Time is the continued sequence of existence and events that occurs in an apparently irreversible succession from the past, through the present, 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 has long been an important subject of study in religion, philosophy, and science, but defining it in a manner applicable to all fields without circularity has consistently eluded scholars.
Nevertheless, diverse fields such as business, industry, sports, the sciences, and the performing arts all incorporate some notion of time into their respective measuring systems.Time in physics is operationally defined as "what a clock reads".The physical nature of time is addressed by general relativity with respect to events in space-time. Examples of events are the collision of two particles, the explosion of a supernova, or the arrival of a rocket ship. Every event can be assigned four numbers representing its time and position (the event's coordinates). However, the numerical values are different for different observers. In general relativity, the question of what time it is now only has meaning relative to a particular observer. Distance and time are intimately related and the time required for light to travel a specific distance is the same for all observers, as first publicly demonstrated by Michelson and Morley. General relativity does not address the nature of time for extremely small intervals where quantum mechanics holds. At this time, there is no generally accepted theory of quantum general relativity. Time is one of the seven fundamental physical quantities in both the International System of Units (SI) and International System of Quantities. The SI base unit of time is the second. Time is used to define other quantities – such as velocity – so defining time in terms of such quantities would result in circularity of definition. An operational definition of time, wherein one says that observing a certain number of repetitions of one or another standard cyclical event (such as the passage of a free-swinging pendulum) constitutes one standard unit such as the second, is highly useful in the conduct of both advanced experiments and everyday affairs of life. To describe observations of an event, a location (position in space) and time are typically noted.
The operational definition of time does not address what the fundamental nature of it is. It does not address why events can happen forward and backward in space, whereas events only happen in the forward progress of time. Investigations into the relationship between space and time led physicists to define the spacetime continuum. General relativity is the primary framework for understanding how spacetime works. Through advances in both theoretical and experimental investigations of space-time, it has been shown that time can be distorted and dilated, particularly at the edges of black holes.
Temporal measurement has occupied scientists and technologists, and was a prime motivation in navigation and astronomy. 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, the swing of a pendulum, and the beat of a heart. Currently, the international unit of time, the second, is defined by measuring the electronic transition frequency of caesium atoms (see below). Time is also of significant social importance, having economic value ("time is money") as well as personal value, due to an awareness of the limited time in each day and in human life spans.
There are many systems for determining what time it is, including 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.
If a Lagrangian has explicit time dependence due to the potential changing, or thrust being applied to the object in question, how does calculus of variations handle this?
It's easy to get the Lagrange equations from:
δL = ∂L/∂x δx + ∂L/∂ẋ δẋ
What is not clear is how this works when t is an...
Long time reader, first time posting here in physics forums.
I know that according to relativity photons have no proper time. I also know that a photon of sufficient energy can interact with a nucleus's nuclear force via the weak interaction resulting in pair production. I was looking at a...
Homework Statement
The graph shows the variation with time t of the acceleration a of an object (graph attached).
What is the change in velocity of the object in the time interval 0 to 4s?
Homework Equations
a = v/t
The Attempt at a Solution
My initial answer was -8 m/s, by simply finding the...
I'm not a Physicist, I just have this question that has bothered me for a long time. This came up again recently when I was listening to some people talking about evolution.
We know the universe is expanding, and we know that time is stretched with it. So if I'm looking back a million years...
Homework Statement
The astronomer fell into the well. The well is circular, depth H = 15m, radius r = 1m. The astronomer has eyes at h = 1.75m. Observation is from the well axis, refraction is neglected.
How many % of sky can an astronomer see at one time?
How many % of sky can an astronomer...
I've got the start of a plot forming in my mind, and I'm wondering if it's likely to be supported by physics at all.
My basic understanding:
when astronauts go to the International Space Station, due to the speed at which it is travelling, time passes ever-so-slightly slower for them. This...
Homework Statement
I need to find the time it takes a load to reach the end of a distance. Velocity and acceleration are also given.
Homework Equations
I was thinking of using t=vf-vi/(a) but this does not incorporate distance which I already have.
I would use the above formula to calculate...
I just watched a physics show that claimed future outcomes can determine past possibilities for subatomic particles. It was Through the Wormhole - Can Time Go Backwards? (Time index 9:18 to 14:33). I hope it's ok to post that link in here it is a cable TV show, moderator kill my link if not ok...
I would like to know what the implications of this paper are https://arxiv.org/abs/1712.10057.
They say " Here we show that, while in nature the complex conjugation needed for time reversal is exponentially improbable, one can design a quantum algorithm that includes complex conjugation and...
Homework Statement
Homework Equations
Rth = R||R+R (not sure?), τ=Rth*C
The Attempt at a Solution
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So, what I ultimately wanted to determine is the time constant of this first order op-amp circuit.
