Time and laser ranging: a window of opportunity for geodesy, navigation and metrology

Recent progress in the domain of time and frequency (T/F) standards requires important improvements of existing time distribution links. Among these, the accuracy of time transfer is actually an important part of the concerns in order to establish and maintain time & space references from ground and/or space facilities. Several time transfers by laser link projects have been carried out over the past 10 years with numerous scientific and metrological objectives. Satellite Laser ranging (SLR) has proven to be a fundamental tool, offering a straightforward, conceptually simple, highly accurate and unambiguous observable. Depending on the mission, LR is used to transmit time over two-way or one-way distances from 500 to several millions of km. The following missions and their objectives employed this technique: European Laser Timing (ELT) at 450 km, Time Transfer by Laser Link (T2L2) at 1,336 km, Laser Time Transfer (LTT) at 36,000 km, Lunar Reconnaissance Orbiter (LRO) at 350,000 km, and MErcury Surface, Space ENvironment, GEochemistry, and Ranging (MESSENGER) at tens of million km. This article describes the synergy between SLR and T/F technologies developed on the ground and in space and as well as the state of the art of their exploitation. The performance and sources of limitation of such space missions are analyzed. It shows that current and future challenges lie in the improvement of the time accuracy and stability of the time for ground geodetic observatories. The role of the next generation of SLR systems is emphasized both in space and at ground level, from the point of view of GGOS and valuable exploitation of the synergy between time synchronization, ranging and data transfer.

Kinetic Relationship between Energy-Dependent Fluorescence Quenching, Light Scattering, Chlorophyll Luminescence and Proton Pumping in Intact Leaves

A measuring system was designed for simultaneous recording of modulated chlorophyll fluorescence and light scattering changes. The kinetic relationship was investigated between lightinduced changes in non-photochemical fluorescence quenching, as determined by the saturation pulse method, and in light scattering, as measured via the apparent absorbance change at 543 nm. Very similar, but not identical kinetics were observed, reflecting a close non-linear relationship between these two indicators of thylakoid membrane energization. Fluorescence was found more sensitive at low levels of energization, while scattering continued indicating further increases in energization when quenching already was saturated. A general relationship between quenching and scattering is demonstrated which holds irrespective of whether energization is varied during induction or via changes in light intensity or CO2 concentration. In the light-off responses, only part of fluorescence quenching was found to relax with the same kinetics as scattering. It is suggested that at high levels of energization slowly reversible membrane changes may be induced which have the potential of non-photochemical quenching at a low level of energization, and which are not accompanied by scattering changes. Neither quenching nor scattering changes displayed kinetics sufficiently fast to be taken as a direct expression of internal thylakoid acidificati on in intact leaves. This conclusion is drawn from comparative measurements of proton-uptake, as reflected by CO2-solubilization upon light-induced stroma alkalization, and of chlorophyll luminescence. Both, the initial CO2 - gulp and the pH-dependent luminescence rise were found to clearly precede the development of energy-dependent quenching.

Light Induced Changes in the Conformation of Spinach Thylakoid Membranes as Monitored by 90° and 180° Scattering Changes: A Comparative Investigation

A series of investigations on light induced thylakoid membrane conformational changes has been carried out using scattering changes measured either at 90° or 180° (transmittance changes) as criteria for changes in membrane structure upon illumination. These two phenomena were found to react similarly to a range of photosynthetic inhibitors, to changes in the ionic (Mg2+, H+) concentration, and to have similar difference spectra. However, it was found possible to distinguish these two phenomena by the differences in the kinetics of the light induced rise times. This could be shown most clearly using an Aminco DW2 spectrophotometer with the measuring beam deflected through 90° by front face mirrors. The dark decay time of the two scattering changes was similar.

These results are discussed in relation to the hypothesis that the light induced narrow angle 90° scattering response represents microconformational membrane changes in response to proton uptake, and the 180° scattering changes represent a mixed response including changes in the stacking of thylakoids.

