- 3275 - GRAVITY WAVES - DeBroglie discovers waves? - We need to perform an experiment that indicates General Relativity isn’t enough, and reveals a hint of the Universe’s theorized “quantum gravitational nature“. The dream of directly detecting gravitons is a much larger prize. One that we expect to be far more impractically difficult to actually achieve.
------------- 3275 - GRAVITY WAVES - DeBroglie discovers waves
- In 1924, a young French noble managed to turn quantum physics on its head, just as it was finding its feet. Even the most conservative physicists were beginning to accept the duality revolution: light is not only a wave, but also behaves like a beam of particles (photons), as Einstein had established with his explanation of the photoelectric effect, which earned him the Nobel Prize in 1921.
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- Then Louis de Broglie (15 August 1892 – 19 March 1987), a novice scientist whose first degree was in history, thought otherwise: what if particles also behaved like waves?
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- A century ago there were still questions as attractive as this, to which one might dedicate a doctoral thesis. And that is exactly what de Broglie did. After studying in depth for several years the bases of quantum physics established by Max Planck and Albert Einstein, he presented his thesis in 1924 with an important theoretical discovery: “electrons behave as waves and, not only that, all particles and objects are associated with matter waves“.
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- This is the “de Broglie Hypothesis“. Putting together Planck’s equations (quantization of energy: E = hν) and Einstein’s (special relativity: E = mc2), de Broglie calculated what the length of these matter waves associated with each particle would be, depending on its velocity and mass.
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- According to de Broglie, “our whole world is quantum“, not just light, a conclusion so bold that it was immediately rejected by many physicists, and ignored by others.
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- Although in 1924 he presented his doctoral thesis the French physicist had already done other research, which had led him to clash with some of the most influential physicists of the moment. Not so with Einstein, who enthusiastically supported de Broglie’s conclusions, but even Einstein’s support was not enough to prove him right: his hypothesis had to be experimentally demonstrated.
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- If the electron were a particle that behaved like a wave, then it would have to show typical properties of waves, such as diffraction and interference. And then some very strange things would happen: for example, one electron would be able to traverse two different holes at the same time. This was demonstrated by the electron diffraction experiment of Davisson and Germer (1927), thus confirming the hypothesis of de Broglie.
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- Louis de Broglie succeeded in laying one of the pillars of quantum physics: the wave–particle duality, which states that waves can behave like particles and vice versa. From his idea of matter waves was born wave mechanics, the new formulation of quantum physics that Schrödinger developed to apply to atoms and molecules.
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- The wave properties of electrons was the basis for inventing the electron microscope (released in 1932), which allows us to see things much smaller than typical optical microscopes permit, because the wavelength of the electron is much shorter than that of photons of visible light.
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- The Universe, if you look at it closely and carefully enough, is fundamentally quantum in nature. If you try and divide matter up into smaller and smaller pieces, eventually you arrive at indivisible components that cannot be broken up any further.
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- These particles interact by exchanging a specific type of quantum that couples to their various charges. Gluons mediate the “strong nuclear force“, interacting with particles that have a color charge.
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- The W and Z bosons mediate the “weak nuclear force“, coupling to the particles that have weak hypercharge and isospins.
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- And the photon mediates the “electromagnetic force“, acting on particles with an electric charge.
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- Gravitation, though, might be the outlier as our theory of gravitation is classical General Relativity. In theory, though, there should be a quantum counterpart, mediated by a hypothetical quantum particle known as the “graviton“. Only, is it possible to find out whether gravitons actually exist?
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- The first quantum particle discovered was the photon: the quantum associated with light. While it’s true that photons mediate the electromagnetic force, the photons that do so are virtual: they provide us with a way of calculating the electromagnetic field that permeates all of space. That stands in contrast to real photons: the photons we can emit, absorb, and otherwise measure in our instruments and detectors.
