Why our universe might exist on a knife-edge | Gian Giudice

996,578 views ・ 2013-10-24

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So last year, on the Fourth of July,
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experiments at the Large Hadron Collider
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discovered the Higgs boson.
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It was a historical day.
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There's no doubt that from now on,
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the Fourth of July will be remembered
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not as the day of the Declaration of Independence,
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but as the day of the discovery of the Higgs boson.
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Well, at least, here at CERN.
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But for me, the biggest surprise of that day
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was that there was no big surprise.
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In the eye of a theoretical physicist,
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the Higgs boson is a clever explanation
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of how some elementary particles gain mass,
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but it seems a fairly unsatisfactory
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and incomplete solution.
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Too many questions are left unanswered.
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The Higgs boson does not share the beauty,
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the symmetry, the elegance,
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of the rest of the elementary particle world.
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For this reason, the majority of theoretical physicists
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believe that the Higgs boson could not
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be the full story.
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We were expecting new particles and new phenomena
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accompanying the Higgs boson.
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Instead, so far, the measurements
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coming from the LHC show no signs of new particles
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or unexpected phenomena.
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Of course, the verdict is not definitive.
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In 2015, the LHC will almost double
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the energy of the colliding protons,
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and these more powerful collisions
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will allow us to explore further the particle world,
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and we will certainly learn much more.
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But for the moment, since we have found
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no evidence for new phenomena, let us suppose
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that the particles that we know today,
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including the Higgs boson,
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are the only elementary particles in nature,
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even at energies much larger
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than what we have explored so far.
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Let's see where this hypothesis is going to lead us.
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We will find a surprising and intriguing result
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about our universe, and to explain my point,
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let me first tell you what the Higgs is about,
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and to do so, we have to go back
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to one tenth of a billionth of a second
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after the Big Bang.
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And according to the Higgs theory,
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at that instant, a dramatic event took place
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in the universe.
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Space-time underwent a phase transition.
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It was something very similar to the phase transition
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that occurs when water turns into ice
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below zero degrees.
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But in our case, the phase transition
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is not a change in the way the molecules
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are arranged inside the material,
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but is about a change
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of the very fabric of space-time.
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During this phase transition, empty space
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became filled with a substance
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that we now call Higgs field.
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And this substance may seem invisible to us,
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but it has a physical reality.
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It surrounds us all the time,
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just like the air we breathe in this room.
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And some elementary particles interact
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with this substance, gaining energy in the process.
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And this intrinsic energy is what we call
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the mass of a particle,
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and by discovering the Higgs boson, the LHC
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has conclusively proved that this substance is real,
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because it is the stuff the Higgs bosons are made of.
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And this, in a nutshell, is the essence of the Higgs story.
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But this story is far more interesting than that.
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By studying the Higgs theory,
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theoretical physicists discovered,
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not through an experiment
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but with the power of mathematics,
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that the Higgs field does not necessarily exist
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only in the form that we observe today.
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Just like matter can exist as liquid or solid,
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so the Higgs field, the substance that fills all space-time,
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could exist in two states.
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Besides the known Higgs state,
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there could be a second state in which the Higgs field
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is billions and billions times denser
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than what we observe today,
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and the mere existence of another state
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of the Higgs field poses a potential problem.
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This is because, according to the laws
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of quantum mechanics, it is possible
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to have transitions between two states,
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even in the presence of an energy barrier
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separating the two states,
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and the phenomenon is called,
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quite appropriately, quantum tunneling.
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Because of quantum tunneling,
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I could disappear from this room
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and reappear in the next room,
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practically penetrating the wall.
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But don't expect me to actually perform the trick
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in front of your eyes, because the probability
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for me to penetrate the wall is ridiculously small.
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You would have to wait a really long time
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before it happens, but believe me,
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quantum tunneling is a real phenomenon,
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and it has been observed in many systems.
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For instance, the tunnel diode,
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a component used in electronics,
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works thanks to the wonders
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of quantum tunneling.
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But let's go back to the Higgs field.
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If the ultra-dense Higgs state existed,
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then, because of quantum tunneling,
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a bubble of this state could suddenly appear
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in a certain place of the universe at a certain time,
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and it is analogous to what happens when you boil water.
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Bubbles of vapor form inside the water,
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then they expand, turning liquid into gas.
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In the same way, a bubble of the ultra-dense Higgs state
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could come into existence because of quantum tunneling.
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The bubble would then expand at the speed of light,
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invading all space, and turning the Higgs field
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from the familiar state into a new state.
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Is this a problem? Yes, it's a big a problem.
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We may not realize it in ordinary life,
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but the intensity of the Higgs field is critical
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for the structure of matter.
