# Generally Speaking: A Primer on General Relativity

##### Apr 13, 2006

“The one sentence statement of general relativity is that ‘gravity is the curvature of spacetime,’” explains Dr. Sean Carroll, assistant professor of physics at the University of Chicago. “Really, the differences come in understanding what that sentence means.”

Carroll says that origin of the theory of general relativity dates to 1905, when scientists, notably including Albert Einstein, realized that space and time are related characteristics of a four-dimensional existence. “When you meet someone for coffee,” says Carroll, “you have to give four numbers of where to meet. Three of them are in space — latitude, longitude, and height above ground — and the fourth is what time to meet.”

However, within this new 4-D framework, says Carroll, Einstein could not understand gravity, and how it worked in spacetime. He decided that rather than being a force, like electromagnetism, gravity must be a property: a geometric curvature. Even though we agree that the angles of a triangle add up to 180 degrees, this rule changes when a curve is involved. When that same triangle is put on a sphere, the angles add up to more than 180 degrees. Likewise, when the curvature of spacetime is recognized, the basic rules thought to apply to gravity are changed.

Lately, though, general relativity has been looked at closely. Carroll says that while no evidence exists for the overthrow of the theory of general relativity, there are some points where general relativity may not apply. “General relativity is doing really well,” he explains to PhysOrg.com, “but there are two places where it might break down.”

These two places, Carroll says, have to do with very short distances and on very large scales. With very short distances, in terms of quantum mechanics, there are problems with gravity and with general relativity. The theory does not apply in the same way as it does with longer spacetime distances. “In classical general relativity, spacetime has a geometry; in quantum gravity, there should be a wave function that tells us what the likelihood is that spacetime has one of various geometries,” Carroll explains. Even though no experiment exists yet that has cracked the theory of quantum gravity, a new test is being developed in Europe to try and work toward just that (read about it on PhysOrg.com: http://phys.org/news12054.html).

The other breakdown might occur on large scales. There is still much about the larger scales that remain hypothetical. General relativity is one of those things. “There is still a question of how much curvature is caused by a certain amount of energy and mass,” says Carroll. “Einstein suggested an equation that related energy to the curvature of spacetime, but it may be right in some circumstances and not in others.” He explains that breaking down dark energy and matter is necessary to understand the implications, but that, so far, their existence is only known through their gravity. “That could be a sign that general relativity breaks down at this scale.”

Carroll also addresses the case of special relativity. “Special relativity is special because it is a special case of general relativity. General relativity is, well, general, and special relativity is one particular case.”

In the case of special relativity, gravity is “turned off.” Carroll explains that gravity can be ignored in this subset because it is such a weak force. “Special relativity deals with the idea that different people moving at different velocities will have different perceptions of what they see, and gravity is not taken into account.” But, he continues, work with particle accelerators show that special relativity is extremely accurate for many experiments.

Understanding general relativity is more a function of realizing that gravity is a property of spacetime, and one of its properties is gravity, which is actually a curvature. The effects we see, explains Carroll, comes from the fact that particles cannot move in a straight line. “Particles are trying to move in straight lines,” he says, “but there are no straight lines because spacetime is curved.”

By Miranda Marquit, Copyright 2006 PhysOrg.com

Explore further: Progress in the fight against quantum dissipation

## Related Stories

#### A Matter of Fact: NASA Finds Direct Proof of Dark Matter

Aug 21, 2006

Dark matter and normal matter have been wrenched apart by the tremendous collision of two large clusters of galaxies. The discovery, using NASA's Chandra X-ray Observatory and other telescopes, gives direct ...

#### Is there another world in the mirror, Case physicist asks

Oct 25, 2005

Like Lewis Carroll’s Alice, who steps through the looking glass into a strange world, Lawrence Krauss, Case Western Reserve University professor of physics, began his search for extra dimensional worlds with the Twilight Zo ...

