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Monday 27 February 2017

Answers for students: Why are scientists and science teachers so obesessed with units?

This is the first in a series of posts designed to help students of all ages with the physics problems they most often often ask me about.

Have you ever felt like feeding your teacher a text book because, after spending hours pouring over the pointless physics problem they set, they gave you no marks – even though you got the answer right?  

You literally just forgot to write down the units. The teacher knows what the units are, they wrote damn question! Hell, you should rip up and eat their favourite book, that'll show them.

Show them they've driven you insane, probably, but I've met students who seemed pretty close to that point.

If you haven’t ever felt like that…. congratulations! You either are a robot from the future, sent back in time to make everyone else feel bad about themselves*, or you have something to blackmail your physics teacher with. Something big I hope. Because I still make that mistake from time to time, and I have a freaking PhD in physics.

But... that said... there’re good reasons why you should make the effort to get the units right - not just in school but in life.  

As an example: Imagine I asked you to buy me "a milk, just a milk, one milk - go, go, go, right now," from the shop down the road. Once you got there you'd find that ‘a milk’ could mean this….

A milk.

...or it could mean this…..

Another milk

…or this….

A recyclable milk

You might be a bit surprised and upset, when you get back, to find I actually mean one of these:


You might ask ‘why has this idiot wasted my time, and sent me on a wild goose chase without telling me exactly what he wanted?’ 

You might be concerned that you were sharing a room with a madman who expected you to get highland cows from the corner shop. 

And, If I then told you it was to demonstrate why you need to put units on your exam questions, you’d probably shout at me and throw the milk at my head**.  

So it’s a good thing this is just happening in our imaginations. 

But it is exactly why your teacher is being so pedantic about the units - because not putting in units has caused some very expensive misunderstandings: NASA once had a computer program give them information in kilometres when they expected it in miles, without putting 'km' after the numbers.

The result? Their $115,000,000 spaceship ended up flat, at the bottom of yet another crater on Mars.

Mars actually has this crater, it's not CGI or anything. Mars is literally laughing at us. Courtesy of NASA.

No-one wants to explain to their boss why they just blew a hole in the planet Mars (or, as a more realistic example, why the expensive door they ordered for the office is too big for its frame) so getting students to put the right units on things is just one of those life-skills teachers try to get you in the habit of doing.

So, while your teacher may or may not be mean, pedantic, dull, or weird smelling, give them a break on this one thing: Putting the right units after your answer really is worth the effort to do.

Which units?

What units to use? Elephants would be cool, but elephants are frowned on as units of weight, length, or smell, because you have to feed them so much, take them for walks, and pay vet bills.

Although, if you're determined to use them as units, 1 elephant is probably equal to about 6 metric tons.

Nor do they fit into a pencil case very well. Courtesy of the BBC.

So in physics we mostly stick to the SI units system, which is a collection of units for basic things that can be used in combinations to cover most situations. 

For example: Distance is measured in meters, and time is measured in seconds. So speed, which is distance travelled per unit of time, gets the unit of 'meters travelled per second', or m/s, or sometimes even ms-1 (they all mean the same thing, it’s just different styles of writing ‘meters per second’). 

Below is a table of the S.I. units.  It's worth the time to learn them, even for things outside of Physics exams:

Unit nameUnit
Definition Dimension
metre m length
  • The distance travelled by light in vacuum in 1299792458 second.
kilogram[n 2] kg mass
  • The mass of the international prototype kilogram.
second s time
  •  The duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.
ampere A electric current
  • The constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would produce between these conductors a force equal to 2×10−7 newtons per metre of length.
kelvin K thermodynamic temperature Θ
mole mol amount of substance N
candela cd luminous intensity
  • The luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 5.4×1014 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.
* That seems a very petty reason to actually go back in time but, if you are, I'm not going to complain about it. You might turn out to have a secondary mission to kill anyone who finds out about the first mission.

** If you actually turned up with a cow, which is very unlikely but not entirely impossible, I would call you 'Master'. Whether you wanted me to or not.

Wednesday 22 February 2017

The Universe in 101 words: What's the fuss about TRAPPIST-1 ?

