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Sunday, 5 November 2017

Yes, We Were There

Yes, We Were There

 Antoine Bret

Many believe that to inform us about the past, science depends on an assumption of uniformitarianism: that the laws of nature we know today were the same in the past. Creationist literature often argues that faith in this “stability principle” is misplaced. For example, the starlight argument, observing that light arriving from stars farther than 6 000 lightyears must have been created more than 6 000 years ago, is attacked this way. Creationists will reason that the argument is only sound if light has always been traveling at its current speed. But if light traveled faster in the past, objects farther than 6 000 light-years away could have been created only 6 000 years ago and yet still be able to send us light. Barry Setterfield became famous in creationist circles in 1981 by “scientifically” exploring the idea (Setterfield 1981).

Radioactive dating methods used to determine the age of Earth, or of the universe, are attacked from the same angle. The uranium–lead dating technique, for example, is instrumental in dating our planet. It relies on the stability of the decay rates involved in the uranium–lead decay chain. How can we be sure these rates have been the same in the past? Can we observe the past? Doubts in clearing up these issues lead to Ken Ham’s rhetorical question “Were you there?”

SN1987A. By ESA/Hubble & NASA - http://www.spacetelescope.org/images/potw1142a/, CC BY 3.0, https://commons.wikimedia.org/w/index.php?curid=17014663

Far, far away

While none of us have blown out 4.6 billion birthday candles, it turns out that we can directly observe the past and check how nature was behaving back then. To illustrate this point, I will first provide some examples of remote astronomical objects whose distance have been measured by the purely geometric parallax method. (Note that by “remote” I mean farther than 6 000 light-years. In the creationist world, such stars are remote. For modern astronomy, they are our neighborhood.) I will then explain how observation shows that the laws of electromagnetism and quantum mechanics were the same at those objects, when the light we observe now was emitted. Finally, I will demonstrate how other observations show that the laws of nuclear physics have not changed since the universe’s birth.

Let’s start with remote objects. The most convincing distance measurement technique is the parallax method. In the simplest terms, this method involves staring at an object and measuring how much you have to turn your head as you move sideways. It is just geometry. If you observe an astronomical object on January 1 and again on July 1, the “side-step” is equal to the Earth’s orbital diameter around the Sun, about 300 million kilometers. The angle through which you had to turn your “head” (telescope) to track the object is correlated with its distance from Earth. For very distant stars, this angle is so small that it cannot be measured. What, then, is the farthest object whose distance has been measured by this method? As of 2012, it’s the star forming region SfR G75.30+1.32, more than 30 000 light years away (Sanna 2012). Also as of that date, at least twenty-eight objects farther away than 6 000 light-years had been measured by the parallax method (Bret 2014). In December 2013, the Gaia spacecraft was launched. It is currently measuring parallaxes for billions of stars up to 30 000 light-years away, so there will soon be far more data available.

So without doubt, billions of objects exist in the universe at distances greater than 6 000 light-years from Earth. If light has always travelled at the same speed, they must have come into existence more than 6 000 years ago. But how do we know whether the speed of light has changed?

The speed of light hasn’t changed lately

An interesting bench experiment can help us to prove it. When a glass tube filled with hydrogen is heated, a specific set of wavelengths is emitted. The set of wavelengths that can be detected is called the hydrogen spectrum, and it is much like a bar code for hydrogen. Other elements have bar codes, or spectra, too, such as helium, carbon, oxygen, and nitrogen. In fact, every element has its own unique spectrum. So do molecules like water and carbon dioxide. These spectra can be computed from the laws of electromagnetism and quantum mechanics. And the speed of light is a parameter of these laws.

When we look at the Sun, we detect the spectra for elements such as hydrogen, helium, carbon, and calcium. This is how we know what the Sun is made of. This is also how we can be sure the laws of electromagnetism and quantum mechanics are the same on the Sun as here on Earth.

