Why Does Time Slow Down or Speed up?

[md65536]

1 hour ago, phyti said:

 From reading his 'relativity' manual, he knows he has two options.

1. remain an anaut moving at .3c, or

2. assume a pseudo rest frame, with Q approaching him at .3c.

Choosing a reference frame doesn't determine what happens. His trip happens in all reference frames. You choose a reference frame  to measure/describe what happens, and the measurements from both reference frames implied here are valid.

1 hour ago, phyti said:

When the anaut twin returns to the earth younger than the other, isn't that proof that the anaut twin biological processes occurred at a reduced rate, even though the anaut was not aware?

Only relative to various observers. For the traveling twin, the biological processes occurred at the usual rate (1 s/s). The traveling twin *is* aware of the difference in aging. She can calculate the aging of Earth, and she can also see it happening.

1 hour ago, phyti said:

Inanimate muons need someone to speak for them.

No. This is just justifying misunderstanding SR.

[phyti]

On 9/8/2019 at 3:54 AM, Strange said:

It depends on what you mean by "real".

When you say things like:

You seem to be saying that something physically changes in the length of the tape and spacecraft. And similarly with the clocks that measure time dilation.

That doesn't work because we are talking about relative measurements. How could another spacecraft speeding up or slowing down cause your astronaut's rule to shrink and expand?

And how much should the ruler shrink? I am stationary (in my frame of reference) so my ruler is 100% of its normal length. When a spacecraft flies by at 10% of the speed of light then my ruler is now 0.5% shorter. But if, at the same time another spacecraft flies past at 20% of c, then my ruler is 3% shorter. So which is it? It can't be both.

So, yes, length contraction is "real" but it isn't something that changes in the "moving" object, it is just a change in relative measurements.

 

[Ships moving past your ship at various speeds don't alter your equipment. Their speed does alter their measurements. It's the same as moving observers assigning different x and t coordinates for the same event, based on their relative speed.]

 

md65536;

Because time dilation is real.

[Yes]

And yet, the atomic clock recorded time at one second per second, never slowing down or "being affected" by time dilation in its own frame.

 

[No. The atomic clock recorded a fixed number of cycles of activity for cesium atoms, which represented a standard unit of time = 1 sec. The cycles differed from those of the ground based clocks due to their relative motion. The clocks frame was that of the aircraft, thus the differences had to have occurred in flight

The purpose of the experiment was to detect differences between static ground based clocks vs moving clocks. They already knew the answer to clocks in a common frame. The evidence was in the clocks, and not the perception of the passengers.]

[If you say the ground based observers detected td via electronic communication, then it occurred in the clock/plane frame and the passengers could not detect it. That is not the same as it didn't occur because none was detected in the plane.]

[Strange]

34 minutes ago, phyti said:

Ships moving past your ship at various speeds don't alter your equipment. Their speed does alter their measurements. It's the same as moving observers assigning different x and t coordinates for the same event, based on their relative speed.

Exactly. So it is purely a matter of how other people measure your length and time. Your rulers and clocks do not change.

And yet you still seem to be saying that there is some change thing place in the clocks and rulers.

[phyti]

19 hours ago, Strange said:

Exactly. So it is purely a matter of how other people measure your length and time. Your rulers and clocks do not change.

And yet you still seem to be saying that there is some change thing place in the clocks and rulers.

How was the MMX resolved?

[Strange]

Just now, phyti said:

How was the MMX resolved?

What do you mean? It was attempting to detect a (relatively) stationary medium that light travelled through. It failed to detect that because it doesn't exist. Not sure how that is relevant.

[phyti]

On 9/10/2019 at 11:56 AM, Strange said:

What do you mean? It was attempting to detect a (relatively) stationary medium that light travelled through. It failed to detect that because it doesn't exist. Not sure how that is relevant.

Wasn't length contraction of the slab in the x axis the solution?

[swansont]

38 minutes ago, phyti said:

Wasn't length contraction of the slab in the x axis the solution?

