A new thought experiment shows how we could get information from a black hole

 

 The event horizon of a black hole has long been thought to be the edge of the knowable universe. Black holes are little pockets of space that information can enter, but never leave. We can learn about black holes indirectly, by looking at their effects on non-black-hole objects around them, or at the extreme distortions in space immediately beyond the event horizon, but direct measurement has traditionally been thought impossible. Now, researchers have come up with a very, very hypothetical experiment which shows that, in principle, we might be able to look into a black hole after all.

The idea centers around the mechanism of Hawking radiation, in which one half of a particle-antiparticle pair can actually escape from a black hole. This is thought to occur because under the intense gravitational conditions just outside a black hole’s event horizon, pairs of virtual particles can be “boosted” into pairs of real, physical ones. In essence, probabilistic possibilities for particle location become concrete realities, thanks to input energy from the black hole. That’s crucial — the energy of these new particle pairs is from the black hole itself, and their creation theoretically decreases the black hole’s mass by some infinitesimally small amount. That’s why Hawking radiation is sometimes referred to as the process by which black holes “evaporate” and eventually die through a slow, slow, slow loss of mass.

The vast, vast majority of these matter-antimatter pairs will immediately annihilate one another, but a small proportion will have precisely the placement and direction needed to be pulled apart before that can happen. One particle heads in, to get sucked up by the black hole, and the other particle goes out — where we could possibly observe it. These observations are unlikely with telescopes, due to the extreme weakness and low frequency of such emissions, but in principle the math suggests that they ought to be there. Some in-lab experiments have claimed to confirm the existence of Hawking radiation, but no proof has been widely accepted by the field just yet.

The new idea for black hole archaeology, published this week in the Arxiv open research database, rests on one important additional fact: These particle-antiparticle pairs, even those pulled apart to such dramatically different fates, ought to be entangled. Entanglement is probably the most dreamed-about part of quantum physics by science fiction authors: Certain pairs of particles can be created or manipulated such that their physical states are related across unlimited physical distance, and with zero actual transfer of information. What this means is that two entangled particles can affect one another instantly, without the need for a signal to traverse the distance between them at the “unbreakable” speed of light. Their communication, if communication is even the right word for what goes on between them, cannot be intercepted, because it fundamentally does not travel from or to anywhere at all.

An artist’s impression of a black hole

That’s an immediately interesting property, in the context of a black hole, since the whole problem with studying them is that we can’t get information out — but entanglement doesn’t seem to involve the transmission of any information. So, if we have two entangled particles, one in the black hole, and one out, then we’ve got a possible inroad to learning about the black hole.

Physicists at CERN and elsewhere are working to understand antimatter, annihilation, and entanglement.

But, things get a little hairier than that. See, simply finding this escaped particle would confirm the existence of Hawking radiation overall, but not tell us much of real value about the black hole that created it. For that, the researchers say we’d need to go on to do three additional things: Measure the overall “spin state” of the black hole itself, drop a pre-measured photon into it, and finally measure the black hole again. The known impact of our added photon should be reflected in the state of our escaped particle, since its entanglement with the black hole via its captured partner particle does not require light or any other form information to actually leave the black hole at any point.
 
You know, maybe.
The extremely hypothetical nature of this paper means that the primary point for non-physicists is very general: Entanglement is weird, black holes are weird, and interactions between the two are even weirder. More than that, the fact that we have to conceive of such outlandish experimental procedures just to imagine some way of learning about the internal conditions of a black hole ought to make it clear just how powerful these singularities really are. Scientists may yet figure out a way to learn about the interior of a black hole, but if they do, it will be a truly monumental achievement.