Warp drive, then where are we?

[aychamo]

Hey guys;

Say that yesterday someone invented a warp drive, that would allow you to shoot across our galaxy in an instant. Say we went up in space today and used it. When you appeared at what you would think would be the other side of the galaxy, how would you know where you are?

My point is: If such a technology even was invented, wouldn't you be all but lost when you got to your destination? Any "signals" coming from Earth to let you know where you are would take however many billion years to reach you. Would you effectivly be lost? Or would there be other mechanisms to locate yourself? (Star locations, etc)

 

[SpaceTiger]

My point is: If such a technology even was invented, wouldn't you be all but lost when you got to your destination? Any "signals" coming from Earth to let you know where you are would take however many billion years to reach you. Would you effectivly be lost? Or would there be other mechanisms to locate yourself? (Star locations, etc)

If such a technology existed, then locating yourself from the stars and galaxies would be a relatively trivial matter (particularly if you were smart enough to make a warp drive). However, current physics suggests that warp drive is not possible.

 

[Entropy]

Well, if the warp drive aloud you to go pass the speed of light and you knew how fast you were traveling and how long you traveled plus the direction you went in, they you could easily find your location.

Other than that, possibly by use of star charts you could locate yourself, but then again stars can look very different in different locations because stars will be in different locations not only because you moved but also because you are closer/further to them so you are seeing "younger" or "older" light coming from them.

 

[ohwilleke]

A guy who doesn't bring a map with him on a galactic length journey, and doesn't know which direction and distance he was traveling into start with, to give him a good hint about where he will be, deserves to be lost.

 

[misskitty]

He, or she, could use the Hitchhiker's Guide to the Universe. I agree with Ohwilleke, anybody who wouldn't bring a bloody map with them is daft.

SpaceTiger does have a point in stating current physics points out no such machine is possible.

 

[DaveC426913]

Actually, current physics does NOT make such a machine impossible.

It makes FTL drives impossible, but not warp drives.

FTL (faster-than-light) drives by definition exceed c.

Warp drives create or make use of wormholes to warp space, making the distance between 'there' and 'here' smaller and traversable without exceeding c.

Nothing in modern day physics says this can't be done. In fact, Einstein's equations predict it as a possibility. (Then again, they also predict that time travel is possible.)

 

[misskitty]

If anything were to exceed c wouldn't it just turn into energy?

 

[Phobos]

When you appeared at what you would think would be the other side of the galaxy,

The whole of the galaxy is made of the same stuff we see on this side...stars, nebulae, etc.

You may just not know the local layout of stuff (particularly on the other side of the galaxy where we don't have a clear view from here...the galactic center obscures our view of the other side).

how would you know where you are?

Presumably, you would target a specific location for your journey. Either you would know it beforehand through observations or you would map it out once you got there relative to areas you do know about. If you did a random shot, then although you might not have any idea about local conditions, you could still easily find the galactic plane and center merely by looking (and thereby orienting yourself) and then you could look outward toward other galaxies and figure out your new position relative to the view you had of those galaxies back on Earth.

Any "signals" coming from Earth to let you know where you are would take however many billion years to reach you.

The galaxy is about 100,000 light years across, so an EM signal would take at most, 100,000 light years (not much help, granted).

 

[ohwilleke]

Actually, current physics does NOT make such a machine impossible.

It makes FTL drives impossible, but not warp drives.

FTL (faster-than-light) drives by definition exceed c.

Warp drives create or make use of wormholes to warp space, making the distance between 'there' and 'here' smaller and traversable without exceeding c.

Nothing in modern day physics says this can't be done. In fact, Einstein's equations predict it as a possibility. (Then again, they also predict that time travel is possible.)

The trouble with wormholes is that the large scale structure of the universe is very close to topologically flat, as shown by the cosmic background radiation studies. A wormhole that would get you any place far away requires a universal that significantly not topologically flat.

 

[SpaceTiger]

Nothing in modern day physics says this can't be done.

Except for the fact that you'd need more than a galaxy's worth of energy to achieve it. I'd say that qualifies as "can't be done".

An interesting link on the subject:

http://www.nasa.gov/centers/glenn/research/warp/socanwe.html

 

[owl3951]

Well...

