Time, Time Travel, and Closed Timelike Curves

[mike]

That’s where the quantum Zeno Effect — a.k.a. Turing’s Paradox — comes in. Taking its name from Zeno’s arrow paradox (a moving arrow can’t actually be seen moving in any single instant, which means it’s not really moving at all), this idea basically states that if you never stop observing a particle that undergoes decay, then that particle will never decay. If that particle never decays, you’ve basically stopped it from doing anything. You’ve stopped time.

Quantum Zeno effect. The quantum Zeno effect (also known as the Turing paradox) is a feature of quantum mechanical systems allowing a particle's time evolution to be arrested by measuring it frequently enough with respect to some chosen measurement setting.

Experiments and discussion
Experimentally, strong suppression of the evolution of a quantum system due to environmental coupling has been observed in a number of microscopic systems.

In 1989, David J. Wineland and his group at NIST[33] observed the quantum Zeno effect for a two-level atomic system that was interrogated during its evolution. Approximately 5000 9Be+ ions were stored in a cylindrical Penning trap and laser cooled to below 250 mK. A resonant RF pulse was applied which, if applied alone, would cause the entire ground state population to migrate into an excited state. After the pulse was applied, the ions were monitored for photons emitted due to relaxation. The ion trap was then regularly "measured" by applying a sequence of ultraviolet pulses, during the RF pulse. As expected, the ultraviolet pulses suppressed the evolution of the system into the excited state. The results were in good agreement with theoretical models. A recent review describes subsequent work in this area.[34]

In 2001, Mark G. Raizen and his group at the University of Texas at Austin observed the quantum Zeno effect for an unstable quantum system,[35] as originally proposed by Sudarshan and Misra.[1] They also observed an anti-Zeno effect. Ultracold sodium atoms were trapped in an accelerating optical lattice and the loss due to tunneling was measured. The evolution was interrupted by reducing the acceleration, thereby stopping quantum tunneling. The group observed suppression or enhancement of the decay rate, depending on the regime of measurement.

In 2015, Mukund Vengalattore and his group at Cornell University demonstrated a quantum Zeno effect as the modulation of the rate of quantum tunnelling in an ultra-cold lattice gas by the intensity of light used to image the atoms.[36]

The Quantum Zeno Effect is used in commercial atomic magnetometers and naturally by birds' magnetic compass sensory mechanism (magnetoreception).[37]

It is still an open question how closely one can approach the limit of an infinite number of interrogations due to the Heisenberg uncertainty involved in shorter measurement times. It has been shown, however, that measurements performed at a finite frequency can yield arbitrarily strong Zeno effects.[38] In 2006, Streed et al. at MIT observed the dependence of the Zeno effect on measurement pulse characteristics.[39]

The interpretation of experiments in terms of the "Zeno effect" helps describe the origin of a phenomenon. Nevertheless, such an interpretation does not bring any principally new features not described with the Schrödinger equation of the quantum system.[40][41]

Even more, the detailed description of experiments with the "Zeno effect", especially at the limit of high frequency of measurements (high efficiency of suppression of transition, or high reflectivity of a ridged mirror) usually do not behave as expected for an idealized measurement.[17]

It was shown that the Quantum Zeno effect persists in the many-worlds and relative states interpretations of quantum mechanics.[42]
Quantum Zeno effect - Wikipedia

'Zeno effect' verified: Atoms won't move while you watch | Cornell Chronicle


It's not just theoretical. The Cornell experiment is just the latest in a series of experiments since then confirming that the quantum Zeno effect really happens. (There's also an "Anti-Zeno Effect," whereby staring at the metaphorical quantum pot brings it to a boil more quickly -- also experimentally confirmed.)
Quantum 'Weeping Angel' Effect Freezes Atoms In Place

 

[Thomas R Morrison]

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.

I don't think you're understanding this clearly. To an observer in coordinate time, the object never reaches the event horizon, as in forever, as in infinite time.

And he wasn't saying that the time dilation is an optical effect, he was saying that the appearance to the coordinate observer that the object never reaches the event horizon is an optical effect - from the POV of the in-falling object, it falls into the singularity without missing a beat. But the observer can't see that happen because the length of the light-path from the event horizon (which in itself is an optical effect induced by the observer being in a rotated reference frame wrt the black hole) to the observer is infinite in length - that's why the external observer can never see the object reach the event horizon.

And yes, to the best of our current understanding, time for the in-falling object looking outward increases until it moves infinitely fast when it reaches the singularity and the universe blinks out of existence (but of course that could never be measured because any clock would be shredded into subatomic particles long before it reached the singularity). A theory of quantum gravity might offer a way out of that conclusion, but GR doesn't. Another way out could be the black hole evaporation that Hawking has proposed, but on those kinds of timescales cosmic acceleration would probably have resulted in the "Big Rip" scenario, so if an observer could survive arriving at the singularity, he would only see an empty lightless universe when the black hole ultimately evaporated.

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?

I'm not sure what the connection here is blowfish, but it's important to note that the "proper time," aka "the rate of time witnessed by an observer locally" never changes - your own watch always appears to run at the same rate no what reference frame you're in. So you can't ever see your own watch slow down, or feel any time-related changes, because for you, personally, it's a constant. So you can't ever experience aging backwards, for example. It's only when you compare your own reference frames to others that these weird relativistic effects come into play.

 

[mike]

But there may be an out to be found in general relativity, Einstein’s theory of gravity that unites space and time as “spacetime”, which curves in the presence of mass. It allows for the possibility of wormholes – a kind of tunnel through spacetime connecting otherwise very distant parts of the universe.


If the “mouths” of the wormhole are moving relative to one another, then traversing the bridge between different points in space would also take a traveller to a different point in time to that in which she started.


However it would still be impossible to go back further in time than the point at which the wormhole was created, limiting the options for travel somewhat - and possibly explaining why we haven’t encountered any visitors from the future. If any natural wormholes were formed in the Big Bang, it might be possible to travel to a limited number of points in the past and in the distant universe, but wouldn’t enable one to flit around the cosmos at will as the Doctor seems to do.

http://www.physics.org/explore-results-all.asp?currentpage=1&q=time travel

Ori's latest research suggests time machines are possible without exotic matter, eliminating a barrier to time travel. His work begins with a donut-shaped hole enveloped within a sphere of normal matter.

