Oct 16, 2016

5 min read

A Tale of 2 Photons

Two photons depart the sun, and head toward Earth. Our sun’s surface is an ephemerally light plasma predominantly filled with electrons and light (Hydrogen, Helium) ions. That boundary between the sun the empty space beyond is shaped by radiation pressure and intense gravity, and it is there where the ions bounce about. The relative motion of those charges on the surface is what stimulates the electromagnetic fields on the atomic scale, and coupled with complex quantum physics, gives rise to discrete photons, such as our 2 travelers.

One photon is almost exactly average among its peers, at 5,700 Kelvin temperature. Another photon, leaving at about the same time and in about the same direction is 1 in 1,000 having a high energy at around 11,00 Kelvin. This is because the emissions from the surface of the sun come in a spectrum, and this, in turn, comes from the physics of those ions. The sun’s gas has a distribution of ion speeds just like any gas, and because of that, it doesn’t product just one energy of photon from its surface, but a full range of energies. This range is the range of the onslaught of photonic radiation that Earth experiences.

The two photons enter Earth’s exosphere. For the high energy photon, this journey ends there. It collides with Hydrogen ion within the Earth’s outer atmosphere, ending its life as a high-energy photon wandering in space. Its energy, however, still persists, being absorbed by the Hydrogen atom. Next, something incredible happens. On most other similar planets (Venus, Mars), there would be a good chance this Hydrogen ion scatters off into space, departing from the planet, probably for good. That does not happen here. Instead, the ion obtains roughly 11 km/s of speed, and rockets away from the planet. However, it falls short. Its energy is not great enough to liberate it from the planet and it becomes trapped in its magnetic field.

Our more average photon continues. It continues through ever-thickening atmosphere, increasingly defying the odds as it goes. As the atmosphere gets thicker, more of its siblings are downgraded to lower energies on scattering events, but a substantial cohort remains. Many strike clouds, or oceans, but not this one. This otherwise ordinary photon loses its energy in a very rare and special interaction with planetary regolith matter — photosynthesis. From there, its energy is recycled in some very strange ways, although a large fraction of it is still lost to inefficiencies. A fraction (even though tiny) goes to build molecules, some more complex than what was there before. It is even transported to different parts of the organism.

Now, you’ll have to suspend an even further degree of disbelief as we continue to follow the disembodied energy of our late photon friend. The appendage of the plant that it was a part of gets snatched up by an even stranger process. It is harvested by human machinery. The material is dried and reconstituted into a pellet form, bid on and traded in complex financial markets, transported great distances on rail cars, and then burned in a large size industrial furnace. The journey ends for these molecules, but the strange story of its energy continues.

After this point, the tiny bit of energy undergoes several energy transitions. It turns into steam, then it is accelerated by one of many nozzles in an array in a stage in a steam turbine of a power plant. Through this process, it converts a fraction of its energy into kinetic, and then imparts it to a moving blade. That blade is a part of a large rotating mechanical apparatus that pushes on electromagnetic fields. These fields then push electrons through a 19 kV difference, and then are pushed and herded through even more fields in an even more complex process, and become a complete commodity.

In the end, that electrical energy is used by a chemical plant, breaking chemical bonds. Remember, the photosynthetic absorption used the photon’s energy to build a bond in the first place. This is a very important detail as we shall see. The peculiar aspect here is that water is broken into 2 components, Oxygen and Hydrogen, and they undergo some cryogenic sub-cooling and get stored in tanks. For some time, this material waits there, the disembodied energy still there until it is loaded onto a vessel.

The rocket ship fills up with fuel in 2 tanks, then waits for checks to finish and the countdown to commence. Engines roar to life, and the energy that went through such a complex process to build up is released at a tremendous rate in a combustion chamber, changing to heat and pressure, and then some amount (again, just a fraction of it) gets converted to raw tangible kinetic energy that goes opposite of the rocket, propelling the rocket into space.

The vast majority of the kinetic energy is spent early on in its journey, but we will suspend disbelief even further, and imagine that this particular unit of disembodied energy remains stored, waiting, until the Earth departure stage is firing in Low Earth Orbit, helping to push its payload out of Earth’s gravity well, and into the great empty space beyond. At that point, being in orbit, there is very little resistance to movement. Only occasional molecules are bouncing around space and can hit the craft.

One of those few particles the craft hits is the ion that the partner, high-energy, photon previously collided with. After billions of years of such ions mostly failing to escape the well of Earth’s influence, this fragile machine, powered by a massive pyramid of chemical reactions, passes by it.

I share this story because it tells of a deep irony that is central to our place in the universe.

The simple reality is that we are made of chemical bonds. I know this is a gross exaggeration, but group together all covalent bonds, and pretend that they all have just about the same energy. Now, we see that we have to satisfy several conflicting requirements if we want to imagine that we both exist, and can make it into space (two things which are already obviously true)

  1. Chemical bonds must be week enough that 2x its energy is not strong enough to get an atom out of our gravity well
  2. Photons from the sun must roughly match the strength of a chemical bond in order to power the complex chemistry of advanced life
  3. Chemical bonds must be used in rocket engines to lift us off-world

These are very fundamental realities, and they show that the engineering challenges of poor mass-ratios in space exploration are entirely fundamental. If we evolved on a different planet, this tension must necessarily exist there.

If you think of Earth as an incubator, you must imagine that needs walls of a certain height to keep the offspring in. But now, the same walls that are strong enough to hold chemical-scale energies in are the same walls that we have to use chemical-scale energies to surmount.

This is The Big First Step. Beyond that, and before that, nothing necessarily needs to have such poor mass economies. This is why that transition is such a fundamental one to society.

Meta: I just recently made a post about a book idea, Capital in Space. This is written as a vague and quick attempt at something like a foreword. Maybe, maybe not, it’s just something I’m throwing out there for now.

Obligatory analytical writing, online participation account for Medium. Engineering, software, books, space, constant daydreaming.

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