Analog SFF, May 2009
Page 12
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Reader's Department: RADIOACTIVE DECAY AND THE EARTH-SUN DISTANCE
by John G. Cramer
About 22 years ago, the physics world was briefly rocked by claims of evidence for a new “fifth force", based on reanalysis of data from an early twentieth century experiment. Baron Roland von Eotvos, a Hungarian nobleman, had performed extensive measurements of the correlation between inertial mass and gravitational mass and published them in 1922. The lead article in the January 6, 1986 issue of Physical Review Letters had the unassuming title: “A Reanalysis of the Eotvos Experiment” by Ephriam Fischbach, et al. Two days later the New York Times ran an article with the headline: “Hints of Fifth Force in Universe Challenge Galileo's Findings” describing the possible implications of Fischbach's work.
Peculiar experimental results from Eotvos’ gravity measurements and from the behavior of “strange” K-mesons (kaons) were explained in the Fischbach paper by introducing a new theory that proposed a “hypercharge” force, a new fifth force of nature that was gravity-like, but that repelled rather than attracted nearby masses. (See my 15th AV column “Antigravity II: A Fifth Force?” published in the September 1986 issue of Analog.) Fischbach's work prompted a number of precision experimental “fifth force” tests. These results, when they became available a few years later, provided compelling experimental evidence that there was no fifth force of the type and strength that Fischbach's group had predicted. The fifth force idea had been falsified.
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Well, Ephriam Fischbach is back with a new reanalysis of old data. In a paper entitled “Evidence for Correlations Between Nuclear Decay Rates and Earth-Sun Distance,” his group at Purdue University has reanalyzed data from long-duration radioactive decay experiments performed at Brookhaven National Laboratory (BNL) in 1986 and at Germany's Physikalisch-Technische Bundesanstalt (PTB) in 1998.
Both of these experiments were precision determinations of the half-lives of long-lived radioisotopes. The BNL experiment was a study of the beta decay of the isotope silicon-32 (32Si), which has a half-life of about 172 years. Data was collected over a period spanning more than four years. As a control, the equipment also monitored the beta decay of the isotope chlorine-36 (36Cl), which has a half-life of 301,000 years. Analysis of the data computed the ratio of 32Si to 36Cl decay rates in order to suppress apparatus-dependent systematic variations.
The PTB experiment was a study of the decay of the alpha-decay radioactive isotope europium-152 (152Eu), which has a half-life of about 13.5 years. The measurement spanned over 15 years, and the equipment also monitored the alpha particle decay of the isotope radium-226 (226Ra), which has a half-life of 1,600 years. The PTB group similarly used the data to compute the ratio of 152Eu to 226Ra decay rates, in order to suppress apparatus-dependent systematic variations.
The reanalysis of these data by the Fischbach group indicated that in both experiments there were similar time-dependent variations in the measured counting rates at the level of about 0.1%. Moreover, during a period of about three years in which both experiments were collecting data at the same time, the observed time variations showed a remarkable correspondence.
The observed time variations had a period of one year and appeared to be roughly seasonal. The Fischbach group compared the variations with a number of variables that had an annual cycle. They settled on one particular variable, the distance R between the Earth and the Sun, which varies annually by about (greater than or equal to) 1.7% because the Earth's orbit is slightly elliptical, so that the Earth is closest to the Sun (R = 147,098,074 km) on about January 3 and farthest from the Sun (R = 152,097,701 km) around July 4. (Curiously, because the seasons are dominated by the tilt of the Earth's rotation axis, the coldest weather in the Northern Hemisphere occurs when the Earth is closest to the Sun and the sunlight is most intense, and the warmest weather occurs when the total sunlight is weakest.)
When the Fischbach group plotted the variations in 1/R2 (i.e., an inverse square law form) with those of the two radioactive decay rate measurements, they found remarkably good correlations. The formal probabilities that the observed correlations could have occurred by uncorrelated random statistical variation were extremely small: 6 x 10-18 for the BNL data and 2 x 10-246 for the PTB data. The variations appear to be real and to be correlated with the Earth-Sun distance.
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Radioactive decays are supposed to be fundamental processes that are well insulated from the influence of environmental effects. The Fischbach work raises the question of what possible physical phenomenon could produce this kind variation in the decay rate of two distinctly different kinds of radioactive decay, one dominated by the weak interaction and the other by the strong interaction? Or alternatively, what kind of unsuspected systematic error could produce almost identical artifact variations in two distinctly different physical measurements performed thousands of miles apart?
One possibility raised in the paper is that there might be a “scalar field” produced by the Sun that modulates the value of the terrestrial fine structure constant a and thereby changes the radioactive decay rates. They cite a theory that predicts such an effect, but note that the coupling strength that would be needed to produce the observed variations is about 14 orders of magnitude larger than the theory would predict.
Another possibility raised in the paper is that the radioactive decays are reacting in some novel way with the flux of neutrinos coming from the Sun. The intensity of solar neutrino flux would follow the inverse square law, and might also be modulated by changes in solar activity. In a follow-up paper, Jenkins and Fischbach noted that a statistically significant drop in the decay rate of the radioactive isotope manganese-54 (54Mn) during the solar flare of December 13, 2006, with several drops in decay rate correlated with spikes in x-rays from the solar flare as detected by GOES satellites.
