Chapter 3, Endnote 1

• Quote: There’s no grid crossing the universe, no array of cosmic streets, that would allow us to state or even define the spatial address of a galaxy, or of anything else.
  
  
• Endnote: Readers familiar with astronomy may wonder about the Cosmic Microwave Background, the diffuse leftover glow from the Big Bang at the universe’s birth. One can indeed specify one’s motion (though not location) relative to this bath of ancient light. But this bath is no more stationary than anything else, even though it is more widespread. Moreover, it would be absent from an isolated bubble, so its existence leaves Galileo’s principle intact. I’ll say more about these issues later.
  

The cosmic microwave background (CMB) poses a subtle and often confusing issue for Galileo’s relativity principle. In my discussion of these subtleties, I will be using language that is only introduced in later chapters (up through chapter 14 and chapter 17). For this reason, you may find it easier to understand this page after you’ve read through chapter 17.

The Cosmic Microwave Background (CMB)

The CMB is the swarm of microwave-frequency photons that fills the universe, a leftover glow from the Big Bang. These photons, unlike the space they move within, form a sort of odd but perfectly ordinary sea — an ordinary medium. In contrast to the hot soup of interacting particles that characterized the Big Bang’s early moments, this sea is as flimsy as it could possibly be, because the photons of the CMB do not interact with one another. Nevertheless, the CMB fills almost the whole universe, making it almost an everywhere-medium. Moreover, the CMB, unlike empty space, is not amotional (for reasons I’ll make clear in a moment.) And so it provides something relative to which one can, in fact, measure one’s speed, even out in deep space.

This would seem to pose a serious threat to Galileo’s principle and this entire book’s premise, and so I need to explain, briefly but carefully, why it isn’t.

The CMB Versus Empty Space: Two Very Different Seas

It is tempting to see empty space as something that makes up the universe, and the CMB as something that fills all of that space and might even be considered a part of it. But that is not correct.

First, one must distinguish the “impossible sea” of empty space from this perfectly ordinary sea of CMB photons that are found within that sea. As I emphasize in chapter 14, you can’t shield yourself from space, or bottle it, or go somewhere where there isn’t any. But you can shield yourself from the CMB — it doesn’t penetrate deep into the ocean or rock, for instance. A simple metal grid keeps the photons inside your microwave oven from leaking out, and for the same reason, a lab or bubble surrounded by a suitable metal grid would reflect all CMB photons back out into space, keeping them from entering the bubble. Similarly, you could take a bottle of CMB photons (with mirrored walls, so that they stay inside) into such a lab, and study them there. None of this is possible for empty space..

The CMB is as widespread an ordinary medium as exists in our universe. But it isn’t an everywhere-medium, the way empty space is. The CMB has natural gaps in it, where space exists but the CMB doesn’t: for instance, there’s no CMB inside planets or stars, even though there’s lots of empty space there. You can make more such gaps in the CMB as I just described, using metal boxes.

Nor is the CMB-as-medium to be confused with the luminiferous aether, which exists everywhere. The CMB is made from photons — a medium made of hordes of wavicles, not so different from any ordinary medium such as air or the ocean. By contrast, the luminiferous aether is an amotional medium, and it is not made from photons. Its vibrations are photons, yes; but it is no more made from photons than the atmosphere is made from sound waves. A medium for photons is not the same as a medium made from photons; the former is amotional, the latter is not.

Using the CMB to Measure Motion (And Why It’s Okay With Galileo)

The importance of the CMB is that it is not hard to measure it once you know how. Although your speed relative to each individual photon is always c, you can easily determine whether you are moving through the CMB as a whole. To see this, let’s imagine a simpler version of the CMB.

Suppose the CMB’s photons all had exactly the same frequency. If you were moving relative to this simple CMB, the photons approaching you from all sides would have speed c, as always, but their frequencies would be change. Those coming from behind you would be “red-shifted” — their frequencies would be reduced slightly — while those approaching toward your front would have slightly increased, or “blue-shifted”, frequencies. It’s easy to measure these frequencies, so it would be easy to use them to measure your speed relative to the CMB.

This same principle works even in the real world, where the CMB photons, rather than having a fixed frequency, have a known pattern of frequencies, and the most common frequency is at 0.15 trillion cycles per second. Measurements of how this most common frequency varies, depending on the direction at which CMB photons are approaching us, teaches us that the Earth and Sun are moving (on average) at about 370 kilometers (230 miles per second) relative to the CMB.

You might then think the CMB violates many statements made in this book — and it does violate a few of them, a very little bit. For instance, the CMB does exert drag on planets and stars. But it’s allowed to — it’s an ordinary medium, not an amotional one. And because the microwave photons each have low energy and are fairly diffuse, the drag they exert is so extremely tiny that it has no measurable effect.

More importantly, this drag does not violate the principle of relativity. In a suitable isolated bubble —perhaps even a nested pair of bubbles, a large one to screen out the CMB photons and counter any drag, and the second bubble to sit within the first and be completely separated from the CMB — there’s no effect of the CMB that can be detected inside the bubble, and so Galileo’s principle holds exactly within that bubble.

That, in turn, means that speed is still relative, and Galileo’s principle is still correct. When we measure Earth’s speed relative to the CMB, that’s not the same thing as the Earth’s “true speed” relative to empty space. It’s just its speed relative to the CMB, no more special than its speed relative to our galaxy’s center or relative to the Sun. All remnants of that relative motion are lost inside an isolated bubble, where steady motion again becomes completely undetectable.

Galileo Still Holds Sway in the Universe

And so, in the end, the central points of this book remain in place. Despite the CMB, empty space and all other truly-everywhere-media for cosmic fields must be amotional and light waves must be nightmarish. Even though the CMB exerts a tiny bit of drag in outer space, it’s not empty space itself that does this deed, nor the luminiferous aether. It’s a bath of objects — photons — that we can escape by ducking inside a metal cage.

Similar issues arise with the cosmic neutrino background, a bath of neutrinos from the early Big Bang, which humans still have not observed but which is widely believed to exist. On the one hand, it is much harder to block neutrinos from entering an isolated bubble, so that line of argument doesn’t really hold up. But conversely, their effects are also incredibly small, to the point that they still have not been observed. Any drag they could exert is insanely tiny. (See also this discussion of an endnote in Chapter 2.) The in-principle issues are similar to those for the CMB, though with different details.