0:00:02 > 0:00:04The universe is falling apart.
0:00:04 > 0:00:06Something is forcing galaxies
0:00:06 > 0:00:10to rush away from each other at ever increasing speeds.
0:00:11 > 0:00:14Ever since this alarming discovery,
0:00:14 > 0:00:17physicists have struggled to understand what might be causing it.
0:00:19 > 0:00:22So far, they've come up with a name.
0:00:23 > 0:00:25They've called it dark energy.
0:00:28 > 0:00:32Dark energy is basically our name for that thing
0:00:32 > 0:00:33that we don't understand.
0:00:36 > 0:00:38It's not the colour dark,
0:00:38 > 0:00:42it's just the expression of our ignorance as to what is this stuff.
0:00:42 > 0:00:47The discovery of dark energy really surprised theoretical physicists
0:00:47 > 0:00:49and remains a deep mystery of nature.
0:00:52 > 0:00:56Until dark energy, we had every reason to be as confident
0:00:56 > 0:00:58in Einstein's theory of general relativity
0:00:58 > 0:00:59as he was himself.
0:01:01 > 0:01:05In the last few days, I've completed one of the finest papers of my life.
0:01:07 > 0:01:10But now things are less certain.
0:01:10 > 0:01:14Einstein doesn't explain dark energy and, so far,
0:01:14 > 0:01:17neither has anyone else.
0:01:17 > 0:01:20We are absolutely still lacking great ideas.
0:01:20 > 0:01:26So, it is crying out for some new breakthrough, new thinking.
0:01:26 > 0:01:30Because it might be that dark energy is not a thing at all,
0:01:30 > 0:01:34but simply evidence that the physics itself is wrong,
0:01:34 > 0:01:36that we need another Einstein.
0:01:39 > 0:01:42What Einstein did was he actually came out
0:01:42 > 0:01:43and looked at the bigger picture
0:01:43 > 0:01:46and put all these different elements together
0:01:46 > 0:01:48to come up with the theories that he had.
0:01:48 > 0:01:53It might be time for the bigger picture to be re-evaluated.
0:01:53 > 0:01:55There's definitely room for another Einstein to come in
0:01:55 > 0:01:58and shake everything up and tell us
0:01:58 > 0:02:01that we've been looking at things completely wrong up until now.
0:02:03 > 0:02:06Would-be latter-day Einsteins the world over are on the hunt
0:02:06 > 0:02:10for answers to modern science's most enduring problem -
0:02:10 > 0:02:13to paint the biggest picture of all,
0:02:13 > 0:02:16to finally solve the mystery of dark energy.
0:02:32 > 0:02:34Energy is all around us.
0:02:35 > 0:02:41It comes from the sun, from chemical reactions, from electricity.
0:02:41 > 0:02:46Energy powers our vehicles, heats our homes, lights our nights.
0:02:47 > 0:02:51Understanding energy has transformed our planet and our lives.
0:02:53 > 0:02:56Dark energy is something altogether different.
0:02:56 > 0:03:01It seems to serve no useful purpose at all, except to show us
0:03:01 > 0:03:03that we understand less than we thought we did.
0:03:09 > 0:03:13Dark energy arrived wholly unexpectedly
0:03:13 > 0:03:15at the very end of the 20th century.
0:03:17 > 0:03:22In 1998, a young scientist called Saul Perlmutter was thinking
0:03:22 > 0:03:24some very big thoughts indeed.
0:03:24 > 0:03:28As a graduate student, I really wanted to find a project that
0:03:28 > 0:03:31would answer some, or that would at least be looking at some,
0:03:31 > 0:03:34very philosophical questions, something that felt like it
0:03:34 > 0:03:37was meaningful about the world we live in
0:03:37 > 0:03:39in some, you know, deep way.
0:03:46 > 0:03:50The question that's been really exciting me is
0:03:50 > 0:03:52whether the universe will last for ever.
0:03:52 > 0:03:55Do we live in a universe that is infinite,
0:03:55 > 0:03:57or, some day, will it come to an end?
0:03:59 > 0:04:01The two big options at that time
0:04:01 > 0:04:04were that the universe could expand for ever,
0:04:04 > 0:04:06but just slow and slow and slow,
0:04:06 > 0:04:08but forever be expanding.
0:04:08 > 0:04:10Or, if there was enough stuff in the universe
0:04:10 > 0:04:12to gravitationally attract it,
0:04:12 > 0:04:16it could slow to a halt and then collapse and come to an end.
0:04:16 > 0:04:19Saul was measuring the way the universe was expanding
0:04:19 > 0:04:23by observing exploding stars called supernovae.
0:04:27 > 0:04:30One particular kind of supernova always explodes the same way
0:04:30 > 0:04:33because it waits until just a critical amount of mass has fallen
0:04:33 > 0:04:38on it and then it explodes, so they all look very similar to each other.
0:04:38 > 0:04:42They brighten as a firework and fade away, and they reach the same
0:04:42 > 0:04:46brightness and you can then use that as an indicator of how far away
0:04:46 > 0:04:49it is, by just looking to see how bright it appears to you.
0:04:53 > 0:04:57Because they explode with exactly the same intensity,
0:04:57 > 0:05:01these supernovae are known as standard candles.
0:05:01 > 0:05:03By comparing their relative brightnesses,
0:05:03 > 0:05:07relative distances can be calculated.
0:05:07 > 0:05:10Saul expected the stars to show what everyone thought
0:05:10 > 0:05:13at the time - that the universe was slowing down.
0:05:14 > 0:05:16Fainter ones are further
0:05:16 > 0:05:19and just like when you watch a car recede into the distance,
0:05:19 > 0:05:23you can tell how far away it is by how faint the tail-lights look.
0:05:24 > 0:05:26If you can use the brightness of the supernova
0:05:26 > 0:05:28to tell you how far away it is,
0:05:28 > 0:05:30that's really telling you how long ago the explosion occurred
0:05:30 > 0:05:33because you know how long it takes for light to travel
0:05:33 > 0:05:35that great distance.
0:05:35 > 0:05:38So, now we have an object where it explodes
0:05:38 > 0:05:41and its brightness tells you when it exploded,
0:05:41 > 0:05:43how far back in time it exploded.
0:05:43 > 0:05:47No matter how good the theory, the practical problem of catching
0:05:47 > 0:05:52an exploding star at just the right time is immense.
0:05:52 > 0:05:56But Saul and his team applied the very latest computer technology
0:05:56 > 0:05:58to the problem.
0:06:02 > 0:06:06Yeah, we think it may be the scuzzy chain is too long.
0:06:06 > 0:06:08Now there's two switches on the back of it.
0:06:10 > 0:06:13We had the computers go down, the computers came back up again,
0:06:13 > 0:06:15but now, finally, we have the analysis completed,
0:06:15 > 0:06:18at least the computer's part of the analysis, and it's beginning
0:06:18 > 0:06:22to show us on the screen what it thinks might be a supernova.
0:06:25 > 0:06:31Eventually, after the team had identified 42 such dying stars,
0:06:31 > 0:06:32the calculations began.
0:06:34 > 0:06:36What Perlmutter discovered shocked him.
0:06:38 > 0:06:40The data was telling the wrong story.
0:06:41 > 0:06:44The universe didn't appear to be slowing down.
0:06:44 > 0:06:46We thought that that's what we would see
0:06:46 > 0:06:49and it looked like the opposite was taking place
0:06:49 > 0:06:53and, in fact, the universe was speeding up in its expansion.
0:06:53 > 0:06:58These distant supernovae were fainter than you would have thought
0:06:58 > 0:07:00and fairly significantly fainter.
0:07:00 > 0:07:04They were probably 20% or more
0:07:04 > 0:07:07and that's the hallmark of a universe
0:07:07 > 0:07:09that's actually speeding up in its expansion.
0:07:13 > 0:07:16To say that this result was a surprise
0:07:16 > 0:07:19would be a masterclass in understatement.
0:07:19 > 0:07:23It was so unexpected that the initial reaction was disbelief.
