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
2
votes
2
answers
2k
views
Why doesn't delayed choice quantum eraser experiment imply retrocausality?
WRT: https://www.youtube.com/watch?v=0ui9ovrQuKE&ab_channel=ArvinAsh
Now I found a paper "Why Delayed Choice Experiments do NOT imply Retrocausality by David Ellerman" on the internet ...
- 179
0
votes
2
answers
69
views
Commutator of energy and unitary time evolution operator
Edited:-
$\hat{H} = -\frac {\hbar^2}{2m} \frac {\partial^2}{\partial^2 x}~+~V$
$\hat{U}(t) = e^{-iHt/\hbar}$
The Robertson Uncertainty Principle states:-
$\sigma^2_A\sigma^2_B \ge (\frac {1}{2i}[\hat{...
- 6,784
1
vote
3
answers
136
views
A unit of time $\frac{GM}{c^3}$ [closed]
I have come across a time unit $M$ that should be equal to $\frac{GM}{c^3}$. I cannot find anything at all about this anywhere.
I came across it while doing flux analysis on the flux from black hole ...
- 10.7k
2
votes
0
answers
366
views
Causality in CPT symmetry analogue of free electron laser (stimulated absorbtion)?
While 2nd law of thermodynamics emphasizes past->future time direction, CPT theorem says that at least microscopic physics has some symmetry between past and future. For example the Feynman-...
- 950
0
votes
4
answers
155
views
Possibility of anti-time
I was just wondering if every particle has an anti-particle (or so I've read/heard), are there other things which could be candidates to have an anti- "form" as well? My example is focused ...
- 651
1
vote
1
answer
51
views
Electromagnetic Induction Effect Affecting Musical Wall Clock
I have a battery-powered musical wall clock which plays the requisite number of musical chimes each time the hour strikes.
I have noticed for years that turning the lights on or off causes the chime ...
- 83.9k
12
votes
2
answers
2k
views
A paradoxical problem in Classical Mechanics
In Kibble-Berkshire Classical Mechanics there is a problem which says:
A ball is dropped from height $h$ and bounces. The coefficient of restitution at each bounce is $e$. Find the velocity ...
- 301
2
votes
1
answer
189
views
Time taken by an air bubble on the sea depth to reach sea level
There is an air bubble at a depth of s meters above sea level.
Suppose the air is an ideal gas and Temperature is constant.
I would like to calculate the time it takes for it to reach sea level.
...
- 188k
0
votes
0
answers
47
views
Time from big bang to here [duplicate]
New to this so apologies for my ignorance, the simpler the answer the better. Here goes.
Light took 13.5 billion years to get to us from the big bang.
On an imaginary neighboring planet that is much ...
- 188k
0
votes
1
answer
636
views
Correlation Amplitude in QM
The following is a section "Correlation Amplitude and the Energy-Time Uncertainty Relation" from Sakurai's Modern Quantum Mechanics book page 79:
Question:
Why does it state that the oscillations ...
CommunityBot
- 1
0
votes
1
answer
105
views
Understanding consequences of spacetime relativity
If I understood right, time flows slower where there is more gravitational force (or to be more precise, as it was pointed out to me, where gravitational potential is lower), compared to where there ...
- 15
-11
votes
2
answers
157
views
Has the Twin Paradox Really Been Resolved? Interesting peer-reviewed paper states that it hasn't been resolved [closed]
Has the Twin Paradox really been resolved? Here is an interesting peer-reviewed paper Has the Twin Paradox Really Been Resolved? (RG) which states that it hasn't been resolved:
Of particular interest ...
- 22.9k
0
votes
6
answers
104
views
Time dilation and understanding which is $\Delta t$ and which is proper time $\tau$
In the textbook that we are using, the definition for proper time $\tau$ is the interval between two events, as measured by an observer who is at rest with the two events. The definition for $\Delta t$...
- 32.7k
2
votes
2
answers
105
views
Measuring the solar system's speed through ether with the help of an eclipse
In A.P. French's Special Relativity, the author said,
Jupiter has
a period of $12$ terrestrial years, and so in half a terrestrial year,
while the earth moves from $A$ to $B$ (Fig. $2$-$7$), ...
6
votes
3
answers
469
views
To what precision can we determine the age of the universe theoretically?
Does it makes sense to expect/hope that one day we will measure the age of the universe (in Earth's frame of reference for example) much more precisely, down to sub-year precision? Is there an ...
