Time Travel - Is It Possible?

Is it possible to travel through time? Various arguments have been presented for and against the possibility. A LOT of science fiction is based on this idea. This article presents evidence that strongly suggests that the laws of physics eliminate all possibility of travel BACK through time. (With a single exception regarding something called the Heisenberg Uncertainty Principle, which would LIMIT any time travel to be less than one one-trillionth of a second, either way, for anything with as much mass as a human body.)

Popular science fiction often refers to time travel as integral parts of plot lines. Even some reporters and media specials create supposedly fact-based segments on the possibility of time travel. The public has come to believe that this capability is an accepted fact. Unfortunately, there ARE no experts in this area, except for us Nuclear Physicists, and so there is no actual solid data to base conjecture on. Or is there?

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Well, if you consider a very broad definition, one version of "time travel" actually IS possible, but only in a forward direction and never backward to an earlier time. Essentially, if you wanted to be alive in some future time, there are a couple of possible methods that might be used. The most obvious is just to have your body put in cryogenic storage when you are about to die, because it is generally believed that organic cells do not degrade very much at very cold temperatures. In that case, in 500 years or whenever, some technician would unfreeze you and fix whatever killed you, and you would be living then! But you would have no way of ever coming back to now!

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The other method does not involve death or freezing. It involves a known relativistic effect that time actually slows down if you travel incredibly fast. In that case, if you could go fast enough in a rocket ship far out into space and then come back, popular arguments say that you (with your slowed time effect) might think that one year had passed, while the Earth had experienced 500 years! So, you would again be in that future world, but again, you would have no possible way to get back here. And, "fast enough" means incredibly fast. You know how the spaceships that we launch into orbit travel around 17,000 miles per hour, three hundred times as fast as cars travel? Now, imagine getting the spaceship up to a speed 20,000 times as fast as the orbiting satellites! You would START getting to the speeds necessary for a serious benefit of time changes.

But THAT idea will NOT actually work! The Time Dilation effect referred to here ONLY exists if you would be TRAVELLING AT VERY HIGH CONSTANT VELOCITY. The problem is that to GET TO such a high velocity, you would have to first ACCELERATE ferociously, and during that process, a DIFFERENT effect exists, called General Relativity, and during that time, time seems to pass with an OPPOSITE effect, in other words, extremely fast. So, even though you might only experience one year of passage of time AFTER you have accelerated, you WOULD HAVE PREVIOUSLY AGED 250 years DURING the acceleration! (and you would probably have died of old age!)

However, even that would NOT work. Virtually everyone MISUNDERSTANDS the popular Time Dilation effect of Special Relativity of Einstein. It does not and can not alter the life or the experience of the PERSON or EVENT being observed. It is ONLY an Effect when SEEN BY SOMEONE ELSE (who is NOT moving). I can cite one of the very first confirmations that Einstein's Special Relativity actually works. These experiments began around 1926. It had been learned that a tiny particle called the Mu Meson only existed for a really short period of time. When very high-energy cosmic rays from outer space impact atomic nuclei high in the Earth's atmosphere, some Mu mesons are often created due to such collisions. It is well-known that Mu mesons decay very rapidly (average lifetime of around 0.00000152 second) and that even at the speed of light, they MUST usually decay within about 0.28 mile or so of where they were created, before naturally decaying into other particles. However, research labs on the surface of the Earth, many miles below, easily detect those Mu-mesons. Until Einstein came along with his explanation, everyone was very confused! The ONLY possible explanation for this is due to Einstein's new Time Dilation effect of Special Relativity. But this was NOT anything which affected the Mu Mesons themselves. It ONLY affected what the scientists, down in their (non-moving) Earth laboratories SAW. For the Mu mesons, their enormous (constant) velocity causes (Time Dilation) time to APPEAR (TO US) to pass much more SLOWLY than it does for us. And so those Mu mesons, TO THE OBSERVING SCIENTISTS IN THEIR EARTH LABORATORIES BELOW, actually appear to have plenty of time to travel the distance of many miles to the labs. To the Mu mesons themselves, THERE DOES NOT SEEM TO BE ANY TIME EFFECTS! To the Mu mesons, our (actually thick) atmosphere APPEARS like it is less than 0.28 mile thick, and so it is totally logical to be able to get to the surface, without disobeying the speed of light being the fastest possible speed for anything to move. Note that in BOTH viewpoints, everything seems to make perfect logical sense, and it is only the OTHER (apparently fast-moving) perspective which seems to have been altered. There have been countless other experimental confirmations of the Time Dilation effects of Special Relativity.

