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Military Has Been Researching "Anti-Gravity" For Nearly 70 Years (thedrive.com)
215 points by tomohawk on Oct 30, 2019 | hide | past | favorite | 158 comments

Physicists have been searching for variations in the coupling of materials to gravity for more than a century, and by some measure, for more than a millenium.


Every experiment anyone has ever tried has come up empty. The equivalence principle is a postulate (Einstein's "happiest idea") that underlies General Relativity, making it essential to test.

(Source: This is literally what I do for a living.)

How much money does society need to spend supporting the work you do before we can safely conclude that Einstein was actually correct and we don't need to test it any more?

That's a great and important question. (doubly so for me, as constrained resources may see me leave the field, to which I have devoted my entire professional life, in the coming months) We are perennially grateful for the support we receive to do the work that we do.

To a high degree of approximation, Einstein's predictions are very, very correct. We are trying to look very carefully at Nature because we don't think we have the whole story.

It is possible that Einstein is exactly right. If so, we would never be able to describe the four forces of Nature with one unified theory. Today, we need two theories to describe everything we see -- the quantum-mechanical/particle-physics description of the Standard Model and gravity. There is no quantum-mechanical fuzziness in the mathematics of gravity, and there is no hint of the differential geometry of gravity in the mathematics of the Standard Model. The aspiration of almost every fundamental-physics physicist is to find a way to either simplify the Standard Model or connect it with gravity.

If the return on investment seems insufficent, know, too, that the technology we develop to push the boundaries of knowledge has important spin-offs. GPS is impossible without corrections from both special relativity and general relativity. The instrumentation we develop to make gravitational experiments possible on earth requires the development of new classes of seismometer [1] that may open new understanding of Earth's dynamics and allow better chip-fabrication instrumentation. The instrumentation developed to test gravity in space is also being used to measure the movement of mass (i.e. ice and water) on Earth's surface [2].

Even more important is the training we provide to students. Just as athletes train in the gym to get stronger, confrontation with the hardest known technical problems provides an efficient path for students and young faculty to become proficient at the entire range of modern measurement science. Alumni from our group have not only gone on to fancy academic positions, but also helped to redefine the kilogram, designed upgraded digital calipers used by tens of thousands of people worldwide, built key components of a major quantum-computing company's infrastructure, and more.

The people who work on these kinds of experiments are making a fraction of what they could make in industry. We are doing it for love, not money, and the returns to society are myriad.

[1] https://arxiv.org/abs/1707.03084 [2] https://gracefo.jpl.nasa.gov/

Good response. I think this is the clearest way to explain to people why researching X is important, the benefits to humanity are almost always in side effects of any research, development and engineering feats: The techniques, understanding and technology required to _attempt_ to figure out or achieve X are usually far more valuable than X itself and are often not known ahead of time, but can be invaluable to the future of humanity.

The problem is researchers are far more likely to express their interested in X to the public - quite right - since that must be their passion for the significant focus required to follow that path. This is what the public find so unintuitive, we need to focus on X to benefit from Y, although we don't know what Y is, but it has the potential to be far more useful than anything more direct.

Reminds me of the more literal explanation in a letter by Ernst Stuhlinger at NASA in response to a similar question:


Really appreciate your tone and patience in sharing you experience here!

> constrained resources may see me leave the field

I am sorry to hear that - it will be a loss for Eöt-Wash.

I'm super bummed about it. It is a complicated story involving impedance mismatches between the goals of two administrative silos. The high-level goals are the same (good science, students, technology), but there are structural differences on how to get there...

"GPS is impossible without corrections from both special relativity and general relativity"

I appreciate your comments in general, but I feel that this point needs to be tempered. Yes there are corrections due to SR and GR effects in GNSS, but they tend to get lost in the morass of the least squares residuals compared to all the other much bigger effects, like clock errors, orbital errors, ionosphere delays, etc, etc.

The relativistic corrections are cumulative (and, if I recall correctly, have opposite sign!). Without correcting for the shifts, it is my understanding that GPS would be degraded within minutes and useless in less than a day.



You have to correct for the shifts, but no knowledge of relativity is required (maybe the SR correction is put in analytically but GR is definitely not). The least squares problem deals with it by lumping it with all the other time offsets for the pseudoranges for each satellite. And of these it is a small part of the error budget and is dealt with by averaging multiple measurements.

The second reference cited above includes the passage:

"There is an interesting story about this frequency offset. At the time of launch of the NTS-2 satellite (23 June 1977), which contained the first Cesium atomic clock to be placed in orbit, it was recognized that orbiting clocks would require a relativistic correction, but there was uncertainty as to its magnitude as well as its sign. Indeed, there were some who doubted that relativistic effects were truths that would need to be incorporated [5]! A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that predicted by general relativity, then the synthesizer could be turned on, bringing the clock to the coordinate rate necessary for operation. After the Cesium clock was turned on in NTS-2, it was operated for about 20 days to measure its clock rate before turning on the synthesizer [11]. The frequency measured during that interval was +442.5 parts in 10^12 compared to clocks on the ground, while general relativity predicted +446.5 parts in 10^12. The difference was well within the accuracy capabilities of the orbiting clock. This then gave about a 1% verification of the combined second-order Doppler and gravitational frequency shift effects for a clock at 4.2 earth radii."

It makes sense that offset corrections can be handled by a lumped model, as any relativistic corrections on timescales longer than an orbit will be constants of the satellite orbit. Making sub-orbit-duration corrections might require a model that captures more of the relevant physics.

Also, we surely would have compensated for relativity experimentally: "The clock seems to run 0.2% more slowly than predicted. We don't know why, but if we subtract 0.2% everything just work. Let's ship it!"

