# The Naked Scientists Forum

### Author Topic: Does Relativity Work Both Ways?  (Read 1977 times)

#### litespeed

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##### Does Relativity Work Both Ways?
« on: 08/01/2010 18:48:30 »
We are all familiar with the 'experiment' of two clocks. One is set on the mantel, and the other is sent out at 86% the speed of light (50% dialation). On return, we find the one sent out comes back having clocked one half the time as the one on the mantel. If the one sent out does not slow up upon returning, we notice through the digital display on the vehicle it is actually ticking at half speeed.

I believe he, on the other hand, would observe our clock ticking faster. This can be illustrated this way. As he streaks out at 86% the speed of light we will observe him travel one light year in, what, 14 months? He, on the other hand, will observe he got there in 7 months, since his time dialation is 50%. His radio communications to us would confirm this.

However, lets assume two cesium clocks in in another circumstance. We have a Cesium clock on the mantel with a digital display on our roof. Out of nowhere a space alien ship streaks by with a similar display. Do we BOTH see the other's clock as showing an equal 50
% dialation? That seems impossible.

I believe time is relative based upon the force needed to achieve a relative dialation of, say 50%.  If this is true, then the space alien and I can compare our observations and determine which one of us has had more accelerative force applied to our respective clocks since the Big Bang.

The only explanation for the fact we are not going the same speed relative to each other is that one of us has had more accelerative force applied. Otherwise, we would be in stasis as the universe expands, and would only notice a red shift in the two digital displays, not a difference in recorded time.

« Last Edit: 08/01/2010 19:04:59 by litespeed »

#### Geezer

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##### Does Relativity Work Both Ways?
« Reply #1 on: 08/01/2010 20:10:16 »
We are all familiar with the 'experiment' of two clocks. One is set on the mantel, and the other is sent out at 86% the speed of light (50% dialation). On return, we find the one sent out comes back having clocked one half the time as the one on the mantel. If the one sent out does not slow up upon returning, we notice through the digital display on the vehicle it is actually ticking at half speeed.

I think, on its return, the travelling clock would, as you say, be "slow" relative to the static clock. But on its return, the travelling clock will be running at the same rate as the static clock. Or, was that what you meant anyway?

#### PhysBang

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##### Does Relativity Work Both Ways?
« Reply #2 on: 08/01/2010 23:43:45 »
We are all familiar with the 'experiment' of two clocks. One is set on the mantel, and the other is sent out at 86% the speed of light (50% dialation). On return, we find the one sent out comes back having clocked one half the time as the one on the mantel. If the one sent out does not slow up upon returning, we notice through the digital display on the vehicle it is actually ticking at half speeed.

I believe he, on the other hand, would observe our clock ticking faster. This can be illustrated this way. As he streaks out at 86% the speed of light we will observe him travel one light year in, what, 14 months? He, on the other hand, will observe he got there in 7 months, since his time dialation is 50%. His radio communications to us would confirm this.
Actually, no. As someone is speeding away from us, we are speeding away from him. To him, we are slowing down.

He will see that the universe is essentially moving past him, and that the distance between him and his eventual goal is not as far away as it was when he set out.

The difference between frames of reference has to take three things into account: time dilation, length contraction, and different simultaneity.
Quote
However, lets assume two cesium clocks in in another circumstance. We have a Cesium clock on the mantel with a digital display on our roof. Out of nowhere a space alien ship streaks by with a similar display. Do we BOTH see the other's clock as showing an equal 50
% dialation? That seems impossible.
It is not, if one properly takes all the translations between reference frames into account.

#### litespeed

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##### Does Relativity Work Both Ways?
« Reply #3 on: 09/01/2010 19:24:34 »

PysBang: I have the impression that relativity depends not just on speed, but how you got to that speed. For instance, in case one we know the second clock was subjected to a given amount of G force over a given time. The first clock experienced no such energy input. Accordingly, ALL the mass/time dialation takes on the SECOND clock according to E=MC2. Acceleration force caused this to happen only on clock number two.

So an astronaut accelerated to 86% the speed of light from a space station will travel one light year in about 14 months. He will report back, however, that it only took him seven months [50% dialation]. Further, while under way, he will tally the clicks on his cecium clock and periodically radio back the results.  We will both notice his tally falls further and further behind as his trip progresses. The difference is entirely due to the accelerative forces applied to his ship that were NOT applied to the space station.

In the second case, however, we don't know how much total accelerative force either craft would have accumulated since the big bang - more precisely the MATTER in our ships. What is clear, however, is that one of us was accelerated more then the other. I believe radio communications between the ships comparing Cesium Clocks would settle the issue. Just as it would in the first case.

But as always, ugh, I could be wrong.

« Last Edit: 09/01/2010 19:33:52 by litespeed »

#### yor_on

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##### Does Relativity Work Both Ways?
« Reply #4 on: 14/01/2010 16:22:29 »
"I believe he, on the other hand, would observe our clock ticking faster."

Yeah, he should, shouldn't he. With the eyes of 'God' :)

But if we take two rockets passing each other you won't be able to say that one is 'contracting' more that the other. No matter that A is traveling twice as fast than B. To both A and B the 'contraction', as observed from their own frame, looking at the other rocket, will be the same.

And if that is true, what does it do to your example?

#### The Naked Scientists Forum

##### Does Relativity Work Both Ways?
« Reply #4 on: 14/01/2010 16:22:29 »