Our expanding universe is not thought to be relativistic. Technically, the galaxies receding away from us are at rest. The space between them is expanding and creating the impression that the galaxies are in motion. However, each galaxy can be treated as a clock, and, it is not likely that any of these clocks tell time at the same rate.
Using the diagram below as a reference, consider Alice and Bob living in a static universe. Alice lives in galaxy A and Bob lives in galaxy B. Each has a photon gun and shoots one photon per second at the other. The vertical red arrows represent the photons fired from one galaxy to the other. When the first photon from Alice (after traveling light years) finally arrives at galaxy B, Bob intercepts it and records it as a unit of time. Since Alice fired photons at regular intervals, Bob only has to wait one second for the next photon and another second for the next one after that and so forth. He receives one photon per second from Alice, and, for similar reasons, Alice receives one photon per second from Bob. This is how Alice and Bob keep track of each other's time rate.
You will note there are some horizontal red arrows at the top of the above diagram. Also located at galaxy A is Gertrude. She also has a photon gun. She fires one photon per second at Alice. Alice therefore intercepts and records one photon per second from Gertrude. Bob also receives photons in the same manner from Norbert who is also lacated at galaxy B (see horizontal red arrows at the diagram's bottom). This is how Alice and Bob keep track of their own time rates.
When the universe is static, both Alice and Bob receive fired photons from all directions at a rate of one photon per second. All clocks seem to agree and equations 1 and 2 provide the relevant math. But suppose the galaxies move away from each other at velocity v due to expanding space. The next diagram represents this scenario:
The vertical red arrows show that the photons take longer to reach their respective destinations and fewer photons are received per second compared to photons represented by the horizontal arrows. Since each photon is counted as a unit of time, Alice and Bob have the impression that the other's time rate is slower. Since Gertrude is located in the same galaxy as Alice, she isn't receding from Alice the way Bob is, so Alice sees no change in her time rate, since she receives the same number of photons (time units) from Gertrude. Ditto for Bob and Norbert at galaxy B.
If we do a little algebra we can derive the Lorentz equation from equation 3 above. The final result is equation 10:
Equation 10 shows each galaxy's proper time (t') shrinking as they accelerate further apart. It does not matter if the galaxies are technically at rest, since the space inbetween and the photons are not at rest. If the space is at rest, here's the result after firing a photon gun for five seconds:
Notice the steady stream of photons. This is where t' = t. The next diagram shows what happens if space expands at a steady rate of v. The photons still have a steady rate but the interval between them has increased, so they are not counted as frequently. Time t' is less than t.
Finally, the next diagram is the most realistic, since the expansion of space is accelerating. Here the the interval between photons continues to grow and they are counted less and less frequently. Time t' is shrinking.
When Alice and Bob are far enough apart, neither will receive any photons from the other. Time t' will be zero. Thus the expanding universe is relativistic if one keeps track of the various time rates at different distances.
"Reality is merely an illusion, albeit a very persistent one."--Albert Einstein
It is apparent from the above quote that reality distinguishes itself from ordinary illusions by its persistent nature. Reality is true even if you choose not to believe in it. Thus, if we are trying to settle the question whether time is real, we should examine time to see, if like reality, it too is a persistent illusion.
The time variable is very persistent and ubiquitous in so many physics equations. On that basis we can claim it's real, but just how real is it compared to things like matter, energy, mass, distance, force, your neighbor's barking dog? It's not like we can grab time out of the air, hold in hand and look at it like a hunk of clay. However, like clay, time can be stretched or compressed depending on how close to the speed of light you are traveling. How is that possible if time is just a product of human imagination? Surely any relative differences in time would also be limited to the human imagination and not an empirical reality.
When examining time, one has to make the distinction between how we measure time and time itself. One popular argument claims that if all particles in the universe stopped changing their states and came to rest, time would stop and cease to exist. This seems reasonable. If nothing happens, how would we experience the "flow of time"?
But what if the "flow of time" is just our experience when we measure time? If your watch stops, you don't assume that time has stopped. You only assume your ability to measure and experience "the flow of time" has stopped. So it seems reasonable to assume that time continues even if every particle comes to a grinding halt. Think of a stalled universe as one big watch that stopped.
So what exactly is time if not a flowing, evolving, ever-changing environment of entropy? The following equation, for me, is a real eye-opener and has forced me to rethink time:
E is energy and psi is the wave function, tp is the Planck time, G, c, and h-bar are the gravitational constant, light speed and Planck's constant, respectively. The above equation shows that it doesn't matter how much or little energy there is, or whether states change frequently or not at all, whether they go forward or backward. No matter what values you plug in for E and psi, you get forward time, specifically, the Planck time. Imagine having zero energy, zero change and still having a Planck time. How is that possible? Thought experiment time:
Imagine a universe with no energy, no distance or space, no charges, no masses, no momentum, no oscillators--just a single zero-dimensional point, a singularity. According to the above equation, time still exists. Why? Because the singularity is persistent--it is real. What exactly is this singularity? It's literally nothing ... except time at a single reference frame, at a single point. No clocks, no observers, just pure time.
