[Bill S]

My studies so far have convinced me of the impossibility of faster than light travel. As well as the impossibility of travel into the past.

After much thought, but no real study, I had reached the point where I concluded that no time travel was possible.  Then, some 20 yrs ago, I decided to try to find out what made some scientists believe it was possible.  I soon realised that time was linked to practically everything else, and I found I was going round in circles.  My wife suggested writing everything down.  I found that the best form of note keeping was to write as though I were trying to explain points to someone else.  20 yrs, and some 250,000 words of (organised) notes later, I am at the same point as you. 

I’m still a bit puzzled by the fact that some scientists seem convinced that past-directed time travel is possible.  Could it be wishful thinking? 

[Bill S]

Zer0, I think you lost me somewhere in your last post.

I'm still a bit stuck on the apparent circularity of the definition of a second.  Hopefully, someone will come up with a simple explanation.

[David Cooper]

Travelling back in time would only be possible with block universe models where the past persists and you can re-enter it, but even then there are problems. In one case there is a circular causality introduced where event one causes event two and event two subsequently causes event one - that's an impossibility. The other way it could happen is where events can change at individual Spacetime locations under an additional Newtonian time, in which case you can go back and "change the past" and thereby lead to the future from that point being changed too as the new chains of causation act forwards from there, at which point the changes to the future will make differences to you when you set off back in time such that you do it differently or don't do it at all, and that leads to new changes being made in the past and another restructuring of events forwards from there, and this would cycle forever with that section of time and its entire future repeatedly being rewritten.

[Bill S]

That’s an interesting line of reasoning, David; I’ll have to give that some thought. 

My initial response would be to ask what, if any, evidence we have that spacetime events are in any way mutable.
 
Even in  “block universe models where the past persists” there are serious problems involved in trying to access that past, much less changing it if you ever got there. 

Another thing that might be worth some thought is the question: “If I go back in time; what happens to you?”  Nothing?  I’m not sure that could be the case.

[David Cooper]

My initial response would be to ask what, if any, evidence we have that spacetime events are in any way mutable.

There is no evidence of it, but there's also no (lasting) evidence against it - any evidence that might be able to emerge would soon become hidden by a rewriting of events.

Even in  “block universe models where the past persists” there are serious problems involved in trying to access that past, much less changing it if you ever got there.

The idea is that you find a way out of the Spacetime block of this universe and re-enter it at a location further back in "time", but to do that without changing the past, your re-entry into it at that point must have happened the first time round too, and as soon as you have a model that accommodates that, it has to tolerate a circularity of causality that is incompatible with real causality.

[Bill S]

The idea is that you find a way out of the Spacetime block of this universe and re-enter it at a location further back in "time", but to do that without changing the past, your re-entry into it at that point must have happened the first time round too, and as soon as you have a model that accommodates that, it has to tolerate a circularity of causality that is incompatible with real causality.

I think we are on the same page there, David.

When I was trying to make some sense of CTCs I found I could not avoid the conclusion that once a loop had formed, there would be no escape from it, and a quasi-infinite accumulation of “versions” of anyone who travelled round the loop would occur at the original start point.  However, because that point is an immutable spacetime event, they must always have been there. 

You can imagine the absurdities into which that threatened to lead me.

[cowlinator (OP)]

And this one you need to expand on " and we would also expect that the CMB radiation would be relatively uniform in temperature. " I guess you consider it following from your first statement?

It does not follow from the first statement, it follows from that fact that the speed of light changes.

If, in the past, the speed of light were greater than it is today, then we would expect that two objects (or sections of fluid, or whatever) would be able to influence each other from farther away than is possible in a constant-c universe.  If the speed of light were great enough, then that would mean that every part of the universe was interacting with every other part of the universe almost instantaneously.  You didn't have to wait 20 minutes for infrared waves to travel 20 light-minutes, it was much quicker. 

This would mean that at that time, the temperature of the entire universe would approach equilibrium much faster in a slowing-c universe than in a constant-c universe.

This would mean that the CMB radiation would be more uniform in temperature in a slowing-c universe than it would be in a constant-c universe.

The discussion would change from "why is the CMB so uniform?", to "why is the CMB so ununiform?"

[Zer0]

Zer0, I think you lost me somewhere in your last post.

I'm still a bit stuck on the apparent circularity of the definition of a second.  Hopefully, someone will come up with a simple explanation.

The second is the base unit of time in the International System of Units. It is qualitatively defined as the second division of the hour by sixty, the first division by sixty being the minute.
Unit system: SI base unit
Unit of: Time
(Wikipedia)
I'm pretty sure this explanation didn't impress you a bit, but hey at least I tried right. 👼

[Zer0]

Time Travel ⌚👣

Realistic Scenario :
New York, NY USA is 13 hours behind Tokyo, Japan.
Calculated air distance between Tokyo n New York is 6,741 miles/10,849 kilometers.

Now...IMAGINE(time for Alice to follow the white rabbit) 🐇

On 31st of December, all of us are having an amazing time partying n dancing around with the Japs in Tokyo.
The clock strikes 12:00am Jan 1st, we wish each other a happy new year n continue partying till around 3:00am n finally get tired n call it quits.