Pressure shift freezing as potential alternative for generation of decellularized scaffolds

Background. Protocols using chemical reagents for scaffold decellularization can cause changes in the properties of the matrix, depending on the type of tissue and the chemical reagent. Technologies using physical techniques may be possible alternatives for the production grafts with potential superior matrix characteristics. Material and Methods. We tested four different technologies for scaffold decellularization. Group 1: high hydrostatic pressure (HHP), 1 GPa; Group 2: pressure shift freezing (PSF); Group 3: pulsed electric fields (PEF); Group 4: control group: detergent (SDS). The degree of decellularization was assessed by histological analysis and the measurement of residual DNA. Results. Tissue treated with PSF showed a decellularization with a penetration depth (PD) of 1.5 mm and residual DNA content of 24% ± 3%. HHD treatment caused a PD of 0.2 mm with a residual DNA content of 28% ± .4%. PD in PEF was 0.5 mm, and the residual DNA content was 49% ± 7%. In the SDS group, PD was found to be 5 mm, and the DNA content was determined at 5% ± 2%. Conclusion. PSF showed promising results as a possible technique for scaffold decellularization. The penetration depth of PSF has to be optimized, and the mechanical as well as the biological characteristics of decellularized grafts have to be evaluated.

Automation of an extracorporeal support system with adaptive fuzzy controllers

For patients suffering from cardiogenic shock cardiopulmonary resuscitation may not be sufficient to restore normal heart function. However, their chances of survival may be increased with the use of an extracorporeal support system. With this system the patient's organs are perfused while being transported to the nearest hospital for proper treatment. In the automation of an extracorporeal support system the patient's vital signals are constantly monitored and proper adjustments are performed to improve organ perfusion. In this paper, an adaptive fuzzy controller is proposed that uses the knowledge and expertise of a perfusionist as a starting point and reference for regulation. Furthermore it is able to adapt to the patient's specific reactions by manipulating the rule base of the fuzzy controller. The performance of the adaptive fuzzy controller is tested with a simulation model of the cardiovascular system.

Design of a fuzzy controller for the automation of an extracorporeal support system with the use of a simulation environment

The automation of a portable extracorporeal support system may greatly help people who suffer from cardiogenic shock by providing them with an optimal oxygen perfusion and avoid mult-organ failure while being transported to a hospital. This however requires the creation of tools that help in the design of the ideal controller. In this paper a simulation environment is described were a cardiovascular model from the ISR Physiome database was used together with a model of a portable extracorporeal support system. Additionally a model of an oxygenator/blender model is introduced to allow the simulation of oxygen perfusion. Fuzzy controlling was used for automation since it allows a straightforward implementation of expert knowledge. Through the simulation environment different scenarios may be created where intensive testing is possible and constant repetition for control optimization. Initial simulation results are given of the fuzzy controller adjusting the extracorporeal flow rate and oxygen administration for a case of low cardiac output.

[Primary stability of the capsule-labrum complex after reconstruction with the Mitek Bioknotless anchor system in human cadaver models]

BACKGROUND

This study examines the postoperative stability of the Mitek Bioknotless anchor system with biomechanical draw-out pulling in human cadaver shoulders.

METHOD

With simulation of anterior shoulder dislocation a test group (n=10, Ø 45 years) was tested against a native group (n=8, Ø 47 years). All shoulders were dissected up to the passive stabilizers. In the test group an artificial Bankart lesion was created and repaired with three Mitek Bioknotless anchors. The humeri of both groups were fixed in 60 degrees glenohumeral abduction and 90 degrees external rotation and then dislocated in a ventral direction. For evaluation purposes the ultimate draw-out strength, mode of failure, translation of humeral head, capsular slope, and bone density in the test group were measured.

RESULTS

In the test group the ultimate strength was a median of 937 N (min. 554 N, max. 1,294 N) with 28 bony anchor dislocations, 1 suture rupture, and 1 capsular rupture, and in the native group with 6 Bankart and 2 HAGL lesions it was 1,214 N (708 N, 1,471 N). The bone density showed a positive correlation to the draw-out strength regarding cortical density and total density.

CONCLUSION

Regarding the high draw-out strength the Mitek Bioknotless anchor system provides enough stability for early functional treatment.

Experiments on robot-assisted navigated drilling and milling of bones for pedicle screw placement

BACKGROUND

This article presents experimental results for robot-assisted navigated drilling and milling for pedicle screw placement. The preliminary study was carried out in order to gain first insights into positioning accuracies and machining forces during hands-on robotic spine surgery. Additionally, the results formed the basis for the development of a new robot for surgery.

