[jeffreyH]

If a mass were to travel at c then length in the direction of travel would be infinitely compressed and time infinitely dilated.

Just a thought, here, Jeffrey; isn't this on a par with arguing that photons don't "experience" time?  An argument to support this would be based on relativity, which doesn't permit massive objects to travel at c; so if it did happen, we would not be able to predict what it would be like.

If you consider length contraction to be valid then at the speed of light coordinate length in the direction of travel reduces to zero. If there is no physical volume then no events can occur. It isn't a case of what a photon 'experiences'. But as you say this is fantasy physics.

[alancalverd]

In each scenario there is a hypothetical space craft, travelling at c

There's the problem. To accelerate a finite mass to c, you need an inifinte amount of energy. This can only be acquired by annihilating all the mass in the observable universe, so the presence of a hypothetical mass travellng at c negates the possibility of there being an external observer - or even the spacecraft itself.

[Bill S (OP)]

Alan, you are, of course, right, it is an impossible scenario within the confines of our current scientific knowledge; of which I was not presuming to try to push the boundaries.   I put the OP in two discussion forums at the same time and, between the two, have found several things to think about. For me this is worth doing.   

This can only be acquired by annihilating all the mass in the observable universe,

This is a good example of what I mean.  Apart from providing another example of why nothing massive can accellerate to c, the thought that arises from this is: “Does this not assume that the observable Universe is infinite?” 

[Bill S (OP)]

If a mass were to travel at c then length in the direction of travel would be infinitely compressed and time infinitely dilated. So in effect you would go nowhere during an infinite time period.

Could this not be interpreted as saying that you would complete your journey in a period of no time?  In other words, you would be in two places at once.

[jeffreyH]

If a mass were to travel at c then length in the direction of travel would be infinitely compressed and time infinitely dilated. So in effect you would go nowhere during an infinite time period.

Could this not be interpreted as saying that you would complete your journey in a period of no time?  In other words, you would be in two places at once.

I think you are more likely to be everywhere at once.

[Bill S (OP)]

I think you are more likely to be everywhere at once.

How much thought have you given to the full implication of that, Jeffrey?

[jeffreyH]

I think you are more likely to be everywhere at once.

How much thought have you given to the full implication of that, Jeffrey?

None.

[Bill S (OP)]

Let’s take just one consideration.  By “everywhere” presumably you mean everywhere in the Universe.  In which case, do you spread out to fill the Universe, or does the Universe, conceptually, contract to “match” you; as might be consistent with Bohm’s “implicate order”, where you are the Universe?

[jeffreyH]

I am not sure what to think about Bohm's ideas. If you have infinite energy do you required an infinite space to contain it?

[Bill S (OP)]

However the question is still open because physicists have realized a way to do it but it requires the use of two cosmic strings passing each other and that's not something one can control and nobody has ever detected a cosmic string yet.

I’m glad you mentioned that, Pete.  I met this idea a few years ago when I read Gott’s “Time Travel in Einstein’s Universe”.  I had difficulties with it then, and would appreciate some guidance. 

My understanding is that the two strings must each be “moving at 0.9999992 times the speed of light”, so your ship has to be travelling at this speed, just to keep pace with the string. 

In the infinitesimal moment when these strings are close enough to make your manoeuvre “possible”, you have to overtake the string without reaching “c”.  You have to pass in front of it without being hit by a string the mass of which is at least 4 x 10^16 tons per inch, travelling at almost c.

After changing direction, you have to pursue the other string, that is travelling at  0.9999992 c, and has a good head start on you.  Again your speed must remain below c. 

Isn’t there something about scenarios needing to be possible.

This means that you're scenario is impossible,

[Bill S (OP)]

I am not sure what to think about Bohm's ideas. If you have infinite energy do you required an infinite space to contain it?

That would seem to be a logical conclusion, but I think that in the implicate order the energy (like everything else) "is" the Universe.