Subtract 2% from what?

Short-termism, and the financialization of research, seems to be a huge problem in all fields of science.

By "financialization," I mean deciding which research projects get funding based on their expected measurable results, i.e., based on some rate of return that can be estimated today. If a research project can lead to more citations soon, or slightly better products soon, or lower costs soon, etc., it will get funding. Otherwise, it will not.

As a consequence, research projects with uncertain payoff but potentially transformational long-term benefits are... neglected.


Your viewpoint is very agreeable, but how do you propose to do it otherwise? Given limited resources (which continues to be the case even today), you have to allocate them somehow. Can you think of a simple and objective measure to distribute the available resources between research projects?

It's all right to decry financialization of research, but it is a very difficult problem to solve.

I would borrow from the "20% free for personal research" deal Google offers software developers: "spend 20% of your time on anything you want." Gmail, for example, was born as a 20%-spare-time project. (The creator of Gmail, Paul Buchheit, is now at Y Combinator.)

People who have dedicated their life to science should get more than 20%. Say, 50%: "Spend 50% of money/time/people on the funded project, 50% however you want."

I think my 50/50 idea would work quite well. Scientists still would have to demonstrate and justify the value of near-term projects to get funding, but would also get the resources they need to fund long-term projects with impossible-to-estimate present value.

Round robin or following a uniform distribution?

Stupid question but what is the two theories and why isn't gravity part of them?

"Today, we need two theories to describe everything we see -- the quantum-mechanical/particle-physics description of the Standard Model and gravity. There is no quantum-mechanical fuzziness in the mathematics of gravity, and there is no hint of the differential geometry of gravity in the mathematics of the Standard Model"

Some argue that keeping talented people out of industry is a downside of the sciences not an upside.

Personally, particle physics irritates me because I don't believe in particles... It's all vibrations, man :)

Academia isn't keeping talented people out of the industry. At any point, a talented person can leave the university, join a company, and easily quintuple their earnings.

It's the industry that's keeping talented people out by giving them mundane or bullshit jobs, and not leaving any space for long-term research and vision.

Though I might say that Academia is making a good job of keeping talented people out of it because their incentive structure is skewed.

This. I did not leave academia to search for fame and fortune. I left it because the bureaucracy and politics of it was terrible.

That's true at the same time. It's a tough choice for those talented with love for science or a will to work on solving bigger problems.

>Some argue that keeping talented people out of industry is a downside of the sciences not an upside.

Yeah, instead of understanding the universe and core applications we could have more BS gadgets...

Think of the ads that could've been served!

I see it as the organizational abstraction of industry is an side-effect artifact of relative material scarcity, and a current, hopefully temporary condition in our species' evolution towards a future when advancing sciences becomes one of the major focal points of our endeavors, and the bulk of industry is largely automated away in the trail of a continuously-growing wavefront of advanced-and-advancing industry. Hopefully capitalism and industry will bootstrap us into that outcome, instead of mediocrely settling for a local maxima of extractive rentier structures. "Money is a sign of poverty", wrote Iain M. Banks, and I suspect the transition to that kind of wealth is an evolving continuum instead of an abrupt, sharp discontinuity.

Hopefully a (theoretically) infinite amount (assuming infinite time) ...theories should continuously be re-tested, although it probably makes sense to spend an exponentially lower and lower amount of money on re-testing theories we know are true. But they should be re-tested, even if some experiments you'd only redo every 50 years or so!

For two important reasons:

1. as a species we must maintain technical capability to perform certain types of experiments - only way to 100% be sure we do this is by re-doing those experiments periodically

2. the fact that the "laws" of physics are fixed in any sense of the word, or that there even are such things as "laws of nature" is at its core a very very likely but UNPROVEN ASSUMPTION... we should always be open to the possibility that we live in a much stranger universe than we imagine and that the laws of physics themselves could vary and that we mush have capability to re-discover the new/changed laws, or to poke deeper at meta-laws

Also (2) is not as much lunacy as it sounds considering that the most likely (in the sense of "mathematically probable" considering the math of probability and information, not in "what we can determine experimentally with our primitive capabilities") setup from our universe models is more like we're in an N-th level "simulation" (a simulation in a simulation ... etc.) maybe running on a something like a "Boltzmann brain" in hell knows what soup of multiverses...

We may live insignificantly short lives and be practically meaningless on a grander scale, but if we dare the play the game of "guessing the laws of the universe" on a large scale, we shouldn't have the arrogance to assume that what we measure and deduce on an insignificantly small space-time volume in our ant-farm corner of the universe actually holds up directly through longer stretches of time and space!

Otherwise we're not really doing science, we're adherents to the religion called scientism, and we're not truly honestly rational, but merely praying in the church of rationalism! These are very very different things!

Wouldn't we very quickly notice if any of the basic laws of phsyics would change, or if any of the physical constants would nudge even a fraction of a million - because our machines (like microchips, radios, like quantum computers, like satellites, GPS, etc. etc.) would stop working as they were intended?

Yeah, we would notice it, but that's not the point of re-running these kinds of experiments.

Just look what humanity was able to do in the Space Race with "computers" less powerful than an ordinary standard wristwatch of today: going to the moon, surviving there and coming back. Today? If it were not for SpaceX, the US would not have a path to access to its own manned transfer capability.

If we do not use abilities, we lose them. Knowledge gets forgotten, data bit-rots, machinery rusts apart.

> "computers" less powerful than an ordinary standard wristwatch of today: going to the moon, surviving there and coming back

This is more of a statement of how little computing power is needed for rudimentary space travel. The technology itself was quite advanced, just in other aspects than raw computing power.

