Now, we are entering the purely theoretical realm of superluminal travel. The spaceship is outracing the light it emits, so when the spaceship takes off, it leaves its own light in the space-dust.

You can think of it like mailing selfies home from long, round-trip vacation. Just as the spaceship was traveling faster than light, you are traveling faster than snail-mail, so you beat some or all of your mailed selfies home.

The day after your homecoming, the mail carrier delivers two of your pictures: one from the trip out, and a second from your trip back.

So, we imagined pushing the ship even faster. The observer watches the spaceship leave the launch pad and head out to the planet, just as before; however, a real pair of spaceships appears on the landing pad, not simply one and an image! Then, one of the ships immediately takes off for the planet, again in reverse, and annihilates with the original ship in a real pair annihilation event at the planet.

Now, you may be wondering: Where did the extra spaceship come from? This seeming contradiction is resolved when we remember that my plane of simultaneity, essentially my plane of now, is skewed in relation to the plane for someone on Earth.

Our versions of now going out into the universe no longer lines up. If you listened to the post, you might want to look at it instead. So, the question for the ship making the instant hyperspace jump is, which plane of simultaneity does it take place on? If the hyperspace jump is in the direction opposite to the one ship is moving in, it will come out of the jump in the past of the stationary observer.

There is no universal now. Special relativity took it away. Or more accurately, revealed that it was never there. But then this does seem a bit, well… lame. Here velocity causes the past to be present somewhere else from that perspective which is solid I guess , and then the ship also gets to that past by appearing in the opposite direction of its movement which seems magic. And yes circling a clock on the Earth surface at.

But unlike for the axis of the orbit this should register some velocity nonetheless. Eric, Your comment about the motion and hyperspace jump makes me thing you still might not quite grasp it.

On your paradox, if you were somehow flying around the Earth at relativistic speeds, the same clock relationships I described above would apply. To an observer on the ground, your clock would be moving slower. There is no movement between them and therefore things occur simultaneously from each clock perspective as in the neutral case of the moving diagram.

Now imagine clock 3 that spins around clock 1 it at a speed of. As I understand it, even though there will be angular velocity between them, each clock will tick the same and things will occur simultaneously from each perspective since there will be no associated velocity.

But now consider the relationship between clocks 2 and 3. There will be velocity between these clocks as their distance fluctuates between 1 and 3 billion meters per orbit. So how can clock 1 tick the same as the other two clocks, if clock 2 and 3 tick differently from each other? Where is my error? I think clock 1 and 3 would still see relativistic effects between them. Yes, the distance is constant, but only because 3 is constantly undergoing insane acceleration to remain in its circular path.

Likewise 3 and 2 would also see relativistic effects. Which is to say that I think the premise of your question is mistaken. This would mean that for clock 3, clock 2 would be gyrating wildly between its future and past. A wild ride. Both of you have been watching too many science programs. A clock is an instrument which itself is defined by a duration of change. The atomic clock is that reference, it has the SI unit of seconds, 1 second is defined to be exactly 9 oscillations of the isotope Cesium This definition refers to a caesium atom at rest at a temperature of 0 Kelvin.

As a practical reference point, the second measured for any atomic clock is also corrected to mean sea level. As long as there are no variables which influence the oscillations of the isotope cesium of either clock 1, clock 2, or clock 3, the measurement made by the instruments would be a constant. That measurement is not contingent upon the velocity or position of each clock in relationship to each other regardless of their distance or velocities.

Lee: The cesium clock is measured at rest relative to the measuring equipment. If such a clock were compared with a cesium passing by at high speed, the passing clock is seen to tick slower. This is verified to great precision by many, many experiments.

With rotation, you have to use the math of GR, because rotation is acceleration. Clock 3 would experience both SR and GR effects. Your conclusion is incomplete and therefore a misrepresentation of the facts, facts which are gathered by the many, many experiments. Your second comments is a non-starter. The only variable which directly affects the oscillation of cesium is referred to as gravitational potential. Both gravitational potential and velocity effects in the context of kinematic time dilations are, for example, routinely incorporated into the calculations used for the Global Positioning System.

Ah, thanks. I knew the acceleration part complicated things which was why I initially tried to evade it , but forgot that acceleration puts us into GR territory. Considering how many things move in curved paths, GR infects everything. Plus gravity same thing as acceleration brings in GR, too. It still blows my mind that the center of the Earth is younger than the surface, because clocks tick relatively slower.

On a fine scale, spacetime is probably warped like smoke or open water waves… something really complex and chaotic with zillions of small inputs. The latest clock, the strontium lattice clock, can detect the GR effect of, IIRC, just centimeters of height difference. So I guess simultaneity is similarly skewed in GR.

