A century ago this year, a young Swiss physicist, who had already revolutionized physics with discoveries about the relationship between space and time, developed a radical new understanding of gravity.
In 1915, Albert Einstein published his general theory of relativity, which described gravity as a fundamental property of space-time. He came up with a set of equations that relate the curvature of space-time to the energy and momentum of the matter and radiation that are present in a particular region.
Today, 100 years later, Einstein’s theory of gravitation remains a pillar of modern understanding, and has withstood all the tests that scientists could throw at it. But until recently, it wasn’t possible to do experiments to probe the theory under extreme conditions to see whether it breaks down.
Now, scientists have the technology to begin looking for evidence that could reveal physics beyond general relativity.
“To me, it is absolutely amazing how well general relativity has done after 100 years,” said Clifford Will, a theoretical physicist at the University of Florida in Gainesville. “What he wrote down is the same thing we use today,” Will told Live Science.
A new view of gravity
General relativity describes gravity not as a force, as the physicist Isaac Newton thought of it, but rather as a curvature of space and time due to the mass of objects, Will said. The reason Earth orbits the sun is not because the sun attracts Earth, but instead because the sun warps space-time, he said. (This is a bit like the way a bowling ball on an outstretched blanket would warp the blanket’s shape.)
Einstein’s theory made some pretty wild predictions, including the possibility of black holes, which would warp space-time to such a degree that nothing inside — not even light — could escape. The theory also provides the foundation for the currently accepted view that the universe is expanding, and also accelerating.
General relativity has been confirmed through numerous observations. Einstein himself famously used the theory to predict the orbital motion of the planet Mercury, which Newton’s laws cannot accurately describe. Einstein’s theory also predicted that an object that was massive enough could bend light itself, an effect known as gravitational lensing, which astronomers have frequently observed. For example, the effect can be used to find exoplanets, based on slight deviations in the light of a distant object being bent by the star the planet is orbiting.
But while there hasn’t been “a shred of evidence” that there’s anything wrong with the theory of general relativity, “it’s important to test the theory in regimes where it hasn’t been tested before,” Will told Live Science.
Testing Einstein’s theory
General relativity works very well for gravity of ordinary strength, the variety experienced by humans on Earth or by planets as they orbit the sun. But it’s never been tested in extremely strong fields, regions that lie at the boundaries of physics.
The best prospect for testing the theory in these realms is to look for ripples in space-time, known as gravitational waves. These can be produced by violent events such as the merging of two massive bodies, such as black holes or extremely dense objects called neutron stars.
These cosmic fireworks would produce only the tiniest blip in space-time. For instance, such an event could alter a seemingly static distance on Earth. If, say, two black holes collided and merged in the Milky Way galaxy, the gravitational waves produced would stretch and compress two objects on Earth that were separated by 3.3 feet (1 meter) by one-thousandth the diameter of an atomic nucleus, Will said.
Yet there are now experiments out there that could potentially detect space-time ripples from these types of events.
“There’s a very good chance we will be detecting [gravitational waves] directly in the next couple of years,” Will said.
The Laser Interferometer Gravitational-Wave Observatory (LIGO), with facilities near Richland, Washington, and Livingston, Louisiana, uses lasers to detect miniscule distortions in two long, L-shaped detectors. As space-time ripples pass through the detectors, the ripples stretch and compress space, which can change the length of the detector in a way that LIGO can measure.
LIGO began operations in 2002 and has not detected any gravitational waves; in 2010, it went offline for upgrades, and its successor, known as Advanced LIGO, is scheduled to boot up again later this year. A host of other experiments also aim to detect gravitational waves.
Another way to test general relativity in extreme regimes would be to look at the properties of gravitational waves. For example, gravitational waves can be polarized, just like light as it passes through a pair of polarized sunglasses.
General relativity makes predictions about this polarization, so “anything that deviates from [these predictions] would be bad” for the theory, Will said.
A unified understanding
If scientists do detect gravitational waves, however, Will expects it will only bolster Einstein’s theory. “My opinion is, we’re going to keep proving general relativity to be right,” he said.
So why bother doing these experiments at all?
One of the most enduring goals of physics is the quest for a theory that unites general relativity, the science of the macroscopic world, and quantum mechanics, the realm of the very small. Yet finding such a theory, known as quantum gravity, may require some modifications to general relativity, Will said.
It’s possible that any experiment capable of detecting the effects of quantum gravity would require so much energy as to be practically impossible, Will said. “But you never know — there may be some strange effect from the quantum world that is tiny but detectable.”
Scientists have long known that light can behave as both a particle and a wave—Einstein first predicted it in 1909. But no experiment has been able to show light in both states simultaneously. Now, researchers at the École Polytechnique Fédérale de Lausanne in Switzerland have taken the first ever photograph of light as both a wave and a particle. The key was a new experimental technique that uses electrons to capture the light’s movement. The work was published today in the journal Nature Communications.