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If the Higgs field were only a few times more intense,
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we would see atoms shrinking, neutrons decaying
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inside atomic nuclei, nuclei disintegrating,
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and hydrogen would be
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the only possible chemical element in the universe.
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And the Higgs field, in the ultra-dense Higgs state,
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is not just a few times more intense than today,
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but billions of times,
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and if space-time were filled by this Higgs state,
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all atomic matter would collapse.
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No molecular structures would be possible, no life.
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So, I wonder, is it possible
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that in the future, the Higgs field
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will undergo a phase transition and,
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through quantum tunneling, will be transformed
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into this nasty, ultra-dense state?
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In other words, I ask myself, what is the fate
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of the Higgs field in our universe?
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And the crucial ingredient necessary
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to answer this question is the Higgs boson mass.
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And experiments at the LHC found that the mass
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of the Higgs boson is about 126 GeV.
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This is tiny when expressed in familiar units,
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because it's equal to something like
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10 to the minus 22 grams,
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but it is large in particle physics units,
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because it is equal to the weight
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of an entire molecule
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of a DNA constituent.
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So armed with this information from the LHC,
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together with some colleagues here at CERN,
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we computed the probability
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that our universe could quantum tunnel
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into the ultra-dense Higgs state,
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and we found a very intriguing result.
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Our calculations showed
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that the measured value of the Higgs boson mass
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is very special.
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It has just the right value
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to keep the universe hanging
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in an unstable situation.
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The Higgs field is in a wobbly configuration
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that has lasted so far
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but that will eventually collapse.
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So according to these calculations,
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we are like campers
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who accidentally set their tent
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at the edge of a cliff.
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And eventually, the Higgs field
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will undergo a phase transition
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and matter will collapse into itself.
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So is this how humanity is going to disappear?
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I don't think so.
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Our calculation shows that quantum tunneling
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of the Higgs field is not likely to occur
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in the next 10 to the 100 years,
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and this is a very long time.
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It's even longer than
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the time it takes for Italy to form a stable government.
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(Laughter)
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Even so, we will be long gone by then.
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In about five billion years,
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our sun will become a red giant,
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as large as the Earth's orbit,
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and our Earth will be kaput,
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and in a thousand billion years,
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if dark energy keeps on fueling
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space expansion at the present rate,
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you will not even be able to see as far as your toes,
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because everything around you
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expands at a rate faster than the speed of light.
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So it is really unlikely
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that we will be around to see the Higgs field collapse.
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But the reason why I am interested
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in the transition of the Higgs field
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is because I want to address the question,
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why is the Higgs boson mass so special?
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Why is it just right to keep the universe
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at the edge of a phase transition?
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Theoretical physicists always ask "why" questions.
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More than how a phenomenon works,
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theoretical physicists are always interested in
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why a phenomenon works in the way it works.
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We think that this these "why" questions
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can give us clues
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about the fundamental principles of nature.
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And indeed, a possible answer to my question
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opens up new universes, literally.
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It has been speculated that our universe
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is only a bubble in a soapy multiverse
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made out of a multitude of bubbles,
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and each bubble is a different universe
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with different fundamental constants
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and different physical laws.
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And in this context, you can only talk about
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the probability of finding a certain value of the Higgs mass.
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Then the key to the mystery
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could lie in the statistical properties
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of the multiverse.
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It would be something like what happens
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with sand dunes on a beach.
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In principle, you could imagine to find sand dunes
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of any slope angle in a beach,
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and yet, the slope angles of sand dunes
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are typically around 30, 35 degrees.
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And the reason is simple:
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because wind builds up the sand, gravity makes it fall.
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As a result, the vast majority of sand dunes
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have slope angles around the critical value,
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near to collapse.
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And something similar could happen
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for the Higgs boson mass in the multiverse.
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In the majority of bubble universes,
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the Higgs mass could be around the critical value,
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near to a cosmic collapse of the Higgs field,
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because of two competing effects,
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just as in the case of sand.
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My story does not have an end,
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because we still don't know the end of the story.
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This is science in progress,
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and to solve the mystery, we need more data,
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and hopefully, the LHC will soon add new clues
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to this story.
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Just one number, the Higgs boson mass,
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and yet, out of this number we learn so much.
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I started from a hypothesis, that the known particles
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are all there is in the universe,
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even beyond the domain explored so far.
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From this, we discovered that the Higgs field
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that permeates space-time may be standing
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on a knife edge, ready for cosmic collapse,
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and we discovered that this may be a hint
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that our universe is only a grain of sand
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in a giant beach, the multiverse.
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But I don't know if my hypothesis is right.
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That's how physics works: A single measurement
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can put us on the road to a new understanding
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of the universe
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or it can send us down a blind alley.
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But whichever it turns out to be,
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there is one thing I'm sure of:
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The journey will be full of surprises.
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Thank you.
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(Applause)
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