0 comments on “G is for Gravity”

G is for Gravity

By Siobhan Fairgreaves

‚ÄúGravity pulls everything down‚ÄĚ

Wrong!

Now we know what gravity doesn’t do- let’s take a look at what it does.

If it helps, you can think of the force of gravity as ‘falling’ into this thing – a gravity potential. If you’re ever floating in the void of space and you’ve lost your keys again, simply having mass would be a good way to get them back | Image: AllenMcC

Before I get too stuck in I‚Äôm going to clarify- we‚Äôve all heard of gravity right? Just in case you missed that class here is a quick reminder. Gravity is one of the many invisible forces acting upon us and our environment every minute of every day. We learn about it in the early days of school, but gravity is still hugely important to physicists. The famous Sir Isaac Newton had a ‚Äėeureka!‚Äô moment concerning the force – more on that later though.

Every object with mass has some gravitational pull (that includes you!) That means your own body is exerting its own force, however tiny, both on the clothes you are wearing, and on distant stars. But for gravity, a bigger mass means a bigger force. So you never need to worry about walking around with lots of small objects stuck to you simply because the gravitational pull created by the Earth is much, much stronger.

For humans, animals and absolutely everything we share our planet with, gravity pulls us towards the centre of Earth. It might seem pedantic but there is an important difference between pulling towards the centre of the Earth and pulling ‚Äėdown‚Äô. An important law in physics, called Newton‚Äôs Third Law, states that ‚Äúfor every action, there is an equal and opposite reaction‚ÄĚ.

Not to be pedantic, but the gravitational pull of Earth isn’t the same everywhere: this image from NASA’s GRACE mission shows the subtle differences across the Earth’s surface | Image: NASA

Applying the law to gravity; the force you exert on the Earth (pulling the Earth towards your body), and the force the Earth exerts on you (pulling your body to the Earth’s centre), are equal and opposite, keeping you firmly in place on the planet’s surface. Understanding this principle helps us understand how gravity works when we scale things up.

So how does this come into play for the rest of the Solar System? Well, the Earth has a bigger mass than our moon Рjust as all the other planets have bigger masses than their moons. But the circular orbits the bodies make around each other mean that gravity doesn’t simply cause the bodies to crash into each other. Instead, the force of gravity maintains the circular paths.

Another one of Newton‚Äôs laws ‚Äď the second this time ‚Äď states that the acceleration of an object is equal to the force acting on it, divided by its mass. For planets and the moons, the force acting on both of them will be the same, but the mass of the planet will always be much larger. This means that the acceleration of the moon is much larger, making the path¬†its‚Äô circular orbit much longer than that of the planet around it.

Fortunately there is one very important celestial body holding this all together (for our Solar System at least) and that is the Sun. The Sun has a by far the biggest gravitational pull which allows it to hold Earth and the other 7 planets (sorry, Pluto) in position. Due to their respective size and distance each of the planets will experience the gravitational pull from the Sun differently.

With so many laws with his name on them, it’s no wonder Isaac Newton was a little uptight. It must have been the apple which probably didn’t actually fall on his head | Image: Eden, Janine and Jim

Newton summed this all up nicely with yet another law: the law of gravitation. The equation calculates the gravitational force between two bodies using their two masses, the distance between them, and a gravitational constant Newton discovered himself. The law is still hugely important to astronomers today.

Hopefully this makes it clear why we shouldn‚Äôt say that gravity pulls everything ‚Äėdown‚Äô! I don‚Äôt know exactly what is ‚Äėdown‚Äô from our position in the cosmos but I‚Äôm not so sure I want to find out. In a way, it would be more correct to say that gravity pulls everything ‚Äėin‚Äô.

I’ve mentioned our old friend Sir Isaac Newton a lot now, and we heard about his famous apple at the very start. Legend has it that an apple fell on Newton’s head and just like that, he understood gravity- but did it really happen? Well it certainly seems to be true that a falling apple did inspire Newton’s work on gravity but there was no instant moment of understanding as the story seems to suggest.

