Questions tagged [time]
Time is defined operationally to be that which is measured by clocks. The SI unit of time is the second, which is defined to be
2,162
questions
146
votes
13
answers
48k
views
Is time continuous or discrete?
I was coding a physics simulation, and noticed that I was using discrete time. That is, there was an update mechanism advancing the simulation for a fixed amount of time repeatedly, emulating a ...
- 2,069
130
votes
11
answers
97k
views
Is anti-matter matter going backwards in time?
Some sources describe antimatter as just like normal matter, but "going backwards in time". What does that really mean? Is that a good analogy in general, and can it be made mathematically precise? ...
- 6,691
123
votes
8
answers
36k
views
What is $\Delta t$ in the time-energy uncertainty principle?
In non-relativistic QM, the $\Delta E$ in the time-energy uncertainty principle is the limiting standard deviation of the set of energy measurements of $n$ identically prepared systems as $n$ goes to ...
- 1,255
115
votes
4
answers
15k
views
Why do electrons, according to my textbook, exist forever?
Does that mean that electrons are infinitely stable? The neutrinos of the three leptons are also listed as having a mean lifespan of infinity.
- 1,938
106
votes
16
answers
17k
views
What is the proper way to explain the twin paradox?
The paradox in the twin paradox is that the situation appears symmetrical so each twin should think the other has aged less, which is of course impossible.
There are a thousand explanations out there ...
100
votes
9
answers
16k
views
What is time, does it flow, and if so what defines its direction?
This is an attempt to gather together the various questions about time that have been asked on this site and provide a single set of hopefully authoritative answers. Specifically we attempt to address ...
86
votes
4
answers
7k
views
What does one second after big bang mean?
Consider the following statement:
Hadron Epoch, from $10^{-6}$ seconds to $1$ second: The temperature of the
universe cools to about a trillion degrees, cool enough to allow
quarks to combine ...
- 7,065
78
votes
4
answers
7k
views
What reference clock is an atomic clock measured against?
I looked at a few of the other posts regarding the accuracy of atomic clocks, but I was not able to derive the answer to my question myself.
I've seen it stated that atomic clocks are accurate on the ...
- 927
74
votes
10
answers
12k
views
Can black holes form in a finite amount of time?
One thing I know about black holes is that an object gets closer to the event horizon, gravitation time dilation make it move more slower from an outside perspective, so that it looks like it take an ...
- 1,295
66
votes
5
answers
23k
views
Why do atomic clocks only use caesium?
Modern atomic clocks only use caesium atoms as oscillators. Why don't we use other atoms for this role?
- 661
59
votes
3
answers
26k
views
What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?
What is the difference between implicit, explicit, and total time dependence, e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?
I know one is a partial derivative and the other is a ...
56
votes
3
answers
15k
views
Why is a second equal to the duration of exactly 9,192,631,770 periods of radiations?
Why is a second equal to the duration of 9,192,631,770 periods of radiation corresponding to the transition between two hyperfine levels of the ground state of the caesium-133 atom?
Why is the number ...
- 687
52
votes
3
answers
10k
views
Do pear-shaped nuclei really have anything to do with time travel?
Recently (in the last week or two), various articles about pear shaped nuclei have appeared, such as this one from Science Alert and this from the BBC
The Science Alert article includes the quote
...
- 1,748
48
votes
10
answers
4k
views
Why is the photon clock equivalent to all clocks?
I can understand why, if the speed of light is invariant, a photon clock would tick slower. I find this explanation very useful in terms of introducing the idea of time dilation (also because it ...
- 2,887
47
votes
4
answers
9k
views
In nuclear physics, what length year in seconds is used?
So I'm working on a nuclear physics problem and am looking at radioactive decay. The common unit used for very long decays is years within the literature. Is this the sidereal or tropical year? I want ...
- 633
44
votes
11
answers
10k
views
How do we measure time?
I'm having a little trouble trying to put to words my problem and I apologize in advance for any causation of trouble in trying to interpret it.
We define periodic events as those events that occur ...
- 565
43
votes
6
answers
19k
views
What is an instant of time?
If we say that an instant of time has no duration, why does a sum of instants add up to something that has a duration? I have a hard time understanding this.
I think of one instant as being a 'moment'...
- 819
43
votes
2
answers
10k
views
How was the first atomic clock calibrated?
