Saturday, May 27, 2023

- 4023 - GRAVITY WAVES - and String Theory?

 

-    4023  -  GRAVITY  WAVES  -  and String Theory?     Blackholes are exotic objects serve as important test studies. If the researchers can discover an important observational difference between “topological solitons” and traditional black holes, this might pave the way to finding a way to test string theory itself.


------------------   4023   -  GRAVITY  WAVES  -  and String Theory?

-  Gravitational-wave detector LIGO is back and can now spot more colliding black holes than ever.  The twin gravitational-wave detectors have started a new observation run after a major upgrade.

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-    The “Laser Interferometer Gravitational-Wave Observatory” (LIGO), which has two massive detectors in Hanford, Washington, and Livingston, Louisiana, is now restarting with improved sensitivity after a multimillion-dollar upgrade. The improvements should allow the facility to pick up signals from colliding black holes every two to three days, compared with once a week or so during its previous run in 2019 and 2020.

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-    Also the “Virgo” detector near Pisa, Italy, which has undergone its own   $9-million upgrade, was meant to join in, but technical issues are forcing its team to extend its shutdown and perform further maintenance.

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-    “KAGRA”, another gravitational-wave detector located under Mount Ikenoyama, Japan, is also restarting on 24 May, 2023. Its technology, although more advanced it is being fine-tuned, and its sensitivity is still lower than LIGO’s was in 2015.  The team will cool the interferometer’s four main mirrors to 20 kelvin.   That sets KAGRA apart from the other detectors that will serve as the model for next-generation observatories.

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-    Gravitational waves are produced by large, accelerating masses, and the waves cyclically stretch and compress the fabric of space as they travel. Starting with LIGO’s historic first detection in 2015, most of the 90 or so gravitational-wave events recorded so far have been from the spiralling motion of pairs of black holes in the process of merging into one; a handful have been produced similarly by the merger of two neutron stars or a neutron star and a black hole.

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-    LIGO, Virgo and KAGRA are all based on the same interferometer concept, which involves splitting a laser beam into two and bouncing the resulting beams between two mirrors at either end of a long vacuum pipe.

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-    At LIGO, the two ‘arms’ of the interferometer are each 4 kilometers long; at Virgo and KAGRA, they are 3 km. The two b eams then come back and are made to overlap at a sensor in the middle. In the absence of any disturbances to space-time, the beams’ oscillations cancel one another out. But the passage of gravitational waves causes the arms to change in length with respect to each other, so that the waves don’t overlap perfectly, and the sensor detects this signal.

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-    Typical gravitational-wave events change the length of the arms by only a fraction of the width of a proton. Sensing such minute changes requires painstaking isolation from noise coming from the environment and from the lasers themselves.

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-    In upgrades carried out before the 2019–2020 run, LIGO and Virgo tackled some of this noise with a technique called light squeezing. This approach deals with inherent noise caused by the fact that light is made of individual particles: when the beams arrive at the sensor, each individual photon can arrive slightly too early or too late, which means that the laser waves don’t overlap and cancel out perfectly even in the absence of gravitational waves.

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-    It’s like dropping a bucket of Bbs, lead pellets.   It’s going to make a loud hiss, but they all hit randomly.   Light squeezing injects an auxiliary laser beam into the interferometer that reduces that effect. Its photons arrive more regularly, with less noise.

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-    The implementation of light squeezing has helped LIGO and Virgo to improve the detectors’ sensitivities to higher-frequency gravitational waves.  But because of the bizarre rules of quantum mechanics, reducing the uncertainty in the arrival time of the photons increases random fluctuations in the laser waves’ intensity.

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-    This causes the lasers to push on the interferometer mirrors and make them jitter, adding a different type of noise and potentially reducing their sensitivity to low-frequency gravitational waves.  You cannot make an infinitely precise measurement: you have to pay the price somewhere else.

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-    To deal with this issue, an important change in the most-recent upgrades of both LIGO and Virgo has been to build extra 300-metre-long vacuum pipes with mirrors at the ends, to store the auxiliary ‘squeezing’ beam for 2.5 milliseconds before injecting it into the interferometer.

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-    The role of these pipes is to shift the waves of the auxiliary laser by distinct amounts depending on their wavelengths. This means that squeezing will be selective: it will decrease the noise at high frequency while also reducing mirror jitter at low frequencies.

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-    With the improved sensitivity of the detectors, researchers will be able to extract more-detailed information about the spiralling objects that produce gravitational waves, including how each spins around its axis and how they revolve around each other.

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-    This means putting Albert Einstein’s general theory of relativity, which predicts the existence of both black holes and gravitational waves, to stricter tests than ever before. The sheer number of observations will improve the big picture of how, and how often, black holes form from massive stars that collapse in on themselves.

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-    Gravitational waves will reveal distinct types of signal in addition to those from black-hole mergers. One major hope is to pick up the gravitational signal of a collapsing star before it manifests as a supernova explosion, a feat that will be possible only if the collapse occurs somewhere in the Galaxy.

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-    Another ambition is to sense the continuous gravitational waves produced by ruggedness in the surface of a pulsar, a spinning neutron star that emits pulses of radiation.

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-   The family of interferometers is due to expand by the end of the decade. The Indian government has announced that it will fund LIGO-India, a replica of the US observatories to be built in part with LIGO’s spare components.

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-    A team of theoretical physicists have discovered a strange structure in space-time that to an outside observer would look exactly like a black hole, but upon closer inspection would be anything but: they would be defects in the very fabric of the universe.

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-    Einstein's general theory of relativity predicts the existence of black holes, formed when giant stars collapse. But that same theory predicts that their centers are singularities, which are points of infinite density. Since we know that infinite densities cannot actually happen in the universe, we take this as a sign that Einstein's theory is incomplete. But after nearly a century of searching for extensions, we have not yet confirmed a better theory of gravity.

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-    But we do have candidates, including string theory. In string theory all the particles of the universe are actually microscopic vibrating loops of string. In order to support the wide variety of particles and forces that we observe in the universe, these strings can't just vibrate in our three spatial dimensions. Instead, there have to be extra spatial dimensions that are curled up on themselves into manifolds so small that they escape everyday notice and experimentation.

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-    Astrnomers found that these “topological solitons” are stable defects in space-time itself. They require no matter or other forces to exist.  They are as natural to the fabric of space-time as cracks in ice.

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-    The researchers studied these “solitons” by examining the behavior of light that would pass near them. Because they are objects of extreme space-time, they bend space and time around them, which affects the path of light. To a distant observer, these solitons would appear exactly as we predict black holes to appear. They would have shadows, rings of light, the works. Images derived from the Event Horizon Telescope and detected gravitational wave signatures would all behave the same.

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-   It's only once you got close would you realize that you are not looking at a black hole. One of the key features of a black hole is its event horizon, an imaginary surface that if you were to cross it you would find yourself unable to escape. Topological solitons, since they are not singularities, do not feature event horizons.

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These topological solitons are incredibly hypothetical objects, based on our understanding of string theory, which has not yet been proven to be a viable update to our understanding of physics.

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May 26, 2023         GRAVITY  WAVES  -  and String Theory?                  4023                                                                                                                        

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