NASA has found a whole solar system of Earth like worlds, around a star called TRAPPIST-1

So what's the fuss about?
  • Only three of these worlds are in TRAPPIST-1's habitable zone, but all have some potential for liquid water.
  • It's a tiny solar system - it's outer edge would be inside the orbit of Mercury. So these potentially habitable worlds would be very close, appearing as big as the Moon in each others skies.
  • The star itself is an ultra cool red dwarf, which will live for trillions of years
It makes our solar system look pretty empty by comparison...

Monday 20 February 2017

The Universe in 101 words : Does anything live on Ceres?

Above: Ceres. Doesn't look promising, but give it a chance

The odds are looking better and better. 

Located in the asteroid belt, Ceres is only a thousand kilometres across. But the Dawn space probe has found signs that It’s had internal warmth, running water, and organic chemistry in the past – possibly even today. 

Those things are thought to be the key ingredients for life to begin… so the odds of finding Cerean life just jumped – comparable, perhaps, to the odds of there having been Martian life. 

Cere’s low gravity and relative closeness would make returning material from its surface easier than for Mars, too. This dwarf planet might surprise us all… 

Above: Ahuna Mons, an ice volcano on Ceres.

Wednesday 15 February 2017

The Universe in 101 words: The 'Oh-My-God' Particle

It's possible the particle came from the kind of exotic objects that lurk in vast nebula, like this one.

Can anything made of matter reach lightspeed? The 'oh-my-god' particle came damn close: A subatomic particle that struck Utah in 1991, it stunned the physicists at the University of Utah's cosmic ray observatory - it was travelling so close to lightspeed that light would only have gained a 1 cm lead in 200,000 years. 

No-one knows how it got such incredible speed, but it must have had a strange trip: So close to lightspeed time slows to a crawl and distances contract - the Universe would seem squashed along the direction of travel, and its whole history would pass in days. 

Above: Such energetic particles can cause cancer in astronauts.

Monday 13 February 2017

6 Of the best images of Mars

Above: Frost on the Martian desert. While the most recent missions hint at rare flows of briny Martian water, we've known since the Viking landers that ice is common enough on the red planet - here Viking 2 captured the early morning frost settling on the soil.

Above: At 27000 meters high  Olympus Mons is the biggest volcano,  and the biggest mountain,  in the solar system.  It's so big that the peak sticks out of the atmosphere and into space -  and, if you were to stand on it's slopes, you wouldn't be able to tell where the real horizon was and what was merely the curve of the mountain.

Above: The two teeny Martian moons whizz across the sky, one in the same direction as the Sun and one against it.  They are so small they're hard to tell from asteroids,  but they give Mars a phenomena we never see on Earth: Moon on moon eclipses

Above: Mars wears the Valles Marineris is a canyon system like a pirate's scar. It's so big it could would swallow Earth's grand canyon like a tic tac: 7Km deep and running for 4000  km the canyon is part giant rift valley,  and part carved by ancient running water,  deep in the Martian past

Above: The Sun sets on Mars. The sunsets on Mars are blue,  thanks to the way the dust in the ochre sky scatters light.  It's beautiful, but beware -  in the Martian night temperatures can easily reach a hundred degrees below freezing.

Above: A massive Martian dust devil, as seen from space. Dust devils on Mars can be hundreds of meters wide and thousands of meters tall. Some even travel in clusters,  wandering the landscape like nomads. Of all Martian weather they are probably the most spectacular and, even though the thin martian wind doesn't carry much force, being caught in one would mean being pelted with flying sand

Wednesday 8 February 2017

The Universe in 101 words: Is it all a hologram?

Do physicists watch the Matrix too much? That‘s the first thing I think when I hear theories about the Universe being a huge 'hologram'. 

And, knowing my fellow physicists... yes. Yes they do.

But give them a chance: What they mean is that holograms are 2D surfaces containing 3D information, and our Universe might be 2D with the data for our third dimension stored on it. If that's so we’re part of the 3D information trapped on its surface... So how could we know? 

Well University of Southampton physicists may have found the signatures of a hologram in the cosmic microwave background...

Monday 6 February 2017

Could we come from space?

Hold on there with the tinfoil hats - I’m not about to start talking about how I was kidnapped and probed back in '01*.

Seriously, who travels thousands of lightyears just to stick things up people's bums? Who does that?!

