Time Travel And Black Holes
In 1963, New Zealand mathematician Roy Kerr used the field equations to analyze a rotating black hole, called a Kerr black hole, and found that the results allowed a path through a wormhole in the black hole, missing the singularity at the center, and make it out the other end. This scenario also allows for closed timelike curves, as theoretical physicist Kip Thorne realized years later.
In the early 1980s, while Carl Sagan worked on his 1985 novel Contact, he approached Kip Thorne with a question about the physics of time travel, which inspired Thorne to examine the concept of using a black hole as a means of time travel. Together with the physicist Sung-Won Kim, Thorne realized that you could have a black hole with a wormhole connecting it to another point in space held open by some form of negative energy.
But just because you have a wormhole doesn’t mean that you have a time machine. Now, let’s assume that you could move one end of the wormhole . You place the movable end on a spaceship, shooting it off into space at nearly the speed of light. Time dilation kicks in, and the time experienced by the movable end is much less than the time experienced by the fixed end. Let’s assume that you move the movable end 5,000 years into the future of the Earth, but the movable end only “ages” 5 years. So you leave in 2010 AD, say, and arrive in 7010 AD.
However, if you travel through the movable end, you will actually pop out of the fixed end in 2015 AD . What? How does this work?
How Was Time Created
The measurement of time began with the invention of sundials in ancient Egypt some time prior to 1500 B.C. However, the time the Egyptians measured was not the same as the time todays clocks measure. For the Egyptians, and indeed for a further three millennia, the basic unit of time was the period of daylight.
The Bond Between Quantum Physics And Time Travel
Dr Samuel Beckett can go back in time in the movie Quantum Leap, but only to the years of his past. Although he swaps places with historical figures like Lee Harvey Oswald, others recognise him as that person. According to the show, Sam was reportedly the Secret Service agent who saved Jackie Kennedy. Once he has corrected what was wrong, he leaps into another body.
There are hand-waving explanations for how he leapt into the bodies, even though the show does not discuss why he did so. It has something to do with quantum mechanics, as the title would imply.
However, it is unlikely to be connected to true quantum leaps. When an electron jumps directly from one atoms orbit to another without stopping in between, that is when it happens. And while thats cool, it wont result in time travel.
In quantum physics, there is a lot of debate over time. Time is significantly more malleable at the micro level, where waves and particles can behave similarly than in our visible classical physics universe.
Consider the clock on the wall. You can move the hands backwards, but this does not cause the time to reverse. Time moves on. However, in the quantum domain, things are easier. If we can express particle activity mathematically in one direction, we can also express it mathematically in the other.
In other words, time travel makes sense when viewed through a quantum lens. Whatever moves forward must also be capable of moving backwards.
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A Form Of Time Travel That Might Be Possible
Of the four dimensions, only time is mysterious. We can play games with the other three, the spatial dimensions imagining, for example, 2- or even 1-dimensional universes, as in Flatland . But in the end, they follow the rules. Time goes in one direction only and time travel moving time in the other direction is easy to imagine in principle but full of nearly impossible conundrums in practice. Astrophysicists struggle to figure it out.
Our brains are adapted to keeping track of the present and the past . But that only tells us how we come to be aware of time. It doesnt tell us what time is. We get a bit closer, says NASA astronomer Sten Odenwald, if we keep in mind that our experience of time is closely linked to entropy, the gradual running down of the universe:
But why do we remember the past and not the future? The reason for this asymmetry has to do with entropy the amount of disorder in the universe. We have memories and historical records only because entropy in the past was lower than the entropy of the present.
Sten Odenwald , The struggle to find the origins of time at Astronomy Magazine
Curiously, the one-way-only view of time does not apply in quantum mechanics:
But it does apply to us, and helps explain why we are all mortal:
Perhaps irresolvable stuff is just what happens if we try to reverse entropy in this universe.
Of course, some physicists argue that if we could revisit the past, we couldnt change it. That might be a way around the problem:
According To Current Physical Theory Is It Possible For A Humanbeing To Travel Through Time
As several respondents noted, we constantly travel throughtime–just forward, and all at the same rate. But seriously,time travel is more than mere fantasy, as noted by GaryT. Horowitz, a professor of physics at the University ofCalifornia at Santa Barbara:
“Perhaps surprisingly, this turns out to be a subtle question.It is not obviously ruled out by our current laws of nature.Recent investigations into this question have provided someevidence that the answer is no, but it has not yet been provento be impossible.”
