The Absurdity of Detecting Gravitational Waves

00:09:07
https://www.youtube.com/watch?v=iphcyNWFD10

摘要

TLDRThis video delves into the fascinating field of gravitational wave detection, stemming from the momentous event of two black holes merging 1.3 billion years ago. As these black holes spiraled together, they emitted gravitational waves with an energy output 50 times greater than the entire observable universe's combined energy at that instant. These waves traveled through space-time and were eventually detected on Earth by measuring the minuscule changes they caused in distances, utilizing interferometers with lasers. Highlighting the extreme difficulty in detecting such waves, the video explains the necessity for the most precise measurements, the smoothest mirrors, and the most stable lasers. Furthermore, it discusses the importance of quantum mechanics and vacuum conditions in achieving the sensitivity needed for detection. By employing strategic measurement techniques, scientists have circumvented quantum uncertainty obstacles—ensuring they can focus on the primary goal of understanding electromagnetic wave effects on space-time. The video concludes with a look toward the future of gravitational wave research, aiming for more comprehensive monitoring of black holes.

心得

  • 🌌 Two black holes merged 1.3 billion years ago, creating gravitational waves.
  • 💥 The energy released was 50 times greater than all energy in the observable universe.
  • 🌀 Gravitational waves were detected for the first time on Earth.
  • 🔬 Detecting these waves involved measuring tiny distance changes.
  • 🎛 Interferometers and lasers were used to detect gravitational waves.
  • 🔍 Measurements were incredibly precise given their tiny size (10^-18 meters).
  • 🔗 Two detectors were used to differentiate between environmental noise and waves.
  • 📡 Quantum mechanics presents unique challenges in wave detection.
  • 🌪 A vacuum state was created to prevent interference in measurements.
  • 🚀 Future detection aims to track all black holes universally.

时间轴

  • 00:00:00 - 00:09:07

    1.3 billion years ago, two black holes merged, sending gravitational waves across the universe, detected on Earth by LIGO, which stretched space-time just enough to be measured, though this detection required overcoming immense challenges, as explained by Professor Rana Adhikari at Caltech.

思维导图

视频问答

  • What are gravitational waves?

    Gravitational waves are distortions in space-time caused by massive objects moving within it, like merging black holes.

  • How much energy was released during the black holes' merger?

    The energy released was 50 times greater than the energy emitted by the entire observable universe at that moment.

  • How are gravitational waves detected on Earth?

    Gravitational waves were detected using interferometers that measure minuscule changes in distances with laser beams.

  • What makes detecting gravitational waves so difficult?

    The changes in space-time caused by gravitational waves are incredibly small, requiring precise measurements over large distances and isolation from environmental noise.

  • Why are two detectors needed for gravitational wave detection?

    Two detectors help distinguish between local environmental noise and actual gravitational waves, ensuring accurate detections.

  • How do lasers play a role in gravitational wave detection?

    Lasers help measure the interference caused by the stretching and squeezing of space at a trillionth of a wavelength, providing the needed sensitivity.

  • What challenges exist due to quantum mechanics in detecting gravitational waves?

    Quantum mechanics introduces uncertainty in photon measurements, but techniques focus this uncertainty on irrelevant aspects, enhancing measurement precision.

  • What is the importance of vacuum in gravitational wave detectors?

    Achieving a vacuum prevents air molecules from interfering with the laser beams, ensuring accurate distance measurements.

  • Why does stretching in space-time require constant laser adjustments?

    Stretching affects the light used for measurement, necessitating constant replenishment with fresh laser light to accurately gauge changes.

  • What is the future goal in detecting gravitational waves?

    The goal is to continuously monitor black holes across the universe, improving current technology for better detection.