METHODS

A simplified anatomical model is used to derive the accuracy requirements. The experimental set-up consists of a navigation system and an impedance-controlled light-weight robot holding the surgical instrument. The navigation system is used to position the surgical instrument and to compensate for pose errors during machining. Holes are drilled in artificial bone and bovine spine. A quantitative comparison of the drill-hole diameters was achieved using a computer.

RESULTS

The interaction forces and pose errors are discussed with respect to the chosen machining technology and control parameters. Within the technological boundaries of the experimental set-up, it is shown that the accuracy requirements can be met and that milling is superior to drilling.

CONCLUSIONS

It is expected that robot assisted navigated surgery helps to improve the reliability of surgical procedures. Further experiments are necessary to take the whole workflow into account.

The selection of a model microalgal species as biomaterial for a novel aquatic phytotoxicity assay

A phytotoxicity assay based on the ToxY-PAM dual-channel yield analyser has been developed and successfully incorporated into field assessments for the detection of phytotoxicants in water. As a means of further exploring the scope of the assay application and of selecting a model biomaterial to complement the instrument design, nine algal species were exposed to four chemical substances deemed of priority for water quality monitoring purposes (chlorpyrifos, copper, diuron and nonylphenol ethoxylate). Inter-species differences in sensitivity to the four toxicants varied by a factor of 1.9-100. Measurements of photosystem-II quantum yield using these nine single-celled microalgae as biomaterial corroborated previous studies which have shown that the ToxY-PAM dual-channel yield analyser is a highly sensitive method for the detection of PS-II impacting herbicides. Besides Phaeodactylum tricornutum, the previously applied biomaterial, three other species consistently performed well (Nitzschia closterium, Chlorella vulgaris and Dunaliella tertiolecta) and will be used in further test optimisation experiments. In addition to sensitivity, response time was evaluated and revealed a high degree of variation between species and toxicants. While most species displayed relatively weak and slow responses to copper, C. vulgaris demonstrated an IC10 of 51 microgL-1, with maximum response measured within 25 minutes and inhibition being accompanied by a large decrease in fluorescence yield. The potential for this C. vulgaris-based bioassay to be used for the detection of copper is discussed. There was no evidence that the standard ToxY-PAM protocol, using these unicellular algae species, could be used for the detection of chlorpyrifos or nonylphenol ethoxylate at environmentally relevant levels.

The combined SPE:ToxY-PAM phytotoxicity assay; application and appraisal of a novel biomonitoring tool for the aquatic environment

Mounting concerns regarding the environmental impact of herbicides has meant a growing requirement for accurate, timely information regarding herbicide residue contamination of, in particular, aquatic systems. Conventional methods of detection remain limited in terms of practicality due to high costs of operation and the specialised information that analysis provides. A new phytotoxicity bioassay was trialled for the detection of herbicide residues in filter-purified (Milli-Q) as well as natural waters. The performance of the system, which combines solid-phase extraction (SPE) with the ToxY-PAM dual-channel yield analyser (Heinz Walz GmbH), was tested alongside the traditional method of liquid chromatography-mass spectrometry (LC-MS). The assay methodology was found to be highly sensitive (LOD 0.1 ng L(-1) diuron) with good reproducibility. The study showed that the assay protocol is time effective and can be employed for the aquatic screening of herbicide residues in purified as well as natural waters.

UV-excited chlorophyll fluorescence as a tool for the assessment of UV-protection by the epidermis of plants

Recently, a new method for estimating epidermal transmission of UV radiation in higher plants has been proposed. The empirical evidence for the usefulness of this method is reviewed here. Direct comparison with spectroscopically determined epidermal transmission yielded equivalent results. A linear correlation to the concentration of epidermal screening compounds has been shown. Relating UV-A and UV-B absorbance allowed some preliminary conclusions about the chemical nature of the screening compounds. A new portable apparatus is presented for the first time, which allows the non-destructive assessment of UV-A screening even under field conditions. Repeated measurements on identical leaves over a time-course of 6 d demonstrated a strong age-dependence in the capacity for the synthesis of UV-A screening compounds upon exposure to UV-B radiation. It is concluded that the new method may provide a valuable tool for the investigation of the acclimation of plants to UV-B radiation and, when accompanied by HPLC analysis, of the reaction of phenolic metabolism to environmental stimuli.