Meaning that the plane of now for the core of the Earth is different than the plane of now for us on the surface. Of course, your plane of now at a different latitude is different than mine. Reality is absurd. Unless one of us lives in Denver. This statement explains your misrepresentation Wyrd. The only thing that model proves is that cesium oscillates slower at the center of the earth then it does on the surface.

We have no tools at our disposal with which to make that determination other than conjecture. Nothing else can be proven utilizing this model. The construct of time is a derivative of human imagination, albeit an ingenious, admirable and useful invention indeed. Mike and Wyrd, So beyond all the complex calculations I presume that clock3 ticks slower than the clock1 it spins around given its accelerated mass rather than ticks the same given no velocity between them as I proposed before.

Sounds good. If clock3 ticks slower still in relation to clock2 because it not only has its accelerated mass, but velocity in relation to clock2, then that would seem paradoxical. Any further thoughts about these two motionless clocks that one spins around? What matters is that clock 3 is traveling in the frame shared by clocks 1 and 2. It does not matter that clock 2 is offset.

Both clocks 1 and 2 see clock 3 moving in a circular orbit at 0. Overall those cancel. You could replicate the effect with sound. A circling siren would have no Doppler shift to someone in the center of the circle, but would to someone near the edge.

Thus no paradox? Works for me. Light cone? Apparently not. Clock 3 does not share their frame. Clock 3 has a very complicated frame due to that acceleration, parts of which orientation are constantly changing. Its dynamics fall under GR, which is a whole other fish kettle. Its done in terms of velocity and acceleration vectors. Acceleration complicates things. You need tensors to describe curved motion in gravity-warped space. Lee, Time being a unit to measure a duration of change is fine.

Any new theory will have to account for the same observations. Change is the only absolute; and because change is a continuum, what we do is build models to measure a duration of that change.

Time is the model which we construct to measure that duration, any duration. The rate of change is not another variable in the equation as such, the rate of change is a derivative of that continuum of change. Both are useful for making highly predictive observations, but in the context of the true nature of reality, both are bullshit just like the model of time itself.

Lee, On the true nature of reality, I can see your point. But the point the post makes only seems reliant on the predictions. The instrument itself is not time, and the instrument does not measure time. The instrument is dependent upon many variables for its own accuracy as an instrument, which simply means that the tick tock of the clock can change depending upon those variables. Of course, those variables which affect the measurement instrument must be taken into account in order to make accurate predictions, and those variables are instrumental in making predictions where the constant of the clock changes.

Actually, I think most science is skeptical of time travel, and FTL. We measure distances along the spatial dimensions with a ruler and along the temporal dimension with a clock. Is it impossible for us to know the geometry of our universe? A new observation could always force us to fine tune, or in severe cases, come up with an entirely new model.

Think of the precession of Mercury under Newtonian laws, ultimately resolved with GR. I think the BU is the only model of the universe that incorporates time though. Life is short, as they say, and an overarching agnosticism may be logically persuasive and shielded from error but I find it emotionally and scientifically unsatisfying. And sometimes contemplation of the greatest puzzles has led to some of our most powerful insights, so it seems to me that the stunning contrast between the static and unchanging BU and our ever-present flowing experience may point the way to a fundamental new understanding of reality and the human condition.

The only exceptions appear to be the Second Law of Thermodynamics entropy and certain interpretations of quantum mechanics that posit the wave function collapse as an objective event. I can see that. What draws me to science is the quest for truth. I never sit around at night pondering what kind of predictive frameworks can be developed.

I ponder what is or what might be. Did they ask Einstein to select one? I have several books of Einstein quotations and many of those are noteworthy and appropriate for an obituary. In all the ERL-related research I did, no one has ever written about what he might have been referring to. Absolutely no one. On presentism, I see what you meant now. Relativity certainly seems to demolish any possibility of a universal now.

But does it demolish even a relative version? The problem is that there certainly seems to be something different about the current instant. The outside world has causal effects which impinge on our mind. Our current mental state seems like a culmination of all our past experiences. Our knowledge of the past is never as solid as we might imagine, but it seems far more substantial than our predictions of the future. For better or worse, our conscious experience seems to include an inescapable now concept.

It also seems hopelessly tangled up with a core self concept. Greene repeatedly impresses me with his ability to convey complex concepts. He explained that the experience of the now means something special for man, something essentially different from the past and the future, but that this important difference does not and cannot occur within physics.

Every moment is. Predictive models are only useful tools; and that useful-ness is contingent upon the objective of that particular predictive model. The most effective predictive model every devised by any institution was during the reign of the Pharaohs, a model which stated that the Pharaoh was a God.

Clearly, this model could make predictions with a precision that no scientific model can match. The only distinction between a predictive scientific model and the predictive Egyptian model is a matter of scope…. Would the wormhole still be a problem if you had to create both ends in the same time close together, before sending off one of them to somewhere else at relativistic speeds? I suppose since hyperspace jumps are fictional, you could just handwave it by saying the drive always kicks the spaceship out back into the original plane it entered it from, just displaced in distance.