Except for the fact that you'd need more than a galaxy's worth of energy to achieve it. I'd say that qualifies as "can't be done".

An interesting link on the subject:

http://www.nasa.gov/centers/glenn/research/warp/socanwe.html

...if you want to get all technical... :tongue2:

 

[JesseM]

The trouble with wormholes is that the large scale structure of the universe is very close to topologically flat, as shown by the cosmic background radiation studies. A wormhole that would get you any place far away requires a universal that significantly not topologically flat.

Why do you say that? I think maybe you are taking the embedding diagrams too literally, wormholes don't require that two regions of space be "near" each other in some higher-dimensional space (in fact general relativity itself doesn't require any such higher-dimensional space for curved 3D space to be 'embedded' in).

 

[ohwilleke]

Don't they? The idea of a wormhole is that you avoid the need to travel at greater than c to get from here to there in a given time by connecting two points via some connection of a shorter length. I have a hard time seeing how that isn't equivalent either a higher dimensional space or a topologically non-flat space.

 

[JesseM]

Don't they? The idea of a wormhole is that you avoid the need to travel at greater than c to get from here to there in a given time by connecting two points via some connection of a shorter length. I have a hard time seeing how that isn't equivalent either a higher dimensional space or a topologically non-flat space.

What do you mean by "topologically non-flat"? I thought flatness referred only to curvature, and would be compatible with a range of topologies. For example, it's possible to have a flat universe with the topology of a torus, as discussed in this article.

 

[turbo]

The galaxy is about 100,000 light years across, so an EM signal would take at most, 100,000 light years (not much help, granted).

Darn! I was hoping to use my Magellen GPS receiver to find my way back. I'll be 99,970 (or so) years too old to do that. :uhh: Unless the wormhole slowed MY time down so that I perceived only a brief passage while the entire external universe experienced 100,000 years of time. That way, a powerful enough beacon in Earth orbit might help me get back...although after about 200,000 years elapsed time on Earth, "catching up with old friends" might be a bit problematic.

 

[JesseM]

Darn! I was hoping to use my Magellen GPS receiver to find my way back. I'll be 99,970 (or so) years too old to do that. :uhh: Unless the wormhole slowed MY time down so that I perceived only a brief passage while the entire external universe experienced 100,000 years of time. That way, a powerful enough beacon in Earth orbit might help me get back...although after about 200,000 years elapsed time on Earth, "catching up with old friends" might be a bit problematic.

You don't need a wormhole to get to the other end of the galaxy in a short subjective time, special relativity will work fine. From the point of view of observers at rest wrt the galaxy, although it will take you over a hundred thousand years to cross it, if you go fast enough your clock will slow down enough so that you have only aged a few years when you reach the other end...from your point of view, you are aging normally but the length of the galaxy has lorentz-contracted down to much less than a hundred thousand light years. You can look at http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html to see a table of how much onboard time it would take to reach various distant locations, like the Andromeda Galaxy, if you accelerated at 1G for the first half of the journey and then decelerated at 1G for the second half (for example, it'd take 20 years to reach the center of the galaxy, 28 years to reach the Andromeda galaxy). But like you said, when you return to Earth huge amounts of time will have passed according to earth-clocks.

 

[DaveC426913]

...the large scale structure of the universe is very close to topologically flat...

Yes, on average. Doesn't mean smaller areas are flat too.

 

[DaveC426913]

Except for the fact that you'd need more than a galaxy's worth of energy to achieve it. I'd say that qualifies as "can't be done".

You'd be wrong. That merely qualifies as "impractical". Before determining whether it is feasible, we must first determine if it is theoretically possible, which it is.

 

[SpaceTiger]

You'd be wrong. That merely qualifies as "impractical". Before determining whether it is feasible, we must first determine if it is theoretically possible, which it is.

No, it qualifies as "can't be done". You can't harness a galaxy's worth of energy to get across a galaxy. That's one of the most ridiculous things I've ever heard. At this point, we don't think it will be physically possible to perform this feat. I acknowledge that physics might change, but it simply can't be done with what we know.

 

[JesseM]

"more than a galaxy's worth of energy" to do what, exactly? To travel across the galaxy fast enough so that the journey only takes a few years of onboard time? To build a wormhole connection from one end of the galaxy to the other? To build an Alcubierre warp drive to get you there? I don't think it's been established that any of these things would require that much energy, although we won't know whether the last two are even physically possible until we have a theory of quantum gravity.

 

[DaveC426913]

No, it qualifies as "can't be done". You can't harness a galaxy's worth of energy to get across a galaxy. That's one of the most ridiculous things I've ever heard. At this point, we don't think it will be physically possible to perform this feat. I acknowledge that physics might change, but it simply can't be done with what we know.

1] Your "more than a galaxy of energy" line is pulled out of thin air. Because you say it takes that much energy, doesn't make it so.
2] I think it should have gone without saying, but apparently not: "it can't be done with current technology". I think the initial poster took that for granted, I don't know why you didn't. Becasue we can't do it today does not mean it can't be done in theory. We know of no reason why it can't be done.
3] For the purposes of labelling this as a serious proposition, as opposed to a "ridiculous" one, note that this type of civilization is actually already defined by Nocolai Kardashev. A type III civilization is defined as one that can harness an entire galaxy as a power source.

It only seems ridiculous because you haven't read up on the subject.

 

[SpaceTiger]

Actually, "more than a galaxy's worth of energy" is a quote from a professor in the department here. His name is J. Richard Gott. I suggest you do a little research on that yourself.

I can't write much now, but will expand later.

 

[JesseM]

Actually, "more than a galaxy's worth of energy" is a quote from a professor in the department here. His name is J. Richard Gott. I suggest you do a little research on that yourself.

What book/paper is it from? Can you give the page number? Gott is known for studying the subject of time travel, so he might have been talking about something like the energy needed to create a Gott loop out of cosmic strings that would allow you to travel back in time, not the energy needed to travel across the galaxy at close to the speed of light, or the energy needed to hold open a stable wormhole.

 

[ohwilleke]

Yes, on average. Doesn't mean smaller areas are flat too.

True, but the extent of non-flatness locally is well understood as a virtue of GR and still doesn't get you to a GR with useful wormholes given what we know about local mass distributions in anywhere we could ever get to.

 

[JesseM]

True, but the extent of non-flatness locally is well understood as a virtue of GR and still doesn't get you to a GR with useful wormholes given what we know about local mass distributions in anywhere we could ever get to.

The question of the large-scale curvature of the universe is irrelevant to wormholes as far as I know--again, it's possible for wormholes to connect distant regions even when an embedding diagram of the universe's large-scale curvature wouldn't show those regions 'close to each other' in the embedding space (the whole idea of an 'embedding space' is just a visual convenience, general relativity itself describes spacetime curvature in intrinsic terms, without the need for any embedding space). As for the local mass distribution, I don't think it's possible to create a wormhole from scratch by pushing matter/energy around into a certain configuration. When people talk about an advanced civilization creating a traversable wormhole, they usually handwave about expanding a wormhole that occurs naturally in the quantum foam that's suggested to exist at the Planck scale according to some ideas about quantum gravity.

 

[DaveC426913]

Actually, "more than a galaxy's worth of energy" is a quote from a professor in the department here. His name is J. Richard Gott. I suggest you do a little research on that yourself.

No, the onus is on you.

 

[SpaceTiger]

not the energy needed to travel across the galaxy at close to the speed of light

I'm not disputing that you could cross the galaxy in a lifetime with much smaller amounts of energy (exploiting SR, as you said), I'm talking about wormhole travel, as Dave was referring to. I promise to give more details and do more research as soon as I return to school. Unfortunately, I've been attending my grandfather's funeral and only have very small intervals of free time.

 

[AcEY]

surely even if we use a warp drive, we would still have the problem of time dilation?

 

[JeffL]

You could use distant galaxies and the center of the Milky Way to extrapolate your coordinates. Local star positions will change but not the more distant objects (at least not that much). So you can still use them to get your bearings. Definitely make sure you have an idea of how far and what direction that spacecraft should go before clicking on the warp drive. I think it's possible.

 

[DaveC426913]

surely even if we use a warp drive, we would still have the problem of time dilation?

Nope. Time dilation is related to acceleration and velocity. A wormhole bypasses them.

Though it has been shown that wormhole in fact can easily act as a time machine, by bringing the two ends near each other (I forget exactly how this works.)

