Reasons to Believe

Fine-Tuning: Responding to a Common Objection

I received an email asking this question:

I have heard another rebuttal to the fine-tuning argument . . . that maybe you can address. The rebuttal goes like this: “Yes, you are right that changing just one constant a little bit would make human life impossible. But there are ways you can change multiple constants at the same time and still have human life arise. So the constants aren’t really as fine-tuned as you say.” Can you respond to that argument?

Virtually all scientists readily acknowledge that tweaking just one of the fundamental constants (e.g., the electromagnetic coupling constant, α), even by small amounts, results in a universe devoid of life. Increasing α causes greater repulsion between like charges, disrupting the stability of elements with more than a few protons. However, decreasing α would result in weaker forces between atoms, negatively affecting the chemical bonding that life requires. Some of these negative consequences are avoided by changing other fundamental constants. Where larger α affects the stability of heavier elements, increasing the strength of the strong nuclear force restores this stability.

When scientists investigate how the universe would change as these two quantities vary, they still find evidence for fine-tuning. Only a small window allows for a universe with sufficient carbon and oxygen, as well as hydrogen, for life to exist. But what if a bunch of parameters are adjusted?

The graphics below illustrate the two commonly considered options. The top image shows that we live on a island in the vast ocean of possible configurations of the constants of physics. Change any one too much and life ends up drowning in the ocean. The key question is, what happens when we zoom out to see the rest of the ocean?

island1

Zooming out, do we get the first photo or the second?

island2    island3

Is our island alone and isolated, or do many islands capable of sustaining life exist?

Maybe Other Islands Remove the Fine-Tuning

Well, one team of physicists asked just that question to see if a universe without a weak nuclear force could support all the life-essential processes. They simulated universes with no weak interaction and adjusted the other parameters (both fundamental—like gravity, electromagnetic, and strong nuclear interactions—as well as cosmological—like baryon number and dark matter density) to try and obtain a universe that behaves similarly to ours. Their research demonstrated that it was possible to find another configuration of parameters that yielded a universe that appeared to be capable of supporting life.1 (Another team disputes that the alternate universe would produce enough oxygen though.)2

In order to find that island, the scientists had to carefully tune a whole bunch of parameters. While the success of this endeavor indicates that other islands might exist, it also shows that the islands are small and have sharp cliffs.

. . . Or Maybe They Don’t

The team also tried a similar analysis focusing on the cosmological constant (or dark energy). The discovery of dark energy in the late 1990s shocked cosmologists because the amount of this bizarre “stuff” was far less than expected. Instead of a value near the Planck scale (as expected from the best model of the physical laws available), the actual value is roughly 120 orders of magnitude smaller! The team tried to find universes capable of supporting life where the dark energy had values near the Planck scale. They found that no adjustment of the other parameters gave a universe that produced stars that burned for billions of years, synthesized elements up to iron, and underwent supernova explosions necessary for distributing the elements for planet formation.

No amount of changing the fundamental constants gives an island where the dark energy has the value predicted by our best scientific theories! In other words, all the livable universes had a fine-tuned value for the dark energy.

Ultimately, the fine-tuning argument doesn’t rest on the premise that our universe is the only possible configuration. It only requires that of all the possible configurations, only a small fraction meets all the necessary conditions for life. Research into finding other potentially livable universes confirms that our universe belongs to a relatively small group (that may contain only one member)—even if a multiverse exists.

Endnotes

  1. Roni Harnik, Graham D. Kribs, and Gilad Perez, “A Universe without Weak Interactions,” Physical Review D 74 (August 2006): 035006, doi:10.1103/PhysRevD.74.035006.
  2. L. Clavelli and R. E. White III, “Problems in a Weakless Universe,” submitted September 5, 2006, arXiv:hep-ph/0609050.

Subjects: Fine-Tuning, Laws of Physics