0:07:26 > 0:07:28Everybody knew Saul and everybody knew the experiment
0:07:28 > 0:07:31he was doing and I remember sitting in the audience and Saul
0:07:31 > 0:07:36getting up and expecting him to present an update on the results
0:07:36 > 0:07:38he'd given a year ago,
0:07:38 > 0:07:41that actually the universe was slowing down.
0:07:41 > 0:07:44And so, I was absolutely amazed that,
0:07:44 > 0:07:49based on only twice as many objects as he had the year before,
0:07:49 > 0:07:51that suddenly he was saying
0:07:51 > 0:07:54that we lived in a universe that was accelerating.
0:07:54 > 0:07:57I remember it just being just incredible.
0:07:57 > 0:08:00I mean, all the astronomers walking around scratching their heads
0:08:00 > 0:08:03saying, "This can't be right. Surely it can't be right?"
0:08:05 > 0:08:08It was not the result that people had been expecting
0:08:08 > 0:08:13and such an extraordinary claim demands extraordinary evidence,
0:08:13 > 0:08:16more than a few data from a handful of stars.
0:08:17 > 0:08:21So, here we are on a beach where there is about a billion pebbles
0:08:21 > 0:08:24and if you think you're trying to understand this beach,
0:08:24 > 0:08:28you wouldn't think you could understand it from 42 pebbles.
0:08:28 > 0:08:29But Saul was right.
0:08:29 > 0:08:32He was able to work out that the universe was accelerating
0:08:32 > 0:08:34just from 42 supernovae,
0:08:34 > 0:08:37which is quite incredible when you think about it.
0:08:41 > 0:08:44On the one hand, this was a good result.
0:08:44 > 0:08:48It was new science and produced a Nobel prize for Saul Perlmutter.
0:08:49 > 0:08:52On the other, it raised an obvious question.
0:08:55 > 0:08:58Once you know that the universe is actually speeding up,
0:08:58 > 0:09:00then you're faced with the question of -
0:09:00 > 0:09:02well, what could make it speed up?
0:09:04 > 0:09:08So far, the only real progress on that question has been
0:09:08 > 0:09:11to give the phenomenon a name.
0:09:11 > 0:09:13It's become known as dark energy.
0:09:15 > 0:09:19Dark energy is just the term we use to describe whatever it is
0:09:19 > 0:09:22that makes the universe accelerate in its expansion,
0:09:22 > 0:09:25what makes it expand faster and faster.
0:09:25 > 0:09:27We don't know what that is.
0:09:27 > 0:09:30It's a mystery and so we call it dark to reflect our ignorance,
0:09:30 > 0:09:33not because the colour is dark.
0:09:37 > 0:09:40The mystery is so deep, so beguiling,
0:09:40 > 0:09:43that wherever there are physicists, there are people hoping
0:09:43 > 0:09:47that they will solve the mystery of dark energy.
0:09:49 > 0:09:52People safe in the infuriating knowledge
0:09:52 > 0:09:55that what they're looking for, if it's there at all,
0:09:55 > 0:09:56is all around them.
0:09:58 > 0:10:01But the fact that no-one has yet been able to identify
0:10:01 > 0:10:04what the dark energy might actually be
0:10:04 > 0:10:08has opened a can of worms not seen in science since the last time
0:10:08 > 0:10:10a physicist got involved in cosmology.
0:10:14 > 0:10:19In 1915, it seemed that the work of physics was nearly at an end.
0:10:19 > 0:10:22Everything made sense.
0:10:22 > 0:10:25Newton had explained the heavens by invoking gravity
0:10:25 > 0:10:29and atoms had been identified as the smallest indivisible units
0:10:29 > 0:10:31of matter.
0:10:31 > 0:10:32Job done.
0:10:37 > 0:10:41But then, a German man, given to musing on trains,
0:10:41 > 0:10:44turned up with a totally new set of ideas.
0:10:47 > 0:10:50I very rarely think in words at all.
0:10:50 > 0:10:51A thought comes...
0:10:55 > 0:10:58..and I might try to express it in words afterwards.
0:11:00 > 0:11:04Einstein called these little flights of fancy
0:11:04 > 0:11:08his "thought experiments" and they would lead him
0:11:08 > 0:11:11to develop his theory of general relativity,
0:11:11 > 0:11:14which totally changed how the workings of the universe
0:11:14 > 0:11:16were understood.
0:11:16 > 0:11:18I sometimes ask myself how did it come that
0:11:18 > 0:11:22I was the one to develop the theory of relativity?
0:11:22 > 0:11:25The reason, I think, is that a normal adult
0:11:25 > 0:11:28never stops to think about problems of space and time.
0:11:28 > 0:11:31These are things which he had thought of as a child.
0:11:32 > 0:11:35But I began to wonder about space and time
0:11:35 > 0:11:37only when I had already grown-up.
0:11:40 > 0:11:44Einstein's theory held that Newton's ideas about gravity,
0:11:44 > 0:11:47though empirically correct in most cases,
0:11:47 > 0:11:49were, in fact, conceptually wrong.
0:11:51 > 0:11:53Gravity, said Einstein,
0:11:53 > 0:11:58was not some nebulous attracting property of mass as Newton supposed,
0:11:58 > 0:12:03but was, in fact, a consequence of mass interacting with space-time -
0:12:03 > 0:12:07the gaps around stars and planets previously known as space.
0:12:09 > 0:12:13According to Einstein, space isn't simply a void.
0:12:13 > 0:12:16It's more like a four-dimensional fabric
0:12:16 > 0:12:19woven from both space and time.
0:12:21 > 0:12:25The mass of planets can warp and distort the fabric,
0:12:25 > 0:12:29gathering other celestial objects, like moons, around them.
0:12:30 > 0:12:35And it's this bending of space-time that creates the effect
0:12:35 > 0:12:37we experience as gravity.
0:12:38 > 0:12:40So, Einstein's theory of general relativity
0:12:40 > 0:12:42is a beautiful theory.
0:12:42 > 0:12:45It's incredibly elegant and has been now around for 100 years.
0:12:45 > 0:12:47It's very predictable.
0:12:47 > 0:12:49You can write things, make predictions of what the universe
0:12:49 > 0:12:52should look like and what objects should look like in the universe,
0:12:52 > 0:12:57and we can test those, and as far as we can tell, it's passed every test.
0:13:01 > 0:13:04The power of general relativity is that, like Newton's
0:13:04 > 0:13:06version of gravity before it,
0:13:06 > 0:13:08it's predictive.
0:13:08 > 0:13:11Bizarre as the curvature of space-time may sound,
0:13:11 > 0:13:14it's eminently testable -
0:13:14 > 0:13:18a fact not lost on Einstein himself.
0:13:18 > 0:13:21I have now come to realise that one of the most important consequences
0:13:21 > 0:13:24of that analysis is accessible to experimental test.
0:13:24 > 0:13:28Accordingly, a ray of light travelling past the sun would
0:13:28 > 0:13:31undergo a deflection amounting to 0.83 seconds of arc.
0:13:37 > 0:13:40In 1919, that prediction was actually observed.
0:13:41 > 0:13:44British astronomer, Arthur Eddington,
0:13:44 > 0:13:47pointed his telescope at a patch of sky near the sun
0:13:47 > 0:13:50during an eclipse and observed a star,
0:13:50 > 0:13:53known to be actually out of view, behind the sun.
0:13:54 > 0:13:59Its rays of light had been bent by the distorted space-time
0:13:59 > 0:14:01created by the sun's mass.
0:14:02 > 0:14:05Einstein's theory had held up.
0:14:05 > 0:14:10A paradigm had shifted and the crowd went wild.
0:14:11 > 0:14:14Einstein was suddenly famous.
0:14:14 > 0:14:19Undoubtedly the cleverest, yet most incomprehensible man on earth.
0:14:20 > 0:14:22This strange world is a madhouse.
0:14:24 > 0:14:28Currently, every coachman and every waiter is debating
0:14:28 > 0:14:31whether relativity theory is correct.