However, there seems to be a problem with this link to solar flares. The first paper suggests that a decrease in neutrino flux with increasing Earth-Sun distance reduces the radioactivity decay rate, while the solar flare paper suggests that the increase in neutrino flux during a solar flare reduces the radioactivity decay rate. You cannot have it both ways.
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Is it plausible that solar neutrinos could modulate the rate of radioactive decays present on the Earth? I would have to say no. There is no known link between solar neutrino flux and radioactive decay processes (particularly those like the 152Eu alpha-decay that proceeds through the strong interaction) and our present understanding of fundamental interactions suggests that such a hypothetical link is extremely unlikely.
This, however, leaves us with two questions: (1) what is producing the variations that the Fischbach group reports? and (2) if such variations do, for whatever reason, depend on the distance from the Sun, are there other ways of looking for them?
Let's consider question 1 first. Decay rates, which are measured as particle detections per second, require a time standard that precisely divides time into well defined intervals. This is usually done with a crystal oscillator that, because of its physical dimensions, oscillates at a frequency that corresponds to an electrical standing wave in the crystal. The dimensions of such a crystal change with temperature. Temperatures, even in a controlled laboratory environment, can have seasonal variations that reflect the external outdoor temperature of environment and the peculiarities of the heating or cooling system.
Therefore, I would be very suspicious that the reported variations on radioactive decay rate might actually be reflecting some tiny seasonal variations in the time standard used in the measurements. I have no reason to believe that this is the problem, but I think that it should be carefully examined before invoking new physics to explain the observations.
As for question 2, the Earth's orbit has a very small eccentricity, so the annual variations in R are small. A better way of testing whether radioactive decay rates depend directly on 1/R2 would b
e to monitor a radioactive decay process within a space vehicle in a long elliptic orbit with a large eccentricity, so that R has a very large variation. As it happens, NASA has a number of space probes that match this description, because many space probes, particularly those that venture into the outer reaches of the solar system, are powered by radioisotope-driven thermoelectric power sources containing a strong radioactive decay source that produces enough energy as heat to power the vehicle. The power levels of such thermoelectric generators are carefully monitored because they constitute the principal power source of the vehicle.
Therefore, as a test of Fischbach's hypothesis, one should ask NASA scientists whether there is any evidence for variations in the output level on radioisotope power sources in vehicles in long elliptic orbits that would correspond to a 1/R2 variation in the radioactive decay rate. However, I suspect that I already know the answer to this question. If such power-level variations were present, they would be large and would long ago have been reported by the scientists and engineers of the space program.
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In any case, it's an interesting situation. We have suggestive but unexplained observations that may reflect a new and unexpected physical phenomenon. It's a situation that is familiar to readers of hard science fiction and usually leads to spectacular discoveries that drive the plot line. That could happen in this case, but I doubt it.
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AV Columns Online: Electronic reprints of over 140 “The Alternate View” columns by John G. Cramer, previously published in Analog, are available online at: www.npl.washington.edu/av.
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References:
“Evidence for Correlations between Nuclear Decay Rates and Earth-Sun Distance", Jere H. Jenkins, Ephriam Fischbach, Hohn B. Buncher, John T. Gruenwald, Dennis Krause, and Joshua J. Mattes, arXiv preprint 0808.3283v1 [astro-ph], August 25, 2008.
“Perturbation of Nuclear Decay Rates During the Solar Flare of 13 December 2006", Jere H. Jenkins and Ephriam Fischbach, arXiv preprint 0808.3156 [astro-ph], August 22, 2008.
Copyright © 2009 John G. Cramer
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A MEASURE OF DEVOTION
Shane Tourtellotte
Full disclosure is not always an option....
“I heard you're having a debate on the starship program, Linny.” Harris Kensil kept his upper body calm and still for the holo, though his foot was twitching out of shot. “I kinda wondered why I had to find out through the newsnets.”
Evelyn Prothro had the grace to flush. “Oh, I'm sorry, Harris. You've been, well, quiet lately, and I guess the foundation's AI took you off the notification list. You're invited, of course. Front row, center.”
Harris almost said yes, but held himself on course. “I had something closer to the action in mind.” Now both feet were shaking. “Something on stage.”
“Really?” Evelyn's eyebrows scrunched, the way they always did when she was dubious about something. “That's ... unexpected, Harris. I mean, it's been a while.”
“I know exactly how long it's been.” Down to the day. “I decided it was too long. So, is there any room for the old guard, or is supporting our brave explorers a job for the youngsters now?”
“You're scarcely old, Harris. That's not—” She stopped and gave him a shrewd look. “Age is not the point.”
Well, trading on their shared maturity had been worth a shot. “Is there another point stopping me?” The tremble had reached one knee, and he casually lowered a hand to press it down.