What I’m talking about is (slightly) less crazy: Naturally occurring space travel. Most of humankind's great inventions have been done first by mother nature, so why should space travel be any different? 

Well… actually, because Earth’s gravity is strong, and it’s atmosphere deep. Only the most violent natural events – massive asteroid strikes – could throw something hard enough to overcome those and reach space. Still… Earth’s history is incredibly long, and filled with such impacts. Could it have happened? 


First a little background: Theories suggesting that Earth life might have naturally migrated to other worlds, or extra terrestrial life might have naturally migrated here, fall under the umbrella term ‘panspermia’. There're a lot of such theories and, yes, many of them belong firmly in the box marked ‘any advocate of this probably wears a tinfoil hat and talks about how he was kidnapped and probed in ‘01’

But others are considered sound science. The least controversial comes from the observation that asteroids and comets, which occasionally hit planets, carry in them some surprisingly (chemically) evolved compounds - exactly the kinds of chemistry we'd expect to see immediately prior to the start of life. Some of these chemical compounds can even form cell-like structures when exposed to water. So it seems quite un-tin-foil-hatish to suggest that asteroids and comets could have brought the chemical ingredients of life to Earth, and to other planets. 

Which, in a roundabout way, gave us this picture....

Another variant of the idea is litho-panspermia, the idea I alluded to earlier: Major asteroid impacts on early Earth and other planets could have splatted large amounts of rocky debris into space, with some of it eventually wandering far enough to fall on other worlds as meteorites. Simulations have shown that the interiors of rocks blown into space this way can stay fairly un-damaged by the forces of the blast - enough for microbes living in pores of the rock to survive the launch and go into hibernation without dying. Then, when one of those rocks eventually drops onto another planet…. 

We know that rocks can travel between planets this way: We’ve found meteorites that were definitely once rocks on the surface of Mars, or the Moon. They’ve told us things about the history of Mars and the Moon that even space probes couldn’t. Some carry evidence of ancient habitable conditions on Mars. 

The journey can be done the other way too: The surface of the Moon is estimated to contain around 200 kg of Earth rocks per square kilometer, blasted free from Earth during various asteroid impacts. The surface of Mars is also thought to hold massive numbers of Earth rocks, scattered across it's surface.

If you can spot one I'll buy you a beer. All the scientists will buy you a beer, for the rest of your life - that's how important it would be. well? Go on, there's a lifetime supply of beer to be had...
Tests and simulations suggest that a micro-organism in hibernation could survive the ‘launch’, and the landing. So it does seem possible that a piece of stone from Earth, thrown onto a course that took it to a nearby world, could transport viable microbes (if the trip was relatively quick – space radiation can damage even a microbe inside a rock several meters across). 

Today there’s a fairly obvious problem with this idea: There are no habitable planets for a wandering space rock from Earth to land on. But 3.5 billion years ago the story was different: A whole host of worlds that are dead today are thought to have hosted habitable environments – we know this from studying rocks from those worlds that made the reverse trip - to Earth! To add fuel to our tin foil hat wearing fire, the rate of asteroid strikes was much higher billions of years ago, allowing many more ‘launch opportunities’ than we see today. 

No–one can say if Earth life has ever made that trip, or if alien life from one of those worlds might have ever landed here. But, if we ever do find microbial life on nearby worlds, we should be cautious before we get too excited – it might not be truly alien, just a long lost cousin… 

*It wasn’t aliens, I just hung around with the wrong crowd. I don’t like to talk about it. 

Wednesday 1 February 2017

The Universe in 101 words: What is 'space brain'?

Above: Earth - protecting us in more ways than we can see.

Earth’s magnetic field forms a bubble which, aside from helping boy scouts get hopelessly lost with a compass, deflects high energy particles away from our planet.

Those are generally bad for people – they cause cancer, radiation sickness, etc - and they damage brain function (or at least it does in lab rats), so astronauts journeying beyond Earth’s magnetic field could expect steadily worsening depression, anxiety, and memory loss

Since insane, cancer ridden astronauts make for bad press NASA are developing shielding against the particles – although matching the power of Earth’s magnetic field will tough. 

Above: The view from a SpaceX launcher, as it leaves Earth behind.