The idea is that an electromagnetic wave has a non-zero energy-momentum tensor, and should therefore curve spacetime, albeit in a small and strange way. In this way, the equations of general relativity imply that the spacetime curvature created by propagating light should influence the propagation of that light itself. This question originally appeared on Quora - the place to gain and share knowledge, empowering people to learn from others and better understand the world.
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Learn about the Moon in a great new book New book chronicles the space program. Dave's Universe Year of Pluto. Groups Why Join? Astronomy Day. The Complete Star Atlas. All objects with mass warp space-time around them; the more massive an object, the more pronounced the warping it causes. When photons travel through the region near a massive object that has caused significant warping, they follow curved paths because the space-time through which they are moving is curved.
While it is true that photons have no mass, it is also true that we see light bend around sources with high mass due to gravity. Once Einstein had formulated the equivalence principle, gravity became less mysterious. He could apply his knowledge of acceleration to better understand gravity. It can also mean a change in direction, like when you go round a roundabout, causing you to lean towards the side of the car. The cylinder is rotated faster and faster until the acceleration eases and the movement stays constant.
But even once the speed is constant, you still feel the accelerated motion—you feel yourself being pinned to the outer edge of the ride. So if someone stood in the very centre of the ride perhaps held by a brace, stopping them from falling to the edge , they would notice all those weird effects we saw under special relativity—that those on the edge will contract in length, and their clocks will tick at a slower rate.
The equivalence principle tells us that the effects of gravity and acceleration are indistinguishable. In thinking about the example of the cylindrical ride, we see that accelerated motion can warp space and time. It is here that Einstein connected the dots to suggest that gravity is the warping of space and time.
Gravity is the curvature of the universe, caused by massive bodies, which determines the path that objects travel. That curvature is dynamical, moving as those objects move. To date, his predictions—as strange as they may sound—have all stood the test of time. Light travels through spacetime, which can be warped and curved—so light should dip and curve in the presence of massive objects. This effect was first observed in , analysing starlight during a solar eclipse. Astronomers found that starlight that passed very close to the sun was very slightly offset in position compared to the same starlight when measured at night.
Similar to how the passage of time is changed under special relativity, general relativity predicts that massive objects will also dilate time. The more massive the object, the more noticeable the effect. On board each satellite is an atomic clock, and your position on the planet can be determined by checking the time broadcast by the satellites above you and comparing those times against the known position of each satellite.
Both effects have been confirmed by a range of experiments , including the Gravity Probe B satellite. Equipped with extremely sensitive gyroscopes, this satellite measured the tiny twists and warps in spacetime made by Earth as it moves and rotates through space.
Since the curvature of spacetime is dynamical, moving objects should create ripples in space that permeate through the universe. Most of these ripples are too small to notice, but the more extreme the event, the higher the chance we can detect it.
Imagine two very massive objects, such as black holes. If those objects were to collide, they could potentially create an extreme disturbance in the fabric of spacetime, moving outwards like the ripples in a pond. But how far away could such waves be felt? Einstein predicted that gravitational waves existed, but believed they would be too small to detect by the time they reached us here on Earth. So it was with great excitement that on February 11 , the scientific community was abuzz with the announcement that a gravitational wave GLOSSARY gravitational wave Ripples in spacetime that propagate outwards like waves had been detected.
We needed instruments capable of detecting a signal one-ten-thousandth the diameter of a proton 10 meter. In the LIGO experiment, a laser is directed into a large tunnel structure. At the end of each arm, a mirror reflects the light from the laser back to where it came from, and the two beams merge back into one. Normally, the laser beams should recombine at exactly the same time. But if a gravitational wave comes rippling through space while the detectors are switched on, that ripple will stretch one arm of the L-shaped structure before stretching the other.
The gravitational wave distorts the passage of the light, resulting in a particular kind of interference light pattern detected at the end. On 11 February , the LIGO teams announced the direct discovery of a gravitational wave matching the signal predicted from the collision of two black holes. Astronomers at the Background Imaging of Cosmic Extragalactic Polarization BICEP2 telescope had supposedly discovered evidence of gravitational waves, but that evidence was later recalled, as it did not pass closer scrutiny.
Rather than listening for the direct signal of a gravitational wave as it rolled past our planet the setup at LIGO , the BICEP2 team analysed swirls of light within the cosmic microwave background GLOSSARY cosmic microwave background The faint remnant of light that permeates the whole universe, left over from the heat of the big bang.
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