1
00:00:01,350 --> 00:00:04,371
Light is the connection
between us and the universe.

2
00:00:04,631 --> 00:00:06,906
Through light, we can
experience distant stars

3
00:00:06,906 --> 00:00:10,069
and look back at the
beginning of existence itself.

4
00:00:10,279 --> 00:00:11,385
But what is light?

5
00:00:19,445 --> 00:00:22,372
In a nutshell, light is the
smallest quantity of energy

6
00:00:22,372 --> 00:00:26,731
that can be transported, a photon,
an elementary particle without a real size

7
00:00:26,731 --> 00:00:29,983
that can’t be split,
only created or destroyed.

8
00:00:30,263 --> 00:00:32,712
LIght also has a wave-particle duality,

9
00:00:32,712 --> 00:00:35,666
being kind of a particle and
a wave at the same time,

10
00:00:35,666 --> 00:00:37,114
although this is a lie.

11
00:00:37,324 --> 00:00:40,569
Also, when we say “light”,
we actually mean visible light,

12
00:00:40,569 --> 00:00:43,475
which is a tiny part of
the electromagnetic spectrum,

13
00:00:43,475 --> 00:00:46,071
energy in form of
electromagnetic radiation.

14
00:00:46,391 --> 00:00:48,652
Electromagnetic radiation consists of

15
00:00:48,652 --> 00:00:51,260
an enormous range of
wavelengths and frequencies.

16
00:00:51,470 --> 00:00:55,147
Gamma rays have the smallest wavelengths,
because they are highest-energy photons.

17
00:00:55,447 --> 00:00:58,295
But most gamma rays are
just under 10 picometers,

18
00:00:58,295 --> 00:01:00,751
which is still way smaller
than a hydrogen atom.

19
00:01:01,261 --> 00:01:04,186
For reference, a hydrogen atom
compared to a cent

20
00:01:04,186 --> 00:01:06,956
is about as big as a cent
compared to the Moon.

21
00:01:07,406 --> 00:01:09,436
Visible light is in
the middle of the spectrum,

22
00:01:09,436 --> 00:01:13,002
in the range of about
400 nanometers to 700 nanometers,

23
00:01:13,002 --> 00:01:14,595
about the size of bacteria.

24
00:01:15,405 --> 00:01:19,911
On the other end of the spectrum, radio
waves can be up to 100 km in diameter,

25
00:01:19,911 --> 00:01:22,091
while the biggest
wavelengths we know exist

26
00:01:22,091 --> 00:01:26,309
can span from 10,000 km
to a baffling 100,000 km—

27
00:01:26,309 --> 00:01:27,741
way larger than Earth!

28
00:01:28,231 --> 00:01:31,122
From a physics standpoint, all these
different waves are the same—

29
00:01:31,122 --> 00:01:35,768
they all have the wave-particle duality
and travel at c, the speed of light,

30
00:01:35,768 --> 00:01:37,611
just at different frequencies.

31
00:01:37,911 --> 00:01:40,456
So what makes visible light special, then?

32
00:01:40,596 --> 00:01:42,406
Well… absolutely nothing.

33
00:01:42,866 --> 00:01:45,316
We just happen to have evolved
eyes that are good at

34
00:01:45,316 --> 00:01:48,756
registering exactly this part
of the electromagnetic spectrum.

35
00:01:49,116 --> 00:01:51,785
This is not a complete
coincidence, though;

36
00:01:51,785 --> 00:01:55,165
visible light is the only set of
electromagnetic radiation

37
00:01:55,165 --> 00:01:56,917
that propagates well in water,

38
00:01:56,917 --> 00:02:00,984
which happens to be where most eyes
first evolved millions of years ago.

39
00:02:01,514 --> 00:02:04,813
That was a smart move, because
light not only interacts with matter,

40
00:02:04,813 --> 00:02:07,229
it’s also altered by it and can be used

41
00:02:07,229 --> 00:02:09,465
to gather information
about the world around us

42
00:02:09,465 --> 00:02:10,839
with almost no delay,

43
00:02:10,839 --> 00:02:14,097
which is arguably really
helpful for survival.