If you had to create the other end, then it would stop you from traveling to arbitrary points in the past. Although if you accelerated one of the ends to near c in the process of moving it to its destination, it would experience relativistic effects and would, from the standpoint of the other end, now be in the future, and from the remote end, the original would be in the past.

The problem is that the spaceship is not changing planes in the jump. Its plane before and after the jump are identical. The one it had before it left the original planet? The one it had just prior to the jump? Or the one it will have when it reaches the destination planet? None of them agree and whichever one chosen will result in it being at a different time than now according to the other planes.

That depends on how they get formed. If they were formed only in stellar-rich environments, or black hole collisions, for examples, then no. This reminds me of discussions we had back when I wrote my Special Relativity series!

I think the bottom line here is that FTL travel is impossible. Back in my series I posited the possibility of ansibles working if both ends were in the same frame of reference, but a kind soul on the internet set me straight. Recent work seems to suggest wormholes are impossible, too. Not everything has a sign.

So, what happens to a ship moving at an appreciable fraction of c that jumps into a wormhole? One possibility is that entering the wormhole represents a new frame of reference, so the surface of simultaneity would shift to whatever is right in the wormhole. Or it may have no meaning in the wormhole, and meaning only returns once the ship leaves. And the traveler is seeing space contract in the direction of travel. Hence they are able to reach the ground before they decay. I imagine most of us are sitting in a room reading this.

It would think the same of you. A useful analogy is to hold a ruler lengthwise in front of you. You see the entire length of the ruler. Now turn the ruler so it appears foreshortened. Now it looks shorter to you of course the ruler is unchanged. It takes many light years to notice the difference!

I learned a lot from your relativity series. On the ansible, the problem of course is actually getting into the same frame. The relative motions of the respective stars and planets make that effectively impossible. You might be able to attempt a conversation, but depending on the distance involved and differences between the frames, it might not be a very linear one. Supposedly they instantly collapse unless you have negative mass to keep them open. The thing I wonder is, just how much negative mass is necessary?

And what would be the consequences for anything trying to traverse the wormhole? Good question on what happens if a relativistic ship traverses a wormhole. I tend to think it would be the same as normal spacetime, although all the energy involved in keeping the wormhole in place and open might make a major difference at the GR level.

Beyond my current level of comprehension. Greene uses the example of Chewbacca sitting in his living room 10 billion light years away. If Chewie goes for a walk at 5 mph, his plane of simultaneity relative to us shifts years! Of course, for closer distances, like within our galaxy, you need to be moving faster for the effect to be noticeable.

But even the relative motions of planets will lead to several seconds or minutes across thousands of light years. That would definitely be a major challenge. My thought at the time was it would involve some zeroing in on the right vector. Maybe the signal got stronger or more coherent as you got closer to exact. It ought to be possible in principle. They would also allow time travel of information.

It takes four parties to demonstrate…. Imagine A and B in Frame 1, separated by light years, using ansibles to communicate, and C and D in Frame 2, also separated by light years, also in communication with ansibles. Frame 1 and Frame 2 are in relative motion. If C passes A , and D passes B , allowing brief message exchange, information can exchanged such that it passes into the past of one of the frames. Apparently, related to dark energy, negative pressure is actually a thing.

I probably just revealed my ignorance at multiple levels. Everyone is pretty ignorant when it comes to dark energy. That one is still very much a blank page. With a bunch of pencil marks in the margins.

In defense of Deep Space Nine, the aliens who lived inside the wormhole did apparently experience time in a different way than the rest of us.

I always thought that was a nice nod to the actual science, even if it was only a trivial nod. In my own science fiction universe, I always wanted time travel to feature prominently anyway. FTL technology paved the way for the first time machines. But those aliens were a nice touch. And they apparently were the source of Bajoran religion, which was kinda cool. For your universe, I like the sound of treating it like a feature instead of just ignoring, as most sci-fi does.

I do have a mechanism like that. In universe, this repulsive force works against both spacecraft and communications that might mess with the timeline. Most Excellent! Used paperbacks from amazon for a measly five bucks. I always viewed the speed of light as moving one quanta of space per one quanta of time. The wavelength of the vibration since waves and particles have a level of interchangeable duality could equal one subatomic particle that conveys a unit of change not charge perhaps some neutrino I will call a chronoton particle.

Much like damming a river only to release a flood later, I suspect we just might be able to save time in a bottle and then use it up quickly. They can only experience one year per year, but from the perspective of those outside their ship, they might not exist The trick, of course, for physicists is coming up with a mathematics for it that are consistent and that predict or at least match observations.