I first tried to calculate the thevenin resistance seen by the capacitor by using
Rth =...
Ok so I've got a question after walking through the time dilation derivation that used 'light clocks' (think a beam of light bouncing back and forth between mirrors) to derive ##\delta t^\prime = \frac{\delta t}{\sqrt{1-\frac{v^2}{c^2}}}##. So my Q is could you derive the same equation if you...
Homework Statement
A positive charge ##q## is fired head-on at a distant positive charge ##Q## that is held stationary. It comes in at speed ##v_0## and comes to an instantaneous halt at distance ##r_f## away from Q. What is the amount of energy radiated due to acceleration in this time...
I saw this question on special relativity on the internet:
"A spaceship traveling to Alpha Centauri, at 0.95C it takes 4.5 years to get there as measured on Earth.how long does it seems to the spaceship passengers?"
the answer was 1.4 years (Lorentz factor of 3.2).
this answer makes no sense...
I've been searching and it seems I am not smart enough to find the solution for this...
Let say I have a fully closed box made of concrete left in air. The pressure inside the box is 0.01atm. Since it is left in air, the air will come inside the box through concrete walls(since concrete is...
The set-up of the system is like the following:
The thing is, a weight was attached to the meter stick and it was placed in different locations. Then the system composed of the meter-stick and the added weight was then released and the time it took for the meter stick to hit the floor was...
In Classical Mechanics, waves produced in linear systems, like EM waves, obey the Superposition Principle in which the wave amplitudes of, say two input waves, “add up” to create one output wave whose varying amplitude is the sum of the two input waves. One example would be Young’s Double Slit...
Homework Statement
A person of mass ##m## stands at the left end of a boat of mass ##M## and length ##L##. Both the person and the boat are initially at rest. The coefficient of kinetic friction between the boat and the water is ##\mu##. If the person starts moving to the right, what is the...
Our 4-D space is ##x^1,x^2,x^3 ,t##.
Our sub-manifold is defined by ##(x^1,x^2,x^3)##
Therefore for this sub-manifold to be maximally symmetric and for which the tangent vector ##\frac{∂}{∂t}(\hat t)## orthogonal to this sub-manifold
The metric becomes...
I have this friend who praises my intelligence sometimes. She always talks about how I'm "really smart" because I get good grades in college. Whenever she praises me, I cannot help but groan silently, because the main reason that I get good grades is because I don't take that many classes per...
This is probably common knowledge to relativity aficionados but at Example 7.3 in this paper:
https://www.farmingdale.edu/faculty/peter-nolan/pdf/relativity/Ch07Rel.pdf
I was surprised to read the author showing that a ship accelerating at 1g from rest for 1 hour and reaching a speed of...
{Moderator's note: Moved to Cosmology forum.]
Dark matter and the fabric of space time,
Can someone with a real knowledge base of physics and the current accepted theories, please explain why the fabric of space is not the candidate for the elusive dark matter?
Having read extensively about...
Is it safe to be exposed to SHF or EHF for a very long time? Like, hypothetically, there was an ear device in my ear that operated in the SHF or EHF frequency band. Would it be safe for me to use it for a long time especially since they're microwave bands(?). Are microwave bands bad for you? I...
Hello,
I have a signal and got the FFT result of that. I have shown them both below along with the MATLAB code.
May I ask if there is any method to find the time zone(s) in the signal that a specific frequency has(have) happened?
The reason I'm asking this is that I want to specify the time...
Homework Statement
The problem asks to convert time in minutes to time in hours, and to also calculate the uncertainty for the time in hours.Specifically 10 minutes +/- 0.2min being converted to hours with uncertainty.
2. Homework Equations & 3.The attempt at a solution
conversion of time...
Homework Statement
Homework Equations
CPU Time= Σ(individual IC time)
The Attempt at a Solution
What I did was first find the integer time, which is 100=75+X, and X is 25. To find the percent reduced,
I did 100(1-x)=75+25(1-0.5). This means x is 0.125 or 12.5 %.
Is this correct?
Since it's possible to choose a coordinate chart where the Schwarzschild metric components are dependent on time, why that's not done? Would'nt there be a scenario where such a choice would be useful?
Hey, I'm interested in temperature recordings for the last 10 years but also for an estimate in the last billion years, this for hobby and fun. There are a lot of public sources but problem is many sites are torn down and others are created, so I feel very in the wild, it's like finding a tree...
I'd like to know how to solve this. I'm pretty lost as to how to solve this. I want to say that the graph would look periodic because of the graph of the time would go down and then go back up again, but I really don't have anything concrete.
The question states:
The circular clock has a...
