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FQXI ARTICLE

June 30, 2015

Time Dilation Gets a Quantum Twist

Quantum vs general relativistic conceptions of time go head-to-head in a proposed table-top test.

FQXi Awardees: Caslav Brukner

October 1, 2012

Caslav Brukner

University of Vienna

Just talking about such an experiment has enticed experimentalists to Brukner’s door. It has long been assumed that the overlap between quantum theory, which governs the behavior of the very small, and general relativity, which deals with how planets and stars warp spacetime on cosmic scales, lies way beyond experimental reach—perhaps only within the center of black holes. But surprisingly, Brukner believes there may be a way to test these two theories much closer to home.

This would not be a test of quantum gravity—a theory uniting quantum mechanics and general relativity—itself, Brukner is quick to add. But he and his colleagues, Magdalena Zych, Fabio Costa and Igor Pikowski, are proposing a test in which the effects of both quantum mechanics and general relativity on a clock are important. Their idea, outlined in the journal

Particle Clocks

The team’s twist on this quantum classic hinges on an important aspect of the double-slit experiment: quantum particles do not like to be spied on. If you watch to see which path a particle takes—whether it passed through the right or left slit—you destroy this wave-like behavior and the interference pattern disappears. Instead, the particles appear to have shot through the slits like bullets. Brukner’s team has combined this effect, known as

This vanishing of interference

will really be a proof that

there was a general relativistic

notion of time involved.

will really be a proof that

there was a general relativistic

notion of time involved.

- Caslav Brukner

But here’s the kicker: quantum complementarity says that the clocks can only continue to behave as waves if there is no significant time dilation effect between the two paths. That’s because, if there is a discernible time dilation, you would be able to look at the clock and deduce which path it had taken, based on whether it seemed to have ticked faster or slower

The experiment pits two conceptions of time—the quantum mechanical and the general relativistic—head to head. On one side, the double-slit experiment puts the clock into a quantum superposition—a blurry confusion of multiple identities. We should not know which path it took during the experiment, and the time shown on the clock is undefined. This is in contrast with general relativity in which time has an objective status: it is well-defined at single points. "In this experiment the time shown by the clock becomes quantum mechanically indefinite, that is, before it is measured it has no predetermined value," says Brukner.

On the Fringe

The standard double-slit experiment creates a distinctive pattern of fringes.

Will the team’s proposed experiment destroy it?

Credit: Jordgette

It is a difficult practical undertaking because the separation so far achieved in experiments that can maintain the required superposition between the two paths is small, so it is tough to accumulate enough of a difference in gravitational potential between the two paths to discern a time dilation effect. The effect would be very tiny—to see a difference of the order of a quadrillionth of a second (10

There’s also the question of what to use as a clock: molecules that have different rotational and vibrational internal dynamics could be used. Arndt is frequently approached by colleagues about the relevance of such ‘internal clocks’ in macromolecules. In that sense, Brukner’s proposal did not come as a full surprise, he says. But their use for exploring time dilation gives it a conceptually important new twist.

Getting a molecule to move slow enough, so that it accumulates sufficient time dilation, is another issue to resolve. "One would first have to prepare a suitable rotational state that acts as the hand of the clock…in small molecules this might be done," says Arndt. In short, the experiment is far from trivial, "nothing for next year…" muses Arndt. "But the proposal addresses conceptual questions of quantum mechanics and should for that reason be experimentally realized."

What will the result of the experiment be? Only time will tell.

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PENTCHO VALEV wrote on May 29, 2015

Did Einstein Tell Einsteinians How to Leapfrog into the Future?

Brian Greene: "Time Travel is Possible (2:48) If you wanted to leapfrog into the future, if you wanted to see what the Earth would be like a million years from now, Einstein told us how to do that."

Brian Cox (03:56): "Time travel into the future is possible".

Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this...

Did Einstein Tell Einsteinians How to Leapfrog into the Future?

Brian Greene: "Time Travel is Possible (2:48) If you wanted to leapfrog into the future, if you wanted to see what the Earth would be like a million years from now, Einstein told us how to do that."

Brian Cox (03:56): "Time travel into the future is possible".

Thibault Damour: "The paradigm of the special relativistic upheaval of the usual concept of time is the twin paradox. Let us emphasize that this...

PENTCHO VALEV wrote on May 28, 2015

Reductio ad Absurdum in Einstein's Relativity

It follows from Einstein's 1905 constant-speed-of-light postulate that, in the bug-rivet scenario, the bug is both dead and alive. Einsteinians camouflage the absurdity by introducing two additional absurdities: 1. The rivet shank length miraculously increases beyond its at-rest length. 2. "The end of the rivet will just keep on going [at 87% the speed of light!] until this wave, typically travelling at the speed of sound, reaches it." ...

Reductio ad Absurdum in Einstein's Relativity

It follows from Einstein's 1905 constant-speed-of-light postulate that, in the bug-rivet scenario, the bug is both dead and alive. Einsteinians camouflage the absurdity by introducing two additional absurdities: 1. The rivet shank length miraculously increases beyond its at-rest length. 2. "The end of the rivet will just keep on going [at 87% the speed of light!] until this wave, typically travelling at the speed of sound, reaches it." ...

PENTCHO VALEV wrote on May 25, 2015

Reductio ad Absurdum in Einstein's Relativity

It follows from Einstein's 1905 constant-speed-of-light postulate that unlimitedly long objects can be trapped inside unlimitedly short containers, and that during the trapping the objects undergo compression and do not undergo compression at the same time:

"The simplest version of the problem involves a garage, with a front and back door which are open, and a ladder which, when at rest with respects to the garage, is too long to fit...

Reductio ad Absurdum in Einstein's Relativity

It follows from Einstein's 1905 constant-speed-of-light postulate that unlimitedly long objects can be trapped inside unlimitedly short containers, and that during the trapping the objects undergo compression and do not undergo compression at the same time:

"The simplest version of the problem involves a garage, with a front and back door which are open, and a ladder which, when at rest with respects to the garage, is too long to fit...

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