Interesting idea on essentially banking time. The other thing that pops in my head is that how could the banking mechanism work without time? You mentioned bottling it, so presumably you could have thousands of units banking time, then with all that stored in a bottle, a ship could use it all in one large jump. Of course, the act of doing that would be equivalent to the hyperspace jump I describe in the post, with all the associated simultaneity issues.

Banking all the time would cause permanent disappearance. I view it more like a curtain blocking most light or a dimmer switch reducing light output. If both the traveler and an observer accelerate away from each other why does the twin-paradox occur i. The twin-paradox has no more credence than the idea that the center of the earth is younger than the surface, or that a mountain peak is older than both the surface and center of the earth.

I have to admit that the twin paradox used to have me tied in knots. The secret is to work it out using spacetime diagrams. If you do it right, it always works out. The simultaneity weirdism only arrives as they try to figure out when each report was made based on the travel time of the communication.

My response — How would anyone ever know? No form of communication could reach back to them and provide evidence of arrival without taking the same amount of time that was displaced getting there. You're still traveling mostly in space, very little in time but now the time you do travel in is actually negative. I forget where I read this vector explanation but it's one that makes the concept tractable in my mind :.

I'd like to add that you're always traveling at the speed of light. If stationary you're traveling through time at exactly the speed of light, but as you move through space you start traveling through time more slowly.

If a photon could talk, it might say "What is this 'time' thing you speak of? A photon is always 'everywhere it's ever been or ever will be' so it might also say "What is this 'space' thing you speak of? The picture you're talking about is a Minkowski diagram and it's very useful for conceptualizing how time and space change in fast reference frames. The way I like to think of it, and I'm not sure how valid my thoughts are I think about what you would experience as an observer in a ship traveling faster and faster.

If the ship had a window to look out, you wouldn't really be noticing anything happen to time, you'd be seeing the universe get squished, with everything getting closer and closer to you.

If you managed to actually achieve the speed of light, the universe would be completely flat. Everything would be contained in a single plane perpendicular to your direction of travel.

So now, what would it mean if you went even faster? The universe would no longer be flat, but would be achieving a negative size. What that really means is not clear to me, but I suppose it's here where the traveling back in time starts to make sense mathematically. My impression was that the wormhole was used as a "shortcut" between two distant places to avoid having to travel very fast.

She can avoid the collision by also instantaneously jumping, but because she is moving what is simultaneous for her is different than what is simultaneous for the actor. If the people at the destination were moving, then what is simultaneous for them is different than what is simultaneous for earth. So indeed, if they could travel instantaneously they could arrive before the catastrophe.

Welcome to Physics Stack Exchange. I honestly don't fully understand the relationship between FTL travel and time travel myself, but special relativity is the key to what's going on. In simplest terms, special relativity postulates that an observer will measure light to travel at the same speed ,, metres per second no matter how fast the observer themselves is travelling.

This sounds strange, but it has been borne out as true by every credible experiment performed to date. This postulate has very important consequences, among them being that facts about time and distance can vary depending on the manner in which an observer is moving.

For example, two events which I observe as occurring simultaneously may be observed as occurring at different times by a person in a spaceship travelling very fast relative to me. Neither of us is experiencing an illusion; the order of events actually changes depending on what reference frame you adopt.

When one considers cases of FTL travel which special relativity forbids under normal circumstances , one sees that any object which is travelling faster than the speed of light in one reference frame is travelling back in time in another reference frame.

Let me know if this makes any sense. You'll probably want to do your own research on this, it's very fascinating. I think that a few diagrams will make this a lot easier to explain, so here is an answer to add to those already present. We will consider some spacetime diagrams. A spacetime diagram is a diagram showing the layout of various events and the movements between them, with time on the vertical axis and spatial distance on the horizontal axis.

But it is important to keep in mind that spacetime is not quite like a pin-board to which events are pinned; it is a bit more subtle than that. Think of it as our name for the idea that we can map out positions and times of events using the temporal and spacial measurements made by any given set of clocks and rulers. Here is a diagram showing the result of such a set of measurements for some particular set of events:. The line labeled "light signal" shows a pulse of light zooming from left to right.

Now we proceed to the next diagram. In order to construct it we will require the mathematics of special relativity. You will have to trust me if you are not already familiar with the subject. We use the Lorentz transformation to discover how these same events would be measured in another frame of reference.

To be specific, consider a frame moving from left to right, at some perfectly allowed less-than-light speed. This takes some getting used to, but it is not too bizarre. It just means that if we plot the same set of events, with their spacial location and timing worked out in the new frame, then the diagram will look like this:. Here is the diagram with this return journey added:. So we enter into all the famous causality paradoxes and we conclude that the whole scenario looks highly implausible, and indeed if the universe makes sense then this sort of thing cannot be happening in it.