 

[SpaceTiger]

Alright, so here's my long-delayed response to the question of warp drive. Again, for easier consumption, I'm going to split it into multiple posts. Also, I know that many of the people here won't need such a pedagogical treatment, but this subject is of great interest to amateurs as well, so I'll keep it in simple terms.

It's true that I'm not an expert on the topic and had to do a little research today to put the wormhole stuff together, but the things I've stated previously in this thread were rushed responses repeated from several people in whose judgement I have a great deal of trust, so I don't think I was in error stating them.

Anyway, here's the situation. We want to get across the galaxy in less than a person's lifetime. There are several things in modern physics that we'll call possible possibilities. I'll try now to determine whether or not they're real possibilities.

1) Near-light speed travel.

This is perfectly workable, but I would not say that it constitutes what we normally understand to be warp drive. The idea behind it is that if we travel near light speed, there will be time dilation/length contraction effects which allow us to quickly traverse large distances in the rest frame of the galaxy. It can be understood entirely with special relativity. In the Earth's rest frame, the minimum time with which anything can cross the galaxy (d~15 kiloparsecs) will be roughly:

$$\Delta t=\frac{d}{c}\sim50,000\ years$$

There is no escaping this. No matter how close you get to the speed of light, the people on Earth will have to wait that long for you to reach your destination (and the same time for you to communicate back). From the traveler's point of view, however, time is dilated:

$$\Delta t'=\sqrt{1-\frac{v^2}{c^2}}\Delta t$$

We want to know the speed requirement to shorten the trip, so let's solve for v:

$$v=c\sqrt{1-(\frac{\Delta t'}{\Delta t}})^2}$$

If we want to cross the galaxy ($$\Delta t=50,000\ years$$) and want the trip to take a human lifetime ($$\Delta t'=100\ years$$), this equation says that you must travel at $$v \sim 0.999998c$$. In general, it's convenient to put relativistic equations in terms of the gamma factor:

$$\gamma=\frac{1}{\sqrt{1-\frac{v^2}{c^2}}}$$

Now, we'll just solve for $$\gamma$$ instead of velocity because it can be easily converted to velocity and because it has more physical meaning. With this, the above equation takes the form:

$$\Delta t'=\frac{1}{\gamma} \Delta t$$

and we find that $$\gamma \sim 500$$ is needed to allow a person to live for the entire trip. For shorter trips, you just multiply this number by the factor by which you want it to be shortened (e.g. $$\gamma\sim 2000$$ for a 25 year trip).

To explore the practical issues surrounding this problem, however, we want to look at it from a different point of view -- that is, we'd like to know how much energy we need to give the ship in order to make the trip. The relativistic equation for kinetic energy is:

$$E_k=(\gamma-1)mc^2 \sim \gamma mc^2$$

if $$\gamma >> 1$$. For a 1000 kg ship and the factor calculated above, this gives an energy of $$\sim 5 \times 10^{22}\ Joules$$. This is about a hundred times the total annual energy use of the United States. A steep cost, but certainly not impossible.

Another issue that JesseM briefly addressed was that of acceleration. If we try to accelerate too fast, then our bodies won't be able to take it (as any fighter pilot can attest to). This means that we can't build a ship that will jump immediately to the speed quoted above -- we have to take into account the extra time it takes to accelerate. JesseM gives the time assuming an acceleration of 1 g, an acceleration we're perfectly capable of handling.

So what's wrong with all this? Well, it depends on what you want. If you want to create an intergalactic civilization, the above means of transportation is completely useless. As I already said, even though the traveler makes the trip in a short amount of time, the earthbound folks will have to wait 100,000 years to hear back from them. If everybody lived on ships of this sort and spent most of their time traveling near the speed of light, then I suppose we could create a star-hopping civilization, but it seems like costs of something like that would far outweigh the benefits. This means of interstellar travel seems like it would be better as a means of escape if something nasty was going on in the solar system (like the sun dying).

So that's the simplest possibility. I'll address wormholes in the next post.

 

[SpaceTiger]

I've had many people ask me why we're so sure that we can't travel faster than the speed of light; after all, Einstein could be wrong at some limit. Maybe in the future we'll create a machine that will actually break the speed of light barrier and penetrate into new physical realms.

