There exist
numerous cases of what look like very strong fine-tuning
in our universe. Both fundamental constants and natural laws are
arranged in a way that allows for us to exist. It seems that the
probability of all of these favorable conditions existing by chance
is incredibly low. It has been argued that the probability of you
existing in a universe as fine-tuned as ours is like the chance of
you surviving a firing squad (having 10 or more soldiers firing their
rifles at you at close range). If you survived a firing squad, it is
argued, you should assume there was some purpose involved in this,
and that it wasn't just a lucky accident.
But
physicist Sabine Hossenfelder disagrees. She has a recent post in
which she attempts to debunk what she calls “the myth that
our universe is 'finetuned for life.'” Her attempt, however, is a
complete failure.
She starts out by giving a
very general armchair argument:
The general argument
against the success of anthropic selection is that all evidence for
the finetuning of our theories explores only a tiny space of all
possible combinations of parameters. A typical argument for
finetuning goes like this: If parameter X was only a tiny bit larger
or smaller than the observed value, then atoms couldn’t exist or
all stars would collapse or something similarly detrimental to the
formation of large molecules. Hence, parameter X must have a certain
value to high precision. However, these arguments for finetuning –
of which there exist many – don’t take into account simultaneous
changes in several parameters and are therefore inconclusive.
This is not
actually correct, as quite a few scientific papers about cosmic
fine-tuning and the biological sensitivity of various fundamental
constants do actually take into account the effects of simultaneous
changes of more than one parameter. An example is the diagram below from a recent article by
physicist Luke Barnes, in which he allows us to view simultaneous
changes in the strong nuclear force and the fine-structure constant,
showing only a tiny area that is compatible with living creatures
such as ours.
Then
Hossenfelder claims to have found “counterexamples” that weaken
the case for cosmic fine-tuning. But she's shooting blanks – her
counterexamples are all duds.
The
first “counterexample” she cites is a 2006 paper called A
Universe Without Weak Interactions. She
says that this paper describes a “universe that seems
capable of complex chemistry and yet has fundamental particles
entirely different from our own.” The paper actually talks about a
universe without a weak nuclear force, one of the four fundamental
forces of the universe. But the weak nuclear force has never been a
very important part of arguments that the universe is fine-tuned.
There are strong reasons for believing that all of the other three
fundamental forces of nature (the strong nuclear force,
electromagnetism, and gravitation) are very fine-tuned, but no one
has claimed that the weak nuclear force is very fine-tuned.
Moreover, the 2006 paper
she refers to was emphatically refuted by a later paper in 2006, a
paper entitled “Problems in a Weakless Universe.” The paper
concluded the following:
We point out, however,
that on closer examination the proposed "weakless" universe
strongly inhibits the development of life in several different ways.
One of the most critical barriers is that a weakless universe is
unlikely to produce enough oxygen to support life. Since oxygen is an
essential element in both water, the universal solvent needed for
life, and in each of the four bases forming the DNA code for known
living beings, we strongly question the hypothesis that a universe
without weak interactions could generate life.
So Hossenfelder's
first “counterexample” doesn't do anything to undermine the case
for cosmic fine-tuning. Her second “counterexample” is no better.
She cites Abraham Loeb's paper “The Habitable Epoch of the Early
Universe.” In that paper Loeb imagined that in the early universe, the
cosmological constant (or the energy density of space) might have for
a while been sufficient to bathe the whole universe with a warmth
suitable for life. He imagined the following scenario:
- The Big Bang occurs 13 billion years ago.
- After about 400,000 years the universe cools enough for atoms to form.
- About 10 million years later, planets and stars form.
- For a few million years it is warm enough for life to exist, because of the cosmological constant, which fills all of space with a pleasant warmth.
- Microbial life forms appear during this relatively brief period.
- A few million years later, the continued expansion of the universe causes the universe to cool sufficiently so that the cosmological constant is no longer sufficient to keep space warm.
- Any microbial life that may have arisen from the warmth of the cosmological constant then dies, as temperatures fall far below freezing.
But this
scenario is nothing like an alternate way for the universe to be
life-friendly, because this “habitable epoch of the early universe”
lasts way too short a time (as the expansion of the universe quickly
causes those early warm temperatures to fade away, being replaced by
deadly cold). It actually lasts (under Loeb's assumptions, as
discussed here) only about two million years, as Loeb admits by saying that
this epoch would last only “a few Myr,” using an abbreviation for
megayears (a million years). Since there was supposedly billions of
years (thousands of millions of years) between the appearance of
earthly microbes and the appearance of man, it is clear that two
million years is not long enough for any intelligent life to evolve.
