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Top 10 Unsolved Mysteries In Physics

Thomas Hornigold

If you’ve ever watched an episode of Star Trek or The Big Bang Theory, then you know that physics can be made accessible to the masses in a fun way. Our favorite sci-fi and comedy writers may not get every detail right, but they do spark our interest in the weirder aspects of scientific theories.

Today, we’re going to talk about 10 real mysteries that physics has yet to explain. From alien communication to time travel to gushing faucets, we’ll try to make these mysteries understandable for everyone.

You may even want to explore these topics further on your own. After all, there are million dollar prizes waiting for the people who solve some cosmic puzzles. (Read on to find out which one of these 10 mysteries could make you rich.) You’ll probably get a Nobel Prize and change the world, too.

10 Where Do Ultra-High-Energy Cosmic Rays Come From?

Photo credit: National Geographic

Our atmosphere is constantly being hit by particles from outer space with high energies. These are called “cosmic rays.” Although they don’t pose much harm to humans, they have fascinated physicists. Observing cosmic rays has taught us a lot about astrophysics and particle physics. But there are some—the ones with the most energy—that are mysterious to this day.

In 1962, at the Volcano Ranch experiment, Dr John D. Linsley and Livio Scarsi saw something incredible: an ultra-high-energy cosmic ray with an energy of more than 16 joules.[1] To give you some perspective, one joule is roughly the energy it takes to lift an apple from the floor onto a table.

All of that energy is concentrated, though, in a particle a hundred million billion billion times smaller than the apple. That means that it’s traveling very close to the speed of light!

Physicists don’t yet know how these particles get this incredible amount of energy. Some theories include the idea that they might come from supernovae, when stars explode at the end of their lives. The particles may also be accelerated in the disks of collapsing matter that form around black holes.

9 Was Our Universe Dominated By Inflation?

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The universe is amazingly flat on large scales. This is something called the “cosmological principle”—the idea that, wherever you go in the universe, there’s roughly the same amount of stuff on average.

But the theory of the big bang suggests that, at the very earliest times, there must have been some big differences in density in the early universe. So it was much lumpier than our universe is today.

The theory of inflation suggests that the universe we see today comes from a tiny volume of the early universe.[2] This tiny volume suddenly and rapidly expanded—far faster than the universe is expanding today.

Just like if you drew on a balloon and then filled it with air, inflation “stretched out” all the lumps in the early universe and explains why we have a fairly flat universe—where conditions are similar wherever you go—today.

Although this explains a lot about what we see, physicists still don’t know what caused inflation. Details of what was happening during this inflation are also sketchy. A better understanding of this era could tell us a lot about the universe as it is today.

8 Can We Find Dark Energy And Dark Matter?

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It’s an amazing fact: Only around 5 percent of the universe consists of the matter that we can see. Physicists noticed some decades ago that the stars on the outer edges of galaxies were orbiting around the center of those galaxies faster than predicted.

To explain this, the scientists suggested that there might be some unseen “dark” matter in those galaxies that caused the stars to rotate more quickly. After this, observations of the expanding universe led physicists to conclude that there must be a lot more dark matter out there—five times as much as the matter we can see.

Alongside this, we know that the expansion of the universe is actually accelerating. This is strange because we’d expect the gravitational pull of matter—both “light” and “dark”—to slow down the expansion of the universe.

Combine this with the fact that the universe is flat—space-time, overall, is not curved—and cosmologists need an explanation for something that balances the gravitational attraction of matter.

“Dark energy” is the solution. Most of the energy in the universe can’t be locked up in matter, but instead, it’s driving the expansion of the universe. Physicists believe that at least 70 percent of the universe’s energy is in the form of dark energy.[3]

Yet to this day, the particles that make up dark matter and the field that makes up dark energy have not been directly observed in the lab. Observing dark matter is difficult because it doesn’t interact with light, which is how observations are usually made.

But physicists are hopeful that dark matter particles might be produced in the Large Hadron Collider (LHC), where they could be studied. It could turn out that dark matter particles are heavier than anything the LHC can produce, in which case it might remain a mystery for a much longer time.