#### Scientists mount ambitious experiments, propose dramatic new theories about dark energy

Feb 18, 2005

A panel of physicists and astronomers will preview emerging theories and experiments aimed at solving the mystery of dark energy, an invisible force that dominates the universe, from 1:45 to 4:45 p.m. Friday, Feb. 18, at ...

## Recommended for you

#### Scientists observe quantum superconductor-metal transition and superconducting glass

1 hour ago

An article published in Nature Physics on March 30, 2014, presents the results of the first experimental study of graphene-based quantum phase transition of the "superconductor-to-metal" type, i.e. transformation of the sy ...

#### New technique detects microscopic diabetes-related eye damage

1 hour ago

Indiana University researchers have detected new early-warning signs of the potential loss of sight associated with diabetes. This discovery could have far-reaching implications for the diagnosis and treatment ...

#### Researchers find tin selenide shows promise for efficiently converting waste heat into electrical energy

3 hours ago

(Phys.org) —A team of researchers working at Northwestern University has found that tin selenide (SnSe) has the highest Carnot efficiency for a thermoelectric cycle ever found, making it potentially a possible ...

#### After 13 years, progress in pitch-drop experiment (w/ video)

5 hours ago

(Phys.org) —As Cyclone Ita hit northern Australia last weekend, a much slower collision occurred in the world's longest-running lab project, The University of Queensland's Pitch Drop Experiment.

#### Robotics goes micro-scale

6 hours ago

(Phys.org) —The development of light-driven 'micro-robots' that can autonomously investigate and manipulate the nano-scale environment in a microscope comes a step closer, thanks to new research from the ...

#### Interview with Gerhard Rempe about the fascination of and prospects for quantum information technology

6 hours ago

Gerhard Rempe, Director at the Max Planck Institute of Quantum Optics in Garching, and his colleagues investigate the fundamentals of quantum information technology.

## More news stories

#### Scientists observe quantum superconductor-metal transition and superconducting glass

An article published in Nature Physics on March 30, 2014, presents the results of the first experimental study of graphene-based quantum phase transition of the "superconductor-to-metal" type, i.e. transformation of the sy ...

#### Researchers find tin selenide shows promise for efficiently converting waste heat into electrical energy

(Phys.org) —A team of researchers working at Northwestern University has found that tin selenide (SnSe) has the highest Carnot efficiency for a thermoelectric cycle ever found, making it potentially a possible ...

#### New technique detects microscopic diabetes-related eye damage

Indiana University researchers have detected new early-warning signs of the potential loss of sight associated with diabetes. This discovery could have far-reaching implications for the diagnosis and treatment ...

#### With neutrons, scientists can now look for dark energy in the lab

It does not always take a huge accelerator to do particle physics: First results from a low energy, table top alterative takes validity of Newtonian gravity down by five orders of magnitude and narrows the ...

#### Scientists capture ultrafast snapshots of light-driven superconductivity

A new study pins down a major factor behind the appearance of superconductivity—the ability to conduct electricity with 100 percent efficiency—in a promising copper-oxide material.

#### Archaeological, genetic evidence expands views of domestication

Many of our ideas about domestication derive from Charles Darwin, whose ideas in turn were strongly influenced by British animal-breeding practices during the 19th century, a period when landowners vigorously ...

#### Pocket-sized anthrax detector aids global agriculture

A credit-card-sized anthrax detection cartridge developed at Sandia National Laboratories and recently licensed to a small business makes testing safer, easier, faster and cheaper.

#### PsiKick's batteryless sensors poised for coming 'Internet of things'

Research from the University of Virginia and the universities of Michigan and Washington is the foundation of a startup company, PsiKick, that plans to manufacture the lowest-power wireless sensors in the ...

#### Researchers discover the seat of sex and violence in the brain

As reported in a paper published online today in the journal Nature, Caltech biologist David J. Anderson and his colleagues have genetically identified neurons that control aggressive behavior in the mouse ...

#### In sex-reversed cave insects, females have the penises

Researchers reporting in the Cell Press journal Current Biology on April 17 have discovered little-known cave insects with rather novel sex lives. The Brazilian insects, which represent four distinct but re ...