Sure but the US has lost so much abilities and especially knowledge. Most people active in the Moon-landing era are pensioners by now and there are at lwast two generations who have to start from scratch now instead of having decades of iterative tech improvements.

Assuming Einstein is correct is not the end, because we still have so many questions. For instance, why is Einstein correct?

It depends where you test.

General relativity should be tested and verified in energies and conditions where it has no been tested yet, because it's likely that there is crack there somewhere.

Many those military research projects mentioned in the article are pure crackpot science. They search where it easy to search and reasons to do so are usually based on misunderstandings of existing theory.

Would be much better to spend that money on another VC funding round for WAaaS (Wiping Asses as a Service)!

Assuming we find dark matter, do you think it will represent some departure from our current understanding of gravity and how it relates to mass? Or do you think it will just be some boring dust clouds?

We have already tested the equivalence principle toward dark matter [1]. It appears to fall like everything else, at the 1% or better level.

It is extremely difficult for modifications of gravity to simultaneously fit the cosmic microwave background, big-bang nucleosynthesis, galactic rotation curves, and the Bullet Cluster. The safe bet these days is on some sort of non-baryonic dark particle (which is why so many people are looking for them). Most other loopholes are closed or in the process of being closed.

The coming decade or two should have something significant to say about dark matter. We will either discover its nature or severely constrain its properties through a plethora of varied experiments and searches.

[1] https://arxiv.org/pdf/1207.2442.pdf

From my understanding `dark matter` is an explanation for what we expected and what we see regarding the mass of the universe?

It's a bit like exploding an apple and measuring all the bits that fly of and missing some bits. Whilst we do not know what the apple looked like initially, we are from our measurements, trying to reconstruct that apple.

How convinced are we that their is dark matter and not a case of how we have measured things so far. Hence, differences in results based upon how we measure are evident, whilst many are close to the same, they just don't exactly match up - hence - dark matter theory.

How likely is it that dark matter as we know it - does exist?

this is all fine and dandy, but when do you think will we have anti-grav drives?

Don't bet more than you can afford to lose.

Make me a hover board, get to work!

He is working on it. Give him more money and he will hire more assistants, and in turn make progress faster.

Of course you are risking that he eventually proves your hover board is impossible.

They will find dark-matter when they unify GR and QM, never :)

So we are litearaly at square 0 still here?

No, sounds like we've eliminated 70 years worth of other possible squares.. if you want to take glass half full approach.

Similar to the realization that saying "no" is one of life's important skills to internalize. Closing off possible avenues of explanation is just as much of an advancement as finding evidence to support an explanation.

Physics took 2 wrong roads and it got stuck in a local minima of the truth plane. Understanding that physics is the most ignorant field of science and how proud the field is of it's models, I don't think this will come anytime soon.


PS: you need to understand how gravity is derived to manipulate it's propagation. Since the mechanisms are very stable due various reasons, only a very good understanding of many processes will allow you to design an experiment that works. I have adopted a very fringe physics model that is unified and explains more using less assumptions. From this perspective I can tell you that contemporary physics is so far from understanding, that most likely we will not see anything in this direction, before we fully trashed our planet.

What's your physics model?

This one: https://www.amazon.de/Basic-Structures-Matter-Supergravitati...

Never could have come up with such a brilliant theory, took me a year to understand it. Now I understand 137 ^^

I love the following Feynman's quote:

``In those days, one of the theories proposed was that the planets went around because behind them were invisible angels, beating their wings and driving the planets forward. You will see that this theory is now modified! It turns out that in order to keep the planets going around, the invisible angels must fly in a different direction and they have no wings. Otherwise, it is a somewhat similar theory!``

The point is that, still today, we have no idea why mass attracts mass. We observe it. We measure it. We model it. We predict it. But, we do not understands its mechanism. At that time, it was making people crazy to think that an object could remotely affect another one. Now, people just accept it. But, when you think about it, it really sounds magic.

> why mass attracts mass

What kind of answer could you get for that kind of question?

Seems to me that "Why?" Is the question you can ask endlessly of any answer.

Often I think a "why?" questions is just a request for a compelling narrative... Maybe that is literaly what "why?" means?

Richard Feynman himself has a reply similar to yours in the documentary called "Fun To Imagine" when he is asked a why question, in that case related to the magnetic force:


Indeed, questions of why are often metaphysical and can't truly be answered within the framework of physics.

> At that time, it was making people crazy to think that an object could remotely affect another one. Now, people just accept it. But, when you think about it, it really sounds magic.

Do people (read physisicts) accept this. My understanding is that, while unconfirmed, the graviton is still a respectable hypothesis.

When you go deeper, it is like that with every part of physics. We don't understand any of it. We just move the horizon of understanding farther and farther, we are not even pretending to know what is behind it. Care to explain where the big bang came from and why did it give birth to these specific sets of physical laws and this specific number of elements and why are there just enough forces and randomness for them ot produce biological life and not kill it instantly with all chaos and random motion etc? Why is life somewhat stable? Science never really "explains" anything, it just learns to describe, model and predict the observations better and better. Observation is all we have.

Thus journey in spirituality starts.

> At that time, it was making people crazy to think that an object could remotely affect another one.


Gravity is very similar to magnetism, a "magically" attracting force of distinct objects. The explanation is very easy via fields. Just trying to explain it via particles will get you into trouble, e.g. the flawed standard model, or the Higgs.

Einstein was pretty close in his spacetime bending explanation, but this still doesn't explain dark matter or the recent gravity experiments with fast rotating magnets in strong supra conductors. There are better gravity field theories out there, but they are lacking experimental verification. Some experiments are cheap, but most are cosmic scale, beyond simple galaxies.