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  • 00:00:00
    1.3 billion years ago
  • 00:00:02
    in a galaxy far, far away
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    two black holes merged
  • 00:00:07
    As they violently spiraled into each other
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    They created traveling distortions in the fabric of space-time
  • 00:00:13
    gravitational waves
  • 00:00:15
    in the last tenth of a second
  • 00:00:16
    the energy released in these waves
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    was 50 times greater
  • 00:00:20
    then the energy being released by everything else
  • 00:00:22
    in the observable universe combined
  • 00:00:24
    It's like an awe-inspiring kind of energy
  • 00:00:27
    after spreading out through the universe at the speed of light for over a billion years
  • 00:00:32
    the waves reached earth, where they stretched and squeezed space
  • 00:00:36
    such that two light beams traveling in perpendicular pipes were put slightly out of step
  • 00:00:40
    allowing humans to detect the existence of
  • 00:00:42
    gravitational waves for the first time.
  • 00:00:51
    That's a simple enough story to tell but
  • 00:00:53
    what I found out when I went to visit
  • 00:00:54
    professor Rana Adhikari at Caltech is
  • 00:00:57
    that it hides the absurdity of just what
  • 00:01:00
    was required to make that detection.
  • 00:01:03
    There's a lot of things about
  • 00:01:04
    gravitational waves which are absurd
  • 00:01:06
    *Humming simulation of gravitational waves
  • 00:01:14
    Is that.... is that it? [RA]: That's it, yeah.
  • 00:01:16
    The main problem with detecting
  • 00:01:17
    gravitational waves is that they're tiny
  • 00:01:20
    they stretched and squeezed space by
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    just one part in 10 to the 21.
  • 00:01:24
    That's the equivalent of measuring the
  • 00:01:26
    distance between here and Alpha Centauri
  • 00:01:28
    and then trying to measure variations in
  • 00:01:31
    that distance that are the width of a human hair.
  • 00:01:34
    To detect such tiny wiggles
  • 00:01:36
    you have to measure over as large
  • 00:01:39
    distance as possible, which is why the
  • 00:01:40
    arms of the interferometers are four
  • 00:01:43
    kilometers. And even with arms this long
  • 00:01:45
    gravitational waves vary the length of
  • 00:01:47
    the arms by at most 10 to the minus 18 meters
  • 00:01:49
    so the detector has to be able to
  • 00:01:51
    reliably measure distances just 1/10000
  • 00:01:55
    the width of a proton. It's the tiniest
  • 00:01:57
    measurement ever made.
  • 00:01:59
    So how is it possible to measure that
  • 00:02:01
    considering all the other sources of
  • 00:02:02
    vibrations and noise in the environment,
  • 00:02:04
    like earthquakes, traffic, and electrical storms.
  • 00:02:07
    Well for one thing the mirrors are the
  • 00:02:09
    smoothest ever created.
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    They weigh 40 kilograms or 90 pounds and
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    are suspended by silica threads just
  • 00:02:16
    twice the thickness of a hair to isolate
  • 00:02:18
    them from their environment
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    and even then the only way to be certain not to
  • 00:02:23
    be tricked by environmental noise was to
  • 00:02:25
    build two detectors far apart from each other
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    in reasonably quiet locations that
  • 00:02:29
    allows you to distinguish between local
  • 00:02:31
    noise which would appear only one side
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    and gravitational waves which would pass
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    through both sides
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    almost simultaneously
  • 00:02:38
    I'm in a building that contains a 1 to 100 scale
  • 00:02:43
    of LIGO, the gravitational wave detector.
  • 00:02:46
    The next challenge is the laser.
  • 00:02:50
    Whoa, whoa.
  • 00:02:52
    That's a lot of stuff.
  • 00:02:54
    You need a laser that can provide one, and exactly one wavelength.
  • 00:02:58
    You can imagine, if your laser
  • 00:03:00
    wavelength is changing and you're trying
  • 00:03:01
    to use interference of light waves to
  • 00:03:03
    make this measurement it's never going
  • 00:03:05