0:14:31 > 0:14:34This mass excitement about my theory is to do with the
0:14:34 > 0:14:37intriguing mystery of incomprehensibility.
0:14:39 > 0:14:42I'm certain that the mystery of not understanding
0:14:42 > 0:14:43is what attracts people.
0:14:46 > 0:14:49This is what all the fuss was about.
0:14:49 > 0:14:52This is the equation that the coachmen and waiters
0:14:52 > 0:14:53were discussing.
0:14:58 > 0:15:02On the one side, the geometry of space-time.
0:15:02 > 0:15:06On the other, the mass and energy of the universe which acts on it.
0:15:06 > 0:15:09Not incomprehensible at all(!)
0:15:09 > 0:15:11At least, not to its author.
0:15:12 > 0:15:17But there was one aspect of general relativity that Einstein himself
0:15:17 > 0:15:19didn't understand.
0:15:23 > 0:15:26The problem that Einstein had is that
0:15:26 > 0:15:30when he solved his equations of general relativity,
0:15:30 > 0:15:34what he found was that he predicted that the universe should
0:15:34 > 0:15:37actually be expanding.
0:15:37 > 0:15:40And that was radically different from the perceived
0:15:40 > 0:15:41wisdom at the time,
0:15:41 > 0:15:45which was that we lived in a static universe, both static in time
0:15:45 > 0:15:46and in space.
0:15:48 > 0:15:52So, he put in an extra term into the equation.
0:15:52 > 0:15:55He called it the cosmological constant.
0:15:55 > 0:15:59He used the Greek variable lambda, but, effectively, it was just
0:15:59 > 0:16:02what a physics undergraduate would call a fudge factor.
0:16:02 > 0:16:05It was just designed to make the equations come out right
0:16:05 > 0:16:10and it would just make the universe sort of stand still.
0:16:10 > 0:16:13The problem is is in science, we'd call that fine tuning.
0:16:13 > 0:16:16And you only have to change the value of the constant
0:16:16 > 0:16:21by a small amount and suddenly, you get back these expanding solutions.
0:16:22 > 0:16:25The general theory of relativity requires the universe to be
0:16:25 > 0:16:27spatially finite,
0:16:27 > 0:16:30but this view of the universe necessitated
0:16:30 > 0:16:34an expansion of equations with the introduction of a new
0:16:34 > 0:16:37universal constant lambda standing in fixed relation to the
0:16:37 > 0:16:39total mass of the universe.
0:16:39 > 0:16:43This is gravely detrimental to the formal beauty of my theory.
0:16:48 > 0:16:51When you add the lambda term,
0:16:51 > 0:16:55it means that the equation is not quite as simple as it was before.
0:16:55 > 0:16:58So, in that sense, it's not as beautiful as an equation.
0:17:01 > 0:17:04The static universe was restored,
0:17:04 > 0:17:09but Einstein always felt he'd added lambda against his better judgment.
0:17:15 > 0:17:18Dear Egrenfest,
0:17:18 > 0:17:21I have perpetrated something in gravitation theory which
0:17:21 > 0:17:25exposes me a bit to the danger of being committed to a madhouse.
0:17:30 > 0:17:35Despite the fudge factor, lambda, the cosmological constant,
0:17:35 > 0:17:40Einstein continued to be celebrated as the world's cleverest man.
0:17:40 > 0:17:44Until, in 1929, he became even cleverer.
0:17:49 > 0:17:52In America, astronomer Edwin Hubble was about to get
0:17:52 > 0:17:56a reputation for scientific cleverness himself.
0:17:56 > 0:18:00He'd been using the world's largest telescope at Mount Wilson in
0:18:00 > 0:18:05California to peer deeper into space than anyone had ever looked before.
0:18:08 > 0:18:12What he discovered completely changed the meaning of the word
0:18:12 > 0:18:15"universe".
0:18:15 > 0:18:20Until Hubble, it had been thought that the universe was our galaxy.
0:18:20 > 0:18:25What Hubble saw was that in fact our galaxy is just one of countless
0:18:25 > 0:18:28millions, but more importantly,
0:18:28 > 0:18:33that all these galaxies were moving apart from each other.
0:18:33 > 0:18:36The universe wasn't static after all.
0:18:36 > 0:18:39This had huge implications.
0:18:39 > 0:18:42It introduced the notion of a beginning
0:18:42 > 0:18:45and an age for the universe.
0:18:45 > 0:18:48But more importantly for Einstein,
0:18:48 > 0:18:52it meant that he could ditch his fudge factor, the cosmological
0:18:52 > 0:18:57constant, and return general relativity to its former glory.
0:18:57 > 0:19:01The lambda that he added to create a static universe
0:19:01 > 0:19:03was no longer required,
0:19:03 > 0:19:07once it was observed by Slipher and Hubble
0:19:07 > 0:19:11that the universe, in fact, was expanding,
0:19:11 > 0:19:14so if, in an expanding universe,
0:19:14 > 0:19:18at the time, the observations could be described
0:19:18 > 0:19:22without the lambda term and so he removed it.
0:19:25 > 0:19:28Einstein was cock-a-hoop.
0:19:28 > 0:19:32In 1931, he went to Mount Wilson to shake Hubble's hand
0:19:32 > 0:19:35and thank him for putting beauty back into his equation.
0:19:35 > 0:19:41Lambda, he later confessed, was the biggest blunder in his career.
0:19:43 > 0:19:49I think that the reason that he said that it was a blunder was
0:19:49 > 0:19:53because if he had just not introduced that term,
0:19:53 > 0:19:58then he would have said that the universe must be expanding
0:19:58 > 0:20:03and done that 14 years before the discovery of the expansion of
0:20:03 > 0:20:08the universe by Edwin Hubble, which would have been a great achievement.
0:20:08 > 0:20:11But despite Einstein's blunder,
0:20:11 > 0:20:14general relativity has stood the test of time.
0:20:14 > 0:20:19It is perhaps the single most successful scientific theory yet.
0:20:19 > 0:20:26Every observation we make of gravity, from the smaller scales
0:20:26 > 0:20:30to solar system scales to galactic scales,
0:20:30 > 0:20:34all the way to the universe, all of that can be described using
0:20:34 > 0:20:37the single theory that Einstein created.
0:20:37 > 0:20:40So, it's the most successful
0:20:40 > 0:20:45and beautiful theory we have of our universe.
0:20:45 > 0:20:47Or at least, it was.
0:20:47 > 0:20:51For all its beauty and simplicity,
0:20:51 > 0:20:55general relativity doesn't account for the effects of dark energy.
0:20:55 > 0:20:59Expansion, as reported by Hubble, works fine,
0:20:59 > 0:21:02but the accelerated expansion of the universe
0:21:02 > 0:21:05that Saul Perlmutter found isn't part of the deal.
0:21:08 > 0:21:12That it's there at all is bad enough, but worse still, the way
0:21:12 > 0:21:16that dark energy seems to work is unlike anything that's been
0:21:16 > 0:21:18observed before.
0:21:18 > 0:21:23The density of anything is the amount of stuff you have
0:21:23 > 0:21:24within a given volume
0:21:24 > 0:21:27and dark energy is an unusual phenomenon,
0:21:27 > 0:21:31in that even though the volume of the universe is increasing
0:21:31 > 0:21:37as it expands, the density is staying the same, constant.
0:21:37 > 0:21:40So, imagine you had say like half a cup of black coffee,
0:21:40 > 0:21:44and then you started adding milk to it,
0:21:44 > 0:21:48and as we pour more and more milk into that cup,
0:21:48 > 0:21:52then the volume of the liquid's getting larger and larger,
0:21:52 > 0:21:55but the density of the coffee is going down, so the coffee
0:21:55 > 0:21:59would be getting lighter and lighter as you added more and more milk.
0:21:59 > 0:22:02But dark energy doesn't behave that way,
0:22:02 > 0:22:05so it's almost as if there's new dark energy being created
0:22:05 > 0:22:08all the time, as the universe expands,
0:22:08 > 0:22:13meaning that its density remains the same. Constant.
