# Tag Info

0

To add to Johannes's succinct answer: there are two other things about time that we believe to be absolute: The topology of any web of causal links between events in space time and The direction in time of causal links. If we postulate that a cause-effect relationship can only propagate at a maximum speed of $c$, then if a cause comes before an effect ...

1

Time might be relative, aging (time passing) is absolute. Run around, jump into a rocket, speed up and circle a few times around a black hole, and do whatever else you fancy, all observers will agree how much you have aged in the process. Here, for 'aging' you can read 'proper time': the time that has passed according to your wristwatch.

0

If time is relative, how could time pass? It doesn't really pass. That's just a figure of speech. Footballers pass. Buses pass. But there is no physical thing called time that actually passes. Instead things move. Things like light and planets and planes and people and hearts and blood and electrochemical signals in your brain. And pendulums and cogs ...

0

The Universe we see around us today including the space was once concentrated to a size smaller than an atom. This infinitely dense object may have had a past before that state but no information would make it from that past into our universe so we may as well say that time began at the big bang.

0

I go with Time is the separation between distinct events that happen in the same place. which is very general and not quantitative at all, but covers the basics. Given three distinct events that happen at the same place we can determine which happened between the other from just the values of the three separations. And it agrees with the notion that ...

0

The 7 fundamental quantities are hard to define. Time Displacement/position Mass Temperature Current Amount of substance (e.i. the mole) Luminous intensity More here: http://gravimotion.info/Physics_seven_basic_quantities.php Time is just one of them. If someone asked me, I would say something like "Time is how long something lasts" or "Time is the ...

-4

Antimatter is matter going backwards through time. No it isn't. Whilst that idea might appear to have some pedigree, (see retrocausality on Wikipedia), it's bunk I'm afraid. Antimatter is like matter, but it has the opposite chirality. Google on positron chirality. Whilst one can mathematically model the positron as a "time reversed electron", it isn't ...

2

Antimatter increase in entropy over time. We can verify this with a thought experiment. Take ten positrons. Put five in one side of a chamber with a barrier and then the other 5 on the other side of the barrier in the same chamber. The chamber and barrier are also made of antimatter. The positrons repel each other and so each have a certain amount of kinetic ...

0

You are correct time does stop at $c$ speed of light, however we (planet) did not place a timer inside the light but are measuring it from outside so time for us passes normally and nothing weird happens and nothing is frozen. As a result we simply are measuring the gap of time from the 1 point to another point whilst a object (light) goes to it so as ...

0

Let's say there are two twins. One on the ground and one moving through the atmosphere at 1,000,000 m/s which is a few thousand times greater than 1,000 km/h. The moving twin does this for 100 years, or about 3 billion seconds, then returns to the ground to compare clocks. The moving twin will have a clock that ticked more slowly by around 16,600 seconds. ...

0

Here is the most prominent example I know: The Russian astronaut Sergej Awdejew was in orbit for a total of 748 days (traveling at approximately 17000 miles per hour). Therefore he has time travelled a whopping 0,02 seconds into the future!

1

The thing is, in relativity you cannot have a reference frame "chasing" a photon. You'll get singularities if you try to view the world from a photon's perspective. A photon cannot move like you and you cannot move like a photon. As a photon, travelling along a light-like world line, experiences no proper time it's proper velocity is simply undefined. ...

4

Light travels at the speed $c$ this speed is finite and with out using any relativity we can calculate the time it takes for something travelling at this speed to reach us: $\text{time} = \frac{\text{Distance}}{\text{speed}}$ or $t= \frac{d}{c} = \text{8 minutes}$ in this case. For a person travelling very close to the speed of light with velocity $v$ from ...

0

This effect is called gravitational time dilation.If you have a spherically symmetric centre of attraction like earth or sun,then you will find that clocks that are closer to the centre tick more slowly as compared to clocks that are far away .This effect is crucial in design of GPS satellites for if this effect is not taken into account satellite and earth ...

0

The notion of anti-particle emerges as soon as special relativity is taken into account. For a relativistic particle of mass $m$, its energy and momentum satisfy $E^2 = p^2 + m^2$. To illustrate where the anti-particles are coming from, let's take the simplest "wave function equation" (that's an abuse of language since it's an equation for fields), i.e. the ...

0

I'd like to add a few words to Domagoj Pandža's excellent answer. He makes this statement: "....Intuition and perception (or the lack of there of) can be a big problem when you're trying to comprehend the implications of special/general relativity ...." I think Domagoj's answer is excellent, but I disagree a little with this statement. Actually almost ...

0

I think other answers are insightful but I would like to elaborate them further. In a nutshell, the question in about an hypothetical brain which has physical dimensions comparable with one light years, and about its perception of time. Since information cannot travel faster than light, the time in which information travel from one side of the brain to the ...

0

A being with 1 lightyear arms or even 1 lightyear across and density similar to flesh would be sufficiently dense to create an enormous black hole. Even planet sized, a being of that size runs into gravity problems, where, lifting it's arm would require significant energy to resist it's on gravitational attraction on it's limbs, so too big doesn't work. ...

1

If there were an enormous being whose arm span is one light year across, how would that being perceive time? Wouldn't what we perceive as a year be virtually nothing to that being? It would probably have to have decenteralized brains spread throughout its volume here and there. Otherwise, yes, there is a distinct problem that a being with a single large ...

1

This is one of those questions that can drive you crazy, since there is a great deal assumed and not stated. Let me try an alternate possibility. Conceivably, the problem wants you to assume that, when moving, the overall speed of the raindrop remains fixed at 5 m/sec, but it travels in a straight line at an angle due to horizontal wind forces, and this may ...

