Einstein's theory of gravity — general relativity — has been very successful for more than a century. However, it has theoretical shortcomings. This is not surprising: the theory predicts its own failure at spacetime singularities inside black holes — and the Big Bang itself.
Unlike physical theories describing the other three fundamental forces in physics — the electromagnetic and the strong and weak nuclear interactions — the general theory of relativity has only been tested in weak gravity.
Deviations of gravity from general relativity are by no means excluded nor tested everywhere in the universe. And, according to theoretical physicists, deviation must happen.
Deviations and quantum mechanics
According to Einstein, our universe originated in a Big Bang. Other singularities hide inside black holes: Space and time cease to have meaning there, while quantities such as energy density and pressure become infinite. These signal that Einstein’s theory is failing there and must be replaced with a more fundamental one.
Naively, spacetime singularities should be resolved by quantum mechanics, which apply at very small scales.
Quantum physics relies on two simple ideas: point particles make no sense; and the Heisenberg uncertainty principle, which states that one can never know the value of certain pairs of quantities with absolute precision — for example, the position and velocity of a particle. This is because particles should not be thought of as points but as waves; at small scales they behave as waves of matter.
This is enough to understand that a theory that embraces both general relativity and quantum physics should be free of such pathologies. However, all attempts to blend general relativity and quantum physics necessarily introduce deviations from Einstein’s theory.
Therefore, Einstein’s gravity cannot be the ultimate theory of gravity. Indeed, it was not long after the introduction of general relativity by Einstein in 1915 that Arthur Eddington, best known for verifying this theory in the 1919 solar eclipse, started searching for alternatives just to see how things could be different.
Einstein’s theory has survived all tests to date, accurately predicting various results from the precession of Mercury’s orbit to the existence of gravitational waves. So, where are these deviations from general relativity hiding?
A century of research has given us the standard model of cosmology known as the Λ-Cold Dark Matter (ΛCDM) model. Here, Λ stands for either Einstein’s famous cosmological constant or a mysterious dark energy with similar properties.
Dark energy was introduced ad hoc by astronomers to explain the acceleration of the cosmic expansion. Despite fitting cosmological data extremely well until recently, the ΛCDM model is spectacularly incomplete and unsatisfactory from the theoretical point of view.
In the past five years, it has also faced severe observational tensions. The Hubble constant, which determines the age and the distance scale in the universe, can be measured in the early universe using the cosmic microwave background and in the late universe using supernovae as standard candles.
These two measurements give incompatible results. Even more important, the nature of the main ingredients of the ΛCDM model — dark energy, dark matter and the field driving early universe inflation (a very brief period of extremely fast expansion originating the seeds for galaxies and galaxy clusters) — remains a mystery.
From the observational point of view, the most compelling motivation for modified gravity is the acceleration of the universe discovered in 1998 with Type Ia supernovae, whose luminosity is dimmed by this acceleration. The ΛCDM model based on general relativity postulates an extremely exotic dark energy with negative pressure permeating the universe.
Problem is, this dark energy has no physical justification. Its nature is completely unknown, although a plethora of models has been proposed. The proposed alternative to dark energy is a cosmological constant Λ which, according to quantum-mechanical back-of-the-envelope (but questionable) calculations, should be huge.
However, Λ must instead be incredibly fine-tuned to a tiny value to fit the cosmological observations. If dark energy exists, our ignorance of its nature is deeply troubling.
Alternatives to Einstein’s theory
Could it be that troubles arise, instead, from wrongly trying to fit the cosmological observations into general relativity, like fitting a person into a pair of trousers that are too small? That we are observing the first deviations from general relativity while the mysterious dark energy simply does not exist?
This idea, first proposed by researchers at the University of Naples, has gained tremendous popularity while the contending dark energy camp remains vigorous.
There is now a large literature on theories of gravity alternative to general relativity, going back to Eddington’s 1923 early investigations. A very popular class of alternatives is the so-called scalar-tensor gravity. It is conceptually very simple since it only introduces one additional ingredient (a scalar field corresponding to the simplest, spinless, particle) to Einstein’s geometric description of gravity.
The consequences of this program, however, are far from trivial. A striking phenomenon is the “chameleon effect,” consisting of the fact that these theories can disguise themselves as general relativity in high-density environments (such as in stars or in the solar system) while deviating strongly from it in the low-density environment of cosmology.
As a result, the extra (gravitational) field is effectively absent in the first type of systems, disguising itself as a chameleon does, and is felt only at the largest (cosmological) scales.
The current situation
Nowadays the spectrum of alternatives to Einstein gravity has widened dramatically. Even adding a single massive scalar excitation (namely, a spin-zero particle) to Einstein gravity —and keeping the resulting equations “simple” to avoid some known fatal instabilities — has resulted in the much wider class of Horndeski theories, and subsequent generalizations.
