Scientists Have Figured Out How to Make Antigravity

In Einstein's theory of general relativity, gravity is always attractive. But according to a new paper, quantum mechanics offers a loophole that can create antigravity. And not only this, if it was measurable that would prove that gravity has quantum properties. How does this work and how can repulsive gravity be compatible with the laws of physics that we know and like? Let's take a look.

Einstein's Theory Has a Problem -- This Idea Solves It

Physicists have been trying to reconcile the differences between Einstein’s theory of spacetime and our observations of quantum mechanics for almost a century. One way that they’ve attempted to do this involves theories that treat space as one-dimensional at very short distances. In a recent paper, physicists claim that they’ve solved a major problem that’s plagued these theories for decades. Does this finally solve the problem? Let’s take a look.

This Spacetime Quasicrystal Could Solve Physicists’ Biggest Problem

What is space, really? That’s one of the biggest questions in science. According to a pair of researchers from the Perimeter Institute, the answer to that is: a quasicrystal. What is a quasicrystal, and how is space a quasicrystal? Let’s take a look.

A New Link Between Quantum Physics and Gravity

We know that Einstein’s general relativity is, strictly speaking, wrong. That’s because it doesn’t account for quantum effects despite the fact that those effects really do exist. In a new paper, physicists say they’ve re-done Einstein’s equations with quantum effects taken into account. According to them, there’s a “clear difference” between the two. Let’s take a look at what they’ve done and what it could mean for physics.

We Thought This Particle Was Impossible To Measure!

The #1 most-wanted particle in physics is the graviton, a quantum of gravity. If physicists were to prove that gravitons exist, they would unambiguously prove that Einstein’s theory is ultimately wrong and must be replaced by a more complete theory that gives quantum properties to space and time. In a recent paper, a physicist came up with an ingenious experiment that could prove that gravitons do exist. Let’s take a look.

String Theory is “Fashion,” Penrose Said. We Finally Have a Response

Roger Penrose is a very well-known mathematician, physicist, and author who won the Nobel Prize a few years back. He’s repeatedly questioned string theory’s prominent role in physics today, most notable in his 2016 book “Fashion, Faith and Fantasy” (the “fashion” part refers to string theory). We finally have a response.

Is Gravity Just an Illusion Caused by Entropy? New Theory Explains How.

Over the past few decades, the idea that gravity is not a fundamental interaction but is instead caused by the increase of entropy has become increasingly popular in the world of physics. Today, we have a paper from a group of physicists who claim that entropic gravity might be the result of space being full of qubits. Let’s take a look.

Causal Order Doesn’t Work, Physicists Find. Now what?

To us normal people, cause and effect are linked together in a distinct order. The cause occurs first, and the effect follows after. But according to a new paper, that causal order breaks down if you apply quantum mechanics to gravity. Join me in my quest to wrap my head around this head-spinning (but also plausible) paper.

New Experiment To Look for Quantum Noise of Space Itself

Physicists are stuck on trying to figure out why gravity and quantum mechanics don’t get along. For almost 100 years now, they have been looking for a theory of quantum gravity to solve the problem. But one of the most general expectations of a quantization of gravity is that space also has quantum fluctuations. And a team of researchers from Caltech now says they’ve got a tabletop experiment which could find those fluctuations. Could this solve the problem? Let’s take a look.



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Why I think the new quantum gravity theory is wrong

Last year, Jonathan Oppenheim’s theory of post-quantum gravity was the only theory that made it into my Best of 2024 summary. Post-quantum gravity is one of the most interesting theories for merging quantum physics with gravity, which in turn is one of the most important problems in physics. Now, researchers say they’ve come up with a way to test the theory, though technology needs to advance before that test is possible. In this video I explain why I think it’s likely that post-quantum gravity will be falsified.

String Theory Isn’t Dead

Heard anything of string theory lately? Well, there have been a few developments. Apparently it’s not dead, going by a recent review article. The article also lists all the things that string theorists have learned recently. And they indeed learned something. Let’s take a look.

Theory of Everything Predicts New Physics For Supermassive Black Holes

Mathematician Stephen Wolfram has attempted to develop a theory of everything using hypergraphs, which are essentially sets of graphs that can describe space-time. Recently, another mathematician named Jonathan Gorard has used hypergraphs to describe what happens if a black hole accretes matter. He claims that evidence for hypergraphs should be observable in the energy that is emitted during the accretion. Big if true, as they say. Let’s take a look.

