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Our future, our universe, and other weighty topics


Tuesday, September 6, 2016

A Physicist Errs on the Limits to Knowledge, Because He Forgets His Physics

Physicist Lawrence Krauss is mainly known for his book A Universe From Nothing. The book was based on a misleading verbal trick. Krauss would repeatedly describe various imaginative speculations about substantive physical states that might have preceded the Big Bang at the beginning of the universe, and he would suggest that such states were “nothing” (even though they were not at all nothing, but instead various types of substantive physical states). He would say that such speculations were descriptions of how the universe could have appeared from nothing. Such reasoning was bogus, because when you describe any type of substantive physical state it is not accurate to use the word “nothing” to describe such a state. The only real nothing is a complete absence of all particles, all fields, all laws, all forces, and every other thing that can possibly be described by a noun.

In a recent Nautilus article, Krauss writes about an equally weighty issue: the issue of whether there are limits to knowledge. Krauss very quickly suggests there are no such limits. He says:

We won’t know if there is a limit to knowledge unless we try to get past it. At the moment, we have no sign of one. We may be facing roadblocks, but those give every indication of being temporary.

As I will show, this statement is completely wrong, partly because of a laws of physics that Krauss has ignored. After making this statement, Krauss suggests that the origin of the universe (the Big Bang) does not provide a limit to human knowledge. Here he is very wrong. The Big Bang described by cosmologists is a state in which the whole universe was packed together in a state of infinite density at the first instant of time. This presents an impenetrable wall, and we will never be able to observe any theoretical state that might have preceded such a state. We will never be able to make observations that stretch back to a time before such a state.

In fact, not only is the Big Bang itself (what cosmologists call Time Zero) unobservable, but actually the first 380,000 years of the universe's history are forever unobservable. The physical reason has to do with what is called the recombination era. Scientists say that in the first 380,000 years of the universe's history, matter and energy were so densely packed that all photons of light coming from the early universe were hopelessly scattered. Imagine you are looking through some crazy telescope that is 50 meters long and has 1000 different lenses at different points in the telescope tube. Each of the 1000 lenses causes the light to scatter in a different way. Of course, such a telescope will not allow you to see anything. Just as such an arrangement would act as an impenetrable optical barrier, the first 380,000 years of the universe's history acts as an impenetrable optical barrier. Each light photon from the Big Bang must have been scattered many times every second, as those particles interacted with other matter and energy particles in the dense early universe.

So Krauss gives us the wrong answer. He suggests that the Big Bang is not an observational barrier, when it is actually almost the most extreme observational barrier imaginable. The Big Bang is like some city protected by a billion-mile steel wall. No matter how big we build our telescopes, we will never be able to look back to a time earlier than 380,000 years after the Big Bang.

In the middle of his article on whether there are limits to knowledge, Krauss makes the very goofy suggestion that the universe “probably has zero total energy.” As it is irrelevant to the matter being discussed, I will save a rebuttal for a later post, merely noting that the idea of a universe with “zero total energy” is pure bunk.

Krauss then goes into a discussion of the theory of cosmic inflation, the idea that the universe underwent an exponential phase of expansion during a fraction of its first second. He refers to “phenomena like inflation,” giving his readers the inaccurate impression that cosmic inflation (exponential expansion of the universe) is something that has been observed (for we should only use the term “phenomenon” in regard to something that has been observed). To the contrary, cosmic inflation (exponential expansion of the universe) is a completely unproven speculation that has never been observed. The expansion of the universe we observe in our telescopes is not exponential, but a much slower type of expansion called linear expansion.

Krauss attempts, pretty ridiculously, to use this cosmic inflation theory as a springboard to the idea that we might be able to get some evidence of another universe. His reasoning is kind of like this: (1) we might be able to observe gravitational waves from what are called primordial b-modes; (2) if we found that, it would be evidence that the cosmic inflation theory is true; (3) one of the many variants of the cosmic inflation theory is an “eternal inflation” theory postulating the existence of other bubble universes; (4) so if we ever see these gravitational waves from primordial b-modes, we will have got some observation perhaps suggesting some other universe.

This reasoning is very bad, because the observation of primordial b-modes would merely tell us about the state of our universe at a time 380,000 years after the Big Bang. They would not prove that the cosmic inflation theory (a theory about what happened in the universe's first second) is true, and they would certainly not prove that a particular variation of the cosmic inflation theory (called eternal inflation) is true. And even if we were ever to see some sign that there was some other “bubble universe” next to ours (something that is very probably impossible, even in theory), that might be a feather in the cap of some cosmologists, but would not at all be anything that would justify the very inaccurate claim Krauss eventually reaches in his essay: "No limits to what we may learn from the application of reason combined with experimental observation are yet known.” To the contrary, if we were to learn there are other, unobservable universes, that would be just a reminder of how limited are the things we can observe.