2

If the raindrop's vertical velocity is constant as the train is both stationary and moving, the time taken for the raindrop to travel down the window would be: $$t = \frac{1\ \text{m}}{5\ \text{m}/\text{s}} = 0.2\ \text{s}$$ Remember, the time $t$ would not depend on the speed of the train. The exercise also specifically states that the raindrop's vertical ...

0

Another way of looking at this: The vertical component of the raindrop's velocity vector is 5 m/sec downward, and the horizontal component is 30 m/sec across. By the Pythagorean method, the resultant velocity vector is 30.41 m/sec diagonal. The vertical component of the raindrop's displacement vector is 1 meter, and as the raindrop is pushed horizontally ...

1

Sitting at infinity, you will see something more and more red-shifted - but never actually stop radiating as you would expect it from a black hole. The reason being that your coordinates (the asymptotically flat ones) diverge at the radius of the event horizon. Specifically, in your coordinates the metric of a Schwarzschild black hole (which is not entirely ...

1

If the Universe has such a time interval, we have not observed it yet. Moreover: if the Universe's "refresh time" is less than a Planck time, roughly 10-43 seconds, we have no direct way of detecting it, as any wave which we created which had that frequency would have a Schwarzschild black-hole radius bigger than its own wavelength, making it very hard to ...

0

One of the more important short half life isotopes which actually occurs (we strongly believe) in nature is $^8Be$ which is part of the helium burning, or tri-alpha, process of fusion in older stars. The half life is about 67 as, so after two alphas fuse, a third one must quickly interact, and apparently it does. This isn't the shortest lived Be, however. ...

-1

I think it is hyrdrogen-7 which is 4n decay, half life is 21x10^-23 seconds

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So, here's the deal. "Time is relative" means a lot of different things to a lot of different people. In order to make a solid step forward, Einstein and company basically needed to clarify what they were trying to say. What they were trying to say looks something like this: "if you see a train passing by you, you're going to see things happen in slightly ...

-4

If you think about it, time as we know it does not actually exist/flow - it is our mental manifestations of the world around us that we think of as time. For example, what we see is not actually there as we view it. The object sends us light-waves (only a small portion possibily of what the object really is), our eyes then have to decode the light waves and ...

4

As mentioned by WhatRoughBeast, caesium offers several advantage over other microwave standards. Its most important feature is the presence of an atomic transition with a very small linewidth. This allows the energy of this transition to be established very accurately (see the uncertainty principle). However, caesium is not the only atom with a narrow ...

0

The notion of absolute time for all observers in all reference frames has been debunked by Einsteins theory if special relativity. Prior to that, scientists believed that there might be an ether that permeated all space, and from which a universal reference frame could be derived. However, when Michelson and Morley did their famous experiments attempting to ...

0

I do not understand what the accuracy of $1$ part in $10^{14}$ means. [...] reviewed the definitions of accuracy and error [...] In definitions of "accuracy" or "error" you should have noticed mentioning of the true value of some particular quantity, referring to the trial(s) under consideration, and the corresponding, commensurate value(s), ...

1

There isn't a such thing as "absolute time." Some events – they are called space-like events – can't even be agreed to happen in an "objective order." Only time-like events can be universally agreed to happen in a particular order, but there's no such thing as "universal time." For you, time will always tick per one second by second, and that will apply to ...

2

Is there an absolute pace of time, no. Is your clock , in a region without gravity, (and at "rest" relative to other objects) "ticking" faster than your alarm clock on the Earth's surface, yes. But obviously you physically cannot escape the effect of gravity, no matter how far away the mass-energy sources are, so this will vary from observer to observer, ...

-1

As other users have said, it has one stable isotope, so that's nice. It's also the SI standard. We define the second by Caesium. Specifically: The second is the duration of 9 192 631 770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom. So, if we were to use another ...

0

I'm afraid you're overcomplicating things, aepryus. Yes, most modern clocks use electromagnetic phenomena, but your pendulum clock employs gravity in much the same fashion as your water-drop clock. The clock rate doesn't depend on gravitational potential, it depends on the first derivative of potential, the "slope" as it were. The force of gravity. And this ...

20

The choice of cesium is due to various factors. It's worth noting that your statement "Modern atomic clocks only use caesium atoms" is simply untrue. At the very least, rubidium and hydrogen clocks are common, and you can get rubidium standards on eBay for well under $200. But the best performance comes from using cesium. In part this is because it was ... 79 "Because that is how the second is defined" is nice - but that immediately leads us to the question "why did Cesium become the standard"? To answer that we have to look at the principle of an atomic clock: you look at the frequency of the hyperfine transition - a splitting of energy levels caused by the magnetic field of the nucleus. For this to work you ... 4 Because one second is defined as (from the SI brochure): the duration of 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom,${}^{133}\mathrm{Cs}\$. Thus, using any other atom is irrelevant (even if calculate some correction time factor).

4

To know what a closed timelike curve looks like, you just do like every spacetime metric. You compute geodesics and field equations and all of that. Unfortunately, things start getting complicated. Closed timelike curves have a lot of weird behaviours, especially when it comes to matter fields upon them. They may not have a properly defined Cauchy problem, ...

1

A closed timelike curve wouldn't actually "look" like anything because it's an abstract thing. You can't actually see any lightcones or worldlines. A metric is an abstract thing too, to do with your measurements of distance and time, typically made using the motion of light. And the crucial point is this: you don't travel along your worldline. You move ...

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