Einstein Was Right, Again: Novel Experiment Proves Antigravity Doesn’t Exist

Dreams of a world powered by antigravity got quashed by a particle physics today.

View of the 'Warp Core,' the engine of the USS Enterprise, in a scene from an episode of the televis...

It turns out that Einstein was right yet again. A recent experiment just proved that antigravity doesn’t exist and we probably won’t ever get to use antimatter to levitate or build a perpetual motion machine or power warp drives (sorry, Star Trek ).

Antimatter itself is very real. Made of particles that mostly behave like regular matter, but their electrical charges are reversed, an anti-proton looks just like a proton but has a negative charge, while an anti-electron (or positron) looks and moves just like an electron but has a positive charge. When a bit of antimatter bumps into a bit of matter, they explode so dramatically that all of their combined mass is converted into energy.

Now we know that matter and antimatter are drawn toward each other — not pushed apart — by gravity. Physicist Albert Einstein predicted this in his theory of general relativity years before the first positron was discovered, but Aarhus University physicist Emma Anderson and her colleagues at ALPHA (the Antihydrogen Laser Physics Apparatus) just tested the theory by watching atoms of anti-hydrogen — a single anti-electron orbiting an anti-proton — fall downward under the pull of Earth’s gravity.

The researchers recently published their work in the journal Nature .

A person in a hard had stands guides a large metal tube into an apparatus in the floor

This is the experimental equipment the ALPHA team used to capture and study anti-hydrogen atoms.

Au Revoir, Antigravity

In a nutshell, Anderson and her colleagues dropped atoms of antihydrogen down a tube, and the atoms fell downward thanks to gravity. That sounds simple, and in fact, it’s exactly what Einstein predicted would happen — but it hadn’t been done before. Until now, there was a lingering chance that antimatter wouldn’t feel the tug of Earth’s gravity, or that matter and antimatter actually experienced a sort of antigravity, pushing each other apart.

To make anti-hydrogen, you need to combine positrons and anti-protons. Anti-protons are produced in high-energy particle collisions, then slowed down in what's called an antiproton decelerator at CERN (which is also home to the Large Hadron Collider). Positrons come from radioactive decay of certain chemical elements, like potassium.

Once you have the antimatter, you face the challenge of working with something that will disappear if it touches even another atom of ordinary matter. The team used an eight-pole magnet to keep antihydrogen atoms swirling along the magnetic field lines and away from the deadly walls of the container. Anti-atoms with enough energy (anti-atoms that were moving fast enough, in other words) could still escape, but slower ones would be trapped inside the magnetic field.

on a blue background, blue atoms fall through a cutaway of a yellow tube with magnetic field lines d...

This illustration shows what anti-hydrogen atoms falling through the magnetic trap might look like if we could actually see them.

When the researchers turned their tube of captured antimatter vertically, they found that the atoms moving downward along the magnetic field lines sped up thanks to the added pull of gravity; the atoms moving upward slowed down, also thanks to gravity trying to pull them Earthward. Anderson and her colleagues couldn’t actually watch the anti-atoms in action, of course, but their instruments counted the tiny flashes of energy every time an anti-hydrogen atom, pulled downward by gravity, gained enough speed to punch through the magnetic field at the bottom of the container and escape, annihilating itself and an unfortunate atom of regular matter in the process.

“To do the experiment, you're actually just turning down the current that makes the magnetic field,” Hangst tells Inverse . “You have a cloud of [anti-hydrogen atoms] bouncing around, and you let them go.”

When that happened, about 80 percent of the anti-hydrogen atoms fell toward Earth. The rest, about 20 percent, were still bouncing upward fast enough to keep going. That’s pretty much the result you’d expect from a tiny cluster of regular hydrogen atoms bouncing around in a magnetic field, too.

That suggests that matter and antimatter both feel the pull of Earth’s gravity in the same way, which means matter and antimatter are attracted, not repelled, by each other’s gravity. In other words, the experiment confirmed that matter and antimatter are drawn together, just like all the other mass in the universe, regardless of their weird properties.

“If you walk down the halls of the department and ask the physicists, they would all say that this result is not the least bit surprising, but most of them will also say that the experiment had to be done because you can never be sure,” says University of California at Berkeley physicist Jonathan Wurtele, a coauthor of the study, in a recent statement. “You don’t want to be the kind of stupid that you don’t do an experiment that explores possibly new physics because you thought you knew the answer, and then it ends up being something different.”

Where’s All The Antimatter?

While we can’t use antimatter to levitate or power a perpetual motion machine, we also can’t blame antigravity for shoving all the antimatter out of the universe we see around us, which would have been a convenient way to explain the one prediction of Einstein’s that antimatter definitely doesn’t seem to obey.

According to general relativity, antimatter and matter should exist in equal amounts. But there’s almost no antimatter in the universe, or at least anywhere in the universe we can see and measure. And that raises important questions, like where the heck did it all go ?

One idea was that shortly after the Big Bang, matter and antimatter basically gave each other a giant shove apart, separating each other once and for all except for a few tiny, scattered particles. But Anderson and her colleagues’ experiment proves that’s just not how antimatter works, leaving physicists with another big mystery to solve.