Please... No more Loop Quantum Gravity

I was recently alerted to a video by my friend and colleague Brian Keating that claims Loop Quantum Gravity (LQG), string theory’s biggest competitor, has been disproven. I was somewhat surprised by this because I was pretty convinced it is for all practical purposes untestable -- much like string theory. I had a look at what he is talking about.


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A new test for quantum gravity

I suspect that one of Einstein’s nightmares was that his theory of gravity would be turned into a quantum theory. I also suspect that we’re inching closer to that nightmare situation every day -- but not how Einstein might have expected. After countless attempts to develop a theory of quantum gravity, physicists are now trying their hand at measuring it through various experiments. Let’s have a look.

Search for Quantum Foam Begins at South Pole

Physicists will soon begin an experiment at the South Pole to test if space has quantum fluctuations. Their new approach looks for decoherence in neutrinos oscillations that are sensitive to what has been dubbed "quantum foam" that could even contain tiny black holes. If successful, this experiment could uncover something that will combine Einstein’s theory of gravity and quantum physics? Let’s have a look.

Three Physics Mysteries Solved At Once: Postquantum Gravity

For the first time in 4 decades, physicists have found a new approach to solving a problem which is almost a century old: How to combine quantum physics with gravity. I told you about this new approach, called “Postquantum Gravity” from Johnathan Oppenheim briefly before Christmas. He and his collaborators are now saying that their idea also explains dark matter and dark energy.



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The experiment that could save physics

Physicists at the University of Warwick in the UK are starting to build an experiment that could just save physics. They could be the first to measure the effects of quantum gravity and unlock progress in a field that has been stuck for more than 4 decades.

String Theory gets Competition: A New Attempt to Solve Physics' Biggest Mystery

Every scientific discipline has its holy grail, and the holy grail of physics is definitely quantum gravity. Quantum gravity is Einstein’s unfinished revolution, the missing unification of General Relativity and Quantum Theory. String theory is the best-known contender, and this week have a strong newcomer.

These Experiments Could Prove Einstein Wrong

[This is a transcript of the video embedded below. Some of the explanations may not make sense without the animations in the video.]


Einstein’s theory of general relativity has made countless correct predictions. And yet physicists are constantly trying to prove it wrong. Why? What would it be good for to prove Einstein wrong? And how could it be done? That’s what we’ll talk about today. First of all, I have to clarify that when I say “proving Einstein wrong”, I mean proving Einstein’s theory of general relativity wrong. Einstein himself has actually been wrong about his own theory, and not only once.

For example, he originally thought the universe was static, that it remained at a constant size. He changed his mind after learning of Hubble’s discovery that the light of distant galaxies is systematically shifted to the red, which is evidence that the universe expands. Einstein also at some point came to think that gravitational waves don’t exist, and argued that black holes aren’t physically possible. We have meanwhile found evidence for both.

I’m not telling you this to belittle Einstein. I’m telling you this because it’s such an amazing example for how powerful mathematics is. Once you have formulated the mathematics correctly, it tells you how nature works, and that may not be how even its inventor thought it would work. It also tells us that it can take a long time to really understand a theory.

General Relativity is now more than a century old, and so far its predictions have all held up. Light deflection on the sun, red shift in the gravitational field, expansion of the universe, gravitational waves, black holes, they were right, right, right, and right again, to incredibly high levels of precision. But still, most physicists are pretty convinced Einstein’s theory is wrong and that’s why they constantly try to find evidence that it doesn’t work after all.

The most important reason physicists think that general relativity must be wrong is that it doesn’t work together with quantum mechanics. General relativity is not a quantum theory, it’s instead a “classical” theory as physicists say. It doesn’t know anything about the Heisenberg uncertainty principle or about particles that can be in two places at the same time and that kind of thing. And this means we simply don’t have a theory of gravity for quantum particles. Even though all matter is made of quantum particles.

Let that sink in for a moment. We don’t know how matter manages to gravitate even though the fact that matter *does gravitate is the most basic observation about physics that we make in our daily life.

This is why most physicists currently believe that general relativity has a quantum version, often called “quantum gravity”, just that no one has yet managed to write down the equations for it. Another reason that physicists think Einstein’s theory can’t be entirely correct is that it predicts the existence of singularities, inside black holes and at the big bang. At those singularities, the theory breaks down, so general relativity basically predicts its own demise.

