The Design of Apparatus: The Need for Specifications.

Madhavi NandimathRoll.No. 06

Apparatus• An experiment involves the examination of some part of the

universe under specially contrived conditions• Apparatus is constructed for providing these required

conditions• Apparatus-Serves to hold certain variables constant and to

change other variables in a prescribed way• It often, but not always involves measurement.• An experiment is based on one or more hypothesis• Sometimes experiments give negative results, not because the

main hypothesis is false, but one of the auxiliary assumptions is either untrue or inapplicable to the situation.

• Originality is rare- Important advances simply by applying scheme developed one field can be effectively applied to other.

Specifications• Designer must make up specifications for every part,

however small, based on the duty that part is to perform.• These design questions can be qualitative or quantitative. If

does not know qualitative possibilities is unlikely to make decisions correctly. The experience is of great help.

• Qualitative decisions are usually harder and may require calculations, sometimes based on theories. Sometimes, estimating the lower or upper limit for some specification, may suffice for the design.

• In many situations theory is either inadequate for design purposes, or the labor of applying it is too great.

• If no previous experience is available to draw on, the choice of the right specifications has then to be left to trial and error.

Gravitational waves (GWs)• In 1916, the year after the final formulation of the field equations of general relativity,

Albert Einstein predicted the existence of GWs

GWs are 'ripples' in the fabric of space-time caused by massive objects moving with violent accelerations.

• Since Albert Einstein first predicted their existence a century ago, physicists have been on the hunt for GWs.

• In principle, GWs could exist at any frequency. Any object with mass that accelerates produces GWs.

• Space-time is very stiff and the distortion is quite small.-Weak signal• Massive & Accelerating particles.• The Universe is filled with incredibly massive objects that undergo rapid accelerations

(black holes, neutron stars, and stars at the ends of their lives).• While the origins of GWs can be extremely violent, by the time the waves reach the

Earth they are millions of times smaller and less disruptive. • In fact, by the time GWs from the first detection reached LIGO, the amount of space-

time wobbling they generated was thousands of times smaller than the nucleus of an atom!

• Such inconceivably small measurements are what LIGO was designed to make.

From prediction to reality: a history of the search for GWs

• 1915 - Albert Einstein publishes general theory of relativity, explains gravity as the warping of space-time by mass or energy

• 1916 - Einstein predicts massive objects whirling in certain ways will cause space-time ripples—GWs.

• 1936 - Einstein has second thoughts and argues in a manuscript that the waves don't exist—until reviewer points out a mistake

• 1962 - Russian physicists M. E. Gertsenshtein and V. I. Pustovoit publish paper sketch optical method for detecting GWs—to no notice

• 1969 - Physicist Joseph Weber claims GW detection using massive aluminum cylinders—replication efforts fail

• 1972 - Rainer Weiss of the Massachusetts Institute of Technology (MIT) in Cambridge independently proposes optical method for detecting waves

• 1974 - Astronomers discover pulsar orbiting a neutron star that appears to be slowing down due to gravitational radiation—work that later earns them a Nobel Prize

• 1979 - National Science Foundation (NSF) funds California Institute of Technology in Pasadena and MIT to develop design for LIGO

• 1990 - NSF agrees to fund $250 million LIGO experiment• 1992 - Sites in Washington and Louisiana selected for LIGO facilities; construction starts 2 years later• 1995 - Construction starts on GEO600 GW detector in Germany, which partners with LIGO and starts taking data

in 2002• 1996 - Construction starts on VIRGO GW detector in Italy, which starts taking data in 2007• 2002–2010 - Runs of initial LIGO—no detection of GWs• 2007 - LIGO and VIRGO teams agree to share data, forming a single global network of GW detectors• 2010–2015 - $205 million upgrade of LIGO detectors• 2015 - Advanced LIGO begins initial detection runs in September• 2016 - On 11 February, NSF and LIGO team announce successful detection of

GWs.

Joseph Weber: The Failed Search for GWs• The first physicist to search for the gravitational waves and attempted to detect

directly.Design & Specifications of the Apparatus

• A GW, will jiggle the shape of any physical body it encounters. To detect such disturbances, Joseph Weber designed an instrument consisted of multiple aluminium cylinders, 2 meters in length and 1 meter in diameter, antennae for detecting GWs.

• These massive aluminium cylinders vibrated at a resonance frequency of 1660 hertz and were designed to be set in motion by GWs predicted by Weber.

• A passing gravitational wave would set one of these cylinders vibrating at its resonant frequency–about 1660 hertz–and piezoelectric crystals firmly attached around the cylinder’s waist would convert that ringing into an electrical signal.

• Weber theorized that, these resonant bar detectors can be used to search for GWs around the 700-3000 Hz region, where they expect to find supernovae, merging neutron star and possibly even mini black holes.

• The basic requirement was to isolate the cylinders from vibration and from local seismic and electromagnetic disturbances.

• Significant source of background noise- Random thermal motion of the aluminum atoms. The vibrations should measure about a 10-millionth of a nanometer—about the same amount the bar would vibrate with thermal energy

• In 1969 he reported in PRL some two dozen coincident detections at the two locations ( one at Maryland another at Chicago, about 1,000 kilometers away) in an 81-day period.

• Coincidences by chance should happen only once in hundreds or thousands of years. This was “good evidence” for GWs.

• However, his experiments were duplicated many times, always with a null result (Prof. Tony Tyson, University of California, Davis). No one besides Weber ever saw anything but random noise.

