- Quote: The ideal isolated bubble would be a thick-walled, windowless spaceship far out in interstellar space, gliding gently with its rocket engine switched off.
- Endnote: Exactly what constitutes “isolated” is a complex issue if we look at it closely. But informally, an isolated bubble should shield anyone inside from all information about objects outside; otherwise, those objects might create effects inside the bubble that would obscure the relativity principle.
This is a remarkably tricky issue, a fact that is conceptually important. The issue is this: what can walls actually shield us from? Certainly not everything.
Light, Visible and Otherwise
Light waves of sufficiently small wavelength can be shielded; a wall made of metal will keep out radio waves, microwaves, visible light, and even X-rays if it is thick enough. But sufficiently high-energy light — gamma rays, in particular, will strike atoms in the wall and cause their nuclei to disintegrate, creating a shower of particles that can penetrate deep into the wall. The more prevalent the gamma rays in the outer universe which might reveal the presence of celestial objects, relative to which we could determine our motion, the thicker the walls our spaceship will need to have.
How bad is this problem? Remember, we’re talking about conceptual issues here. In a calm universe where life might exist, there had better not be too many gamma rays at super-high energies. And so, if our spaceship’s interior is small enough and its walls are thick enough, the gamma rays and their effects when they strike the exterior of the walls can be kept out of the craft’s living quarters.
Almost.
Neutrinos
The “almost” refers to another problem: neutrinos. Neutrinos can arise in the gamma-ray collisions with the ship’s walls, or from other processes far away in the universe. And neutrinos can go right through walls — more precisely, if their energy is low (not high!) then most neutrinos can travel all the way through the Earth. Up to a point, high-energy neutrinos actually travel shorter distances than low-energy ones do (thanks to subtle properties of the weak nuclear force described here.) But the big picture is this: we’re not going to be able to shield ourselves from all the neutrinos. Why can’t we use them to determine our motion?
We could do so, in principle, if we could measure them. The very same properties that make it easy for neutrinos to pass through matter make it extremely difficult to detect them. Without a very large experiment and lots of time to run it, we have no chance of detecting enough neutrinos to learn about the wider universe. This can’t be done in the short term on a small spaceship. Typical neutrino detectors (such as Super-Kamiokande) are huge and run for years.
And this seems to be a general rule: if something is so elusive that it can penetrate thick walls, it also tends to be too elusive to detect within a reasonably-sized spaceship.
But this argument, you notice, tends to the practical rather than the principle. That, in turn, is not entirely satisfying. Since we’re trying to put a foundation under the relativity principle, having to make a somewhat practical argument doesn’t seem completely convincing — to me, anyway.
Gravity
Even in an imaginary universe that has empty space but no neutrino-like particles, there may always be gravity. Gravity cannot be shielded. Gravitational forces, which arise (in Einstein’s view) from the very shape of space and time, cannot be blocked by material objects.
And yet, strangely, gravity is, in a way, self-shielding. This is a feature of Einstein’s relativity principle that is already clear in Newton’s gravity. If a spaceship is sufficiently small and its rockets are off, then gravity will certainly affect its motion and the motion of everything inside it — in precisely such a way that no one inside the spaceship will know. That’s why astronauts in the international space station float — it’s not that they are without gravity, but that gravity causes their motion to be exactly the same as the space station’s motion. (See discussion of Chapter 4, Endnote 6.) Because of this, shielding of gravity isn’t necessary, as long as the spaceship is small enough.
Gravitational waves — ripples in space itself — potentially complicate this story. These cannot be shielded either, even in principle, no matter how thick the walls. But like neutrinos, they are exceedingly difficult to detect in a small spaceship. Gravitational wave detectors used today are many kilometers across. So again, we have a practicality issue — and again, that’s not completely satisfying.
The principle of relativity seems so essential to the cosmos that there ought to be a precise notion of an isolated bubble, one that closes these apparent loopholes and turns issues of practice into issues of principle. If anyone has ideas on how to improve the discussion I’ve given here, either conceptually or mathematically, please leave a comment.
