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Einstein's Equivalence Principle Put to Test

An orbital experiment is about to test to see if objects really fall at same speed in a gravitational field -- or wither some exotic physics may be detected.

In the late 1500s, the Italian scientist Galileo Galilei conceived of an experiment that changed a foundation of physics. He mulled -- and by some accounts actually tested -- what would happen if two spheres with different weights were dropped at the same time from the Leaning Tower of Pisa.

At the time, the prevailing theory of gravity, developed almost 2,000 years earlier by the Greek philosopher and scientist Aristotle, attributed the speed of falling object to its proportional weight, with heavier objects falling faster than lighter ones.

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Galileo believed that mass was immaterial to an object's falling speed. All would hit the ground at the same time no matter how much they weighed. From that, he deduced that in a vacuum, all bodies would fall at the same speed, an idea that underpins Albert Einstein's general theory of relativity, published 100 years ago.

The concept, called the equivalence principle, has been well tested on Earth, but scientists wonder if it breaks down when measurements are precise enough.

Putting the principle under a proverbial microscope is the goal of a French-backed space experiment called, appropriately, Microscope. The 668-pound satellite flew as a secondary payload aboard a Soyuz rocket which launched last week from Europe's Kourou, French Guiana, spaceport.

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Microscope contains two cylindrical test masses -- one made of titanium and the other a platinum-rhodium alloy -- which will be electrostatically levitated and stabilized so sensors can measure accelerations equal to a millionth of a billionth of Earth's gravity. Experiments on Earth have been about 100 times less sensitive, mostly because of random, seismic vibrations from naturally occurring and human activities.

"We expect to open a new window beyond Einstein," Microscope lead scientist Pierre Touboul wrote in an email to Discovery News.

If the equivalence principle breaks down, the door opens for new physics to complement general relativity, maybe a new type of interaction or a new type of particle for this interaction, he said.

"If there is no violation, this is a new constraint for quantum gravity theory, and we open the way to perform a better physics experiment," in space, Touboul added.

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"Any violation of the equivalence principle would be of vital importance," the French space agency CNES wrote in a summary of the experiment posted on its website.

"It would be the first sign of new physical phenomena ... which are not explained by our standard physics model. It would thus bring into question our knowledge at the interface between the field quantum theory and relativity theories of gravitation, as well as the application of these theories to astrophysics and cosmology," CNES said.

Scientists plan to compare the relative motion of the Microscope masses for two years. The instrument was switched on this week, the start of a two-month checkout. Science operations are slated to begin in July.

Artist’s concept of the Microscope spacecraft in orbit.

Exactly 100 years ago on Nov. 25, 2015, physicist Albert Einstein, then 36, presented a fourth and final lecture to the Prussian Academy of Sciences about his new general theory of relativity. The idea not only redefined the concept of gravity, but also ended up reshaping humanity’s perspective on reality. Here’s a look at the theory in thought and action.

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Einstein was famous for his thought experiments, which often played out for years in his imagination. From the gedankenexperiment, as it is known in German, Einstein grasped fundamental concepts about the physical world that could be verified by observation and experiments. One of his most famous ones began in 1907 when Einstein pondered if a person inside a windowless elevator could tell if he was in a gravitational free-fall, or if the elevator was being hauled up by a constant acceleration. Einstein decided the laws of physics must be the same in both cases. The mathematical equation he derived to explain this so-called principle of equivalence, which equated the effects of gravitation with acceleration in zero-gravity, became the basis for general relativity.

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A total solar eclipse on May 29, 1919, gave astronomers an opportunity to verify Einstein’s general theory of relativity by proving that the sun’s gravitational field was bending the light of background stars. The effect was only observable during time when the sun’s light was dim enough for stars to become visible. British astronomer Arthur Eddington led an expedition to the island of Principe, off the West Coast of Africa, to photograph the eclipse, which lasted nearly seven minutes. The images of stars in the region around the sun proved that Einstein’s interpretation of gravity trumped the 200-year old Newtonian model, which interpreted gravity as a force between two bodies. Einstein saw gravity as warps and curves in space and time.

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In 1917, Einstein amended his general relativity theory to introduce what he called the “cosmological constant,” a mathematical way to counter the force of gravity on a cosmological scale and stave off the collapse of the universe, which the general relativity theory posited. At the time, astronomers believed that the Milky Way was surrounded by an infinite and static void. In 1923, Edwin Hubble and other astronomers find the first stars beyond the galaxy and by 1929 Hubble provides evidence that space is expanding. Einstein realized the cosmological constant was a blunder. Or perhaps not. In 1998, scientists made the startling discovery that the expansion of the universe is speeding up, driven by an anti-gravity force called dark energy, which in many ways acts like Einstein’s cosmological constant. Pictured here is the Hubble Space Telescope’s extreme deep field view, which contains about 5,500 galaxies. The telescope is named after Edwin Hubble.

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One of the first implications of the general relativity theory was the realization that if an object is compressed enough, the dimple it generates in the fabric of space and time will be too strong for even photons of light to escape. Thus, the idea of black holes was born. Though they can’t be directly observed, astronomers have found black holes of all sizes by measuring how they affect nearby stars and gas. Pictured here is an artist’s rendering of a black hole named Cygnus X-1, siphoning matter from a nearby star.

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Like ripples in a pond, scientists believe that gravity transmits in waves, deforming space and time across the universe. It is similar to the movement of electromagnetic radiation, which propagates in waves, except that gravitational waves are moving the fabric of space and time itself. So far, attempts to find gravitational waves, such as those caused by two black holes colliding for example, have been unsuccessful. Next week, the European Space Agency plans to launch a prototype space-based observatory called the evolved Laser Interferometer Space Antenna (eLISA) to test a technology to find gravitational waves. Pictured above is an artist's rendering of two merging galaxies rippling space and time.

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