-4
votes
1
answer
57
views
Can a big enough Giant travel many lightyears in just a few seconds? [closed]
We, as humans, given our height and size, view the world from the same general perspective. An ant, on the other hand, will understand the same world in a completely different way, given how limited ...
- 188k
-4
votes
2
answers
104
views
Relativity theory inconsistency
Suppose I have two synced clocks, light-based for simplicity (light bouncing between mirrors up and down).
A train comes by me at some velocity u and I put one of the clocks on the moving train.
...
- 384
0
votes
1
answer
112
views
If someone flies away from me quickly, and then I catch up to them, will our clocks be in sync or out of sync?
At time 0, Alice and Bob are stationary in space with their clocks synced. Then Alice, for $t=\text{1 day}$ (measured by herself), flies in one direction at the speed $v=0.99 c$, and then stops. Due ...
- 22.9k
0
votes
1
answer
82
views
Why is causal order not related to directionality of time?
Hans Reichenbach argues for the causality and causal chain to define a topological coordinative definition of time order. Here is an excerpt from his textbook, The Philosophy of Space and Time, Dover(...
- 617
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 ...
- 53.1k
16
votes
6
answers
7k
views
Are we really moving at the speed of light in the time dimension? [duplicate]
I have seen a vector explanation of time dilation that our net velocity in space-time is the speed of light. Most of that velocity for slow spatial speeds is in the time dimension.
As you go faster ...
- 53.1k
0
votes
2
answers
81
views
How do we account for gravity affecting the actual measurement of time?
Let's suppose we are in Newtonian Mechanics, and we have a clock, say a mechanical stop watch. Then, it is that the gears of the stopwatch is affected by gravity, and hence the time ticks faster at ...
- 12k
-3
votes
1
answer
71
views
Photons that escape the universe [closed]
According to my current understanding of special relativity, photons do not experience the passage of time. It is as though the universe is completely 'paused' for them. I know that objects with mass ...
- 42.1k
3
votes
1
answer
183
views
What is the shortest controllable time?
I was reading http://en.wikipedia.org/wiki/Planck_time and noticed a reference to: http://phys.org/news192909576.html where it is stated that:
12 attoseconds is the world record for shortest ...
- 111
-1
votes
0
answers
75
views
Big Bang at time equal to negative infinity
What are the consequences of transforming the cosmological coordinate system of the Big Bang from $t=-13.8 bny$ to $t=-\infty$? Has this ever been investigated? Would the equations/solutions look ...
- 7,290
1
vote
0
answers
117
views
Isn't this article "Is the universe is a Vaccum Fluctuation?" by Edward P Tyron misinterpreting Energy-Time uncertainty principle? [duplicate]
I have learnt the Energy-time uncertainty principle from Griffiths which says:
$Δt$ represents the amount of time it takes the expectation value of $Q$ to change by one standard daviation. $Δt$ ...
7
votes
3
answers
1k
views
How about an exact solution for the position of a planet as a function of time?
Recently I was surprised to discover that no exact solution for the position of a planet as a function of time exists. I am referring to the two-body problem in a gravitational field where Newtons law ...
- 21
-1
votes
1
answer
75
views
Can time exist without change? [duplicate]
Time seems to be simply a measure of change, or rather how we perceive time is by observing the rate of change. If no change can be detected, no motion, no chemical change, no change of state, not ...
3
votes
1
answer
203
views
$\partial F_2/dt$ part of a time dependent canonical transformation
Suppose we have a time-dependent canonical transformation - say generated by a function of the type $F_2(q,P,t)$. The resulting Kamiltonian picks up an extra partial $\partial F_2/\partial t$:
\...
CommunityBot
- 1
1
vote
9
answers
457
views
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?
Sabine Hossenfelder says time dilation is due to acceleration in the twin's paradox. Is this true?
At 12 minutes into this video https://www.youtube.com/watch?v=ZdrZf4lQTSg, Hossenfelder states, &...
- 9,848
-1
votes
1
answer
70
views
If Inflation never happened before the CMB, and the present expansion were projected backwards, how old would the Universe be since the Big Bang?
The Cosmic Microwave Background Radiation is the oldest thing we can directly observe. To explain the near uniformity of temperature of the CMBR and the flatness of space, Cosmic Inflation was ...
- 122k
0
votes
1
answer
400
views
Is time the rate at which one moves through space? [closed]
I'll start out with the cliche attempt in a protective shield of my dignity. I am a young highschool kid just eager to learn and understand. If I'm way off or this is already a known idea, or maybe ...