That was the ad-hoc explanation from Fitzgerald-Lorentz contraction. This was unsatisfying, as ad-hoc explanations tend to be, and was tossed when relativity was found to be a viable alternative and no undetectable aether was required.

[Farid]

If the light from the clock reaching you slower because of you almost moving at the speed of light means that times has slowed down. Then why doesn't just taking out the battery in a clock mean that time has stopped? 

[Ghideon]

11 minutes ago, Farid said:

If the light from the clock reaching you slower because of you almost moving at the speed of light means that times has slowed down. Then why doesn't just taking out the battery in a clock mean that time has stopped? 

Time dilation has nothing to do with the mechanical operations of clocks so the question does not apply. And the first sentence does not seem to agree with what would be observed.
-Speed of light is invariant; moving at high velocity relative to something does not affect the measured speed of light. So the light from a clock would not be measured to move slower between two observers even if they move at high relative velocity. 
-Any observer will measure time moving at regular pace in their own frame of reference and see time run slower in moving frames. No-one will measure time slowing down in their own frame of reference.  

In what frame of reference do you state that time has slowed down?

 

[Farid]

Quote

In what frame of reference do you state that time has slowed down?

I am not saying that time slows down in that case. I saw a documentary on this. It had to with time slowing down to a person moving at or near the speed of light. 

[Strange]

3 minutes ago, Farid said:

I am not saying that time slows down in that case. I saw a documentary on this. It had to with time slowing down to a person moving at or near the speed of light. 

It is nothing to do with moving "near the speed of light". It happens at all speeds. GPS satellites have to account for it, for example.

[Mordred]

To give a more profound example they have even measured time dilation at an average humans head height compared to his feet

https://www.nist.gov/news-events/news/2010/09/nist-clock-experiment-demonstrates-your-head-older-your-feet

[iNow]

1 minute ago, Mordred said:

To give a more profound example they have even measured time dilation at an average humans head height compared to his feet

I’d not read/heard this before, but of course it makes sense due to gravitational potential. Similarly AFAIK, there’s also a pretty decent voltage difference between the head and feet, but this isn’t the thread for that

[swansont]

11 hours ago, Farid said:

I am not saying that time slows down in that case. I saw a documentary on this. It had to with time slowing down to a person moving at or near the speed of light. 

Driving across the US at highway speeds, from one coast to the other and back, you will accumulate around 1 ns of time difference from the kinematic effects of time dilation. As with Mordred's example, seeing the effect is just a matter of having a good enough clock 

[koti]

5 minutes ago, swansont said:

Driving across the US at highway speeds, from one coast to the other and back, you will accumulate around 1 ns of time difference from the kinematic effects of time dilation. As with Mordred's example, seeing the effect is just a matter of having a good enough clock 

Side question - whats is the current resolution we can measure with atomic clocks? Say using your highway example, what is the shortest distance (assuming some speed easy to use for calculations) that we could travel to see the time dillation difference?

[swansont]

3 hours ago, koti said:

Side question - whats is the current resolution we can measure with atomic clocks? Say using your highway example, what is the shortest distance (assuming some speed easy to use for calculations) that we could travel to see the time dillation difference?

It depends on the clock. The best ones aren't built to be portable — one must make sacrifices in clock performance in order to do that. The clock in Mordred's link and the kind of clocks I work on wouldn't run if you put them in a vehicle and drove around. So you'd have to drive faster to see the change with a clock with lesser performance.

Long-term performance of clocks can get to parts in 10^17 fractional frequency stability. Timing stability would be better than a nanosecond at one month. Some devices can do better in the short term, but they tend not to run for very long.

But if we were able to move a really good clock at say ~100 km/hr (27 m/s), gamma would be ~4 x 10^-15. If your clock performed better than that you could resolve the accumulated time difference, but it would have to run long enough to get the statistics. IIRC the NIST clock measuring the height difference ran for about 5 hours to be able to resolve the time dilation, with a stability roughly less than an order of magnitude better than the dilation effect 

 

edit: the paper is  Chou et al SCIENCE VOL 329 24 SEPTEMBER 2010 p1631

The stability is only slightly better than the shift, so that's why it took several hours to discern the timing difference