Okay, so we have some reason to think general relativity is wrong, but how can we find out whether that’s indeed the case? The best way to do this is by testing the assumptions that Einstein based his theory on. The most important assumption is that the speed of light is the same in all directions and everywhere in the universe. To be precise, that refers to the speed of electromagnetic radiation at all frequencies, not just in the range of visible light, and it’s the speed in vacuum, usually denoted c. The speed of light in a medium depends on the rest frame of the medium.

According to Einstein, the speed of light in vacuum doesn’t depend on the energy of the light or its polarization. If the speed depends on the energy, that’s called dispersion, and if it depends on the polarization that’s called birefringence. We know that these effects both exist in medium. If we’d also see them in vacuum, that would mean Einstein was wrong indeed.

The currently best experiments for this come from analyzing electromagnetic radiation from gamma ray bursts. This is mostly because gamma ray bursts are bright, short, and can be far away, often several billion light years. Moreover, they emit electromagnetic radiation up to really high energies. Since one knows that the light must have been emitted in the burst at about the same time regardless of its energy, one can then test whether it also arrives at the same time. If it doesn’t, that would be evidence that the speed of light depends on the energy.

Since the gamma ray bursts are so far away, even tiny differences in the speed of light can add up to a noticeable delay. The most recent data on this were published just a few months ago by a group from Oxford and Stockholm. So far there is no indication that Einstein was wrong. You already knew this of course because otherwise you’d have seen the headlines! But that’s one way it could happen.

In general relativity it further turns out that gravitational waves also move with the speed of light. This is quite difficult to test because it requires one to measure both gravitational waves and light from the same source. Even if you manage to do that, it’s difficult to tell whether they were really emitted simultaneously. There is really only one measurement of this at the moment, which is a gravitational wave event from August 2017.

This is believed to have been a merger of two neutron stars, and it was accompanied by an electromagnetic signal. The electromagnetic signal was detected by the Fermi and INTEGRAL spacecraft beginning at about 1.7 seconds after the gravitational wave event began.

This is compatible with what Einstein predicts. However, it is pretty much impossible to prove Einstein wrong this way. Because if the two signals do not arrive together you don’t know whether that’s because one arrived 5 years earlier, or will arrive 100 years later, or maybe because you just didn’t measure it because it was too weak. Indeed, so far none of the other observed gravitational wave events came with an electromagnetic counterpart, and no one’s claimed that this means Einstein was wrong.

So that’s not a very promising way to prove Einstein wrong. But gravitational waves offer another opportunity to do that. In Einstein’s theory of general relativity, the black hole horizon is not a physical thing. It’s just the location of a surface that, once you’re inside, you can’t get out. It’s really just a name we give to this boundary much like city limits. But if Einstein’s theory is not fundamentally correct, then the black hole horizon could have physical properties, for example created by quantum effects in that yet-to-be-found theory of quantum gravity.

If that was so, then the gravitational waves emitted from black hole mergers would look different from what Einstein predicts. Because if the horizon is a physical thing, then it can ring and that creates echoes, not of sound waves, but of gravitational waves. In the gravitational wave data, this would look like a regular repetition of the original signal with decreasing amplitude.

There are a number of people who have looked for those. Niayesh Afshordi and his group at Perimeter Institute, some people from the LIGO collaboration, and a few others. They actually did find a signal that looked like an echo in the previously mentioned gravitational wave event from August 2017. Depending on whom you ask, the statistical significance is between 2 and 4.2 sigma.

However, after analyzing the data some more, astrophysicists seem to have mostly agreed that the alleged echo didn’t have anything to do with the black hole horizon itself. Remember this was a neutron star merger. People from Luciano Rezolla’s group have argued what happened is that the collapse to a black hole was somewhat delayed. This looks like an echo, but only once, and is also why the electromagnetic signal came 1.7 seconds after the gravitational wave signal had started.

In a new paper which just appeared a few weeks ago, Niayesh’s group claims again they’ve found a signal of a black hole echo. They just can’t give up trying to prove Einstein wrong. This time they say they found it in a different gravitational wave event and at 2.6 sigma, so that’s about a 1 in 200 chance to be coincidence. Personally I think it’s very implausible that we will find evidence that Einstein was wrong in black hole signals, but it’s worth looking for.

Another way physicists try to find ways to prove general relativity wrong is by showing that it doesn’t correctly work together with quantum mechanics. The major challenge for doing this is that in the experiments that we have been able to do so far, we either measure quantum effects, but then the masses of the objects are so small that we can’t measure the gravitational field. Or we can measure the gravitational field, but then the objects are so massive we can’t measure their quantum effects.