What’s Next

By varying the strength of the magnetic field — and thus varying how fast anti-atoms had to move in order to punch through it and escape — Anderson and her colleagues managed to measure how much the pull of gravity accelerated the antimatter. The answer turned out to be around 32 feet per second (per second), which is roughly how much Earth’s gravity accelerates ordinary falling matter, too.

One of the next steps is to measure that even more precisely to make sure there’s not really any small difference in how much antimatter accelerates as it falls downward. In other words, that it feels gravity’s pull just as strongly as regular matter, not more or less.

But for now, Anderson and her colleagues are focused on studying how antimatter interacts with radiation. If Einstein’s predictions are correct, anti-hydrogen should absorb, emit, and reflect the same spectrum of light as regular hydrogen — so Anderson and her colleagues will spend the next year or so zapping antimatter with lasers and microwaves to find out.

This article was originally published on Sep. 27, 2023

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Why dark energy could keep the dream of anti-gravity alive

A new antiproton decelerator experiment has revealed stunning new insights into gravity and antimatter.

Photo credit: Getty

Prof Jon Butterworth

According to first Newton, then Einstein, and now an experiment at CERN , gravity is an attractive force that exists between all objects in the Universe.

That includes objects that have no mass, because gravity acts on energy, and mass is just one form of energy (as Einstein’s most famous equation states, energy is equal to mass multiplied by the square of the speed of light ). This is why even massless photons of light, travelling from distant stars, have their paths bent as they pass massive galaxies on the way.

Antigravity is a hypothetical repulsive gravitational force. In some ways, it sounds obvious that it should exist. There are both attractive and repulsive electric forces, so why not the same for gravity?

The difference is that electric charge comes in two types, positive and negative. Positive and negative charges attract each other, while charges that are alike repel each other. The equivalent of ‘charge’ for gravity is energy, and it only comes in one type – positive.

As these positive energies attract each other there, doesn’t seem to be room for antigravity, which is a pity because it would be a great way of flying around without the need for rockets, jet engines or even wings.

However, there is (or was, until this month) a possible get-out clause for antigravity: antimatter.

Why antimatter matters

Antimatter is not hypothetical, it is very real. Particles such as electrons have an antimatter equivalent. The antiparticle of the electron is the positron, and it has not only been observed, but is regularly used in hospitals for diagnostic purposes.

Positrons emitted from unstable elements injected into a patient’s body will give off a very distinctive energy signal when they meet an electron and annihilate. The signal is so distinctive that the point of annihilation can be identified very precisely.

The whole process, known as Positron Emission Tomography, gives doctors unique information on the soft tissues and flow of material around a body.

A new antigravity mystery

Antimatter has the opposite electric charge to matter, so does it also have the opposite gravitational charge, and so experience antigravity? This was the question the ALPHA-g experiment at CERN was designed to answer. Does antimatter fall down or up?

Producing antiparticles is quite easy. Accelerators such as those at CERN can make many positrons and anti-protons. That’s fine, but these particles have electric charge, and they are also in general moving at high speed. Neither of those things is good if you want to measure the effect of gravity, because gravity is really, really weak.

Just think: your muscles, which use the electromagnetic force, can pick up a pen or paper, thereby counteracting the combined gravitational attraction of an entire planet.

So any tiny stray electric field in your experiment could easily obscure the effect of gravity on a charged particle like a positron or antiproton. And anyway, they will have sped away before you could see which way they fall.

The antiproton decelerator at CERN is designed to combat this; to slow down antiprotons, and eventually bring them together with positrons to make electrically neutral antihydrogen. In a similar way in which an atom of hydrogen is made up of a single proton and an electron, an atom of antihydrogen is made up of a single antiproton and a positron.

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The ALPHA experiment has been collecting antihydrogen atoms and studying them since 2013, and this month they published results from a new setup, called ALPHA-g, where the g stands for gravity.

The idea is very simple – trap a few hundred antihydrogen atoms in a vertical tube, let them diffuse around, and measure how many come out of the top and how many come out of the bottom.

The experimental set-up is such that if gravity affects antimatter in the same way it affects matter, 80 per cent of them should drop out of the bottom, while 20 per cent would diffuse out of the top of the experiment by 'bouncing' up.

Within the precision of the experiment, this is what happened. Antimatter falls down, like normal matter.

Now, is this the end of the road for antigravity?

Not really.

But it is the end for a certain type of antigravity. We won’t be getting antigravity rockets (or hoverboards) riding on a cushion of antimatter.

However, while most scientists are profoundly unsurprised by this result, a form of antigravity is actually built into our current best understanding of cosmology.

Astrophysical measurements indicate that the rate of expansion of the Universe is increasing, meaning that some force is counteracting the gravitational attraction between the matter in the Universe, and actually pushing it apart. We call this dark energy but we could just as well have called it “antigravity”.

In fact, there were even cosmological theories which proposed that half the Universe was made up of antimatter, and this was repelling the matter and thus providing the dark energy effect.

Such ideas also potentially solved some other problems with our understanding of cosmology – although they created a whole bunch more. Either way, in light of the ALPHA-g result, it seems they are wrong, and there must be something else behind the antigravity effect of dark energy.

Read more about physics:

  • The parallel worlds of quantum mechanics
  • Dead and alive: why it's time to rethink quantum physics
  • The quest for quantum gravity: why being wrong is essential to science

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