A Brief History of Time travel in Scientific American of September 2015 page 58

This document contains comments about the article A Brief History of Time travel by Tim Folger In Scientific American of September 2015.
We already have the means to skip ahead in time but going backward is a different wormhole

"Introduction"

In Newton's Universe, time was steady everywhere and everywhen: it never sped up or slowed down. But for Einstein, time was relative.

What does that mean?

Time travel is not only possible, it has already happened though not exactly as H.G. Wells imagined.

This requires a clear definition of what "Time travel" is.

As Einstein proved, time passes more slowly for objects in motion than for those at rest.

The physical issue is that moving clocks tick more slowly than clocks at rest. However this requires a carefull definition what a clock is (how it operates) and what "at rest" means. Two relative moving clocks cannot both be called at rest.
The next step is the relation between a clock versus time.

While Krikalev was in orbit, he aged 1/48 of a second less than his fellow eartlings.

From the perspective of the eartlings Krikalev watch run 1/48 second behind.

From another perspective he traveled 1/48 of a second into the future.

From the perspective of Krikalev the eartling watches run 1/48 second before.

If Krikalev left earth in 2015 and made a round-trip to Betelgeuse - a star that is about 520 light-years from Earth - at 99.995 percent the speed of light, by the time he returned to Earth he would be only 10 years older.

This is mathematical a very simpel statement but physical a very difficult exercise.
How do you know that the spaceship is travelling of a speed of 99.995 % of the speed of light?
How do you know that a space ship is "travelling" at 0 % of the speed of light ?
This whole issue is linked to the physical process of how the clock internal operates as a function of its speed.

"Time travel to the future, we know we can do," Gott says. "It's just a matter of money and engineering!"

This is a too simple statement for any scientist.

Jumping a few nano seconds into the future is relative straightforward. But going backward in time is harder.

This completly depends if you consider "being at rest" an absolute concept. If you consider "being at rest" an absolute concept than if you consider the Sun and the Earth either the observer at the Sun or at the Earth can be at rest. Not both.
When "being at rest" is an absolute concept than when:

• you are at rest all the moving clocks run slower and your clock runs forward in time.
  
• you are not at rest then there are clocks which move slower or are at rest. These clocks run faster than your clock and your clock runs backward in time.

Einstein's special theory of relativity forbade it.

A theory in principle can never be in conflict with the results of any experiment.
All experiments start with reference system or a reference frame. If you do not have a reference than you can not make any prediction.

After another decade of work, Einstein unveiled his general theory of relativity, which finaly lifted that restriction.

GR discusses matter (gravity). SR is without matter. Newton's Law discusses matter (gravity). In reality all experiments involve matter and gravity, this raises the question how important is SR.

How someone would actual travel back in time, however, is a vexing problem because the equations of GR have many solutions.

If your clocks runs slower going to the "right" compared to your reference system than you have a chance that your clock runs faster going to the "left".
A clock which is based on light signals cannot operate (count) faster than the speed of light. Such a clock will always tick faster when the speed decreases, until its speeds becomes zero. That is the moment when the clock ticks the fastest.

Different solutions assign different qualities to the universe and only some of the solutiuons create conditions that permit time travel into the past.

A new way of looking at time.

First, he argued though all motion is relative, the laws of physics must look the same for everyone any where in the universe.

The laws of physics are descriptions of physical processes how these physical processes change in time. It does not matter if you live on earth, on the moon or on any other star, the evolution and the motions of our solar system is identical.
The difinition of a second, a light year and a kilometer could be different, but after these units adaptations the laws of physics should be identical.
It should be mentioned that this concept does not make any predictions what the laws of nature are.

Secondly, he realized that the speed of light must be similarly unchanging from all perspectives: if everyone sees the same laws of physics operating, they must also arrive at the same result when measuring the speed of light.

What every sees versus what is actual happening are two different issues. The difference is explained because it takes time before we actual see what was happening. Part of this problem is the speed of light which is also a physical process. There is no necessity that the (one way) speed of light should be always and every where identical. The problem is that everyone should agree about these differences because if not, to calculate what is happening based on what we see, will be different. And that is not what we want.

To make light a universal speed limit, Einstein had to jettison two commonsense notions:
that all observers would agree on the measurement of a given length and that they would also agree on the duration of time's passage.

Why should there be a speed limit? Why specific light ie radiation or photons?

He showed that a clock in motion, whizzing past someone at rest, would tick more slowly than a stationary clock at the person's side.

A clock which internal working uses light signals, will not operate (tick) at the speed of light.
Its ticking rate is a function of the speed of the clock. How faster its speed how slower the ticking rate.
This is a physical effect. If two clocks move from A to B, the clock following the longest path will run behind when they meet at B. There after this time delay will stay constant.

And the length of a ruler moving swiftly by would shorten.

The question is if Einstein considered this a physical effect. In order to demonstrate this physical effect you should not use any clock.

Yet for anyone who was traveling at the same speed as the clock and ruler, the passage of time and the length of the ruler would appear normal

This is a tricky sentence.
I expect what they mean is that if you measure the speed light with a clock and a ruler both at rest versus with both moving the result should be the same.

At ordinary speeds, the time-and-space-distorting effects of SR are negligible.

What do they mean with time-and-space-distorting effects? Do they mean that a moving clock runs slower and or that a moving ruler is shorten?

For example, many experiments have confirmed that the decay rate of unstable particles called muons slows by an order of magtitude when they are traveling at clcose to the speed of light.

That is the same as a moving clock but does not include length contraction.

Gödels strange universe

Those speedy clocks and rulers and muons are all racing forward in time.

IMO you should only mention that speedy clocks run slower.

Can they be thrown into reverse?

Or different written: Is it possible that a "moving away" clock runs faster (Comparing with my clock). IMO this is only possible if the concept "absolute" at rest is considered. See also in section "Introduction" absolute concept

The universe Gödel described to his skeptical friend had two properties: It rotated, which provided centrifugal force that prevented gravity from crunching together all the matter in the cosmos, creating the stability Einstein demanded of any cosmic model.

How do you know that the universe is stable?
A stable universe is one which exists forever in the past and in the future everywhere
An expanding universe is in principle not stable.

But it also allowed for time travel into the past, which made Einstein deeply uneasy.

I expect that the center of such a rotating universe is at absolute-rest. A clock at the center of such an rotating universe runs the fastest.
It is important to make a distinction between traveling into the past or in the past.
Traveling into the past always should imply that your clock ticks forward.
Traveling in the past implies that your clock ticks backward, and that is clearly impossible
It should be remembered that a clock is a physical process.

In Gödel's cosmos, space travelers could set out and eventually reach a point in their own past etc.

That is impossible. Gödel should build an appartus to demonstrate this. The same reasoning applies to a perpetual motion machine. Build one to demonstrate that it exists.

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