0:22:13 > 0:22:16So, you can think of it, as you get more space,
0:22:16 > 0:22:19you actually get more dark energy, which is
0:22:19 > 0:22:22like getting something for nothing, which is clearly ridiculous.
0:22:22 > 0:22:27It's clearly against all our training as physicists.
0:22:30 > 0:22:34There is one way to adapt general relativity to cope with this
0:22:34 > 0:22:37magically constantly self-replenishing force
0:22:37 > 0:22:41and that is to simply add it to the equation.
0:22:41 > 0:22:45100 years after Einstein's "biggest blunder",
0:22:45 > 0:22:48the cosmological constant is back.
0:22:48 > 0:22:51Lambda is being written once more.
0:22:51 > 0:22:55This time, not to keep the universe still,
0:22:55 > 0:22:59but to account for its unexplained accelerating expansion.
0:22:59 > 0:23:04The values are different, but the concept is exactly the same.
0:23:04 > 0:23:07All this leads cosmologists
0:23:07 > 0:23:11to one of two equally alarming conclusions -
0:23:11 > 0:23:13either we need another Hubble,
0:23:13 > 0:23:16or we need another Einstein.
0:23:16 > 0:23:20But before we consign Albert to the scientific scrapheap,
0:23:20 > 0:23:23there is a branch of physics which might help.
0:23:23 > 0:23:27An area where things popping in and out of existence is quite normal.
0:23:33 > 0:23:38This is the strange and wonderful world of Clare Burrage
0:23:38 > 0:23:40and of quantum mechanics.
0:23:40 > 0:23:44Quantum mechanics is the theory of what happens to really,
0:23:44 > 0:23:45really small things.
0:23:45 > 0:23:49It's a theory of how the fundamental particles in the universe work.
0:23:49 > 0:23:52Atoms, electrons, protons.
0:23:53 > 0:23:58And quantum mechanics is intrinsically uncertain.
0:23:59 > 0:24:03Einstein hated quantum mechanics.
0:24:03 > 0:24:04He disliked the probabilistic
0:24:04 > 0:24:08"now you see it, now you don't" nature of the idea.
0:24:08 > 0:24:12"God," he famously declared, "does not play dice."
0:24:12 > 0:24:15In 1930, he paid a visit to Nottingham,
0:24:15 > 0:24:18where Clare now does her research. He didn't say much.
0:24:23 > 0:24:26He probably didn't say anything about quantum mechanics.
0:24:26 > 0:24:30He came to give a talk on general relativity.
0:24:30 > 0:24:35His actual chalk writing is preserved for devotees to marvel at.
0:24:35 > 0:24:38But even though Einstein didn't like it,
0:24:38 > 0:24:42quantum mechanics could shed light on dark energy
0:24:42 > 0:24:46and come to the aid of his once-more-under-fire theory.
0:24:46 > 0:24:48In theory.
0:24:50 > 0:24:52Quantum mechanics tells us
0:24:52 > 0:24:55that particles can come in and out of existence in the vacuum.
0:24:55 > 0:24:58And the fact that those particles have mass
0:24:58 > 0:25:02and potentially are moving around, they have a little bit of energy.
0:25:02 > 0:25:06And so, when they pop into existence,
0:25:06 > 0:25:08they give a little bit of energy to the vacuum and yes,
0:25:08 > 0:25:12they disappear again, but the fact that that process is going
0:25:12 > 0:25:16on all of the time means that there is some energy stored in the vacuum.
0:25:16 > 0:25:18And because Einstein told us that energy
0:25:18 > 0:25:20and mass are the same thing,
0:25:20 > 0:25:23having lots of energy stored in space affects space-time
0:25:23 > 0:25:28that cause the expansion of the universe to accelerate.
0:25:28 > 0:25:32So, it seems that quantum mechanics should, in theory,
0:25:32 > 0:25:36be able to explain how the cosmological constant works.
0:25:36 > 0:25:40And how dark energy appears in the vacuum of space
0:25:40 > 0:25:44and is driving the acceleration of the universe.
0:25:44 > 0:25:47But there's a problem.
0:25:47 > 0:25:50When they came to calculate this vacuum energy,
0:25:50 > 0:25:54they discovered how spectacularly wrong they were.
0:25:54 > 0:25:57If you were to say there was one pebble on this beach,
0:25:57 > 0:26:00you'd be wrong by one part in a billion.
0:26:00 > 0:26:04If you were to say there was one particle in the universe,
0:26:04 > 0:26:07you'd be off by ten to the 80.
0:26:07 > 0:26:13But the vacuum energy was calculated to be off by ten to the 120.
0:26:13 > 0:26:15That is a google.
0:26:15 > 0:26:18That is spectacularly wrong.
0:26:21 > 0:26:24The fact that our predictions are so far off from what we see
0:26:24 > 0:26:27tells us that there's something fundamentally missing
0:26:27 > 0:26:31in the way that we understand physics, that we understand
0:26:31 > 0:26:34the world around us, so there's still a mystery,
0:26:34 > 0:26:35still a puzzle there.
0:26:35 > 0:26:39It might be tempting to simply ignore dark energy.
0:26:39 > 0:26:43You could argue that the apparent accelerated expansion
0:26:43 > 0:26:45is, in fact, a trick of the light, that it
0:26:45 > 0:26:50may be a function of other inaccessible dimensions at play.
0:26:50 > 0:26:56That it just looks like dark energy, but is actually...something else.
0:26:56 > 0:27:00But dark energy isn't just an irritating threat
0:27:00 > 0:27:03to Einstein's beautiful equations.
0:27:03 > 0:27:06It's also a very practical solution to a fundamental
0:27:06 > 0:27:12question in cosmology, namely - what is the universe made of?
0:27:12 > 0:27:16When Einstein was busy thinking about gravity on trains,
0:27:16 > 0:27:18the answer was simple.
0:27:18 > 0:27:21The universe was made of the same stuff that you
0:27:21 > 0:27:26and I are made of, the stuff of stars, planets, Coke cans,
0:27:26 > 0:27:28tennis rackets, atoms,
0:27:28 > 0:27:33made, in turn, from electrons, protons and neutrons.
0:27:33 > 0:27:37But physics was about to get a shock.
0:27:37 > 0:27:41It turned out that there was something else out there
0:27:41 > 0:27:45that the universe was also made of,
0:27:45 > 0:27:47matter of a different kind.
0:27:47 > 0:27:52In 1975, an astronomer called Vera Rubin made an unexpected discovery.
0:27:54 > 0:27:56If we plot
0:27:56 > 0:28:02the velocity of the planets as a function of distance from the sun,
0:28:02 > 0:28:07Mercury, Venus, Earth, Mars,
0:28:07 > 0:28:13Jupiter, Saturn, Uranus, Neptune, Pluto,
0:28:13 > 0:28:18and you can see that Mercury orbits much more rapidly
0:28:18 > 0:28:21than Pluto.
0:28:23 > 0:28:26The graph is called a rotation curve.
0:28:26 > 0:28:29It is the embodiment of the law of gravity.
0:28:29 > 0:28:31The further away you travel from the sun,
0:28:31 > 0:28:34the weaker its gravitational force.
0:28:34 > 0:28:39Galaxies work in the same way as our solar system.
0:28:39 > 0:28:43Except that instead of planets orbiting a central sun,
0:28:43 > 0:28:45in a spiral galaxy,
0:28:45 > 0:28:49stars are held in orbit by a gravity-providing black hole.
0:28:50 > 0:28:55Vera decided to plot the rotation curves in galaxies.
0:28:55 > 0:28:58She trained her telescopes on Andromeda,
0:28:58 > 0:29:01the galaxy closest to our own.
0:29:01 > 0:29:03I came out with sets of numbers
0:29:03 > 0:29:05and I plotted them on pieces of paper and
0:29:05 > 0:29:09I discovered that the stars as you went further and further out
0:29:09 > 0:29:10did not slow down,
0:29:10 > 0:29:14they were moving just as fast as the stars near the centre.