Dark energy is supported by many different observations of the universe, but it’s still deeply mysterious. In a very real sense, it may be that “space just likes to expand” and we can only see it expanding when we look at very large scales.

Or maybe the dark matter and dark energy explanations are incorrect, and an entirely new theory is needed. But it would have to explain everything we see better than the current theory before physicists will adopt it. Even so, it’s incredible to think that we may know very little about 95 percent of the universe.

7 What’s At The Heart Of A Black Hole?

Black holes are some of the most celebrated objects in astrophysics. We can describe them as regions of space-time with such strong gravitational fields that even light cannot escape.

Ever since Albert Einstein showed that gravity “bends” space and time with his theory of general relativity, we have known that light is not immune to gravitational effects. In fact, Einstein’s theory was proved during a solar eclipse that demonstrated the Sun’s gravity was deflecting distant rays from far-off stars.

Since then, many black holes have been observed, including a huge, supermassive one at the heart of our own galaxy. (Don’t worry. It won’t swallow up the Sun any time soon.)

But the mystery of what occurs at the heart of a black hole is still unsolved. Some physicists thought that there might be a “singularity”—a point of infinite density with some mass concentrated down into an infinitely small space. It’s difficult to imagine. Worse yet, any singularity leads to a black hole in this theory, so there’s no way we could observe a singularity directly.

There is still debate about whether information is lost inside black holes.[4] They absorb particles and radiation and emit Hawking radiation, but the Hawking radiation doesn’t seem to contain any further information about what’s occurring inside the black hole. Some information about the particles that fall beyond the event horizon into the black hole seems to be lost.

The fact that it seems impossible, at least at the moment, to understand what is at the heart of black holes has made sci-fi authors speculate for decades about whether they could contain different universes or be used for teleportation or time travel.

Since being absorbed by a black hole involves being stretched into a string of atoms (“spaghettification”), we’re not volunteering to venture inside and find out.

6 Is There Intelligent Life Out There?

People have been dreaming of aliens for as long as they’ve looked up at the night sky and wondered what might exist out there. But in recent decades, we have discovered plenty of tantalizing pieces of evidence.

For a start, planets are far more common than people originally thought, with most stars having a planetary system. We also know that the time gap between our planet becoming habitable and life emerging on it was quite small. Does this suggest that life is likely to form? If so, we have the famous “Fermi paradox”: Why haven’t we communicated with aliens yet?

There are plenty of solutions to the Fermi paradox, ranging from the wild to the more sad and mundane. It really shows the difficulty of reaching any good scientific conclusions when you only have one data point: us.

We know that intelligent life evolved on this planet (okay, maybe it’s debatable), which means that it can happen. But we can’t know if we just got incredibly lucky. Or maybe there’s something special about our planet that makes it extremely rare but suitable for hosting life. Or perhaps the probability of life beginning is extremely low, so there are few, if any, alien civilizations out there.

Astronomer Frank Drake put together his “Drake equation”[5] as a way of looking at all the different aspects of this problem. Each of the terms represents a reason why we may not be communicating with intelligent life.

Perhaps life is common, but intelligent life is rare. Maybe, after a while, all civilizations decide against communicating with other life-forms. They’re out there, but they don’t want to talk to us.

Or, chillingly, maybe this shows that many alien civilizations destroy themselves shortly after becoming technologically advanced enough to communicate. We can worry about this happening on Earth with nuclear weapons or out-of-control AI.

It’s even been suggested that the lack of communication from aliens is proof that the world was created—either by a God or as part of a computer simulation. This would explain why there’s only us. The cosmic gamers are playing in single-player mode.

The reality is that we haven’t been looking for all that long, and space is unimaginably vast. Signals can easily get lost, and an alien civilization would have to send a powerful radio signal for us to pick it up. But it’s exciting to think that the discovery of an alien civilization could happen tomorrow and change our understanding of the universe forever.