There is one other possibility that gets mentioned occasionally. There have never yet been seen any "black holes" in space, but some physicists are very convinced they must exist. There are actually a bunch of assumptions one has to make to permit them to exist, but an equation that Einstein derived 90 years ago has one mathematical solution that might suggest that they could exist. If they actually exist, and if (some more assumptions), then two of these peculiar objects could (at least mathematically) be connected with each other, even if they were pretty far apart in actual space. A noted physicist, Kip Thorne, and some of his colleagues believes this to be possible. Once all those assumptions are accepted, and we have this connected pair of singularities, Thorne calls it a "worm-hole". His reasoning is that, possibly, an object entering one end of this thing, would instantly appear at the other end, even if it was an incredible distance away.

I personally am certain that Kip Thorne does NOT fully understand Special Relativity and General Relattivity, where he might then realize that his speeculation about 'worm holes' is simply silly (and totally impossible).

As a result, all kinds of strange results can be had in thought experiments, and there are some concerns about objects or people being duplicated (which is also impossible). Thorne speculates on a method of time travel that might be possible with such an object. Again, there are several dozen serious weak assumptions that are necessary to even suggest such things, and if any of the assumptions are wrong, it couldn't exist. Other assumptions involve whether any actual object going into one end would not be destroyed in the process of passing through it, etc.

Not the remotest chance of actually being true, and as a Physicist, I am ashamed of Kip Thorne for even publicizing such silly ideas.


We DO know that, as our future progresses, the physical laws of Nature must and will not be violated. This includes Conservation of Mass, conservation of Angular Momentum, the Laws of Newton, the Laws of Thermodynamics, and similar bases of logical thinking. In addition, certain constants, such as the number Pi and the speed of light, must also be consistent with logical thinking.


Speculate now, that time travel into the past was possible, and that people a thousand years in our future learned how to do it and control it. For the moment, let's take a big example, that they decided to transport a medium sized hill from their time to ours, a thousand years earlier. Or a million people, which also would have a total mass of around 100,000 tons.

There IS a basic scientific law that is the Conservation of Mass. But we would now have an EXTRA 100,000 tons of material (or people) which would now simultaneously exist with the earlier existence of those same things or people. That would be an EXTREME Violation of the Conservation of Mass, so it is NOT remotely possible! But it gets worse! The Earth has a characteristic called Moment of Inertia, which is related to determining how fast the Earth rotates! IF an EXTRA 100,000 tons of material suddenly came into existence, then the Earth's Moment of Inertia will necessarily ALSO suddenly change. The Earth would suddenly change in its rate of rotation, meaning that the length of the day would also suddenly change (but only by a very small amount).

No matter WHAT process might be involved to try to do such things, it is certainly clearly necessary to know exactly where each atom was at some specific instant in time. This would be an astoundingly difficult accomplishment, but we will assume that those people a thousand years in our future would have figured out some way to do it.

Since we would need to somehow re-construct the structure of the hill, we would certainly need to know the position of each atom REALLY accurately. If two atoms were supposed to be right next to each other (as in a compound, like hydrogen and oxygen forming a water molecule), the location of the atoms would obviously have to be known to about 10-12 cm (or one one-trillionth of a centimeter). This figure comes from nuclear physics, where this is a common distance of interaction between individual atomic nucleons.

OK. So the future technology is good enough to do this, we are assuming! Now, keep in mind that all of those atoms had to be precisely at those (known) locations.

Our test hill is 1000 feet tall and 2000 feet in each horizontal direction. Its volume would be 1/3 times its height times the area of its base, (volume of a cone) or 1.3 billion cubic feet. With an average density of stone and dirt, this would give a weight of the hill at about 100 million tons.

In ANY time, our Earth must rotate in compliance with the law of conservation of angular momentum. If an ADDITIONAL 100 million tons were suddenly (now) added to the Earth near the Equator, the Earth's mass and rotational inertia would be measurably changed (increased). Since the amount of angular momentum that the Earth has must be constant, this means that the rate of rotation of the Earth would have to have slowed down to preserve the amount of angular momentum of the Earth. The day would therefore get longer in length by a fraction of a second. This is pretty easily calculable.

