While sitting in my hot tub this morning watching the sun brightening the dawn hours I seem to have slipped a bit down that old rabbit hole. I don’t think there is any value in what I found down that little hole, but I am going to write about it just so I don’t totally forget the thoughts.

I was contemplating the fact that our eyes are sensitive enough to register a single photon of light, which I think is is pretty amazing. I noticed that as the sky brightened, the stars all disappeared except for a couple of exceptionally bright ones that I could spot, but then lose again if I moved my eye. After a few minutes those disappeared too, sort of. The “sort of” part was that I could still catch glimpses of stars far from the center of my field of view, off to the sides. When I shifted my eyes to ‘look’ at them, I could no longer see them. This because of the different sensitivities between the rods and cones in the eye. Rods do good with dim black and white, cones are better at brighter colors.

Among these observations I found the rabbit hole. I drifted off into wondering what a single photon is and realized that it isn’t a particle of any sort, rather it is a single wave. Of course then the next question was “what is waving?” And the obvious answer is that the universe is waving. The universe is a four dimensional (or more) shape in what we call space-time (ST). The space time is filled with fields, at least gravity and electromagnetic fields, very likely many more. The single photon is a wave in the electromagnetic field. Simple, no problem with this.

However, Einstein postulated that the speed of light is a constant from every point of view. That is a really odd thing, if we are moving within the universe, moving relative to the ST axis, then if a photon is a wave on the field its speed should vary depending upon the motion of the observer. This is the “mind twist” the sits in the middle of general relativity. The rat hole that I fell into this morning is that means that nothing (no thing) moves relative to ST. If we (the observers here in our hot tub in the back yard) aren’t moving relative to the fields that are somehow “pinned” to ST, then of course the speed of light is a constant for everything because nothing is moving relative to any other “things” from the vantage point of ST. It is all stationary – sort of.

This “sort of” is slightly different because ST is expanding, it is getting bigger but not in the sense of getting bigger dimensionally, it is getting bigger timely (actually space-timely). In 1931 an astronomer named Edmin Hubble proposed that the frequency (and wavelength) of light changes as it travels through space. The further it travels, the longer the wavelength becomes – providing a nice measure of distance if you happen to know the wavelength of light at its source. Handily, we have a few standards that might fit this criteria (assuming things work the same everywhere). The hydrogen alpha line is an example of one. We know the frequency/wavelength up close, and we assume that it would be the same “up close” at a distant location. Because there is so much hydrogen in the universe it is bright and readily identifiable. What astronomers noticed was that this easily identifiable feature shows up at different wavelengths depending upon how far the source is from us. The further the source is, the longer (redder) the wavelength is – giving a nice measuring stick for distance.

My rabbit hole adventure revolved around the realization that the reason that the wavelength gets longer is that the wave has been waving for a longer time, and that while the wave was racing across the electromagnetic field that is “pinned” to ST space got bigger and therefore the wavelet got longer. It is because the universe had been expanding everywhere while the wave was making its “speed of light” traverse to our eye (or telescope).

That seems obvious, and perhaps was way back when I was struggling through my physics major – but this morning it suddenly because intuitively obvious, not just logically obvious. But that still leaves a little problem of nothing (no thing) moving with respect to ST (and therefore the electromagnetic field pinned to the universal ST). Light doesn’t “travel” – there is no thing traveling. Light waves across (or through) the field that is stationary in ST. Since we, and everything else, is stationary (in some dimension other than our normal 3-dimensional point of view) the speed of that wavelet is always the same for everything.

All of this seems to make sense – but it leaves a little nagging question behind; “How can everything be stationary when things clearly move about?” From what point of view, in what kind of multidimensional universe, can it appear that things are moving when in the reality they are not? What is moving if it is not the “things”? While this sounds like a crazy question (and perhaps it is), it doesn’t sound any crazier than to say that the speed of light is a constant that does not change due to the relative motions of the source and receiver. THAT makes zero sense to me in exactly the same way that the idea that no things move relative to the universe and hence relative to each others.

So there is my rabbit hole. The speed of light being constant and all of the weird things described in Special and General Relativity make perfect sense if the universe is stationary but expanding.