5 Can Anything Travel Faster Than The Speed Of Light?

Since Einstein changed the face of physics with his theory of special relativity, physicists have been sure that nothing can travel faster than the speed of light. In fact, relativity predicts that for anything with mass to even travel at the speed of light, infinite energy is required.

We see this in the ultra-high-energy cosmic rays mentioned earlier. They have extraordinary energies relative to their size, but they still don’t travel this fast. The speed of light as a hard limit might also explain why communications from alien civilizations are unlikely. If they’re also limited by this, signals might take thousands of years to arrive.

But people are continually questioning whether there might be some ways around the universe’s speed limit. In 2011, the OPERA experiment had some preliminary results that suggested neutrinos were traveling faster than the speed of light. But researchers later noticed some additional errors in their experimental setup that confirmed the results were incorrect.

If any way of communicating matter or information faster than the speed of light exists, it would undoubtedly change the world. Faster-than-light travel violates something called causality—the relationship between the causes and effects of events.[6]

Due to the way that time and space are interrelated in special relativity, information traveling faster than the speed of light would allow one person to receive information about an event before it has “happened” (according to them)—a type of time travel.

Faster-than-light communication would create all kinds of paradoxes that we don’t know how to resolve. So it seems likely that it doesn’t exist. But if you do manage to develop it, please tell us about it yesterday.

4 Can We Find A Way To Describe Turbulence?

Moving back down to Earth, there are still plenty of things that occur in our everyday lives that are difficult to understand. Try playing with the faucets in your home.

If you let the water flow gently, you’re looking at solved physics—a type of flow we understand well that’s called “laminar flow.” But if you turn up the water to maximum pressure and watch it sputter and spurt, you’re looking at an example of turbulence. In many ways, turbulence is still an unsolved problem in physics.[7]

The Navier-Stokes equation determines how fluids like water and air should flow. This equation is a little bit like a force balance. We imagine that the fluid is broken up into little parcels of mass. Then the equation takes into account all of the various forces that act on this parcel—gravity, friction, pressure—and tries to determine how the parcel’s velocity should respond.

For simple or steady flows, we can find solutions to the Navier-Stokes equation that completely describe the flow. Physicists can then write down an equation that tells you the velocity (speed and direction) of the fluid at any point in the flow.

But for complicated, turbulent flows, these solutions start to break down. We can still do a lot of science with turbulent flows by solving the equations numerically with large computers. This gives us an approximate answer without a formula that fully explains how the fluid is behaving.

We forecast the weather this way. But until we find those elusive solutions, our knowledge will be incomplete. By the way, this is one of the unsolved Clay Institute prize problems. So if you manage it, there’s a million dollars in it for you.

3 Can We Build A Room-Temperature Superconductor?

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Superconductors could be some of the most important devices and technologies that humans ever discover. They are special types of material. When the temperature drops low enough, the electrical resistance of the material drops to zero.

This means that you can obtain huge currents for a tiny application of voltage across the superconductor.[8] If you set the electrical current flowing in a superconducting wire, it can continue flowing for billions of years without dissipating because there’s no resistance to its flow.

A good deal of power is lost in our current power cables. They aren’t superconducting and have electrical resistance, which causes them to heat up when you pass a current through them. Superconductors could reduce these losses to zero.

But the possibilities of superconductors are even more exciting than this. The magnetic field produced by a wire has a strength that depends on the current flowing through that wire. If you can get very high currents in a superconductor cheaply, you can get really powerful magnetic fields.

These fields are currently being used in the Large Hadron Collider to divert the fast-moving charged particles around its ring. They are also used in experimental nuclear fusion reactors, which could provide our electricity in the future.

The problem is that all known superconductors need to be at these very low temperatures to work. Even our hottest-temperature superconductors need to be at -140 degrees Celsius (-220 °F) before they start exhibiting this wonderful property.

Cooling them down to these low temperatures usually requires liquid nitrogen or something similar. Therefore, it’s very expensive to do. Many physicists and materials scientists across the world are working on developing the holy grail—a superconductor that could work at room temperature. But no one has managed it yet.

