Talk:Time dilation/Archive 6

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Readability

I think this article is well done. I have just two comments - here and in the next section.

First, I will basically repeat what I said for another physics article. Reading this article, I wonder if the authors were considering who the typical reader was and crafting the article for maximum readability for that audience or if they were sometimes writing for their peers and/or just writing what they know.

Someone once said, “Women make better teachers than men, because women tend naturally to focus on helping the student learn whereas as men are interested in showing what they know.” I don’t know if that’s true, but it makes a good point. As I read, some sections of the current article, I get the impression that the author “knows it and wants to show it” as opposed to being really focused on what the typical reader is looking for.

Since you all are interested in physics, you've probably heard that when Stephen Hawking discussed writing his “A Brief History of Time”, his savvy editor told him that for each equation in the book, expected sales would be cut in half – not just that people would skip those sections, but that when they skimmed the book and saw equations, their eyes would glaze over and they would go elsewhere. I think it’s a rare reader who will come to the Wikipedia Time Dilation article wanting to crawl through a set of equations. Even for that rare reader, the best approach is probably to guide him to some source intended for technical exposition as in “For a more in depth discussion see Jones’ ‘ABC of Relativity’ and for the full treatment see Smith’s ‘A To Z of Relativity’.” The typical reader may come to the article because there was some reference to the "Time Dilation" in a newspaper, magazine, scifi book, etc., but he probably will not be using it to supplement his reading a special relativity text book.

I think it would be good for all the major points to be made in readily understandable English before hitting a mass of equations and/or technical discussion.

I really doubt that any reader will get anything out of the Time dilation at constant acceleration section and strongly suggest it be deleted or at the very least moved to the very end where it will scare off the fewest number of readers. I liked the Simple inference of time dilation section, but I would suggest it follow the Time dilation is symmetric between two inertial observers section which is very good and the The spacetime geometry of velocity time dilation section which is also very good and maybe both of those sections should be moved up. I'll leave it to others to change or not. TwPx 02:52, 14 November 2007 (UTC)

Seriously, it's wikipedia, someone put a laymen explanation summary near the top of the article. -donald duck —Preceding unsigned comment added by 75.185.132.42 (talk) 03:51, 8 February 2008 (UTC)

Apparent contradiction

Under "Velocity and gravitational time dilation combined-effect tests" (specifically the bullet about GPS), the article states that "the in-orbit clocks are corrected for both special and general relativistic time-dilation effects so they run at the same (average) rate as clocks at the surface of the Earth."

A few paragraphs later, under "Time dilation and space flight", it says "at the velocities presently attained, however, time dilation is not a factor in space travel."

These two statements appear to contradict one another. If differences in clocks on moving GPS satellites are found, then certainly differences in clocks on our spaceships can be found as well. What does "not a factor" mean? Would a two-second difference on a moon mission qualify as "not a factor"? Certainly the astronauts wouldn't notice when they return to earth. But it's still a factor, I would argue, even if it's only a fraction of a second.

I cannot find any valid reference on the internet to the difference in time that was observed between the clock placed on Apollo 11 and the stationary clock on earth. However, I do recall reading, in the 1969 Worldbook, that a difference was observed. I will keep looking. Dbrashj (talk) 16:32, 28 November 2007 (UTC)

From Global Positioning System#Relativity:

"For the GPS satellites, general relativity predicts that the atomic clocks at GPS orbital altitudes will tick more rapidly, by about 45.9 microseconds (μs) per day, because they are in a weaker gravitational field than atomic clocks on Earth's surface. Special relativity predicts that atomic clocks moving at GPS orbital speeds will tick more slowly than stationary ground clocks by about 7.2 μs per day. When combined, the discrepancy is about 38 microseconds per day; a difference of 4.465 parts in 10¹⁰.[18] To account for this, the frequency standard onboard each satellite is given a rate offset prior to launch, making it run slightly slower than the desired frequency on Earth; specifically, at 10.22999999543 MHz instead of 10.23 MHz.[19]"

The Apollo missions went somewhat farther out of Earth's gravity well of course, but I kind of doubt they carried a clock capable of measuring measuring the difference over the few days of the missions.

