When giving the coordinates for a location, most people provide the latitude, longitude and perhaps altitude. But there is a fourth dimension often neglected: time. The combination of the physical coordinates with the temporal element creates a concept known as space-time, a background for all events in the universe.

"In physics, space-time is the mathematical model that combines space and time into a single interwoven continuum throughout the universe," Eric Davis, a physicist who works at the Institute for Advanced Studies at Austin and with the Tau Zero Foundation, told Space.com by email. Davis specializes in faster-than-light space-time and anti-gravity physics, both of which use Albert Einstein's general relativity theory field equations and quantum field theory, as well as quantum optics, to conduct lab experiments.

"Einstein's special theory of relativity, published in 1905, adapted [German mathematician] Hermann Minkowski's unified space-and-time model of the universe to show that time should be treated as a physical dimension on par with the three physical dimensions of space — height, width and length — that we experience in our lives," Davis said. [Einstein's Theory of Relativity Explained (Infographic)]

"Space-time is the landscape over which phenomena take place," added Luca Amendola, a member of the Euclid Theory Working Group (a team of theoretical scientists working with the European Space Agency's Euclid satellite) and a professor at Heidelberg University in Germany. "Just as any landscape is not set in stone, fixed forever, it changes just because things happen — planets move, particles interact, cells reproduce," he told Space.com via email.

## The history of space-time

The idea that time and space are united is a fairly recent development in the history of science.

"The concepts of space remained practically the same from the early Greek philosophers until the beginning of the 20th century — an immutable stage over which matter moves," Amendola said. "Time was supposed to be even more immutable because, while you can move in space the way you like, you cannot travel in time freely, since it runs the same for everybody."

In the early 1900s, Minkowski built upon the earlier works of Dutch physicist Hendrik Lorentz and French mathematician and theoretical physicist Henri Poincare to create a unified model of space-time. Einstein, a student of Minkowski, adapted Minkowski's model when he published his special theory of relativity in 1905.

"Einstein had brought together Poincare's, Lorentz's and Minkowski's separate theoretical works into his overarching special relativity theory, which was much more comprehensive and thorough in its treatment of electromagnetic forces and motion, except that it left out the force of gravity, which Einstein later tackled in his magnum opus general theory of relativity," Davis said.

## Space-time breakthroughs

In special relativity, the geometry of space-time is fixed, but observers measure different distances or time intervals according to their own relative velocity. In general relativity, the geometry of space-time itself changes depending on how matter moves and is distributed.

"Einstein's general theory of relativity is the first major theoretical breakthrough that resulted from the unified space-time model," Davis said.

General relativity led to the science of cosmology, the next major breakthrough that came thanks to the concept of unified space-time.

"It is because of the unified space-time model that we can have a theory for the creation and existence of our universe, and be able to study all the consequences that result thereof," Davis said.

He explained that general relativity predicted phenomena such as black holes and white holes. It also predicts that they have an event horizon, the boundary that marks where nothing can escape, and the point of singularities at their center, a one dimensional point where gravity becomes infinite. General relativity could also explain rotating astronomical bodies that drag space-time with them, the Big Bang and the inflationary expansion of the universe, gravity waves, time and space dilation associated with curved space-time, gravitational lensing caused by massive galaxies, and the shifting orbit of Mercury and other planetary bodies, all of which science has shown true. It also predicts things such as warp-drive propulsions and traversable wormholes and time machines.

"All of these phenomena rely on the unified space-time model," he said, "and most of them have been observed."

An improved understanding of space-time also led to quantum field theory. When quantum mechanics, the branch of theory concerned with the movement of atoms and photons, was first published in 1925, it was based on the idea that space and time were separate and independent. After World War II, theoretical physicists found a way to mathematically incorporate Einstein's special theory of relativity into quantum mechanics, giving birth to quantum field theory.

"The breakthroughs that resulted from quantum field theory are tremendous," Davis said.

The theory gave rise to a quantum theory of electromagnetic radiation and electrically charged elementary particles — called quantum electrodynamics theory (QED theory) — in about 1950. In the 1970s, QED theory was unified with the weak nuclear force theory to produce the electroweak theory, which describes them both as different aspects of the same force. In 1973, scientists derived the quantum chromodynamics theory (QCD theory), the nuclear strong force theory of quarks and gluons, which are elementary particles.

In the 1980s and the 1990s, physicists united the QED theory, the QCD theory and the electroweak theory to formulate the Standard Model of Particle Physics, the megatheory that describes all of the known elementary particles of nature and the fundamental forces of their interactions. Later on, Peter Higgs' 1960s prediction of a particle now known as the Higgs boson, which was discovered in 2012 by the Large Hadron Collider at CERN, was added to the mix.

Experimental breakthroughs include the discovery of many of the elementary particles and their interaction forces known today, Davis said. They also include the advancement of condensed matter theory to predict two new states of matter beyond those taught in most textbooks. More states of matter are being discovered using condensed matter theory, which uses the quantum field theory as its mathematical machinery.

"Condensed matter has to do with the exotic states of matter, such as those found in metallic glass, photonic crystals, metamaterials, nanomaterials, semiconductors, crystals, liquid crystals, insulators, conductors, superconductors, superconducting fluids, etc.," Davis said. "All of this is based on the unified space-time model."

## The future of space-time

Scientists are continuing to improve their understanding of space-time by using missions and experiments that observe many of the phenomena that interact with it. The Hubble Space Telescope, which measured the accelerating expansion of the universe, is one instrument doing so. NASA's Gravity Probe B mission, which launched in 2004, studied the twisting of space-time by a rotating body — the Earth. NASA's NuSTAR mission, launched in 2012, studies black holes. Many other telescopes and missions have also helped to study these phenomena.

On the ground, particle accelerators have studied fast-moving particles for decades.

"One of the best confirmations of special relativity is the observations that particles, which should decay after a given time, take in fact much longer when traveling very fast, as, for instance, in particle accelerators," Amendola said. "This is because time intervals are longer when the relative velocity is very large."

Future missions and experiments will continue to probe space-time as well. The European Space Agency-NASA satellite Euclid, set to launch in 2020, will continue to test the ideas at astronomical scales as it maps the geometry of dark energy and dark matter, the mysterious substances that make up the bulk of the universe. On the ground, the LIGO and VIRGO observatories continue to study gravitational waves, ripples in the curvature of space-time.

"If we could handle black holes the same way we handle particles in accelerators, we would learn much more about space-time," Amendola said.

## Understanding space-time

Will scientists ever get a handle on the complex issue of space-time? That depends on precisely what you mean.

"Physicists have an excellent grasp of the concept of space-time at the classical levels provided by Einstein's two theories of relativity, with his general relativity theory being the magnum opus of space-time theory," Davis said. "However, physicists do not yet have a grasp on the quantum nature of space-time and gravity."

Amendola agreed, noting that although scientists understand space-time across larger distances, the microscopic world of elementary particles remains less clear.

"It might be that space-time at very short distances takes yet another form and perhaps is not continuous," Amendola said. "However, we are still far from that frontier."

Today's physicists cannot experiment with black holes or reach the high energies at which new phenomena are expected to occur. Even astronomical observations of black holes remain unsatisfactory due to the difficulty of studying something that absorbs all light, Amendola said. Scientists must instead use indirect probes.

"To understand the quantum nature of space-time is the holy grail of 21st century physics," Davis said. "We are stuck in a quagmire of multiple proposed new theories that don't seem to work to solve this problem."

Amendola remained optimistic. "Nothing is holding us back," he said. "It's just that it takes time to understand space-time."

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