Time, Time Travel, and Closed Timelike Curves

[marduk]

Look up the concept of "four-velocity." We're always moving at C. When we're at rest (which is a relative concept, but useful nevertheless) we're moving at C in units of time: 1 second per second. As we approach a relative of velocity of C through space, the time component approaches zero. But in all reference frames, our four-velocity is constant, C.

I'm with you there - my point is that they're still not equivalent. I'm free to vector in any direction in 3-space relative to an object. I'm not free to vector in any direction with time. I'm constrained to only go forward, at a varying rate relative to something else.

Yep - that's it exactly. The really neat part though, is that the curvature of spacetime induces stress in the "paper," so there's actually energy in the curvature of spacetime. And that's why the equations of general relativity are nonlinear - the mass-energy of an object curves its spacetime environment, and that curvature itself contains energy which further contributes to the gravitational curvature of spacetime a little bit, and on and on ad infinitum. That's why it takes tensor calculus to model the curvature to high precision. But we can get by quite well with the "linearized weak-field-limit equations" in most scenarios - the nonlinear contributions are minuscule until you get close to neutron stars and black holes, where the nonlinear contributions become substantial.

Isn't this what is meant by mass/energy equivalence, though? The energy to warp space comes from matter containing a whole hell of a lot of it.

 

[Thomas R Morrison]

Does this work?

The force of gravity is equivalent to the force of acceleration. The causation of the force are not equivalent, though - me accelerating when I jump off a cliff is because the earth warps space/time (until 'sudden deceleration trauma' when I hit the ground). But me taking the leap off the cliff is because I've converted potential energy in the form of sugar in my muscles into kinetic energy when I jump off of it, accelerating from zero when standing to do so.

Gravity is the bending of space/time (that's not an analogy, that's it's definition within relativity)

That's right, but you have to be careful, you said "the force of acceleration." the force and the acceleration are two different things. Gravity is an acceleration, but not a force. A force is something that you feel, gravity isn't - we only feel gravity because the ground is pushing against our free-fall acceleration.

We can 'spoof' gravity by accelerating something, or spinning it. It will feel the same to the observer. But it's cause isn't the same.

Is that a better answer?

The causes are different, but they're all accelerations generated by space and time relationships, so technically "linear acceleration" and "centripetal acceleration" and "gravitational acceleration" are all just accelerations; they're just produced differently. But their physical characteristics are all equivalent because physics doesn't care how an acceleration is produced, only the magnitude distinguishes them (and in the case of gravitation, the tidal force, which is negligible in the weak-field limit).

 

[mike]

Regardless of the accuracy of the predictions, analogies that work for calculations don't translate to them actually being that way in the real world. The closest example I can think of to illustrate this point is that from a relative point of view, gravity is indistinguishable from acceleration. The math can be worked out identically. To quote Eistein:

"Einstein’s ground-breaking realization (which he called “the happiest thought of my life”) was that gravity is in reality not a force at all, but is indistinguishable from, and in fact the same thing as, acceleration."
However it is obvious that while the math can be worked out to make acceleration and gravity appear to be identical, we can all plainly see that the reason we all stick to the Earth isn't because we're standing on an ever accelerating flat plane hurtling straight ahead through space. The same thing applies to the idea that space is curved. What's happening there is that the math uses the curvature analogy to predict how things behave, but that doesn't necessarily translate to space actually being curved like a flexible rubber sheet.

I think the gravity probe B experiment shows it does


If the object spins, another distortion is introduced, "in the same way as the elastic sheet would be twisted by a spinning heavy wheel on it."
Einstein's Warped View of Space Confirmed

The satellite also confirmed the frame-dragging effect, in which the rotating Earth twists the surrounding spacetime. It's as if the spinning Earth were immersed in honey, Everitt explained. "When it spins, the Earth will drag the honey with it," he said. "In the same way, the Earth drags spacetime with it."

http://www.sciencemag.org/news/2011/05/long-last-gravity-probe-b-satellite-proves-einstein-right


Geodetic effect - Wikipedia
Gravity well - Wikipedia

Does acceleration warp space?

Does a moving object curve space-time as its velocity increases?

 

[mike]

The probability of time travel...............
Is even greater than the EtsH. imo.

Its possible according to the TOR.

So if its been invented just once, it might have been on another world a million years ago, or a million years from now. Its then a reality right now......................

 

[marduk]

D’oh!

Nice catch Tomas, wasn’t careful with my words.

 

[marduk]

The probability of time travel...............
Is even greater than the EtsH. imo.

Its possible according to the TOR.

