People say that no one has all the answers. In fact, no one can have all the answers, even if they have some place to store all that information. Some things are simply unknowable. This isn’t just my opinion – this is according to accepted theory. Strangely, some unknowable information occurs in physics, which is contrary to the idea that we can learn as much as we want about something, as long as we have adequate technology.
It may be no big surprise about the unknowable existing at the quantum scale, the scale of subatomic particles. Subatomic particles are of course smaller than atoms, at least for the sake of argument here. Experience tells us that manipulating something very small can be difficult, like with a splinter in your finger or a piece of lead from a mechanical pencil. You might need to resort to tweezers or a piece of adhesive tape. Since we already have that much trouble with small objects we can see, it’s reasonable to assume it gets trickier for smaller objects we can’t see.
Dealing with unimaginably small items, quantum theory of course says many strange things, like there is a minimum amount of uncertainty about the location and momentum of a subatomic particle.[1] Location is easy enough to understand, and momentum is basically a measure of motion that takes mass into consideration. A simple way of calculating the momentum of everyday visible objects is to multiply the object’s velocity times its mass. For example, when traveling at the same speed, heavier objects have more momentum than others. At the same mass, faster objects have more momentum. According to quantum theory, the more you know about a subatomic particle’s location, the less you can now about its momentum and vice versa. You can’t observe both with high precision at the same time.
This uncertainty relation between location and momentum is known as the Heisenberg uncertainty principle. The math for this principle can be adjusted to relate other quantities like elapsed time and energy, but the uncertainty is still there. To some extent, it’s understandable that something on the scale of light waves is going to be disturbed when you try to observe it with light. We might reasonably expect to overcome this limitation with better technology. However, quantum theory says these two values are mathematically related in such a way that they cannot both be “localized” (observed in great detail) at the same time. Furthermore, this observation limitation doesn’t just apply to us, but to any physical interactions. So that higher precision isn’t just hidden from us humans and our clumsy instruments, but from the rest of the universe as well. Nothing can measure both values at the same time with high precision. Some things in the quantum realm are apparently unknowable, and it’s not our fault.
Instead of very small scales, what if we looked at very large scales, say maybe billions of light years? A light year is the distance light can travel in one Earth year. Surely quantum theory can’t impose its uncertainty limitation at such a large scale, can it? Not that tiny distances in nanometers would mean much when we’re talking about light years. To see how different these scales are, I highly recommend the interactive Scale of the Universe web site.[2] Its visual way of comparing the size of one thing to another is very enlightening (more like mindblowing). This web site requires a Flash plug-in for your web browser, which is well worth the minor inconvenience.
A distance of billions of light years can be said to be on a cosmological scale. It would be an understatement to say the universe – the cosmos – is a very big place. Strangely, it seems to be getting even bigger. In the early part of the twentieth century, astronomers started finding evidence that most other galaxies are moving away from our own galaxy.[3] Georges Lemaitre and Edwin Hubble concluded that the rate at which galaxies move away from us is based on their distance from us. Galaxies that are farther away are fleeing from us at a higher speed. Why?
That relation between the distance to a galaxy and its velocity away from us can be described by a number called the Hubble constant. Even though there are different calculated estimates for that constant, there seems to be solid agreement that the universe is expanding. Scientists say the “metric” of space is expanding, which might be interpreted as saying space itself is getting bigger. The more space (distance) between us and a galaxy, the more accumulated expansion there is in between, which is what causes galaxies that are farther to move away from us at a faster speed.
So cosmic expansion is moving galaxies away from us and moving the more remote galaxies away from us at higher speeds. Presumably we would eventually develop space travel technology that can transport us such long distances. What’s the catch? When summing up this expansion-based “motion” between us and very remote galaxies, some of those galaxies are moving away faster than the speed of light. This should signal multiple mental alarms. The first is the speed issue. You may know that Einstein’s special theory of relativity says that nothing can move faster than the speed of light in a vacuum.[4] How can galaxies be moving away from us faster than light?
If the inability to surpass the speed of light is interpreted as a physical law, then cosmic expansion has a loophole or two for getting around that law. While the various technical details are somewhat out of scope, here are a few examples. The special relativity loophole says that special relativity and its speed limit only apply to inertial reference frames – situations without significant gravity that involve only constant-speed motion in a straight line (including zero speed).[3] However, our situation of changing space and receding galaxies instead falls under general relativity, which covers gravity, acceleration, and other conditions seen in a more realistic universe.
Similarly, the locality loophole says that special relativity applies to local conditions and not at the scale of galaxies.[5] From my non-expert point of view, maybe this is just the same loophole as before. But even locally, high relative speeds don’t have to be an issue. There’s nothing to prevent two sufficiently advanced spaceships from moving away from each other in a straight line, each moving at 60% of the speed of light relative to some stationary observer in between. Light from either spaceship would eventually reach the observer, and light from the observer would eventually catch up with either space ship. Neither spaceship is going the speed of light relative to the observer. What happens if the observer isn’t there? According to special relativity, the two spaceships also are not going faster than light relative to each other either, in spite of their speeds apparently adding up to 120% of the speed of light.[6]
Loopholes considered, cosmic expansion surely can move galaxies away from us at speeds faster than light, and we somehow have seen light from some of those galaxies. That makes it seem not so bad, but it gets worse. As space continues to expand, the rate at which a galaxy moves away from us continues to increase as well, since that also increases the amount of expanding space between us and that galaxy. As a result, galaxies are effectively accelerating away from us.
Galaxies moving faster and faster away from us brings up the other mental alarm that should be going off. How fast is too fast? The recession speed and acceleration of the most distant galaxies prevent light generated by them today from ever reaching us. The distance at which this happens is called the cosmic even horizon, and it’s currently about 16 billion light years.[7] We will (probably) never know the future of such galaxies, and many will never be known to us to begin with. The unknowable truly exists at the cosmic scale. Yes, there are places and events in the universe we might not ever learn about, and it’s not because of any limitation of technology or humans.
From the tiny scale of subatomic particles to the scale of the universe itself, there is information that is physically unknowable. The universe is keeping secrets from us, and not even the Freedom of Information Act can get them released for us.[8] Of course, this unknowable information is only unknowable according to theory. Maybe we can find a way to retrieve it. How can we not try?
References
[1] https://en.wikipedia.org/wiki/Uncertainty_principle
[2] https://scaleofuniverse.com/
[3] https://en.wikipedia.org/wiki/Expansion_of_the_universe
[4] http://scienceworld.wolfram.com/physics/SpecialRelativity.html
[5] https://www.space.com/33306-how-does-the-universe-expand-faster-than-light.html
[6] https://www.physicsforums.com/threads/two-spaceships-in-opposite-direction-at-near-c.655083/
[7] https://en.wikipedia.org/wiki/Cosmological_horizon
(c) Copyright 2019 by Mike Ferrell
Odd STEM 
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