If this new hill remained as part of OUR time's Earth for a few months, this slight change in the length of the day will have an additive effect. With the Earth's surface rotating at a rate of over 1,000 mph, it wouldn't take long for the Earth to have rotated differently enough for it to have rotated a full mile less far than it would have without the presence of the (new) hill. Let's say that after that time interval (of a few months) in our time, the hill was removed to exist again in its original time or in some other time, so the differential rotational rate would revert to zero so that the length of the day reverted to its previous value.

What would be the consequences of this? The Earth's rotation would then be forever at the correct rate but shifted by one mile off on the actual locations of all future objects on the Earth. In other words, a thousand years in the future, the Earth would have rotated a mile differently than it would have if the hill had not appeared for those few months! When the people intended to transport that hill back to our time, the hill could not be where they expected it to be! It would be a mile away from there! Actually, the entire earth's surface would be in "wrong" places by that mile! This now means that the process of first selecting the hill FOR time transport would had to have failed!

Whatever mechanism would have been used to select the particular atoms and molecules of the hill would necessarily have to have failed. The actual PROCESS of time travel would have unintentionally changed very subtle things such that the effect HAS TO BE that the molecules of the Earth or the molecules of the hill necessarily can NOT be in the precise locations they should have been at for the process to succeed.

OK. A human has far less mass than a hill. Would that still have this effect? Yes, but on a much smaller level. So, you're in this lab, a thousand years from now. Some machine identifies all of the atoms that are parts of you. The machine has to EXACTLY know where all those atoms are in order to collect them and do the time-travel thing. That means the machine must know within a trillionth of a centimeter the exact location of each of those atoms. However, when your 200 pounds was placed on the Earth of a thousand years earlier, the Earth's rotation again must slow down a microscopic amount, in order to conserve angular momentum. It doesn't seem like much, but that change is cumulative, just like for the mountain.

You probably weigh about one-billionth of our hill. So, instead of causing a shift of a mile, it is only one-billionth of a mile. That's about 1/20,000 inch. That's HUGE compared to the need to know where all the atoms were to better than one trillionth of an inch! Let's say that the effect, for all the time you were on (our) ancient Earth, caused a total change of 2.5 billionths of a second in how far the Earth had rotated. This would cause the Earth to have rotated about the 1/20,000 of an inch less far than it would have if you had not appeared. NOTHING will make that difference up during the next thousand years! As you are being hooked up to the time-travel machine, what it thinks are the precise locations of each of your molecules, are actually now OFF by 1/20,000 of an inch! The machine would now not be able to capture your atoms properly to start the process of sending you back in time! You are toast! Or rather, scrambled eggs! You get the point. The consequent result of the shift back in time would make the original experiment fail! Every time!

OK! You're persistent! Instead of staying back there for a few months, what if you only stayed a few seconds? Well, do the math! A few months is around 8 million seconds. If you only went back for eight seconds, the effect would only be one-millionth as great. True! So, now, the result of the experiment causes your atoms to be (1/20,000) * (1/1,000,000) or about 50 trillionths of an inch shifted. (That's about 125 trillionths of a centimeter. But we are pretty sure that we would need to know the locations of every one of your atoms to about ONE trillionth of a centimeter. STILL, no dice!

If you had perfect equipment, and you decided to only go back for less than 1/20 of a second, THEN the earth's rotation would only have been shifted by an acceptable amount, where the atoms could (probably) all still be found by the equipment. But to set up huge collections of involved and expensive equipment, only to be able to go back for 1/20 of a second, GEE, doesn't sound like much of a good idea. What could you do, or even see, in 1/20 second?

There really is no way around this situation. No matter HOW good future technology gets, it would HAVE to know where all of your atoms are at some instant. But then, during the process of the time travel experiment, the Earth's rotation is slowed so it is even one-trillionth of a centimeter back, the equipment necessarily has to then fail to find your atoms to do the experiment. Poof!

In case you think this is silly speculation, it's definitely not. It turns out that we human beings like to pump water up into reservoirs up on mountains, so we can make electricity when it falls down through turbines. By raising all this water up, we are unintentionally changing the rotational inertia of the whole planet! Scientists have confirmed that this action has contributed a measurable amount to making the Earth rotate slightly more slowly, and therefore the day is getting longer. Just from pumping water! This effect described here is real, and the effects are fairly easily calculated. Won't work!

A Separate Problem

In science fiction, they often refer to being careful to avoid "contaminating the time line." They are rightfully addressing a valid concern. If someone was able to go back a thousand years, and either intentionally or accidentally contribute to someone's death, the future (their past) would be necessarily and irreversibly changed.