As we all know, atomic clocks are being used to measure time and the GPS system.
But I was wondering based on what was the first atomic clock calibrated and how accurate this calibration was based on ...
- 696
43
votes
2
answers
7k
views
Is there a time operator in quantum mechanics?
The question in the title has been asked many times on this site before, of course. Here's what I found:
Time as a Hermitian operator in QM? in 2011. Answer states time is a parameter.
Is there an ...
- 628
42
votes
1
answer
5k
views
Why is the period of a geostationary satellite not exactly 1440 minutes?
When reading about Astra satellites on Wikipedia (https://en.wikipedia.org/wiki/Astra_1KR), I saw that the period of the Astra 1KR satellite, positioned at 19.2° E, is 1,436.1 minutes (source: NORAD ...
- 481
40
votes
5
answers
12k
views
Does old light contain clues to its age?
Light from celestial objects is old. In the case of galaxies, it's millions of years old. It seems plausible to me that light might show signs of its age.
I was surprised that a Google search only ...
- 4,681
40
votes
4
answers
19k
views
The meaning of imaginary time
What is imaginary (or complex) time? I was reading about Hawking's wave function of the universe and this topic came up. If imaginary mass and similar imaginary quantities do not make sense in physics,...
- 561
37
votes
7
answers
9k
views
If we suddenly lost track of time, how would we know what time is now? [duplicate]
If we hypothetically lost all watches and all devices that keep track of time, how would we say what is the current time? Or we actually don't and time is just a convention?
- 489
36
votes
3
answers
4k
views
Symmetrical twin paradox in a closed universe
Take the following gedankenexperiment in which two astronauts meet each other again and again in a perfectly symmetrical setting - a hyperspherical (3-manifold) universe in which the 3 dimensions are ...
- 3,671
35
votes
7
answers
7k
views
Why are leap seconds needed so often?
In Coordinated Universal Time (UTC), leap seconds are added to account for the slowing down of Earth's rotation. But the slowing down is said to be of the order of milliseconds in a century. Then why ...
- 931
34
votes
7
answers
285k
views
How exactly does time slow down near a black hole?
How exactly does time slow down near a black hole? I have heard this as a possible way of time traveling, and I do understand that it is due in some way to the massive gravity around a black hole, but ...
- 919
33
votes
8
answers
8k
views
Do the laws of physics evolve?
Hubble's constant $a(t)$ appears to be changing over time. The fine stucture constant $\alpha$, like many others in QFT, is a running constant that varies, proportional to energy being used to measure ...
- 1,770
33
votes
4
answers
4k
views
Intuition for multiple temporal dimensions
It’s easy, relatively speaking, to develop an intuition for higher spatial dimensions, usually by induction on familiar lower-dimensional spaces. But I’m having difficulty envisioning a universe with ...
- 457
32
votes
7
answers
7k
views
Would there be no time in a universe with only light?
It is sometimes said, that if you stand still (in space), you travel through time at the speed of light. On the other side light never stands still, so it always only travels through space (at the ...
- 9,554
32
votes
3
answers
3k
views
The age of the universe
Many times I have read statements like, "the age of the universe is 14 billion years" . For example this wikipedia page Big Bang.
Now, my question is, which observers' are these time intervals? ...
- 5,899
31
votes
6
answers
8k
views
Is time travel possible? Is it possible to go back in time?
I read somewhere that according to relativity, it is possible - involving black holes and other stuff - to jump into the past. Is it possible for anything to go back in time either continuously or by ...
- 5,207
31
votes
3
answers
9k
views
Time as a Hermitian operator in quantum mechanics
In non-relativistic QM, on one hand we have the following relations:
$$\langle x | P | \psi \rangle ~=~ -i \hbar \frac{\partial}{\partial x} \psi(x),$$
$$\langle p | X | \psi \rangle ~=~ i \hbar \...
- 1,445
30
votes
3
answers
23k
views
How do grandfather clocks keep going?
How do grandfather clocks keep going? The pendulum is what makes the clock go. However, the pendulum will slow down due to friction. What energy source keeps the pendulum from eventually stopping?
- 2,839
30
votes
5
answers
3k
views
What do you really see on a line of clocks as you pass by them at high speed?
According to my understanding of SR, if I travel at 0.8c relative to a line of clocks, I should see the clocks in front of me going 3 times faster than my own, and those behind me going 3 times slower ...