So if its been invented just once, it might have been on another world a million years ago, or a million years from now. Its then a reality right now......................

Sorry, how is it possible?

 

[Thomas R Morrison]

I'm with you there - my point is that they're still not equivalent. I'm free to vector in any direction in 3-space relative to an object. I'm not free to vector in any direction with time. I'm constrained to only go forward, at a varying rate relative to something else.

They're equivalent - think of it this way: you can only move with a positive velocity; the change in position is always a positive unit of space per unit time. You can't have a negative velocity. So just like linear motion is always a positive unit of space per unit time, motion through time is always positive as well (in special relativity - it gets messier with GR).

Isn't this what is meant by mass/energy equivalence, though? The energy to warp space comes from matter containing a whole hell of a lot of it.

Yep. This raises an interesting question about inertial mass, actually - I'm not sure that matter would have any inertial mass if it didn't produce a gravitational field. Just as an electrical charge requires energy to accelerate it, which is stored in the creation of the magnetic field, matter may only have inertial mass because the gravitoelectric charge undergoing acceleration produces and gravitomagnetic field. The analogy between electric charge and gravitational charge is actually very fascinating, and has been a topic of great interest ever since Oliver Heaviside first explored the strange analogy between the two back in the late 19th century. Even today the laws of electromagnetic induction are commonly used to explore analogous phenomena with gravitation - only the sign of the interaction is flipped (and the common convention is limited to the weak field limit to avert the nonlinearities of GR).

The probability of time travel...............
Is even greater than the EtsH. imo.

Its possible according to the TOR.

So if its been invented just once, it might have been on another world a million years ago, or a million years from now. Its then a reality right now......................

I wouldn't say "greater" than the EtsH - the technology is more or less equivalent. It's easier to produce warp field propulsion than it is time travel into the past, and there are probably lots more civilizations that can travel here, than our own single timeline (and we have to consider that we may not make it far enough to achieve either warp field propulsion or time travel.) So while it's possible that devices and/or travelers could arrive here from our our Earthly future, and that may account for some of the exotic aerial devices we've observed, I suspect that most of the sightings are of extraterrestrial origin.

 

[mike]

Sorry, how is it possible?

General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. The equations, however, might be difficult to physically achieve.
Also, under Einstein's theory of general relativity, gravity can bend time.

Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board.

Time Travel Machine Outlined
Time Travel: Theories, Paradoxes & Possibilities
[1310.7983] The Blue Box White Paper

 

[mike]

And on the flip side.

Smolin: A star that collapses into a black hole very quickly squeezes down to infinite density and time stops — that's according to general relativity. And basically that moment when time stops is deferred by quantum mechanics, by quantum uncertainty, and rather than collapsing to infinite density, the star collapses to a certain extreme density, and then bounces back and begins to expand again. And that expanding star becomes the birth of a new universe. The point where time ends inside a black hole becomes joined to the point where time begins in a Big Bang in a new universe.
Do Black Holes Create New Universes? Q&A With Physicist Lee Smolin

 

[marduk]

They're equivalent - think of it this way: you can only move with a positive velocity; the change in position is always a positive unit of space per unit time. You can't have a negative velocity. So just like linear motion is always a positive unit of space per unit time, motion through time is always positive as well (in special relativity - it gets messier with GR).

Sure I can have negative velocity. Velocity is a vector, right? So if I’m moving in a negative direction, I have negative velocity.

I can’t have negative speed, is that what you mean?

 

[marduk]

General relativity also provides scenarios that could allow travelers to go back in time, according to NASA. The equations, however, might be difficult to physically achieve.
Also, under Einstein's theory of general relativity, gravity can bend time.

Picture a four-dimensional fabric called space-time. When anything that has mass sits on that piece of fabric, it causes a dimple or a bending of space-time. The bending of space-time causes objects to move on a curved path and that curvature of space is what we know as gravity.

Both the general and special relativity theories have been proven with GPS satellite technology that has very accurate timepieces on board.

Time Travel Machine Outlined
Time Travel: Theories, Paradoxes & Possibilities
[1310.7983] The Blue Box White Paper

That top link didn’t work for me, but I think maybe you’re referring to CTCs created by things like rotating wormholes?

The basic question is that even if it may be possible to do so, it would be really hard, and you might not be able to change or do anything when you got there.

If you think of the universe as a state machine, you’d have to still allow it to return to the state that allowed you to use the time machine to begin with.

Chaos is a big problem there, and one we have no idea how to get around to my knowledge - unless you go back to the MWH.

 

[marduk]

And on the flip side.