0:29:16 > 0:29:21We find that the velocities remain flat all the way to the
0:29:21 > 0:29:23edge of our observations.
0:29:23 > 0:29:26And that was a surprise.
0:29:26 > 0:29:28And a surprise that had to be explained.
0:29:29 > 0:29:33By rights, the stars should have flown off into space,
0:29:33 > 0:29:37but they didn't and wherever spiral galaxies were measured,
0:29:37 > 0:29:40the same flat curves appeared.
0:29:40 > 0:29:43It was decided that the only explanation
0:29:43 > 0:29:46was that there must be more stuff out there that we couldn't see,
0:29:46 > 0:29:49providing the extra gravity,
0:29:49 > 0:29:51holding the galaxies together
0:29:51 > 0:29:53and flattening the curves.
0:29:58 > 0:30:01They called this stuff
0:30:01 > 0:30:02dark matter.
0:30:08 > 0:30:12The new dark matter was a shock, in more ways than one.
0:30:12 > 0:30:16The very fact of its existence was almost overshadowed
0:30:16 > 0:30:20by the fact that when the calculations were made, this new
0:30:20 > 0:30:26form of matter outweighed the atomic form of stuff by about 90 to one.
0:30:28 > 0:30:30In the 1980s,
0:30:30 > 0:30:34when new ways of measuring dark matter were developed,
0:30:34 > 0:30:37it was discovered that there simply wasn't enough of it
0:30:37 > 0:30:41to make the universe work as it clearly does.
0:30:41 > 0:30:44The universe was short of stuff
0:30:44 > 0:30:47to the tune of about 70%.
0:30:48 > 0:30:50Cosmology scratched its head.
0:30:51 > 0:30:53Then, in 1998,
0:30:53 > 0:30:57a young scientist called Saul Perlmutter
0:30:57 > 0:31:00was thinking some very big thoughts indeed.
0:31:00 > 0:31:02Something that felt like it was
0:31:02 > 0:31:07meaningful about the world we live in, in some deep way.
0:31:07 > 0:31:10The universe was speeding up in its expansion.
0:31:10 > 0:31:14The dark energy that earned Saul his Nobel Prize was an interesting
0:31:14 > 0:31:16and troubling concept,
0:31:16 > 0:31:18but it also had a number
0:31:18 > 0:31:22and that number was highly significant.
0:31:22 > 0:31:24We know from Einstein - him again -
0:31:24 > 0:31:27that energy and mass are related,
0:31:27 > 0:31:31that energy, E, equals mass times the speed of light squared.
0:31:31 > 0:31:34E = MC2.
0:31:35 > 0:31:39Plug dark energy into that equation and you get the missing mass
0:31:39 > 0:31:42that dark matter couldn't account for.
0:31:45 > 0:31:48The universe was complete.
0:31:48 > 0:31:51It was made about 4% baryonic matter,
0:31:51 > 0:31:54the stuff that we're made from,
0:31:54 > 0:31:5525% dark matter
0:31:55 > 0:32:00and the gaping 70%-sized hole was filled with dark energy.
0:32:09 > 0:32:14So far, despite heroic efforts to find it, and overwhelming
0:32:14 > 0:32:18evidence that it exists, no-one has identified what dark matter is.
0:32:24 > 0:32:28And, of course, dark energy, both useful and confounding,
0:32:28 > 0:32:32is barely in its infancy when it comes to a convincing explanation.
0:32:32 > 0:32:35There's radiation damage.
0:32:35 > 0:32:40We may not be quite there with the shielding yet to provide
0:32:40 > 0:32:42the right radiation environment.
0:32:42 > 0:32:47But there is an idea in cosmology that dark matter
0:32:47 > 0:32:50and dark energy may be linked by more than just a common
0:32:50 > 0:32:55adjective and if they are, a new European spacecraft called
0:32:55 > 0:32:59Euclid may shed light on what that link might be.
0:33:00 > 0:33:06The Euclid Consortium is staffed by 1,200 scientists from 14 countries.
0:33:06 > 0:33:08These are some of them
0:33:08 > 0:33:12having their picture taken at their annual conference in Lausanne.
0:33:14 > 0:33:17They're hoping that by taking pictures of the universe,
0:33:17 > 0:33:22they'll be able to work out how it's expanded over its lifetime and
0:33:22 > 0:33:26that by determining that, the nature of dark energy will become clearer.
0:33:28 > 0:33:32The way we think about it is that it's either some new
0:33:32 > 0:33:34stuff in the universe,
0:33:34 > 0:33:39some particle or even just a new field that you
0:33:39 > 0:33:43put in to the universe to explain the properties of the universe.
0:33:45 > 0:33:49Alternatively, you could say that the equation you wrote down
0:33:49 > 0:33:52is not correct. It's not wrong,
0:33:52 > 0:33:54but we like to say it's "incomplete".
0:33:54 > 0:33:58So, you could sort of fiddle with the mathematics of the equation,
0:33:58 > 0:34:01so actually what you could do is maybe come up with a natural
0:34:01 > 0:34:04explanation for it.
0:34:04 > 0:34:09So, Euclid should be able to tell us which of those alternatives it is.
0:34:10 > 0:34:15The satellite will launch and start sending data back to Earth in 2020.
0:34:19 > 0:34:23The all-important camera for the Euclid space telescope is being
0:34:23 > 0:34:28built and tested in the UK, in this country house in the Surrey Hills.
0:34:34 > 0:34:39These are going to be the biggest images that come down from in orbit.
0:34:39 > 0:34:43You have an image of 625 megapixels,
0:34:43 > 0:34:46so that's roughly 300 HD
0:34:46 > 0:34:50television screens full of data and that comes down every ten minutes.
0:34:51 > 0:34:55This imager takes roughly the same amount of data that Hubble
0:34:55 > 0:34:58has taken and will take in its entire lifetime in one day.
0:35:03 > 0:35:05It's an astonishing leap forward,
0:35:05 > 0:35:09given what's available from current space telescopes.
0:35:09 > 0:35:12The huge data sets will provide information from the universe
0:35:12 > 0:35:14in almost every direction.
0:35:16 > 0:35:19The Hubble space telescope is the biggest
0:35:19 > 0:35:22and best telescope we have available at the moment
0:35:22 > 0:35:25and the amount of sky that's covered is about the size
0:35:25 > 0:35:28of your little fingernail, if you held it up at arm's length.
0:35:28 > 0:35:32And Euclid will do the same type of imaging as the Hubble space
0:35:32 > 0:35:35telescope, but instead of just covering that small patch
0:35:35 > 0:35:40of sky, it will cover practically every bit of sky that you can see.
0:35:41 > 0:35:45Not only will Euclid be able to measure the historic
0:35:45 > 0:35:49acceleration of stars and galaxies in all directions,
0:35:49 > 0:35:52it's hoped it will also provide data about how dark matter
0:35:52 > 0:35:56around galaxies has expanded over time.
0:35:56 > 0:36:02This is possible because of an effect called gravitational lensing.
0:36:02 > 0:36:08So, in general relativity, mass bends space and time
0:36:08 > 0:36:12and then light is bent around large massive objects,
0:36:12 > 0:36:15just like Eddington measuring the star behind the Sun,
0:36:15 > 0:36:18and so, we used the same technique for Euclid.
0:36:18 > 0:36:20I can illustrate it using this wine glass
0:36:20 > 0:36:22and this image of the universe,
0:36:22 > 0:36:27so as we draw the wine glass across the image,
0:36:27 > 0:36:32what you see is that the galaxies behind the wine glass
0:36:32 > 0:36:37get distorted and that distortion is caused by the lens.
0:36:37 > 0:36:42In general relativity, the lens is mass, because it bends the light.
0:36:42 > 0:36:45And that can be shown in this picture.
0:36:45 > 0:36:49You have a large clump of mass here, which is like the lens,
0:36:49 > 0:36:51like the bottom of the wine glass,
0:36:51 > 0:36:56and what you can see are all the distorted galaxies behind that lens.