The truth is that, whilst sheltering from the wrath of the bubonic plague at his family home in the countryside, a young Newton witnessed an apple fall from a tree. This sight led him to wonder, ‚ÄúWhy should that apple always descend perpendicularly to the ground?‚ÄĚ However it was not until around 20 years later that Newton first published this principle.

So that’s one myth busted, and another classic example of science, or scientists, being inspired by nature.

See you next time for a futuristic post all about holograms!

 

0 comments on “G is for Gravity Waves”

G is for Gravity Waves

By Jonathan Farrow from the Thoughtful Pharaoh

Deep in Antarctica, right on top of the geographic South Pole, there is a research station that peers back in time to the very beginning of our universe. Named the Amundsen-Scott Station, it is home to instruments such as the creatively named South Pole Telescope (SPT), the Keck Array, and the BICEP experiments.

The temperature is currently sitting at about -30C and it’s the height of summer. ¬†The sun won’t set at the station until March 23rd and once it sets, it won’t rise again until September. ¬†So why the heck (or, one might say…Keck)¬†would we build an observatory there?

Because the temperature is so low and the altitude is so high (2743m) at the South Pole, the air is thin and dry, reducing blurriness normally caused by¬†the atmosphere. ¬†There are no cities nearby to cause light pollution and there are months of nonstop night, allowing for¬†continuous observation. ¬†It’s an astronomer’s dream. ¬†Except the nearly-constant -30C temperatures. ¬†And the remoteness. ¬†But otherwise, dreamlike.

Damn that looks cold! Photo from 2003
The perfect place to set up a top-secret laboratory from which to take over the world!… I mean… uh… from which to observe the beginnings of the universe. ¬†Yeah, that’s what we’re doing. ¬†Definitely that. ¬†Image credit: NASA

So what are astronomers looking for all the way down there at the end of the world? ¬†They are searching for clues as to how the universe started. ¬†Ever heard of the Big Bang Theory? ¬†No, not these clowns, the theory about the beginnings of the universe. Although, come to think of it, the theory is actually pretty well summed up by the first line of the Barenaked Ladies’ theme song to the Big Bang Theory (yes, those clowns):

Our whole universe was in a hot dense state,
Then nearly fourteen billion years ago expansion started. Wait…

That’s really the core of the¬†theory: ¬†everything used to be really hot and dense and now its¬†not. ¬†What happened in between is what the astronomers at the South Pole are trying to figure out.

History of Universe
For a bit of a primer on this diagram, check out A Short History of (The Universe), an essay I wrote which introduces the origins of the universe.  Image by NASA

Astronomy is awesome because when we look up, we are actually looking back in time. ¬†The distances involved are so great that it can take years (or billions of years) for light to reach us. ¬†So, what if we just looked as far as we could, wouldn’t we be able to see the Big Bang happening? ¬†What would that even look like?

Unfortunately, because everything was so hot and dense right at the start of the universe, nothing could stick together so the universe was just a soup of energetic particles.  Any light that was emitted was bounced around like the light from a flashlight in thick fog.  About 380 000 years after the Big Bang , the universe had cooled and expanded enough to let atoms form and collect electrons.  Atoms are mostly empty space, which means that unless they are packed very close together like in a solid or liquid, they are transparent.  What resulted was light spreading pretty evenly throughout the universe, starting 13.7996 billion years ago.  This is what is called the Cosmic Microwave Background Radiation (CMBR).  Cosmic because it comes from space, Microwave because it has lost a lot of energy since the Big Bang and is now only 2.7 degrees above absolute zero, Background because it is there no matter which direction you look, and Radiation because it is light.

WMAP
A map of the Cosmic Microwave Background Radiation. The different colours represent slight anomalies (about 10^-5 degrees C difference). Red is a little bit hotter, dark blue is a little bit colder.  Image by NASA

So, no matter how far you try to look, this map is all that you see.  It is all that can be seen because it is the oldest light that escaped.  Sounds kind of disappointing, but astronomers think that that image (what some refer to as the baby picture of the universe) holds clues to what happened before.

If there was inflation, faster-than-light expansion of space and time (again, check out my essay on the history of the universe¬†if you’re confused), that process should have produced gravitational waves.

