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FQXI ARTICLE

August 2, 2015

Breaking the Universe’s Speed Limit

If we give up the idea that time exists and the speed of light is constant at the fundamental level, then we could find a theory of quantum gravity.

FQXi Awardees: John Donoghue

April 21, 2011

JOHN DONOGHUE

University of Massachusetts, Amherst

But what happens if you zoom in on the Rubik’s cube until you can see the very atoms that make it up? Suddenly the laws of physics have changed completely. What we saw on the macroscale is not the same on this microscale.

Merging the vastly different laws that govern the macro and the micro has been a huge challenge for physics. Now, John Donoghue, a physicist at the University of Massachusetts, Amherst, thinks he may have the answer. Perhaps, he argues, the familiar view of spacetime as a four-dimensional fabric, which we inherited from Einstein, is not fundamental, but only emerges on large scales—just like our picture of a solid and symmetrical Rubik’s cube disappears and re-appears depending on the perspective that we look at it. If he is correct, physicists may have to rethink one of their more cherished beliefs: that the speed of light has always been constant.

This idea would change

99.9% of physics research.

99.9% of physics research.

- John Donoghue

However, there’s good reason to think that our understanding of spacetime and, in turn, the speed of light, may need to be rewritten. The two cornerstones of modern physics, Einstein’s general relativity, which explains the behavior of stars and planets on the largest scales, and quantum mechanics, which governs the interactions of subatomic particles, each paint a different picture of the role of space and time. General relativity weaves space and time together into a four-dimensional fabric that can be warped by matter, while the equations of quantum mechanics use an immutable absolute clock to measure out the regular ticks as time passes. This difference has led some physicists to ponder whether spacetime changes character on different scales.

Emerging Spacetime

Physicists use the term

THE SPEED OF GRAVITY

Artist’s impression of the gravitational waves caused by two

orbiting black holes.

Credit: K. Thorne & T. Carnahan, Caltech/NASA

Jan Ambjørn, a physicist at the Niels Bohr Institute in Copenhagen, Denmark, is a fan of Donoghue’s work. Asking whether the speed of light is emergent is, "a totally legal question," he says. We still have trouble understanding what is happening in the early universe, so "it might be that some new perspective is needed," he adds.

Eleanor Knox, an expert on emergent theories of spacetime, at King’s College London agrees that Donoghue’s ideas are "a good way forward." However, she notes that until he and his colleagues have a more specific driving theory, it will be difficult to know where to look for evidence of an emerging speed of light.

Making Waves

With a grant of almost $90,000 from FQXi, Donoghue hopes to address that issue, by refining his theory so that he can make specific predictions about where to look for experimental signs of an emerging speed limit. Unfortunately, most of the effects of differing speeds of light would only be noticeable at extremely high energies—far greater than even the famed Large Hadron Collider, the particle accelerator in Geneva, Switzerland, can test.

Asking whether the speed

of light is emergent

is a totally legal question.

of light is emergent

is a totally legal question.

- Jan Ambjørn

Other FQXi researchers are studying how different theories about character of spacetime should affect some high-energy particles. Cosmic rays and bursts of high-energy radiation, known as gamma rays, travel huge distances across the universe, and over the course of their long journey the tiny effects of unusual spacetime structure might accumulate to an observable level. Donoghue has a student looking at this too. "I’d like to be able to report that we found something that showed evidence, but we haven’t," he says. But the team has set a limit on how big any effects due to emergence could be. "That’s also a bit of progress because we’ve made a connection that people hadn’t thought of before," Donoghue says.

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AMRIT SRECKO SORLI wrote on March 28, 2015

speed of light is variable and depend on energy density of quntum vacuum.

speed of light is variable and depend on energy density of quntum vacuum.

PENTCHO VALEV wrote on February 7, 2014

The Most Crucial Question in Relativity

A light source emits a series of pulses the distance between which is d (e.g. d=300000km).

A stationary observer/receiver measures the frequency of the pulses to be f=c/d.

An observer/receiver moving with speed v towards the light source measures the frequency of the pulses to be f'=(c+v)/d.

The most crucial question:

Why does the frequency shift from f=c/d to f'=(c+v)/d ?

Answer 1 (fatal for relativity): Because...

The Most Crucial Question in Relativity

A light source emits a series of pulses the distance between which is d (e.g. d=300000km).

A stationary observer/receiver measures the frequency of the pulses to be f=c/d.

An observer/receiver moving with speed v towards the light source measures the frequency of the pulses to be f'=(c+v)/d.

The most crucial question:

Why does the frequency shift from f=c/d to f'=(c+v)/d ?

Answer 1 (fatal for relativity): Because...

PENTCHO VALEV wrote on January 18, 2014

Special Relativity Is Obviously False II

The observer starts moving away from the light source with speed Vo, and accordingly the speed of the light waves relative to him shifts from c to c'=c-Vo, in violation of special relativity.

As a result, " in a time t the number of waves which reach the observer are those in a distance (c-Vo)t, so the number of waves observed is (c-Vo)t/lambda, giving an observed frequency f'=f(1-Vo/c) ":

Tony Harker, University College London: "The...

Special Relativity Is Obviously False II

The observer starts moving away from the light source with speed Vo, and accordingly the speed of the light waves relative to him shifts from c to c'=c-Vo, in violation of special relativity.

As a result, " in a time t the number of waves which reach the observer are those in a distance (c-Vo)t, so the number of waves observed is (c-Vo)t/lambda, giving an observed frequency f'=f(1-Vo/c) ":

Tony Harker, University College London: "The...

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