Anyone who saw the 2014 space opera Interstellar will remember that planets featured in the film, visited by a crew from Earth, orbited not a star but a massive black hole called Gargantua, its accretion disc the source of heat and light. Much of the science in the film was calculated by renowned physicist Kip Thorne and indeed the treatment of black holes in the film was a source of inspiration for scientists even here in the Czech Republic.
As it happens, they had been grappling with a similar problem: could a cold black hole (as opposed to the one in the film) also provide sufficient energy for nearby orbiting planets to be habitable or sustain life? Their findings, which caused something of a stir in the international scientific community, suggest yes. The project’s Tomáš Opatrný, a physicist and professor at Palacký University in Olomouc, told me more on a line from Scandinavia this week:
“We had been dealing with a thermodynamic question of what would be the conditions for getting useful energy, capable, for example, of supporting life, if you were in an otherwise empty universe orbiting a black hole. Normally, a star would provide you with lots of energy but being near a black hole you couldn’t benefit from that.”
But a black hole itself can provide plenty of energy needed, as was posited in Interstellar. Black holes, despite the name, emanate plenty of energy and light and are among the brightest objects in the universe. The reason is because as and matter sucked into a black hole heats up and radiates as it accretes: in Interstellar, the fictional Gargantua features a massive accretion disc which provides heat and light for nearby planets, including the one called Miller’s – not quite habitable but agonizingly close.
But what about a much older black hole, with no accretion disc to speak of, the kind considered by the Czech team? Professor Opatrný again:
“What makes black holes bright is the accretion disc: this is a lot of material falling into the black hole which is heated considerably and radiates. What we considered was an old black hole of an old constellation of stars where there would be no accretion material left and all the nearby material was spent. The black hole, in short, would be really black and almost nothing would get out, with the exception of Hawking radiation which is very weak. That means completely black and completely cold. All radiation which reaches the hole is absorbed.”
“We had been dealing with a thermodynamic question of what would be the conditions for getting useful energy if you were in an otherwise empty universe orbiting a black hole.”
Opatrný and the team of scientists worked out energy factors within the vicinity of cold black holes: chief in their calculations was Cosmic Microwave Background – leftover thermal radiation which dates back to the Big Bang itself. Normally, Cosmic Microwave Background radiation (CMB) is extremely cold, says Opatrný, but in the neighbourhood of a black hole, the radiation would heat up significantly, providing both heat and visible light. Such a scenario posits a ‘bizarro’ version of conditions here on Earth. Life as we know it benefits from the difference in temperature between the sun and a cool sky; in the Czech scientists’ scenario, the conditions would be reversed: a hot sky and cool object orbited.
“For useful energy you need a certain imbalance, a contrast between heat and cold. On Earth, we benefit from a hot sun and cold sky; the situation near a black hole would be very different. At the same time, everywhere in the Universe you have Cosmic Microwave Background (thermal radiation dating back to the Big Bang) which is very cold, just barely three degrees above absolute zero.
“But that is still warmer than a black hole and you still have some imbalance we were playing with the idea how this energy could still be useful. The radiation, at the right distance from the black hole, would be slightly accelerated, heated up, closer but not too close to a black hole. You can’t be too close for the radiation to be at a reasonable value.”
In other words, a world near a cold black hole could benefit from hot/cold conditions provided by the interplay of the black hole and the heated CMB radiation. Within the so-called Goldilocks zone (or circumstellar habitual zone), Professor Opatrný explains a reasonable level of energy and heat could be capable of sustaining life. Not necessarily a planet where life took hold or had time to evolve but perhaps a planet to migrate to, in the far distant future, when humans presumably master space travel by bending space-time, or travelling through a wormhole as they do in Interstellar. Conditions, including visible light on such a planet, would be determined by the heated CMB radiation.
“If you take the microwave radiation near a black hole you have a blue shift effect which means that whatever waves fall to you is accelerated. All the radiation is accelerated and shifted to the blue and because the planet would also be orbiting quickly you would also see a Doppler shift. Even though you would start with microwave radiation it would shift to high frequencies to visual light as we see on Miller’s planet in Interstellar. These microwaves are shifted to visible and moreover to extreme ultraviolet.”
Gauging what that alien sky would look like near a black hole sounds like a gargantuan task in itself.
“It was necessary, through very precise calculations, to trace all the rays which would hit the planet. This was a job for the people at Opava University, Pavel Bakala, an astrophysicist, to examine what the sky would look like close to a black hole. He traced all possible light rays and from that we would see exactly where the light was coming from. It would look like a very small bright spot, a tiny spot like when we see Neptune from Earth. Small, but extremely bright.”
“If I were a cosmic ‘real estate agent’ I would recommend a small star with a span of billions years rather than a planet by a black hole!”
Miller’s planet, landed on by the crew in Interstellar, is in many ways a different and also fictional beast. In the film, the planet is mostly or all shallow sea affected by enormous tidal forces which call up a tsunami of nightmarish proportions. Otherwise, it might have been an almost pleasant stop during the holidays, a giant paddling pool.
What the filmmakers did not take into account was precisely that studied by the Czech scientists – the universe’s background radiation. Under the estimates of the Czech researchers, Miller’s planet would have looked very different heated to a temperature of almost 900 degrees Celsius. And the unforgettable tidal wave? That would more likely be molten aluminium than water. Ouch.
So in some respects, the cold black hole scenario could perhaps be more inviting and in any case, potential conditions near cold black holes are something the Czech team will hope to investigate further. Of course, even if a suitable planet orbiting a black hole were one day found, it wouldn’t be Professor Opatrný’s choice by far for Earthlings to found an off-world colony.
“It’s an interesting idea but I would wait a few hundred billion years before we need to consider life near a black hole! There are plenty of stars which will still be around. If I were a cosmic ‘real estate agent’ I would recommend a small star with a span of billions years for a long-term investment!
“One day of course the energy near black hole will be more important but unfortunately the scenario in our paper will no longer be true then, because by then the cosmic microwave radiation will be too cold to be of any use.
“Then perhaps there will be an opportunity for other scenarios, such as the thermodynamic imbalance between two black holes, one small and one large, each with a different temperature, each radiating Hawking radiation. That will be very slow and weak, but by then life will probably already be very quiet and very slow, too. Basically, one day such scenarios will be the only source of energy left.”