Theorists have spent the last decade extracting physical consequences from these theories. The recent detections of gravitational waves have provided a way to constrain the physical class of modifications of Einstein gravity allowed.
However, much work still needs to be done, with the hope that future advances in multi-messenger astronomy lead to discovering modifications of general relativity where gravity is extremely strong.
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PhD in Astrophysics, supervisor George F.R. Ellis, worked in relativity, cosmology, and alternative theories of gravity for 30 years, been at Bishop's University for 16 years, currently full professor in the Physics & Astronomy Department. Author of 210 refereed journal articles and 7 books, funded by NSERC and volunteered extensively for NSERC, the Canadian Association of Physicists, and occasionally for other organizations worldwide.
It has always bugged me that the illustrations of "mass bending space" to explain gravity actually use the normally understood effects of gravity for that illustration. It always looks like a bowling ball hanging in an elastic fish net. And we are supposed to envision something like a ball bearing rolling around that depression because gravity is pulling it "down" into the "gravity well". That is not showing us bent lines that are the way thing going "straight" in "space" somehow end up going in circles around massive objects - it is just showing us that masses attract each other, which we already know.Reply
Worse, when trying to explain how light cannot escape the event horizon of a black hole, the popular media illustration shows light being swept into the hole by space flowing into the hole faster than the speed of light. But, then theorists argue that space does not really flow at all, it just bends and warps. And that really pulls the rational rug out from under that depiction of light not being able to go "upstream" fast enough to get out of a black hole's event horizon.
Even worse, it you ask whether a photon acts like a particle or a wave at the event horizon, you tend to get a different conceptual image. If it is a particle, then it would lose energy as it traveled above the event horizon, but would not get to zero energy until it got to infinite distance - the effect would be increasing red shift as a function of distance above the event horizon. But, if it is a wave, then what does a photon do when emitted directly outward at an event horizon? What does "gravity" do to "waves" in a HIggs Field?
We need some better explanations.
Nothing fits. Because our concept of light is false. Light has a duty cycle, not a frequency. This duty cycle has the property of a wave and the intermittence of a particle.Reply
Once one realizes that the shift of light is a duty cycle shift, not a frequency shift, things start to fit together. The duty cycle of light keeps a constant on time. But the off time is modulated with emitter motion. Space Width Modulation.
No physicality can exist as a point. All physicality have area and volume. NOTHING can condense to a point. Only EM fields can superposition, mass never can.
These are the true basics. and Always have been.
Unclear Engineer said:We need some better explanations.
You're absolutely right. Fortunately, they do exist, and are readily available.
It's true that those image depicting gravity like a "bowling ball hanging in an elastic fish net" are everywhere, and it's also true that they are completely wrong. For one thing, although gravity bends spacetime, it's biggest effect by far is on bending time - not space.
PBS Space Time (hosted by physicist Matt O'Dowd) is one of the best physics channels on YouTube. As you can see in the following video, he shows how gravity really works. I think you should find the cover image on the video rather gratifying. (Note: In order to see the following embedded videos, you need to click on "VIEW ALL x COMMENTS" below.)
The following video, made two weeks later, goes into more detail about how gravity is caused.
As this is a rather tricky subject to get an intuitive grasp of, Matt also recommends Nick Lucid's video on the subject, which I found gave an additional useful perspective.
F5PfjsPdBzg, list: PLOVL_fPox2K83_36YgnGisn4rxNvgq1iR
Regarding black holes, you say:
Unclear Engineer said:But, then theorists argue that space does not really flow at all, it just bends and warps. And that really pulls the rational rug out from under that depiction of light not being able to go "upstream" fast enough to get out of a black hole's event horizon.
This is true. As we're talking about spacetime, it makes no sense to talk about the "flow" of spacetime, as that would imply spacetime flowing through time, which is rather impossible.
Instead, it's more useful to recall that all objects composed of mass have a certain escape velocity. In the case of black holes, that escape velocity is simply greater than the speed of light, which is why nothing can make it out of them. But what does that mean for the poor photon in a black hole moving toward the event horizon? Does it simply reach a point where it runs out of energy and falls back toward the black hole's center?
Photons don't act like that, of course. But just like a rocket ship on Earth that doesn't have enough fuel to reach escape velocity, a lack of sufficient energy is the photon's problem as well. Just as a rocket uses up energy in its climb into space, so does a photon lose energy in its trajectory toward the event horizon. Yet the speed of light is always constant, so what happens?
Here it's more useful to consider the photon's wave properties. Light waves lose energy by becoming more and more red shifted, i.e., their frequency decreases. Eventually, in this situation, the light wave runs out of energy, shedding it to the black hole's gravitational field, and its frequency becomes zero. In other words, at that point it just disappears.
