And both observers would be right. Einstein later illustrated this point with another thought experiment. Imagine that you once again have an observer standing on a railway embankment as a train goes roaring by. Because the lightning strikes are the same distance from the observer, their light reaches his eye at the same instant.

So he correctly says that they happened simultaneously. Meanwhile, another observer on the train is sitting at its exact midpoint. From her perspective, the light from the two strikes also has to travel equal distances, and she will likewise measure the speed of light to be the same in either direction. But because the train is moving, the light coming from the lightning in the rear has to travel farther to catch up, so it reaches her a few instants later than the light coming from the front.

Since the light pulses arrived at different times, she can only conclude the strikes were not simultaneous—that the one in front actually happened first. Once you accept that, all the strange effects we now associate with relativity are a matter of simple algebra.

Einstein dashed off his ideas in a fever pitch and sent his paper in for publication just a few weeks later. Einstein kept obsessing on relativity all through the summer of , and in September he sent in a second paper as a kind of afterthought. It was based on yet another thought experiment. And now imagine that it spontaneously emits two identical pulses of light in opposite directions. Now, said Einstein, what would this process look like to a moving observer?

From her perspective, the object would just keep moving in a straight line while the two pulses flew off.

With a little more algebra, Einstein showed that for all this to be consistent, the object not only had to lose energy when the light pulses departed, it had to lose a bit of mass, as well. Or, to put it another way, mass and energy are interchangeable.

## 10 Things Einstein Got Right

Einstein wrote down an equation that relates the two. By Mitch Waldrop. May Lightning Strikes a Moving Train.

LIGO and Virgo regularly look for these gravitational monotones by targeting hundreds of known pulsars learn about our standard search for continuous waves from known pulsars. The detection of a continuous wave would offer a unique opportunity to study gravitational-wave polarizations. This is because the long duration of such a signal would make it possible to distinguish the effect of the different polarizations, even with a single detector. This is generally not possible with the current network of gravitational-wave detectors and the short-lived signals detected so far.

In this study, we have for the first time looked for gravitational waves coming from a set of pulsars without assuming that the signals are polarized as predicted by Einstein. As in previous studies , we have used information about these pulsars obtained through radio and gamma-ray observations, allowing us to accurately track any potential gravitational wave signal in our data over the whole length of the three-month run a technique called "coherent integration". While we did not detect any signals, we have produced the first upper limits for beyond-Einstein strain from any of these pulsars.

This is important because, due to the nature of continuous-wave searches, previous analyses would have missed signals of nonstandard polarizations, even if they were loud. Also, these limits can, in principle, be translated into constraints on specific extensions of general relativity.

With these results, we have also demonstrated the robustness of the search method, which uses rigorous statistics to allow us to look for all polarizations in an efficient way. In the future, the search will be expanded to be sensitive to signals at more frequencies. Representation of the six polarizations permitted in general "metric" theories of gravity.

In these two cases, the spacetime distortion is in the plane perpendicular to the direction in which the gravitational wave travels i. Panels c to f denote polarizations that are not permitted in general relativity. Panel c again shows a transverse polarization, while panels d to f illustrate distortions that propagate in a direction shown by the arrow that lies in the same plane as the spacetime distortion.

### Gravitational radiation

For more information on how this figure was generated and its meaning, see the preprint at arXiv. These plots show our upper limits on the amplitude of continuous waves as a function of the expected gravitational-wave frequency. Each dot represents one analyzed pulsar, while the lines represent the sensitivity of the detectors. The top two plots correspond to signals with non-standard polarizations, while the bottom plot corresponds to the polarizations predicted by general relativity.

First Search for Continuous Gravitational Waves Beyond General Relativity Einstein's general relativity is our most successful theory of space, time, and gravity.

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## General Relativity

Glossary Metric theory : Metric theories are a large class of theories of gravity characterized by the fact that they describe the effect of gravity on matter and energy via a simple mathematical object called a metric tensor. General relativity is a metric theory, and so are essentially all its viable alternatives e. Brans-Dicke gravity. For more on alternative theories , see this review technical. Neutron star : The extremely dense remnant of the core of a massive star, born in a supernova explosion. Pulsars : Neutron stars that have been observed through the pulses of electromagnetic radiation usually in the radio band that they emit.

A large fraction of the neutron stars we expect to exist cannot be observed as pulsars, either because they do not emit electromagnetic radiation, or because their electromagnetic radiation is not emitted in the direction of Earth.

## Gravity - Gravitational fields and the theory of general relativity | ipixaradugit.tk

Observing run : A period of observation in which data are taken. Upper limit : A statement on the maximum value some quantity can have while still being consistent with the data. Here, the quantity of interest is the maximum intrinsic gravitational-wave strain amplitude of a given continuous-wave signal arriving at Earth. Strain : Fractional change in the distance between two measurement points due to the deformation of spacetime by a passing gravitational wave.

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