0:36:56 > 0:36:59And what you could do with an image like this is you can
0:36:59 > 0:37:04calculate how much mass would I need within the lens to create
0:37:04 > 0:37:07the distortions that I see and what you find is quite remarkable.
0:37:07 > 0:37:12What you find is that there is about 100 times more mass here
0:37:12 > 0:37:16than you see from the light in the image and that missing mass,
0:37:16 > 0:37:20that mass you cannot see, is what we call dark matter.
0:37:23 > 0:37:28So, Euclid will make an image of the whole sky at this resolution
0:37:28 > 0:37:33and it will find all these distorted background galaxies
0:37:33 > 0:37:35and from that, it can infer
0:37:35 > 0:37:38the distribution of dark matter in the universe.
0:37:38 > 0:37:44Euclid will compare lensing all over the universe and by doing so,
0:37:44 > 0:37:47will help paint an accurate picture of how the universe is
0:37:47 > 0:37:51tearing itself apart under the influence of dark energy.
0:37:51 > 0:37:57So, Euclid may tell us that it's the cosmological constant
0:37:57 > 0:38:00and then we have to explain that,
0:38:00 > 0:38:04it might tell us that our theory
0:38:04 > 0:38:08of gravity is not complete,
0:38:08 > 0:38:11and we'd have to explain that,
0:38:11 > 0:38:14it could tell us that actually the dark matter
0:38:14 > 0:38:20and dark energy are two sides of the same coin and that actually there
0:38:20 > 0:38:24might be a unified dark sector, but we'd have to explain that.
0:38:24 > 0:38:30It could be another theory that we haven't even come up with yet.
0:38:30 > 0:38:32And so Euclid will give us
0:38:32 > 0:38:36a coherent data set that we can test all these theories against.
0:38:39 > 0:38:43Whatever the case, the devil's in the detail, and these days,
0:38:43 > 0:38:47the detail can be interrogated to degrees not thought possible
0:38:47 > 0:38:51when Einstein first reluctantly inserted his cosmological
0:38:51 > 0:38:55constant into general relativity.
0:38:55 > 0:38:59Cosmology is one of the fields that is actually pushing
0:38:59 > 0:39:04the boundaries of cosmology itself, but also statistics and computing.
0:39:04 > 0:39:07It is the frontier, I think.
0:39:07 > 0:39:10Euclid will be pushing the boundaries like never before.
0:39:10 > 0:39:14It will stream more data from space than has ever been
0:39:14 > 0:39:15processed in the past.
0:39:15 > 0:39:19In the end, it will have about one and a half billion galaxies.
0:39:19 > 0:39:23It will observe one and a half billion galaxies, so it's huge.
0:39:23 > 0:39:27And a lot of the time, your eyes cannot just pick up patterns,
0:39:27 > 0:39:31so this cannot be possible without computers and statistics.
0:39:31 > 0:39:35The computer-aided searches should give unprecedented clarity
0:39:35 > 0:39:39on how science should be thinking about dark energy.
0:39:39 > 0:39:42There will be winners and losers.
0:39:42 > 0:39:46The amount of data that we have on dark energy hasn't been enough
0:39:46 > 0:39:49to be able to tell us which path we have to go down,
0:39:49 > 0:39:51so we have lots of theories
0:39:51 > 0:39:55and hundreds of models that could still fit our data.
0:39:55 > 0:39:59When Euclid comes, lots of these can be thrown away and it could
0:39:59 > 0:40:02narrow down the possibilities of what this dark energy is.
0:40:12 > 0:40:14Euclid is not the only show in town
0:40:14 > 0:40:18when it comes to mapping the expansion of the universe.
0:40:19 > 0:40:24At Kit Peak in Arizona, Risa Wechsler is hoping to use
0:40:24 > 0:40:27the proposed dark energy spectroscopic instrument,
0:40:27 > 0:40:31DESI, to make a map of part of the universe, like this one.
0:40:35 > 0:40:38But 100 times more accurate,
0:40:38 > 0:40:41so that she can check the validity of computer
0:40:41 > 0:40:44simulations of the universe that she's created.
0:40:46 > 0:40:51One of the things that I do is try to simulate the entire universe
0:40:51 > 0:40:55and tie what we think about the physics of the evolving
0:40:55 > 0:40:58universe to what we actually see with surveys like DESI.
0:41:01 > 0:41:05What we're trying to do in these simulations is take a whole
0:41:05 > 0:41:07bunch of hypothetical universes, some of them
0:41:07 > 0:41:11will have a cosmological constant, some of them
0:41:11 > 0:41:14will have a different time evolving dark energy, some of them
0:41:14 > 0:41:16will have more or less amount of dark matter,
0:41:16 > 0:41:20and then when we compare that to what we actually see,
0:41:20 > 0:41:23we can rule out a lot of these ideas,
0:41:23 > 0:41:26so some of them will not be consistent with what we measure
0:41:26 > 0:41:30and then we can determine that that's not the universe we live in.
0:41:30 > 0:41:34When DESI starts producing data in 2020,
0:41:34 > 0:41:38it might be that one of Risa's simulations strikes gold.
0:41:38 > 0:41:42It'll be up against a lot of competition.
0:41:42 > 0:41:47In the absence of hard data, this is boom time for theories.
0:41:47 > 0:41:50Multi-Galileons, ghost condensates,
0:41:50 > 0:41:53and the higher co-dimensional brane worlds theory
0:41:53 > 0:41:58jostle for attention in the race to explain dark energy.
0:41:58 > 0:42:03Many of these theories usually try to provide a global solution
0:42:03 > 0:42:08to the dark energy problem, a fix to general relativity,
0:42:08 > 0:42:10but Clare Burrage is working on an idea
0:42:10 > 0:42:13that says that Einstein may have been both
0:42:13 > 0:42:16right and wrong at the same time,
0:42:16 > 0:42:19depending on where you are.
0:42:19 > 0:42:23We know that Einstein's theory works very well on Earth
0:42:23 > 0:42:24and in the solar system.
0:42:24 > 0:42:27We've tested it and it works phenomenally well.
0:42:27 > 0:42:30But we don't have ways of testing that theory on the kinds
0:42:30 > 0:42:34of distance scales that are relevant to cosmology
0:42:34 > 0:42:37and so it could be that whilst relativity is a good description
0:42:37 > 0:42:42of what's happening around us, it doesn't work as a description of
0:42:42 > 0:42:46the universe as a whole system and maybe you need to change the theory.
0:42:48 > 0:42:54Clare's solution involves something called a chameleon,
0:42:54 > 0:42:57a particle that tries to blend in not by changing colour,
0:42:57 > 0:43:00but by changing how it exerts its force.
0:43:02 > 0:43:04There are two types of particles in the universe.
0:43:04 > 0:43:08There are the ones that make up matter, like electrons and protons
0:43:08 > 0:43:11and neutrons and quarks, and then there's another set
0:43:11 > 0:43:13of particles, and those are the ones that transmit forces.
0:43:13 > 0:43:16So, for example, the photon, which makes up light,
0:43:16 > 0:43:19also carries the electro-magnetic forces.
0:43:20 > 0:43:23It's exactly like what we're doing with the ball and the magnet.
0:43:23 > 0:43:26We don't see the photons transmitting the force directly
0:43:26 > 0:43:29but we see the fact that the magnet makes the ball move.
0:43:29 > 0:43:33In physics, the greater a particle's mass, the smaller the distance
0:43:33 > 0:43:38over which it's able to exert any force or field it might have.
0:43:38 > 0:43:43The mass of a particle tells you how far it can carry information.
0:43:43 > 0:43:45If a particle that's transmitting a force is heavier,
0:43:45 > 0:43:48it only transmits the force over a shorter distance scale.
0:43:48 > 0:43:51So the range that you can transmit the force over changes
0:43:51 > 0:43:53depending on where you're looking.
0:43:55 > 0:43:59The idea is that when the chameleon comes into contact with other stuff,
0:43:59 > 0:44:02it interacts with it and becomes heavy
0:44:02 > 0:44:07and its force-transmitting capability all but disappears.