Now, when we observe all those objects farther than 6000 light years from Earth, we spot the very same spectra we see in our labs under controlled conditions. Hydrogen, helium, oxygen, carbon, nitrogen, calcium, nickel, iron, cobalt, water, ammonia, methanol … the list of detected elements and molecules is endless. Indeed, every atom or molecule we know on Earth has been detected in astronomical observations. All the astronomically observed spectra are the same as they are on Earth (ignoring some Doppler shift which may result from any relative motion between the source and Earth; for the distances at stake here, the redshift resulting for the expansion of the universe is completely negligible).

So the laws of the universe we observe 30 000 light years away are exactly the same we observe at home. This means the speed of light and the laws of electromagnetism and quantum mechanics have been the same for at least the last 30 000 years. There is no way around this: a star located more than 6 000 light years away came into existence more than 6 000 years ago. It is not an assumption. It is a conclusion based on empirical evidence. Of course, one can still argue light was created in transit. This is completely irrefutable (as is the idea that the universe was created miraculously last Thursday, with a perfect appearance of age!) and appealing only to the unshakable YEC, who moreover will now have to deal with the theological problem of a deceptive God.

The decay rates haven’t changed lately, either

Nuclear decay rates depend on the laws of nuclear physics. Although they are more involved than those of electromagnetism, we do know them, and they explain very well the decay rates we observe. Here I will review a number of observations telling us that these laws haven’t changed in a very long time—if ever.

Supernovae are exploding stars that can be detected very far away. Among the many kinds of supernovae, the so-called Type Ia supernovae (SNs) do something pretty interesting when they explode—they release a very large amount of nickel-56 (an isotope with 28 protons and 28 neutrons in its nucleus) into space. But nickel-56 is radioactive. It decays to cobalt-56 (which has 27 protons and 29 neutrons) with a half-life of 6 days. (A half-life is the amount of time it takes for one-half of a sample of radioactive atoms to decay into its daughter product.) Cobalt-56 is also radioactive and decays to iron-56 (with 26 protons and 30 neutrons) with a half-life of 77 days. Then iron-56 is stable.

We know this all happens because we have observed all of the relevant spectra (Kuchner 1994). At each step along the decay chain of nickel-56 → cobalt-56 → iron-56, energy is released, which results in light that can be observed. After reaching a maximum soon after the explosion, the amount of light sent by a Type Ia SN decays as the number of nickel → cobalt-56 decay events drops. From the slope of the light curve, the decay rate can be inferred. This turns out to be a half-life of 6 days. The cobalt-56 created in the process starts decaying to iron-56. Here again, the light curve drops as more and more cobalt decays to iron. And here again, the slope of the light curve allows us to measure the half-life: 77 days (Filippenko 1997).

Luckily for us, type Ia SNs are anything but rare events. As of June 8, 2015, the Harvard online database (www.cbat.eps.harvard.edu/lists/Supernovae.html) counted 2 788 of them, 332 of which have been observed since 2012. One of the closest Type Ia SNs, called SN 2011fe (supernovae are named after the year they were observed followed by a, b, c, … aa, ab, ac … and so on) is just 21 million light-years away. Even if a YEC were to dispute the distance measurement, it must be farther away than 30 000 light-years because the parallax method does not work for it. Some could argue the explosion is too quick for any parallax to be measured. After all, any object needs to be observable for at least six months, for the technique to work. So, would there be enough time to measure a parallax? Yes. As an illustration, the remnant of the Type Ia SN observed by Tycho Brahe in 1572 is still visible today. There has therefore been ample time to try the parallax method. But it doesn’t work.

Type Ia SNs don’t tell us just about the nickel-56 → cobalt-56 and cobalt-56 → iron-56 decay rates. They also tell us about the laws of nuclear physics. If these laws had been different 21 million years ago, when SN 2011FE exploded, the observed decay rates would be different.

