Now that you mention this, would an accelerator in space need walls? Could you make it from a series of big circles (maybe solenoids?) at distances of a million kilometers apart or so? Neptune's orbit is about 15 billion km long, so, you'd need about 15k of such loops. In free fall they would just go around the Sun, so you'd only need to maintain their perfect alignment with some minute adjustments.
Is there some reason to think that the only way to get higher energy collisions is by scaling the accelerators? Is there any possibility that we may be able to get to higher energies with earth-scale experiments?
The limiting factor is the strength of magnetic fields you can achieve. The more momentum the particles have, the stronger a field you need to bend them into a circular orbit. The attainbale strength is determined by material properties and there is nothing to suggest that we can get orders of magnitude improvement here. The improvements have been quite linear for decades, see the figure here:
And you eventually run into rather fundamental problems of tearing your atoms apart with the magnetic field you create. But I am not up to speed on how close we actually are to these limits.
Ultra-high-energy cosmic rays are possible candidate. For example, Oh-My-God particle had energy of a baseball traveling at 100 km/h. 40 million times higher than particle energies of terrestrial accelerators.
For the effort it would be easier to build one around the Earth. Double it up as a driveable highway, rail and electrical infrastructure (and then you can use solar power sold on a global market where one side is always in daylight).
It won't be surprising if we eventually do. A particle accelerator beyond Earth has been often discussed, and recently a paper[0] even sketched such a project for Moon.
Hollywood has vastly overplayed the actual density of asteroid fields. It’s kind of disappointing.
If you build something in a vacuum then you don’t necessarily need a single continuous piece to create a vacuum. If you specced an accelerator a million miles in diameter, the diameter matters from a standpoint of whether we can accelerate the particle sideways fast enough to keep it in the track, but they are also saying they need 3.1 million miles of accelerator hardware. They are implying they can’t lay it out as a crazy straw, but is that because of the lateral acceleration, the interaction between the coils, or the limits of manufacturing?
What if you built a collider in a spherical arrangement, accelerating in three dimensions at once, but 1/3 the diameter? What if the accelerator were broken into sections to dodge asteroids, with a cumulative segment length that added up to the desired total? What if you laid it out like a Spirograph? What if you laid it out like a truncated Spirograph (just bits of the outer circumference)? What if you laid it out like a 3 dimensional truncated Spirograph?
