Posted on 12/13/2018 2:07:09 PM PST by ETL
The concept of time travel has always captured the imagination of physicists and laypersons alike. But is it really possible? Of course it is. We're doing it right now, aren't we? We are all traveling into the future one second at a time.
But that was not what you were thinking. Can we travel much further into the future? Absolutely. If we could travel close to the speed of light, or in the proximity of a black hole, time would slow down enabling us to travel arbitrarily far into the future. The really interesting question is whether we can travel back into the past.
I am a physics professor at the University of Massachusetts, Dartmouth, and first heard about the notion of time travel when I was 7, from a 1980 episode of Carl Sagan's classic TV series, "Cosmos." I decided right then that someday, I was going to pursue a deep study of the theory that underlies such creative and remarkable ideas: Einstein's relativity. Twenty years later, I emerged with a Ph.D. in the field and have been an active researcher in the theory ever since.
Now, one of my doctoral students has just published a paper in the journal Classical and Quantum Gravity that describes how to build a time machine using a very simple construction.
Closed time-like curves
Einstein's general theory of relativity allows for the possibility of warping time to such a high degree that it actually folds upon itself, resulting in a time loop. Imagine you're traveling along this loop; that means that at some point, you'd end up at a moment in the past and begin experiencing the same moments since, all over again – a bit like deja vu, except you wouldn't realize it. Such constructs are often referred to as "closed time-like curves" or CTCs in the research literature, and popularly referred to as "time machines." Time machines are a byproduct of effective faster-than-light travel schemes and understanding them can improve our understanding of how the universe works.
Over the past few decades well-known physicists like Kip Thorne and Stephen Hawking produced seminal work on models related to time machines.
The general conclusion that has emerged from previous research, including Thorne's and Hawking's, is that nature forbids time loops. This is perhaps best explained in Hawking's "Chronology Protection Conjecture," which essentially says that nature doesn't allow for changes to its past history, thus sparing us from the paradoxes that can emerge if time travel were possible.
Perhaps the most well-known amongst these paradoxes that emerge due to time travel into the past is the so-called "grandfather paradox" in which a traveler goes back into the past and murders his own grandfather. This alters the course of history in a way that a contradiction emerges: The traveler was never born and therefore cannot exist. There have been many movie and novel plots based on the paradoxes that result from time travel – perhaps some of the most popular ones being the "Back to the Future" movies and "Groundhog Day."
Exotic matter
Depending on the details, different physical phenomena may intervene to prevent closed time-like curves from developing in physical systems. The most common is the requirement for a particular type of "exotic" matter that must be present in order for a time loop to exist. Loosely speaking, exotic matter is matter that has negative mass. The problem is negative mass is not known to exist in nature.
Caroline Mallary, a doctoral student at the University of Massachusetts Dartmouth has published a new model for a time machine in the journal Classical & Quantum Gravity. This new model does not require any negative mass exotic material and offers a very simple design.
Mallary's model consists of two super long cars – built of material that is not exotic, and have positive mass – parked in parallel. One car moves forward rapidly, leaving the other parked. Mallary was able to show that in such a setup, a time loop can be found in the space between the cars.
So can you build this in your backyard?
If you suspect there is a catch, you are correct. Mallary's model requires that the center of each car has infinite density. That means they contain objects – called singularities – with an infinite density, temperature and pressure. Moreover, unlike singularities that are present in the interior of black holes, which makes them totally inaccessible from the outside, the singularities in Mallary's model are completely bare and observable, and therefore have true physical effects.
Physicists don't expect such peculiar objects to exist in nature either. So, unfortunately a time machine is not going to be available anytime soon. However, this work shows that physicists may have to refine their ideas about why closed time-like curves are forbidden.
Explore further: Stephen Hawking's final book suggests time travel may one day be possible – here's what to make of it
Consider for a moment that when you are traveling in a commercial airliner, the pilot and crew are navigating to your destination with the aid of data from the Global Navigation Satellite System (GNSS), of which the United States NAVSTAR Global Positioning System (GPS for short) is the most familiar component.