Evelyn thought for a moment. “This deserves more attention than a simple holocall provides. Why don't we meet for dinner tonight? A new place just opened on the north side, and—”
“I'd prefer Dalton's,” Harris said, “if that's okay. Something close, and familiar.” She gave Harris another searching look. “Of course, I could always cook you up something here. That's been an even longer while.”
That flustered her for an instant. “No, Dalton's is fine. Make it six thirty?”
“Perfect. I'll see you then, Linny. Good-bye.” He waited until her image had faded completely from the platform before giving in to a single full-body shudder.
But he'd done it. At least the first part, the easiest one.
* * * *
It was natural, he told himself, to be anxious about meeting Linny tonight. He was angling to speak in front of an audience, both live and telepresent, while others tried to tear down everything he said. It was enough to give anyone butterflies, even discounting the two-year layoff. Fortunately, the walk to the restaurant was pleasant and calming. At least with a healthy slug of scotch in him it was.
He could see the Dalton's sign a block away when a noisy group of college-age kids rounded a corner and plowed into him. He felt an elbow, then the brickform of a wall against his back. The gabbling, solipsistic clot passed by without an apology, without even a taunting laugh.
Harris wanted to be mad at them, but the jitters took hold of him again, and he settled for being mad at himself. It took a long look at his wrist-multi and a mantra of “You'll be late” to pry him off the wall and push him the last few hundred feet.
Things felt much better inside Dalton's. The low light off the dark-paneled walls, the muffled clink of glasses and cutlery, the hint of hearty smells wafting from the kitchen all worked to settle him. He wanted a couple of minutes to draw it all in and reach equilibrium, but the hostess, briskly solicitous, insisted on showing him straight to his table.
Linny was already there. She rose and took his hands in hers. “Good to see you again, Harris.” Harris began to offer his cheek, but there was no kiss coming, so he just sat down.
The menu appeared in the tabletop before him. “So how's the work going, Linny?” he asked as he tapped out his choices. “I don't mean the public relations stuff. How's it really going?”
Linny's eyes lingered on her menu. “It could be better. The election's not so far off, and there's a fair chance that all the candidates for the top spot could end up Homers. That's why we arranged the debate: not just to persuade, but to remind any party that'll listen that there's a real constituency for interstellar exploration.”
“A majority, last I saw,” Harris said, “even if it is getting thin.”
“It's getting very thin. A lot of people drawn by the novelty of FTL flight have drifted away, and others who joined us in the flush of the first mission's success have—” She sighed. “—gotten bored, I suppose.”
“Can't see how that's possible.”
Linny gave him one of those looks. “Harris...” She hesitated, then looked down. “Any suggestions for what I should order?”
“Oh, don't go by me.” He hadn't ventured outside three or four favorites in a long while, but he wasn't going to tell her that.
She gave the menu a few taps, and it went away. She looked back up, more resolute now. “Okay, Harris. Did you get bored?”
“What? No, no.” His foot gave a twitch. “I wanted to get back into physics, at least as a consultant. And I have been writing on the foundation's behalf. Some speeches, some essays.”
“All anonymous.”
“Yes. I've still been helping the cause, just more modestly, more quietly. And now, I think it's time that I stepped up and did more. I was hoping that you'd be glad I'd decided that.”
Linny studied him, not even shifting her gaze when the waiter rolled up to deliver their drinks and a bread basket. Harris knew she was measuring him, contemplating whether his old self was back. He bore the scrutiny passively, even stilling his foot.
“How much more were you thinking of doing?”
He sighed and reached for a roll. “I'm not aiming to hog the spotlight. If you had me just deliver opening and closing statements for our side at the debate, that would be fine.” Was he being too meek again? “Anything more would be up to you and the debaters.”
“Hm. Bookending the debate might come across as grandstanding. If you're going
to be part of the team, it needs to be as a full member. Can you handle that?”
He pulled some bread free from the roll. “My debate skills are probably rusty. But if that's what it takes for me to participate, that's what I'll do.” He made himself eat the bread, despite a churning stomach.
Her frowning mouth lifted into a wry smile. “A few people at Far Flight will think you're trying to grab back power. They won't like it.”
“I'm not trying to grab power, Linny. Earn it back, maybe, but that's for the future. One step at a time,” he said, matching her smile.
She nodded. “All right,” she said, reaching for the rolls. “Say you're assigned the opening statement. What kind of statement will it be? What will you emphasize?” She took a bite and chewed slowly.
Harris knew what this was: an audition. He could have taken offense, maybe should have, but he put that aside. He began talking, laying out points and arguments as Linny gradually worked through two rolls.
As he talked, an old feeling bubbled up within him, something close to exhilaration. Writing essays in a quiet corner of his home didn't bring it. Hearing others say the things he believed so zealously never produced it. Making the case himself, face to face, did. It made him think he could actually pull off this return from the shadows.
She didn't say anything after he finished, but her face told him he had passed the test. In the fading rush of the moment, he dared some frankness. “Time was, Linny, when I didn't have to prove myself to you.”
She turned aside, covering her unease with a dab of her napkin. “That's true. That was two years ago.” She looked back. “What happened, Harris? What took you away? What ... hurt you?”