Homework Statement
How to calculate time taken (rough approximation) for pressure equalization between two tanks.
Both the tanks have same fluid (Air).
Homework Equations
Rough equilibrium pressure can be achieved by using equations Ptot = (P1V1+P2V2)/Vtot.
Thought of using exponential...
Homework Statement
For any harmonic load:
$$F(t)=F_0\cdot \sin(\omega t)$$
What is the corresponding Frequency domain equivalent?
My lecture notes is suggesting:
$$ F(t)=F_0 \cdot e^{i \omega t} $$
But I am failing to see how they are equal?
The lesson is about Stochastic Response of...
Here's the problem:
https://i.stack.imgur.com/rUVvu.png
What I did:
[2]: https://i.stack.imgur.com/AX2Ye.png
[3]: https://i.stack.imgur.com/K1Zbi.png
To those who could not understand what I read: So basically I used some geometry to to get $l_1$ and $l_2$ in terms of $r$ and the assumed...
So I see EM radiation , visible spectrum including being portrayed as two sinewave vectors each perpendicular to other one being that of the E field the other being the B field, the field is carried by photons, let's take the visible spectrum as an example.
So having a specific frequency the...
Homework Statement
"Using the time module, write a program that tells one the current time in hours, minutes, and seconds, and time since January the first of 1970."
Homework Equations
import time
% = mod division
int = returns integer value of number
The Attempt at a Solution
import time...
Homework Statement
Two objects ##1## and ##2## move at constant speeds ##v_1## and ##v_2## along of two mutually perpendicular lines. At the moment ##t = 0## the particles are located at distances ##l_1## and ##l_2## from the point of intersection of the lines. At what time will the two objects...
I have been attempting to open up files on NASA’s GRS and neutron data,
http://pds-geosciences.wustl.edu/missions/messenger/grs_edr.html
but have been running into some roadblocks. The data is all stored in .dat IEEE binary format (which IEEE format they do not specify in their documentation...
Are all arrows of time special cases of the thermodynamic one? The arrows of time I am referring to are the psychological arrow of time and the cosmological arrow of time. Thanks.
[Mentors' note: This thread was split off from another thread about the cosmic ray muon measurements]
The particle at rest in the moving ship is moving against the Earth.
The particle at rest on the Earth is moving against the moving ship.
A scientist in the moving ship measures the life time...
Homework Statement
A spaceship of proper length L is moving with respect to the ground with speed v. As measured on the ground, how much time does a light signal need to get from the front to the end of the spaceship.
Homework Equations
L'=L/γ
t'=tγ
The Attempt at a Solution
The right answer...
In QM, states evolve in time by action of the Time Evolution Unitary Operator, U(t,t0). Without the action of this operator, states do not move forward in time. Yet even stationary states, like an eigenstate of energy, still contain a time variable – they oscillate in time at a fixed...
Hello, I have a 8 gram cartridge of N2O meant for kitchens. Long story short from what I understand it has 10 cm cube of N2O inside. I also know it has 900 psi / 60 bar of pressure inside. The cylinder is exactly like this image.
I am trying to find out how much time it would take if I...
i don't understand, how does quantum eraser not imply time travel? i firmly don't believe in retrocausality, but it really does seem to imply it . can someone please explain in layman terms?
in here paul davies says observers now can "constrain" the past, does this mean our perception of...
As per my intro post, I don’t consider myself to have any science background beyond high school education.
In a philosophy thread about time on City-Data Forum, where most of the entries are just general unscientific musings, I saw the following post:
“Time is a distance in spacetime: v =x/t...
How can I calculate the Hubble Parameter in time. I know that it decreases in time and approaches to some constant value but I am not sure to what value, Is there any graph for that ?
Homework Statement
Ql: Which sound wave will have its crests farther apart from each other - a wave with frequency 100 Hz or a wave with frequency 500 Hz?
Homework Equations
Frequency= 1/ periodic time
The Attempt at a Solution
I did it like that:
I just found the periodic time for each...
I have been reading some fairly mind bending stuff about the principle or least time (and those of least action) raising questions about causality and free will.
Can anyone explain this to me? Is this total 'woo woo' psuedo science, or are these philosophical questions widely accepted?
Thanks...
Homework Statement
Homework EquationsThe Attempt at a Solution
I have found expressions for the unit vectors for cylindrical coordinates in terms of unit vectors in rectangular coordinates.
I have also found the time derivatives of the unit vectors in cylindrical coordinates. However, I am...
Peeling this out into its own thread for clarity:
How is time dilation of extreme reference frames (photons, black holes, intergalactic space-time) taken into account in Big Bang cosmology? Since from the POV of a singularity or a photon, their clocks have effectively stopped and any lower...