At 5:00am Tokyo time we get onboard NASA's X-43A ScramJet n request the captain to take it easy n cruise at Mach2 speed cause we are in no hurry n want to doze off.
(Mach2 = 2469.6/kmph, flight duration 5hrs, distance covered - 12,345kilometers)

Flight lands in New York 9:00pm local time 31st DEC.
We rest for an hour n reset our wrist watches n get back to partying once again, clock strikes 12:00am once again n hence we wish each other a very happy new year...Once Again!
🎉🎈🎇🎈🎆🎉
(We might have had a blast of a celebration for new year's but we shall all definitely end up having an epic hangover on the 1st of Jan) 😴

[Zer0]

Time Travel ⌚👣

Animated Scenario : 👯➡🐇➡🍥

Imagine, we are all a bunch of Frogs hanging around in the arctic region with a bunch of froggy chicks.
(woodfrog aka rana sylvatica)

Somehow we come across a secret underground hole which leads us into a supermarket, we find an automated refrigerator that can be easily used to simulate winter conditions for a set period of 6 months.

So, we go on a hogging frenzy n eat food n fill up our tummies n stop just before another bite could burst our stomachs.
We then jump into the automated refrigerator, set time for 6months n go into hibernation.

Wake up next season n go out n hang around with a new bunch of chicks, return n repeat the same hog food frenzy automated refrigerator hibernation cycle.

Resurrect next season n go hang around with a newer set of chicks until we're too old n have had more than our fair share respectively n decide to call it quits.

Eventually the smarty scientifically intellectual froggys that we are, we would have managed to live thrice longer than other dumbo frogs n ended up having ten times more fun compared to them. 🐸
(I know it indeed sounds like a lot of fun but, after the first hibernation cycle we might have hung around with our daughters, n by the end of the second cycle those froggy chicks could have been our grand daughters...geez!) 😱😵😱

[Zer0]

Time Travel ⌚👣

MultiVerse Scenario : (use excessive imagination)

IMAGINE, the whole Universe is a Symphony Orchestra hall. 〽
I am a miniscule single musical note, a very small part of a very large symphonic play of grandeur scale. 🎵

Somehow I receive a message of alien origin, trying to trace its originating point I am dragged out of the orchestra hall(out of my universe)

I meet an alien creature from another universe, realizing multiverses are in fact a reality. 🌐
I'm gifted with an iPod sort of a device which the mutiverse civilization beings call an energetron, n they teach me how to use it. 🎦

Now, I plug into the energetron n point it at my universe n I can hear sweet symphony flowing harmoniously.(steady state of universal energy) 🎧

Then I hit the pause button 🔵 and the music stops.
(flow of universal energy stops)
Next I press the rewind button ⏪ n seems like the soundtrack is moving backwards.
(flow of universal energy is reversed)

Considering I'm outside the orchestra auditorium the energetron has no effects on me.
(my tone n sound remain constant) 🎵

Finally I see the orchestra has reached the point where its just about to begin so I hit the pause 🔵 n play button ▶
(just before the big bang) 🎬

And then I comeback to the location of the orchestra hall n take the front seat which is yet to come into formation n await holding my breath until the curtains rise n I would get to watch a harmoniously symphonic universal once in a lifetime performance to which I may never ever be able to stop applauding for.
🎶 🎼 🎻 🎹 🎷 🎺 🎸 🎼 🎶
(no doubt would cherish n love the show, but would also have a shallow hollow feeling of being left out) 🎵

[Bill S]

Zer0, I applaud your imagination. What else can one say? 

Do you want to tackle these points seriously?  If so, we might need a slightly different approach.

[Zer0]

Zer0, I applaud your imagination. What else can one say? 

One could sing a song titled Imagine by John Lennon or simply say Hey U, Dream On!
👼

[Zer0]

Do you want to tackle these points seriously?  If so, we might need a slightly different approach.

Yes I'm up for it, let the games begin. 💪👊
But just to clarify the Rules of the Game, I'm Hoping the discussions would be Strictly pertaining to Scientific Rationality n not Metaphysical or Philosophical in nature. 🚬
Bill[edited] if you resort to Wonderlanding 🍥 theories n start acting all Alicey 👯 with me, then even I'm gonna go full Rabbity White 🐇 on you.

[Bill S]

But just to clarify the Rules of the Game,

[OK.  Rule one.  Refrain from referring to me as "Billy".  There are few things to which I take exception, but being called Billy is one of them  I let it pass first time, but don't make a habit of it.

[Bill S]

Eventually the smarty scientifically intellectual froggys that we are, we would have managed to live thrice longer than other dumbo frogs n ended up having ten times more fun compared to them

How does hidernating make you live longer?

[yor_on]

And this one you need to expand on " and we would also expect that the CMB radiation would be relatively uniform in temperature. " I guess you consider it following from your first statement?

It does not follow from the first statement, it follows from that fact that the speed of light changes.