Even the slight possibility of time travel exerts suchfascination that many physicists continue to study not onlywhether it may be possible but also how one might do it.
One of the leading researchers in this area is William A. Hiscock, a professor of physics at MontanaState University. Here are his thoughts on the matter:
“Is it possible to travel through time? To answer this question,we must be a bit more specific about what we mean by travelingthrough time. Discounting the everyday progression of time, thequestion can be divided into two parts: Is it possible, within ashort time , to travel into thedistant future? And is it possible to travel into the past?
“Our current understanding of fundamental physics tells us thatthe answer to the first question is a definite yes, and to thesecond, maybe.
JohnL. Friedman of the physics department at the University ofWisconsin at Milwaukee has also given this subject a great dealof consideration:
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General Relativity And Gps Time Travel
The difference that low earth orbit makes in an astronaut’s life span may be negligible better suited for jokes among siblings than actual life extension or visiting the distant future but the dilation in time between people on Earth and GPS satellites flying through space does make a difference.
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The Global Positioning System, or GPS, helps us know exactly where we are by communicating with a network of a few dozen satellites positioned in a high Earth orbit. The satellites circle the planet from 12,500 miles away, moving at 8,700 mph .
According to special relativity, the faster an object moves relative to another object, the slower that first object experiences time. For GPS satellites with atomic clocks, this effect cuts 7 microseconds, or 7 millionths of a second, off each day, according to American Physical Society publication Physics Central .
Then, according to general relativity, clocks closer to the center of a large gravitational mass like Earth tick more slowly than those farther away. So, because the GPS satellites are much farther from the center of Earth compared to clocks on the surface, Physics Central added, that adds another 45 microseconds onto the GPS satellite clocks each day. Combined with the negative 7 microseconds from the special relativity calculation, the net result is an added 38 microseconds.
Why Einstein Said Time Is An Illusion
Albert Einstein once wrote: People like us who believe in physics know that the distinction between past, present and future is only a stubbornly persistent illusion. Time, in other words, he said, is an illusion. He says he thinks time is real and that the laws of physics may not be as permanent as we think they are.
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What Did Stephen Hawking Say About Time Travel
A more fundamental objection to time travel schemes based on rotating cylinders or cosmic strings has been put forward by Stephen Hawking, who proved a theorem showing that according to general relativity it is impossible to build a time machine of a special type (a time machine with the compactly generated Cauchy.
Quantum Mechanics Of Time Travel
Until recently, most studies on time travel are based upon classicalgeneral relativity. Coming up with a quantum version of time travel requires physicists to figure out the time evolution equations for density states in the presence of closed timelike curves .
had conjectured that once quantum mechanics is taken into account, self-consistent solutions always exist for all time machine configurations, and initial conditions. However, it has been noted such solutions are not unique in general, in violation of determinism, unitarity and linearity.
The application of self-consistency to quantum mechanical time machines has taken two main routes. applied to the density matrix itself gives the Deutsch prescription. Applied instead to the state vector, the same rule gives nonunitary physics with a dual description in terms of post-selection.
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What Happens When You Drift Off Into Unconsciousness
Your mind is in chaos, you stagger around and accidentally enter a strange looking cubicle. You drift off into unconsciousness. After a while, you can not tell how long, you drift back into consciousness and stagger out of the cubicle. You see someone in the distance looking at you calmly and fixedly.
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Quantum Mechanics To The Rescue
There has been a particularly clear treatment of time travel in thecontext of quantum mechanics by David Deutsch in which it is claimed that quantummechanical considerations show that time travel never imposes anyconstraints on the pre-time travel state of systems. The essence ofthis account is as follows.
A quantum system starts in state S1, interacts with itsolder self, after the interaction is in state S2,time travels while developing into state S3, theninteracts with its younger self, and ends in stateS4 .