Consider for a moment your personal genealogy. You have 2 parents (2 to the first power), 4 grandparents (2 to the second power), 8 great grandparents (2 to the third power), and so on. Just 20 generations ago, about 400 years, there were EXACTLY 2 to the 20th power (1,048,576) contributors of exactly who you are today! (NOTE: In the real world, there is a LOT of duplication (sort of inbreeding) that occurs, so there were probably far fewer than a million different individuals in your family tree of 20 generations ago. But STILL a LOT of different people had to exist even just 400 years ago for you to be the person you are today. If even ONE of them accidentally got killed before continuing the family line, YOU WOULDN'T EVEN EXIST TODAY!)

Those science fiction writers are therefore quite correct in expressing concerns at not contaminating the time line, as the result could have been that the future time travelers might never have been born! What we're saying here is that the problem of contaminating the time line is even broader than they comment on. A single person sent back in time does not have the mass of a hill, but the effects still apply. Even a 200-pound human appearing at a previous time in history would cause extremely subtle changes, like the one already referred to, of the rotation period of the Earth. Not much, but since all future activities and configurations of all the molecules that make up the Earth and all its surroundings, are absolutely dependent on the conditions that existed just prior to that instant, even the tiniest of changes necessarily affect the future in exponentially great ways as time proceeds.

Notice that in the Earth rotation example, we're not even addressing about whether the INDIVIDUAL still shows up at the precise location necessary in the future for the process of time travel to have been initiated. We have been explaining that the Earth itself would necessarily have rotated a slightly different amount, as a result of the time travel, and therefore would necessarily have caused a discontinuity in history at that instant. Either the Earth would have had to physically JUMP (which violates quite a few laws of Nature) or there would have to be a time hiccup noticeable at that instant (also violating laws of Nature). In either case, a necessary consequence of sending someone back in time would have to be a world-wide effect of a discontinuity of time or space. This necessary consequence makes certain that a time travel mechanism can never be possible.

The possibility of contamination of the time line is also much more troublesome than it first might appear. What if our imaginary time traveler unintentionally and unknowingly killed a single bacterium during his trip. That tiny bacterium could have been the source of a future world-wide plague (or even just a small infection) that was going to occur hundreds of years in the future, (but still in THEIR past). Again, the consequences can be huge, even when the initial incident seems irrelevant.

The sum total of the results of a visit to the distant past would be multiple. The oxygen the visitor consumed during his visit might instead have been used by some other organism, or the slight resultant lack of oxygen and surplus of carbon dioxide might have inspired a few additional blades of grass to start to grow. As in the cases already mentioned, the long-term consequences of this could be unexpectedly huge.

So far, we have just been discussing time travel into the past. Could time travel into the future be possible? There are actually two possibilities here. If this were to involve any mass vanishing from one time and appearing in another, then the same difficulties would probably exist (but differently), which would make it impossible. If, instead, the premise was to put the person in some sort of suspended animation, this is certainly possible, and even practical. This mechanism does not violate any laws of Nature.

There have been a number of people who have spent LOTS of money to have their bodies "frozen" when they die, in the hope that future doctors, hundreds of years from now, will be able to revive them and "cure their death!" I have doubts about the current technologies being used, but the concept is quite valid. But it would be a one-way trip into the future!


This entire concept only first arose due to the discovery in Nuclear Physics that some truly weird things can and DO occur in Nuclear Physics, but they are necessarily LIMITED by something that was discovered in the 1930s by Heisenberg. It is called the Heisenberg Uncertainty Principle.

So, if the limits of time and mass and motion is within the Heisenberg Uncertainty Primciple, then it IS possible that an proton can be going along and it can BECOME TWO protons, BUT ONLY VERY BRIEFLY, less than a billionth of a second, before the two have to re-combine to again form the original proton. THAT reasoning made it clear that the rate of TIME passage can also be affected, but again, only for extremely TINY particles (invisibly small) and only for extremely brief intervals of time.

When Reporters and science-fiction writers learned about this, and NEVER REALLY UNDERSTOOD THE DETAILS, they started to dream up the ideas of time travel. It did not help that a few Physicists around 1960 made a number of their own logical errors in dreaming up another idea which caught the public's fascination, that of a Twins Paradox, where the rate of time travel appears to be changeable. Another page in this Domain shows and explains WHY that Twins Paradox is absolutely wrong.


First Published on the Web: May 16, 1999

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C Johnson, Theoretical Physicist, Physics Degree from Univ of Chicago