Are there other astronomical phenomena allowing for a test of the laws of nuclear physics in the past? Yes. Here are just a few examples:
  • Titanium-44 is radioactive and decays to calcium-44. In doing so, it emits a very special kind of light, a gamma photon with an energy of 1 160 keV (one keV is one thousand electron-volts, an energy unit used very commonly among chemists and physicists). This element, together with the expected gamma photon, is commonly detected with supernova explosions like that of Cassiopeia A (type IIb), about 11 000 light-years away.
  • Aluminum-26 decays to magnesium-26 by emitting a gamma photon at 1 808.6 keV. This nuclear phenomenon has been observed from the center of our galaxy, about 20 000 light years away.
  • Iron-60 decays to cobalt-60, also emitting signature gamma photons that have been detected from the center of our galaxy.
 The list of nuclear-astrophysical observations is extremely long. On June 13, 2012, the Nuclear Spectroscopic Telescope Array (NuSTAR) satellite was launched. A Google search on August 5, 2015, for “NuSTAR radioactive” returns more than 26 000 results. Nuclear astrophysics is a rapidly developing discipline, standing at the intersection of its two parents. Observations leave us no doubts: the laws of nuclear physics, and with them all the decay rates we know of, have not changed over a time span very much larger than 6 000 years.

Conclusion

Physicists do not hold that the laws of physics haven’t changed over the last 30 000+ years because of a uniformitarian prejudice. They hold it because this is what they observe. It is worth emphasizing the routine aspect of these observations. We are not talking of a few stars between 6 000 and 30 000 light years away, among which some spectra and decay rates coincide with some of ours on Earth. There are trillions of documented observations of stars farther away from Earth than 6 000 light years. Atomic or molecular “bar code” observations are routine. We have made millions of them, and are making more every day. Decay rates and nuclear events are routinely monitored. We have notations on thousands of them, and are making more every day.

We do not simply suspect the universe is more than 6 000 years old. We know that it is about 13.8 billion years old, and our knowledge is based on observations made by people who, regardless of their age, culture or religion, all come to the same conclusion. And yes, Ken Ham, for all practical purposes, “we were there” to see it.

References
 
Bret A. 2014. The World Is Not Six Thousand Years Old—So What? Eugene (OR): Wipf and Stock.
Filippenko AV. 1997. Optical spectra of supernovae. Annual Review of Astronomy and Astrophysics 35:309–355.
Kuchner MJ. 1994. Evidence for Ni-56 yields Co-56 yields Fe-56 decay in type IA supernovae. The Astrophysical Journal Letters 426:L89–L92.
Sanna A. 2012. Trigonometric parallaxes of massive star-forming regions. ix. The outer arm in the first quadrant. The Astrophysical Journal 745(1):82–88.
Setterfield B. 1981. The velocity of light and the age of the universe, part 1. Ex Nihilo 4(1):52–93.
 
About the author

Antoine Bret is a physicist and associate professor at the University Castilla-La Mancha in Spain. His researches focus on fundamental aspects of plasma physics, with applications in inertial confinement fusion and astrophysics. He was a visiting scholar at the Harvard-Smithsonian Center for Astrophysics in 2012 and 2014. He has been teaching a course on climate, energy, and related issues
for ten years. He is the author of The World Is Not Six Thousand Years Old— So What? (Eugene [OR]: Wipf and Stock, 2014) and The Energy-Climate Continuum: Lessons from Basic Science and History (Cham [Switzerland]: Springer, 2014).

Author’s address
Antoine Bret
Universidad Castilla-La Mancha
ETSI Industriales
Avda Camilo Jose Cela, s/n
13 071 Ciudad Real
Spain

Copyright 2015 by Antoine Bret; licensed under a Creative Commons Attribution-Non-Commercial-
NoDerivs 3.0 Unported License. http://creativecommons.org/licenses/by-nc-nd/3.0/

This article appeared in The Reports of the National Center for Science Education Volume 24, No 5 (2015)