If, in the past, the speed of light were greater than it is today, then we would expect that two objects (or sections of fluid, or whatever) would be able to influence each other from farther away than is possible in a constant-c universe.  If the speed of light were great enough, then that would mean that every part of the universe was interacting with every other part of the universe almost instantaneously.  You didn't have to wait 20 minutes for infrared waves to travel 20 light-minutes, it was much quicker. 

This would mean that at that time, the temperature of the entire universe would approach equilibrium much faster in a slowing-c universe than in a constant-c universe.

This would mean that the CMB radiation would be more uniform in temperature in a slowing-c universe than it would be in a constant-c universe.

The discussion would change from "why is the CMB so uniform?", to "why is the CMB so ununiform?"

There are several difficulties with that idea Cowlinator. One main come from how a 'Big Bang' is presumed to act. A inflation without a 'center' is not something that need to relate to the geometry we define, and a 'lightspeed' would not be acting the way one might expect there. There is no problem in letting 'c' be 'c' for it to connect 'nodes'. And the 'nodes' making this universe exist is defined by 'c'. You could just as easily imagine a Big Bang to be what creates the connections allowing us a universe to define.  It's very hard discussing a 'small universe' inflating unless one step away from how a 'Big Bang' is thought to act in mainstream physics.

[yor_on]

You might think of it this way. As a inflation containing 'energy', creating a 'hot universe', expanding from and in every 'dot' of its 'known geometry'. But the 'geometry' will be limited by information, and the information will propagate at 'c'. So assuming that this is a evenly isotropic distribution of 'energy', then what  inform other 'nodes' of its existence will firstly be 'photons' and 'waves', as well as other manifestations propagating at that 'speed', and as there is no real definition of a center then it has to happen 'everywhere'. Whether we should state that this happens 'simultaneously' is another question, depending on how you define it, globally (meaning ideally from the viewpoint of a 'God' existing 'outside' this universe) or locally.
=

Presume we define it locally.

In that case the universe we measure in 'light years' from Earth will be getting older and older the further out we go. If you now imagine yourself at the very 'edge' of what we can see, then in a isotropic homogeneous universe you will find the exact same to be true, but now with Earth and its sun being the new edge. Although you won't see them of course, as they are later builds than the information light present you with, you being teleported to that 'edge'. Also I might add, this 'surround view' of a 'universe' will be existent at all directions, both from Earth and at that 'edge', implying a 'infinite universe'.

What one can notice here is that what makes a Big Bang plausible is not the question of how big, or small, this universe might be, or have been. Actually it's the age we're looking at. And that is a entirely different thing.

[cowlinator (OP)]

And this one you need to expand on " and we would also expect that the CMB radiation would be relatively uniform in temperature. " I guess you consider it following from your first statement?

It does not follow from the first statement, it follows from that fact that the speed of light changes.

If, in the past, the speed of light were greater than it is today, then we would expect that two objects (or sections of fluid, or whatever) would be able to influence each other from farther away than is possible in a constant-c universe.  If the speed of light were great enough, then that would mean that every part of the universe was interacting with every other part of the universe almost instantaneously.  You didn't have to wait 20 minutes for infrared waves to travel 20 light-minutes, it was much quicker. 

This would mean that at that time, the temperature of the entire universe would approach equilibrium much faster in a slowing-c universe than in a constant-c universe.

This would mean that the CMB radiation would be more uniform in temperature in a slowing-c universe than it would be in a constant-c universe.

The discussion would change from "why is the CMB so uniform?", to "why is the CMB so ununiform?"

There are several difficulties with that idea Cowlinator. One main come from how a 'Big Bang' is presumed to act. A inflation without a 'center' is not something that need to relate to the geometry we define, and a 'lightspeed' would not be acting the way one might expect there. There is no problem in letting 'c' be 'c' for it to connect 'nodes'. And the 'nodes' making this universe exist is defined by 'c'. You could just as easily imagine a Big Bang to be what creates the connections allowing us a universe to define.  It's very hard discussing a 'small universe' inflating unless one step away from how a 'Big Bang' is thought to act in mainstream physics.

The statement
"The CMB radiation would be more uniform in temperature in a slowing-c universe than it would be in a constant-c universe."
is true in an universe with or without inflation, and is also true in a universe with or without a big bang. 

[yor_on]

I sort of lose sight of how you think here cowlinator?

The CMB has to be 'uniform' if the universe 'inflates', light being of a constant 'c', as it seems to me? Using your definition, if I now understand it correctly, of light being 'slower' now? Then shouldn't early light be blue shifted? If we turn it around and assume a 'slower early light', red shifted? And shouldn't those two both imply different temperature graduations as measured by us, changing the further out (earlier in time) you try to observe.

If we on the other hand accept the idea of a Big Bang happening 'everywhere' with its subsequent 'inflation', stipulating 'c' to be unchanging, then the CMB should be of a uniform measure as I think of it.
=

It might be possible though to test? If we presume a blue shifted early light, uniformly dispersed. I don't know, but presuming that 'c' and other constants give us one value over time, you might be able to prove that this value comes out to 'low' unless the speed of light and other constants using it have changed their value over time. It would be a very theoretical exercise though.