond) that can account for material being spewed out of this black hole?

  7. Angel says:

    Robert, I think you’re describing the phenomenon of phase velocity, which actually differs from wave velocity. Interactions occur FTL all the time; there is simply no net information transmission.

  8. Pingback: 19 Facts You Didn’t Know About Light | Display Lighting Blog

  9. Drake says:

    One: Wave velocity CAN be used to describe a wave feature such as phase velocity. Also phase velocity in vacuum is the speed of light not faster and if in medium it is lowered by the refractive-index which is frequency-dependent.
    Two:General relativity describes the ever expanding shape of the universe as “curved slightly like the seat of a saddle.” there fore the presence or lack there-of of matter is not a constant factor in the presence of gravity, the object (black-hole) does not have gravity escaping it rather it bends the shape of the universe slightly more making the gravity due to presence or lack-there-of of matter a part of the surrounding space-time which in essence is irrevocable and non-changeable.

  10. Anirudh Kumar Satsangi says:

    One possible condition is that the speed of gravity is much much higher at black holes than the speed of light. Thus, black holes can easily emit gravity. And as it goes away from the black holes the speed of gravity gradully dereases.

  11. Angel says:


    What justifies your presumption that gravity has a velocity to begin with?

  12. Anirudh Kumar Satsangi says:

    Thanks for asking a very good question, Angel. Do you mean gravity is static?

  13. Robert Lucien Howe says:

    If you read my earlier post you will find my burbling on about an FTL gravity theory.. The discovery of gravity waves sunk that one.. and prove beyond doubt that gravity does move and moves strictly at the speed of light.
    I had an auxiliary solution that now seems to be the correct one. The spatial frame is absolute at FTL speeds – this allows a coherent physics that works inside black holes. The existence of an absolute frame rules out general relativity at FTL speeds, and the existence of black holes could be a tentative proof of this.. That gravity wave detection is also a pretty convincing proof for the existence of black holes.

  14. Anirudh Kumar Satsangi says:

    Would you agree Robert that the escape velocity at black holes is greater than the speed of light?

Leave a Reply

Your email address will not be published. Required fields are marked *