"We're talking about these closed loops of time, and the simplest kind of closed loops are circles, which is why we have this ring-shaped hole," Ori explained.

Inside this donut-shaped vacuum, space-time could get bent upon itself using focused gravitational fields to form a closed time-like curve. To go back in time, a traveler would race around inside the donut, going further back into the past with each lap.

"The machine is space-time itself," Ori said. "If we were to create an area with a warp like this in space that would enable time lines to close on themselves, it might enable future generations to return to visit our time."

Ori emphasized one significant limitation of this time machine—"it can't be used to travel to a time before the time machine was constructed." His findings are detailed in the Aug. 3 issue of the journal Physical Review D.

But what if you used a natural wormhole to take one of these machines to a temporal co-ordinate of your choice.............................

Put one back at the big bang, and another at the restaurant at the end of the universe..........

 

[mike]

Timespeak

You and I know it as a time machine. Physicists, on the other hand, call it a "closed timelike curve." Below, feast on the concepts and conjectures, the dialects and definitions that physicists rely on when musing about the possibility of time travel. If this list only whets your appetite for more, we recommend you have a gander at the book from which we excerpted this glossary: Black Holes and Time Warps: Einstein's Outrageous Legacy, by Kip S. Thorne (Norton, 1994).
NOVA Online | Time Travel | Timespeak

 

[mike]

Recent experiments offer tentative support for time travel's feasibility—at least from a mathematical perspective. The study cuts to the core of our understanding of the universe, and the resolution of the possibility of time travel, far from being a topic worthy only of science fiction, would have profound implications for fundamental physics as well as for practical applications such as quantum cryptography and computing.

Closed timelike curves
The source of time travel speculation lies in the fact that our best physical theories seem to contain no prohibitions on traveling backward through time. The feat should be possible based on Einstein's theory of general relativity, which describes gravity as the warping of spacetime by energy and matter. An extremely powerful gravitational field, such as that produced by a spinning black hole, could in principle profoundly warp the fabric of existence so that spacetime bends back on itself. This would create a "closed timelike curve," or CTC, a loop that could be traversed to travel back in time.
"It's intriguing that you've got general relativity predicting these paradoxes, but then you consider them in quantum mechanical terms and the paradoxes go away," says University of Queensland physicist Tim Ralph. "It makes you wonder whether this is important in terms of formulating a theory that unifies general relativity with quantum mechanics."

Time Travel Simulation Resolves “Grandfather Paradox”

 

[mike]

I see the grandfather paradox in the non quantum context like this.

Imagine a rich kid gets given a muscle car for his 16th birthday. He takes it for a spin and before he even gets to the end of the street he hits a brick wall at 130mph and is killed.

Now take 100 time machines with pilots, 10 of these pilots go back and kill their grandfathers when the grandfathers are 2 years old. 10 of those pilots have hit the wall and no longer exist.

The rest "driving carefully" have no such issues.

If for example you traveled to another inhabited planet and time traveled up and down its timeline sightseeing, you wouldn't have any problems.

Don't kill your grandad and everything will be fine.

 

[mike]

"Time travel into the past, which is what people usually mean by time travel, is a much more uncertain proposition. There are many solutions to Einstein's equations of General Relativity that allow a person to follow a timeline that would result in her (or him) encountering herself--or her grandmother--at an earlier time. The problem is deciding whether these solutions represent situations that could occur in the real universe, or whether they are mere mathematical oddities incompatible with known physics. No experiment or observation has ever indicated that time travel is occurring in our universe. Much work has been done by theoretical physicists in the past decade to try to determine whether, in a universe that is initially without time travel, one can build a time machine--in other words, if it is possible to manipulate matter and the geometry of space-time in such a way as to create new paths that circle back in time.

"How could one build a time machine? The simplest way currently being discussed is to take a wormhole (a tunnel connecting spatially separated regions of space-time) and give one mouth of the wormhole a substantial velocity with respect to the other. Passage through the wormhole would then allow travel to the past.

"Easily said--but where does one obtain a wormhole? Although the theoretical properties of wormholes have been extensively studied over the past decade, little is known about how to form a macroscopic wormhole, large enough for a human or a spaceship to pass through. Some speculative theories of quantum gravity tell us that space-time has a complicated, foamlike structure of wormholes on the smallest scales--10^-33 centimeter, or a billion billion times smaller than an electron. Some physicists believe it may be possible to grab one of these truly microscopic wormholes and enlarge it to usable size, but at present these ideas are all very hypothetical.

"Even if we had a wormhole, would nature allow us to convert it into a time machine? Stephen Hawking has formulated a "Chronology Protection Conjecture," which states that the laws of nature prevent the creation of a time machine. At the moment, however, this is just a conjecture, not proven.

"Theoretical physicists have studied various aspects of physics to determine whether this law or that might protect chronology and forbid the building of a time machine. In all the searching, however, only one bit of physics has been found that might prohibit using a wormhole to travel through time. In 1982, Deborah A. Konkowski of the U.S. Naval Academy and I showed that the energy in the vacuum state of a massless quantized field (such as the photon) would grow without bound as a time machine is being turned on, effectively preventing it from being used. Later studies by Hawking and Kip S. Thorne of Caltech have shown that it is unclear whether the growing energy would change the geometry of space-time rapidly enough to stop the operation of the time machine. Recent work by Tsunefumi Tanaka of Montana State University and myself, along with independent research by David Boulware of the University of Washington, has shown that the energy in the vacuum state of a field having mass (such as the electron) does not grow to unbounded levels; this finding indicates there may be a way to engineer the particle physics to allow a time machine to work.

According to current physical theory, is it possible for a human being to travel through time?

 

[marduk]

I don't think you're understanding this clearly. To an observer in coordinate time, the object never reaches the event horizon, as in forever, as in infinite time.

And he wasn't saying that the time dilation is an optical effect, he was saying that the appearance to the coordinate observer that the object never reaches the event horizon is an optical effect - from the POV of the in-falling object, it falls into the singularity without missing a beat. But the observer can't see that happen because the length of the light-path from the event horizon (which in itself is an optical effect induced by the observer being in a rotated reference frame wrt the black hole) to the observer is infinite in length - that's why the external observer can never see the object reach the event horizon.

And yes, to the best of our current understanding, time for the in-falling object looking outward increases until it moves infinitely fast when it reaches the singularity and the universe blinks out of existence (but of course that could never be measured because any clock would be shredded into subatomic particles long before it reached the singularity). A theory of quantum gravity might offer a way out of that conclusion, but GR doesn't. Another way out could be the black hole evaporation that Hawking has proposed, but on those kinds of timescales cosmic acceleration would probably have resulted in the "Big Rip" scenario, so if an observer could survive arriving at the singularity, he would only see an empty lightless universe when the black hole ultimately evaporated.

I still disagree. 'Tending towards infinity' is not infinity.

Would time go by infinitely fast when crossing the event horizon of a black hole?

Now suppose the observer tries a different trajectory after crossing the horizon, accelerating outward as much as possible in order to see more of the future history of the external universe. This will only work up to a point: the best the observer can do is hug the outgoing light ray (diagonally from lower-left to upper-right) as much as possible... but since the observer is not actually allowed to go at the speed of light, seeing all of the future of history will be impossible. The best the observer can do is to meet the singularity a bit more on the right of the diagram.

You may see it as a distinction without a difference, but it's not. Just like you can't go faster than the speed of light in space, you can't go to zero in time. The event horizon isn't a mystical place where you can start going faster than the speed of light or exist in no time. It just means you can't get back out again, and nothing you can do can effect the rest of the universe ever again. Unless of course you have a heechee can opener.

Interestingly, time would probably dilate enough that the black hole would evaporate as you fell into it. Not sure what would happen to you then.

 

[marduk]

That’s where the quantum Zeno Effect — a.k.a. Turing’s Paradox — comes in. Taking its name from Zeno’s arrow paradox (a moving arrow can’t actually be seen moving in any single instant, which means it’s not really moving at all), this idea basically states that if you never stop observing a particle that undergoes decay, then that particle will never decay. If that particle never decays, you’ve basically stopped it from doing anything. You’ve stopped time.

Quantum Zeno effect. The quantum Zeno effect (also known as the Turing paradox) is a feature of quantum mechanical systems allowing a particle's time evolution to be arrested by measuring it frequently enough with respect to some chosen measurement setting.

Experiments and discussion
Experimentally, strong suppression of the evolution of a quantum system due to environmental coupling has been observed in a number of microscopic systems.

In 1989, David J. Wineland and his group at NIST[33] observed the quantum Zeno effect for a two-level atomic system that was interrogated during its evolution. Approximately 5000 9Be+ ions were stored in a cylindrical Penning trap and laser cooled to below 250 mK. A resonant RF pulse was applied which, if applied alone, would cause the entire ground state population to migrate into an excited state. After the pulse was applied, the ions were monitored for photons emitted due to relaxation. The ion trap was then regularly "measured" by applying a sequence of ultraviolet pulses, during the RF pulse. As expected, the ultraviolet pulses suppressed the evolution of the system into the excited state. The results were in good agreement with theoretical models. A recent review describes subsequent work in this area.[34]

In 2001, Mark G. Raizen and his group at the University of Texas at Austin observed the quantum Zeno effect for an unstable quantum system,[35] as originally proposed by Sudarshan and Misra.[1] They also observed an anti-Zeno effect. Ultracold sodium atoms were trapped in an accelerating optical lattice and the loss due to tunneling was measured. The evolution was interrupted by reducing the acceleration, thereby stopping quantum tunneling. The group observed suppression or enhancement of the decay rate, depending on the regime of measurement.

In 2015, Mukund Vengalattore and his group at Cornell University demonstrated a quantum Zeno effect as the modulation of the rate of quantum tunnelling in an ultra-cold lattice gas by the intensity of light used to image the atoms.[36]

The Quantum Zeno Effect is used in commercial atomic magnetometers and naturally by birds' magnetic compass sensory mechanism (magnetoreception).[37]

It is still an open question how closely one can approach the limit of an infinite number of interrogations due to the Heisenberg uncertainty involved in shorter measurement times. It has been shown, however, that measurements performed at a finite frequency can yield arbitrarily strong Zeno effects.[38] In 2006, Streed et al. at MIT observed the dependence of the Zeno effect on measurement pulse characteristics.[39]

The interpretation of experiments in terms of the "Zeno effect" helps describe the origin of a phenomenon. Nevertheless, such an interpretation does not bring any principally new features not described with the Schrödinger equation of the quantum system.[40][41]

Even more, the detailed description of experiments with the "Zeno effect", especially at the limit of high frequency of measurements (high efficiency of suppression of transition, or high reflectivity of a ridged mirror) usually do not behave as expected for an idealized measurement.[17]

It was shown that the Quantum Zeno effect persists in the many-worlds and relative states interpretations of quantum mechanics.[42]
Quantum Zeno effect - Wikipedia

'Zeno effect' verified: Atoms won't move while you watch | Cornell Chronicle


It's not just theoretical. The Cornell experiment is just the latest in a series of experiments since then confirming that the quantum Zeno effect really happens. (There's also an "Anti-Zeno Effect," whereby staring at the metaphorical quantum pot brings it to a boil more quickly -- also experimentally confirmed.)
Quantum 'Weeping Angel' Effect Freezes Atoms In Place

I love the weeping angel reference from Doctor Who, but I don’t think the QZM will actually freeze time.

It can halt interactions or decays for a very short period of time - or speed it up. But it doesn’t curve space/time to my knowledge.

I’m thinking it’s more akin to a stasis field?

 

[Randall]

I think the gravity probe B experiment shows it does ...

That's the way it's often related it to people, but a closer look reveals that the actual measurements didn't measure space distortion. What really happened is they used stuff in space ( a probe ) to look at how other stuff in space ( large rotating masses ) relate to yet other stuff in space ( light ) from other stuff in space ( distant stars ). From that, the assumption is made that because all that stuff in space behaves like space is distorted, that space must therefore be distorted.

But that's jumping to a conclusion that could be completely wrong. For example maybe space is completely uniform and stuff in space behaves the way it does because it's governed by the rules of some vast construct that tells it how to behave according to its own rules ( e.g. the fundamental forces of nature ), and we humans just happened upon an analogy ( curved space ) that can be used to make accurate predictions.