Osmotic stress induces inactivation of photosynthesis in guard cell protoplasts of Vicia leaves

Guard cell protoplasts isolated from Vicia leaves showed a strong suppression of the photosynthesis under hypotonic conditions, as reflected by changes in the chlorophyll fluorescence characteristics. The response was reversible as well. Mesophyll cell protoplasts did not show any lowering of photosynthetic activity under hypo-osmotic conditions. This result indicates that the response was guard cell specific.

Nitrogen starvation-induced chlorosis in Synechococcus PCC 7942. Low-level photosynthesis as a mechanism of long-term survival

Cells of the non-diazotrophic cyanobacterium Synechococcus sp. strain PCC 7942 acclimate to nitrogen deprivation by differentiating into non-pigmented resting cells, which are able to survive prolonged periods of starvation. In this study, the physiological properties of the long-term nitrogen-starved cells are investigated in an attempt to elucidate the mechanisms of maintenance of viability. Preservation of energetic homeostasis is based on a low level of residual photosynthesis; activities of photosystem II and photosystem I were approximately 0.1% of activities of vegetatively growing cells. The low levels of photosystem I activity were measured by a novel colorimetric assay developed from the activity staining of ferredoxin:NADP+ oxidoreductase. Photosystem II reaction centers, as determined by chlorophyll fluorescence measurements, exhibited normal properties, although the efficiency of light harvesting was significantly reduced compared with that of control cells. Long-term chlorotic cells carried out protein synthesis at a very low, but detectable level, as revealed by in vivo [35S]methionine labeling and two-dimensional gel electrophoresis. In conjunction with the very low levels of total cellular protein contents, this implies a continuous protein turnover during chlorosis. Synthesis of components of the photosynthetic apparatus could be detected, whereas factors of the translational machinery were stringently down-regulated. Beyond the massive loss of protein during acclimation to nitrogen deprivation, two proteins that were identified as SomA and SomB accumulated due to an induced expression following nitrogen reduction.

Epidermal transmittance of leaves of Vicia faba for UV radiation as determined by two different methods

Leaves of Vicia faba were collected from the field and the greenhouse and transmittance of epidermal peels from adaxial and abaxial sides was determined in the wavelength range from 250 to 800 nm using a spectrophotometer equipped for the measurement of turbid samples. From the same leaves, epidermal transmittance was estimated by a recently developed fluorometric method. Both methods gave highly correlated results with a slope of the regression line between both methods close to 1 and an intercept close to 0. Transmittances at around 310 nm as low as 3% were detected in the adaxial epidermis of field-grown leaves, while transmittance could be as high as 70% in the abaxial epidermis of greenhouse-grown leaves. There was a strong correlation between UV-A (ca. 366 nm) and UV-B (ca. 310 nm) transmittance detected by both methods which could be explained by the pigment composition in methanolic extracts where flavonols accounted for 90% of the absorption at 310 nm in the extract, while hydroxycinnamic acid derivatives which absorb only at the shorter wavelength constituted about 5%. It is concluded that the fluorescence method which allows rapid measurements on intact leaves can provide a quantitative estimate of epidermal transmittance for UV-B (280-320 nm) and UV-A (320-400 nm) radiation.

Light-induced dynamic changes of NADPH fluorescence in Synechocystis PCC 6803 and its ndhB-defective mutant M55

Blue-green fluorescence emission of intact cells of Synechocystis PCC6803 and of its ndhB-defective mutant M55 was measured with a standard pulse-amplitude-modulation chlorophyll fluorometer equipped with a new type of emitter-detector unit featuring pulse-modulated UV-A measuring light and a photomultiplier detector. A special illumination program of repetitive saturating light pulses with intermittent dark periods (10 s light, 40 s dark) was applied to elicit dynamic fluorescence changes under conditions of quasi-stationary illumination. The observed effects of artificial electron acceptors and inhibitors on the responses of wild-type and mutant M55 cells lead to the conclusion that changes of NAD(P)H fluorescence are measured. In control samples, a rapid phase of light-driven NADP reduction is overlapped by a somewhat slower phase of NADPH oxidation which is suppressed by iodoacetic acid and, hence, appears to reflect NADPH oxidation by the Calvin cycle. Mercury chloride transforms the light-driven positive response into a negative one, suggesting that inhibition of NADP reduction at the acceptor side of PSI leads to reduction of molecular oxygen, with the hydrogen peroxide formed (via superoxide) causing rapid oxidation of NADPH. The new fluorescence approach opens the way for new insights into the complex interactions between photosynthetic and respiratory pathways in cyanobacteria.

