What the heck is energy?

Electricity, fuels, heating: energy is all around us. Physicists talk about it all the time to decode the mechanisms behind basically every process. Yet a good definition of energy is hard to come by: what is this “energy”?

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Phenomenal cosmic power, itty bitty living space

Wouldn’t it be great to take the universe in the lab? Astronomy is one of the most captivating parts of physics. I mean, one can’t scoff at the idea of unveiling the mysteries of the cosmos. Unfortuntely, galaxies and black holes don’t exactly cooperate as far as experimenting goes.

A group of physicists is working on a solution. Continue reading

All physics is wrong!

Quantum Mechanics: wrong! General Relativity: wrong! The standard model of particle physics: wrong, wrong, wrong.

All physics (actually, science in general) is wrong—to some extent. And scientists know about it, too! Here’s the thing: science has to be wrong. Because it doesn’t find The Truth, instead it explains what we see around us as well as possible.

Even when we have an explanation for what we see, there might be a better one we missed.

Newton thought gravity was a force between objects with mass. That’s pretty much right. Enough to get you to the Moon at least. He never thought of mass warping spacetime. But he also never saw gravity bending light (which has no mass), or affect the passing of time. Einstein, with his General Relativity explained everything, including these things, which he never even observed!

You can describe the motion of all planets, perfectly, keeping Earth at the center. It’s just very complicated and more wrong than Newton’s gravity. credit: wikimedia

Indeed, a good theory must predict new stuff, stuff we haven’t seen. Before Newton, people described the motion of stars and planets as circles moving around circles, around circles. Whenever something didn’t fit, they’d add a circle. It perfectly described, everything we could see, but couldn’t predict anything new. Newton’s laws told astronomers where to find a new planet: Neptune.

Whenever a prediction turns out wrong, scientists find out a new way to explain the new facts, then move to new prediction, and the cycle restarts.

Soner or later, something will come along and prove General Relativity wrong. Personally, I think dark matter (an invisible, untouchable substance that just has to be there) will be the next battleground. Challengers are stepping up.

What our universe is made of (according to current theories): 95% is “dark” stuff (a fancy way to say we have no clue what it is). credit:nasa.gov

Just like Relativity, all other theories will eventually fall. No theory is perfect, but each newly-accepted one is better than the last. At any time in history (well, at least since we’ve had the scientific method at least), scientific facts were the best explanations of the world. Ever. And that’s still true now.

It’s good to keep an open mind, but also to keep in mind why facts are considered facts. If you open your mind too much, you risk your brain falling out.

If you want more
  • You can literally write books on all the stuff we don’t know. Jorge Cham did.
  • A detailed explanation of what works and what doesn’t around dark matter on PBS Spacetime:


Cover photo: Facepalm, CC-BY Brandon Grasley/flickr

Two equations are enough to go to the Moon

Even though going to the Moon seems hard (I think someone mentioned it), it actually takes just two simple rules. Both were discovered by world-renowned physicist and a-hole Isaac Newton, whose birthday is at some point during the holidays.

What a jolly festive fellow! credit: csamuel.org

First off, the mighty a=F/m (more widely known as F=m a). It simply means that dividing the force (F) acting on an object by its mass of the object (m), gives by how much the object accelerates (a). This general formula tells you your rocket will move, so it’s clearly rather important for your journey to the moon.

Not only that, it’s also at the core of how rockets move at all. In fact, rocket propulsion is based on that weird “equal and opposite reaction” business you probably heard of.

Take a balloon and inflate it: if you let it go, it flies away (making a fart noise). The air inside it is pushed out by pressure. However, if you take the balloon and air combined, no new force is acting when you release the balloon, so, as a whole, balloon and air must have zero acceleration. Because the balloon is pushing air out, there must be a force as intense (equal) pushing back (opposite) on the balloon. Rockets are the same, just with fancy tech to be more efficient.

CC-BY-ND mfrascella/flickr

The other equation Newton found was the one to calculate the force of gravity. That was pure genius. And it’s pretty important for your lunar journey, since gravity most of the force you’ll have to navigate: Earth’s, chaining your rocket to the ground and yanking it off the sky, and the Moon’s, tugging it to its destination. Know gravity’s workings and you can start charting your way to the stars.

