Time Travel/Bootstrap Paradox?

[Hans Blaster]

Edited my post a bit that you quoted.

We could record the two locations on different camera's or instruments and later on compare. There's a way to do it for those who are truly interested in it and/or truly care about it, etc.

What two locations? The distant event and the observer? Two observers? Two different distant events?

They are all still subject to the travel time for light.

Either way, however far away it is is how much were seeing it in the past, and to see it as it "now", you have to account for that, etc. And the universe is the same equal age, equally everywhere, etc.

We do account for it. If you want to see "now" in some distant location you have to wait for the signal to arrive. Some things are so far away that we will never see their now because of the expansion of the universe.

Now if that's too confusing for you, or confuses you too much, then I am truly, truly sorry.

Things work a lot better when you use standard terminology.

 

[SelfSim]

I can see this thread has now become about:

'I demand that responders to my posts tell me what I mean when I string a bunch of words together in an incomprehensible way .. and they'd better get it right .. or else!'

Its more about illogical semantics expressed with attitude .. than it is about Physics.

 

[Aryeh Jay]

I avoid all these paradoxes by traveling to different realities in the multiverse when I go back.

 

[SelfSim]

For anyone still interested in wrapping their minds around the details, the below diagrams and their accompanying explanatory words, may be helpful.

They include an example of a galaxy located at the edge of the observable universe (the Hubble radius or the Hubble sphere).

- The particle horizon is the boundary of the observable universe, representing the farthest distance from which light could have traveled to us since the Big Bang. It defines the limit of what we can see because light from objects beyond this distance hasn't had enough time to reach us.

- The (cosmological) Event horizon marks the boundary beyond which events can no longer affect an observer, even in the infinite future. It represents the furthest distance from which light emitted now could potentially reach the observer. (Note this is different from the particle horizon, which defines the furthest distance from which light emitted in the past, has already reached the observer).

The black solid lines indicate our present position. The blue line is the particle horizon through time, the red line is the event horizon, the green area is the Hubble sphere. The dotted black line is a co-moving galaxy that is currently located on the Hubble radius. Its photons that we observe today have travelled on the cyan path (they were emitted at =4.3Gy). Its photons that it emits today (=13.8Gy) will travel on the purple path (they will reach us at =49Gy).

 

[GoldenKingGaze]

One may consider a machine that can go forwards in time, collect information and came back after it left. There would be no time warp but it may change the future.

I think the past has infinite enertia. You could either not go back, past a barrier as tough as a black hole, or if you could go back to the crucifixion for example, people would not be able to see you and you could not gain attention by standing in front of them, they'd just knock you down. No change, not normal matter anymore.

To change something in the past would take an enormous amount of energy. It would have to result in two different organizations of atoms, as now and the new alternate, somehow in the same space, because reality now can't change, it has already happened. We are and can't cease to exist via differently arranged marriages and fights...

In an NDE one lady experienced seeing the crucifixion. She couldn't change anything.

 

[SelfSim]

For anyone still interested in wrapping their minds around the details, the below diagrams and their accompanying explanatory words, may be helpful.

They include an example of a galaxy located at the edge of the observable universe (the Hubble radius or the Hubble sphere).

- The particle horizon is the boundary of the observable universe, representing the farthest distance from which light could have traveled to us since the Big Bang. It defines the limit of what we can see because light from objects beyond this distance hasn't had enough time to reach us.

- The (cosmological) Event horizon marks the boundary beyond which events can no longer affect an observer, even in the infinite future. It represents the furthest distance from which light emitted now could potentially reach the observer. (Note this is different from the particle horizon, which defines the furthest distance from which light emitted in the past, has already reached the observer).

The black solid lines indicate our present position. The blue line is the particle horizon through time, the red line is the event horizon, the green area is the Hubble sphere. The dotted black line is a co-moving galaxy that is currently located on the Hubble radius. Its photons that we observe today have travelled on the cyan path (they were emitted at =4.3Gy). Its photons that it emits today (=13.8Gy) will travel on the purple path (they will reach us at =49Gy).

View attachment 365598
View attachment 365599

So, further to my above post, Cosmic Redshift is related to the time dilation and can be seen directly from the above diagrams, as follows.

Since;
1+ z = a(t0b)/a(tem)

where z is redshift;
a(t0b) is the scale factor at the time of observation;
a(tem) is the scale factor at the time of emission

The light from the co-moving galaxy at the current Hubble radius that we observe today, (from the diagrams), was emitted when;

a(tem)=0.403, and thus, z=1.48, and the observed events from this galaxy are thereby time dilated by a factor 2.48.

So, the light it emits today will be observed when a(t0b)=8.07, with redshift z=7.07.

Time dilations of this magnitude have also been observed, ie: we literally see waayyy distant supernovae explode in slow motion.

(I'm still wrapping my mind around the even broader picture, (see below), where the scale factor and redshift are related to the other horizons/light cones etc, using the same colour scheme as in the co-moving diagram):

 

[sjastro]

For anyone still interested in wrapping their minds around the details, the below diagrams and their accompanying explanatory words, may be helpful.

They include an example of a galaxy located at the edge of the observable universe (the Hubble radius or the Hubble sphere).

- The particle horizon is the boundary of the observable universe, representing the farthest distance from which light could have traveled to us since the Big Bang. It defines the limit of what we can see because light from objects beyond this distance hasn't had enough time to reach us.

- The (cosmological) Event horizon marks the boundary beyond which events can no longer affect an observer, even in the infinite future. It represents the furthest distance from which light emitted now could potentially reach the observer. (Note this is different from the particle horizon, which defines the furthest distance from which light emitted in the past, has already reached the observer).

The black solid lines indicate our present position. The blue line is the particle horizon through time, the red line is the event horizon, the green area is the Hubble sphere. The dotted black line is a co-moving galaxy that is currently located on the Hubble radius. Its photons that we observe today have travelled on the cyan path (they were emitted at =4.3Gy). Its photons that it emits today (=13.8Gy) will travel on the purple path (they will reach us at =49Gy).

View attachment 365598
View attachment 365599

This looks very familiar, my knowledge of particle and event horizons comes from Tamara Davis PhD thesis which has been condensed in this version.

https://arxiv.org/pdf/astro-ph/0310808

She frequently appears on the ABC (the Australian channel) in science programs.

 

[SelfSim]

This looks very familiar, my knowledge of particle and event horizons comes from Tamara Davis PhD thesis which has been condensed in this version.

https://arxiv.org/pdf/astro-ph/0310808

She frequently appears on the ABC (the Australian channel) in science programs.

Yes .. thanks for that! As I was reading another dissertation on all this, I was reminded of that very paper (which I poured over years ago, now). Everytime this subject comes up, I learn a bit more than what I got from the previous time.

The diagrams I posted however, came from a Physics Forum debate from someone who can clearly navigate their way through the math and the physical theory side also. I decided to post them because they looked familiar from distant (& dormant) memories of the Davis/Lineweaver paper) ... and the colours also helped to make them clearer.