Radon and thoron in cave dwellings (Yan'an, China)

222Rn and 220Rn concentrations were measured in cave dwellings and brick houses in the region of Yan'an (China) during summer 1997. The underground dwellings are built into Quaternary loess, and all investigated houses are founded on it. The median values of indoor 222Rn and 220Rn concentrations are 42 (n = 18) and 77 Bq m(-3) (n = 15) for brick houses and 92 (n = 23) and 215 (n = 17) Bq m(-3) for cave dwellings. To classify the dwellings in respect to their "cave-character," the fraction of walls having a direct contact to the loses is calculated for each dwelling. While the 222Rn concentrations are increasing with higher fractions, the 220Rn concentrations are not correlated with this fraction. On the other hand, due to the short half-life of 220Rn the distance from the measuring point to the walls is negatively correlated with the 220Rn concentration, while there is no correlation with the 222Rn concentration. Therefore, concentric isolines of 220Rn concentrations showing a strong gradient were detected in cave dwellings. An influence of the ventilation rate is distinct for 222Rn but weak for 220Rn. The effective dose rates for 222Rn and 220Rn and their progenies are calculated for brick houses (2.7 mSv y(-1)), cave dwellings (7.1 mSv y(-1)), and for traditional cave dwellings with a bed foundation built with loess (16.7 mSv y(-1)). These calculations are based on summer measurements only. It is expected that the true effective dose rates will be significantly higher.

In vitro investigations of drug release and penetration--enhancing effect of ultrasound on transmembrane transport of flufenamic acid

Percutaneous absorption studies are performed in various in vitro models to determine the rate of drug absorption via the skin. We designed an phonophoretic drug delivery system to investigate the influence of ultrasound on transmembrane transport of different drugs. Phonophoresis is defined as the migration of drug molecules, contained in a contact agent, through the skin under the influence of ultrasound. We investigated the absorption of flufenamic acid in a buffer medium in dependence of ultrasound energy and application time. For evaluating membrane penetration of flufenamic acid, the concentration range of buffer solution was measured. Flufenamic acid was determined by using a fluorimetric method. Ultrasound energy was supplied for between 5 and 30 min at a range of intensities (0; 0.3; 0.6; 0.9; 1.2; 1.5 W/cm2). energy levels commonly used for therapeutic purpose. The pronounced effect of ultrasound on the transmembrane absorption of the drug was observed at all ultrasound energy level studied. The time of application was found to play an important role in delivery and transport of drug. Dependent on time, we observed an arise of temperature up to 4.5 degrees. It appears that there was no difference between an intensity of 0.3 and 1.5 W/cm2 and the measured drug concentrations in solution. The highest penetration was observed at an intensity of 1.0 W/cm2 after 30 min. These results were not significantly different from concentration in measurements after 30 min and 0.5 and 1.5 W/cm2. It seems that the arise of drug concentration is caused by effects of temperature and by variation of membrane delivery in dependence of temperature.

Two distinct states of the thylakoid bf complex

Under normal physiological conditions the state of the cyt bf complex is characterized by rapid reoxidation kinetics of cyt b-563 following flash-illumination. It is known that these kinetics are dramatically slowed down under oxidizing conditions. Here we show that this slow-down of cyt b-563 oxidation is the consequence of a relatively slow (half-time of several minutes) transformation of the cyt bf complex into a distinctly different state (termed state-s). Reversal to the normal state requires strong reductive treatment or light-induced electron transport. The results are in line with a recent model of functional cyt bf dimers [Cramer et al., Annu. Rev. Plant Physiol. Plant Mol. Biol. 47 (1996), 477-5081, if it is assumed that state-s reflects the monomeric state of the bf complex.

