Why relativity

Even something as basic as inertia the resistance of your car to move until forced to by the engine, and its tendency to keep moving after you take your foot off the accelerator can be thought of as connected to the gravitational field of every other particle in the universe.

Consider a thought problem, closely related to the one that originally led Einstein to this idea in What if the universe were entirely empty except for two astronauts?

One of them is spinning, the other is stationary. The spinning one feels dizzy, doing cartwheels in space. But which one of the two is spinning? Without any external reference, Einstein argued, there is no way to say which one is correct, and no reason why one should feel an effect different from what the other experiences.

The distinction between the two astronauts makes sense only when you reintroduce the rest of the universe. In the classic interpretation of general relativity, then, inertia exists only because you can measure it against the entire cosmic gravitational field. What holds true in that thought problem holds true for every object in the real world: the behaviour of each part is inextricably related to that of every other part. It is also, Smolin thinks, a promising way to obtain bigger answers about how nature really works, across all scales.

Smolin is keenly aware that he is pushing against the prevailing devotion to small-scale, quantum-style thinking. Much as all of the parts of the universe are linked across space, they may also be linked across time, he suggests. His arguments led him to hypothesise that the laws of physics evolve over the history of the universe. Over the years, he has developed two detailed proposals for how this might happen.

His theory of cosmological natural selection, which he hammered out in the s, envisions black holes as cosmic eggs that hatch new universes. More recently, he has developed a provocative hypothesis about the emergence of the laws of quantum mechanics, called the principle of precedence — and this one seems much more readily put to the test.

If you perform an experiment that has been performed before, you expect the outcome will be the same as in the past. Strike a match and it bursts into flame; strike another match the same way and… you get the idea.

Smolin hypothesises that those laws actually may emerge over time, as quantum systems copy the behaviour of similar systems in the past. One possible way to catch emergence in the act is by running an experiment that has never been done before, so there is no past version that is, no precedent for it to copy.

Such an experiment might involve the creation of a highly complex quantum system, containing many components that exist in a novel entangled state. If the principle of precedence is correct, the initial response of the system will be essentially random. As the experiment is repeated, however, precedence builds up and the response should become predictable… in theory. Although precedence can play out at the atomic scale, its influence would be system-wide, cosmic.

Getting the two classes of physics theories to work together, though important, is not enough, either. What he wants to know — what we all want to know — is why the universe is the way it is. Why does time move forward and not backward? How did we end up here, with these laws and this universe, not some others? Like Hogan, he is less concerned about the outcome of any one experiment than he is with the larger programme of seeking fundamental truths.

For Smolin, that means being able to tell a complete, coherent story about the universe; it means being able to predict experiments, but also to explain the unique properties that made atoms, planets, rainbows and people. Here again he draws inspiration from Einstein. The most likely way to get the big answers is to engage with the universe as a whole. If you wanted to pick a referee in the big-small debate, you could hardly do better than Sean Carroll, an expert in cosmology, field theory and gravitational physics at Caltech.

He knows his way around relativity, he knows his way around quantum mechanics, and he has a healthy sense of the absurd: he calls his personal blog Preposterous Universe. Right off the bat, Carroll awards most of the points to the quantum side. That has been the prevailing view ever since the s, when Einstein tried and repeatedly failed to find flaws in the counterintuitive predictions of quantum theory. Taking a larger view, the real issue is not general relativity versus quantum field theory, Carroll explains, but classical dynamics versus quantum dynamics.

Relativity, despite its perceived strangeness, is classical in how it regards cause and effect; quantum mechanics most definitely is not. Einstein was optimistic that some deeper discoveries would uncover a classical, deterministic reality hiding beneath quantum mechanics, but no such order has yet been found. The demonstrated reality of spooky action at a distance argues that such order does not exist. In all these ways, he is true to the mainstream, quantum-based thinking in modern physics.

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UTC Offset:. Picture of the Day Image Galleries. Watch : Mining the Moon for rocket fuel. Queen guitarist Brian May and David Eicher launch new astronomy book. Last chance to join our Costa Rica Star Party! 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. Einstein's theory of general relativity replaced Newton's gravity. Now, gravitational wave observations of black holes might stretch the limits of Albert's masterpiece.

The event horizon of the supermassive black hole at the center of the Milky Way appears dark against a bright maelstrom of swirling gas and infalling matter.

The Laser Interferometer Gravitational-wave Observatory LIGO and Virgo collaborations had caught ripples in space-time itself: the wake of two black holes that collided and merged more than a billion light-years away. The advance illustrated here is exaggerated to show detail. Is it complete? Although relativity has passed every test with flying colors, gaps exist that have driven research for decades.

Each of the other three forces is mediated by particles: Photons carry the electromagnetic force, gluons carry the strong nuclear force, and W and Z bosons carry the weak nuclear force. But no one has yet observed the corresponding particle that should carry the gravitational force — the graviton — though current theories say it should exist. Forging a black hole To make a black hole, you need to compress a lot of mass into a very small space.

All stars spend most of their lives fusing hydrogen into helium in their cores. The energy this produces creates an outward pressure that balances the inward pull of gravity.

After a star exhausts its core hydrogen, it eventually starts to fuse helium into carbon. The 1. Astronomers hope to compare such models with images of the black hole made with the Event Horizon Telescope, to look for any deviations from general relativity. A weighty particle Gravitational waves also may reveal physics beyond relativity in other ways, notes Kent Yagi, a theoretical astrophysicist at the University of Virginia.

One way is simply by constraining parameters like the mass of the graviton. If this particle has no mass, then gravitational waves should move at the speed of light, he says. Strong aurorae dazzle astronauts on space station.

Long trips to space linked to possible brain damage. First crewed Artemis Moon landing delayed until at least Gift ideas for astronomy lovers and stargazers — holiday gift guide. Snapshot : ALMA spots moon-forming disk around distant exoplanet.

The first 'space hotel' plans to open in Cosmos: Origin and Fate of the Universe. Astronomy's Moon Globe. Galaxies by David Eicher. Astronomy Puzzles. Jon Lomberg Milky Way Posters. Astronomy for Kids. One of the problems is that it is hard to predict definitely what the payback of basic physics will be, though few dispute that physics is somehow "good.

Physicists have become adept at finding good examples of the long-term benefit of basic physics: the quantum theory of solids leading to semiconductors and computer chips, nuclear magnetic resonance leading to MRI imaging, particle accelerators leading to beams for cancer treatment.

But what about Einstein's theories of special and general relativity? One could hardly imagine a branch of fundamental physics less likely to have practical consequences. But strangely enough, relativity plays a key role in a multi-billion dollar growth industry centered around the Global Positioning System GPS.

When Einstein finalized his theory of gravity and curved spacetime in November , ending a quest which he began with his special relativity, he had little concern for practical or observable consequences. He was unimpressed when measurements of the bending of starlight in confirmed his theory. Even today, general relativity plays its main role in the astronomical domain, with its black holes, gravity waves and cosmic big bangs, or in the domain of the ultra-small, where theorists look to unify general relativity with the other interactions, using exotic concepts such as strings and branes.

But GPS is an exception. The system is based on an array of 24 satellites orbiting the earth, each carrying a precise atomic clock.