So basically, I say so what if the effect on stuff in space that is associated with massive rotating objects in space can be calculated by drawing lines that look like a ball spinning in a liquid that's a little bit sticky? That still doesn't prove space itself is curved or sticky. To quote the ESA site: " Gravity is best explained using Albert Einstein's Theory of General Relativity, which is not a quantum theory. Instead, it imagines that gravity is generated when matter distorts space, like a heavy object would stretch a rubber sheet."

Notice the words "imagines" and "like". It's clearly an analogy. But it works in practical applications, so let's keep using it where it's helpful. Just don't fall prey to the logical fallacy of assuming analogies equal realities, because somewhere the analogy will break down, and you'll run the risk of thinking other predictions are also possible in the real world, ( when they're not ), and I think that's the territory we're getting into when we start talking about warp bubbles and time travel.

 

[Randall]

I wonder: If time reversed? Would we have reverse experiences? It doesn't seem so. But it's good for making your brain on a Saturday morning after staying up too late on Friday decide that going back to bed seems like a good idea.

 

[Thomas R Morrison]

That still doesn't prove space itself is curved or sticky. To quote the ESA site: " Gravity is best explained using Albert Einstein's Theory of General Relativity, which is not a quantum theory. Instead, it imagines that gravity is generated when matter distorts space, like a heavy object would stretch a rubber sheet."

Notice the words "imagines" and "like". It's clearly an analogy. But it works in practical applications, so let's keep using it where it's helpful. Just don't fall prey to the logical fallacy of assuming analogies equal realities

You're misreading this: the analogy is "like a heavy object would stretch a rubber sheet." Yes, that is an analogy: nobody's saying that space is actually a rubber sheet.

But spacetime does curve in the presence of matter, and yes - it even twists when matter is spinning.

Think of it this way: spacetime is a field, and the presence of matter distorts that field. This curvature of the spacetime metric is real and physical and it has been confirmed by a very wide range of experiments. The simplest and most direct test of this is the deflection of starlight around the Sun during a solar eclipse, first conducted by Sir Arthur Eddington in 1919, and subsequently confirmed by far more precise observations. If gravity were simply a force acting in a Euclidean spacetime, the deflection of light would only be 1/2 of the observed value, but because gravity is an actual curvature of the background geometry of spacetime, light follows that geometry around a massive body, resulting in a value twice as great as the value predicted by Newtonian physics.

We recently recorded a Physics Frontiers episode called "The Parameterized Post-Newtonian Formalism" which we'll publish in a few weeks, that discusses the wide range of experiments and theoretical alternatives to general relativity. Those experimental and theoretical arguments can be found in this excellent 2014 review paper: The Confrontation between General Relativity and Experiment, Sections 3 and 4 (pp. 26-53).

Have a look at it: all non-metric theories of gravitation have been ruled out. That means that even if the curvature of spacetime produced by matter is some kind of illusion produced by other physics, then that illusion is so complete that it's physically indistinguishable from a metric curvature of spacetime - so we might as well treat it that way, because a more complicated underlying theory couldn't possibly explain our observations any better than general relativity anyway. But there's also something to be said for simplicity: if it looks like a duck and quacks like a duck, then it makes more sense to treat it as a duck, than to try to explain how a pigeon could look like a duck and quack like a duck and float like a duck.

 

[Randall]

You're misreading this: the analogy is "like a heavy object would stretch a rubber sheet." Yes, that is an analogy: nobody's saying that space is actually a rubber sheet.

I'm not saying space is a rubber sheet either, so you're misinterpreting what I'm saying. The rubber sheet analogy used to get across the idea that things in space behave like or as if space itself is non-uniform. But again, saying A is like B does not mean that A = B.

But spacetime does curve in the presence of matter, and yes - it even twists when matter is spinning.

We don't really know that. All we know is that things in space behave as if that's the case. Generally these are natural things such as particles, waves, and free-floating objects with mass.

Think of it this way: spacetime is a field, and the presence of matter distorts that field.

That's another analogy.

This curvature of the spacetime metric is real and physical and it has been confirmed by a very wide range of experiments. The simplest and most direct test of this is the deflection of starlight around the Sun during a solar eclipse, first conducted by Sir Arthur Eddington in 1919, and subsequently confirmed by far more precise observations. If gravity were simply a force acting in a Euclidean spacetime, the deflection of light would only be 1/2 of the observed value, but because gravity is an actual curvature of the background geometry of spacetime, light follows that geometry around a massive body, resulting in a value twice as great as the value predicted by Newtonian physics.

I know about gravitational lensing. And again, those examples doen't prove that space is curved. They just prove we have better calculations for how light behaves near massive objects in space. We could just as easily say that it's the effect of the massive object on the light that causes light to behave that way, and apply the same calculations. The results would be identical and no space imaginary space warping would be necessary.

Consider this thought experiment as it relates to your example:

In the example the apparent position of a distant star is being altered as it's light passes by a massive object. So hypothetically, if instant travel between points A and B were possible, there's no reason we couldn't use these calculations to plot the actual position of the star, and then fly straight to the star, thereby eliminating the extra distance caused by the arc that the star's light made as it passed near the massive object. But if space itself were actually curved, that wouldn't be possible.

BTW: Yes I realize we'd have to figure in the other variables like the movement of the target star during the period that it took for it's light to get there from here and so on, but the thought experiment doesn't need those to make the point.

We recently recorded a Physics Frontiers episode called "The Parameterized Post-Newtonian Formalism" which we'll publish in a few weeks, that discusses the wide range of experiments and theoretical alternatives to general relativity. Those experimental and theoretical arguments can be found in this excellent 2014 review paper: The Confrontation between General Relativity and Experiment, Sections 3 and 4 (pp. 26-53).

Have a look at it: all non-metric theories of gravitation have been ruled out. That means that even if the curvature of spacetime produced by matter is some kind of illusion produced by other physics, then that illusion is so complete that it's physically indistinguishable from a metric curvature of spacetime - so we might as well treat it that way, because a more complicated underlying theory couldn't possibly explain our observations any better than general relativity anyway..

OK that part is sensible up to a point. But it breaks down in the thought experiment I proposed above. In other words, it's fine for looking at free-floating natural objects, but as soon as we can compensate for the distortion of light by gravity, we can plot a more direct path, apply propulsion and take a more direct route. Plus I really have my doubts that the analogy can be mapped to things like warp bubbles for spaceships in the real world.