So one of the ways to prove Einstein wrong is to bring more massive objects into quantum superpositions and then measure their gravitational field. If the gravitational field is also in a quantum superposition, then that means general relativity is out and Einstein wrong. This avenue is pursued for example by the group of Markus Aspelmeyer in Vienna.

A related idea is to show that gravity can cause entanglement. Entanglement is a quantum effect and if it can be caused by gravity, then this means gravity must have quantum properties too, which it can’t in Einstein’s theory. So that too would prove Einstein wrong. This is a good idea in principle, but I suspect that in practice it will be very, very difficult to show that the entanglement didn’t come about in other ways.

Another rather straight-forward test is to check whether the one-over-R-squared law holds at very short distances. Yes, that’s known as Newton’s law of gravity, but we also have it in general relativity. Whether this remains valid at short distances can be directly tested with high precision measurements. These are done for example by the group of Eric Adelberger in Washington DC.

This image shows the key component of their measuring device. These two parts are rotated against each other while the gravitational attraction between them is being measured. This creates a periodically changing force which is a really clever way to filter out noise. Their most precise measurement yet was published in 2020 and confirms that one-over-R-squared law is correct all the way down to 57 micrometers. So again, they didn’t find anything out of the order so far, but this is another way Einstein could turn out to be wrong. 

 Finally, one can test a key assumption underlying general relativity, which is the equivalence principle. The equivalence principle says loosely speaking that all objects should fall the same and, most importantly, that how fast they fall doesn’t depend on their mass. This is much easier to measure than the gravitational field of particles because when you test the equivalence principle you are looking for a difference.

You can make your life even easier by looking for a difference between two objects that are very similar except for their mass, like two different isotopes of the same atom. This has been done most recently by a group in Stanford, California who looked for a difference in how two isotopes of Rubidium fall in the gravitational field of Earth. Again you already know they didn’t find any violation of the equivalence principle because otherwise you’d have heard of it. But this too is a way that Einstein could turn out to be wrong.

What would it be good for to prove Einstein wrong? Well, first of all it would give us experimental guidance to develop a theory of quantum gravity, and that could help us understand the quantum properties of space and time, as well as what’s inside black holes or what happened at the big bang.

Many physicists also hope that it will shed light on other puzzles, such as dark matter and dark energy, or explain some nagging anomalous observations in cosmology, like the presence of too many large structures in the universe, which we talked about in an earlier video, or that different measurement of the Hubble rate don’t give the same results.

Personally I think the most promising way to prove Einstein wrong is the approach pursued by the group of Aspelmeyer. And if they succeed they’ll almost certainly win a Nobel Prize. But it’s quite possible that in the end the breakthrough will happen in a way that no one saw coming.

Is the universe REALLY a hologram?

[This is a transcript of the video embedded below.]


Do we live in a hologram? String theorists think we do. But what does that mean? How do holograms work, and how are they related to string theory? That’s what we will talk about today.

In science fiction movies, holograms are 3-dimensional, moving images. But in reality, the technology for motion holograms hasn’t caught up with imagination. At least so far, holograms are still mostly stills.

The holograms you are most likely to have seen are not like those in the movies. They are not a projection of an object into thin air – however that’s supposed to work. Instead, you normally see a three-dimensional object above or behind a flat film. Small holograms are today frequently used as a security measure on credit cards, ID cards, or even banknotes, because they are easy to see, but difficult to copy.

If you hold such a hologram into light, you will see that it seems to have depth, even though it is printed on a flat surface. That’s because in photographs, we are limited to the one perspective from which the picture was taken, and that’s why they look flat. But you can tilt holograms and observe them from different angles, as if you were examining a three-dimensional object.

Now, these holograms on your credit cards, or the ones that you find on postcards or book covers, are not “real” holograms. They are actually composed of several 2-dimensional images and depending on the angle, a different image is reflected back at you, which creates the illusion of a 3-dimensional image.

In a real hologram the image is indeed 3-dimensional. But the market for real holograms is small, so they are hard to come by, even though the technology to produce them is straightforward. A real hologram looks like this.

Real holograms actually encode a three-dimensional object on a flat surface. How is this possible? The answer is interference.

Light is electromagnetic waves, so it has crests and troughs. And a key property of waves is that they can be overlaid and then amplify or wash out each other. If two waves are overlaid so that two crests meet at the same point, that will amplify the wave. This is called constructive interference. But if a crest meets a trough, the waves will cancel. This is called destructive interference.

Now, we don’t normally see light cancelling out other light. That’s because to see interference one needs very regular light, where the crests and troughs are neatly aligned. Sunlight or LED light doesn’t have that property. But laser light has it, and so laser light can be interfered.

