Re: [Phys-L] another DIY relativity experiment

["Tom Van Baak" <tvb@LeapSecond.com>]

Hi Don,

Thanks for your insights. Let me share some thoughts, but I know there are many on this list who are real physicists.

First, on a technical note...

The experiment I did with my family in 2005 [1] on Mt Rainier was a "up-and-down" experiment. That is, the main clock(s) stayed at home and we "exposed" portable clock(s) to mountain elevation for a weekend before returning back home for the time comparison. They came back roughly 20 ns ahead.

Doing it this way seemed completely natural and also practical. Think of it as a blue shift experiment.

The experiment I did for PBS/Hawking in 2016 [2] on Mt Lemmon was a "down-and-up" experiment. The main clock(s) stayed on the summit and we exposed portable clock(s) to Tucson elevation at hotel for a day before returning back to the summit for the time comparison. They came back roughly 20 ns behind. More like a red shift experiment.

Doing it this way seemed unnatural at first. But it allowed the producer to film the final comparison scenes at sunset at an observatory overlooking Tucson. And besides, the main clocks were in my car, so where we sync'd the clocks before dilation and where we compared the clocks after dilation was defined by where my car was. For me, either way was fine (same result, different sign).

In the end, doing two experiments, each a different way, contributes to the educational value of both.

Anyway, back to your issue...

So clocks slow down due to velocity, and clocks slow down due to gravity. On the PBS show, as well as any number of articles and books, there's talk about clocks running faster at higher elevation. True. But maybe a safer way to look at it is this -- gravity can only make clocks run slower; gravity just slows clocks less up on a mountain than it slows clocks down in a valley.

One reason we think that clocks run faster on a mountain is that we all tend to live in the valley. In fact, we have defined the SI second, as well as UTC, at sea level. If instead, you take a more universal view, you could define your "standard second" far away from earth and then clocks anywhere near earth would only run slower. We would then talk about clocks being more slow or less slow, depending on elevation, instead of clocks being fast or slow, depending on elevation.

And so with this view there's less problem with past or future. No one is moving to the past. We are all moving towards the future, and all moving a bit slower than a unperturbed clock in deep space. Here on earth you just move a little more or less slower depending on your elevation. To me that's not time travel per-se but perhaps merely traveling in time at slightly different rates.

So I think I agree with your concerns / explanation.

In that context perhaps our defining the "future" in terms of person(s) who are already most ahead is not unlike the problems with the parochial definition of the SI second at sea level on planet Earth. Instead if you think of, if you define, the future as the time that a clock in deep space keeps, then we're all late to the future, just more or less late. Not Back to the Future, but Late for the Future.

For those of you who missed the episode, it's online [3] and the cesium clock scenes are the last 15 minutes.

/tvb

[1] http://leapsecond.com/great2005/tour/
[2] http://leapsecond.com/great2016a/photos.htm
[3] http://www.pbs.org/video/2365757267/

----- Original Message ----- 
From: "Donald Polvani" <dgpolvani@verizon.net>
To: <Phys-L@Phys-L.org>; "'Tom Van Baak'" <tvb@leapsecond.com>
Sent: Thursday, May 19, 2016 2:52 PM
Subject: RE: [Phys-L] another DIY relativity experiment


> On Tuesday, May 17, 2016 9:51 PM, Tom Van Baak wrote:
>
>
> > Wednesday night (May 18) look for a TV show on National Geographic or PBS
> > called "Genius by Stephen Hawking". Episode 1: Can We Time Travel?
>
> I did watch the show with interest and also was interested in the URLS Tom
> supplied.  I do have a question.  Hawking made a big point of how time
> travel back to the past is impossible because we would see a copy of
> ourselves moving backwards and how could there be a copy of us apparently
> made from nothing.  However, the time dilation experiment purported to show
> that time travel to the future was possible by having one person with a
> precise pre-synchronized clock stay some time at the bottom of a mountain
> and then (in the show after one day to reduce experimental error) bring his
> clock to the previously synchronized clock at the top of the mountain. Due
> to gravity induced time dilation, the clock at the top was 20 nanoseconds
> ahead of the clock which had spent the day at the bottom of the mountain and
> so the person previously at the bottom (and his clock) had traveled to the
> "future".  If you accept this (and I believe the time dilation result, but
> I'm not sure about the traveling to the future interpretation), why couldn't
> the people at the top carry their clock down the mountain, again find that
> it is 20 nanoseconds ahead of the clock at the bottom and so claim that they
> had traveled back in time?