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Time Travel: some thoughts

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TimFox:
Some recent news about replacing cesium-beam clocks with another process at an ever higher frequency mentioned that the slight difference in frequency between several cesium clocks at NIST in Colorado were due to different altitudes of the systems.

MikeK:

--- Quote from: Bicurico on May 02, 2022, 07:19:42 pm ---Our universe, all that is in it, is just an infinitely small disturbance in the nothingness, a singularity.
--- End quote ---

I don't see how our universe can be infinite when we know to high precision how old it is.  It's very likely that our universe is one among countless.  Also likely that our universe was created by a black hole in another universe...The Big Bang *was* a singularity and a black hole *contains* a singularity.

As far as I know, time travel is not possible because it violates entropy.  Time moves in one direction only.

SiliconWizard:
While "traveling backwards", as in being physically present at some point in the past while being in the present, is a contradiction in itself and can probably only be solved through some kind of metaphysics, observing the past is almost trivial. When you look at someone 1m away from you, what you see of them is already 3.33ns in the past. :horse:

Rick Law:

--- Quote from: eugene on May 05, 2022, 04:37:17 am ---I get that. I'm just surprised that it didn't make the list of things to think about. It's the only thing with any basis at all in science that suggests time travel in any form might be possible.

--- End quote ---

You are right, I should include that - I omitted it because of my own bias.  For one reason or another, somehow, I just consider Tachyon (complex mass) too far a stretch.

So, per request, here is my thought on it

3a. Time travel by time dilation

Per Einstein's relativity, we know and have confirmed that we each have our own personal clock.  My one second is not the same as your one second.  If I travel fast enough, one second for me could be 2 seconds or even years for the rest of the people on Earth.  This is a way to time travel to the future Earth - get into a space craft and accelerate to incredible speed to return to Earth with your time being an hour later but Earth time being a years later.  It is quite possible in theory, but I don't know of any practical way to accelerate anything other than elementary particles to a speed for meaningful time travel (meaningful as in 1 second for you but wrist watch measurable > 1 second for others). 
 
Interesting note:  Current theory is light speed (c) is the universe's speed limit.  There is actually a complication here that is important to point out.  Say the plan route is just a bit (say 1km) under 2 light seconds away.  On Earth, the observer saw me cover that 2 light second (minus 1km) distance in 2 seconds.  Earth observers saw me traveled almost two light seconds in 2 seconds and traveled at speed below c.  All Laws of Physics are obeyed and all is well.  I am the traveler and my clock ran slow, so I could cover that Earth-observer measured 2 light seconds (-1km) in my 1 second.  It would seem I observed myself traveled almost 2x light speed and broke the Laws of Physics.  Not so.  From my traveling frame of reference, Relativity say distances in my direction of travel is compressed accordingly.  My observations is actually myself traveled the distance of 1 light second (minus 0.5km) in 1 second, almost at light speed but not 2x light speed.
 
Current theory say only objects with zero mass can travel as the speed of light (c).  We have mass, and in fact our mass increase as our speed increase.  The closer we approach light speed, the closer our mass increased to near infinity.  So we can't get near light speed with conventional means.  If we could travel at light speed, our clock will stop relative to the rest of the folks on Earth.  We can go anywhere and since our clock doesn't move, we got there instantly.

Interesting note:  For a photon and photons always move at light speed, the moment of it's birth and it's death is the same moment.  For us, we can see that photon being emitted, travel to the moon, 1.3 seconds later, it hit and bounce on the mirror left by our Astronauts and return to earth another 1.3 seconds later and absorb by our photo detector.  For that photon, the birth, the bounce, the death (re-absorption) are all the same moment.  No time has passed in-so-far as the photon is concerned.

To go back in time, we need to travel faster than light speed.  For something to go faster than light speed, it will need to have complex mass - that is, it's mass is an imaginary (complex) number.  Imaginary as in the imaginary number square root of -1.  Tachyon is the name given to the imaginary object (field) with imaginary mass.  Can we use it?  I rather doubt that.  To know more, we have to study Tachyon, but to study it, we have to find it or make it.  Thus far, the possibility exists only in the mathematics.

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