It is almost certainly not a sufficient time for any life at all to
develop. So the possibility discussed by Loeb is irrelevant.
When
discussing whether the universe is fine-tuned to allow for
intelligent life, we don't care whether there might have been some
brief two-million year window in the very early universe (a
short-lived period of warmth) that might have allowed mere microbes
to appear before being wiped out when the universe becomes super-cold
again. We care about the possibility of intelligent life appearing.
In fact, it is overwhelmingly likely that the higher radiation and
asteroid concentrations in any early universe would not even allow
microbes to have appeared in the early universe.
Loeb's paper
(discussing something both extremely improbable and irrelevant) did
nothing to upset the idea that the cosmological constant is
fine-tuned. The fact that the cosmological constant is enormously
fine-tuned is reaffirmed by a recent scientific paper noting that the
cosmological constant is 10123 times smaller than its
“natural value,” and that “there is no satisfactory solution
yet for this problem.” The paper's graphs suggest that there would
no observers if the cosmological constant were a few hundred times
smaller or larger. Given the natural value so much larger, having a
cosmological constant within this range is like hitting the exact
center of a target 1000 yards distant.
So
Hossenfelder's second “counterexample” is a dud that does nothing
to undermine the case for cosmic fine-tuning. Hossenfelder's third
“counterexample” is no better. She cites a 2016 paper by Adams
and Grohs discussing the “triple alpha process,” a case in which
nuclear physics has to be just right in order for carbon to be
produced, because of what is called a resonance fine-tuning. The
authors imagine other universes that might not require this
particular type of resonance fine-tuning.
Hossenfelder
claims that this paper is a “demonstration that a chemistry complex
enough to support life can arise under circumstances that are not
anything like the ones we experience today.” That's wrong, because
the paper actually describes universes very much like ours, but in
which there are small changes in fundamental constants. And also,
the paper does not actually describe an alternate universe with a
chemistry complex enough to support life, because it fails to
describe a universe in which oxygen is produced in sufficient
quantities. This is made clear on page 26 of the paper, in which the
authors say, “This
set of simulations
does not include nuclear reactions that produce oxygen, neon, and
heavier elements.”
It
has long been recognized that the fundamental constants have to be
just right for nuclear reactions in stars to produce large quantities
of both
oxygen and carbon, both of which are requirements for life. By
failing to discuss alternate reactions in which stars produce oxygen,
the paper of Adams and Grohs does nothing to undermine that
fine-tuning requirement. The fine-tuning requirement is stated in a
2014 scientific paper which tells us on page 16 that in order for you
to have abundant quantities of oxygen and carbon, you need for the
quark masses to be within 2 to 3 percent of their current values, and
you also need for the fine-structure constant to be within 2.5% of
its current value. You could therefore say nature has to hit two
different “holes in one,” and these aren't the only “holes in
one” nature has to hit in order to end up with intelligent life.
In short, all
of Hossenfelder's “counterexamples” are empty duds. She has done
nothing whatsoever to weaken the case that the universe is
fine-tuned. Far from being a “myth” as she claims, the finding
that the universe is incredibly fine-tuned for life is one of the
fundamental achievements in the past 50 years of physics, and is
something that has been acknowledged by many physicists and
cosmologists.
As all of her
“counterexamples” are failures, Hossenfelder doesn't actually
succeed in providing even a single example of some alternate universe
as life-friendly as ours. Even if she were to provide such a thing,
it would do nothing to discredit the claim that our universe is
fine-tuned for life. Arguments about cosmic fine-tuning never claim
that there is only one possible universe consistent with life, but
merely claim that it is incredibly improbable that any particular
random universe would be compatible with the appearance of
intelligent life. There are many possible universes that would allow
intelligent life to appear, but the set of all possible universes
that would allow intelligent life to appear is almost infinitely
smaller than the set of all possible universes, making it almost
infinitely improbable that any particular random universe would
accidentally meet the many requirements for intelligent life. You do
not damage such reasoning in the least by showing a few other
possible universes that might allow intelligent life to appear.
It's rather
like this. A man may point to a car, point out its fitness for a
purpose, and say, “Wow, that sure is fine-tuned” or “that sure
wasn't produced by some set of accidents.” You do not at all
discredit such reasoning by demonstrating that there are other
possible cars with a very different appearance.
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