Gravitational Waves vs. Gravity Waves: Know the Difference!

Gravity waves, gravitational waves and primordial gravitational waves... what do they mean? Is there a difference?

So it looks like we'll be talking a lot about gravitational waves over the coming days, but why can't they be called "gravity waves"? In this social media world where brevity is key, it may seem that chopping "gravitational" to "gravity" is no big deal - it saves a whole six characters for an even more concise tweet!

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Though you'll likely see many news headlines heralding the wonders of "gravity wave science", do not fall into the trap! While both have gravity in common, gravity waves and gravitational waves are two very different beasts. Read on to find out why and then show off your gravitational smarts to your friends the next time you're down the pub.

Gravitational Waves are, in their most basic sense, ripples in spacetime. Einstein's theory of general relativity predicted them over a century ago and they are generated by the acceleration (or, indeed, deceleration) of massive objects in the cosmos. If a star explodes as a supernova, gravitational waves carry energy away from the detonation at the speed of light. If two black holes collide, they will cause these ripples in spacetime to propagate like ripples across the surface of a pond. If two neutron stars orbit each other very closely, energy is carried away from the system by - you guessed it - gravitational waves. If we could detect and observe these waves, a new era of gravitational wave astronomy may be possible, allowing us to differentiate between gravitational wave signatures and work out which phenomenon is generating them. For example, a sudden pulse of gravitational waves may indicate they came from a supernova explosion, whereas a continuous oscillating signal may indicate two closely-orbiting black holes before merging.

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So far, gravitational waves are theoretical, even though strong indirect evidence for their existence is known. Interestingly, as gravitational waves propagate through spacetime, they will physically warp the "fabric" of space, very slightly shrinking or expanding the space between two objects. The effect is minuscule, but using laser interferometers - such as the Laser Interferometer Gravitational-Wave Observatory, or LIGO, that measures the tiniest perturbations in lasers reflected along 2.5 mile-long L-shaped vacuum tunnels - the propagation of gravitational waves through our planet may be detected. In the case of LIGO, there are 2 stations located on opposite sides of the US separated nearly 2,000 miles. If a gravitational wave signal is real, its signature will be observed at both locations; if it's a false positive (i.e. a truck driving past) only one station will detect it. Though LIGO started operations in 2002, it has yet to detect gravitational waves, but in September 2015, the system was upgraded to Advanced LIGO and hopes are high that, finally, physicists may have some good news for us on Thursday.

Extra Credit: Primordial gravitational waves. You may remember the kerfuffle about the BICEP2 "discovery" (and then un-discovery) of gravitational waves in the weak primordial "glow" of the Big Bang - known as the cosmic microwave background (CMB). Although the BICEP2 "discovery" turned out to be a dud, it is believed that tiny gravitational perturbations around the time of the Big Bang may leave their "fingerprint" in this ancient radiation as a special kind of polarized light. Should the fingerprint of primordial gravitational waves (i.e. gravitational waves produced by the Big Bang) be observed, certain models for cosmic inflation and quantum gravity may be confirmed. However, these are not the gravitational waves that LIGO is hunting for - LIGO (and other observatories like it) is looking for gravitational waves being generated by energetic cosmic events happening right now in our modern universe. The hunt for primordial gravitational waves is more of an archaeological dig into our universe's past.

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Gravity Waves are physical perturbations driven by the restoring force of gravity in a planetary environment. In other words, gravity waves are specific to planetary atmospheres and bodies of water. In the case of atmospherics, as air blows across an ocean and then encounters an island, for example, that air will be forced to rise. Downwind from the island, the air will be forced to a lower altitude by gravity, but its buoyancy will work against gravity forcing it aloft again. The result is often a region of oscillating air in the atmosphere that can produce clouds in the waves' crests (or highest points) as moisture from lower altitude condenses. Also, in the case of oceans, surface gravity waves form at the atmosphere/water interface; wind blows the surface out of equilibrium causing the restoring force of gravity to force the surface back down, while the water's buoyancy pushes it back up. Wind-driven waves, tides and tsunamis are all examples of gravity waves.

So, the upshot is that gravity drives both gravitational waves and gravity waves, but they have very different properties that shouldn't be confused.

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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