—wwoods (talk) 21:11, 28 November 2007 (UTC)

Of course it couldn't possibly be due to a doppler effect through a medium, to think so is heretical. 98.165.6.225 (talk) 20:23, 14 April 2008 (UTC)

Time dilation is symmetric...

Common sense would dictate that if time passage has slowed for a moving object, the moving object would observe the external world to be correspondingly "sped up". Counterintuitively, special relativity predicts the opposite.

A clock is a frequency, therefore doppler shift applies. Events happen faster in the forward direction and slower in the backward direction. Referring to the twin paradox example, the traveling twin records every event that happens for the homebound twin, but in a shorter time span by his clock. The same number of events happened in a shorter time interval, thus the rate of activity was greater for the moving twin. The moving twin (or a detection device) has altered perception due to time dilation, it's not magic. —Preceding unsigned comment added by Phyti (talk • contribs) 23:20, 6 March 2008 (UTC)

Formula for proper time is wrong.

The article lists the formula for proper time as:

$t^{*}={\frac {c}{g}}\cdot \ln \left(\left({\sqrt {c^{2}+v_{0}^{2}}}-{\frac {v_{0}}{\sqrt {1-{\frac {v_{0}^{2}}{c^{2}}}}}}\right)\cdot {\frac {{\sqrt {c^{2}+(g\cdot t+v_{0}/{\sqrt {1-{\frac {v_{0}^{2}}{c^{2}}}}})^{2}}}+g\cdot t+v_{0}/{\sqrt {1-{\frac {v_{0}^{2}}{c^{2}}}}}}{c^{2}}}\right)$

Any value of v₀ greater than 235682253.9 m/s will cause sqrt(c²+v₀²)-v₀/sqrt(1-v₀²/c²) to be negative, causing the value inside the ln() function to be negative, resulting in an invalid result. It appears to be correct for v₀=0. I suspect the formula is wrong for any non-zero v₀. Unfortunately, I don't understand the article referenced by the section to pull the proper formula out. 70.95.251.55 (talk) 10:01, 16 March 2008 (UTC)

In the spacetime slice of a single map frame used to define simultaneity, proper time τ elapsed during constant proper acceleration g from a co-linear initial velocity v_o follows from the integrals g = Δw/Δt = cΔη/Δτ and the velocity conversions η=tanh^-1[v/c]=sinh^-1[w/c]. The result in terms of elapsed map time Δt is $\Delta \tau ={\frac {c\Delta \eta }{g}}={\frac {\sinh ^{-1}[{\frac {g\Delta t}{c}}+\sinh[\eta _{o}]]-\eta _{o}}{g/c}}$. I think the expression above does work for v_o=0, as you suggest, since ln[sqrt[1+t²]+t]=sinh^-1[t]. Thermochap (talk) 12:09, 17 March 2008 (UTC)

Note: The accelerated motion expressions for x (compared to Δx=c²Δγ/g) and v (considering Δη=gΔτ/c) in that same section are correct for any v_o, while the expression for t (compared to Δt=Δw/g) also works only when v_o=0. Thermochap (talk) 12:01, 18 March 2008 (UTC)

Overview

"It is only when an object approaches speeds on the order of 30,000 km/s (1/10 the speed of light) that time dilation becomes important."

I believe the speed of light is only around 300km/sRMFan1 (talk) 12:53, 28 March 2008 (UTC)

You're misreading or misremembering. The speed of light it 3x10^8 m/s = 300,000 km/s = 300 Mm/s. Consider, the U.S. is ~5,000 km across; how many seconds does it take light to cross that distance?