This army guy came up with nice and cheap experiments, similar to Tajmar's experiments. We will see what will come out of it. Tajmar is also holding an old patent of an Anti-Gravity device, which nobody built so far. The army device with two rotating fields seems to be much better.

> Gravity is very similar to magnetism

The analogy doesn’t help, even ignoring that what we experience day-to-day is more like an electric field and there is a different gravitational analogy for magnetism, AKA gravitomagetism. It’s harder to notice than gravitational time dilation.

> easy via fields

Only for people who find non-Euclidian geometry that mixes time and space and is defined by a bunch of 4x4 tensors to be “easy”.

> [particles bad fields good]

“Particle” is a convenient human-scale name for things, but everything’s fields these days anyway. Even the Higgs.

In fact, GR is field-only — it’s QM which says “by the way, those things you call particles are a certain type of excitation in a field” and “if the gravity field has a particle it must be a massless spin-2 boson, also every massless spin-2 field is indistinguishable from gravity”.

> doesn’t explain dark matter

Doesn’t need to. It says there’s something to look for. GR is “stuff does X”, the observation is “X”, so there must be stuff. We’re can’t see stuff, but GR doesn’t say we have to be able to see it.

> fast rotating magnets

I don’t know what you’re referring to, the closest pattern I am matching your words to is Podkletnov whose work was never reproducible by third parties, and you can bet SpaceX will have tried.

> better gravity field theories out there, but they are lacking experimental verification

Science doesn’t use verification, it uses falsification. When an hypothesis can’t be falsified even in principle it isn’t science; when it can but only in principle, it’s a hypothesis; when it has been tested and the attempt to falsify it results in evidence that persistently doesn’t falsify it to a standard of statistical significance, then it gets published and perhaps elevated to the level of “theory”.

>Gravity is very similar to magnetism, a "magically" attracting force of distinct objects. The explanation is very easy via fields.

The explanation of how gravity is applied is easy via fields.

Not why gravity attracts and what causes that...

> Gravity is very similar to magnetism

But magnets, how do they work? (C)

They don't understand the dirac see and how field lines work, nor how a CL node operates when it's in magnetic mode. In fact, they even think that you can describe EM with vector math, which is half bollocks. Maxwell tried to formulate Vector equations from his Quaternion equations and failed, because it's not possible without loosing certain effects.

Certain Atoms or combinations cause their sourounding CL nodes to be in paramagnetic mode. (They are dislocated from groundstate to the left part of the diagram). If you heat the material under external magnetic field, the atoms and grains order to the external field. Once the matter is cold enough, the external Electron bindings hold the atoms in place that one large magnetic field is created. Everything is basically a phase looked loop oscillator and stabilizes its frequency to neighboring nodes.

Since you most likely only had contact with the standard model, giving enough explanation for you to understand the mechanics behind it would definitely be to long.

Dr. Stoyan Sargs - Basic Structures of Matter Supergravitation unified Theory, Chapter 2, 3, 6 and partially 12 (Planet magnetic field) should answer your question :)

When I was at uni my mother had a job as a nursery school teacher. One of the projects they did was to look at forces (quite good really!) So, they pulled ropes, and dropped apples, and sailed boats and used magnets. She asked me to think of a simple explanation of how the magnets worked so that the children could understand them. It is painful to me that I could not do that, and I can't do it now and I believe that no one can!

If you do not work on an important problem, it's unlikely you'll do important work. It's perfectly obvious. Great scientists have thought through, in a careful way, a number of important problems in their field, and they keep an eye on wondering how to attack them. Let me warn you, `important problem' must be phrased carefully. The three outstanding problems in physics, in a certain sense, were never worked on while I was at Bell Labs. By important I mean guaranteed a Nobel Prize and any sum of money you want to mention. We didn't work on (1) time travel, (2) teleportation, and (3) antigravity. They are not important problems because we do not have an attack. It's not the consequence that makes a problem important, it is that you have a reasonable attack. That is what makes a problem important. When I say that most scientists don't work on important problems, I mean it in that sense. The average scientist, so far as I can make out, spends almost all his time working on problems which he believes will not be important and he also doesn't believe that they will lead to important problems.

Richard Hamming, You and Your Research


Time travel back into the past is understood to be impossible.

Physicist here (by degree, not profession). What you state is not true: by currently accepted physical theories, time travel to the past IS very much possible. It's known as a "closed, time-like curve" in the literature. Now, most mechanisms explored for doing so wouldn't allow travel back further than the initial creation of the time travel device, so you could say that time travel to our past is not possible since the construction of such a thing is necessarily in the future. But as a matter of physics, general relativity allows it.

Now that said, most physicists are convinced that time travel cannot be possible, because if it were then paradoxical situations could be constructed--e.g. going back in time and killing yourself (the earlier you) before you entered the machine, or killing your grandfather before your father was born, etc. There are as many theorized rules preventing or limiting time travel as there are physicists that have looked at it, but critically they are all ADDITIONAL rules, postulated not to explain observable phenomenon but purely to think up a way time travel paradoxes might be avoided. Physics, as currently understood from available experimental evidence, 100% allows time travel to the past, paradoxes included.

> Physics, as currently understood from available experimental evidence, 100% allows time travel to the past, paradoxes included.

General Relativity, which is where the term "closed timelike curve" is used, does not allow paradoxes. A spacetime with closed timelike curves in it (for example, the Godel universe) still has only one thing happening at each event.

The real problem conceptually with such solutions is that they appear to contradict free will: since only one thing can happen at each event, if you experience visiting that event multiple times, you cannot freely choose what to do at that event.