Easy, ain’t it?

Not so much: astronauts—freaking jet-fighter pilots with engineering degrees—take theoretical classes to learn how to steer spacecrafts. And even before that, you’ll need a spacecraft. It’ll need enough oomph to escape Earth, but be sturdy enough to not explode in the process, and take you back in one non-crispy piece.

That’s why Newton never went to space.

Still, at its core, space travel is all about his equations. All the research from all the super-smart people in space agencies: it’s all aimed at improving our use of those two simple rules.

Thanks and happy birthday, you insufferable genius! Whenever it is.

And to you all: happy holidays!
If you want more
  • If you get a chance, watch episode 3 of Cosmos: you haven’t heard about Newton’s work on gravity if you haven’t heard Neil deGrasse Tyson tell you about it
  • Space tech might be on the way to surpass Newton. But it’s all very vague—and frankly weird

Cover photo: CC0 27707/pixabay

The relativity experiment you hold every day

After the discovery of gravitational waves, there’s a lot of talk about Einstein’s General Relativity. We usually talk about it in the context of black holes and other things we don’t quite see every day, but I bet you held a relativity experiment in your hand in the past 10 minutes. Indeed, if you used a smartphone or anything with a GPS, you effectively performed a general relativity experiment.


Indeed, the 31 GPS satellites actually spend their days broadcasting the time on the super-accurate atomic clock each has on board.

The signal takes a few hundredths of a second to reach you on the ground. So comparing very accurately the time on your watch to the signal from the satellite, you can calculate how far it is. Putting together the distance from enough satellites, you will find your own position on the planet.

Only one point on the surface of the Earth can be simultaneously at certain distances from four GPS satellites. That’s where you are. Credit: ingegnerando.it

It’s all nice and fine up to here, but what does this have to do with relativity?

According to the theory, higher up in gravitational fields (say, when you’re orbiting in space) time flows ever-so-slightly faster. A minute in orbit is a teensy weensy shorter than a minute on Earth. As usual with relativity, the effect is small, so small you wouldn’t notice.

Because the effect is so tiny, GPS satellites were first deployed with relativistic corrections turned off. Scientist figured it wouldn’t make a difference. Boy were they wrong: in a short time, the localization was off by kilometers.

Since they also thought this might happen, the scientists had made possible to switch on the corrections from the ground.

So every time your satnav tells you where to turn, every time Google Maps tells you accurately how far the nearest pub is, you’re actually confirming that General Relativity is correct.



Coverpote photo: CC0 Sylwia Bartyzel, via unplash

Why galaxies are flat (and Earth isn’t)

The universe teems with flat stuff. Most galaxies, including the Milky Way, are quite flat and (relatively) thin pancakes of stars. All planets of the solar system (real planets, not Pluto) orbit pretty much on the same plane. Unsurprisingly, it’s no coincidence.

The plane along which all (real) planets orbit around the Sun. credit: pics-about-space.com

The plane along which all (real) planets orbit around the Sun. credit: pics-about-space.com

Galaxies and star systems form the same way: coagulating clouds of gas—though at obviously different scales.

Imagine throwing a plume of gas or atoms in space. Push them in random directions: some one way, some another, some up, some down. Unless you cheated, they bump into each other and, because of gravity, clump together. When the atoms didn’t collide head-on (ie, most of the times), these clumps spin. Clumps themselves attract each other and collide into bigger spinning blobs.

After each collision, the atoms and chunks of atoms align, canceling out all of their opposing motion, but keep spinning (in fancy physics terms, it’s called angular momentum conservation). You can see the blobs in the video up here as a forming galaxy seen from “above”.

Slowly but surely, the whole cloud flattens to a plane. If it’s a galaxy, it forms stars on that plane, whereas in the Solar System it became that begot the orbital plane.

Other planetary systems and galaxies spin too, but each inclined its own way, because they formed from different clouds of gas.

A lot of galaxies, photographed by the Hubble Space Telescope. They’re spinning in every which way. Credit: NASA/wikimedia

But if planets and stars also form by congealing gas, why aren’t they flat as well?

The reason is that planets and stars are much denser than galaxies. Being closer to each other, their clumps of gas feel a stronger gravitational pull to the center of the blob, that wins over the mechanism that would keep them flat. So planets and stars become spheres.

Saturn formed across all the stages: most matter coalesced in the huge (clearly spherical) planet, but a little formed some of its many more or less round moons, finally the last faint leftovers ended up as the iconic, extremely flat rings.

Round, flat, round: Saturn, its rings, and four of its many moons. Credit: NASA/wikimedia

If you want more
  • A long but excellent post on planets, galaxies, and roundness by the great Neil DeGrasse Tyson
  • Minutephysics has a cool video that explains this more technically, and shows why it can only happen in a 3D universe


Cover photo: CC0 WikiImages/pixabay

Four fundamental things about gravitational waves

The team at LIGO (the Laser Interferometry Gravitational-wave Observatory) annouced they directly measured the gravitational waves emitted by two black holes merging into one. What are they talking about? Here’s the answer to 4 of the most common questions (plus 2 extra-credit, if you feel up to it).

The merging of the two black holes, and the resulting gravitational wave. NASA

What are gravitational waves?

Gravitational waves are ripples in space-time predicted by Einstein’s General Relativity theory. If you heard anything about this theory, it probably is that, by their mass, objects warp space-time around them.

If mass is moving in the right way, it should (in theory) create gravitational waves. These propagate in space like ripples on a pond. They periodically make it longer and narrower, then shorter and broader, and so on.

Credit: MOBle/English Wikipedia

Credit: MOBle/English Wikipedia

Edit: Confused by the “stretching space” thing? I unpacked this (using puppies!) in a later post.

How big is the effect?

Tiny. Less than tiny: unfathomable catastrophes (crashes between black holes, supernovae exploding, stuff like that), relatively close (which means within our galaxy) change the distance between Earth and the Moon by about a thousandth the width of an atom.

As you can figure, an effect this tiny is pretty hard to measure.

How did we find them?


LIGO is a laser interferometer. It works by splitting a laser beam in two: one goes on its original way, the other bends 90 degrees away. Each of them bounces back and forth a few times a few hundred times along a tube. Then the two laser lights meet again, so that their lightwaves interfere with each other, perfectly canceling each other.

If a gravitational wave is to pass through the device, it stretches and shortens each leg, alternatively. This breaks the perfect synchronism between the two laser waves. The scientists looked for this sort of signal, where the two laser branches were out of sync.

To be absolutely sure they eliminated every disturbance, then, they looked for identical traces to appear in both a detector in Northwestern US and one in the Southeast. An additional detector, the Italian VIRGO, will join soon.

If the effect is so tiny, why bother?

Because gravitational waves provide a completely new way to perceive and study the universe. As Catherine Man, of the French Observatoire de la Côte d’Azur, said

Now we are no longer observing the universe with telescopes using ultraviolet light or visible light but we are listening to the noises produced by the effects of the gravitation of celestial bodies on the fabric of space-time

Among the things we could “listen” to there’s the echo of our universe’s youth. Until it became 380 thousand years old, the universe was opaque: it didn’t let light through. So we couldn’t observe anything earlier than that time using electromagnetic waves. But gravitational waves already existed, if we could hear them, we could learn much.

Extra credit for confident readers

Didn’t someone find gravitational waves a few years ago?

Yes and no: gravitational waves carry energy, and in 1993 Russel Hulse and Joseph Taylor did indeed win a Nobel prize in physics for observing that very energy. But, until now, nobody measured gravitational waves directly.

Also, last year, the BICEP2 project announced they found them. They were wrong. They didn’t look good that time.

So what changes with eLISA now?

Likely, not much. eLISA is a space observatory for gravitational waves planned by the European Space Agency, slated to launch in 2024. Even if gravitational waves won’t be new by then, eLISA is much much more sensitive than LIGO.

Basically, it will be a larger ear, capable of measuring different, fainter, farther away sources. And, being in space, it will experience much less disturbance.

If you want more
  • For the first anniversary of the discovery, we dug deeper: what does it mean to stretch space? how do you measure the stretch at all? And explained it with puppies!!

Cover photo: CC0 Austin Schmid, via unsplash.