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Let's say a black hole of mass $M$ and a very compact lump of anti-matter (not a singularity) also of mass $M$ are traveling toward each other. What does an outside observer see when they meet?

Will they blow themselves apart in a matter/anti-matter reaction? Or will their masses combine, never quite meeting in the infinite time dilation at the event horizon?

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    $\begingroup$ It will behave like regular matter $\endgroup$
    – ClassicStyle
    Commented Sep 28, 2015 at 19:17
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    $\begingroup$ Whatever happens inside the event horizon stays inside the event horizon. $\endgroup$
    – Solomon Slow
    Commented Sep 28, 2015 at 19:25
  • $\begingroup$ related: physics.stackexchange.com/q/7290 $\endgroup$
    – arivero
    Commented Sep 28, 2015 at 19:34
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    $\begingroup$ This question came up while discussing how to defend Earth from attackers that can throw black holes at you. IMO, Worldbuilding-related questions on phys and chem.SE should link back to the original, for reference and so interested people can maybe contribute to the original. Obviously they need to be well asked and able to stand on their own to have a place on phys/chem/bio.SE, though. $\endgroup$
    – Peter Cordes
    Commented Sep 29, 2015 at 3:44
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    $\begingroup$ Perhaps the "anti-matter" would react with the other matter while it's circling the black hole in its accretion disk. It would never reach the Schwarzschild radius. $\endgroup$
    – Peter - Reinstate Monica
    Commented Sep 29, 2015 at 10:21

3 Answers 3

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Whether the infalling material is matter or antimatter makes no difference.

Fundamentally, the confusion probably comes from thinking of black holes as normal substances (and thus retaining the properties of whatever matter went into making them). Really, a black hole is a region of spacetime with certain properties, notably the one-way surface we call an event horizon. That's it. Whatever you envision happening on the inside of a black hole, whether it be a singularity or angels dancing on the head of a pin, is completely irrelevant.

The reason spacetime is curved enough to form an event horizon is essentially the due to the density of mass and energy in the area. Antimatter counts just the same as matter when it comes to mass and energy. Anti-protons have the same, positive mass as normal protons, and at a given speed they have the same, positive kinetic energy too.

Even if you wanted matter and antimatter to annihilate somewhere near/inside a black hole, the resulting photons would cause no less curvature of spacetime, as all particle physics reactions conserve energy and momentum. This is related to how you could form a black hole from nothing but radiation.

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    $\begingroup$ Angels dancing on the head of a pin...is that a reference to something? $\endgroup$
    – Horus
    Commented Sep 29, 2015 at 6:48
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    $\begingroup$ @Horus en.wikipedia.org/wiki/… $\endgroup$
    – zovits
    Commented Sep 29, 2015 at 7:02
  • $\begingroup$ Wouldn't the annihilation of the mass reduce the gravity to 0, allowing photons&co to escape? $\endgroup$
    – algiogia
    Commented Sep 29, 2015 at 9:36
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    $\begingroup$ @algiogia Annihilation doesn't destroy energy - it simply releases it. The released energy still has exactly the same mass (and thus gravity - Relativity 101 :)) as the two particles that annihilated each other (and for more energetic particles than electrons and positrons, the output isn't raw photons either). Energy is always preserved. $\endgroup$
    – Luaan
    Commented Sep 29, 2015 at 10:06
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    $\begingroup$ @JanDvorak Well, it's not one of the firmest facts in science by far, but there's been some experiments where antimatter distinctly behaves as normal matter as far as gravity is concerned. It's not outright proven (nothing in physics really is), but I don't think anybody expects any serious surprises on that front anymore. Of course, gravity is incredibly weak and the amounts of antimatter we deal with is tiny, so the uncertainty isn't that small, but since we're already assuming the antimatter falls into the black hole, it probably fits the question. $\endgroup$
    – Luaan
    Commented Sep 29, 2015 at 17:37
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Annihilation conserves everything

What might possibly be unintuitive is that during matter-antimatter annihilation nothing disappears - the particles simply get converted to other particles and energy. From the point of gravity, however, energy is mass - so from the point of an outside observer, if no particles escape the system, then it doesn't matter if the annihilation reaction happened or not - the total mass=energy of the system doesn't change.

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To add just a little to what Chris has said, when the antimatter falls into the black hole - let's say it's a positron - it annihilates with regular matter. In this case, the positron would presumably annihilate an electron, creating two gamma rays (very high energy light) of energy 2*(0.511 MeV). Just as matter cannot escape a black hole, photons (our gamma rays) can't either. Spacetime is so curved that they have no path out! So, essentially, the mass-energy of the anti-matter can't escape, even if it turns into 'massless' photons.

Curved spacetime causes other interesting effects, including gravitational lensing where large objects like galaxies or stars bend the path of light passing nearby them.

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  • $\begingroup$ Why would there be an electron inside the black hole, unless its from matter that just fell in & hasn't yet reached the centre of the BH? $\endgroup$
    – PM 2Ring
    Commented May 2, 2019 at 8:20

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