Luckily "free will" as would fit a layman's interpretation of the term is already incompatible with physics, so the existence of CTCs doesn't pose additional problems.

I think most people would interpret free will to mean something that is perfectly compatible with any definition of determinism.

If I decide to raise my right hand, I raise it. Don't get pedantic in regards to disabled people. It doesn't matter if the universe is completely deterministic, that doesn't mean that I'm not the entity that decided to raise my right hand. The deterministic universe may have produced me, but the software I'm running is me. The reason I raised my hand is because of me. Sure there are an infinite number of factors that have fed into the circumstances of my birth, and everything that's happened to me, but everything that I do is specifically as a consequence of my free will. I am the software, deterministic or not.

That's an interpretation of free will that I completely subscribe to, but I do not think it is a majority view.

It's still the unavoidable consequence of the past.

> "free will" as would fit a layman's interpretation of the term is already incompatible with physics

That depends on which layman you talk to and which theory of physics you are using.

> you cannot freely choose what to do

But that's already the case. It could be my free will to walk on the ceiling, but the laws of physics won't let me.

Yes, and in the case of a closed timelike curve, the laws of physics won't let you do something different the second time you are at a particular event in spacetime than you did the first time. Both are straightforward constraints from the point of view of the laws of physics.

But that still does not mean they look the same to us, intuitively speaking. For example, the laws of physics won't let you walk on the ceiling at all, whether your worldline is a closed timelike curve or not. But if your worldline is not a closed timelike curve, then, intuitively, it seems like the laws of physics will let you choose which direction you want to walk on the floor. However, if your worldline is a closed timelike curve, and you come around again to the event in spacetime where you chose to walk to the left last time around, the laws of physics will not let you choose to walk to the right this time; they constrain you to walk to the left again, since from the standpoint of the laws of physics you are at the same event in spacetime and that event can only happen one way. To many people that seems like a much stronger constraint than the constraint that you can't choose to walk on the ceiling.

Definitely not a physicist here but isn't forward time travel somewhat possible by approaching speed of light? I read about effects where if I left earth, travelled very fast a year to me might be many to earth. Could a person not therefore 'travel fast' somewhere and come back so a place like earth had advanced significantly in time relative to them?

Not sure if that is time travel in a 'jump to a new time' sense but it effectively is. And I wonder how far you could push that with theoretical technology. 1 day for 1000 years type thing...

We are doing forward time travel right now at a rate of 1sec/sec.

Only from your point of view. My head and my feet travel at slightly different speeds.

What are your head and your feet? Fluids are flowing in and out of them, outer layers are sloughing off, they're warm enough to be quite bright in infrared, potassium-40 in your head and feet are beta-decaying and so on. A decent fraction of the momentary mass in your brain may be in your feet four heartbeats later (and vice-versa). Are you implicitly considering them in some time-averaged way?

(A comment similar to yours would be that your head is older than your feet because of general relativistic effects. Even if we ignore everything before you reached adulthood, there's a Ship of Theseus thing going on thanks to tissue turnover on timescales of weeks to months e.g. https://en.wikipedia.org/wiki/Bone_remodeling_period ).

Whose second? You and I at different points on Earth and our seconds differ. Even a decade ago we could show that a second precisely defined in the middle of the floor of a laboratory could not apply to the whole of the laboratory.

"General relativity states that time speeds up for objects as gravity weakens. To demonstrate this, Chou and his colleagues raised one optical clock 33 centimetres above another. The slightly lower gravity at that height meant that compared with the reference clock, the raised clock ticked with a fractional boost in frequency of 4 × 10^–17, equivalent to a gain of 90 billionths of a second over 79 years."

Even ignoring the distance between different edges of your brain and that there are humans in orbit and in mountain communities as well as below sea level, there are a number of processes in human brains that produce movement an order of magnitude faster (over short distances) not to mention people driving or flying much than the speed in this paragraph:

"To demonstrate special relativity, which says that time slows down for moving objects, the researchers jolted the single atom in their optical clock so that it oscillated at relative speeds of less than 10 metres per second, or 36 kilometres per hour. This time, the clock's ticks seemed to drop by a fractional frequency of almost 6 × 10^–16."

https://www.nature.com/news/2010/100923/full/news.2010.487.h... (Chou et al.'s article is available at http://spiff.rit.edu/classes/phys200/lectures/dilation/chou/... and starts on the bottom of the second page of the PDF) [and cf my Finally paragraph below]

Importantly when relating an ultraprecise clock on the ground with an ultraprecise clock passing above it in a blimp at, say, a constant linear ten metres per second, a Lorentz transform will need general-relativistic corrections.

These corrections seem negligibly small but it seems safe to bet that in a few years they'll matter in a rack full of high end computers.

Clock precision has also gotten much better http://iopscience.iop.org/article/10.1088/1742-6596/723/1/01... -- "Measurements made with the rubidium fountains (and other clocks at USNO) after the first 1.5 years of continuous operation resulted in the most precise test of the universality of the gravitational redshift and enabled constraints on the coupling of fundamental constants to gravity to be improved by on average a factor of four"

Meanwhile, just to plug some of the work HN's user ISL's group is doing for an extreme setup where the dependence of second not just on position on Earth but on dynamical processes in different stratified-planet layers already matters for practical purposes: https://www.npl.washington.edu/eotwash/node/17 ("seismic Newtonian noise ... the gravitational attraction of an optic to high density regions of the ground caused by passing seismic waves").