0:44:07 > 0:44:11But in regions of deep space where there's very little in the way
0:44:11 > 0:44:15of anything, the chameleon has no stuff with which to interact and
0:44:15 > 0:44:21so is very light and can transmit its force over vast distances.
0:44:21 > 0:44:26It's a neat idea, but evidence is hard to come by.
0:44:26 > 0:44:31Then, in 2014, Clare came up with an experiment
0:44:31 > 0:44:35that might unmask the chameleon.
0:44:35 > 0:44:39The experiment that we proposed last year is that you'd
0:44:39 > 0:44:42specifically design your experiment to look for chameleons,
0:44:42 > 0:44:45which means that you look in a very high vacuum
0:44:45 > 0:44:49and you use tiny, tiny particles, so we're using individual atoms.
0:45:00 > 0:45:04But wrangling individual atoms isn't easy.
0:45:06 > 0:45:09It takes an enormous amount of scientific hardware,
0:45:09 > 0:45:13specially configured in a highly precise way.
0:45:14 > 0:45:17Just six months after Clare's paper was published,
0:45:17 > 0:45:21atomic physicist Holger Muller got in touch.
0:45:21 > 0:45:25As it happened, he explained, he had exactly the right equipment needed
0:45:25 > 0:45:29to perform Clare's experiment, right here in Berkeley.
0:45:29 > 0:45:33From where, in 1998, Saul Perlmutter's group
0:45:33 > 0:45:36discovered dark energy in the first place.
0:45:36 > 0:45:39It might be that the conundrum could be solved at the same
0:45:39 > 0:45:42institution that it was discovered.
0:45:45 > 0:45:48We've been setting up this experiment for several years
0:45:48 > 0:45:52when my post-doc colleague came across Clare Burrage's paper
0:45:52 > 0:45:54on the pre-print server and we read the paper
0:45:54 > 0:45:57and we found that, "Wow, they're describing
0:45:57 > 0:46:01"exactly the experiment we've been building for all these years."
0:46:01 > 0:46:04And we got excited about it so we stopped doing our original
0:46:04 > 0:46:07experiment and started doing the dark energy measurement.
0:46:08 > 0:46:11It's an amazing feeling to have that kind of quick response
0:46:11 > 0:46:14because it almost never happens like that in science.
0:46:14 > 0:46:16Things take a long time to go
0:46:16 > 0:46:20from theory to somebody actually doing an experiment.
0:46:20 > 0:46:22To have a measurement of something
0:46:22 > 0:46:25we proposed in the space of six months is phenomenal.
0:46:25 > 0:46:28The experiment involves using a vacuum chamber
0:46:28 > 0:46:31and a cunning chameleon trap.
0:46:31 > 0:46:35The animal, the chameleon, changes its colour in order to hide, right?
0:46:35 > 0:46:38And in the same sense, the chameleon particle
0:46:38 > 0:46:41changes its mass in order to hide.
0:46:43 > 0:46:48At the centre of the vacuum chamber is a marble-sized sphere.
0:46:48 > 0:46:51If there are chameleon particles around,
0:46:51 > 0:46:53they will interact with the mass of the ball
0:46:53 > 0:46:58and produce very little in the way of force but perhaps just
0:46:58 > 0:47:02enough to affect something very small like an individual atom.
0:47:02 > 0:47:08The heart of the experiment is this little sphere inside there
0:47:08 > 0:47:13and so the experiment works by first collecting a cloud of caesium atoms
0:47:13 > 0:47:17on top of the sphere so here's the sphere and about one centimetre
0:47:17 > 0:47:22on top there's a little cloud of about 100 million caesium atoms.
0:47:24 > 0:47:28The machine contains the atom cloud using infrared lasers,
0:47:28 > 0:47:30invisible to the naked eye.
0:47:30 > 0:47:34The beams need to be very precisely controlled, so they're sent around
0:47:34 > 0:47:36a complicated series of mirrors
0:47:36 > 0:47:39on what's known as an optics table.
0:47:40 > 0:47:43We use lasers to control the atoms
0:47:43 > 0:47:46and so to do that, we need to pass them through this table of optics.
0:47:46 > 0:47:50The laser beam kind of takes a snake-like path throughout
0:47:50 > 0:47:54all these optics but eventually gets into something like this,
0:47:54 > 0:47:55an optical fibre.
0:47:55 > 0:47:58The light can then travel through here
0:47:58 > 0:48:00and is brought over to interact with the atoms.
0:48:02 > 0:48:05So, you see here the sphere
0:48:05 > 0:48:09and we trap the cloud of atoms just on top of the sphere,
0:48:09 > 0:48:13and then we release the trap and the atoms are free to fall
0:48:13 > 0:48:16subject only to the Earth's gravity
0:48:16 > 0:48:18and the potential chameleon force.
0:48:20 > 0:48:22When the atoms are released,
0:48:22 > 0:48:25they will fall towards the ball, which will contain chameleon
0:48:25 > 0:48:30particles, if they exist, and if they do exist, they will be
0:48:30 > 0:48:34busy interacting with the mass of the ball, making themselves
0:48:34 > 0:48:39heavy and reducing the distance over which their force can be felt.
0:48:39 > 0:48:43Which isn't to say that the force is completely non-existent.
0:48:43 > 0:48:47According to chameleon theorists, there'll be a tiny region
0:48:47 > 0:48:51on the surface of the sphere where the force is active
0:48:51 > 0:48:54and given that the atoms are so tiny,
0:48:54 > 0:48:58they will be affected by that force.
0:48:58 > 0:49:00If it exists at all.
0:49:00 > 0:49:04All the team need to do is to precisely measure the difference
0:49:04 > 0:49:09between the speed the atoms fall with and without the ball in place.
0:49:16 > 0:49:18Now we want to measure the chameleon only
0:49:18 > 0:49:21and not the combination of gravity and the chameleon,
0:49:21 > 0:49:25so what we do is we will move this sphere out
0:49:25 > 0:49:29and then do the measurement again, this time measuring gravity only.
0:49:30 > 0:49:33The experiment is set up to compare the difference between how
0:49:33 > 0:49:36fast the atoms fall towards the ball
0:49:36 > 0:49:40and how fast they accelerate through empty space.
0:49:40 > 0:49:43If the tracking reveals an unexplained acceleration, this
0:49:43 > 0:49:48could be due to the force associated with the chameleon particle.
0:49:48 > 0:49:52The experiment has now been running for over six months
0:49:52 > 0:49:54and they're starting to get their first results.
0:49:54 > 0:49:59Right now we have seen no evidence for chameleons, which means
0:49:59 > 0:50:01they either don't exist or they hide
0:50:01 > 0:50:04in a region of the perimeter space
0:50:04 > 0:50:07that we can't yet measure. So, what does that mean?
0:50:07 > 0:50:11If either the chameleon force is extremely weak
0:50:11 > 0:50:15or it's even heavier than we thought, then we can't see them.
0:50:15 > 0:50:18The team at Berkeley are now adjusting the experiment to rule out
0:50:18 > 0:50:24any theoretical nooks and crannies where the chameleon might be hiding.
0:50:24 > 0:50:28Well, if we make the experiment more and more and more sensitive,
0:50:28 > 0:50:31we will either discover the particle
0:50:31 > 0:50:33or rule it out once and for all.
0:50:35 > 0:50:38A scientist might be like a drunk who lost his keys
0:50:38 > 0:50:42and is now looking for it under the next lamppost...
0:50:43 > 0:50:47..and it's not because he knows that the key is there,
0:50:47 > 0:50:50but because it's hopeless to look in the dark anywhere else.
0:50:50 > 0:50:55People have searched for dark energy in cosmology and in astrophysics,
0:50:55 > 0:51:00and now we start looking for it under the atomic physics lamppost.
0:51:00 > 0:51:03Whether this is a good idea or not, we will know in a couple of years
0:51:03 > 0:51:08when it has either been found or not, but it's always exciting to
0:51:08 > 0:51:11have... It's like a new window that you can open and look through
0:51:11 > 0:51:14and you don't know what you will see before you've tried to do it.