Time is so essential to reality, that no "persistent illusion" can persist without it. Time can persist without anything else we would deem real, but nothing we deem real can persist without time. The words "reality," "existence," "persistence," "presence" all imply the passage of time. At this juncture, one could argue that time is not only real, but reality's most essential component. And, when we perform the above thought experiment, we witness time in its purest form.
So if you ever encounter a skeptic who believes time isn't real, that particles exist without time, ask the following question (but don't hold your breath):
We begin with equation 1, Einstein's famous general relativity equation, then we do a little algebra and contract the tensors to simplify the math:
Next, we put together the metric. Equation 12 below is the finished product:
Coordinates y and z are set to zero because the spaceship is moving along the x axis. Velocity v is further defined below. It is a function of the warp-bubble shaping function and the difference between the spaceship's position and the total distance along x.
At 15 notice the value inside the warp bubble is 1 and outside is zero. This allows time to dilate outside the warp bubble, while inside the bubble there is no time dilation. More on this later. For now, let's continue defining the other components:
Below is a crude diagram representing the warp bubble moving along the x axis:
"It is then easy to see that for the spaceship’s trajectory we will have:
dτ = dt ."--Miguel Alcubierre
From Alcubierre's metric we can derive a Lorentz equation (see 19 below) that shows how time dilation is avoided. Since the value of the bubble shaping function is 1 inside the warp bubble the velocity v and velocity x/t cancel each other, so there's no time dilation inside the warp bubble. The function value is 0 on the outside, so there x/t is not cancelled and any time dilation would take place outside the warp bubble.
At 20 above, we see that proper time (t') equals coordinate time (t).
"The metric I have just described has one important drawback, however: it violates
all three energy conditions (weak, dominant and strong). Both the weak and the
dominant energy conditions require the energy density to be positive for all observers."--Miguel Alcubierre
It appears that negative energy density is required to achieve speeds greater than light speed. Negative energy density implies negative energy and negative mass. Check out the following equation system:
Notice that when v > c, the only way to get a value for m' that is real and not imaginary is to plug in -mv^2, a negative energy that has negative mass (-m). For photons, where v = c, mass m' is zero as expected. Particles that move faster than light (e.g. tachyons) have negative mass. Thus it seems such exotic matter is required to achieve superluminal speed. However, consider a satellite orbiting a black hole at radius r:
The black hole has positive mass, i.e., positive energy density. If we increase that positive energy density, we can imagine the satellite orbiting faster and faster. There seems to be no upper limit to velocity v in this gadanke experiment, since there is no apparent upper limit to positive mass m. Further, using Einstein's field equations we can show that unlimited positive energy density (pressure, heat, etc.) can yield unlimited velocity:
At equation 30 above it is plain to see that any increase in positive energy density on the right side will cause a corresponding increase in velocity on the left side. So it appears exotic matter is not really needed to achieve superluminal speed. But then there's this:
"[L]ight itself is also being pushed by the distortion of
spacetime."--Miguel Alcubierre
This is an unfortunate choice of words. It gives the impression that the distortion of spacetime can push light faster than light. At least that's what needs to happen if the spaceship is going faster than light; otherwise, the ship's electromagnetic energy could be left behind! Let's see what happens to photons when spacetime is distorted:
On the left side of equation 36 we have a mass particle with velocity v. On the right we have a photon with frequency f. If velocity v increases due to spacetime distortion, the velocity of the photon does not increase; rather, its frequency increases. Now, just for fun, what would happen if we assume the photon's velocity could increase?
On the right side of equation 38, the denominator has c'--a special photon velocity that can rise above c. What is the consequence? A paradox! When energy density increases, so does the spacetime curvature on the left side. This causes c' to increase wich causes the spacetime curvature to reduce which causes c' to reduce and so on. Bottom line: you end up with some very screwed-up physics!
If we can't show that photons go faster than c, at least we can show that other particles can ... or can we? Imagine the spaceship moving toward a star system at velocity nc, which is greater than c. The star system sends photons toward the spaceship at velocity c. What is the combined velocity? It's not nc + c. The velocity addition formula reveals the answer:
As Einstein himself could have told you, the combined velocity is no faster than light! This means no matter how fast distorted spacetime moves the spaceship, if there are any photons along its path (and there will be!), it will move no faster than light.
So how should we interpret equations 30 and 33? Why does it appear that superluminal speed is possible? Equation 33 makes sense if we place ourselves inside the spaceship and allow time dilation to happen.
At 43 above, we multiply the real velocity by t to get the distance. The perceived velocity v is that distance over proper time t', the time experienced by the observers in the spaceship. Let's see what we can derive:
At 51 above, we see that as epsilon gets closer to 1, the perceived velocity gets closer to infinity. The real velocity (epsilon-c) never goes faster than light.