Finally, and also because GPS is mentioned here and there in this thread, I recommend slides 16-19 of http://web.stanford.edu/group/scpnt/pnt/PNT18/presentation_f... If we are to insist on the notion of the flow of time, we should be aware that it poses some serious practical problems as one increases the density of marks on a chosen time axis in a dynamical spacetime like ours and populated with sources (in the Einstein Field Equation sense) like those around us. Approaching the limit of ultradense time coordinates we have to consider how parts of ourselves are sources, some organs and regions of organs being denser than others just for starters. And that's true even without getting anywhere near metaphysics and philosophical issues.

Regarding the time paradoxes, the second part of your comment reminded me of a very interesting (and good) 1977 Czech film called "Tomorrow I'll Wake Up and Scald Myself with Tea"[0], in which some Nazis survive WWII. They get ahold of some anti-aging pills, and after centuries, hijack a time machine back to World War II in order to give Hitler the bomb, reversing the course of the war. However, their efforts are repeatedly stymied by comical time paradoxes.

[0] https://en.wikipedia.org/wiki/Tomorrow_I'll_Wake_Up_and_Scal...

I've never understood why "paradoxes make cause-and-effect-reasoning difficult" concerns are a valid argument that something could not be possible. Does physics care whether the workings of the universe are accessible to our logic?

If I look at a random numeric oracle of 1-10, and consider the result as N, and apply to it some kind of mathematical formalism of:

If ( N + 10 = 20 ) then eat(last_bagel)

My wife comes home and is cross with me for eating the last_bagel. So, I hop in my handy time machine, travel back to before the time where I access the oracle, and alter it to only produce a random number from 1-9.

If I succeed, my wife is no longer cross with me. But, if she is no longer cross with me, then why would I alter past events? Likely, I would have no cause to travel back in time in this scenario, so the oracle remains programmed to produce up to 10.

Now my wife is cross with me again.

Which one happens?

Of course there is no answer to this question yet, but thats the point. Just because a thought experiment doesn't solve it, doesn't mean it cannot be physically produced.

For all we know in actual implementation you would simply eat the bagel, time travel back and bias the oracle, but your conscious experience only ever consists of finding that your oracle was modified and has your fingerprints on it somehow.

Yet quantum mechanics is full of phenomenon that operate counter to our understanding of cause and effect. Physicists accept things like double slit interference - why do they seem to consider these time travel induced causal paradoxes so off-limits?

Mostly because a bagel is not an electron, and slit experiments illustrate wave-particle duality, do they not?

Anyhow, the message was about how paradoxes make reasoning hard, not about how we can wave away the uncertainties. Next time I'll just skip all the time machine nonesense and head to the nearest rectory.

I think the point made by user jbattle is that physics already allows, for example, causal relationships that go either “back in time” or “faster than light” from certain valid reference frames. Physicists have found ways of coping with this reality. Why should the introduction of a time machine make it fundamentally any different?

> Does physics care whether the workings of the universe are accessible to our logic?

the universe itself doesn't care, but physics, that is, our math based theories, certainly they do, in fact they require it.

Not in the way the post you are replying to means. Our “math based theories” of physics do not require a world without cause-effect paradoxes. Indeed they seem to explicitly permit them.

> by currently accepted physical theories, time travel to the past IS very much possible

Only if you make use of exotic matter, which nobody has ever observed and which there are good reasons to think cannot exist.

You can't get CTCs in Minkowski space.

Hawking 1982 https://doi.org/10.1103/PhysRevD.46.603 has an excellent argument based on gravitational backreaction about the implausibility of CTCs (and closed null curves) in any finite spacetime, including expanding spacetimes in which there are cosmic horizons at finite distances. In our standard cosmology you'd need extremely exotic matter.

> general relativity allows it

General Relativity requires neither the WEC nor the Lorentzian (1,3,0) metric signature (e.g. one can use a signature discontinuity on some hypersurface or atypical values for (virt,phys,rank)). You can describe low-entropy "objects" with all sorts of unaligned directions for their arrows-of-time that way, but good luck explaining the observations of the cosmic microwave background or recovering Special Relativity in a lab test. AWEC is there to give us large scale structure at all, not "purely to think up a way time travel paradoxes might be avoided".

> paradoxical situations could be constructed--e.g. going back in time and killing yourself

Start much more easily. Set up a worldtube containing the timelike and null paths for para-positronium in an anti-de Sitter_{3} space in 1+3 Minkowski space by tracing the worldtubes of the leptons and the photons as they loop around for some small multiple of the lifetime of para-positronium. You'll want the block universe view: at any given point on each closed there is a frozen configuration. Coarsely, at each point (t,x,y,z) on the loop of each CTC there is an electron, a positron, or neither; more technically the conjugate variables at each point on a closed curve are fixed. So you need a way to recover the para-positronium from the photons. There seems to be a lot fewer degrees of freedom than in the absence of the CTC!



You'll quickly find that thinking of a human being (radiating in the IR while metabolizing, shedding skin and hair, perspiring, and so on) "lucky" enough to be near a supersymmetric charged rotating black hole in AdS_{5} with microscopic parts on a large set of worldtubes some of which are closed (some closed and timelike, some closed and null) and some of which might not be, is so intractable that you can only make extremely wild guesses about interactions with a grandparent whose microscopic components are almost entirely not on closed curves.

> Physics, as currently understood from available experimental evidence, 100% allows time travel to the past

No time travellers showed up at Hawking's champagne party afaict, but maybe they were sneakily stealing socks out of his washing machine rather than trying to make small talk.

> paradoxes

Most of these vanish in a block universe view where everything is encoded in a nontrivial set of spacetime-filling fields. Lots of apparent paradoxes arise from incomplete values surfaces in initial values formulations. Lots of others relate to something similar to pareidolia where given a block universe people will insist that they see faces and cats and planets without admitting that they're imposing a particular time axis (and orientation) and using that as a basis for segmenting a huge bunch of worldtubes (that collectively they take as being a single object).

> then paradoxical situations could be constructed--e.g. going back in time and killing yourself (the earlier you) before you entered the machine, or killing your grandfather before your father was born

Can't the paradox be resolved by considering it as a fork/branching of a new reality/timeline where the grandfather is killed, kind of similar to many world interpretation in QM. I'd speculate that using model of lazy fork it may be possible to get some ballpark estimates on energy requirements/etc. below the typical "all the energy in the Universe". Such forks/perturbations possibly also dissipate, like a small wave in the pond, and the forked reality gets naturally merged into the mainline ( i think this is how QM's many world while possibly true is still compatible with "realistic" situation of limited number, may be even just 1, Universe mainlines)

PBS Space Time just did an episode on this. Turns out, based purely on the maths it is possible, however highly improbable for us to do it.


Which he alludes to with the phrase

"They are not important problems because we do not have an attack."

That said, closed time like curves are not excluded as possible by General Relativity.

Lots of things were understood to be impossible until they weren’t.

Certain kinds of it.

Time travel to the past in a parallel universe at least doesn't cause any logical problems ('paradoxes') to arise.


And where would the information about parallel universes be contained?

Far, far away in our infinite space. Further than we can see, because if we look more than 14B light years away we're blinded by the Big Bang of our own creation.

Max Tegmark calculated out that parallel universes are probablistically present around 10^21 an 10^23 light years out from us, if matter's about as dense and as randomly distributed as in our universe.

You mean everettian worlds? In the wave function.

i would figure in that parallel universe lol, or if you want to be more general, in some overarching metaversal container leading out to the omniverse at greeter degrees. but that's just me ¯\_(ツ)_/¯

So the universe would be composed of infinite energy?

Yes, a more in depth analysis of this can be found at https://en.wikipedia.org/wiki/No-communication_theorem

this should properly be called a conjecture, as I have never encountered a formal proof of this "theorem"

The math say time travel is possible, but the energy required would consume the universe, depending on what paper your read. There is no reason math wise the arrow of time only goes in one direction.

Erm, there is something fundental discovered in thermodynamics that we call entropy. Energy only flows in one direction and time as well.

So you know, I in general try not to pull comments out of thin air.




The MATH allows it.

> Throughout his career, Witten conducted research into gravitation, quantum gravity, and general relativity. The last one of these is the theory first put forward by Albert Einstein that proposes that gravity is essentially a warp or curve in the geometry of space-time caused by mass.

You're assuming that your readers are already familiar with quantum gravity, but need an introduction to general relativity?

They're assuming that the readers will infer, from the name "quantum gravity", that it has to do with gravity. From just the name, "general relativity," people who don't know what it is probably wouldn't guess it's a theory of gravity.

> Throughout the expansive Project Outgrowth document, Mead and the other scientists also explored field propulsion, defined as those concepts which use “electric and/or magnetic fields to accelerate an ionized working fluid,

hall effect thrusters


totally compliant with conventional physics

There are Gravity Stones from the 60's scattered among US colleges to

> "remind students of the blessings forthcoming when a semi-insulator is discovered in order to harness gravity as a free power and reduce airplane accidents"

Decent way to get students thinking about it.


Oh the story behind that is a wild ride from start to finish. Babson's sister drowned when they were children, and he decided the real reason that happened is anti-gravity technology wasn't sufficiently advanced to prevent her drowning. He spent a large amount of his philanthropy funding anti-gravity research for the rest of his life.

Edit: Just realized this is mentioned in the article you linked.

It's still worth a call-out. The other line in the article that always gets me is "Sometimes, attendees [of the Gravity Research Foundation] sat in chairs with their feet higher than their heads, to counterbalance gravity."

It feels like absurd lore in Fallout, but probably actually happened.

I took a look and found plenty of semi-insulators out there, but no peep about what association those materials have to do with taming the gravitational force - or am I missing the point?

Maybe it means a gravitational insulator?

Why did he think to mention airplanes? The whole concept of airplanes would be obsolete with anti-gravity.

I feel like in 40% of SF stories it will at one point be important that your spaceship not have the glide ratio of a buttered brick, even if it has an antigravity drive.

Depends on the nature of the anti-gravity drive. If it's like an engine, which needs some sort of process to operate, then yes, it can fail to work and you want a backup system. If it's a specific material, which works like a magnet in that the anti-gravity effect is present as a result of the passive configuration of matter, less so.

The Enemy could still shoot and destroy your cavorite plates, or the mechanism which turns them so that it does or doesn't block gravity from the desired direction.

This is a terrible article that is little more than conspiracy theorist baiting. For example, the author takes this quote from Ben Rich:

>“We have some new things. We are not stagnating. What we are doing is updating ourselves, without advertising. There are some new programs, and there are certain things, some of them 20 or 30 years old, that are still breakthroughs and appropriate to keep quiet about [because] other people don’t have them yet.”

and by throwing the quote at the end of a long series of anti-gravity/EM papers tries to give the impression that the military is hiding something wild like a secret anti-gravity device. It's very National Enquirer.

There are other examples of this. The discussion of the WEAV pulls the quote "no moving parts and assures near-instantaneous response time", which presumably is to hint that this explains those instantly-accelerating discs seen in various videos. That "near-instantaneous response time" refers to the ability to change thrust quickly and not velocity is, of course, obscured.

>This is a terrible article

From what I have read up till now on thedrive, the whole outlet is terrible, all their articles seem to be drivel like this.

What is the currently accepted answer to the question, "what is gravity"?

