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New Device Going to be Ready that will Scrutinize Einstein's Century-Old Equivalence Principle, According to SLAC Theorists

Despite decades of attempts, gravitational waves continue to elude direct detection. However, one new technology could soon change that. SLAC theorists are watching closely as their experimentalist colleagues at Stanford make ready a device that will scrutinize Einstein's century-old equivalence principle, which says objects with different mass and compositions accelerate at the same speed under gravity.

The precision of the Stanford experiment will be the highest ever achieved for a test of the equivalence principle. While the Stanford researchers' immediate goal is to examine this principle, the experiment will also demonstrate technology proposed for use in the search for gravitational waves.

"Directly observing gravitational waves would revolutionize astrophysics," said SLAC graduate student Surjeet Rajendran. "They could offer a snapshot of the big bang, as well as other early universe processes."

Gravitational waves are ripples in space-time that travel at the speed of light. Current research suggests that these waves are created by massive objects, particularly by two-body systems composed of neutron stars, white dwarves or black holes, to name a few. Since the effects of gravitational waves are infinitesimal, their observation can be disrupted by almost anything—variations in atmosphere, the earth's shaking. Although several experiments have tried, none have directly observed these elusive waves. In the early stages of one gravitational wave detector, unknown signals were eventually traced back to nearby logging activity.

The hunt for direct detection of gravitational waves requires both extreme sensitivity and the ability to silence background noise. Advancements in atomic technology, which will be showcased in the Stanford experiment, offer both of these features.

The experiment's magnetically shielded vacuum pipe, which extends through a 30-foot shaft in the Varian Physics Building's basement, represents a pristine environment—"a tube of nothing," so to speak, that shields out almost all background noise. Researchers will launch millions of atoms to the top of this cylinder of nothingness. After atoms, each with slightly different mass, are pulled down the tube by gravity, measurements of their states will be conducted down to an accuracy of 15 decimal places.

Proof that this extreme precision can be achieved makes possible new types of gravitational wave detectors, which would extend the measuring capability of current methods. The collaboration that has formed between SLAC theorists and Stanford experimentalists has proposed two such detectors, one terrestrial and another satellite-based, that aim to utilize the core atomic technology of the Stanford experiment.

For the Stanford experiment, changes in trajectories and velocities will be crucial to revealing possible violations of the equivalence principle. The same is true of the proposed detectors, where the paths of atoms will potentially echo reverberations thought to be found in gravitational waves.

"The success of this experiment will be a dramatic turning point for our proposal," postdoctoral student Peter Graham said. "It will serve as a wake-up call and prove the power of this technology."

Note for Gravitational Wave
In physics, a gravitational wave is a fluctuation in the curvature of spacetime which propagates as a wave, traveling outward from a moving object or system of objects. Gravitational radiation is the energy transported by these waves. Important examples of systems which emit gravitational waves are binary star systems, where the two stars in the binary are white dwarfs, neutron stars, or black holes.

Although gravitational radiation has not yet been directly detected, it has been indirectly shown to exist. This was the basis for the 1993 Nobel Prize in Physics, awarded for measurements of the Hulse-Taylor binary system.

In Einstein's theory of general relativity, the force of gravity is due to curvature of spacetime. This curvature is caused by the presence of massive objects. Roughly speaking, the more massive the object is, the greater the curvature it causes, and hence the more intense the gravity. As massive objects move around in spacetime, the curvature will change. If the objects move around in a certain way, ripples in spacetime can spread outward like ripples on the surface of a pond. These ripples are gravitational waves.

The simplest example of a strong source of gravitational waves is a spinning neutron star with a small mountain on its surface. The mountain's mass will cause curvature of the spacetime. Its movement will "stir up" spacetime, much like a paddle stirring up water. The waves will spread out through the Universe at the speed of light, never stopping or slowing down.

As these waves pass a distant observer, that observer will find spacetime distorted in a very particular way. Distances between objects will increase and decrease rhythmically as the wave passes. The magnitude of this effect will decrease the farther the observer is from the source. Any gravitational waves expected to be seen on Earth will be quite small; the change in size of any object will never be much more than 1 in 1020. Still, scientists are attempting to measure the effects of these waves using extraordinarily precise experiments.

By measuring these waves, astrophysicists hope to learn about systems they could not observe with more traditional means such as optical telescopes, radio telescopes, etc. Gravitational waves can penetrate regions that the more familiar waves cannot, providing us with a view of black holes and other mysterious objects in the distant Universe. Using precise measurements of gravitational waves in this way will also allow us to test the general theory of relativity more thoroughly.

In general terms, gravitational waves are radiated by objects whose motion involves acceleration, provided that the motion is not perfectly spherically symmetric (like a spinning, expanding or contracting sphere) or cylindrically symmetric (like a spinning disk).