In fact, “GPS” is often synonymous with satellite navigation, even it is now one of three global satellite navigation systems in operation along with the Russian GLONASS and EU Galileo satellite systems (they will be joined by the Chinese BeiDou-2 system when it expands to global scale in the early 2020s), While this article is specifically about NAVSTAR GPS, the basic operating principles are similar across the various GNSS implementations.
GPS was developed by the United States Department of Defense to provide a satellite-based navigation system for the U.S. military. It was later put under joint DoD and Department of Transportation control to provide for both military and civilian navigation uses, and has become a part of daily life.
Most recent-model cars are equipped with built-in GPS navigation systems (increasingly as standard equipment), you can purchase hand-held GPS navigation units that will give you your position on the Earth (latitude, longitude, and altitude) to an accuracy of 5 to 10 meters that weigh only a few ounces and cost around $100, and GPS technology is increasingly found in smartphones (though not all smartphones derive location information from GPS satellites).
The nominal GPS configuration consists of a network of 24 satellites in high orbits around the Earth, but up to 30 or so satellites may be on station at any given time.
Each satellite in the GPS constellation orbits at an altitude of about 20,000 km from the ground, and has an orbital speed of about 14,000 km/hour (the orbital period is roughly 12 hours - contrary to popular belief, GPS satellites are not in geosynchronous or geostationary orbits). The satellite orbits are distributed so that at least 4 satellites are always visible from any point on the Earth at any given instant (with up to 12 visible at one time). Each satellite carries with it an atomic clock that “ticks” with a nominal accuracy of 1 nanosecond (1 billionth of a second).
A GPS receiver in an airplane determines its current position and course by comparing the time signals it receives from the currently visible GPS satellites (usually 6 to 12) and trilaterating on the known positions of each satellite[1]. The precision achieved is remarkable: even a simple hand-held GPS receiver can determine your absolute position on the surface of the Earth to within 5 to 10 meters in only a few seconds. A GPS receiver in a car can give accurate readings of position, speed, and course in real-time!
More sophisticated techniques, like Differential GPS (DGPS) and Real-Time Kinematic (RTK) methods, deliver centimeter-level positions with a few minutes of measurement. Such methods allow use of GPS and related satellite navigation system data to be used for high-precision surveying, autonomous driving, and other applications requiring greater real-time position accuracy than can be achieved with standard GPS receivers.
To achieve this level of precision, the clock ticks from the GPS satellites must be known to an accuracy of 20-30 nanoseconds. However, because the satellites are constantly moving relative to observers on the Earth, effects predicted by the Special and General theories of Relativity must be taken into account to achieve the desired 20-30 nanosecond accuracy.
Because an observer on the ground sees the satellites in motion relative to them, Special Relativity predicts that we should see their clocks ticking more slowly (see the Special Relativity lecture). Special Relativity predicts that the on-board atomic clocks on the satellites should fall behind clocks on the ground by about 7 microseconds per day because of the slower ticking rate due to the time dilation effect of their relative motion [2].
Further, the satellites are in orbits high above the Earth, where the curvature of spacetime due to the Earth’s mass is less than it is at the Earth’s surface. A prediction of General Relativity is that clocks closer to a massive object will seem to tick more slowly than those located further away (see the Black Holes lecture).
As such, when viewed from the surface of the Earth, the clocks on the satellites appear to be ticking faster than identical clocks on the ground. A calculation using General Relativity predicts that the clocks in each GPS satellite should get ahead of ground-based clocks by 45 microseconds per day.
The combination of these two relativitic effects means that the clocks on-board each satellite should tick faster than identical clocks on the ground by about 38 microseconds per day (45-7=38)! This sounds small, but the high-precision required of the GPS system requires nanosecond accuracy, and 38 microseconds is 38,000 nanoseconds.
If these effects were not properly taken into account, a navigational fix based on the GPS constellation would be false after only 2 minutes, and errors in global positions would continue to accumulate at a rate of about 10 kilometers each day! The whole system would be utterly worthless for navigation in a very short time.