I know about the Alcubierre drive, but at present I don't think that it can work, or that if it does, the actual principle will be just as mysterious as gravity itself, and it will not work by actually warping space, but by projecting some sort of gravitational effect ahead of the craft and some sort of anti-gravitational effect behind it, and that it can all be looked at in the analogy of warped space geometry, but it still won't necessarily mean space is actually being warped. We still aren't certain how gravity is imparted onto objects in the universe ( whether or not it's carried by some boson or not ). Maybe we'll never figure that out. I don't know.

But there's also something to be said for simplicity: if it looks like a duck and quacks like a duck, then it makes more sense to treat it as a duck, than to try to explain how a pigeon could look like a duck and quack like a duck and float like a duck

From that perspective I'd say that if space itself is the duck, then what it looks like is that it is volumetric. That quacks and floats just fine. Adding that it's made of something ( nobody has specified what and that we cannot see ) but that can be distorted, doesn't quack and float like anything. But let's look a little closer at it anyway:

All we see is how stuff inside space behaves. But because the math that makes predictions about that behavior can be plotted on a grid that results in curves, some people have jumped to the conclusion that space must look like those curves if we could see it. So to test that they have done the experiments mentioned in this thread

However those experiments don't actually provide any support for the idea that space is curved because they aren't measuring space itself. They only measure the way stuff inside space is behaving. So saying the experiment confirms the hypothesis is just confirmation bias of a conclusion drawn on a possibly false premise.

To illustrate using the eclipse experiment as an example for the duck metaphor: If space is the "duck" then the experiment isn't looking at the duck. It's looking at shadows and light on a surface that to them resembles a duck. But we really don't know that's it's actually a duck because shadows can look like all sorts of things and in reality be caused by something completely different.

On the other hand we can plainly see the massive object with an associated gravitational effect, and it's pretty obvious that it's the cause. It quacks and floats much more like a duck than thinking space is really an invisible unknown undetectable substance that distorts itself in the presence of mass.

Anyway I don't want to upset anyone because my views differ here. So maybe I should just let you folks carry on without me putting a damper on anything. Here's a really great video showing how shadows of things aren't always the things they appear to be.

​

 

[Thomas R Morrison]

In the example the apparent position of a distant star is being altered as it's light passes by a massive object. So hypothetically, if instant travel between points A and B were possible, there's no reason we couldn't use these calculations to plot the actual position of the star, and then fly straight to the star, thereby eliminating the extra distance caused by the arc that the star's light made as it passed near the massive object. But if space itself were actually curved, that wouldn't be possible.

Postulating instant travel isn’t reasonable. Because the only even theoretically rational way to do that, is by jumping through a wormhole - which isn’t actually travel through spacetime at all; it’s a short-cut that obviates the whole premise of the example: to elucidate the true nature of the intervening space. The theoretical maximum rate of motion through space is nearly the speed of light, so that’s a rational scenario. And in that scenario, you’d have to follow the curved path that light follows around a massive body (called a geodesic), or you’d have to use lateral thrusters to navigate a straight line through the curved spacetime, which would be cheating (and it wouldn't be the shortest path through spacetime either, so it would take you longer to arrive at the same speed).

Plus I really have my doubts that the analogy can be mapped to things like warp bubbles for spaceships in the real world.

Unless it’s not an analogy at all. And it certainly doesn’t appear to be an analogy: by every method that we’ve ever conceived to test whether or not spacetime actually curves, we’ve found that it does in fact curve. So until someone can show us another way to simulate a curved spacetime with different underlying physics (and many brilliant minds have tried, yet all have failed), then it makes the most sense to just accept the theory of curving spacetime because it works perfectly. Ultimately that’s why Einstein is so famous – it’s not because his theories are so beautiful and original (which they are), it’s because they describe all of our observations and experiments so perfectly that we have yet to find any hint that they’re in any way wrong. After a century of rigorous challenges, both observationally and theoretically. That’s astounding.

I know about the Alcubierre drive, but at present I don't think that it can work, or that if it does, the actual principle will be just as mysterious as gravity itself, and it will not work by actually warping space, but by projecting some sort of gravitational effect ahead of the craft and some sort of anti-gravitational effect behind it, and that it can all be looked at in the analogy of warped space geometry, but it still won't necessarily mean space is actually being warped. We still aren't certain how gravity is imparted onto objects in the universe ( whether or not it's carried by some boson or not ). Maybe we'll never figure that out. I don't know.

Those fundamental questions are wonderful – there’s so much we still don’t know. We still have no idea what the nature of electric charge is either, or why the physical constants have the values they have. I would love to see answers to the really fundamental questions like that before I check out

I actually agree that the warp bubble and wormhole solutions appear to be impractical – though both have been so vociferously assaulted by the physics community, and left standing when the dust settled, that I trust that they’re theoretically consistent with the established and well-tested physics of general relativity. So I tend to think that they’re probably attainable for radically advanced civilizations.

But the thing is, you don’t actually need a warp bubble for warp travel. All you need is a positive mass and an effective negative mass, and you’re good to go. In fact, if you have a positive inertial mass, and an equal negative inertial mass, they don’t even have to interact gravitationally in order to accelerate your spacecraft – they can interact via the far more powerful field of electrical charge to really accelerate your vehicle in a jiffy, without any huge gravitating mass-energies. Robert L. Forward presented this elegant and simple approach (which is virtually identical to the idea you posted above) in his 1990 paper “Negative Matter Propulsion,” and I still favor that solution over the vastly more difficult warp bubble approach, because Forward’s concept appears much closer to something that we could actually build in the foreseeable future – and like warp bubble propulsion, it costs no energy to travel that way. And we just recorded a podcast tonight called “Negative Mass in General Relativity” that covers two fairly recent papers (this one and this one) that show that negative effective inertial mass (and the associated repulsive gravitation) is physically realizable within GR because the positive energy theorems don’t apply to accelerating universes like our own.

Anyway I don't want to upset anyone because my views differ here. So maybe I should just let you folks carry on without me putting a damper on anything.

I always enjoy hearing from you, and I welcome lively rational debates. I only get frosty when somebody employs deceitful and underhanded debate tactics in lieu of a rational argument, but you never do that, so we’re good.

I’ve been through the process of seriously questioning the base assumptions of general relativity, as you’re doing right now. It took 20-30 years of really digging into this stuff before I was finally convinced of its physical reality. So I know where you’re coming from: it does seem like BS that space can have curvature and other physical qualities. But the harder I tried to find an alternative explanation for our observations, the more I discovered the power and elegance and internal logical consistency to the theory. And now I wish I’d appreciated it sooner, because I wasted so many years farting around studying fringe theories, and I could’ve spent that precious time making meaningful progress with the theories that actually work.

 

[Randall]

Postulating instant travel isn’t reasonable ...

It's reasonable in the context of the thought experiment to illustrate the point that the apparent position of the star and it's actual position are different and that the apparent position is due to a curvature in the path of the light rays around a massive object in space, the total distance of which compared to a direct line must logically be longer, and that hypothetically, knowing the variance would allow the actual position to be calculated so that a direct path could be taken. We don't need to invoke wormholes or anything else to get this point.

I always enjoy hearing from you, and I welcome lively rational debates. I only get frosty when somebody employs deceitful and underhanded debate tactics in lieu of a rational argument, but you never do that, so we’re good.

I'm glad to hear that because I've been beaten down by complainers who have used appeals to authority rather than their intellect to restrict my participation here. I think your contributions are top-notch and that the forum is lucky to have you as a member. At this point I don't see either of us changing our present views on the nature of space itself. As this article states: "The 3D/4D controversy is part of a general disagreement among philosophers of time concerning the degree to which time is dissimilar from the dimensions of space.That general disagreement has been an important theme in the philosophy of time during the last one hundred years, and will most likely continue to be so for some time to come."

But that doesn't mean that we can't move along to the idea of time travel. I was looking at time as change within a given frame of reference. That doesn't require space to be curved or not curved. It just requires that for no time to be the case, nothing within the frame of reference changes. Is there a way we can begin there, or what other approach might you suggest?

 

[DROBNJAK]

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.

Yeah, your reasoning here is wrong. Einstein was only using acceleration to illustrate meaning of inertial mass, because acceleration acts on inertial mass, but not on gravitational mass.

This does a nice job with some concepts in special and general relativity, but I cringe when people talk about "nuclear gravitation" theories - we've got a pretty good handle on the nuclear strong force and its mediating particles: one would have to explain away all of that data and simultaneously present a formal differential geometry model of nuclear gravitation to make such a claim reasonable, and that all seems very unlikely. I'd love to see somebody pull it off though, if that's even possible, because it would be so much fun to play with and it would open up so many prospective new technological frontiers.

I would like to hear more detailed rebuttal of the idea that strong force is not gravity. Since reading Daniel Fry's book, honestly, I was more impressed with his experience of touching the surface of the craft whereupon he felt oily repulsive force, than later by his tank circuit artificial gravity. GR's EM stress-energy tensor is only faintly coupled with gravity, while strong force has much more umphhh. I've spent a whole two weeks digging up everything about strong force, quarks, gluons and Frank Wilczek.

On the end of the day, gluons contain huge relativistic energies and can bend space-time with ease. Its a fact that 99% of proton's inertial mass comes purely from energy of gluons. As illustrations go, a single gluon pulls on a quark with a force of 18 tons. That means that a single gluon, can lift a whole railroad car full of coal and accelerate it faster than a jet engine. A single subatomic bozon!!! It would be extremely strange to say that gluons don't bend space-time. And gluons already produce inertial mass and gluons have the space warping energy levels. Practically gluons are 2 in 1 as far as equivalence principle goes. QFT states that gluons produce inertial mass, and they have energy to bend space-time. Its natural to assume that they produce gravity through space-time bending.

I am very curious what would happen if photons were kicked out from the General Relativity's Stress-Energy tensor and were replaced with gluons. Gluons and photons are both bozons and Einstein didn't know about gluons. Maybe GR would look different if he did know about them?

Upon Daniel Fry saying that aliens "inflated" some particle from inside atomic nuclei to create "oily" repulsive force, I found that, theoretically, something possibly similar exists in QFT in a form of glueballs. Unfortunately they quickly self-destruct.

As well I found that inside gluons there are no quantum fluctuations. Practically, gluon "flux tubes" are only space in the whole universe where there is no ZPE. I can't bend my head around what does that mean. Here is the video explaining the concept.

 

[Randall]

Yeah, your reasoning here is wrong. Einstein was only using acceleration to illustrate meaning of inertial mass, because acceleration acts on inertial mass, but not on gravitational mass.

Not really. You're taking the illustration I used with the equivalence principle out of context. It was meant to demonstrate how mathematical expressions based on analogies, although accurate in terms of numbers, do not necessarily mean what's happening in reality is the same as what's happening in the analogy. But regardless of that, we're still talking about the equivalence principle in this way: "That is, being on the surface of the Earth is equivalent to being inside a spaceship (far from any sources of gravity) that is being accelerated by its engines. Source

I would suggest that before you assume someone is wrong about something, that you make a greater effort to be sure you understand the context and to cross reference your sources. A lot of misunderstandings happen because of these sorts of communication problems. One way to help avoid that is to ask someone for clarification and sources before drawing conclusions.

 

[Thomas R Morrison]

It's reasonable in the context of the thought experiment to illustrate the point that the apparent position of the star and it's actual position are different and that the apparent position is due to a curvature in the path of the light rays around a massive object in space, the total distance of which compared to a direct line must logically be longer, and that hypothetically, knowing the variance would allow the actual position to be calculated so that a direct path could be taken. We don't need to invoke wormholes or anything else to get this point. I'm glad to hear that because I've been beaten down by complainers who have used appeals to authority rather than their intellect to restrict my participation here. I think your contributions are top-notch and that the forum is lucky to have you as a member. At this point I don't see either of us changing our present views on the nature of space itself. As this article states: "The 3D/4D controversy is part of a general disagreement among philosophers of time concerning the degree to which time is dissimilar from the dimensions of space.That general disagreement has been an important theme in the philosophy of time during the last one hundred years, and will most likely continue to be so for some time to come."

