Gravitational Machines
I think this idea is essentially an example of a "gravitational slingshot".
I found it interesting that such a system could be used to "accelerate delicate and fragile objects to a velocity of 2000 km/sec at an acceleration of 10,000 g, without doing any damage to the objects. ... So a large space ship with human passengers and normal mechanical construction could easily survive the 10,000 g acceleration." This seems counterintuitive, but since the object is in freefall the entire time, I guess it makes sense.
Dyson was quite the visionary. LIGO / Virgo gravitational wave detectors have confirmed all this (with much more development from people like Caltech's Kip Thorne and many others):
> "The energy source of the machine is the gravitational potential between the stars A and B. As the machine continues to operate, the stars A and B will gradually be drawn closer together, their negative potential energy will increase and their orbital velocity V will also increase... the loss of energy by gravitational radiation will bring the two stars closer with ever-increasing speed, until in the last second of their lives they plunge together and release a gravitational flash at a frequency of about 200 cycles and of unimaginable intensity."
The Ligo Lab's youtube channel has lots of great videos on the topic, from the sounds made by a pair of colliding black holes to long talks about how certain elements are mostly made by colliding neutron stars:
In a similar vein, John Kraus (Of Antennas... textbook fame) described a gravitational transmitting and receiving system as a fun (?) diversion near the end of the book.
It has been a few years, but I seem to recall that the transmitter was a 500T steel bar spun at very close to the maximum RPM the tensile strength of steel allowed; the radiated energy was something like a fraction of an attowatt. (An attowatt = 1*10^-18W)
There are more efficient transmitting schemes out there.
I always wondered how you might be able to extract energy from the expansion of space. It's particularly interesting because conservation of energy does not hold on such large scales.
Reminds me a lot of the Three Body Problem by Cixin Liu. Lot of space travel concepts explored there similar to this.
https://www.amazon.com/Three-Body-Problem-Remembrance-Earths...
Does anyone have an idea what sort of design could achieve the proposal near the end:
> Clearly the immense loss of energy by gravitational radiation is an obstacle to the efficient use of neutron stars as gravitational machines. It may be that this sets a natural limit of about 108 cm/sec to the velocities that can be handled conveniently in a gravitational technology. However, it would be surprising if a technologically advanced species could not find a way to design a nonradiating gravitational machine, and so to exploit the much higher velocities which neutron stars in principle make possible.
From my dim memory of Kip Thorne's popular book, a spinning black hole could be used in this way, which would mean there's at least one solution.
This talks about two bodies, but would the Halo Drive (https://arxiv.org/abs/1903.03423) also be one such thing? I mean, I guess the photons are the other bodies?
Of course there are also more mundane ways of utilizing gravity such as Tidal and Hydroelectric power, or just walking (controlled falling) for that matter.
how exactly is energy extracted from such a system?
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It's an article from 1962:
"After the detection of the gravitational wave GW170817, Jason T. Wright (Physics Today, 72, 5, 12, 2019) reminded the community that many of its features had been predicted by Dyson more than half a century earlier. Dyson’s article was published only once, in Cameron’s long out of print collection, though a scan may be found at the web site of the Gravity Research Foundation (https://www.gravityresearchfoundation.org). Dyson thought it had been reprinted (in his Selected Papers, AMS Press, 1996, forward by Elliot H. Lieb) but it was not. Hoping to make the article easier to find, I wrote Dyson for his permission to post it at the arXiv"
It's about using two big bodies, A and B, to accelerate objects: "The energy source of the machine is the gravitational potential between the stars A and B. As the machine continues to operate, the stars A and B will gradually be drawn closer together, their negative potential energy will increase and their orbital velocity V will also increase."