Now, if the only possible states of the system were and , then there is aconstraint on initial states: initial state1 isimpossible. For if1 interacts with3 then itwill develop into2, which, during time travel, will developinto3, which inconsistent with the assumedstate3. Similarly if3 it will develop into2, whichwill then develop into3 which is also inconsistent. Thus thesystem can not start in state1.
Now whatever one thinks of the merits of many worldsinterpretations, and of this understanding of it applied to mixtures,in the end one does not obtain genuine time travel in Deutsch’saccount. The systems in question travel from one time in one world toanother time in another world, but no system travels to an earlier timein the same world. Thus, even if it were a reasonable view, it is not quite asinteresting as it may have initially seemed.
Alternate Time Travel Theories
While Einstein’s theories appear to make time travel difficult, some researchers have proposed other solutions that could allow jumps back and forth in time. These alternate theories share one major flaw: As far as scientists can tell, there’s no way a person could survive the kind of gravitational pulling and pushing that each solution requires.
Infinite cylinder theory
Astronomer Frank Tipler proposed a mechanism ) where one could take matter that is 10 times the sun’s mass, then roll it into a very long, but very dense cylinder. The Anderson Institute , a time travel research organization, described the cylinder as “a black hole that has passed through a spaghetti factory.”
After spinning this black hole spaghetti a few billion revolutions per minute, a spaceship nearby following a very precise spiral around the cylinder could travel backwards in time on a “closed, time-like curve,” according to the Anderson Institute.
The major problem is that in order for the Tipler Cylinder to become reality, the cylinder would need to be infinitely long or be made of some unknown kind of matter. At least for the foreseeable future, endless interstellar pasta is beyond our reach.
Theoretical physicist Amos Ori at the Technion-Israel Institute of Technology in Haifa, Israel, proposed a model for a time machine made out of curved space-time a donut-shaped vacuum surrounded by a sphere of normal matter.
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Can You Take Back To The Future
No one can take Backto the Future away from you. In trippy new research, scientists say theyve confirmed what they call the Avengers: Endgame model of time travel. They did this by running a quantum time travel simulation that runs backward and forward, letting them damage the past and see what resulted.
Can We Use Time Travel In Everyday Life
We can’t use a time machine to travel hundreds of years into the past or future. That kind of time travel only happens in books and movies. But the math of time travel does affect the things we use every day.
For example, we use GPS satellites to help us figure out how to get to new places. NASA scientists also use a high-accuracy version of GPS to keep track of where satellites are in space. But did you know that GPS relies on time-travel calculations to help you get around town?
GPS satellites orbit around Earth very quickly at about 8,700 miles per hour. This slows down GPS satellite clocks by a small fraction of a second .
GPS satellites orbit around Earth at about 8,700 miles per hour. Credit: GPS.gov
However, the satellites are also orbiting Earth about 12,550 miles above the surface. This actually speeds up GPS satellite clocks by a slighter larger fraction of a second.
Here’s how: Einstein’s theory also says that gravity curves space and time, causing the passage of time to slow down. High up where the satellites orbit, Earth’s gravity is much weaker. This causes the clocks on GPS satellites to run faster than clocks on the ground.
The combined result is that the clocks on GPS satellites experience time at a rate slightly faster than 1 second per second. Luckily, scientists can use math to correct these differences in time.
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How Can Time Not Exist
The problem, in brief, is that time may not exist at the most fundamental level of physical reality. As Rovelli explains it, in quantum mechanics all particles of matter and energy can also be described as waves. And waves have an unusual property: An infinite number of them can exist in the same location.
How About The Issue Of Constraints In The Time Travel Region T
How about the issue of constraints in the time travel region T? Prima facie, constraints in such a region would not appear to be surprising. But one might still expect that there should be no constraints on states on a spacelike surface, provided one keeps the surface small enough. In the physics literature the following question has been asked: for any point p in T, and any space-like surface S that includes p is there a neighborhood E of p in S such that any solution on E can be extended to a solution on the whole space-time? With respect to this question, there are some simple models in which one has this kind of extendibility of local solutions to global ones, and some simple models in which one does not have such extendibility, with no clear general pattern. The technical mathematical problems are amplified by the more conceptual problem of what it might mean to say that one could create a situation which forces the creation of closed timelike curves. . What are we to think of all of this?
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