- 3064 - INTERFEROMETERS - light and atoms? In astronomy we can put light and atom interferometers in space to be used to detect gravity waves. Gravity waves radiate from binary Blackholes and binary Neutron Stars. These gravity wave “telescopes” could see back to the beginning of the Universe.
--------------- 3064 - INTERFEROMETERS - light and atoms
- Photons behave both as waves and as particles. So do atoms. So does everything. It is just that in a larger mass the waves are too small to be detectable. You have to get to small masses, atomic size, and waves become predominant.
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- Light interferometers were invented over 200 years ago. Just recently science has began using this old technology in new ways.
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- Radios have been around a long time too. They are everywhere today. Your cell phone is a radio. Your remote control is a radio. AM and FM radios operate at frequencies of 1000 kilohertz and 300 Megahertz.
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- These frequencies correspond to wavelengths of 300 yards down to 4 inches. The TV remote uses infrared waves that have wave lengths of 0.000001 meters, which is about the size of a bacteria.
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- Radio waves and light waves are the same electromagnetic medium as radio waves, why don’t we have radios that operate at light wave frequencies? In fact ,we do, and simple ones are operating in fiber optic cables.
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- Science is learning how to use light wave frequencies to build comb generators, and heterodyne mixers that are very analogies to traditional radios. The frequencies are 10^15 cycles per second, 1,000,000,000,000,000 oscillating cycles in each second. This corresponds to wavelengths of 3*10^-7 meters, 0.0000003 meters.
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- The light wave interferometer is one of the most accurate instruments known. The new comb generators will make them more accurate then today’s atomic clocks. A radio’s simple comb generator generates a comb of harmonic frequencies by pulsing energies into a simple diode.
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- The rapid on and off pulse is a square wave containing all the harmonics of the original signal. The optical comb generator does the same thing only at light frequencies. Using these optical comb generators will allow more precise atomic clocks to be invented.
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- Also, there will be new ultra sensitive chemical detectors. Lasers will control chemical reactions. Higher capacity telecommunications will be possible at higher frequencies. Improved light detectors and light range finders will be invented. And, there will be more applications that we have not thought of yet.
- Optical Comb generators start with mode-locked lasers that can create ultra-short pulses of light. The shortest pulses from these lasers have durations less than 10 femtoseconds. ( See Review 361 “Time is Getting Short”).
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- The carrier frequency for the light consists of one pure frequency, or color. The amplitude pulse of this single frequency consists of light of many frequencies all traveling together. The shorter the pulse, the broader the spread of frequencies.
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- The mode-locked laser is set to emit a train of these pulses at a regular rate, called the “Repetition Frequency“. If you plot these frequencies the spectrum appears as the teeth on a hair comb, spaced precisely at the laser’s Repetition Rate.
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- Typically the repetition rate is set at 1 GHz, or 1,000,000,000 cycles per second. The optical comb has 400,000 teeth spaced out at 1 GHz apart. High-speed photodiodes can accurately measure frequencies at 1 GHz. Now, with this optical comb we can translate the accuracy up into the frequencies of light.
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- One problem that had to be overcome was the phase shifting that occurred with a train of pulses. This amounted to creating an Offset Frequency after a multiple of pulses. Scientists had to know this “Offset Frequency” in order to calibrate their instrument for ultra-precise measurements.
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- Using a cesium beam atomic clock one second is defined as a cesium oscillation of 9,192,631,770 cycles, exactly. A cesium clock is based on 9 GHz microwaves being absorbed by cesium atoms. An optical comb generator could be calibrated to 60,000 times the cesium frequency.
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- To calibrate out the Offset Frequency the scientists needed a mode-locked laser that had a bandwidth of at least one octave. An octave is a factor of 2 in frequency. An octave spanning spectrum enables the comparison of frequencies of two comb lines at opposite ends of the spectrum.
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- Any deviation from the second frequency being exactly twice the first frequency is a measure of the phase shift and the Offset Frequency. This is called “self-referencing” because you are comparing the comb frequency against itself. If these two frequencies are mixed together, or beat together, the difference frequency is the Offset Frequency, and this process is called “heterodyne detection“. The same as in an old heterodyne radio.
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- Another new tool is the atom interferometer. It is similar to the light interferometer except it uses the waves from atoms instead of the waves from photons. Gyroscopes using this system will improve 200 fold. This will allow navigation cross country that is more accurate than GPS.
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- Interferometers use interference patterns from mixing two waves to calculate the differences in the distances that the two waves travel. In the 1890’s Albert Michelson and Edward Morley tried this experiment to detect if Earth’s orbit was passing through an ether.
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- Science at that time thought that an ether medium was needed for light waves to travel across space. They discovered there was no ether.
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- In 1920 Louis-Victor de Broglie discovered that all particles behaved as waves depending on the particles mass and its velocity. The heavier and faster the particle the shorter its wavelength.
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- An atoms wavelength is around 1/100th of a nanometer. Visible light has wavelengths of 400 to 700 nanometers. If the path of an atom varies 1/1000th of a nanometer an atom interferometer can detect it. To detect these waves of atoms they must be cooled to nearly absolute zero Kelvin temperature.
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- On the Earth’s surface, gravity’s acceleration is 9.8 meters / second. But, this varies slightly depending on the composition of the rock at the surface. By detecting the variations in the strength of gravity these instruments can detect underground oil.
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- 100,000 tons of oil buried 1 kilometer underground will decrease gravity by a few hundred thousands of a percent. Not just oil. The same technique can detect underground diamonds. Not just diamonds, The atom interferometer can detect underground caves where terrorists are hiding.
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- However, the most immediate use will be the military using it for accurate navigation without the use of satellites. They can even navigate under the Artic ice with precision.
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- In astronomy we can put light and atom interferometers in space to be used to detect gravity waves. Gravity waves radiate from binary Blackholes and binary Neutron Stars. These gravity wave “telescopes” could see back to the beginning of the Universe.
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- The Cosmic Microwave Background allows us to see back to 300,000 years after the Big Bang. But, gravity waves could see past that opaque wall even further back in time to the Big Bang.
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- Whenever you have a greater ability to see, you are bound to see something new. Light interferometers and atom interferometers are ultra-precise measurements that will discover new things. Get ready to learn.
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- See Review 2684 - Wave-Particle Duality
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February 27, 2021 INTERFEROMETERS - light and atoms 915 3056
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