“Woah, woah, woah. ¬†Hold up. ¬†I understand gravity, apples falling on heads, etc etc… ¬†How the Keck could there be gravity¬†waves?”

One of Einstein’s key contributions to science was the understanding that space and time are¬†linked and that they are¬†influenced by mass. ¬†He described space-time as a fabric that could be warped by the presence of mass. ¬†All that¬†gravity is, he said, is the curvature of space-time around mass. ¬†A simplified way to understand this is by thinking of¬†space-time as a trampoline. ¬†If you put a mass on the trampoline, it will create a depression. ¬†The heavier the mass, the more extreme the depression. ¬†Now, if you have an extreme depression and move it very quickly back and forth, it will create waves in the same way that a moving hand in a pool will create waves. ¬†Astronomers think that inflation must have created gravity waves with a very specific signature. They also think that very heavy stars moving quickly, like binary neutrino stars, would create these gravitational waves.

Loooook into my gravity waaaaves.  You are not getting sleepy.  You are paying attention, commenting below, and sharing this with your friends.
Loooook into my gravity waaaaves. You are not getting sleepy. You are paying attention, commenting below, and sharing this with your friends.  Image by NASA

If (or, once they are discovered for sure, when) gravity waves pass through you, it is space itself which is expanding and contracting.  You are not moving, but as the wave passes through your arm, your arm will be closer to your body than it was before and time for it will move slower.

The thing about gravity, though, is that it is by far the weakest of the fundamental interactions (Electromagnetic, Weak, Strong being the other, stronger ones).  By a factor of about a nonillion (1 with 30 zeroes after it).  This makes the waves it creates very difficult to detect.  While your arm is probably having a taste of timelordery as you read this, there is no way you could possibly feel it.  Gravity waves are not interesting for how they make us feel, but rather for the challenge they present in detecting, for the possible confirmation of our current physical model, and for what they can tell us about the origin of the universe.

So let’s come back¬†back to the barren, frigid wasteland of Antarctica and the astronomers freezing their buns off for science. ¬†BICEP2, the second iteration of the¬†Background Imaging of Cosmic Extragalactic Polarization experiment, looked at the¬†CMBR and looked for patterns in the light. ¬†These patterns, called b-mode polarization, can be produced by gravity waves, but also by interstellar dust.

In order to cancel out the effect of dust, the BICEP2 team used data from Planck, a European satellite launched in 2009 with a very similar mission: to study the early universe. ¬†Whereas BICEP2 could only look at one particular wavelength with high sensitivity, Planck could look in a few different wavelengths but didn’t have quite as much sensitivity for these b-modes. ¬†Dust doesn’t leave the same polarization patterns in¬†light in¬†different wavelengths, so by comparing the results from different wavelengths from Planck, the BICEP2 team was able to show that the b-modes weren’t from dust and so had to be from gravity waves from the early universe. ¬†Proof of inflation! ¬†Proof of the standard model! A possible Nobel Prize!

./b_over_b_rect.eps
Those swirls are the b-mode polarization that astronomers were looking for. ¬†This diagram, while confusing as Keck (it’s going to catch on!) and quite complicated, was EVERYWHERE when the announcement was made. ¬†Image by BICEP2 team

So, understandably excited and with a positive result in hand, there was a big announcement at the Harvard-Smithsonian in March of last year.  Unfortunately, the data they used was preliminary.  In September, new data was released and the effect of dust seems to have been larger than they thought.  The team reduced the confidence in their findings but still stood by a significant result.  Just last month, in January 2015, another set of data was released that makes the BICEP2 findings inconclusive.

It seems the team jumped the gun a little bit, were blinded by the impact of their apparent discovery, and had too much confidence in preliminary data. The result of all this is that there is still no direct evidence of inflation or of gravitational waves and the teams at Planck and BICEP are going to work together now with the strengths of their instruments.  Within a few years, the effect of dust should be able to be cancelled out and we will be able to see whether we were right about the beginning of the universe.  And all the frostbite will have been worth it.