But that doesn't mean that we can't move along to the idea of time travel. I was looking at time as change within a given frame of reference. That doesn't require space to be curved or not curved. It just requires that for no time to be the case, nothing within the frame of reference changes. Is there a way we can begin there, or what other approach might you suggest?

I’m sorry I didn’t respond to this sooner. I’d written a really cool response and was almost done tweaking it, when I had to reboot my computer…and I was so sure that I’d saved it…but it was gone when I went to pull it up again. That just sucked the wind right out of my sails.

In one of Daniel Fry’s books about physics, he refers to time as the first dimension (it’s usually called the fourth dimension), because without it, nothing can be observed or measured – not even space. And that’s always rung true to me. As I mentioned on the first page of this thread, theoreticians have tried all kinds of ways to formulate an alternative theory for gravitation, and many of those efforts involve a flat spacetime instead of a curved one. There’s only one problem: they’ve all failed:

We recently recorded a Physics Frontiers episode about the constraints on alternative gravitation theories, and the long and short of it is that all non-metric models of gravitation have been ruled out. Some slightly different types of metric theories are still possible, but the constraints on them are very, very tight. It appears from every scientific experiment and observation in the canon of physics and astrophysics that Einstein’s model is the correct one (which is a little disappointing for people like myself who are hungry for new physics, but it’s a testament to Einstein’s model that even after a century, we can still find no way to “improve” his theory, or even alternative that actually works).

So I strongly recommend reading sections 3 and 4 of this marvelous review paper, so you can see for yourself why the only remaining viable models of gravity are metric curvature theories, and why even the alternative metric curvature theories to general relativity have been all but ruled out completely:

“The Confrontation between General Relativity and Experiment,” Clifford M. Will, 2014
https://arxiv.org/pdf/1403.7377.pdf

It’s frankly absolutely stunning just how accurately general relativity has described every experimental/observational scenario, both direct and indirect, that we’ve been able to test.

And DROBNJAK – in another thread you asserted that dark energy and cosmic inflation are strikes against general relativity (GR), but that’s not true. General relativity encompasses a range of possible cosmic geometries, including accelerating de Sitter universes like the one we inhabit. And GR actually gives us the bedrock that we need to explain cosmic inflation, because it shows that under some circumstances gravitation can be strongly repulsive. So those are actually points in favor of GR. Dark matter is still a sticking point, but the Vegas money is on an independent cause, not a flaw in GR itself. They’ve tried to model dark matter through alternative gravitation theories, and those efforts have all failed. So at this point it looks like there is something we need to learn about galactic dynamics, but it doesn’t look like the problem is with GR. But I suppose that all we know for sure at this point, is that we’re not sure at this point wtf is going on – so the solution will definitely be exciting and refreshing once we figure it out.

Yeah, your reasoning here is wrong. Einstein was only using acceleration to illustrate meaning of inertial mass, because acceleration acts on inertial mass, but not on gravitational mass.

I would like to hear more detailed rebuttal of the idea that strong force is not gravity. Since reading Daniel Fry's book, honestly, I was more impressed with his experience of touching the surface of the craft whereupon he felt oily repulsive force, than later by his tank circuit artificial gravity. GR's EM stress-energy tensor is only faintly coupled with gravity, while strong force has much more umphhh. I've spent a whole two weeks digging up everything about strong force, quarks, gluons and Frank Wilczek.

On the end of the day, gluons contain huge relativistic energies and can bend space-time with ease. Its a fact that 99% of proton's inertial mass comes purely from energy of gluons. As illustrations go, a single gluon pulls on a quark with a force of 18 tons. That means that a single gluon, can lift a whole railroad car full of coal and accelerate it faster than a jet engine. A single subatomic bozon!!! It would be extremely strange to say that gluons don't bend space-time. And gluons already produce inertial mass and gluons have the space warping energy levels. Practically gluons are 2 in 1 as far as equivalence principle goes. QFT states that gluons produce inertial mass, and they have energy to bend space-time. Its natural to assume that they produce gravity through space-time bending.

I am very curious what would happen if photons were kicked out from the General Relativity's Stress-Energy tensor and were replaced with gluons. Gluons and photons are both bozons and Einstein didn't know about gluons. Maybe GR would look different if he did know about them?

Upon Daniel Fry saying that aliens "inflated" some particle from inside atomic nuclei to create "oily" repulsive force, I found that, theoretically, something possibly similar exists in QFT in a form of glueballs. Unfortunately they quickly self-destruct.

As well I found that inside gluons there are no quantum fluctuations. Practically, gluon "flux tubes" are only space in the whole universe where there is no ZPE. I can't bend my head around what does that mean. Here is the video explaining the concept.

There’s a lot of interesting stuff in this post so I’m going to have to mull it over before posting a more comprehensive reply.

But we can definitely imagine that a more advanced nuclear physics technology could give us powerful new tools for manipulating the spacetime physics of GR. After all, nucleons are vastly denser than any of the matter that we can manipulate with today’s technology, so if we could control mass-energy densities in that regime we’d be able to generate all kinds of fascinating gravitational effects in the lab and industry. Quark-gluon plasma is even denser than nuclear matter by a couple of orders of magnitude. But how we might harness energy densities at that scale, beyond creating it for incredibly fleeting instants of time and at nuclear scales of volume, remains a complete mystery. And those tiny volumes of extreme energy densities are still insufficient for producing any observable gravitational effects, because the coupling between mass-energy and spacetime is so incredibly weak (governed by the constant G/c^4 – which is roughly 10e^-50 orders of magnitude in cgs units, which is why it takes so *much* mass-energy to produce any observable gravitational effects).

In Fry’s books he does talk about a form of nuclear technology that forces out “the binding energy particle” from the nucleus, so it encompasses the entire atom, and yields a very short-range repulsive gravity effect – in his books he says that’s how the hull of advanced spacecraft repel any interstellar dust or micrometeorites that would be hazardous to the craft. It’s a fascinating idea, but we have no idea how to do it, and of course we can’t even be sure that it’s a valid model.

But I think you’re too dismissive of the electromagnetic stress-energy tensor. Sure, it’s weaker than the stress-energy tensor by the mass equivalence term of c^2, but then again, we are creating quark-gluon plasma using EM technology. So it would be foolish to write off everything that we’ve achieved with EM technology – and the energy densities and precision that we’re attaining with it are still rising all the time. If nothing else, it’s clearly the best and only “handle” that we have for exploring and exploiting the rest of physics, including nuclear physics /the strong force.