- 1,902
30
votes
2
answers
5k
views
Can physics be done without the use of time?
Carlo Rovelli is interviewed in this article (The illusion of time):
Alongside and inspired by his work in quantum gravity, Rovelli puts forward the idea of 'physics without time' This stems from the ...
29
votes
4
answers
14k
views
Lagrangian to Hamiltonian in Quantum Field Theory
While deriving Hamiltonian from Lagrangian density, we use the formula
$$\mathcal{H} ~=~ \pi \dot{\phi} - \mathcal{L}.$$
But since we are considering space and time as parameters, why the formula
$$\...
- 1,747
29
votes
3
answers
6k
views
Is there an actual proof for the energy-time Uncertainty Principle?
As I understand, the energy-time uncertainty principle can't be derived from the generalized uncertainty relation. This is because time is a dynamical variable and not an observable in the same sense ...
- 7,271
27
votes
19
answers
11k
views
Why should a clock be "accurate"?
Having read that atomic clocks are more accurate than mechanical clocks as they lose a second only in millions of years, I wonder why it is necessary for a reference clock to worry about this, if the ...
- 437
27
votes
5
answers
38k
views
The Time That 2 Masses Will Collide Due To Newtonian Gravity [closed]
My friend and I have been wracking our heads with this one for the past 3 hours...
We have 2 point masses, $m$ and $M$ in a perfect world separated by radius r. Starting from rest, they both begin to ...
- 493
27
votes
6
answers
6k
views
Is time dilation due to the travel time of light?
I'm trying to think about special relativity without "spoiling" it by looking up the answer; I hope someone can offer some insight - or at least tell me I'm wrong.
Suppose I have an ordinary clock ...
- 1,335
26
votes
4
answers
6k
views
With what velocity are we moving along the time dimension?
Does the question make sense? Velocity along time axis means $v_t=\mathrm dt/\mathrm dt$? If it doesn't, please explain where the flaw is. Taking time as measure like length? Or do we need to ...
- 659
26
votes
7
answers
20k
views
Why are objects at rest in motion through spacetime at the speed of light? [closed]
I read that an object at rest has such a stupendous amount of energy, $E=mc^2$ because it's effectively in motion through space-time at the speed of light and it's traveling through the time dimension ...
- 4,517
26
votes
9
answers
5k
views
Time is the only dimension that has an arrow, and the only dimension which contributes an opposite sign to the metric. Is that just a coincidence?
Time is different from space in these two seemingly independent ways.
One of them is generally believed to have to do with special boundary conditions at the beginning of time.
But if you knew nothing ...
- 1,771
25
votes
9
answers
6k
views
Telescopes and Time: Please Explain
I'm a psychotherapist by training so go easy on me here. I would like to know, in simple terms if possible, the basic mechanics of how Hubble can see back in time.
I pretty much understand, in this ...
- 383
25
votes
4
answers
3k
views
Time dilation all messed up!
There is a problem with my logic and I cannot seem to point out where.
There's a rocket ship travelling at close-to-c speed v without any acceleration (hypothetically), and there is an observer AA ...
- 351
25
votes
3
answers
7k
views
What is the length of 1 second in meters
If time is treated as a fourth dimension of spacetime, what is relation between length and time units?
Or in other words, how can I convert time units to length units, for instance seconds to meters?
- 421
25
votes
6
answers
2k
views
Why does time not run backwards inside a refrigerator?
The arrow of time is often associated with the fact that entropy always increases. On the other side that should mean, if entropy decreases time should run backwards. But inside a refrigerator we have ...
- 9,554
25
votes
8
answers
6k
views
What grounds the difference between space and time?
We experience space and time very differently. From the point of view of physics, what fundamentally grounds this difference?
Dimensionality (the fact that there are three spatial dimensions but only ...
- 431
23
votes
7
answers
9k
views
Is there a maximum frames per second (FPS)?
Take a video camera and crank up the frames per second rate.
Disregarding current technological advancements, could a camera's FPS go so fast that any two captured images be identical? Would ...
- 575
23
votes
7
answers
11k
views
Does real life have "update lag" for mirrors?
This may sound like a ridiculous question, but it struck me as something that might be the case.
Suppose that you have a gigantic mirror mounted at a huge stadium. In front, there's a bunch of people ...
- 371