Smolin: A star that collapses into a black hole very quickly squeezes down to infinite density and time stops — that's according to general relativity. And basically that moment when time stops is deferred by quantum mechanics, by quantum uncertainty, and rather than collapsing to infinite density, the star collapses to a certain extreme density, and then bounces back and begins to expand again. And that expanding star becomes the birth of a new universe. The point where time ends inside a black hole becomes joined to the point where time begins in a Big Bang in a new universe.
Do Black Holes Create New Universes? Q&A With Physicist Lee Smolin

Time doesn’t quite ever get to zero in the black hole relative to the universe. Just really close to it.

It’s asymptotic.

 

[mike]

Traversable Achronal Retrograde Domains In Spacetime
Benjamin K. Tippett
∗
University of British Columbia, Okanagan, 3333 University Way, Kelowna BC V1V 1V7, Canada
David Tsang
†
Physics Department, McGill University, Montreal, QC, H3A 2T8, Canada
There are many spacetime geometries in general relativity which contain closed timelike curves. A layperson
might say that retrograde time travel is possible in such spacetimes. To date no one has discovered a spacetime
geometry which emulates what a layperson would describe as a time machine

. The purpose of this paper is to propose such a space-time geometry.
In our geometry, a bubble of curvature travels along a closed trajectory. The inside of the bubble is Rindler
spacetime, and the exterior is Minkowski spacetime. Accelerating observers inside of the bubble travel along
closed timelike curves. The walls of the bubble are generated with matter which violates the classical energy
conditions. We refer to such a bubble as a Traversable Achronal Retrograde Domain In Spacetime.

I. INTRODUCTION
A. Exotic spacetimes
How would one go about building a time machine? Let us begin with considering exactly what one might mean by “time machine?” H.G. Wells (and his successors) might describe an apparatus which could convey people “backwards in time”. That is to say, convey them from their current location in spacetime to a point within their own causal past. If we describe spacetime as a river-bed, and the passage of time as our unrelenting flow along our collective worldlines towards our future; a time machine would carry us back up-stream. Let us describe such motion as retrograde time travel
.
In the context of general relativity, just as in the river, it is the underlying geometry which determines where the world-
lines can flow. In general, it is possible for spacetime geometry to permit retrograde time travel

https://arxiv.org/pdf/1310.7985.pdf

 

[Thomas R Morrison]

And on the flip side.

Smolin: A star that collapses into a black hole very quickly squeezes down to infinite density and time stops — that's according to general relativity. And basically that moment when time stops is deferred by quantum mechanics, by quantum uncertainty, and rather than collapsing to infinite density, the star collapses to a certain extreme density, and then bounces back and begins to expand again. And that expanding star becomes the birth of a new universe. The point where time ends inside a black hole becomes joined to the point where time begins in a Big Bang in a new universe.
Do Black Holes Create New Universes? Q&A With Physicist Lee Smolin

Lee Smolin is one of my favorite physicists on the planet - I'm reading his marvelous "The Trouble with Physics" right now. It's kind of hilarious to read the way a true, sensible physicist makes minced meat of superstring theory =D But he has lots of crystal clear insights into all kinds of fundamental physics topics.

Thanks mike, I look forward to reading that.

Time doesn’t quite ever get to zero in the black hole relative to the universe. Just really close to it.

It’s asymptotic.

Time actually stops at the event horizon (which is why it's called the "event" horizon - time ceases to pass from our reference point, at the horizon). Interestingly, and pertinent to our discussion here, space and time actually switch roles inside of the event horizon: "forward in time" becomes "forward in space - toward the singularity," which is why no paths lead out of the black hole.

Sure I can have negative velocity. Velocity is a vector, right? So if I’m moving in a negative direction, I have negative velocity.

I can’t have negative speed, is that what you mean?

That's right - you can't move through a negative distance of space: distance is always positive, just as time is always positive (until you get into weird situations with GR). The choice of "positive" and "negative" with regard to vectors is purely arbitrary, and therefore physically meaningless.

 

[marduk]

Lee Smolin is one of my favorite physicists on the planet - I'm reading his marvelous "The Trouble with Physics" right now. It's kind of hilarious to read the way a true, sensible physicist makes minced meat of superstring theory =D But he has lots of crystal clear insights into all kinds of fundamental physics topics.

Thanks mike, I look forward to reading that.


Time actually stops at the event horizon (which is why it's called the "event" horizon - time ceases to pass from our reference point, at the horizon). Interestingly, and pertinent to our discussion here, space and time actually switch roles inside of the event horizon: "forward in time" becomes "forward in space - toward the singularity," which is why no paths lead out of the black hole.

I don’t think time stops at the event horizon.

