Will We Reach the Stars? A Physicist’s Analysis (Part II)


In my previous post, I presented some simple calculations showing how much energy it would take to send a space shuttle to the nearest star, Proxima Centauri, in ten years. It turned out that we would need the amount of energy that the world’s largest power plant produces in 820 years (if we could run it for that long). This led me to conclude tentatively that we’re not likely ever to reach other star systems; but I promised that in my next post I would examine potential breakthroughs in science and technology that may one day make interstellar travel possible. So here we go.

Assuming we manage to keep from destroying ourselves in a nuclear war, humanity will certainly accomplish some impressive breakthroughs in science and technology in the future. For example, we can expect significant advances in the miniaturization of electronic and optical devices. And, as has long been predicted, we will almost certainly see the successful integration of biological systems (e.g., the human brain) with artificial systems (e.g., computers). At first glance, these advancements seem unrelated to interstellar travel, but I think it will turn out that, if interstellar travel is at all possible, these things will play an integral role.

In light of my previous post, however, the developments that seem most relevant to space travel will be those related to energy production, including the development of novel energy sources and improved efficiency of existing technology. The question at the heart of our discussion here, then, is this: Will these advancements be enough?

The way I see it, one of three possible developments will be necessary in order to make interstellar travel possible. Let’s consider each one in turn.

The first possibility is simply to find a better source of energy. Thus far, the vast majority of our energy has come from burning hydrocarbons. The burning of fuel, whether it be gasoline or the solid rocket propellant used by the space shuttle, is simply a chemical reaction. Energy is released because the atoms and molecules start out bonded together in one configuration and end up in a different, lower-energy, configuration. The energy difference between the two configurations is the amount of energy that we get out of the reaction and can use to power our devices.

Other means of production include harvesting the energy of mechanical motion, such as the motion of air (wind energy) or water (hydroelectric power), or collecting sunlight. These are all great sources of energy, but the fact is that you have to have a huge number of collection devices spread all over the place in order to get an appreciable amount of energy. That won’t help us with space travel unless we can store all of that energy in a compact battery that can fit on our space shuttle. And once again, the energy stored in a battery is chemical in nature and has a limited density.

What we need is something with a high energy density — a lot of energy packed into a small amount of volume and mass.

Modern physics places a limit on this. The total amount of energy contained in a given amount of mass — and hence the absolute maximum amount of energy that can be extracted from said mass — is given by Einstein’s famous equation, E=mc^2. This equation governs how much energy is produced in nuclear power plants.

Nuclear reactors work by converting a tiny fraction of the fuel’s mass into energy. (In fact, the same is true of chemical fuels as well, but the change in mass is so tiny that nobody ever talks about it.) However, since only a tiny fraction of the mass is converted to energy, nuclear reactors are not very efficient. We need something even better than conventional nuclear power.

According to modern physics, the absolute best that we could ever hope to achieve would be to convert all of a fuel’s mass into energy. The best way to do this is to combine matter and anti-matter so that all of the mass is annihilated, leaving nothing but energy. Producing the amount of energy that we need for our journey to Proxima Centauri would require the annihilation of about 6,500 kilograms of mass, half of which would have to be anti-matter.

So why don’t we do that?

Well, the problem is where to get the antimatter. Producing or even harvesting the antimatter in the first place would take a tremendous amount of energy. So that really puts us in a catch-22: We need energy to get energy.

Thus, barring some absolutely revolutionary breakthrough in our understanding of the nature of matter and energy, it appears as though nature has put an upper limit on how much energy we can extract from a given amount of material. And even if we’re able to reach that absolute limit, we’ll find ourselves hard-pressed to use that energy to send a ship to another star. I therefore conclude that our first option — finding a better source of energy — is not very promising.

Let’s look at the second possibility, then.

The second advancement that might enable interstellar travel would the development of the ability to bend space-time somehow — i.e., create a wormhole or something similar. We’ve all seen this in science fiction movies, and if you’ve read any popular literature about general relativity, then you have some conceptual idea about how wormholes work in principle. The problem here is that even if it is possible to create a wormhole, doing so would probably require more energy than simply sending a ship the required distance.

That doesn’t mean we won’t ever be able to do it. I can imagine, for instance, setting up a huge power plant — perhaps a space station that orbits a star and directly harvests nuclear power from it — dedicated to opening and closing wormholes. It would serve as a sort of interstellar space port that builds up and stores energy and then releases it in huge amounts on occasions for which the creation of a wormhole is desired.

But that is probably something we’ll only be able to do after we already manage to travel to other stars. So let’s keep thinking.

The heart of our problem thus far is finding the means and the energy to transport a certain amount of mass (i.e., our bodies) over a great distance. My third proposal represents not a solution to this problem but a reformulation of the problem: What if, instead of transporting our bodies across space, we first converted ourselves into something much lighter? Then a much smaller amount of energy would be required for the transport.

By our current understanding of reality, we are composed, at the most fundamental level, of information. In principle, you or I could be converted into pure information, which could then be encoded in a beam of light. This would be helpful because light has no mass at all and travels at the maximum possible speed (the speed of light). And according to relativity, if you were converted into light and traveled the 4.24 light-year distance to Proxima Centauri, no time at all would pass for you, while exactly 4.24 years would pass on earth.

There is one problem with this, though. There’s no device on Proxima Centauri that can receive the signal in which you are encoded and convert you from light back into a more preferable form. When you hit Proxima Centauri, your photons will be absorbed by the matter in the star and disappear forever, which is the same outcome that you would get if you just plunged into the star at 39% of the speed of light while riding in a shuttle!


Although I think this is the most exciting possibility, it once again requires that we first find some way to transport mass across distances from one star to another. And so here I come back to my earlier mention of miniaturization and bionics: specifically, the miniaturization of optoelectronics and the development of brain-computer interfaces.

Rather than trying to send people at first, we could begin by sending robots (i.e., computers) to another star as pioneers. This way, we could take advantage of miniaturization of technology to make these robots so tiny and lightweight that a relatively small amount of energy would be needed. (And they wouldn’t need food and water for the journey, either.) Once there, these robots could build the hardware necessary to receive future signals sent from the earth. Then we could begin sending people (and, in principle, anything else) encoded in beams of light. Hence, we could truly realize interstellar travel by means of teleportation.

There is one obvious and very basic philosophical problem here: If you are physically disintegrated at one location and reintegrated at another, is the new you still you? Or did you die, and is the new you just a copy that other people won’t be able to distinguish from you? Or, if you are merely copied without disintegrating the original you, what’s the difference between the new you and the old you?

It’s a disturbing question. My own graduate quantum mechanics professor commented on quantum teleportation by saying that if the technology ever reaches the level at which humans can be teleported, he would never volunteer for it because he couldn’t be sure that what came out of the other end would really be him.


Well, at least it’s a cool idea. And we might be able to watch other people be teleported. (There are, after all, people signing up for the Mars One suicide mission.)

In the end, maybe it’s just that I’m a pessimist, but if I had to make a bet, I’d say we’re much more likely either to blow ourselves up with nuclear weapons or to permanently strand ourselves on this rock by exhausting all of our energy supplies than to make it to another star system. So I have to conclude that in all likelihood, we’re never going to make it to another star system.

I do hope someone will prove me wrong, though.

One thought on “Will We Reach the Stars? A Physicist’s Analysis (Part II)

  1. Pingback: Will We Reach the Stars? A Physicist’s Analysis (Part I) | Mycroft's Journal

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