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- Every time you see something, that’s a result of a photon exciting a molecule in the rods or cones present in the retinas of your eyes, which then stimulates an electrical signal to your brain, which interprets the set of data coming in and constructs an image of what you observed.
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- The act of ‘seeing” is an inherently quantum act, with each photon carrying a specific amount of energy that either will or won’t be absorbed by particular molecules. Although the photoelectric effect, first described by Einstein, was what demonstrated the quantum nature of light, it’s important to recognize that all light is quantum in nature.
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- Alternating expansion and contraction of space is due to passing gravitational waves.
We can describe many of the phenomena associated with light perfectly well by viewing light as a wave, however, and gravitation has what’s quickly become a well-known analogue: gravitational waves.
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- Just as a charged particle moving through an electromagnetic field will emit electromagnetic waves (in the form of photons), a mass moving through a region of curved spacetime (which is the analogue of a gravitational field) will emit gravitational radiation, or gravitational waves.
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- When the advanced “LIGO detectors’ began taking data in 2015, they quickly began discovering the strongest sources of gravitational radiation in the Universe in the frequency range that the interferometers were sensitive to merging blackholes.
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- Over the past 5 years, 2015 - 2020, those detectors were upgraded, joined by the Virgo detector, and have to date discovered more than 50 total gravitational wave events. From merging blackholes to merging neutron stars to, quite possibly, neutron stars merging with blackholes, they’ve demonstrated that gravitational radiation is very real, and in agreement with Einstein’s predictions.
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- The big question then becomes, once we know gravitational waves are real, whether they exhibit wave-particle duality as well? In other words, just as photons exhibit wave-like properties but also particle-like, quantum properties, is the same thing true for gravitational waves? Is there a particle-like counterpart that this radiation is made of, with the tremendous amounts of energy carried by gravitational waves distributed into individual, discrete quanta?
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- It’s a compelling and eminently reasonable idea. Water waves, for example, are made of particles, even though they don’t appear that way. But if you were to float, say, a bunch of ping pong balls atop the surface of the water, you can get an idea for visualizing what’s truly occurring.
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- Individual ping pong balls would move up-and-down, back-and-forth, etc., along the surface of the water, and you can imagine that the individual molecules along a wavy surface of water are doing something similar.
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- Even though there’s an awful lot we don’t yet know about gravitational waves, including whether they’re made of individual quanta or not, there are a lot of properties we have been able to discern:
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----------------------- gravitational waves do carry real, finite, measurable amounts of energy that can be deposited into detectors,
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----------------------- gravitational waves propagate at a specific speed through space, specifically, the speed of gravity, which differs from the speed of light by no more than 1 part in 10^15,
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----------------------- gravitational waves compress-and-expand the space they travel through in mutually perpendicular directions, which enables LIGO and Virgo to detect them,
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----------------------- and they ought to interfere with any other ripples in space both constructively and destructively, obeying the same rules that any other wave would obey.
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- They have already observed that gravitational waves, just like photons, do indeed stretch their wavelengths as they travel through the expanding Universe. As the background of the underlying space expands, so do the wavelengths of the gravitational waves we observe.
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- As the fabric of the Universe expands, the wavelengths of radiation get stretched as well. But all of this would be true whether gravitation were purely classical in nature or whether there were a more fundamental quantum theory of gravity that Einstein’s General Relativity is only an approximation for. If it’s quantum, that implies that every gravitational wave we see, in analogy with every light wave that we see:
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----------------------- is made of a large number of quantum particles,
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----------------------- where each quantum has an inherently zero rest mass,
meaning that it propagates at the speed of light (which equals the speed of gravity).
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- There are a few properties that would be unique to gravitons: properties that it wouldn’t share with photons. One of them is that, owing to the nature of the theory of gravitation, the particle that mediates the gravitational force would have to have a spin of 2, rather than a spin of 1 like the photon.
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- Because it’s massless, its spin can only be +2 or -2; it can have no intermediate value.