To get this snapshot, the researchers shot laser pulses at a nanowire. The wavelengths of light moved in two different directions along the metal. When the waves ran into each other, they look liked a wave standing still, which is effectively a particle.
In order to see how the waves were moving, the researchers shot a beam of electrons at the nanowire, like dropping dye in a river to see the currents. The particles in the light wave changed the speed at which the electrons moved. That enabled the researchers to capture an image just as the waves met.
“This experiment demonstrates that, for the first time ever, we can film quantum mechanics – and its paradoxical nature – directly,” said Fabrizio Carbone, one of the authors of the study, in a press release. Carbone hopes that a better understanding of how light functions can jumpstart the field of quantum computing.
Technology has changed rapidly over the last few years with touch feedback, known as haptics, being used in entertainment, rehabilitation and even surgical training. New research, using ultrasound, has developed an invisible 3-D haptic shape that can be seen and felt.
The research paper, published in the current issue of ACM Transactions on Graphics and which will be presented at this week’s SIGGRAPH Asia 2014 conference [3-6 December], demonstrates how a method has been created to produce 3D shapes that can be felt in mid-air.
The research, led by Dr Ben Long and colleagues Professor Sriram Subramanian, Sue Ann Seah and Tom Carter from the University of Bristol’s Department of Computer Science, could change the way 3D shapes are used. The new technology could enable surgeons to explore a CT scan by enabling them to feel a disease, such as a tumour, using haptic feedback.
The method uses ultrasound, which is focussed onto hands above the device and that can be felt. By focussing complex patterns of ultrasound, the air disturbances can be seen as floating 3D shapes. Visually, the researchers have demonstrated the ultrasound patterns by directing the device at a thin layer of oil so that the depressions in the surface can be seen as spots when lit by a lamp.
The system generates an invisible 3D shape that can be added to 3D displays to create something that can be seen and felt. The research team have also shown that users can match a picture of a 3D shape to the shape created by the system.
Dr Ben Long, Research Assistant from the Bristol Interaction and Graphics (BIG) group in the Department of Computer Science, said: “Touchable holograms, immersive virtual reality that you can feel and complex touchable controls in free space, are all possible ways of using this system.
“In the future, people could feel holograms of objects that would not otherwise be touchable, such as feeling the differences between materials in a CT scan or understanding the shapes of artefacts in a museum.”
Tom Broadhurst, an Ikerbasque researcher at the University of the Basque Country (UPV/EHU), has participated alongside scientists of the National Taiwan University in a piece of research that explores cold dark matter in depth and proposes new answers about the formation of galaxies and the structure of the Universe. These predictions, published in the journal Nature Physics, are being contrasted with fresh data provided by the Hubble space telescope.
In cosmology, cold dark matter is a form of matter the particles of which move slowly in comparison with light, and interact weakly with electromagnetic radiation. It is estimated that only a minute fraction of the matter in the Universe is baryonic matter, which forms stars, planets and living organisms. The rest, comprising over 80%, is dark matter and energy.
The theory of cold dark matter helps to explain how the universe evolved from its initial state to the current distribution of galaxies and clusters, the structure of the Universe on a large scale. In any case, the theory was unable to satisfactorily explain certain observations, but the new research by Broadhurst and his colleagues sheds new light in this respect.
As the Ikerbasque researcher explained, “guided by the initial simulations of the formation of galaxies in this context, we have reinterpreted cold dark matter as a Bose-Einstein condensate.” So, “the ultra-light bosons forming the condensate share the same quantum wave function, so disturbance patterns are formed on astronomic scales in the form of large-scale waves.”
This theory can be used to suggest that all the galaxies in this context should have at their center large stationary waves of dark matter called solitons, which would explain the puzzling cores observed in common dwarf galaxies.
The research also makes it possible to predict that galaxies are formed relatively late in this context in comparison with the interpretation of standard particles of cold dark matter. The team is comparing these new predictions with observations by the Hubble space telescope.
The results are very promising as they open up the possibility that dark matter could be regarded as a very cold quantum fluid that governs the formation of the structure across the whole Universe.
This is not Thomas Broadhurst’s first publication in the journal Nature. In 2012, he participated in a piece of research on a galaxy of the epoch of the reionization, a stage in the early universe not explored previously and which could be the oldest galaxy discovered. This research opened up fresh possibilities to conduct research into the first galaxies to emerge after the Big Bang.
Tom Broadhurst has a PhD in Physics from the University of Durham (United Kingdom); until joining Ikerbasque he did his research at top research centres in the United Kingdom, United States, Germany, Israel, Japan and Taiwan. He has had 184 papers published in leading scientific journals, and so far has received 11,800 citations. In 2010, he was recruited by Ikerbasque and carries out his work in the UPV/EHU’s department of Theoretical Physics. His line of research focusses on observational cosmology, dark matter and the formation of galaxies.