Unclear Engineer said:Even worse, it you ask whether a photon acts like a particle or a wave at the event horizon, you tend to get a different conceptual image. If it is a particle, then it would lose energy as it traveled above the event horizon, but would not get to zero energy until it got to infinite distance - the effect would be increasing red shift as a function of distance above the event horizon. But, if it is a wave, then what does a photon do when emitted directly outward at an event horizon? What does "gravity" do to "waves" in a HIggs Field?
The Higgs field has no effect on bosons such as photons, so fortunately we don't have to worry about that aspect of things.
If a photon is emitted sufficiently far away from the event horizon of a black hole in a direction pointing away from it, then your description of what happens to it is accurate. And I've described above how gravity affects the energy of light waves.
What is "sufficiently far away" for the photon? It needs to be outside the photon sphere.
ZZZ, Thanks for the effort you put into your reply. I will take some time to watch and think before considering a response.Reply
One thing I am not very knowledgeable about is the Higgs Field. You posted that it doesn't act on bosons. But bosons are just zero spin objects that are not prohibited from occupying the same quantum state as others, right? Isn't a helium-4 atom a boson? Helium definitely has rest mass, making it fundamentally different from a photon. Any similarly good explanations of the Higgs Field and Higgs Bosons would be appreciated.
As I recall, quantum gravity is not tested and verified like many GR experiments, including white dwarfs and neutron stars. Space.com did publish a report on this in the past.Reply
"A team of researchers affiliated with several institutions in France and one in the U.S. has found that objects of different mass dropped in space fall at a rate within two-trillionths of a percent of each other. In their paper published in the journal Physical Review Letters, the group describes their satellite-based physics study and what they learned from it. Most everyone has heard the story of Galileo dropping two different sized cannon balls from the Tower of Pisa in the 17th century to demonstrate his theory that in the absence of air resistance, two objects will fall at the same rate. Einstein later refined the theory and added it to his Theory of General Relativity. Since that time, many people have tested the theory, and it has always been confirmed. Still, some physicists believe that there are bound to be exceptions to the theory because of the disconnect between general relativity and quantum mechanics. In this new effort, the team in France devised an experiment to measure two objects dropping together for two years—specifically, two chunks of metal in a satellite—to see if they could spot an exception."
Let me proffer a real intellectual fine point.Reply
Science can not presume or expect rational explanations for everything or anything,
that sets up a bias.
We like to have 'rational' explanations for things because that settles, calms our imaginations our psychologies.
Myths of gods will often suffice.
Determinism (would) makes things predictable.
If one finds it (something close enough to it) great,
but real science must be open to non-determinism.
Time seems to have some limited non-determinism.
The Universe might exist as a small gap between a yin and a yang.
Yin or yang might give an approximate guess, but the actuality is not exactly either,
and that is a very simple example.
Science finds coherent explanations for things,
and as long as they hold with reasonable accuracy they provide tools to work with
..... until they don't.
It's not satisfying, but science offers probabilities,
not the certainty of religion.
Mostly this falls on the ears of deaf conclusionists.
Just like mathematicians thinking math's 'perfection' was proof of god,
Until Godel proved otherwise.
Physicists are still expecting absolute perfect closure/certainty when in the most and more perfect realm of pure imagination their primary bias has already been proved impossible.
Our psychological needs will almost always override the delicate fine tuning of rational comprehension.
You can take the ape out of the jungle,
but you can't take the jungle out of the ape.
Please pass the bananas.
Unclear Engineer said:One thing I am not very knowledgeable about is the Higgs Field. You posted that it doesn't act on bosons. But bosons are just zero spin objects that are not prohibited from occupying the same quantum state as others, right? Isn't a helium-4 atom a boson? Helium definitely has rest mass, making it fundamentally different from a photon. Any similarly good explanations of the Higgs Field and Higgs Bosons would be appreciated.
You're right about the definition of a boson and how that applies to objects such as helium-4 atoms. The more precise answer is that the Higgs Field acts on particles with mass, and only on particles with mass. The greater a particle's mass, the stronger its interaction with the Higgs field is.
The one exception here is neutrinos, which have mass, but which don't get it from interacting with the Higgs field. Nobody knows for sure where neutrinos get their mass from, although there are a number of theories. The Standard Model predicts that neutrinos should have no mass, but the fact that they do have mass has been observed experimentally.
A Higgs boson is simply a quantum excitation of the Higgs field in the same way that all fundamental particles are simply quantum excitations of their associated quantum fields. It is the Higgs field that is responsible for giving all particles their mass; the Higgs boson is not involved in this process. However, detection of the Higgs boson confirmed the existence of the Higgs field, and the properties of the Higgs boson (including its various decay modes) tell us something about the way the Higgs field works.