The “no limits” claim Krauss makes is dead wrong. Meandering around in multiverse speculations, the physicist Krauss failed to consider what he should have considered before making such a claim: the laws of the known universe. One such law is Heisenberg's uncertainty principle, which says we can never even in theory do so simple a thing such as measuring both the momentum and position of any subatomic particle. This places severe limits on how much we can learn on the subatomic level. There is also another law of our universe that imposes the most severe and drastic limitations on what we may learn about our universe.

The law I refer to is the law that nothing can travel faster than the speed of light. This law is part of Einstein's theory of special relativity. A consequence of the law is that we absolutely cannot learn about the current state of anything more than the tiniest fraction of the physical universe. The observable universe is larger than 10 billion light years. When astronomers report observations at a distance of 10 billion light years, they are not seeing the current state of the universe, but the way things were 10 billion years ago.

The law that nothing can exceed the speed of light is one that tells us: you can never observe the current state of any part of the universe except the part very close to you. It works like this: we cannot observe the past thousand years of any astronomical object except in an area within a radius of 1000 light-years (a small fraction of our galaxy); we cannot observe the past million years of any astronomical object except in an area within a radius of a million light-years (still only a tiny fraction of the observable universe); and we cannot observe the past billion years of any astronomical object except in an area within a radius of a billion light-years (still less than a hundredth of the observable universe).

That is an absolutely gigantic limit on what we may know. There is therefore no truth in this claim of Krauss: “No limits to what we may learn from the application of reason combined with experimental observation are yet known.” We absolutely do know of such a limit, and it is a gigantic one. Among the consequences of the speed of light limit are the following: (1) if we are alone in the universe or galaxy, we can never find out such a thing; (2) we can never know what is the most advanced species in the universe; (3) we can never know how common intelligent life is in the universe as a whole, and will always be uncertain about such a thing by a factor of at least 100.

Consider, for example, a fleet of spaceships trying to explore the universe, traveling at the speed of light. Whatever conclusions such a fleet of ships reached after a million years would only be an exploration of less than a millionth of the universe; and by the time such a survey was finished, the ships would no longer know the current state of the stars they had explored in the past million years. 

 

We may observe some distant galaxy 100 million light years away, but we are seeing it the way it was 100 million years ago. That galaxy may now look totally different because of events that occurred in the past 100 million years– it could have been completely transformed by a technological takeover by some galactic supercivilization, and may now look totally different. The current state of such a galaxy is forever unknowable to us.

There are many other additional limits on what we can learn. One limit is created by what we may call the Law of Transience. It is not a formal law of physics, but it was eloquently described by the science fiction writer Brian Aldiss in his novel Galaxies Like Grains of Sand:

Of the laws we can deduce from the external world, one stands above all: the Law of Transience. Nothing is intended to last. The trees fall year by year, the mountains tumble, the galaxies burn out like tall tallow candles. Nothing is intended to last — except time.

A corollary of this Law of Transience is that over millions of years, previous physical states are destroyed by natural activity and natural changes that cause turnover in which previous states of matter are lost. In many ways, this imposes a limit on what we can learn. We cannot learn exactly how life first originated in our galaxy or our planet, or what was the first planet in the universe to first develop life, or exactly what our galaxy looked like 10 billion years ago, or exactly what the first star looked like. We cannot learn many other things relating to the very distant past. Trying to discover such things is like trying to find out what was written in the wet sand at the seashore by the ancient Romans. Natural transience imposes the most severe limits on what we can learn about the distant past.

There are also vast limits on human knowledge imposed by the very limited size of our own intellects. Our minds are a poor match for the complexities of nature, as shown when physicists sometimes say no one really understands quantum mechanics. 

Then there is the limit imposed by the pure size and immensity of the universe. Galaxies billions of light-years distant appear as such dim blurry blotches in our telescopes that we will never be able to have a very detailed knowledge of even their contents billions of years ago. Even if we were to somehow magically get a machine capable of viewing the current contents of any planet in the universe (in defiance of the speed of light limit), there would be so many planets to look at (more than 1,000,000,000,000,000,000,000) that we would never be able to analyze the endless oceans of data.

So I have discussed some obvious and gigantic limits on what we may learn: limits about what we can learn about the universe's beginning, limits on what we can learn about the subatomic world, the limit that we can only learn about the current state of a little part of the universe near us, the limit that much of the distant past's secrets can never be uncovered, and the limits imposed by the universe's size and immensity. So the claim of Krauss that "no limits to what we may learn from the application of reason combined with experimental observation are yet known” is very far from being true.