0:51:20 > 0:51:22Having more information is always a good thing
0:51:22 > 0:51:26so ruling out possibilities. Although on a personal level
0:51:26 > 0:51:29maybe it's a little bit upsetting because it's a nice theory
0:51:29 > 0:51:32but it means that you've got more information and you can go
0:51:32 > 0:51:36on from there and build something better, build a better theory.
0:51:39 > 0:51:43With all these theories, it's really a question of taste.
0:51:43 > 0:51:47You either like a cosmological constant or you can explain
0:51:47 > 0:51:49it through a chameleon effect.
0:51:49 > 0:51:55None of these as yet give us the elegant solution
0:51:55 > 0:51:58that we are looking for and that's what really we're looking for.
0:51:58 > 0:52:02We're looking for this simple, elegant solution to this strange
0:52:02 > 0:52:07accelerated universe and nothing yet has given us that.
0:52:07 > 0:52:12Chameleon? Yeah, maybe, but as yet, there's no evidence for it.
0:52:12 > 0:52:16Dark energy? Yeah, we can sort of understand it,
0:52:16 > 0:52:18but we can't get the number right
0:52:18 > 0:52:21so we're still grasping in the dark
0:52:21 > 0:52:24for an elegant, simple solution to what we see.
0:52:28 > 0:52:31Where that simple solution will eventually come from
0:52:31 > 0:52:35is anyone's guess.
0:52:35 > 0:52:39That is one of the infuriating things about science.
0:52:39 > 0:52:44It can't always produce the rabbit from the hat on time and on budget.
0:52:46 > 0:52:51Sometimes it takes an unexpected turn of events, or what the
0:52:51 > 0:52:55media like to call a "genius".
0:52:55 > 0:52:57Though the geniuses themselves have
0:52:57 > 0:53:00a rather different take on their exploits.
0:53:03 > 0:53:06I'm not more gifted than anybody else.
0:53:06 > 0:53:09I'm just more curious than your average person
0:53:09 > 0:53:11and I will not give up on a problem
0:53:11 > 0:53:14until I have found the proper solution.
0:53:14 > 0:53:17I think that curiosity is what drives...what drives most
0:53:17 > 0:53:22cosmologists and physicists, a curiosity about the universe - why?
0:53:22 > 0:53:25What is the universe made out of? Why are we here?
0:53:25 > 0:53:27How did the universe begin?
0:53:27 > 0:53:29What will happen to the universe in the future?
0:53:29 > 0:53:32All of these are questions which are driven by curiosity.
0:53:37 > 0:53:39I have no special talent.
0:53:39 > 0:53:42I am only passionately curious.
0:53:43 > 0:53:46Curiosity, I think, is...
0:53:46 > 0:53:50Well, it's the best motivating force, OK?
0:53:50 > 0:53:55Working hard doesn't necessarily get you to an answer.
0:53:55 > 0:53:59Working too hard can actually stifle creativity.
0:54:00 > 0:54:03With our work, you know it's a mixture of inventiveness
0:54:03 > 0:54:05and persistence in the hard work.
0:54:05 > 0:54:07It's a combination.
0:54:13 > 0:54:17It's the end of the Euclid conference in Lausanne.
0:54:17 > 0:54:20The conference organisers have laid on a social evening,
0:54:20 > 0:54:23cruising around Lake Geneva.
0:54:23 > 0:54:25It's a chance for the delegates to unwind
0:54:25 > 0:54:30and maybe even think a little about the biggest picture of all.
0:54:30 > 0:54:34Yeah, so Einstein's theory was motivated for a reason, right?
0:54:34 > 0:54:36He had an equivalence principle.
0:54:36 > 0:54:37Yeah, and, I mean, we're going to measure
0:54:37 > 0:54:40a lot of things about the nature by looking at how it evolves,
0:54:40 > 0:54:42how dark energy actually evolves with red shift.
0:54:42 > 0:54:44But the problem is the zero-point energy,
0:54:44 > 0:54:46the vacuum energy, the quantum
0:54:46 > 0:54:48mechanical part that you add there.
0:54:48 > 0:54:50Try and study the nature of dark energy
0:54:50 > 0:54:53and at the same time, try and test if general relativity works.
0:54:53 > 0:54:54So, there's like a lot of work
0:54:54 > 0:54:57and a lot of discoveries that are going to happen down the road.
0:54:57 > 0:54:59- Exactly.- And I'll drink to that.
0:54:59 > 0:55:01Exactly that.
0:55:08 > 0:55:11The process of scientific discovery sometimes makes progress
0:55:11 > 0:55:13through sheer hard work
0:55:13 > 0:55:18and sometimes it needs someone to take an inspired alternative view.
0:55:19 > 0:55:22We learned an awful lot about animals
0:55:22 > 0:55:25and plants by simply observing them, but it took Darwin,
0:55:25 > 0:55:27with a radical idea,
0:55:27 > 0:55:31to give us a context to understand life itself.
0:55:31 > 0:55:34And in our efforts to understand the wider world
0:55:34 > 0:55:38and even the universe, observations are critical.
0:55:39 > 0:55:43The ideas of dark matter and dark energy come courtesy of people
0:55:43 > 0:55:49watching stars but just as Einstein musing on his train managed
0:55:49 > 0:55:51to take all the known science and
0:55:51 > 0:55:55see it from a different, more useful, angle,
0:55:55 > 0:55:58it might be that to solve the dark energy problem, someone needs
0:55:58 > 0:56:01to pull off a similar trick
0:56:01 > 0:56:05and come up with an even better idea.
0:56:05 > 0:56:08There are an awful lot of very smart people in the world.
0:56:08 > 0:56:11I wouldn't be surprised if we end up with another Einstein, you know,
0:56:11 > 0:56:13somewhere along the line here.
0:56:13 > 0:56:16I don't know whether it'll be in our lifetime but we...
0:56:16 > 0:56:19I think we have a good shot at it.
0:56:19 > 0:56:21We need teams like Euclid.
0:56:21 > 0:56:24That's the only way you can get the data that you need.
0:56:24 > 0:56:28But to understand that data, to give it some interpretation,
0:56:28 > 0:56:30to give it an idea,
0:56:30 > 0:56:32could come from one person.
0:56:32 > 0:56:34That could be the next Einstein.
0:56:35 > 0:56:38A genius could come up and put all the observations that we have
0:56:38 > 0:56:41so far, put it together, and come up with a new theory.
0:56:41 > 0:56:44Yeah, it is quite possible.
0:56:44 > 0:56:46I'm kind of hoping it's me.
0:56:52 > 0:56:55The tantalising truth is that all it might take to solve
0:56:55 > 0:56:59the mystery of the dark energy is one big idea,
0:56:59 > 0:57:04for someone out there to see things differently,
0:57:04 > 0:57:08someone perhaps like you.
0:57:08 > 0:57:11And if that new Einstein is you,
0:57:11 > 0:57:15if you manage to solve the mystery of dark energy,
0:57:15 > 0:57:18you're likely to become very famous indeed,
0:57:18 > 0:57:21as famous as the original Einstein.
0:57:23 > 0:57:27Wherever I go and wherever I stay,
0:57:27 > 0:57:30there's always a picture of me on display.
0:57:30 > 0:57:34On top of the desk or out in the hall,
0:57:34 > 0:57:37tied round a neck or hung on a wall.
0:57:37 > 0:57:40Women and men they play a strange game,
0:57:40 > 0:57:43asking, beseeching, "Please, sign your name."
0:57:43 > 0:57:46From the erudite fellow they brook not a quibble,
0:57:46 > 0:57:50but firmly insist on a piece of his scribble.
0:57:50 > 0:57:53Sometimes, surrounded by all this good cheer,
0:57:53 > 0:57:56I'm puzzled by some of the things that I
0:57:56 > 0:58:01and wonder, my mind for a moment not hazy,
0:58:01 > 0:58:05if I, and not they, could really be crazy.