A simple example is the spinning dumbbell. Set upon one end, so that one side of the dumbell is on the ground and the other end is pointing up, the dumbbell will not radiate when it spins around its vertical axis but will radiate if it tumbles end-over-end. The heavier the dumbbell, and the faster it tumbles, the greater is the gravitational radiation it will give off. If we imagine an extreme case in which the two weights of the dumbbell are massive stars like neutron stars or black holes, orbiting each other quickly, then significant amounts of gravitational radiation would be given off.

Some more detailed examples:
Two objects orbiting each other in a quasi-Keplerian planar orbit (basically, as a planet would orbit the Sun) will radiate.
A spinning non-axisymmetric planetoid — say with a large bump or dimple on the equator — will radiate.
A supernova will radiate except in the unlikely event that it is perfectly symmetric.
An isolated non-spinning solid object moving at a constant speed will not radiate. This can be regarded as a consequence of the principle of conservation of linear momentum.
A spinning disk will not radiate. This can be regarded as a consequence of the principle of conservation of angular momentum. On the other hand, this system will show gravitomagnetic effects.
A spherically pulsating spherical star (non-zero monopole moment or mass, but zero quadrupole moment) will not radiate, in agreement with Birkhoff's theorem.

Note for Neutron Star
A neutron star is formed from the collapsed remnant of a massive star; i.e. a Type II, Type Ib, or Type Ic supernova. Models predict that neutron stars consist mostly of neutrons, hence the name. Such stars are very hot, as supported by the Pauli exclusion principle indicating repulsion between neutrons. A neutron star is one of the few possible conclusions of stellar evolution.

A typical neutron star has a mass between 1.35 and about 2.1 solar masses, with a corresponding radius between 20 and 10 km, respectively — 30,000 to 70,000 times smaller than the Sun. Thus, neutron stars have overall densities of 8.4×1016 to 1×1018 kg/m³, which compares with the approximate density of an atomic nucleus of 3×1017 kg/m³. The neutron star's density varies from below 1×109 kg/m³ in the crust increasing with depth to above 6 or 8×1017 kg/m³ deeper inside.

In general, compact stars of less than 1.44 solar masses, the Chandrasekhar limit, are white dwarfs; above 2 to 3 solar masses (the Tolman-Oppenheimer-Volkoff limit), a Quark star might be created, however this is uncertain. Gravitational collapse will always occur on any star over 5 solar masses, inevitably producing a black hole.

As the core of a massive star is compressed during a supernova, and collapses into a neutron star, it retains most of its angular momentum. Since it has only a tiny fraction of its parent's radius (and therefore its moment of inertia is sharply reduced), a neutron star is formed with very high rotation speed, and then gradually slows down. Neutron stars are known to have rotation periods between about 1.4ms to thirty seconds. The neutron star's compactness also gives it very high surface gravity, 2×1011 to 3×1012 times stronger than that of Earth. One measure of such immense gravity is the fact that neutron stars have an escape velocity of around 150,000 km/s, about 50% of the speed of light. Matter falling onto the surface of a neutron star would be super-accelerated by this gravity and the force of impact would likely destroy the object's component atoms, rendering all its matter identical, in most respects, to the rest of the star.

Current understanding of the structure of neutron stars is defined by existing mathematical models. A neutron star is so dense that one teaspoon (5 millilitre) of its material would have a mass over 5×1012 kg. On the basis of current models, the matter at the surface of a neutron star is composed of ordinary atomic nuclei as well as electrons. The "atmosphere" of the star is roughly one meter thick, below which one encounters a solid "crust". Proceeding inward, one encounters nuclei with ever increasing numbers of neutrons; such nuclei would decay quickly on Earth, but are kept stable by tremendous pressures. Proceeding deeper, one comes to a point called neutron drip where free neutrons leak out of nuclei. In this region, there are nuclei, free electrons, and free neutrons. The nuclei become smaller and smaller until the core is reached, by definition the point where they disappear altogether. The exact nature of the superdense matter in the core is still not well understood. While this theoretical substance is referred to as neutronium in science fiction and popular literature, the term "neutronium" is rarely used in scientific publications, due to ambiguity over its meaning. The term neutron-degenerate matter is sometimes used, though not universally as the term incorporates assumptions about the nature of neutron star core material. Neutron star core material could be a superfluid mixture of neutrons with a few protons and electrons, or it could incorporate high-energy particles like pions and kaons in addition to neutrons, or it could be composed of strange matter incorporating quarks heavier than up and down quarks, or it could be quark matter not bound into hadrons. However so far observations have neither indicated nor ruled out such exotic states of matter.

In figure, Stanford graduate student Jason Hogan and SLAC's Surjeet Rajendran (center) and Peter Graham (right) discuss an experiment that may have vast scientific implications.