How can spacetime be curved

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. They theorised that during the early expansion of the universe, tiny gravitational waves would have disturbed the light around them, which would have been amplified into a larger pattern as the universe expanded, coalescing into these patterns in the cosmic microwave background.

The announcement was made before the BICEP2 data went through more rigorous analysis and feedback from their colleagues. Instead, it looked likely that the patterns of light were not caused by gravitational waves, but instead by the dust inside our own galaxy as it interacted with magnetic fields. The successful LIGO experiment has ushered in a new era of astronomy. Before now, astronomers have largely focused on the study of the electromagnetic spectrum including light and radio waves.

What we do know is that this technique will allow us to better understand the most massive objects in the universe such as black holes, neutron stars, and supernovae; and it will provide us with a new window to study how the universe formed. One unanswered question is whether or not gravity is propagated by the graviton—the proposed but so-far undetected particle responsible for gravitational interactions.

Even more pressing, we know that general relativity is, in its current form, incompatible with the other pillar of modern physics: quantum mechanics GLOSSARY quantum mechanics A branch of physics that explains how the universe works on incredibly tiny scales atomic and subatomic. But it has produced many unexpected, unintuitive predictions that have been confirmed again and again for over a hundred years. This has been one of the greatest journeys in the history of science, involving not just Newton and Einstein, but thinkers and doers all around the world who have worked to put these theories to the test.

Even so, the schism between relativity and quantum mechanics remains. However, there are a few theories—stringy, loopy, multi-dimensional theories—unproven but with promise of becoming the next milestone in understanding our cosmos. Understanding gravity—warps and ripples in space and time Expert reviewers.

Isaac Newton and Albert Einstein were pivotal in advancing our understanding of gravity. Newton and the laws of gravity Newton published one of the most celebrated works of science, the Principia , in Newton realised that gravity was responsible for objects falling to the ground and for the orbit of celestial objects.

Newton was well aware of this when he said, Gravity must be caused by an agent acting constantly according to certain laws; but whether this agent be material or immaterial, I have left to the consideration of my readers.

Isaac Newton. Experiments in a smooth-moving vehicle yield the same results as experiments conducted on land. Space and time are linked Almost years after Galileo, Einstein pondered the consequences of relativity in the context of an important factor: the speed of light.

Show labels Stop Start. In this theory, space is not an empty void, but an invisible structure called spacetime. Nor is space simply a three-dimensional grid through which matter and energy moves. It is a four-dimensional structure whose shape is determined by the presence of matter and energy. Wherever he puts any object—himself, a ruler, a triangle, or anything—the thing stretches itself because of the thermal expansion.

Everything is longer in the hot places than it is in the cold places, and everything has the same coefficient of expansion. Now we are going to imagine that our bugs begin to study geometry. They can draw lines, and they can make rulers, and measure off lengths. They learn how to make a straight line—defined as the shortest line between two points.

Our first bug—see Fig. But what happens to the bug on the sphere? He draws his straight line as the shortest distance— for him —between two points, as in Fig. He just knows that if he tries any other path in his world it is always longer than his straight line.

So we will let him have his straight line as the shortest arc between two points. It is, of course an arc of a great circle. Finally, our third bug—the one in Fig. We think you get the idea now that all the rest of the analysis will always be from the point of view of the creatures on the particular surfaces and not from our point of view.

You can figure out how they could do it. Then our first bug the one on the normal plane finds an interesting fact. Then he discovers another interesting thing. See Fig. Then he invents the circle. A circle is made this way: You rush off on straight lines in many many directions from a single point, and lay out a lot of dots that are all the same distance from that point.

Of course, its equivalent to the curve you can make by swinging a ruler around a point. Anyway, our bug learns how to make circles. Then one day he thinks of measuring the distance around a circle.

If he follows the prescription we gave above, he would probably think that the result was hardly worth the trouble. He gets a figure like the one shown in Fig. Get a sphere and try it. A similar thing would happen to our friend on the hot plate. If he lays out four straight lines of equal length—as measured with his expanding rulers—joined by right angles he gets a picture like the one in Fig.

Then as they tried to make accurate squares on a larger scale they would discover that something was wrong. The point is, that just by geometrical measurements they would discover that something was the matter with their space. We define a curved space to be a space in which the geometry is not what we expect for a plane. The geometry of the bugs on the sphere or on the hot plate is the geometry of a curved space.

The rules of Euclidean geometry fail. You can find out that you live on a ball by laying out a square. If the square is very small you will need a lot of accuracy, but if the square is large the measurement can be done more crudely.

Our friend on the sphere can find triangles that are very peculiar. He can, for example, find triangles which have three right angles. Yes indeed! One is shown in Fig.

Suppose our bug starts at the north pole and makes a straight line all the way down to the equator. Then he makes a right angle and another perfect straight line the same length.

Then he does it again. For the very special length he has chosen he gets right back to his starting point, and also meets the first line with a right angle. As the triangle gets bigger the discrepancy goes up.

The bugs on the hot plate would discover similar difficulties with their triangles. They make circles and measure their circumferences. For example, the bug on the sphere might make a circle like the one shown in Fig. Our bug on the hot plate would discover a similar phenomenon.

Suppose he was to draw a circle centered at the cold spot on the plate as in Fig. You can think of others. We have given two different examples of curved space: the sphere and the hot plate. But it is interesting that if we choose the right temperature variation as a function of distance on the hot plate, the two geometries will be exactly the same.

It is rather amusing. We can make the bug on the hot plate get exactly the same answers as the bug on the ball. If you assume that the length of the rulers as determined by the temperature goes in proportion to one plus some constant times the square of the distance away from the origin, then you will find that the geometry of that hot plate is exactly the same in all details 1 as the geometry of the sphere.

There are, of course, other kinds of geometry. We could ask about the geometry of a bug who lived on a pear, namely something which has a sharper curvature in one place and a weaker curvature in the other place, so that the excess in angles in triangles is more severe when he makes little triangles in one part of his world than when he makes them in another part.

In other words, the curvature of a space can vary from place to place. It can also be imitated by a suitable distribution of temperature on a hot plate. We may also point out that the results could come out with the opposite kind of discrepancies. First of all, we could have a hot plate with the temperature decreasing with the distance from the center. Then all the effects would be reversed. But we can also do it purely geometrically by looking at the two-dimensional geometry of the surface of a saddle.

Imagine a saddle-shaped surface like the one sketched in Fig. This circle is a curve that oscillates up and down with a scallop effect. Spheres and pears and such are all surfaces of positive curvatures; and the others are called surfaces of negative curvature.

In general, a two-dimensional world will have a curvature which varies from place to place and may be positive in some places and negative in other places. In general, we mean by a curved space simply one in which the rules of Euclidean geometry break down with one sign of discrepancy or the other. The amount of curvature—defined, say, by the excess radius—may vary from place to place. We might point out that, from our definition of curvature, a cylinder is, surprisingly enough, not curved.

If a bug lived on a cylinder, as shown in Fig. By continuing to use the site you are agreeing to our use of cookies. OK Find out more about our cookie policy. Toggle navigation Toggle navigation. Toggle mission navigation. Missions Show All Missions. Asset Publisher Spacetime curvature. Spacetime curvature.