Photovoltaics and time near a black hole (as a key story element)

[Daniel Sellers]

I'm a physics student and science fiction writer, and I've never been to this section of PF before! But I have an idea I think is cool for a novel or short story and I'd like some input on the physics involved. As I'm not overly worried about this idea being stolen I'm going to provide a short plot setup:

The setting is the distant future where human technology has progressed to near god like capabilities including, if we wanted to, the ability to go 'close' the event horizon and maintain a large enough radial acceleration to stay in one place (not close enough to become spaghettified).

In a physics lecture, a professor explains that in such a gravitational field you would experience time dilation. One problem you would have to contend with is that, depending on how close to the event horizon you were sitting, you might have thousands or even millions of years worth of stellar radiation bearing down on you at once.

A student naively suggests that we could place photovoltaics in such a position and then retrieve them with millions of years worth of energy stored in a battery (assuming the energy to avoid falling in was less than the energy you were collecting... blah blah hand waving)

"No," the professor explains, "because we, far from the black hole, would still have to wait millions of years before the battery was so charged. We might as well just build solar panels and wait a million years, but I have a lunch meeting this afternoon so I can't."

Another professor in the future-university overhears, and is inspired, because she is working on some project that requires an extreme amount of power (could go a lot of different directions with this). She thinks: I myself could go down near the event horizon, collect millions of years worth of solar power, and then come back up. Millions of years would have passed, but I could potentially have the largest concentration of energy in the universe in my possession. So she starts starts gathering a team to do just that.

If you've read all of this then thanks for your time! I'm mostly asking for input on the physics, what kind of technology this future-physicist would need etc... but there's a lot of room here for action and human interest so plot ideas are welcome too.

Thanks!

 

[Daniel Sellers]

I just realized this thread belongs in writing and world building, not the general sci-fi section. Sorry! Can it be moved?

 

[Drakkith]

If you've read all of this then thanks for your time! I'm mostly asking for input on the physics, what kind of technology this future-physicist would need etc... but there's a lot of room here for action and human interest so plot ideas are welcome too.

I haven't done much math for the following, so it's mostly a best-guess based on my limited knowledge of physics, especially GR.

The key things would be the propulsion technology to get your team there and back, the capability to store all of that energy (preferably in a compact form), and the photovoltaics. You have many options, it's more a question of 'how big' you want your spaceship(s). Standard chemical rockets would work, but you'd have to build a truly gargantuan spacecraft just to have enough fuel to get your ship to the star system and deliver it back to wherever they wanted to go after their little trip.

You also need run into a size and mass problem with storing the energy. A million years worth of energy (assuming it's not a trivial amount per year) requires a staggeringly huge mass of whatever you're going to store it in, whether it's batteries, reactive chemicals, radioactive isotopes, or whatever. We're talking MUCH more energy storage than the entire world currently has.

The photovoltaics are subject to similar scaling issues. Assuming you need more energy than the typical planet produces in, say, a few decades or centuries, you'd need solar arrays at least the size of large cities. Luckily I think this is the least problematic of your issues. However, we're also assuming that the majority of the light hasn't been blueshifted right past the visible region and into the x-ray or gamma ray regions. The kind of time dilation you're talking about would likely do just that. Even if you limit time dilation to 1000x slower than 'real time' you still blueshift light by that same factor. So deep red light at 700 nm in wavelength is now 0.7 nm, which is in the far x-ray range.

On top of all that, your spaceship and its equipment needs some way to survive the tremendous flux of heat and particles bombarding it.

My view is that you should invent some technologies that go some way towards solving these scaling issues. More efficient engines and fuel, higher-density and lower mass energy storage, and more efficient ways to collect solar energy. Or, you could always ignore these issues and focus on the characters and the story, hand-waving away or not mentioning the technology behind the venture. That's also a perfectly viable option.

 

[Daniel Sellers]

Thanks for the very thoughtful response!

Indeed, I write stories for characters and adventures in interesting settings, but this story would be enhanced by at least some exploration of the physics I think. It was inspired by in actual discussion of GR with one of my professors after all.

I would probably go with hand-waving for the energy storage system.

I suppose in theory you could pick what spectrum of light you wanted to intercept by how deep in the gravity well you set your photo cells. But is there anything inherently un-physical about panels designed to receive far x-ray light?

Also, what about instead of propulsion the system was allowed to stay in place by surrounding the black hole and balancing the gravity? Kind of like an inverted Dyson-sphere or maybe just a Dyson-ring. Of course then I invite all the challenges associated with a Dyson sphere, but other people have pondered these challenges quite a lot already.

As for all the heat bombarding the craft, I'd be comfortable saying 'very advanced futuristic radiation shielding' and the ambient heat onboard could actually be an additional source of power generation.

On the story side, our million+ year old physicist might re-emerge to find she's become a legend, and find people waiting for her and the unprecedented power bank she's acquired - should she anticipate this and find some way to weaponize the super-battery? Just thinking out loud now.

Thanks again!

 

[Drakkith]

I suppose in theory you could pick what spectrum of light you wanted to intercept by how deep in the gravity well you set your photo cells. But is there anything inherently un-physical about panels designed to receive far x-ray light?

Well, they wouldn't be your everyday solar panels like we have now, but there's no reason they have to be photovoltaics. Perhaps these panels are filled with a fluid that absorbs the x-rays and heats up, with a very efficient heat engine driving the power production.

Also, what about instead of propulsion the system was allowed to stay in place by surrounding the black hole and balancing the gravity? Kind of like an inverted Dyson-sphere or maybe just a Dyson-ring. Of course then I invite all the challenges associated with a Dyson sphere, but other people have pondered these challenges quite a lot already.

Sure. Go ahead.

On the story side, our million+ year old physicist might re-emerge to find she's become a legend, and find people waiting for her and the unprecedented power bank she's acquired - should she anticipate this and find some way to weaponize the super-battery?

The story aspect is much more difficult to help with. You have an almost unlimited number of options, and you'll have to figure out what kind of story you want to tell.