I think the event horizon is the location whereby no event can take you back outside of it. Typically, Light is used - where light can’t escape a black hole, we say that is the event horizon. Interestingly, no event inside the horizon can impact events outside of the horizon as a consequence.

Time gets very close to stopping, but not quite. It tends to zero. There is no way that I know of to absolutely stop time.
In general relativity (GR), does time stop at the event horizon or in the central singularity of a black hole?

 

[mike]

There is no way that I know of to absolutely stop time.

In short, we don't know. There are a few indications that time started at the big bang, or at least it had some form of discontinuity. This might be wrong though.

• According to General Relativity, there is no such thing as an absolute time. Time is always relative to an observer, without the universe there would be no corresponding concept of time. All observers within the universe would have their clocks "slowed down" the nearer they are to the big bang (nearer in time). At the big bang point, their clock would stop. This said, we know that GR doesn't apply as-is all the way to the Big Bang.
  
• Some cosmological theories like CCC predict a series of aeons and some form of cyclic universe. These predict a discontinuity (CCC predicts a conformal scale change) of time at the big bang, and at the end of the universe.

Did time exist before the creation of matter in the universe?

 

[mike]

The Quantum Zeno Effect Explains How You Can Stop Time Using Physics | Inverse

 

[Thomas R Morrison]

I don’t think time stops at the event horizon.

I think the event horizon is the location whereby no event can take you back outside of it. Typically, Light is used - where light can’t escape a black hole, we say that is the event horizon. Interestingly, no event inside the horizon can impact events outside of the horizon as a consequence.

Time gets very close to stopping, but not quite. It tends to zero. There is no way that I know of to absolutely stop time.
In general relativity (GR), does time stop at the event horizon or in the central singularity of a black hole?

That's just a semantic technicality (which physicists love - most of them seem to take great pride in being annoying), because you can't actually *see* time stop. And ultimately, this is where the "relativity" in "general relativity" comes in. There is no event horizon for the object falling into a black hole - the rate of "proper time" for the object never changes; the object just gets drawn faster and faster toward the singularity until the tidal forces shred it into quarks and then it goes splat. But to an external observer, their "coordinate time" makes it look like time is passing more slowly for the in-falling object: it gets more and more red-shifted as it approaches the event horizon, and it never reaches the event horizon from the POV of the observer. This is why people say that "time stops" at the event horizon - it's just an illusion produced by the shifting angle of the reference frames, but by all outward indications, time appears to slow to a stop at the event horizon:

"So if you, watching from a safe distance, attempt to witness my fall into the hole, you'll see me fall more and more slowly as the light delay increases. You'll never see me actually get to the event horizon. My watch, to you, will tick more and more slowly, but will never reach the time that I see as I fall into the black hole. Notice that this is really an optical effect caused by the paths of the light rays."
What happens to you if you fall into a black hole

 

[marduk]

That's just a semantic technicality (which physicists love - most of them seem to take great pride in being annoying), because you can't actually *see* time stop. And ultimately, this is where the "relativity" in "general relativity" comes in. There is no event horizon for the object falling into a black hole - the rate of "proper time" for the object never changes; the object just gets drawn faster and faster toward the singularity until the tidal forces shred it into quarks and then it goes splat. But to an external observer, their "coordinate time" makes it look like time is passing more slowly for the in-falling object: it gets more and more red-shifted as it approaches the event horizon, and it never reaches the event horizon from the POV of the observer. This is why people say that "time stops" at the event horizon - it's just an illusion produced by the shifting angle of the reference frames, but by all outward indications, time appears to slow to a stop at the event horizon:

"So if you, watching from a safe distance, attempt to witness my fall into the hole, you'll see me fall more and more slowly as the light delay increases. You'll never see me actually get to the event horizon. My watch, to you, will tick more and more slowly, but will never reach the time that I see as I fall into the black hole. Notice that this is really an optical effect caused by the paths of the light rays."
What happens to you if you fall into a black hole

I disagree.

It’s really actually a fundamental question. There’s a big difference between zero relative time and really close to zero relative time.

It’s the same difference between really big and infinite. Or countably infinite or uncountably infinite.

It’s not just an optical effect, either. If you were to fall into a black hole, you could watch the galaxy around you wither and die, and even maybe the heat death of the universe. But it wouldn’t all happen at once.

If you got close to the horizon and backed back out, you’d be in the future. But not infinitely far.

 

[ALPHA ROMEO UNIFORM]

Thomas,
We might not be able to see time but can we feel the change in temperature when it occurs. Late Ingo Swan and his science work inside laboratory studies?