## Archive for the ‘**RELATIVITY**’ Category

## Sound clocks and sonic relativity

**Scott L. Todd and Nicolas C. Menicucci**

Sound propagation within certain non-relativistic condensed matter models obeys a relativistic wave equation despite such systems admitting entirely non-relativistic descriptions.

A natural question that arises upon consideration of this is, “do devices exist that will experience the relativity in these systems?”

We describe a thought experiment in which ‘acoustic observers’ possess devices called sound clocks that can be connected to form chains.

Careful investigation shows that appropriately constructed chains of stationary and moving sound clocks are perceived by observers on the other chain as undergoing the relativistic phenomena of length contraction and time dilation by the Lorentz factor, γ, with c the speed of sound. Sound clocks within moving chains actually tick less frequently than stationary ones and must be separated by a shorter distance than when stationary to satisfy simultaneity conditions.

Stationary sound clocks appear to be length contracted and time dilated to moving observers due to their misunderstanding of their own state of motion with respect to the laboratory. Observers restricted to using sound clocks describe a universe kinematically consistent with the theory of special relativity, despite the preferred frame of their universe in the laboratory.

Such devices show promise in further probing analogue relativity models, for example in investigating phenomena that require careful consideration of the proper time elapsed for observers…

Read more at https://arxiv.org/pdf/1612.06870v1.pdf

## Echoes from the Abyss

**Evidence for Planck-scale structure at black hole horizons**

Jahed Abedi, Hannah Dykaar, Niayesh Afshordi

In classical General Relativity (GR), an observer falling into an astrophysical black hole is not expected to experience anything dramatic as she crosses the event horizon. However, tentative resolutions to problems in quantum gravity, such as the cosmological constant problem, or the black hole information paradox, invoke significant departures from classicality in the vicinity of the horizon. It was recently pointed out that such near-horizon structures can lead to late-time echoes in the black hole merger gravitational wave signals that are otherwise indistinguishable from GR. We search for observational signatures of these echoes in the gravitational wave data released by advanced Laser Interferometer Gravitational-Wave Observatory (LIGO), following the three black hole merger events GW150914, GW151226, and LVT151012. In particular, we look for repeating damped echoes with time-delays of 8MlogM (+spin corrections, in Planck units), corresponding to Planck-scale departures from GR near their respective horizons. Accounting for the “look elsewhere” effect due to uncertainty in the echo template, we find tentative evidence for Planck-scale structure near black hole horizons at 2.9σ significance level (corresponding to false detection probability of 1 in 270). Future data releases from LIGO collaboration, along with more physical echo templates, will definitively confirm (or rule out) this finding, providing possible empirical evidence for alternatives to classical black holes, such as in firewall or fuzzball paradigms.

Read more at https://arxiv.org/pdf/1612.00266v1.pdf

Read also https://briankoberlein.com/2016/12/03/echoes-from-the-abyss/

## Relativity Gets Thorough Vetting from LIGO

**The gravitational wave signal observed by the LIGO detectors shows no deviation from what general relativity predicts.**

## Detecting “Christodoulou memory effect” with LIGO

**Detecting gravitational-wave memory with LIGO: implications of GW150914**

It may soon be possible for Advanced LIGO to detect hundreds of binary black hole mergers per year. We show how the accumulation of many such measurements will allow for the detection of gravitational-wave memory: a permanent displacement of spacetime that comes from strong-field, general relativistic effects. We estimate that Advanced LIGO operating at design sensitivity may be able to make a signal-to-noise ratio 3(5) detection of memory with ∼35(90) events with masses and distance similar to GW150914. Given current merger rate estimates (of one such event per ∼16 days), this could happen in as few as ∼1.5(4) years of coincident data collection. We highlight the importance of incorporating higher-order gravitational-wave modes for parameter estimation of binary black hole mergers, and describe how our methods can also be used to detect higher-order modes themselves before Advanced LIGO reaches design sensitivity.

Read more at http://arxiv.org/pdf/1605.01415v1.pdf

## Is General Relativity a (partial) Return of Aristotelian Physics?

**Herbert Pietschmann**

Aristotle has split physics at the sphere of the moon; above this sphere there is no change except eternal spherical motion, below are two different kinds of motion: Natural motion (without specific cause) and enforced motion. In modern view motion is caused by gravity and by other forces. The split at the sphere of the moon has been definitely overcome through the observation of a supernova and several comets by Tycho Brahe. The second distinction was eradicated by Isaak Newton who showed that gravitational motion was caused by a force proportional to the inverse square of the distance. By the theory of General Relativity, Albert Einstein showed that there is no gravitational force but motion under gravity (i.e. Aristotles ) is caused by the curved geometry of spacetime. In this way, the Aristotelian distinction between natural motion and enforced motion has come back in the form of two great theories: General Relativity and Quantum Field Theory which are today incompatible. To find a way out of this dilemma is the challenge of modern physics.

Read more at https://arxiv.org/ftp/arxiv/papers/1604/1604.06491.pdf

## LIGO Magazine

LIGO Magazine is published twice a year by the LIGO Scientific Collaboration and details the latest research, news and personalities across the diverse group of members. It is available in pdf format as a free download from this website. The next issue will be published in September 2016.

**Issue 8, March 2016**

– See more at: http://www.ligo.org/magazine/#sthash.c8lh3ge3.dpuf

## The black-hole collision that reshaped physics

**A momentous signal from space has confirmed decades of theorizing on black holes — and launched a new era of gravitational-wave astronomy.grav**Read more at www.nature.com