 

[DROBNJAK]

Yeah, but just imagine this. Einstein derived General Relativity GR, from equivalence principle.

Now, gluons pull on quarks with 18 tons of force inside a radius r= 1 x 10^-15m, and make them to move close to speed of light. Because E= gamma * m * c^2 that process creates inertial mass. That's mainstream physics explanation of where inertial mass comes from. No contention here.

Now, same as the above, gluons have relativistic levels of energy. Gluons have enough energy to bend space-time according to GR. By bending space-time, gluons create gravity. Its simply a question of plugging in Quantum Chromo Dynamics QCD into GR e'voila one got gravitational mass.

Because the gluon energy that creates both inertial and gravitational mass is the same, inertial and gravitational mass are equal and one gets the equivalence principle out of QCD. Practically, QCD and gluons are source of the equivalence principle.

This approach has an additional beauty point for Miss Universe Pageant that atomic nucleus is already perfect for creation of very high frequencies like Gamma Rays. Gravity has one nasty property in a sense that it is not shielded by other masses. When Earth falls between Sun and Moon, Moon's trajectory stays the same, since gravity from Sun passes through Earth as if Earth is not there. Now, this is only possible if gravity has a really, really high frequency and can pass through matter unchanged. Quarks and Gluons are already so tiny that they can easily produce required high frequencies for gravity.



But here is the show stopper. Contradiction here is in that previous post video and the above photo, where they say that gluon flux tubes are erasing Quantum Fluctuations QF. The above photo shows QCD model of photon, confirmed by experiments to 12 decimal places. Small balls are quarks and you can see a big cavity between quarks. That's because quarks and gluons erased QFs.

Now, is space-time = Quantum Fluctuations? Is the space without QFs bendable or unbendable under GR? ...

Now, this is all 'sounds good' qualitative reasoning. Somebody has to do rigorous mathematical model. That's much higher that anything my noodles produced so far ;-)

And those tiny volumes of extreme energy densities are still insufficient for producing any observable gravitational effects, because the coupling between mass-energy and space-time is so incredibly weak (governed by the constant G/c^4 – which is roughly 10e^-50 orders of magnitude in cgs units, which is why it takes so *much* mass-energy to produce any observable gravitational effects).

As you said, even nucleons are still weaker than needed for UFOs. But if we, for a moment, put to a side hadron (protons & neutrons) engineering, even if one has to face with 1 x 10^50 scaling down factor, we will be multiplying with number of protons & neutrons in the Earth, so hopefully one would be reproducing g (Earth's grav. field strength).

I haven't abandoned the EM Poynting vector approach, its just that the above idea was so attractive that I had to do some research. EM Poynting vector approach is the only chance with technology we've got, at least till we can do with hadrons what we can do with electrons.

As you said before, huge EM fields that surround UFOs might be a byproduct of some other process. That process might be "hadron engineering" or something else, etc.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

As Time Goes By: UFOs and Time Distortions

These cases practically sum up most of the main observation trends in the UFO investigations.

There are two or three fields that turn up in UFO incidents that don't exist in our physics. I'll quickly mention few.

- UFOs are replete with EM effects, and we understand those.

- But there are several cases where rocks were thrown or bullets were fired at UFOs and they either bounced, or flu in curved trajectories (GR geodesics?) around the UFO. Is that repulsive gravity? We don't even have field like that in our modern physics.

- Frequently witnesses find themselves inside these frozen time bubbles, where time had stopped, but witnesses are no gravitational effects that would otherwise rip them apart. GR requires these two effect to be together.

- Just by wearing very thin skinned suits aliens can hover above the ground. Suits provide a weak artificer gravity and produce strong EM interference with cars, radios and headlights.

This is a list of UFO non-understendable effects that Jacq Valle put together:

 

[Randall]

I’m sorry I didn’t respond to this sooner. I’d written a really cool response and was almost done tweaking it, when I had to reboot my computer…and I was so sure that I’d saved it…but it was gone when I went to pull it up again. That just sucked the wind right out of my sails ... theoreticians have tried all kinds of ways to formulate an alternative theory for gravitation, and many of those efforts involve a flat spacetime instead of a curved one. There’s only one problem: they’ve all failed ...

That is so frustrating when that happens. Sorry to hear that. I've often considered getting a UPS. Even a short term one that would allow one to save everything before shutting down. Then I think, "Na the chances of it happening don't justify the expense." But maybe there's some boxing week sales on where you can get one cheap. Anyway. I digress.

As already stated. I don't have any problem with the idea of thinking of space as curved for the purpose of making accurate calculations for the way things work. But that situation only means that the things that are measured in space behave as if space is curved, not that space itself is actually curved, and to reiterate the salient example. Let's assume the amount of gravitational lensing is calculated using this curved space method, and do a thought experiment. To do this we don't need to include all the variables such as how everything is moving in space and the time it takes light to get here from there, because those aren't relevant to the point being made.

In this thought experiment we can simply draw out a situation where we have light from a source passing near a massive object that results in the rays appearing to bend when they get near the object. However simply because light rays from a source bend around a massive object doesn't mean the actual position of the source has changed. Therefore we are now faced with two situations. There is an actual position and an apparent position of the source, and logically only one of them is the actual position in space.

Logically, to determine the actual position of the light source in space, we can simply calculate the amount of lensing adjust the angle accordingly, and then draw a straight line through space from the point of observation to the actual position of the source. Consequently being able to draw a straight line through space to the actual position of the source means there's no actual space curvature. To emphasize this point. Hypothetically if some sort of instant point to point beaming were possible, we could plot the position of the source adjusted for the lensing and all the other variables and transport directly there in a straight line.

This situation has to be true because otherwise, the apparent position and the actual position would have to be identical ( but we know they're not ). So to reiterate. What is really happening is not that space is actually curved, but that a method of calculation is being used to describe how light rays passing near a massive object behave. That method says, "Let's think of space as if it's curved." However we know it's not because we can adjust for the effect of the lensing and plot a straight line through space to the actual destination. Consequently taking that extra step and saying, "Space really is curved." is not justifiable. It's also not a moot point because knowing the actual position and the apparent position of a destination are important real world considerations.