In vitro investigations of drug release and penetration--enhancing effect of ultrasound on transmembrane transport of flufenamic acid

Although topical drugs are usually applied at a convenient site, the target for the drug interaction may be systemic. Phonophoresis is the use of ultrasound to enhance the delivery of topical applied drugs. The purposes of our study were to investigate the in vitro penetration and the in vivo transport of flufenamic acid in dependence of ultrasound. Percutaneous absorption studies are performed in various in vitro models to determine the rate of drug absorption via the skin. We designed a phonophoretic drug delivery system to investigate the influence of ultrasound on transmembrane transport of different drugs. We investigated the absorption of flufenamic acid in a buffer medium in dependence of ultrasound energy and application time. For evaluating membrane penetration of flufenamic acid, the concentration range of buffer solution was measured. Ultrasound energy was supplied for between 5 and 30 min at a range of intensities up to 1.5 W/cm2, energy levels commonly used for therapeutic purpose. The pronounced effect of ultrasound on the transmembrane absorption of the drug was observed at all ultrasound energy levels studied. The time of application was found to play an important role in delivery and transport of drug. Dependent on time, we observed a rise of temperature up to 4.5 degrees C. It appears that there was no difference between an intensity of 0.3 and 1.5 W/cm2 and the measured drug concentrations in solution. The highest penetration was observed at an intensity of 1.0 W/cm2 after 30 min. These results were not significantly different from concentration measurements after 30 min and 0.5 and 1.5 W/cm2. It seems that the arise of drug concentration is caused by effects of temperature and by variation of membrane delivery in dependence of temperature. Using this in vitro model we note it is possible to compare the transdermal penetration and absorption of commercial flufenamic ointment in volunteers.

[Pain intensity and power densities of therapeutic ultrasound--a serial, comparative pilot study in patients with low back pain]

INTRODUCTION

A pilot study was conducted to provide an indication of the power density (PD) dependent analgesic effect of ultrasound (US) under serial application in patients with low back pain.

METHODS

Thirty patients (n=3 x 10) were randomly assigned to one of three PDs (0.3, 0.7 or 1.0 W/cm(2)). Twelve treatments were applied once a day for 2 weeks. The sonation time per treatment was 10 minutes. Patients were unaware of their grouping. Pain intensity (PI) was assessed using a visual analogue scale for each treatment. The relationship between the different PI profiles was tested by repeated measures by ANOVA.

RESULTS

The averaged duration of pain was over 10 years (chronic low back pain). Twenty-seven (n=3 x 9) patients could be analyzed (three drop-outs). In the medium (0.7 W/cm(2)) and high (1.0 W/cm(2)) PD groups the PI showed a clearly decreasing trend, while in the low (0.3 W/cm(2)) PD group, the PI remained stationary. None of the contrasts showed a locally significant effect (Bonferroni adjustment).

CONCLUSIONS

The observed effect is presumably overestimated. In the low PD group, there were more people out of work, and fewer took analgesic drugs. No conclusive judgement is possible. The concept of the PD-dependent effect of US must be evaluated in trials with greater sample sizes.

Kinematic and neuromuscular changes of the gait pattern after Achilles tendon rupture

After long immobilization periods in equinovalgus with operated Achilles tendon rupture, long-lasting changes of motor patterns in functional movement can be expected. In the present study, possible alterations in gait pattern have been analyzed based on kinematic and neuromuscular parameters. Ten patients 1 year after surgery and a healthy control group performed 10 gait cycles in natural walking cadence. Ankle motion, pressure distribution, and electromyographic data were recorded and analyzed in defined phases. Kinematic and neuromuscular changes are still evident 1 year after surgery with a temporal phase shift and a neuromuscular deficit of the lateral gastrocnemius muscle. The objective of rehabilitation should be the facilitation of the temporal innervation pattern of the lateral gastrocnemius muscle in the functional movement.

Two fundamentally different types of variable chlorophyll fluorescence in vivo

Upon onset of saturating continuous light only the first part of the observed polyphasic fluorescence rise follows Q(A) reduction (photochemical phase), whereas the remaining part (thermal phases) is kinetically limited by relatively slow reactions with light saturated half-times in the order of 10-50 ms. A simple hypothesis is presented for the interpretation of these fundamentally different types of variable fluorescence. The hypothesis, which is based on the reversible radical pair model of PSII, assumes stimulation of both prompt and recombination fluorescence upon Q(A) reduction, with only recombination fluorescence being in competition with nonradiative energy loss processes at the reaction centers. It is proposed that changes in the rate constants of these processes modulate the yield of recombination fluorescence in closed centers, thus causing large variations in the maximal fluorescence yield and also giving rise to the 'thermal phases'. This hypothesis can reconcile numerous experimental findings which so far have seemed difficult to interpret.