2 Why Is There More Matter Than Antimatter?

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In some ways, we still don’t know why anything exists at all. A bold statement but true! For every particle, there is an equal and opposite particle called an antiparticle. So for electrons, there are positrons. For protons, there are antiprotons. And so on.

If a particle ever touches its antiparticle, they annihilate and turn into radiation. Since you probably don’t want to get annihilated, it’s a good thing that antimatter is incredibly rare. Sometimes, it falls in cosmic rays. We can also make antimatter in particle accelerators for trillions of dollars a gram. But on the whole, it seems to be incredibly rare in our universe.

This is a real mystery. We simply don’t know why matter dominates in our universe and not antimatter. Every known process that changes energy (radiation) into matter produces the same amount of matter and antimatter. So if the universe began dominated by energy, why didn’t it then produce equal amounts of matter and antimatter?

We can imagine a universe where energy turns into matter-antimatter pairs. Then they would annihilate each other and turn back into energy forever. But there would be no structure, no stars, and no life.

There are some theories that might explain this. Scientists probing the interactions of particles at the Large Hadron Collider are looking for examples of “CP violation.”

If they occur, these interactions could show that the laws of physics are different for matter and antimatter particles.[9] Then we can imagine that perhaps there are processes out there that are slightly more likely to produce matter than antimatter and this is why we see an asymmetrical universe dominated by matter.

Wilder theories suggest that there might be whole regions of the universe that are dominated by antimatter. Interestingly, it might be more difficult to dispute this than you think.

Antimatter and matter interact with radiation in the same way, and so they look exactly the same. Our telescopes could not distinguish between an antimatter galaxy and a matter galaxy.

But these theories have to explain how the matter and antimatter became separated and why we don’t see evidence of lots of radiation being produced when the matter and antimatter collide and annihilate.

Unless we discover evidence for antimatter galaxies, CP violation in the early universe looks like the best solution. But we still don’t know exactly how it works.

1 Can We Have A Unified Theory?

In the 20th century, two great theories were developed that explained a lot about physics. One was quantum mechanics, which detailed how tiny, subatomic particles behaved and interacted. Quantum mechanics and the standard model of particle physics have explained three of the four physical forces in nature: electromagnetism and the strong and weak nuclear forces. Its predictions are amazingly accurate, even though people still argue about the philosophical implications of the theory.

The other great theory was Einstein’s general relativity, which explains gravity. In general relativity, gravity occurs as the presence of mass bends space and time, causing particles to follow paths that are curved due to space-time being bent out of shape. It can explain things that occur on the grandest of scales—the formation of galaxies and the dance of the stars.

There’s only one problem. The two theories are incompatible. We can’t explain gravity in a way that makes sense with quantum mechanics, and general relativity does not include the effects of quantum mechanics. As far as we can tell, both theories are correct. But they do not seem to work together.[10]

Since this was realized, physicists have been working on some kind of solution that can reconcile the two theories. This is called a Grand Unified Theory (GUT) or just the Theory of Everything.

Scientists are used to the idea of theories that only work within certain limits. For example, Newton’s laws of motion are what you get when you take a low-speed limit of special relativity. Also, electricity and magnetism used to be considered completely different theories until Maxwell unified them into electromagnetism.

Physicists are hoping to be able to “zoom out” and see that quantum mechanics and general relativity are both part of a greater theory, like patches in a quilt. String theory is an attempt that can reproduce features of general relativity and quantum mechanics. But it is difficult to test its predictions with experiments, so it cannot be confirmed.

The search for a fundamental theory—one that can explain everything—goes on. Perhaps we will never find it. But if physics has taught us anything, it’s that the universe is truly remarkable and there are always new things to discover.

Thomas Hornigold is a physics student at the University of Oxford. In his spare time, he publishes a physics podcast—Physical Attraction—which attempts to explain physics one chat-up line at a time. You can find it on iTunes, and follow the podcast on Twitter.


Read about more intriguing theories made possible by physics on 10 Impossible Things Physicists Just Made Possible and 10 Mind-Bending Discoveries In Physics.

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