The Equivalence Principle Says That

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May 18, 2007: Standing on the Moon in 1971, Apollo 15 astronaut Dave Scott held his easily out at shoulder summit, a hammer in i hand and a plume in the other. And as the world looked on via live television, he let go.

Information technology was an odd sight: the feather didn't migrate to the ground, it plummeted, falling but as fast as the hammer. Without air resistance to slow the feather, the 2 objects hit moondust at the aforementioned instant.

Right: Astronaut Dave Scott drops a feather and hammer on the moon. [Video] [Transcript]

"What practise y'all know!" exclaimed Scott. "Mr. Galileo was right."

Scott was referring to a famous experiment of the 16th century. Depending on who tells the story, Galileo Galilei either dropped balls from the top of the Leaning Belfry of Pisa or he rolled balls down slopes at abode. Either way, the result was the aforementioned: Although the balls were made of different materials, they all reached lesser at the same time.

Today, this is known as "the equivalence principle." Gravity accelerates all objects equally regardless of their masses or the materials from which they are made. It'south a cornerstone of modern physics.

But what if the equivalence principle (EP) is wrong?

Galileo's experiments were just accurate to near i%, leaving room for doubt, and skeptical physicists take been "testing EP" ever since. The best modernistic limits, based on, due east.g., laser ranging of the Moon to measure out how fast it falls around Earth, testify that EP holds within a few parts in a trillion (10¹²). This is fantastically accurate, yet the possibility remains that the equivalence principle could neglect at some more subtle level.

"Information technology's a possibility we must investigate," says physicist Clifford Volition of Washington University in St. Louis, Missouri. "Discovering fifty-fifty the slightest difference in how gravity acts on objects of dissimilar materials would take enormous implications."

In fact, it could provide the first real evidence for string theory. String theory elegantly explains fundamental particles as different vibrations of infinitesimal strings, and in doing so solves many lingering problems of modern physics. But string theory is highly controversial, in role considering well-nigh of its predictions are most impossible to verify with experiments. If it's not testable, information technology'south non science.

The equivalence principle could offering one way to test cord theory.

"Some variants of string theory predict the existence of a very weak force that would make gravity slightly different depending on an object'due south composition," says Will. "Finding a variation in gravity for different materials wouldn't immediately prove that cord theory is right, only it would requite the theory a dose of supporting evidence."

Right: Modernistic tests of the Equivalence Principle. Figure based on a similar diagram in a review article from Physics World. [More]

This new facet of gravity, if information technology exists, would exist and so astonishingly weak that detecting it is a tremendous challenge. Gravity itself is a relatively weak forcefulness—it's a trillion trillion trillion (10³⁶) times more feeble than electromagnetism. Theorists believe the new force would be at least ten million 1000000 (ten¹³) times weaker than gravity.

Just as magnetism acts on objects made of iron but not plastic, the new strength wouldn't touch on all thing every bit. The force's pull would vary depending on what the object is fabricated of.

For example, some versions of cord theory advise that this new strength would interact with the electromagnetic energy contained in a material. Ii atoms that have the same mass can comprise different amounts of electromagnetic free energy if, say, one has more protons, which have an electric charge, while the other has more neutrons, which have no charge. Traditional gravity would pull on both of these atoms as, but if gravity includes this new force, the pull on these two atoms would differ e'er and then slightly.

No experiment to date has detected this tiny difference. But now 3 groups of scientists are proposing space-borne missions that would chase for this effect with greater sensitivity than ever before.

"What y'all want to do is accept two test masses made of different materials and watch for very modest differences in how fast they autumn," Will says. "On World, an object tin only fall for a short time before it hits the footing. Only an object in orbit is literally falling around the Earth, so it can fall continuously for a long fourth dimension." Tiny differences in the pull of gravity would accumulate over time, perhaps growing large plenty to be detectable.

Ane test mission, called the Satellite Examination of the Equivalence Principle (STEP), is being developed by Stanford University and an international team of collaborators. STEP would be able to detect a deviation in the equivalence principle every bit small as ane role in a meg trillion (x¹⁸). That'due south 100,000 times more sensitive than the current best measurement.

Right: An artist's concept of STEP in orbit. [More]

Step's pattern uses four pairs of test masses instead of just ane pair. The redundancy is to ensure that any difference seen in how the test masses autumn is truly caused by a violation of the equivalence principle, and not by some other disturbance or imperfection in the hardware.

"When trying to measure out such a miniscule effect, you have to eliminate equally many external disturbances every bit possible," Volition explains. STEP'south design places the test masses inside a big tank of liquid helium to insulate them from external temperature fluctuations, and surrounds the masses with a superconducting shell to shield them from magnetic and electric interference. Microthrusters annul the effects of atmospheric elevate on the orbiting satellite, making the complimentary fall of the test masses nearly perfect.

In this pristine environment, each pair of test masses should stay perfectly aligned with each other as they fall around the Earth—that is, if the equivalence principle holds. But if this new component of gravity does exist, ane examination mass will fall at a slightly different charge per unit than its partner, then the pair will drift slightly out of alignment over time.

Currently, STEP is still in the pattern phase. Some other satellite-based experiment, the French-adult Micro-Satellite à traînée Compensée pour fifty'Observation du Principe d'Equivalence (MICROSCOPE), is scheduled to launch in 2010. MICROSCOPE will take two pairs of exam masses instead of iv, and will be able to detect a violation of the equivalence principle as modest equally 1 function in a million billion (x^xv).

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The tertiary experiment is the Italian satellite Galileo Galilei ("GG" for short), which volition operate in much the same way every bit Footstep and MICROSCOPE, except that it uses merely one pair of test masses. To improve its accuracy, the Galileo Galilei satellite volition spin about its central axis at a rate of 2 rotations per 2nd. That way, any disturbances inside the spacecraft will pull in all directions equally, thus canceling themselves out. The experiment should be able to attain a sensitivity of one part in a hundred million billion (10¹⁷).

Whether any of these missions stand a run a risk of detecting a violation of the equivalence principle is hard to say. Will says that he expects the experiments won't find any deviation, in part because finding 1 would be such a major revolution for modern physics. And string theory makes a range of predictions virtually how strong this new strength would be, so information technology's possible that the effect would be besides small for fifty-fifty these space-borne instruments to detect.

Finding no deviation would still be helpful: information technology would rule out some variants of string theory, inching physicists toward the correct "Theory of Everything." Just finding a deviation, however small, would be a behemothic leap.

Author: Patrick Barry | Editor: Dr. Tony Phillips | Credit: Science@NASA

More than Information

Annotation: The equivalence principle discussed in this story is now referred to by physicists equally the "weak equivalence principle" or WEP for short. In 1907, Einstein formulated a more comprehensive potent equivalence principle (SEP) for his General Theory of Relativity. The WEP is a subset of the SEP.

Space missions to "exam EP" -- STEP, MICROSCOPE, GG

The dwelling page of Clifford M. Will -- professor of physics at the University of Washington; President of the International Society on General Relativity and Gravitation; member of the National Academy of Sciences.

The legend of the leaning tower -- Historians are not certain if Galileo e'er carried out experiments at the Leaning Tower of Pisa. And then why, asks Robert P Crease, has the story become part of physics folklore?

Tests of the Weak Equivalence Principle (PhysicsWorld)

The Eöt-Wash Grouping: Laboratory Tests of Gravitational and sub-Gravitational Physics -- learn more almost recent laboratory tests of the Equivalence Principle

NASA's Future: The Vision for Space Exploration