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

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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.
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10answers
13k views

Is time continuous or discrete?

While working on physical system simulation software, I noticed that I had implemented discrete time, which means that there was an update mechanism advancing the simulation for a fixed amount of time ...
54
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5answers
8k 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?
45
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3answers
9k 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 ...
45
votes
7answers
34k views

Is anti-matter matter going backwards in time?

Or: can it be proved that anti-matter definitely is nót matter going backwards in time? From wikipedia: There is considerable speculation as to why the observable universe is apparently almost ...
39
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8answers
7k 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 ...
35
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7answers
6k 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?
33
votes
6answers
4k 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 ...
29
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5answers
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 ...
29
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6answers
3k 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 ...
28
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2answers
2k 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 ...
27
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3answers
7k views

What is the difference between implicit and explicit time dependence e.g. $\frac{\partial \rho}{\partial t}$ and $\frac{d \rho} {dt}$?

What is the difference between implicit and explicit 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 total ...
25
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2answers
2k 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? ...
23
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7answers
6k 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 ...
23
votes
6answers
6k views

Should I abandon my thought experiment about time?

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 ...
23
votes
3answers
2k 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 ...
22
votes
3answers
1k views

Symmetrical twin paradox

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 ...
21
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5answers
13k 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,...
20
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3answers
9k views

Oh-my-god particle: How can it get through Milky way in 10 seconds?

My question is concerning wikipedia article on Oh-my-God particle, to be precise, this paragraph: This particle had so much kinetic energy it was travelling at 99.99999999999999999999951% the ...
20
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2answers
4k views

Time as a Hermitian operator in QM?

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 \...
20
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7answers
3k 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 ...
19
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10answers
4k views

What are the mechanics by which Time Dilation and Length Contraction occur?

What are the mechanics of time dilation and length contraction? Going beyond the mathematical equations involving light and the "speed limit of the universe", what is observed is merely a phenomenon ...
19
votes
6answers
116k 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 ...
19
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1answer
1k 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 ...
18
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3answers
2k views

Would computers accelerated to high speeds compute “faster” from our point of view?

I woke up to this thought yesterday: Lets say Computers A and B have exactly the same specifications and at time T both are set to process an algorith that would normally take exactly 1 year and ...
18
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6answers
4k views

Is time travel possible? Is it possible to go back in time?

I read somewhere that according to relativity, black holes and other space related stuff it is possible to jump into past. Is it possible for anything to go back in time either continuously or by ...
17
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3answers
2k 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?
16
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9answers
3k views

Is the flow of time regular?

Is the flow of time regular? How would we come to know if the our galaxy along with everything in it stops for a while(may be a century) w.r.t to the galaxies far beyond our reach. Is there a way to ...
16
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2answers
1k views

Inflation and the Meaning of Time

I'm not quite sure how to ask this so that it can be answered in layman's terms, but I have lately seen, in several places, that with cosmological inflation, there was a point where the universe ...
16
votes
1answer
288 views

If time isn't continuous, what is the best-known upper bound on the length of time intervals?

There have been several questions about whether time is continuous or not and it seems like the answer isn't currently known. I know quantum mechanics treats time as continuous and any mathematics ...
15
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9answers
10k views

Is there a proof of existence of time?

It seems to me that there is no such thing as time. There is only movement in the universe and we compare our own movement to a different object to have a sense of time. It can be a clock or a atomic ...
15
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5answers
4k views

Is the future already determined?

I've always wondered (and was re-inspired to explore further from these two videos) that if at a single point of time we know about the complete state (position, momentum, spins, everything.) of every ...
15
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3answers
3k views

Has everything we see happened in the past?

Whatever we see is basically based on the light that hits our eyes, right? When we look at the Moon we are looking at the Moon as it was couple of seconds ago, as the light takes some seconds (~2 ...
15
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5answers
10k views

Did time exist before the creation of matter in the universe?

Does time stretch all the way back for infinity or was there a point when time appears to start in the universe? I remember reading long ago somewhere that according to one theory time began shortly ...
15
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7answers
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Clocks in special relativity

One book on special relativity says: Any observer at rest relative to his own timepiece will see that other clocks moving with respect to him run fast - the greater their speed, the faster they ...
14
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4answers
2k views

Time dilation all messed up!

Okay! 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 ...
14
votes
9answers
649 views

How to explain (pedagogically) why there is 4 spacetime dimensions while we see only the 3 spatial dimesions?

I have been asked this question by a student, but I was able and in the same time incapable to give a good answer for this without equations, so do you have ideas how one can explain this in a simple ...
14
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3answers
407 views

Is there a prediction of quantum mechanics that could be construed as representing an “energy-time uncertainty relation?” [duplicate]

As the title suggests. Is there a prediction of quantum mechanics that could be construed as representing an "energy-time uncertainty relation?" Does there exist any reference to such a prediction, or ...
14
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1answer
239 views

Is period of rotation relative?

My question is inspired by the following answer by voix to another problem: "There is a real object with relativistic speed of surface - millisecond pulsar. The swiftest spinning pulsar currently ...
13
votes
3answers
989 views

Is “now” or “the present moment” properly defined in GR?

My question is about the extent to which "now" is defined in GR. In Minkowski spacetime, it's possible to define a "now" for an inertial observer by finding a spacelike 3-plane such that, in the ...
13
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1answer
1k views

What could explain the measurements that the Methuselah star is older than the universe?

So there has been talk in the news of a star named Methuselah that is "older than the universe". Moreover, this star happens to belong to our very own Milky Way. The article mentions that Methuselah ...
13
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4answers
1k views

Energy is actually the momentum in the direction of time?

By comparatively examining the operators a student concludes that `Energy is actually the momentum in the direction of time.' Is this student right? Could he be wrong?
13
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5answers
14k views

How can time be relative?

I don't understand how time can be relative to different observers, and I think my confusion is around how I understand what time is. I have always been told (and thought) that time is basically a ...
13
votes
6answers
764 views

More than one time dimension

We know that space-time dimensions are 3+1 macroscopically, but what if 2+2? Obviously it is tough to imagine two time dimensions, but mathematically we can always imagine as either having two ...
13
votes
2answers
504 views

What accounts for the discrepancies in my calculations of year lengths?

A common exercise in many introductory astronomy texts is to use the lengths of various kinds days to calculate the approximate length of the corresponding year. For example, ratio $k$ of the length ...
13
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3answers
317 views

What makes now special?

I recently came across the paper Relation between the psychological and thermodynamic arrows of time (arXiv). Their argument makes sense to me, however their concept doesn't seem to address what 'now'...
13
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1answer
350 views

How can two time theories be compactified to 3+1 without any Kaluza-Klein remnants

I have recently been looking into the two-time theories and the implied concepts. For me this seems slightly hard to grasp. How can I see the basic concept in this theory in a fundamental way based ...
12
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3answers
20k views

Is time a Scalar or a Vector?

In Wikipedia it's said that time is a scalar quantity. But its hard to understand that how? As stated that we consider only the magnitude of time then its a scalar. But on basis of time we define ...
12
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3answers
590 views

If time standard clocks and any memories about the time standard are destroyed, can we recover the time standard again?

Assume the time standard clocks and any memories about the time standard are destroyed. Can we recover the time standard again exactly? Recovering the time standard again means we can determine the ...
12
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4answers
4k views

Does the future already exist? If so, which one?

In the NOVA Fabric of the Cosmos program, Brian Greene explains a theory in which there is no "now", or more specifically, now is relative. He describes an alien riding a bicycle on a far off planet ...