The engineers who designed the GPS system included these relativistic effects when they designed and deployed the system. For example, to counteract the General Relativistic effect once on orbit, the onboard clocks were designed to “tick” at a slower frequency than ground reference clocks, so that once they were in their proper orbit stations their clocks would appear to tick at about the correct rate as compared to the reference atomic clocks at the GPS ground stations.
Further, each GPS receiver has built into it a microcomputer that, in addition to performing the calculation of position using 3D trilateration, will also compute any additional special relativistic timing calculations required [3], using data provided by the satellites.
Relativity is not just some abstract mathematical theory: understanding it is absolutely essential for our global navigation system to work properly.
http://www.astronomy.ohio-state.edu/~pogge/Ast162/Unit5/gps.html
However, if infinite mass is the only requirement, Michael Moore and Rosie O'Donnell could very well be from the future.
Oh, I thought it said infinite ASS.
Every time I see an article like this I want to ask the same question..
Which came first? MATTER OR SPACE ?
Musta been space, cuz the matter has to have a place to exist?
If matter, then no time cuz time is something that measures what matter does(I.E.Changes in/of, no?..No Matter, then No Time,no?
Words are fun— UNICORN...MERMAID....
Noticable with a wrist watch or a calendar, yes. But if tou have two atomic clocks you can notice much smaller changes. I saw a show where scientists syched up two atomic clocks, drove one up a mountain, stayed there overnight and drove back down with it. It was a detectable amount of time off from the other one.
GPS satellites have to be constantly adjusted because of this difference. If not, their error would increase until they were worthless.
Mass approaches infinity as an object approaches the speed of light. This is why an object (with mass) can never reach the speed of light.
Precisely at the speed of light the oblect's mass would be infinite (an impossibility of course).
Two other things happen as well. The object's length approaches zero, and a tick of the clock grows to infinity.
All of these things are realized from an outside stationary observer's point of view.
Are words actually equal to fun...or is that statement a metaphor? How about n words or f words? Are those also fun? Sorry...just had to get COSMICALLY precise.
Like energy and matter, via e=mc^2, matter and space may be manifestations of the same thing. ie, space and energy may be coupled in a similar way as matter and energy.
I’ve always thought there is no such thing as time in the sense that there is mass or gravity. As you say, it’s something we perceive, but does not exist as it’s own thing. Then again, I’m not a physicist.
Every time I see an article like this I want to ask the same question... Which came first? MATTER OR SPACE?
Like energy and matter, via e=mc^2, matter and space may be manifestations of the same thing. ie, matter and space may be coupled in a similar way as matter and energy.
Running into any spec of dust at those speeds would be quite a bit more severe than a bug on the windshield......
Nah.
IIRC, FWIW, in the 1960s, the US Navy took two matched Cesium clocks, and put one aloft on a plane for some period of time.
Eventually, the clocks were brought back together, and the one with flight time was behind the one that stayed on the ground.
I think a MIT student, also in the 1960s came up with a theory to travel back in time, but hit the "infinite mass" issue as well.
Thanks ETL.
“object of infinite mass”? Wouldn’t criminal & liar hillary fit in there somewhere? She is putting-on a few pounds after all.
When you accelerate an object (change its velocity, velocity being a two-component vector consisting of speed AND direction) the object moves into a different space-time reference frame. Its clock slows down relative to the clock in the previous frame. ie, its sense of time progressively slows the more it is accelerated.
But in order to change an object’s velocity, an outside force must be applied (F=ma). Without any outside forces being applied, the object would remain in an ‘inertial’ state, move at a constant speed AND direction.
The law of inertia states that an object will remain at rest or in a state of uniform motion (constant speed AND direction) unless acted upon by an outside force.
Went out on a blind date with a woman that had so much mass that it seemed like it took forever for me to get away.
Nah. IIRC, FWIW, in the 1960s, the US Navy took two matched Cesium clocks, and put one aloft on a plane for some period of time. Eventually, the clocks were brought back together, and the one with flight time was behind the one that stayed on the ground.
I meant noticeable to us, without relying on an atomic/cesium clock. As for the airplane experiment, see my post 38. And also 41, regarding the GPS satellite network and Relativity.
Interesting OP. Thanks.
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