- 188k
2
votes
0
answers
62
views
Radial quantization and time order
In CFT, one ususally begins quantization by defining radial ordering on the complex plane, with the notion of radial ordering being the equivalence of time ordering. This is often "motivated"...
- 109
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 ...
-2
votes
2
answers
50
views
Is it true that time dilation in a universe with no gravity could not be calculated without knowing how much mass is present in the universe?
Looking for a quick clarification on something. I am a layman and I have been trying to find out how much time dilation would exist if there was no gravity anywhere, and ignoring what seem to be ...
- 6,432
0
votes
0
answers
24
views
Does time slow down for light according to relativity? [duplicate]
According to Einstein's theory of relativity
Time slows for a individual who moves at the speed of light and time goes backward if the individual is faster than the speed of light.
So , is it that for ...
- 188k
0
votes
2
answers
192
views
Heating and Cooling down time of an object
Let's say I have 2 identical metal bars, same properties and everything, both are at 100 degrees Celsius. Is the time heating up one metal bar to 120 the same as cooling one down to 80 degree Celcius?
CommunityBot
- 1
0
votes
0
answers
32
views
Finding the missing charge
I'm doing a lab report. I have to find the charge integrating some pulses. Unfortunately some pulses are partially saturated (see the photo). My idea is trying to find the charge in function of ...
1
vote
2
answers
3k
views
Explanation for frequency graph of the Doppler Effect
Just wanted an explanation for why the frequency graph for the doppler effect (as a source approaches an observer) looks like so:
Let the central $x$-axis value be the time at which the source is ...
- 188k
4
votes
2
answers
164
views
Difference between proper time and coordinate time
Lately, I have been studying General Relativity, and I am very confused about the difference between proper time ($\tau$) and coordinate time ($t$).
Also, whenever we write any line element given a ...
3
votes
5
answers
9k
views
Confusing definition of proper time – which is correct?
I have googled for „definition of proper time“
This source https://www.collinsdictionary.com/dictionary/english/proper-time gives the following definition:
proper time ... measured by a clock that ...
0
votes
2
answers
40
views
Defining the loudness in terms of mass and time
The unit of loudness is $$\rm\frac{W}{m^2},$$ which is equivalent to $$\rm\frac{kg}{s^3}.$$ Since kg is the SI unit for mass and s is the SI unit for time, this led me to the following question:
Can ...
- 44.7k
2
votes
1
answer
95
views
Why is cosmological time unique?
According to the definition I have encountered for the concept of cosmological time, it is defined in the following way:
The cosmological principle states that, at each location in the universe, it ...
0
votes
0
answers
34
views
Would time exist to a static observer in a static vacuum? [duplicate]
I lack the knowledge about physics to know if this question could even be answered, apologies if the question is a little strange and specific.
An observer with zero mass is in a perfect vacuum in a ...
- 15
-5
votes
4
answers
122
views
Is it possible to define time in a more universal way, such as, time is the outward manifestation of the activities inside an atom? [closed]
Is it possible to define time as the outward manifestation of the activities inside an atom?
For example,
one second is defined as the unperturbed ground state hyperfine
transition frequency of the ...
- 22.9k
0
votes
0
answers
48
views
Was Big Bang the "START" of time? [duplicate]
I know that this question has been repeated a lot. But I still don't understand this concept.
Big bang created matter and space but how could it possibly create time?
If Big bang didn't create time ...
- 188k
2
votes
3
answers
81
views
What does the $v$ term in the proper time equation represent exactly?
I'm trying to intuitively understand proper time which is defined as the time that is always measured in a moving observer’s or particle's rest frame and is given by the equation.
$\Delta\tau = \sqrt{...
- 10.7k
5
votes
1
answer
266
views
Why did Dirac say that atomic time is different from relativistic time, and that gravity is becoming weaker? What is the relation between the two?
In this gem of an interview in 1982 with Friedrich Hund, Dirac says at 09:17 that there is some theoretical basis and observational evidence that atomic time and distances are different from ...
- 22.9k
2
votes
0
answers
78
views
What is Dirac talking about here? [duplicate]
https://www.youtube.com/watch?v=xJzrU38pGWc&ab_channel=mehranshargh
"I might say that my recent work has been very much concerned with Einstein's general relativity and I believe that the ...
- 188k
1
vote
0
answers
68
views
Why cant one-way speed of light be measured using a timer start that is electronically connected at midpoint? [duplicate]
Why can't one-way speed of light be measured using a timer start that is electronically connected at midpoint?
I'm sure I'm missing something, but why can't an experiment be built with two timers that ...
