Tuesday, February 1, 2022

3440 - QUANTUM GRAVITY - more we know stranger it gets?

  -  3440   -  QUANTUM  GRAVITY  -  more we know stranger it gets?     By demonstrating that particles display the “Aharonov-Bohm effect” for gravitational forces, previously only seen with electromagnetic ones, we might have our first clue to gravity's quantum nature.       



          

-------  3440  -   QUANTUM  GRAVITY  -  more we know stranger it gets?    

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-     See Review 3435 -  “Quantum Gravity  -  have we demonstrated it”  for more on this subject.

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-  Three of our fundamental forces of nature, the electromagnetic and strong and weak nuclear forces, are known to be “quantum’ in nature. Quantum meaning the forces are carried by particles.  However, the oldest known fundamental force, gravity, has only been shown to exhibit behavior described by Einstein's general relativity: a classical and continuous theory. 

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-  If you were to break down the matter in our Universe to its smallest and most fundamental subatomic constituents, you’d find that everything was made up of individual “quanta“, each of which possesses both wave and particle properties simultaneously.

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-   If you pass one of these quantum particles through a double-slit and don’t observe which slit it passes through, the quantum will behave as a “wave“, interfering with itself  and leaving us with only a probabilistic set of outcomes to describe its ultimate trajectory. Only by observing it can we determine precisely where it is at any moment in time.

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-  This bizarre, “indeterminate probabilistic” behavior has been thoroughly observed, studied, and characterized for three of our fundamental forces: the electromagnetic force and the strong and weak nuclear forces.

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-   However, it’s never been tested for gravitational forces, which remains the one remaining force that only has a classical description in the form of Einstein’s general relativity. Although many clever experiments have attempted to reveal whether a quantum description of gravity is required to account for the behavior of these fundamental particles, none has ever been performed decisively.

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-  The “Aharonov-Bohm effect“, has just been discovered to occur for gravity as well as electromagnetism.  It could be our first clue that gravity is truly quantum in nature.

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-  In general relativity, the presence of matter and energy determine the curvature of space. In quantum gravity, there will be quantum field theoretic contributions that lead to the same net effect. So far, no experiment has been able to establish whether gravity is quantum in nature or not.

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-  In the world of quantum physics, few experiments are more demonstrative of the bizarre nature of reality than the double-slit experiment. Originally performed with photons more than 200 years ago, shining light through two thin, closely-spaced slits resulted not in two illuminated images on the screen behind the slits, but rather in an interference pattern. The light that went through each of the two slits must be interacting before they reach the screen, creating a pattern that displays light’s inherent wave-like behavior.

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-  This same interference pattern was shown to be generated with electrons as well as photons; for single photons, even as you passed them through the slits one at a time; and for single electrons, again even as you passed them through the slits one at a time. 

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-  As long as you don’t measure which slit the quantum particles go through, the wave-like behavior is easily observable. It is evidence of the counterintuitive, but very real, quantum mechanical nature of the system.   Somehow, an individual quantum is capable of going through “two slits at once” where it must interfere with itself.

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-   And, if you do measure which slit these quanta pass through, you see no interference pattern at all. Instead, you just get two “clumps” on the far side of the screen, which correspond to the set of quanta that went through slit 1 and slit 2, respectively.

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-   You cannot simply ascribe definite quantities like a “position” and a “momentum” to each particle, as you would in a classical, pre-quantum treatment of those quantities. Instead, you have to treat position and momentum as quantum mechanical operators, mathematical functions that “operate” on a quantum wave function of probabilities.

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-  When you “operate” on a wave function, you get a probabilistic set of outcomes for what is possible to observe. When you actually make that key observation, when you cause the quantum you’re “observing” to interact with another quantum whose effects you then detect, you recover only a “single collapsed value“.

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-   You can perform this experiment with electrons, particles with a fundamental, negative electric charge, and you send them through these slits one at a time. If you measure which slit the electron goes through, it’s easy to describe the electric field generated by the electron as it goes through that slit. But even if you don’t make that critical measurement  you can still describe the electric field that it generates. 

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-  The reason you can do this is because it isn’t just the individual particles or waves that are quantum in nature, but the physical fields that permeate all of space are quantum in nature as well.   Yhey obey the rules of “quantum field theory“.

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-  For the electromagnetic interaction, as well as the strong and weak nuclear interactions, science has verified and validated the predictions of quantum field theory many times over. The agreement between theoretical predictions and the results of experiments, measurements, and observations is spectacular, agreeing in many cases to better than 1-part-in-a-billion precision.

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-    What happens to the gravitational field of an electron as it goes through a double slit?  Theoretically, without a working quantum theory of gravity, we cannot make a robust prediction, while experimentally, detecting such an effect goes far beyond our current capabilities.

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-   At present, we do not know whether gravity is an inherently quantum force or not, as no experiment or observation has been able to make such a critical measurement.

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-  If you have a charged particle in motion, it can be affected by both the presence of electric fields and magnetic fields.  The electric field will accelerate the charged particle along the direction of the field, in direct proportion to the strength of the field and proportional to the charge of the particle, causing it to either speed up or slow down in the process.

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-  The magnetic field accelerates the charged particle perpendicular to both the magnetic field and the direction of motion of the particle, causing it to bend but not to increase or decrease its speed.

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-  If your electric and magnetic fields are both zero, your electron won’t accelerate; it will just continue along in constant motion.

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-  But in the quantum Universe, there’s another effect that comes into play that can change the behavior of your quantum particle, even when the electric and magnetic fields are both zero,  the “Aharonov-Bohm effect“.

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-  Electric potential is more commonly known as “voltage“. Changes in voltage, from one region to another, are what creates electric fields and compels electric currents to flow. You can get the electric field from the “electric potential” simply by taking the gradient, which details how the field changes, directionally, throughout space.

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-  “Magnetic potential” is a little more complicated because it doesn’t have a common analog like voltage, and also because the magnetic field itself doesn’t come about from a simple gradient, but rather from a mathematical operation known as the “curl” of the magnetic potential.

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-  You can have a non-zero electric or magnetic potential in a region even where the electric and magnetic fields are both zero.   The potential couples to the phase of a charged particle’s wave function, and if you measure the phase of that charged particle you’ll find that it does depend on the electromagnetic potential, not just on the electric and magnetic fields.

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-  The Aharonov-Bohm effect states that a particle’s phase will change as it moves around a region containing a magnetic field, even if the field itself is zero everywhere the particle is present. The phase shift has been robustly detected for decades now, leading many to pursue extensions of the original physics, which applied only to the electromagnetic force. 

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-  The way we typically measure the Aharonov-Bohm effect is to set up a cylindrical region of space that contains a substantial but highly confined magnetic field: something that’s easy to create with a long coil of wire, like a solenoid. You then set up a charged particle in motion around that magnetic field, but carefully, so that the particle itself doesn’t pass through the region containing the field.

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-  The wave function will still experience a phase shift that can be observed experimentally. This is true even though the electric and magnetic fields are negligible outside the confined region containing the field, and the probability of finding the particle within the field-containing region is also negligible.

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-  The same effect that’s been observed for the magnetic potential should be observable for any force that arises as a consequence of a potential. This includes not only the electric force and the other known quantum forces, but also the gravitational force.

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-   When you want to experiment with the gravitational force, the biggest problem is always that gravitational effects are so small. Although people have been designing experiments for many decades with a view toward detecting this effect, an enormous breakthrough came in 2012.

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-  The idea was that you can create ultra-cold atoms and control their motion by pulsing a laser beam, including into a region where the gravitational potential, but not the field, is different from other locations. Even in regions where the gravitational force is zero, which can be arranged by a careful setup, the non-zero potential could still have an effect.

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-  If you can then split a single atom into two matter waves, move them into areas with different potentials, and then bring them back together, you could observe an interference pattern, measuring their phase and quantifying the gravitational Aharonov-Bohm effect.

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-   January, 2022,  a team took multiple ultra-cold rubidium atoms, put them into quantum superpositions with one another, and compelled them to trace two different paths inside a vertical vacuum chamber. Because there was a heavy mass at the top of the chamber, but one that was axially symmetric and completely outside of the chamber itself, it only changed the gravitational potential of the atoms, with the atom that reached a higher trajectory experiencing a greater change in potential.

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-  Then, the atoms are brought back together, and from the interference pattern that is produced, a phase shift emerges. The amount of the phase shift that’s measured should correspond to:

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--------------------------  How separated the two atoms are from one another,

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--------------------------  How close they each come to the top of the chamber

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-------------------------  The external mass which alters the gravitational potential is present or not.

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-  Measure the phase shifts of these atoms and compare them with the theoretical predictions for the gravitational Aharonov-Bohm effect the match is dead on.

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-   This is a remarkable achievement. But the analysis could be applied to any force or field that’s derivable from a potential, both quantum and classical. It’s a tremendous triumph for quantum mechanics under the influence of gravity, but it isn’t quite enough to demonstrate the quantum nature of gravity itself. 

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-  When you “operate” on a wave function, you get a probabilistic set of outcomes for what is possible to observe. When you actually make that key observation, when you cause the quantum you’re “observing” to interact with another quantum whose effects you then detect, you recover only a single value.  How does all this happen?

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January 31, 2022      QUANTUM  GRAVITY  -   stranger it gets?                 3440                                                                                                                                               

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