• By the end of the 1970s it was generally thought that Weber had not actually detected GWs.

It seems to stem from the subtlety of statistics-simultaneous events.

• The random fluctuations happen to match by chance. Distinguishing a real signal from randomly paired events is hard, and it turned out that Weber’s analysis wasn’t sophisticated enough to distinguish the two.

• The project did stir up interest in GW detection. Modern efforts to detect GWs such as LIGO have expanded on Weber’s idea of measuring motion induced by GWs.

• Which just goes to show that even a failed experiment can be a success.

Pulsars: Gravitational Radiation & Indirect Evidence

• On 2 July 1974 the first signals were discovered from a binary pulsar (PSR1913+16), two neutron stars that orbit each other using a Arecibo radio telescope (Russell Hulse and Joseph Taylor).

• Pulsars- Rapidly rotating neutron stars that emit radio pulses in our direction. The timing of these pulses is very precise, so they can be used to measure the pulsar’s position and motion extremely accurately.

• The radio waves from a pulsar are emitted in two bunches which sweep across space at the same rate as the pulsar rotates (upper figure). From a binary pulsar, GWs are also emitted (lower figure).

Hypothesis: Gravitational Radiation (GR)• If an electric charge is accelerating, it will emit radiation. Correspondingly,

according to GR, accelerating masses should emit GR, a propagation deformation of space-time. Again, the variations are, however, small.

• A very important property of the new pulsar is that its pulse period, the time between two beacon sweeps (0.05903 see) has proved to be extremely stable, as opposed to what applies to many other pulsars. The pulsar's pulse period increases by less than 5% during 1 million years. This means that the pulsar can be used as a clock which for precision can compete with the best atomic clocks

• The two neutron stars in PSR1913+16 are moving so fast and close together in elliptical orbits. Their separation was very small.

• When the stars pass close to each other they will predictably emit large amounts of GR. This makes them lose energy: Their orbits will therefore shrink and their orbiting period will shorten. The change is very small. It corresponds to a reduction of the orbit period by about 75 millionths of a second per year, but, through observation over sufficient time, it is nevertheless fully measurable.

• The binary pulsar has been observed continuously since its discovery, and the orbiting period has in fact decreased. Agreement with the prediction of GR is better than ½ %.

• The good agreement between the observed value and the theoretically calculated value of the orbital path can be seen as an indirect proof of the existence of gravitational waves.

Ground Based Interferometers-LIGO: First direct detection of GWs

• On September 14, 2015 at 09:50:45 UTC, the LIGO observatories at Hanford, WA, and Livingston, LA detected a signal occurring within the 10-ms intersite propagation time named as GW150914.

• On February 12, 2016 LIGO scientific collaboration announced that the signal is indeed a ‘GW’.

•  The basic feature of GW150914 point to it being produced by the coalescence of two black holes.

• The Source: About 1.3 billion years ago and as many light years away, two spinning black holes, each about thirty times the mass of the sun ended their lives as a single entities.

• Einstein tells us that mass distorts spacetime, warping distance and duration. And an accelerating mass releases some of its energy in ripples of this distortion.

• Over the billions of years of their shared lives, the two black holes lost energy to these GWs and their orbit decayed. They slowly, inevitably , spiralled towards each other .

• As the partners approached, their orbit speed up and their orbit contracted. Eventually the partners came within about 10 kilometres of each. By this time, they were orbiting each other about thirty-five times per second!

• In-spiral, Merger, Ringdown. After millions of years in a slowly decaying orbit, the final plunge took less than a fifth of a second.

• In those last moments, GWs carried away 5.4x10*47 Joules. That’ s three times the energy contained in our Sun. Three suns, released as ripples in spacetime.

Design, Specifications and Detection:• At least two detectors, in different locations, operated in unison, to

confirm the results. It must detect deviations in distance as small as one thousandth the diameter of a proton.

How it Works:• Michelson Interferometer• Each arm is 2.5 miles (4 km) long (vacuum-sealed, seismically

isolated, super-cooled laser arms that measure distance incredibly accurately ). If you compare the distances measured in the two arms the measurement in the difference is accurate to better than one part in 10>22. This means they can measure a change in distance one one-thousandth of the width of a proton.

• The laser light is allowed to bounce back and forth multiple times.• If the path is the same length for the laser in both the arms, the light

will be directed back toward the laser.• If any difference in path is detected, the photo-detector will produce a

signal.

• Interferometers used in LIGO are the worlds largest precision optical instruments.

• This requires very precise instruments, including the vacuum tubes, lasers, mirrors, and mechanical systems.

• LIGOs vacuum system is one of the largest, with a volume of about 300,000 cubic feet.

• Pressure inside the tube-one trillionth of atmosphere: minimal gases to interfere with laser beams.

• The solid state laser, 1064 nm wavelength Nd:YAG laser which is stabilized in amplitude, frequency and beam geometry are used (In 0.01 second, the frequency varies by less than few millionths of a cycle).

• To minimize additional noise sources, all components (other than laser) are mounted on vibration isolation stages in ultrahigh vacuum.

• 30+ control systems involved in keeping mirrors and lasers aligned without human intervention.

• To monitor environmental disturbances and their influences on the detectors, each observatory site is equipped with an array of sensors: seismometers, accelerometers, microphones, magnetometers, radio receivers, weather sensors, ac power line monitors, and a cosmic-ray detector.

Thank You