Time is the continued sequence of existence and events that
occurs in an apparently irreversible
succession from the past, through the present, and into the future. It is a component quantity of various measurements used to sequence events, to compare the duration of
events or the intervals between them, and to quantify rates of change of quantities in material reality
or in the conscious experience. Time is often referred to as a fourth
dimension, along with three spatial
dimensions.
Time in physics is operationally defined as "what a clock reads". This operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event constitutes one standard unit, such as the second, is useful in the conduct of both advanced experiments and everyday affairs of life. There are many systems for determining what time it is. Periodic events and periodic motion have long served as standards for units of time. Examples include the apparent motion of the sun across the sky, the phases of the moon, and the passage of a free-swinging pendulum. More modern systems include the Global Positioning System, other satellite systems, Coordinated Universal Time, and mean solar time. In general, the numbers obtained from different time systems differ from one another, but with careful measurements, they can be synchronized.
A clock or chronometer is a device that measures and displays time. The clock is one of the oldest human inventions, meeting the need to measure intervals of time shorter than the natural units such as the day, the lunar month, and the year. Devices operating on several physical processes have been used over the millennia. Clocks can be classified by the type of time display, as well as by the method of timekeeping.
Clocks have different ways of displaying the time. Analog clocks indicate time with a traditional clock face and moving hands. Digital clocks display a numeric representation of time. Two numbering systems are in use: 12-hour time notation and 24-hour notation. Most digital clocks use electronic mechanisms and LCD, LED, or VFD displays. For the blind and for use over telephones, speaking clocks state the time audibly in words. There are also clocks for the blind that have displays that can be read by touch.
Specific types of Clock
Doomsday Clock
The Doomsday Clock is a symbol that represents the
likelihood of a human-made global catastrophe,
in the opinion of the members of the Bulletin of
the Atomic Scientists. Maintained since 1947, the clock is a metaphor, not a prediction, for threats to
humanity from unchecked scientific and technological advances. That is, the
time on the clock is not to be interpreted as actual time. A hypothetical
global catastrophe is represented by midnight on the clock, with the Bulletin's
opinion on how close the world is to one represented by a certain number of
minutes or seconds to midnight, which is then assessed in January of each year.
The main factors influencing the clock are nuclear warfare, climate change, and artificial
intelligence. The Bulletin's Science and Security Board
monitors new developments in the life sciences and technology that could
inflict irrevocable harm to humanity.
The clock's original setting in 1947
was 7 minutes to midnight. It has since been set backward 8 times and forward
17 times. The farthest time from midnight was 17 minutes in 1991, and the
nearest is 90 seconds, set on January 2023.
The clock was moved to 150 seconds
(2 minute, 30 seconds) in 2017, then forward to 2 minutes to midnight
in January 2018, and left unchanged in 2019. In January 2020, it was moved
forward to 100 seconds (1 minute, 40 seconds) before midnight. In
January 2023, the Clock was moved forward to 90 seconds
(1 minute, 30 seconds) before midnight and remained unchanged in
January 2024.
Basis
for settings
"Midnight"
has a deeper meaning besides the constant threat of war. There are various
elements taken into consideration when the scientists from The Bulletin of
the Atomic Scientists decide what Midnight and "global
catastrophe" really mean in a particular year. They might include
"politics, energy, weapons, diplomacy, and climate science";
potential sources of threat include nuclear threats, climate change, bioterrorism, and artificial intelligence.
Members of the board judge Midnight by discussing how close they think humanity
is to the end of civilization. In 1947, at the beginning of the Cold War,
the Clock was started at seven minutes to midnight.
Fluctuations
and threats
Before January 2020, the two tied-for-lowest points for the
Doomsday Clock were in 1953 (when the Clock was set to two minutes until
midnight, after the U.S. and the Soviet Union began testing hydrogen bombs) and
in 2018, following the failure of world leaders to address tensions relating to
nuclear weapons and climate change issues. In other years, the Clock's time has
fluctuated from 17 minutes in 1991 to 2 minutes 30 seconds in 2017.
Discussing the change to 2+1/2 minutes in 2017, the first use of a fraction in the
Clock's history, Lawrence Krauss,
one of the scientists from the Bulletin, warned that political leaders
must make decisions based on facts, and those facts "must be taken into
account if the future of humanity is to be preserved". In an announcement
from the Bulletin about the status of the Clock, they went as far to
call for action from "wise" public officials and "wise"
citizens to make an attempt to steer human life away from catastrophe while
humans still can.
On January 24, 2018, scientists moved the clock to two
minutes to midnight, based on threats greatest in the nuclear realm. The
scientists said, of recent moves by North Korea under Kim Jong-un and the administration of Donald Trump in the U.S.: "Hyperbolic
rhetoric and provocative actions by both sides have increased the possibility
of nuclear war by accident or miscalculation".
The clock was left unchanged in 2019 due to the twin
threats of nuclear weapons and climate change, and the problem of those threats
being "exacerbated this past year by the increased use of information
warfare to undermine democracy around the world, amplifying risk from these and
other threats and putting the future of civilization in extraordinary
danger".
On January 23, 2020, the Clock was moved to 100 seconds (1
minute, 40 seconds) before midnight. The Bulletin's executive chairman, Jerry Brown, said "the dangerous rivalry and
hostility among the superpowers increases the likelihood of nuclear blunder...
Climate change just compounds the crisis". The "100 seconds to
midnight" setting remained unchanged in 2021 and 2022.
Climate Clock
The Climate Clock is a graphic to demonstrate how
quickly the planet is approaching 1.5 °C of global warming, given current
emissions trends. It also shows the amount of CO2 already emitted,
and the global warming to date.
The Climate Clock was launched in 2015 to provide a
measuring stick against which viewers can track climate change
mitigation progress. The date shown when humanity reaches 1.5°C will
move closer as emissions rise, and further away as emissions decrease. An
alternative view projects the time remaining to 2.0°C of warming. The clock is
updated every year to reflect the latest global CO2 emissions trend
and rate of climate warming. As of April 2, 2024, the clock counts down to July
21, 2029 at 12:00 PM. On September 20, 2021, the clock was delayed to July 28,
2028, likely because of the COP26
Conference and the land protection by indigenous peoples.
The clock is hosted by Human Impact Lab, itself part of Concordia University.
Organisations supporting the climate clock include Concordia University, the David Suzuki
Foundation, Future Earth, and the
Climate Reality Project.
Relevance
1.5 °C
is an important threshold for many climate impacts, as shown by the Special
Report on Global Warming of 1.5 °C. Every increment to global
temperature is expected to increase weather extremes, such as heat waves and extreme
precipitation events. There is also the risk of irreversible ice sheet loss.
Consequent sea level rise also increases sharply around 1.75 °C, and
virtually all corals could be wiped out at 2 °C warming.
The
New York Climate Clock
In late September 2020, artists and
activists, Gan Golan, Katie Peyton Hofstadter, Adrian Carpenter and Andrew Boyd
repurposed the Metronome
in Union Square
in New York City to show the Climate Clock. The goal
was to "remind the world every day just how perilously close we are to the
brink." This is in juxtaposition to the Doomsday Clock, which measures a variety of
factors that could lead to "destroying the world" using
"dangerous technologies of our making," with climate change being one of the smaller factors.
This specific installation is expected to be one of many in cities around the
world. At the time of installation, the clock
read 7 years and 102 days. Greta Thunberg, Swedish environmental activist,
was involved in the project early on, and reportedly received a hand-held
version of the climate clock.
Since its inception, the New York Climate Clock has added a second set of numbers for the percentage of the world's energy use that comes from renewable energy sources.