1
00:00:06,716 --> 00:00:10,397
The coldest materials in the world 
aren’t in Antarctica.

2
00:00:10,397 --> 00:00:12,521
They’re not at the top of Mount Everest

3
00:00:12,521 --> 00:00:14,376
or buried in a glacier.

4
00:00:14,376 --> 00:00:15,897
They’re in physics labs:

5
00:00:15,897 --> 00:00:20,382
clouds of gases held just fractions
of a degree above absolute zero.

6
00:00:20,382 --> 00:00:25,367
That’s 395 million times colder 
than your refrigerator,

7
00:00:25,367 --> 00:00:28,073
100 million times colder 
than liquid nitrogen,

8
00:00:28,073 --> 00:00:31,550
and 4 million times colder
than outer space.

9
00:00:31,550 --> 00:00:35,901
Temperatures this low give scientists a
window into the inner workings of matter,

10
00:00:35,901 --> 00:00:39,637
and allow engineers to build 
incredibly sensitive instruments

11
00:00:39,637 --> 00:00:41,292
that tell us more about everything

12
00:00:41,292 --> 00:00:43,130
from our exact position on the planet

13
00:00:43,130 --> 00:00:46,135
to what’s happening in 
the farthest reaches of the universe.

14
00:00:46,135 --> 00:00:48,928
How do we create such 
extreme temperatures?

15
00:00:48,928 --> 00:00:51,989
In short, by slowing down 
moving particles.

16
00:00:51,989 --> 00:00:55,951
When we’re talking about temperature,
what we’re really talking about is motion.

17
00:00:55,951 --> 00:00:57,716
The atoms that make up solids,

18
00:00:57,716 --> 00:00:58,458
liquids,

19
00:00:58,458 --> 00:00:59,338
and gasses

20
00:00:59,338 --> 00:01:00,869
are moving all the time.

21
00:01:00,869 --> 00:01:05,616
When atoms are moving more rapidly,
we perceive that matter as hot.

22
00:01:05,616 --> 00:01:08,827
When they’re moving more
slowly, we perceive it as cold.

23
00:01:08,827 --> 00:01:12,563
To make a hot object 
or gas cold in everyday life,

24
00:01:12,563 --> 00:01:15,960
we place it in a colder environment, 
like a refrigerator.

25
00:01:15,960 --> 00:01:20,498
Some of the atomic motion in the hot
object is transferred to the surroundings,

26
00:01:20,498 --> 00:01:22,251
and it cools down.

27
00:01:22,251 --> 00:01:23,788
But there’s a limit to this:

28
00:01:23,788 --> 00:01:27,685
even outer space is too warm
to create ultra-low temperatures.

29
00:01:27,685 --> 00:01:32,823
So instead, scientists figured out a way 
to slow the atoms down directly –

30
00:01:32,823 --> 00:01:34,204
with a laser beam.

31
00:01:34,204 --> 00:01:35,751
Under most circumstances,

32
00:01:35,751 --> 00:01:38,464
the energy in a laser beam 
heats things up.

33
00:01:38,464 --> 00:01:40,533
But used in a very precise way,

34
00:01:40,533 --> 00:01:44,813
the beam’s momentum can stall 
moving atoms, cooling them down.

35
00:01:44,813 --> 00:01:49,403
That’s what happens in a device 
called a magneto-optical trap.

36
00:01:49,403 --> 00:01:51,954
Atoms are injected into a vacuum chamber,

37
00:01:51,954 --> 00:01:55,415
and a magnetic field 
draws them towards the center.

38
00:01:55,415 --> 00:01:58,090
A laser beam aimed 
at the middle of the chamber

39
00:01:58,090 --> 00:02:00,623
is tuned to just the right frequency

40
00:02:00,623 --> 00:02:06,170
that an atom moving towards it will absorb
a photon of the laser beam and slow down.

41
00:02:06,170 --> 00:02:09,089
The slow down effect comes from
the transfer of momentum

42
00:02:09,089 --> 00:02:11,108
between the atom and the photon.

43
00:02:11,108 --> 00:02:14,208
A total of six beams, 
in a perpendicular arrangement,

44
00:02:14,208 --> 00:02:18,375
ensure that atoms traveling 
in all directions will be intercepted.

45
00:02:18,375 --> 00:02:21,018
At the center, where the beams intersect,

46
00:02:21,018 --> 00:02:24,840
the atoms move sluggishly 
as if trapped in a thick liquid —

47
00:02:24,840 --> 00:02:29,924
an effect the researchers who invented it 
described as “optical molasses.”

48
00:02:29,924 --> 00:02:32,315
A magneto-optical trap like this

49
00:02:32,315 --> 00:02:35,405
can cool atoms down 
to just a few microkelvins —

50
00:02:35,405 --> 00:02:38,785
about -273 degrees Celsius.

51
00:02:38,785 --> 00:02:41,609
This technique was developed in the 1980s,

52
00:02:41,609 --> 00:02:43,913
and the scientists 
who'd contributed to it

53
00:02:43,913 --> 00:02:47,931
won the Nobel Prize in Physics in 1997
for the discovery.

54
00:02:47,931 --> 00:02:52,751
Since then, laser cooling has been 
improved to reach even lower temperatures.

55
00:02:52,751 --> 00:02:55,990
But why would you want 
to cool atoms down that much?

56
00:02:55,990 --> 00:02:59,786
First of all, cold atoms can make
very good detectors.

57
00:02:59,786 --> 00:03:01,530
With so little energy,

58
00:03:01,530 --> 00:03:04,961
they’re incredibly sensitive 
to fluctuations in the environment.

59
00:03:04,961 --> 00:03:09,562
So they’re used in devices that find 
underground oil and mineral deposits,

60
00:03:09,562 --> 00:03:12,203
and they also make 
highly accurate atomic clocks,

61
00:03:12,203 --> 00:03:15,093
like the ones used 
in global positioning satellites.

62
00:03:15,093 --> 00:03:18,152
Secondly, cold atoms hold 
enormous potential

63
00:03:18,152 --> 00:03:20,243
for probing the frontiers of physics.

64
00:03:20,243 --> 00:03:22,662
Their extreme sensitivity 
makes them candidates

65
00:03:22,662 --> 00:03:27,300
to be used to detect gravitational waves 
in future space-based detectors.

66
00:03:27,300 --> 00:03:31,624
They’re also useful for the study 
of atomic and subatomic phenomena,

67
00:03:31,624 --> 00:03:35,894
which requires measuring incredibly 
tiny fluctuations in the energy of atoms.

68
00:03:35,894 --> 00:03:38,046
Those are drowned out 
at normal temperatures,

69
00:03:38,046 --> 00:03:41,090
when atoms speed around 
at hundreds of meters per second.

70
00:03:41,090 --> 00:03:45,265
Laser cooling can slow atoms to just 
a few centimeters per second—

71
00:03:45,265 --> 00:03:49,122
enough for the motion caused by
atomic quantum effects to become obvious.

72
00:03:49,122 --> 00:03:53,599
Ultracold atoms have already 
allowed scientists to study phenomena

73
00:03:53,599 --> 00:03:56,150
like Bose-Einstein condensation,

74
00:03:56,150 --> 00:03:59,631
in which atoms are cooled almost 
to absolute zero

75
00:03:59,631 --> 00:04:02,200
and become a rare new state of matter.

76
00:04:02,200 --> 00:04:05,791
So as researchers continue in their quest 
to understand the laws of physics

77
00:04:05,791 --> 00:04:07,925
and unravel the mysteries of the universe,

78
00:04:07,925 --> 00:04:12,161
they’ll do so with the help 
of the very coldest atoms in it.