The mechanism by which the Higgs field gives particles their mass is not particularly intuitive. How would a field that makes particles more massive as the interactions get stronger (and the particles get harder to accelerate as well) not slow down their motion? The answer is in the math.
One of the most notable features of the Higgs field is that unlike the quantum fields associated with all the other Standard Model particles, the lowest energy state of the Higgs field is not the state where the field is completely absent. As a result, the Higgs field has a nonzero value everywhere. This property is crucial for the Higgs field's ability to give particles mass throughout the entire universe.
There are so many ways that the Higgs boson and the Higgs field are central to modern particle physics that one could write a book about them (and many people have). Needless to say, I can't do justice to this topic in a forum post. If you don't know at least the basics of quantum field theory, learning about it is a good place to start.
I'm not sure which approach works best for you, so here's a playlist from YouTube on quantum field theory; you can sample these and see which you like best. A number of them don't directly deal with QFT, but still contain fascinating related material.
ATcrrzJFtBY, list: RDQMgIyGuUz7dgg
And here a couple of videos specifically about the Higgs mechanism from PBS Space Time:
Einstein has taken us down a wrong path, and 100 years later, physics has not recovered from the consequences. We need to look at the clear evidence and go back to working on real physics instead of science fiction! Theory and experiments show Special Relativity and General Relativity are optical illusions. Space and time are absolute as denoted by Galilean Relativity.
Hi my name is Dr William Walker and I am a PhD physicist and have been investigating this topic for 30 years. It has been known since the late 1700's by Simone Laplace that nearfield Gravity is instantaneous by
analyzing the stability of the orbits of the planets about the sun. This is actually predicted by General Relativity by analyzing the propagating fields generated by an oscillating mass. In addition, General Relativity predicts that in the farfield Gravity propagates at the speed of light. The farfield speed of gravity was recently confirmed by LIGO.
Recently it has been shown that light behaves in the same way by using Maxwell's equations to analyze the propagating fields generated my an oscillating charge. For more information search: William Walker Superluminal. This was experimentally confirmed by measuring radio waves propagating between 2 antennas and separating the antennas from the nearfield to the farfield, which occurs about 1 wavelength from the source. This behavior of gravity and light occurs not only for the phase and group speed, but also the information speed.
This instantaneous nature of light and gravity near the source and been kept from the public and is not commonly known. The reason is that it shows that both Special Relativity and General Relativity are wrong! It can be easily shown that Instantaneous nearfield light yields Galilean Relativity and farfield light yields Einstein Relativity. This is because in the nearfield, gamma=1since c= infinity, and in the farfield, gamma= the Relativistic gamma since c= farfield speed of light. Since time and space are real, they can not depend on the frequency of light used. This is because c=wavelength x frequency, and 1 wavelength=c/frequency defines the nearfield from the farfield. Consequently Relativity is an optical illusion. Objects moving near the speed of light appear to contract in length and time appears to slow down, but it is just what you see using farfield light. Using nearfield light you will see that the object has not contracted and time has not changed. For more information: Search William Walker Relativity.
Since General Relativity is based on Special Relativity, General Relativity must also be an optical illusion. Spacetime is flat and gravity must be a propagating field. Researchers have shown that in the
weak field limit, which is what we only observe, General Relativity reduces to Gravitoelectromagnetism, which shows gravity can be modeled as 4 Maxwell equations similar in form to those for electromagnetic fields, yielding Electric and Magnetic components of gravity. This theory explains all gravitational effects as well as the instantaneous nearfield and speed of light farfield propagating fields. So gravity is a propagating field that can finally be quantized enabling the unification of gravity and quantum mechanics.
YouTube presentation of above argument:
William D. Walker, PhD Thesis - Gravitational Studies, ETH Zurich, 1997
William D. Walker, Superluminal Electromagnetic and Gravitational Fields Generated in the Nearfield of Dipole Sources, 2006
William D. Walker, Nearfield Electromagnetic Effects on Einstein Special Relativity, 2007
Z. Wang, ‘New Investigations on Superluminal Propagation of Electromagnetic Waves in Nondispersive Media’, Nov. (2003).
J. C. Sten and A. Hujanen, ‘Aspects on the Phase Delay and Phase Velocity in the Electromagnetic Near-Field’, Progress In
Electromagnetics Research, PIER 56, 67-80, (2006).
Hans G. Shantz, "Near Field Phase Behavior", 2005
A spherical piece of space can have a much larger volume than one would think based on its radius.Reply
The space within the sphere is funnel-shaped and ultimately tubular, stretched to an arbitrary length.
The matter density has a maximum, but the total amount of matter does not.
So a black hole is not a singularity (not infinitely dense)