—WWoods (talk) 20:27, 28 March 2008 (UTC)

Remove section Time dilation in popular culture

I propose we get rid of this section Time dilation#Time dilation in popular culture with its potentially endless and i.m.o. rather silly list of science fiction titles. I don't think that this belongs here. For those who really want to keep the list for easy reference, the entire section can be moved to Science fiction#Usage of time dilation or something. Comments? DVdm (talk) 19:10, 15 May 2008 (UTC)

Since there was no comment, I went ahead and removed the science fiction section. DVdm (talk) 11:32, 16 May 2008 (UTC)

congratulations! ErNa (talk) 20:37, 18 May 2008 (UTC)

Someone re-inserted the section without much of a reason other than "Why the hell not?". Since there appeared to be a silent consensus on having it removed, I undid the re-insertion. DVdm (talk) 11:50, 20 June 2008 (UTC)

Silence doesn't mean there's is a consensus to have it removed, silence just means that no one has looked at the discussion page regarding it. I think a link to 'Time dilation in popular culture' in the 'See also' section would be reasonable. --118.92.227.17 (talk) 21:19, 7 August 2008 (UTC)

I don't think it is reasonble to allow for yet another silly list of science fiction titles describing stories "where they used time dilation to go to 47 Ursae Majoris and back in a mere 6 years". DVdm (talk) 16:51, 8 August 2008 (UTC)

Just a thought

Just a thought.. it would be completely valid to presume as foundational postulate to special relativity that all objects age/move the same in space-time. That (dt) ^2 +(ds)^2 is exactly the same for all objects observed in a particular inertial frame.. where dt is how much i perceive the body to have "aged".. while ds is how much i percieve the body to have "moved" spatially in that interval. Distance obviously being measured in 'c' units. Needless to say, time dilation results immediately follow from that assumption. A more elegant way of understanding the result, i feel, than the usual textbook derivation. Lets the reader appreciate that space and time are not two separate entities but things we perceive as separate - a limitation imposed by human senses. While the concept space-time is what truly makes sense.

time dilation results follow intuitively.. ( clock (1) at rest vs clock(2) moving at velocity v:

(dt1)^2 = (dt2)^2 +((v*dt1)/c )^2

dt2=dt1*sqrt(1-(v/c)^2)

Dilip rajeev (talk) 22:25, 2 July 2008 (UTC)

Don't understand this - more explanation needed

I am by no means an expert on this subject, which is why I referred to the article. I was specifically looking for an explanation as to why it is commonly claimed that travelling at speed away from some location (e.g. London), and then back to the same location will result in less time having passed for the travelling observer than an observer remaining in London. This claim is also made in at least 2 places in the article ("Velocity and gravitational time dilation combined-effect tests", first bullet; "Time dilation and space flight", second bullet) but I can see no explanation for why the observer in London (in my example) should be designated the "stationary" one, while the guy on the spaceship is the "traveller". Surely the guy on the spaceship feels like he is stationary and the guy in London is moving away from him? I think the article would be substantially improved by an attempt to explain this in straightforward terms. Gavingalloway (talk) 15:01, 21 August 2008 (UTC)

Ignoring the gravitational effects, the guy who stays in London does not have to "change direction", so during the entire process he will "feel nothing" and remains inertial. The traveller will have to physically change direction a few times (or constantly), and feel the effects of that. He cannot remain inertial. That is the essential asymmetry.

Remember that the subject of this article is Time dilation, which describes an essentially symmetric effect, whereas this particular stay/travel-effect is explained in the Twin paradox article, with a strong emphasis on the asymmetry. DVdm (talk) 15:57, 21 August 2008 (UTC)

OK, thanks for pointing me in the right direction. In that case, would it make sense for the links to the Twin paradox article to be more prominent? For example, there is no mention of it in the "Velocity and gravitational time dilation combined-effect tests", first bullet; "Time dilation and space flight", second bullet sections I mentioned above, where, to me at least, the discussion has strayed well into the realm of the twin paradox. In this article, the twin paradox is only referenced when discussing temporal coordinate systems. Additionally, it is the last reference in the "see also" section, suggesting that it is only loosely related to this article. Gavingalloway (talk) 12:06, 22 August 2008 (UTC)

Indeed, there is no mention of it, since these sections are about the so-called "combined velocity and gravitational effect" (- which is actually a misnomer, since there really is only one effect, and we just happen to be able to calculate it as a combined effect because it is so small). The Twin paradox article o.t.o.h. is not about that combined effect. It focuses on the asymmetry of the situation, and in absence of gravitation. So I guess this section is really not the best place to point to the twin paradox article. You find pointers to it elsewhere in the article. - DVdm (talk) 14:58, 22 August 2008 (UTC)