We can determine the value of alpha for equations 30 and 33 (see equation 55):
Now equations 30, 33 and 52 make sense when you consider that observers outside the spaceship will see it going no faster than c, but observers inside the spaceship will swear they covered distance vt in time t', which could be faster than light.
But then again, we have a time dilation problem, since the clock inside the spaceship won't agree with the clock of outside observers. This is definitely true if the spaceship moves and its departure point A and arrival point B are at rest. We illustrate this in the diagram below. The arrow represents the spaceship.
But what if the spaceship, points A and B move at the same rate?
If points A, B and the spaceship all move at the same rate as in the above diagram, their clocks will agree. All observers along that path will be under the impression that the spaceship traveled distance vt in time t', which could be faster than light, at least on paper.
Now, if the observers in the spaceship view the star system they're heading for, according to the velocity addition formula, the combined velocity of the ship and the photons coming from the star will not exceed light speed, so what do they see? We can speculate they see a shorter distance or distance vt' instead of distance vt. If they don't look where they are going, they might assume the distance is vt and conclude they covered it faster than light.
Given what we now know, we should be able to visit any star system in record time, and when we get back home, our clock will agree with clocks on earth. Albeit, Miguel Alcubierre's paper makes no mention of the impact the needed energy will have on the spaceship's clocks. Things like mass and energy also slow time. According to Varieshi and Burstein's paper (click here to read), the amount of energy (mass) needed to manipulate spacetime to achieve a seemingly superluminal speed is approximately 3.42 X 10^38 solar masses! Many orders of magnitude greater than our observable universe! So time dilation is a problem even if we can successfully get the spaceship and points A and B moving at the same rate.
Conclusion: Warp drive makes wonderful science fiction--and it will be quite some time before it becomes a scientific fact.
Update: So do we really need the spaceship and points A and B moving at the same rate to cure time dilation? Take the muon time dilation experiment. When muons are moving at velocity v, their time slows. But if an observer were riding the back of a muon and considered herself at rest, she would see the scientists moving at velocity v, so why doesn't their time dilate as well? If the muons and scientists that observe them are moving at v relative to each other, shouldn't their clocks agree?
But then a closer look at the Lorentz factor reveals what's going on. Time dilation really depends on how fast the muons and scientists are moving relative to the speed of light--and not to each other. Thus the scientists really are at rest (or slower than the muons)--a photon has no trouble catching them, but does have trouble catching a fast muon (according to a coordinate-time observer; a proper-time observer [the muon or scientist] will see the photon approaching at c). Thus, for their clocks to agree, points A, B and the spaceship must move at the same velocity relative to the speed of light, not to each other.
If you are familiar with Max Planck's work or Albert Einstein's photo-electric effect, then you know energy comes in discrete packets called quanta. These discrete energy levels are represented by nice, neat integers, where n = 1, 2, 3 ... But then there is this equation:
The above equation's second term is the ground state--but why isn't the frequency (f) and Planck's constant (h) multiplied by a nice, neat integer? Why 1/2? That's odd. And why ain't the ground state zero? Surely if you remove everything there should be nothing, nada, zip! Definitely not 1/2.
To understand what's going on, let's try a thought experiment. Imagine you want to push a couch a distance of x. You put your hands on the couch and apply zero force or energy. You gradually increase the energy applied until you reach a critical value where the couch begins to move. Let's label that critical value 1E.
Now suppose you want to push two couches along distance x. You gradually increase the applied energy to, say, 1.7E:
Unfortunately, 1.7E is not enough energy to do the work, so gradually increase the applied energy to 2E:
Assuming the couches are identical, the critical values of energy needed to push one or two couches are 1E and 2E. The critical coefficients are integers. If we want to move n couches we need nE. Below is a diagram of our thought experiment:
Note that the energy you applied is continuous, but the critical values (in red) are discrete. Also note the lowest energy is zero, so again, where does the (1/2)fh come from? Consider a single die. It has six discrete states: 1, 2, 3, 4, 5, 6. If we add up these states, we get 21:
This time, instead of a continuous line, the following die diagram has discrete steps:
Now, let's imagine a die with an infinite number of sides (or states) ranging from zero to six. Its states are no-longer discrete, but continuous. However, like the couch experiment, there are critical values marked in red (see diagram below).
Once again note the ground state is zero, but also note when we add up all those energies (see equation 3) we don't get 21 like before. We get 18! The following diagram illustrates the difference between the six-sided die and the infinite-sided die. The six-sided die clearly has more area under the curve. That extra area is indicated in gray:
So how do we fix this discrepancy? Let's include a y-intercept as they say in calculus parlance:
At equation 5 we use an intercept of 1/2. Doing so gives us a sum of 21--equal to the six-sided die! The diagram below illustrates this point. Note the gray areas cancelling each other:
