Is Space-Time Fluid?

Is space not as immaterial as we thought?

Physicists have gathered evidence that space-time can behave like a fluid. Mathematical evidence, that is, but still evidence. If this relation isn’t a coincidence, then space-time – like a fluid – may have a substructure.

We shouldn’t speak of space and time as if the two were distant cousins. We have known at least since Einstein that space and time are inseparable, two hemispheres of the same cosmic brain, joined to a single entity: space-time. Einstein also taught us that space-time isn’t flat, like paper, but bent and wiggly, like a rubber sheet. Space-time curves around mass and energy and this gives rise to the effect we call gravity.

That’s what Einstein said. But turns out if you write down the equations for small wiggles in a medium – such as soundwaves in a fluid – then the equations look exactly like those of waves in a curved background.

Yes, that’s right. Sometimes, waves in fluids behave like waves in a curved space-time; they behave like waves in a gravitational field. Fluids, therefore, can be used to simulate gravity. And that’s some awesome news because this correspondence between fluids and gravity allows physicists to study situations that are otherwise experimentally inaccessible; for example, what happens near a black hole horizon or during the rapid expansion of the early universe.

This mathematical relation between fluids and gravity is known as “analog gravity.” That’s “analog” as in “analogy” not as opposed to digital. But it’s not just math. The first gravitational analogies have been created in a laboratory. 

Most amazing is the work by Jeff Steinhauer at Technion, Haifa. Steinhauer used a condensate of supercooled atoms that “flows” in a potential of laser beams which simulate the black hole horizon. In his experiment, Steinhauer wanted to test whether black holes emit radiation as Stephen Hawking predicted. The temperature of real, astrophysical, black holes is too small to be measurable. But if Hawking’s calculation is right, then the fluid-analogy of black holes should radiate too.

Black holes trap light behind the “event horizon.” A fluid that simulates a black hole doesn’t trap light; instead it traps the fluid’s soundwaves behind what is called the “acoustic horizon.” Since the fluid analogies of black holes aren’t actually black, Bill Unruh suggested calling them “dumb holes.” The name stuck.



"What if the fluid analogy is more than an analogy? Maybe space-time really behaves like a fluid; maybe it is a fluid.'"


But whether the horizon catches light or sound, Hawking-radiation should be produced regardless, and it should appear in form of fluctuations (in the fluid or quantum matter fields, respectively) that are paired across the horizon.

Steinhauer claims he has measured Hawking-radiation produced by an acoustic black hole. His results are, at present, somewhat controversial – not everyone is convinced he has really measured what he claims he did – but I am that sure sooner or later this will be settled. More interesting is that Steinhauer’s experiment showcases the potential of the method.

Of course fluid-analogies are still different from real gravity. Mathematically, the most important difference is that the curved space-time which the fluid mimics has to be designed. It is not, unlike real gravity, an automatic reaction to energy and matter; instead, it is part of the experimental setup. However, this is a problem which, at least in principle, can be overcome with a suitable feedback loop.

The conceptually more revealing difference is that the fluid’s correspondence to a curved space-time breaks down once the experiment starts to resolve the fluid’s atomic structure. Fluids, we know, are made of smaller things. Curved space-time, for all we know at present, isn’t. But how certain are we of this? What if the fluid analogy is more than an analogy? Maybe space-time really behaves like a fluid; maybe it is a fluid. And if so, the experiments with fluid-analogies may reveal how we can find evidence for a substructure of space-time.

Some have pushed the gravity-fluid analogy even further. Gia Dvali from LMU Munich, for example, has proposed that real black holes are condensates of gravitons, the hypothetical quanta of the gravitational field. This simple idea, he claims, explains several features of black holes which have so-far puzzled physicists, notably the question how black holes manage to keep the information that falls into them.

We used to think black holes were almost featureless round spheres. But if they are instead, as Dvali says, condensates of many gravitons, then black holes can take on many slightly different configurations in which information can be stored. Even more interesting, Dvali proposes the analogy could be used to design fluids which are as efficient at storing and distributing information as black holes are. The link between condensed matter and astrophysics, hence, works both ways.

Physicists have looked for evidence of space-time being a medium for some while. For example, by studying light from distant sources, such as gamma-ray bursts, they tried to find out whether space has viscosity or whether it causes dispersion (a running apart of frequencies like in a prism).  A new line of research is to search for impurities – “space-time defects” – like crystals have. So far the results have been negative. But the experiments with fluid analogies might point the way forward.

If space-time is made of smaller things, this could solve a major problem: how to describe the quantum behavior of space time. Unlike all the other interactions we know of, gravity is a non-quantum theory. This means it doesn’t fit together with the quantum theories that physicists use for elementary particles. All attempts to quantize gravity so-far have either failed or remained unconfirmed speculations. That space itself isn’t fundamental but made of other things is one way to approach the problem.

Not everyone likes the idea. What irks physicists most about giving substance to space-time is that this breaks Einstein’s bond between space and time which has worked dramatically well – so far. Only further experiment will reveal whether Einstein’s theory holds up.

Time flows, they say. Maybe space does too.

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Cuthbert Simpkins 15 June 2019

This is where I am now on gravity and the theory of everything. I would appreciate your critique. I am trying to figure out how to express the idea mathematically. Whether one calls it an aether or spacetime we are talking about something that has structure in the absence of mass and energy. I prefer to use the term aether because it hearkens us back to the Michelson-Morley experiment. After this experiment it was concluded that there is no aether because the velocity of light was the same regardless of its direction. Einstein did not need an aether for special relativity. He did need to create space-time for general relativity. The underlying assumption of the experiment was that the aether has a directional flow. However, the same negative result would have been obtained if the aether were moving chaotically without direction. In that case the speed of light would also be the same regardless of direction. So, let us hypothesize that the aether is chaotic in the absence of mass or energy. Let us further propose that the aether inside of a mass is not as chaotic in its movement or in some other critical property as the aether outside of the mass. Let us further propose that aether inside is connected to the aether outside of the borders of the mass. A field of aether that was less chaotic would then surround the mass and the greater the distance from the mass the more it would resemble the aether in which there was no mass or energy. If two or more such masses were brought into proximity to each other their fields of reduced chaotic aether would overlap. The result would be a region of reduced chaotic aether between the masses (RCE) and chaotic aether outside of this region (CE). Because of the difference between CE and RCE the two masses would move toward each other with the degree of chaos becoming less and therefore the force of gravity increasing the closer they came to each other. The difference between CE and RCE could be what we call gravity. This chaotic aether hypothesis provides a model for the other forces too. Electromagnetic energy is created by the vibration of electrons. Perhaps when an electron vibrates it causes the chaotic aether to become organized into a self-perpetuating wave. The strong force could be a manifestation of gravity at a subatomic scale. This possibility emerges when one considers the fact that protons and neutrons have densities that are the same as the density of a black hole. These densities are 4.7 x 10^18 kg/m^3, 6.25 x 10^18 kg/m^3, 6 x 10^18 kg/m^3, respectively. This fact is consistent with the idea that protons and neutrons are very small black holes which are surrounded by highly ordered space which constitutes a gravitational field over a short distance that is stronger than the electrostatic repulsion one would expect from protons that are in such close proximity. In other words, this could be the strong force. In contrast the density of an electron at 1.66 x 10^16 kg/m^3 is two orders of magnitude less than that of a black hole. This allows its charge to disrupt the ordered space and keeps from crashing into the nucleus. Finally, under this model the weak force could be result of instability or disruption of the aether within a neutron. This model let's call it the chaotic aether model (CAM) is consistent with the observation that light bends when it passes close to a mass because the aether out of which the light is formed is less chaotic close to the mass. This more ordered aether would change the velocity of light and thereby cause it to bend.
This is in agreement with Xinhang Shen's idea below. I think the aether would probably be like a chaotically moving liquid. I have tried to use the Navier-Stokes equations to model it. But that model is based on a liquid that is in a gravitational field with terms for gravitational force and density that don't make sense if you are modeling gravity itself. In its derivation it also assumes that the smallest unit of the liquid is a cube. I think the essential question is what model of the aether would explain all of the forces that we can measure and unify them under one theory?

Zephir AWT 7 October 2017

So.. - is Sabine Hossenfelder aetherist at the end? Nope, she is a conjuncturalist - she doesn't predict or even creates a waves, she follows and rides them. Just before few years she banned me from her blog for the promotion of the same concept. Now she just describes the research of others (mostly males BTW - the ladies may be smart, but rarely willing to risk their neck as forerunners), which she had no courage neither invention to support by her own just before few years. But (with compare to L. Motl and another conservative pathoskeptics) at least she doesn't lack the self-preservation instinct and switched to analog gravity recently (of course [she needs some fresh mind]( to do the actual work instead of her).