    to work because it's something like
  • 00:03:07
    trying to measure this distance but your
  • 00:03:08
    ruler stick is constantly changing back
  • 00:03:10
    and forth you can't tell how many inches this is.
  • 00:03:13
    All this equipment, at least
  • 00:03:16
    three-quarters of it, all we're trying to
  • 00:03:17
    do is make the laser more stable, and by
  • 00:03:20
    the end of the day what we've achieved
  • 00:03:21
    is something which has a stability of
  • 00:03:24
    one part in 10 to the 20. What does that mean...
  • 00:03:28
    That's a hundred billionth of a trillion.
  • 00:03:30
    That's kind of what we end up with.
  • 00:03:32
    The best lasers for this purpose have a
  • 00:03:33
    wavelength of 1064 nanometers. That's
  • 00:03:36
    infrared light. But this presents a problem.
  • 00:03:39
    How can you measure 10 to the minus 18 with 10 to the minus 6
  • 00:03:43
    wavelength of light?
  • 00:03:44
    Yes, I wish more people would ask this question.
  • 00:03:47
    It's great for this animation
  • 00:03:48
    to show such a large shift in the
  • 00:03:50
    wavelength but the reality is, it's only
  • 00:03:52
    one trillionth of a wavelength that the
  • 00:03:54
    arms are shifting in length.
  • 00:03:56
    It seems obvious that you can measure half a
  • 00:03:59
    wavelength because that will cause the
  • 00:04:01
    light to interfere with itself.
  • 00:04:02
    Yeah, but that's fully. That will go from
  • 00:04:03
    completely dark to completely bright
  • 00:04:05
    So are you looking at, like, slightly darker
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    and slightly brighter?
  • 00:04:09
    Yeah and the limit here at how good we
  • 00:04:12
    can measure this difference between dark
  • 00:04:14
    and bright has to do with the, the fact
  • 00:04:17
    that the light is discrete. It comes in
  • 00:04:19
    discrete chunks which are called photons.
  • 00:04:21
    The variation in the number of photons
  • 00:04:23
    hitting the mirrors at any instant due
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    this quantum uncertainty is proportional
  • 00:04:26
    to the square root of the total number
  • 00:04:28
    of photons. What this means is the more
  • 00:04:31
    photons you use, the smaller the
  • 00:04:33
    uncertainty gets, that's a fraction of the total.
  • 00:04:36
    This is why the laser power in the arms is one megawatt.
  • 00:04:41
    That is enough energy to power a thousand homes, in a light beam.
  • 00:04:47
    And a megawatt, you know
  • 00:04:48
    *snap* boom, they won't even rip your head off.
  • 00:04:50
    Just, vaporized be just a smoking stump.
  • 00:04:54
    Even with a perfect laser and one
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    megawatt of power, anything the light
  • 00:04:57
    hits would interfere with it,
  • 00:04:59
    even the air, so all the air in the arms
  • 00:05:01
    of the detectors had to be eliminated
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    and it took 40 days to pump down to just
  • 00:05:06
    a trillionth of atmospheric pressure and
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    the tubes were heated up to the
  • 00:05:11
    temperature of the oven to expel any residual gases.
  • 00:05:14
    They pumped out enough
  • 00:05:15
    air to fill up two and a half million
  • 00:05:17
    footballs, making it the second largest
  • 00:05:19
    vacuum in the world after the Large Hadron Collider.
  • 00:05:22
    Now here's something most people don't think about
  • 00:05:24
    which is that gravitational waves stretch
  • 00:05:26
    space-time so light traveling through
  • 00:05:29
    that space should be stretched as well.
  • 00:05:31
    If everything is stretching how do you
  • 00:05:33
    know anything is stretching?
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    How do you know anything is stretching?
  • 00:05:35
    That's the conundrum.
  • 00:05:36
    It doesn't make any sense!
  • 00:05:39
    This whole thing is bogus shut it down!
  • 00:05:42
    I would send a laser beam down this tube
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    and then wait for it to come back and
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    then i would say "well nothing happened"
  • 00:05:49
    because the space got stretched and the
  • 00:05:51
    laser wavelength got stretched
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    Its...It looks the same if you got it stretched or not stretched.
  • 00:05:55