44
00:02:14,507 --> 00:02:16,453
Okay, where does light come from?

45
00:02:16,733 --> 00:02:21,063
A vast range of electromagnetic waves
are created when atoms or molecules

46
00:02:21,063 --> 00:02:23,527
drop from a higher state of
energy to a lower one.

47
00:02:23,857 --> 00:02:26,835
They lose energy and emit it
in the form of radiation.

48
00:02:27,185 --> 00:02:31,383
At the microscopic level, visible light is
created when an electron within an atom

49
00:02:31,383 --> 00:02:34,892
that is in an excited state drops
to a lower energy state

50
00:02:34,892 --> 00:02:36,980
and loses this excess energy.

51
00:02:37,180 --> 00:02:41,066
The same way, incoming light can elevate
an electron to a higher state of energy

52
00:02:41,066 --> 00:02:42,585
by being absorbed by it.

53
00:02:42,995 --> 00:02:45,478
Macroscopically, the moving charge
of the electron

54
00:02:45,478 --> 00:02:47,718
creates an oscillating magnetic field,

55
00:02:47,718 --> 00:02:50,826
which creates an oscillating electric
field perpendicular to it.

56
00:02:51,076 --> 00:02:53,527
These two fields move
themselves through space,

57
00:02:53,527 --> 00:02:55,972
transferring energy from
one place to another,

58
00:02:55,972 --> 00:02:59,299
carrying information about its
place of creation with it.

59
00:02:59,539 --> 00:03:03,479
Why, of all the things in the universe, is
light the fastest thing there is?

60
00:03:03,909 --> 00:03:05,042
Let’s change the question:

61
00:03:05,042 --> 00:03:08,077
what is the fastest way to travel
through space in the universe?

62
00:03:08,307 --> 00:03:16,159
It’s c, exactly 299,792,458 m/s
in a vacuum,

63
00:03:16,159 --> 00:03:18,615
one billion kilometers per hour.

64
00:03:18,615 --> 00:03:22,658
Electromagnetic radiation just
happens to move this fast.

65
00:03:22,658 --> 00:03:27,813
Any particle that has no mass travels at
c, without acceleration or any in-between.

66
00:03:28,153 --> 00:03:29,838
The light that’s being
released from a candle

67
00:03:29,838 --> 00:03:32,350
does not speed up until
it reaches lightspeed;

68
00:03:32,350 --> 00:03:35,642
at the very instant of its creation,
its speed is c.

69
00:03:36,042 --> 00:03:39,564
So why is c, or the
speed of light, finite, then?

70
00:03:39,734 --> 00:03:41,307
Well… nobody knows.

71
00:03:41,457 --> 00:03:43,405
Our universe is just built this way.

72
00:03:43,405 --> 00:03:45,010
We don’t have a smart answer here.

73
00:03:45,320 --> 00:03:47,108
So light is a part of a spectrum,

74
00:03:47,108 --> 00:03:49,948
an elementary particle that
also behaves like a wave,

75
00:03:49,948 --> 00:03:52,187
propelled by two perpendicular fields

76
00:03:52,187 --> 00:03:54,649
traveling at the speed
limit of the universe.

77
00:03:55,119 --> 00:03:57,676
Okay, that’s nice and all, but what
about all the crazy stuff about

78
00:03:57,676 --> 00:04:00,018
traveling at the speed of light and time,

79
00:04:00,018 --> 00:04:02,929
the twin paradox, quantum stuff,
things like that?

80
00:04:03,409 --> 00:04:05,391
We have to save them for another video.

81
00:04:05,801 --> 00:04:08,565
For now, let’s just be happy that we
evolved eyes that pick up

82
00:04:08,565 --> 00:04:10,979
waves of information
permeating the universe,

83
00:04:10,979 --> 00:04:15,339
making us see things that help us to put
our existence into perspective.