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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8answers
2k views
Is time continuous?
I was making universe simulations, and I noticed that I implemented discrete time (the only type possible on computers). By that, I mean that I had an update function, that was called many times per ...
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7answers
739 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 ...
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3answers
741 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?
...
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3answers
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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 ...
14
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4answers
2k views
How is the classical twin paradox resolved?
I read a lot about the classical twin paradox recently. What confuses me is that some authors claim that it can be resolved within SRT, others say that you need GRT. Now, what is true (and why)?
14
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2answers
392 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 ...
13
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2answers
557 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 ...
13
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1answer
149 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 ...
12
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9answers
444 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 ...
12
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4answers
576 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?
10
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13answers
1k 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 ...
10
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5answers
6k 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 ...
10
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1answer
370 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 ...
9
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3answers
53 views
Seeing cosmic activity now, really means it happens millions/billions of years ago?
A Recent report about a cosmic burst 3.8 billion light years away. It is written as though it is happening now. However, my question is, if the event is 3.8 billion light years away, doesn't that mean ...
9
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4answers
562 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 ...
9
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3answers
446 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 ...
9
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0answers
224 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 ...
8
votes
4answers
522 views
Is a 1D vector also a scalar?
A vector in one dimension has only one component. Can we consider it as a scalar at the same time?
Why time is not a vector, although it can be negative and positive (when solving for time the ...
8
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2answers
248 views
Question about proper time in general relativity
I think I may have some fundamental misunderstanding about what $dt, dx$ are in general relativity.
As I understand it, in special relativity, $ds^2=dt^2-dx^2$, we call this the length because it is ...
7
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3answers
355 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 ...
7
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3answers
677 views
Can black holes actually merge?
If time stops at the event horizon, can we ever detect two black holes merging? In other words, if you are a short distance away, would you encounter a spherically symmetric gravitational field, or a ...
7
votes
2answers
96 views
How to create unusual sundial?
I am considering small "artsy" project. I would like to create sundial by placing gnomon on the window and painting hour lines on the window facing wall.
Since this is to be placed in bedroom I am ...
7
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4answers
599 views
Special Relativistic Time Dilation — A computer in a very fast centrifuge
Ok, I've stumbled onto what I think is a bit of a paradox.
First off, say you had some computer in a very fast(near light speed) centrifuge. You provide power to this computer via a metal plate on ...
7
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4answers
1k 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 ...
7
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1answer
110 views
Cancelling special & general relativistic effects
We know that for a GPS we need to make a correction for both general and special relativity: general relativity predicts that clocks go slower in a higher gravitational field (the clock aboard a GPS ...
7
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1answer
316 views
Time crystals : fake or revolution?
This article about "crystals of time" just appeared on the PRL website.
Viewpoint: Crystals of Time (http://physics.aps.org/articles/v5/116)
The authors (including famous Frank Wilczek) claim that ...
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3answers
115 views
Do velocity and acceleration time dilation factors add?
For a spinning space station such as in 2001, A Space Odyssey, what would be the time slowing in the perimeter of the spinning space station with respect to the center axis of the station?
The ...
6
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8answers
484 views
Can you completely explain acceleration to me?
I understand what acceleration is, and I know the formula, and I understand it's a vector.
I just don't understand how the equation works exactly. I'm kind of picky, I know, but bear with me.
...
6
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3answers
253 views
If the entire universe is the same age, can one assume that any potential life elsewhere is at the same stage of development as us?
There was some discussion in another question regarding whether you can consider all parts of the universe to have existed for the same length of time, and the conclusion seemed to be that, because of ...
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4answers
1k views
How can gravitational forces influence time?
How does it work that gravitational forces can affect time and what usable applications could arise from this?
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7answers
608 views
Time, what is it? [closed]
If you ask any person about time, she/he will give you some answer.
I suspect that it is extremely difficult, (if not impossible) to define time.
Is there a definition of what it is in physics? Is it ...
6
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5answers
359 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 ...
5
votes
2answers
380 views
What's the difference between space and time?
I'm having a hard time understanding how changing space means changing time. In books I've read people are saying "space and time" or "space-time" but never explain what the difference is between the ...
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6answers
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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 ...
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4answers
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The Time That 2 Masses Will Collide Due To Newtonian Gravity
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 ...
5
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2answers
52 views
Tropical year is 27s/year off from Gregorian year?
http://hpiers.obspm.fr/eop-pc/models/constants.html states the
tropical year is $365.242190402$ days.
The Gregorian calendar's average year is $365.2425$ days (every 4th year
a leap year, except ...
5
votes
3answers
481 views
Does this Zeilinger group result provide experimental proof of backward-in-time causation?
Does this recent Zeilinger group delayed choice entanglement experiment imply backward-in-time influences?
http://arxiv.org/abs/1203.4834
From the abstract: "This can also be viewed as “quantum ...
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2answers
124 views
Is there any scientific evidence that demonstrates why time passes?
Is there any scientific evidence that demonstrates why time passes?
Or is it just an opened question?
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3answers
140 views
How can we know, today, that there's something from 100 light-years from here?
In my understanding, to take a picture of something that is 100 light-years from here, our "camera" would have to travel 100 years at light speed, take the picture, send to us, and 100 years later we ...
5
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1answer
225 views
What is the speed of time
When we measure the speed of a moving element we do it with the help of a reference frame. Now if we need to measure the speed of time, is it possible? Is time really has speed?
Thanks in advance.
5
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1answer
331 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 ...
5
votes
5answers
408 views
Is time fundamentally different from space?
Note: This is a rewrite of the original question, which was titled What would time be for 2D beings?
In my current, non-physicist's understanding, every instant of our three‑dimensional world ...
5
votes
2answers
424 views
What is the fastest process or shortest time in nature?
We know about some events that happen very quickly. For example, the dielectric relaxation time is about $10^{-14}\, \mathrm{seconds}$.
I'm interested in other processes that switch extremely fast ...
5
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2answers
313 views
The analogy between temperature and imaginary time
There are many statements about the relation between time and temperature in statistical physics and quantum field theory, the basic idea is to interpret (inverse) temperature in statistics as "time" ...
5
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1answer
120 views
Does the collapse of the wave function increase entropy of the atomic system itself?
Does wave-function collapse cause the entropy of the atom (ie. the sub-atomic particle system that makes up the atom) to increase?
5
votes
0answers
214 views
Why is my approach to the equation of time off by a constant?
I'm trying to better understand the causes for the equation of time by deriving an approximation from first principles.
My naive approach, $EOT_{NAIVE}$, is to take the difference between the right ...
4
votes
5answers
936 views
How many colors exist?
How many "colors" do exist?
Our perception:
As far as I know, colors are just different frequencies of light. According to wikipedia, we can see wavelengths from about 380 nm und 740 nm. This means ...
4
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5answers
493 views
Why isn't the symmetric twin paradox a paradox?
Two twin sisters synchronize their watches and simultaneously (from the earth frame) depart earth in different directions. Following a predetermined flight plan, each sister accelerates identically to ...
4
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2answers
2k views
How might clock synchronization work with RSA SecurID tokens?
My workplace uses these things to generate one-time passwords which only work within a short time period. I have always been curious about how the clock synchronisation between the authentication ...
4
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5answers
301 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 ...