And this interference can be used to create holograms. For this, one first splits a laser beam in two with a semi-transparent glass or crystal, called a beam-splitter, and makes each beam broader with a diverging lens. Then, one aims one half of the beam at the object that one wants to take an image of. The light will not just bounce off the object in one single direction, but it will scatter in many different directions. And the scattered light contains information about the surface of the object. Then, one recombines the two beams and captures the intensity of the light with a light-sensitive screen.

Now, remember that laser light can interfere. This means, how large the intensity on the screen is, depends on whether the interference was destructive or constructive, which again depends on just where the object was located and how it was shaped. So, the screen has captured the full three-dimensional information. To view the hologram, one develops the film and shines light onto it at the same wavelength as the image was taken, which reproduces the 3-dimensional image.

To understand this in a little more detail, let us look at the image on the screen if one uses a very small point-like object. It looks like this. It’s called a zone plate. The intensity and width of the rings depends on the distance between the point-like object and the screen, and the wavelength of the light. But any object is basically a large number of point-like objects, so the interference image on the screen is generally an overlap of many different zone plates with these concentric rings.

The amazing thing about holograms is now this. Every part of the screen receives information from every part of the object. As a consequence, if you develop the image to get the hologram, you can take it apart into pieces, and each piece will still recreate the whole 3-dimensional object. To understand better how this works, look again at the zone plate, the one of a single point-like object. If you have only a small piece that contains part of the rings, you can infer the rest of the pattern, though it gets a little more difficult. If you have a general plate that overlaps many zone plates, this is still possible. So, at least mathematically, you can reconstruct the entire object from any part of the holographic plate. In reality, the quality of the image will go down.

So, now that you know how real holograms work, let us talk about the idea that the universe is a hologram.

When string theorists claim that our universe is a hologram, they mean the following. Our universe has a positive cosmological constant. But mathematically, universes with a negative cosmological constant are much easier to work with. So, this is what string theorists usually look at. These universes with a negative cosmological constant are called Anti-de Sitter spaces and into these Anti-de Sitter things they put supersymmetric matter. To best current knowledge, our universe is not Anti De Sitter and matter is not supersymmetric, but mathematically, you can certain do that.

For some specific examples, it has then been shown that the gravitational theory in such an Anti de Sitter universe is mathematically equivalent to a different theory on the conformal boundary of that universe. What the heck is the conformal boundary of the universe? Well, our actual universe doesn’t have one. But these Anti-De Sitter spaces do. Just exactly how they are defined isn’t all that important. You only need to know that this conformal boundary has one dimension of space less than the space it is a boundary of.

So, you have an equivalence between two theories in a different number of dimensions of space. A gravitational theory in this anti-De Sitter space with the weird matter. And a different theory on the boundary of that space, which also has weird matter. And just so you have heard the name: The theory on the boundary is what’s called a conformal field theory, and the whole thing is known as the Anti-de Sitter – Conformal Field Theory duality, or AdS/CFT for short.

This duality has been mathematically confirmed for some specific cases, but pretty much all string theorists seem to believe it is much more generally valid. In fact, a lot of them seem believe it is valid even in our universe, even though there is no evidence for that, neither observational nor mathematical. In this most general form, the duality is simply called the “holographic principle”.

If the holographic principle was correct, it would mean that the information about any volume in our universe is encoded on the boundary of that volume. That’s remarkable because naively, you’d think the amount of information you can store in a volume of space grows much faster than the information you can store on the surface. But according to the holographic principle, the information you can put into the volume somehow isn’t what we think it is. It must have more correlations than we realize. So it the holographic principle was true, that would be very interesting. I talked about this in more detail in an earlier video.

The holographic principle indeed sounds a little like optical holography. In both cases one encodes information about a volume on a surface with one dimension less. But if you look a little more closely, there are two important differences between the holographic principle and real holography:

First, an optical hologram is not actually captured in two dimensions; the holographic film has a thickness, and you need that thickness to store the information. The holographic principle, on the other hand, is a mathematical abstraction, and the encoding really occurs in one dimension less.

Second, as we saw earlier, in a real hologram, each part contains information about the whole object. But in the mathematics of the holographic universe, this is not the case. If you take only a piece of the boundary, that will not allow you to reproduce what goes on in the entire universe.

This is why I don’t think referring to this idea from string theory as holography is a good analogy. But now you know just exactly what the two types of holography do, and do not have in common.