    It doesn't make any sense
  • 00:05:57
    well it's sort of a matter of timing is
  • 00:05:59
    how it works.
  • 00:06:00
    So the amount of time it takes for light
  • 00:06:02
    to go down this tube and come back is
  • 00:06:06
    very short.
  • 00:06:07
    However the wave... the gravitational wave
  • 00:06:09
    when it comes through its doing the slow thing like
  • 00:06:11
    *low humming*
  • 00:06:13
    this noise I made, which is low. Its this
  • 00:06:15
    it's this slow stretching, its only a
  • 00:06:17
    hundred times per second. And it's true
  • 00:06:19
    when the wave comes through the light
  • 00:06:22
    which is in there it actually does get stretched.
  • 00:06:25
    And... and then that part doesn't...
  • 00:06:27
    doesn't do the measurement for us but
  • 00:06:29
    um.. now that the space is stretched that
  • 00:06:31
    laser light is like come and gone
  • 00:06:33
    it's out of the picture. We're constantly shooting
  • 00:06:35
    the laser back into the system so the
  • 00:06:37
    new fresh light now goes through there
  • 00:06:39
    and has to travel a bigger distance than the light before.
  • 00:06:42
    And so by looking at how this interference changes with time
  • 00:06:46
    and keeping the laser wavelength from
  • 00:06:48
    the laser itself fixed, we're able to do the measurement.
  • 00:06:51
    So what was needed to detect gravitational waves?
  • 00:06:54
    Well, a megawatt of laser power to
  • 00:06:56
    minimize shot noise of exactly one wavelength
  • 00:06:59
    because we're trying to measure just a trillionth of that wavelength,
  • 00:07:01
    continually inserted to replace older light
  • 00:07:04
    that's been stretched and squished,
  • 00:07:06
    in the world's second-largest vacuum chamber at just a
  • 00:07:08
    trillionth of atmospheric pressure,
  • 00:07:10
    hitting the smoothest mirrors ever created,
  • 00:07:12
    suspended by silica threads,
  • 00:07:14
    at two distant sites to eliminate noise,
  • 00:07:16
    with four kilometer long arms to
  • 00:07:17
    increase the magnitude of gravitational
  • 00:07:19
    waves to just a thousand of the width of a proton.
  • 00:07:23
    You know what we already do
  • 00:07:25
    daily in here is what I would have said
  • 00:07:27
    is impossible if you asked me about it 20 years ago.
  • 00:07:30
    One of the things that was
  • 00:07:31
    most interesting for me to learn was
  • 00:07:33
    what is limiting the sensitivity of the
  • 00:07:35
    detectors today, and it turns out it is
  • 00:07:37
    quantum mechanics and essentially you can think of it
  • 00:07:39
    like a Heisenberg uncertainty principle, we've got two
  • 00:07:42
    things and together their uncertainty
  • 00:07:44
    has to be bigger than a certain value.
  • 00:07:46
    Luckily for us we are only trying to
  • 00:07:48
    measure one thing here, we're not trying
  • 00:07:50
    to measure two things at the same time.
  • 00:07:51
    All we want to know is how much more
  • 00:07:54
    this arm get stretched from that arm
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    and that's... that's the key point which people
  • 00:07:58
    did not understand until recently.
  • 00:08:00
    The way to build these systems is such that
  • 00:08:01
    they're extremely good in measuring one
  • 00:08:04
    thing and that all of the uncertainty
  • 00:08:06
    which comes from quantum mechanics is
  • 00:08:08
    completely crammed into this other thing
  • 00:08:10
    that we don't care about.
  • 00:08:12
    I feel like we're getting down to these levels of
  • 00:08:14
    nature where it seems like nature
  • 00:08:16
    doesn't want you to go any further.
  • 00:08:18
    But, through our ingenuity we're figuring out
  • 00:08:20
    ways to engineer quantum noise, I think
  • 00:08:24
    that's such a remarkable concept
  • 00:08:25
    and I look forward to the results
  • 00:08:27
    that it's going to bring.
  • 00:08:28
    I think the next logical step is to go from
  • 00:08:31
    two signals to detecting all the black holes
  • 00:08:34
    in the universe all the time.
  • 00:08:35
    It's not like an alien civilization level of
  • 00:08:38
    technology it's just... we have to do a lot better
  • 00:08:41
    than what we're doing now but it's
  • 00:08:42
    I see it sort of within... within our grasp
标签
  • gravitational waves
  • black hole merger
  • detection
  • interferometers
  • quantum mechanics
  • space-time
  • energy
  • vacuum
  • lasers
  • sensitivity