From perspective od dense aether model the isomorphism of [gravitomagnetism, Maxwell's and Navier-Stokes equations]( is the emergent, i.e. scale dependent property and vacuum in general behaves more like the elastic foam, rather than fluid. The space-time is random environment composed of fluctuations, the collective behavior of which resembles the behavior of fluid at certain energy-density scales, the foam or metamaterial at another ones. It has no intrinsic behavior hardwired in it.

jimmy wells 6 October 2017

Regardless of the theory -- gravity must be or have some kind of a mechanism. The mechanism must actually be something. Whatever that something is -- it must be filling space.
Space is NOT empty. There is an all encompassing lattice-type thread particle network in otherwise empty space. It is gravity centered -- Einstein called it Space-Time. Otherwise known as the Fabric of Space. (yes, the fabric of space must actually be made from something)
A good 2-D model would be something like a spiders web made of the finest web-silk-thread filament. Now imagine a 3-D web.
It is made from individual yet connected thread particles and of course the web has tension on it (that's were gravity gets its pull). The speed vibrations travel through the web is the speed of light (light is a just a vibration travelling in a thread particle network)

Here is a regular thread tension formula...
Tension = velocity squared x mass / Length
If we plug in c and rearrange we get the one-inch formula...

TL = mc^2

If the tension goes up... gravity goes up, and so does the speed of light and everything else with it.
That includes any type of measuring device, the speed your brain is working and time itself.
Increase or decrease tension and it changes everything along with it, that's all electro-magnetic phenomena, vibrations... everything.
It's like being a character in a movie and you don't know the speed the projector is running... fast, slow, stop, start... you don't know.

Now we know were energy comes from and why light travels at c.
Tesla was correct...
"There is no energy in matter other than that received from the environment." – Nikola Tesla

Mnemonic memory device...
E for Einstein: E = mc^2
TL for Tesla: TL = mc^2

"Matter" is just balled-up / clumped up parts of the same particle network.
What they call "Dark Matter" is completely filling space -- it's the particle network itself. The clumps around galaxies and other spots are excessive amounts.
Dark Energy is tension on the network as a whole. Everything is being pulled on equally. Any masses in the system create a higher tension pull between them -- that's gravity

Electrons form a thread mesh-type cage around the nucleus. Electrons actually are something but everyone mistakenly thinks the vibration travelling around the thread is the electron -- that's what has caused all the confusion.
The electron is conveying vibrations but the material it is made from (threads) are NOT moving..
EXAMPLE: Think guitar string -- the string itself would be the electron but everyone thinks the vibration or note is the electron. That is why there is all kinds of probability and uncertainty -- the vibration is traveling around a spherical thread mesh cage -- where exactly is the vibration? No way to know for sure.

Here is a link to the whole theory...

Xinhang Shen 3 October 2017

The extra gravitation is generated by the mass of aether. The acceleratingly expansion of the visible part of the universe is pushed by the pressure of aether. Light in space can be bent by the change of the density of aether. The phenomenon that an electron emits photons when it moves relative to aether just as a boat generates waves when it moves relative to the water. The surface slower rotation of the sun than its core is the effect of viscosity of aether. The differences of the displayed times of the atomic clocks in Hefele-Keating experiment are probably the result of the effects of aether when they had moved in different velocities relative to aether. The increased lives of particles in accelerators or going through atmosphere are also the effects of aether when the particles have moved through aether in high velocities.

Xinhang Shen 3 October 2017

The space of the visible part of the universe is indeed filled up with a compressible viscous fluid called aether, but not spacetime which does not exist in nature at all. The concept of spacetime is derived from Einstein's relativity which theory has already been disproved both logically and experimentally (see "Challenge to the special theory of relativity", March 1, 2016 on Physics Essays and a press release "Special Theory of Relativity Has Been Disproved Theoretically" on Eurekalert website: The problem of Einstein's relativity is that it has redefined time and space through Lorentz Transformation. The newly defined time is no longer the physical time measured with physical clocks, which can be easily demonstrated by the following thought experiment of candle clocks:

There are a series of vertically standing candles with the same burning rate and moving at different constant horizontal velocities in an inertial reference frame of (x, y, z, t) where x, y, z, t are relativistic positions and time. At any moment t of relativistic time, all candles have the same height H in the reference frame of (x, y, z, t) and the height has been calibrated to physical time as physical clocks. Therefore, we have the simultaneous events of the observation measured in both relativistic time and physical time in the frame of (x, y, z, t): (Candle1, x1, y1, H, t), (candle2, x2, y2, H, t), …, (CandleN, xN, yN, H, t). When these events are observed on anther horizontally moving inertial reference frame (x', y', z', t'), according to special relativity, these events in the reference frame of (x', y', z', t') can be obtained through Lorentz Transformation: (Candle1, x1′, y1′, H, t1′), (Candle2, x2′, y2′, H, t2′), … , (CandleN, xN', yN', H, tN') where t1′, t2′, …, and tN' are relativistic times of the events in the frame of (x', y', z', t'). It is seen that these events have different relativistic times after Lorentz Transformation in the frame of (x', y', z', t'), i.e., they are no longer simultaneous measured with relativistic time in the frame of (x', y', z', t'), but the heights of the candles remain the same because the vertical heights here do not experience any Lorentz contraction. Since the heights of the candles are the measures of the physical time, we can see these events still have the same physical time, i.e., they are still simultaneous measured with the physical time. Therefore, the physical time is invariant of inertial reference frames, which is different from relativistic time. As relativistic time is no longer the physical time we measure with physical devices, the description of special relativity is irrelevant to the physical world.

Now let's have a look at the symmetric twin paradox. Two twins made separate space travels in the same velocity and acceleration relative to the earth all the time during their entire trips but in opposite directions. According to special relativity, each twin should find the other twin’s clock ticking more slowly than his own clock during the entire trip due to the relative velocity between them because acceleration did not have any effect on kinematic time dilation in special relativity. But when they came back to the earth, they found their clocks had exact the same time because of symmetry. Thus, there is a contradiction which has disproved special relativity. This thought experiment demonstrates that relativistic time is not our physical time and can never be materialized on physical clocks.

Now let's look at clocks on the GPS satellites which is thought as one of the strong evidences of Einstein's relativity. Many physicists claim that clocks on the GPS satellites are corrected according to both special relativity and general relativity. This is not true because the corrections of the atomic clocks on the GPS satellites are absolute changes of the clocks, none of which is relative to a specific observer as claimed by special relativity. After all corrections, the clocks are synchronized not only relative to the ground clocks but also relative to each other, i.e., time is absolute and special relativity is wrong.

This is a fact as shown on Wikipedia. But some people still argue that the clocks on the GPS satellites are only synchronized in the earth centered inertial reference frame, and are not synchronized in the reference frames of the GPS satellites. If it were true, then the time difference between a clock on a GPS satellite and a clock on the ground observed in the satellite reference frame would monotonically grow due to their relative velocity while the same clocks observed on the earth centered reference frame were still synchronized. If you corrected the clock on the satellite when the difference became significant, the correction would break the synchronization of the clocks observed in the earth centered frame. That is, there is no way to make such a correction without breaking the synchronization of the clocks observed in the earth centered frame. Therefore, it is wrong to think that the clocks are not synchronized in the satellite frame.

Hefele-Keating experiment is also considered as another evidence of relativistic effects. It is clear that all the differences of the clocks after flights in Hefele-Keating experiment were absolute (i.e., they were the same no matter whether you observe them on the earth, on the moon or on the space station). But according to relativity, if the clocks were observed on the earth, the two clocks after flights had experienced the equivalent paths of same velocity and same distance in same elevation, and thus should generate the same kinematic time dilation and the same gravitational time dilation, directly contradicting the experimental result. Therefore, the differences of the clocks were nothing to do with the velocities relative to each other or relative to the earth as claimed by relativists, but were the result of the velocities relative to one medium which seems fully dragged by the earth on its surface but partially dragged on the altitude of the airplanes. It is wrong to interpret the differences of the displayed times of the clocks as the results of relativistic effects.

The increase of the lives of muons in a circular accelerator or going through the atmosphere are also absolute changes which are the same observed in all reference frames.

All so-called proofs of relativistic effects are just misinterpretations of experiments and observations without exception, and all what relativity describes is irrelevant to physical phenomena, including the speed of light which in special relativity is constant in all inertial reference frames, but which in real physical world still follows Newton's velocity addition formula (see the paper).

That is, time is absolute and space is 3D Euclidean. There is nothing called spacetime continuum in nature, not to mention the ripples of spacetime.

kyoung21b 3 October 2017

Uh oh, the ether is back ! Nice piece though... One thing that's confusing is that, in my ignorance, I was under the impression that Hawking radiation was dependent on pair production at the event horizon - if that's the case what's the fluid/sound wave analog of pair production ?