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- Additionally, gravitons would only interact through the gravitational force. They’d respond to any other quantum that had mass or carried energy, but they should be uncharged and would be unaffected under all of the other fundamental interactions.
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- All massless particles travel at the speed of light, irrespective of energy or wavelength, including the photon and gluon.
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- One way that the Universe could surprise us would be if gravitons turned out to actually have a very tiny, non-zero rest mass. Just as many of the fundamental particles (even including some of the force-carrying bosons, such as the W-and-Z bosons from the weak interactions) have a finite mass inherent to them, it’s possible that a graviton might as well.
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- From our current gravitational wave measurements, however, and the energy received by our detectors, we’ve constrained the graviton’s mass to be mind-bogglingly tiny. If it does have a mass, it’s got to be less than 1.6 × 10^-22 eV/c2, or some 10^28 times lighter than the electron.
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- The key place to look for gravitons, or a signature of the “particle” part of the nature of these gravitational waves that we’ve demonstrated exist, would be where quantum gravitational effects are anticipated to be strongest and most pronounced: at the shortest distance scales and where gravitational fields are strongest. There’s no better place in the Universe to probe this regime than where two blackholes merge, as close to their singularities as you can conceivably get.
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- General Relativity is perfectly adequate, for all the blackholes expected to exist in our Universe, for describing the entirety of the effects that happen outside of a blackhole’s event horizon.
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- But when you get very close to a singularity, or specifically when two singularities merge together to create a different singularity, we anticipate that quantum effects may show up: quantum effects that signal a departure from the predictions of General Relativity.
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- If we wanted to realistically do that, we’d have to be able to take data right around the exact moment the singularities merged, and we’d have to do it on extremely fast timescales.
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- Today, LIGO is sensitive to events that occur on millisecond timescales, but if we could probe the Universe on sub-picosecond timescales, including at the very end of the inspiral phase, at the moment of the merger, and at the start of the subsequent ringdown phase , that might be possible.
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- We presently have laser pulses that hit the femtosecond or even attosecond timescales (10^-15 seconds to 10^-18 seconds), and with enough interferometers working at once, we might be sensitive enough to actually detect any signatures of quantum gravity.
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- Detecting the much sought-after B-modes predicted by cosmic inflation would indirectly demonstrate that gravitation is inherently quantum in nature, but there would be no direct detection of gravitons.
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- If you fired an electron through a double slit and could measure whether its gravitational field passed through both slits or just one, that would reveal whether gravity was quantum in nature or not, but, we wouldn’t detect gravitons.
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- Other schemes exist as well, and they’re very clever. If you passed photons of various wavelengths through a crystal and the “steps” the crystal moved were discrete instead of continuous, you could prove that space was quantized.
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- If you brought masses into a quantum superposition of states and the energy levels were dependent on gravitational self-energy, you could determine whether gravity was quantized or not. And there are other potential signatures as well that could indirectly reveal whether gravity is inherently quantum in nature.
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- If we could demonstrate that gravity is inherently quantum in nature, that would be tremendous. If we could demonstrate that space is quantized, that would change how we view our reality. And if we could perform an experiment whose results disagreed with the straightforward predictions of General Relativity, that would spur us towards tremendous developments and new advances.
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- But none of that would be the same as demonstrating that gravitons actually exist, anymore than measuring the orbital decay of pulsing neutron stars demonstrated that gravitational waves really exist. That discovery was consistent with everything we now think about gravitational waves. But it didn’t prove that gravitational waves existed; we needed direct detection for that.
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- For right now, our next step should be to perform an experiment that indicates General Relativity isn’t enough, and reveals a hint of the Universe’s theorized quantum gravitational nature.
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- The dream of directly detecting gravitons is a much larger prize: one that we expect to be far more impractically difficult to actually achieve. The everlasting mystery of “gravity”.
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- September 13, 2021 GRAVITY WAVES - DeBroglie discovers waves 3275
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