It is an illusion. You are floating freely in the space, but massive mass near-by is causing curvature in space-time. In other words: Time goes more slowly in your shoes, because they are closer to the mass. This causes any movement to be circular movement, which is pressing you to the near-by mass, which you experience as "gravity".

Wow, what an excellent way of phrasing it. Give me more. Why "any movement is circular"?

It seems to me that, as soon as there is mass in the universe which curves space-time, all paths of movement are closed. To a first approximation, for anything close to a single large mass that means an elliptical path.

That assumes, of course, that an instantaneous "path of movement" is something that is even worth considering, given that it ignores the time dimension and the fact that all those other objects aren't remaining in a static configuration around the object whose path we are considering.

I would think that parabolic trajectories only close in infinity, and hyperbolic not even there? Or both close only in infinity? The math on that is currently out of my brain's cache.

They all close. Worst case is they'll get longer and longer as the universe expands and the objects will never be able to complete them and come back for a second lap.

Einstein believed gravity isn't really a force in the same sense as the other three fundamental forces, but a property of space-time. That seems to be a problem because it causes some kind of "incompatibilty" with the others. Trying to reconcile those, efforts has been made to create quantum gravity theories, that view space itself as quantized and in this direction you find strings and quantum loop gravity.

Disclaimer: I'm no expert, just parroting what I found in Wikipedia.

IANAP either but as far as I understand the incompatibility is not on wether gravity is a force or not (that's just a way to interpret the equations). The issue is that all attempts to quantize gravity have failed so far (due to the specific mathematical properties of the gravitational field) and QFT would require gravity to be quantized to interact with other quantum fields.

Gravity is one of the essential forces in nature. If you want to explain it in simpler terms it is not possible because gravity is the simpler thing. You can use gravity to explain other things, but you cannot use other things to explain gravity.

That’s not really true. The other 3 “fundamental” forces are irreducible yet still entirely explained by the standard model.

Well, gravity is similarly explained by general relativity.

The dent in the rubber sheet that the earth currently sits on - hence the whole curvature in spacetime thing. As far as I know no one has figured out or found gravitons yet, so it's just a theory.

Which I assume means there should be a few gravity-deniers out there to give themselves a break from global heating denial. :)

That which maketh the apple bonk the skull most uninvitedly.

Apple bonkers?

Curvature in spacetime.

This video by Vsauce really put it in a way that made it stick for me.


Is this article just a derivative of the patents that were discussed on HN several weeks ago?

I liked that discussion. Plenty of varying perspectives such as "Bob Lasar was proven right!" and "in an arms race, keep your enemy busy [with disinformation]".

Is there new info here?

I agree it's just a rehashing of information they know will get a lot of clicks. I have a vested interest in this topic so I've been following it closely and I don't see anything new here.

I checked that this article wasn't posted on 1 April.

Not surprising in the 1950's. Every other science fiction author was a believer - they wrote endlessly about shields that 'blocked' gravity, fields that interacted with gravity, materials that had variable gravitational force, and on and on. Their speculation helped fuel/was fueled by science investigation of that era.

For anyone interested, a book was written on this about 15 years ago by a reporter from "Janes Defense Weekly". "The Hunt for Zero Point: Inside the Classified World of Antigravity Technology". Not as tin-hat as you might think, mostly a story of a lot of dead ends while trying to research some DARPA dark projects.

If you like this, do check out Rex Research. You have to sift through a lot of BS but there's some great stuff buried in it.


Not sure how this topic could be covered without a mention of Dr. Ning Li's work:


They must have some reason to think it's possible. I've also always wondered if Coral Castle was really constructed using some ancient lost technique to harness this power. Takes foil hat back off...

They'll at some point stumble across a Goa'uld device that does precisely that. But for the love of God, don't just plug it in!

To really be able to manipulate gravity, you would also need to manipulate mass, and so far nuclear is the only way I know of, in which we do that, but uhh.. I don't think we'll get anti gravity until we can control mini stars... Little nuclear engines.

It is a common myth that nuclear reactions "turn mass into energy". What actually happens is that the products of the reaction have less binding energy than the initial nucleus. Since energy gravitates, this reduces the mass of the products. The exact same thing happens in chemical reactions. If you burn something, the product are lighter than the initial substances by the mass equivalent of the energy released.

"...energy gravitates, this reduces the mass of the products." "...the product are lighter than the initial substances by the mass equivalent of the energy released."

How are these not restating "turn mass into energy"?

Both scenarios approximate: High Mass => Low Mass + Energy

https://youtu.be/Xo232kyTsO0 check out this PBS spacetime explanation for what it means for energy to have mass. It's impossible to convert energy into mass, adding up all the mass inside a composite does not equal the true mass, because energy itself has mass. When we split an atom, we are releasing some of that extra (energy) that was stored as potential energy holding the parts together, not converting energy into matter, just moving states of energy to other types.

I change the location of my gravitational field every time I move my body. I modify my gravitational field with every breath as I breath out heavier gases and breath in lighter gases.

The thing about gravitational energy is that it's the weakest of all fundamental forces. Nuclear energy is more about releasing the energy stored in the Strong/Weak Nuclear Interaction than it is about gravity.

Gravity is a property of energy, of which mass is one form. That means a laser beam attracts other energy via gravity.

What if gravity isn't always relative to mass? I mean, all of our scientific evidence suggests it is... but I suppose it's possible that's not quite right.

Actually we know that gravity in fact not due to mass. Mass is just one component of what actually interacts with gravity.

Gravity is related to time and entropy.

oh, you mean since Roswell? ok.


The problem is that the package hasn’t been updated for python3 :D

nobody liked this? This makes me so sad :D

I hear the research has been looking up.

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