This might happen, of course, and I'm never one to completely exclude possibilities, but it should be understood that the theoretical problems surrounding faster-than-light travel (FTL) go far beyond a simple mechanical limit (like the sound barrier). Rather, the existence of FTL would require a complete shift in our current understanding of the way the universe works. According to Einstein's theory, an object that moved faster than the speed of light would, in some reference frames, be traveling back in time, and any time you introduce the possibility of time travel, you run into all sorts of paradoxes.

The most common and easily understood paradox is called the "grandfather paradox". Basically, it says that if you could travel back in time, you could kill your grandfather when he was young and render your own birth impossible. Since it was you that killed your grandfather, your non-existence wouldn't make a whole lot of sense. If you ask me, this simple argument should be enough to suggest that we need new physics (or, at least, a better understanding of the existing physics) to fully deduce the theoretical possibilities of any means of time travel (including wormholes). However, we'll put that aside for the moment.

What are wormholes? Well, they can be thought of as a sort of "tunnel" in spacetime. In Dr. Gott's book on time travel, he gave a great analogy that went something like the following (I don't have the book with me at the moment). If you have two ants on opposite sides of a table, what's the quickest way for them to reach each other? Well, they can take the long route in which they crawl all the way to the edge of the table and then around to the other side. This is analogous to traveling at sublight speeds to the other end of the galaxy. There is a quicker way, however. Given a suitable amount of strength/energy, the ants can break right through the table and create a tunnel that brings them to their destination in a much shorter time. This is analogous to a wormhole. You just need to bend spacetime enough to get where you want to go. This does not technically constitute FTL travel, but it does lead to the same time travel paradoxes.

There are many other problems with this picture. The first is that, in the conventional ideas surrounding warp drive, you would need both ants to break through the table. In other words, you would need someone to "open up" the wormhole at your destination as well as at your point of origin. Unless the opening was already in place, this would require someone to travel to your destination and the only other way to do this is at sublight speeds. Another problem is that Stephen Hawking's recent work seems to suggest that quantum fluctuations would prevent the wormhole from even opening. This is part of his "Chronology Protection Conjecture", which basically asserts that the laws of physics conspire to prevent the possibility of time travel.

Thus, I would say that both Dave and I were wrong. Our current understanding of physics does not suggest that warp drive is possible, nor does it suggest that it's impossible. Rather, we're still unsure whether the mainstream theories of physics predict its possibility.

As for my comment about the "galaxy of energy", I was indeed referring to the "Gott loop". Since the possibilities of time travel and warp drive are intimately connected and this is the time travel theory that is experiencing the least opposition, then I assume it is what he was referring to in his quote to me.

 

[mapper]

For the galaxy worth of energy poster.
http://www.nasa.gov/centers/glenn/research/warp/possible.html

"Zero Point Energy"
Zero Point Energy (ZPE), or vacuum fluctuation energy are terms used to describe the random electromagnetic oscillations that are left in a vacuum after all other energy has been removed. If you remove all the energy from a space, take out all the matter, all the heat, all the light... everything -- you will find that there is still some energy left. One way to explain this is from the uncertainty principle from quantum physics that implies that it is impossible to have an absolutely zero energy condition.

For light waves in space, the same condition holds. For every possible color of light, that includes the ones we can’t see, there is a non-zero amount of that light. Add up the energy for all those different frequencies of light and the amount of energy in a given space is enormous, even mind boggling, ranging from 10^36 to 10^70 Joules/m3.

In simplistic terms it has been said that there is enough energy in the volume the size of a coffee cup to boil away Earth’s oceans. - that’s one strong cup of coffee! For a while a lot of physics thought that concept was too hard to swallow. This vacuum energy is more widely accepted today.

What evidence shows that it exists?

First predicted in 1948, the vacuum energy has been linked to a number of experimental observations. Examples include the Casimir effect, Van der Waal forces, the Lamb-Retherford Shift, explanations of the Planck blackbody radiation spectrum, the stability of the ground state of the hydrogen atom from radiative collapse, and the effect of cavities to inhibit or enhance the spontaneous emission from excited atoms.
The Casimir Effect:

The most straight-forward evidence for vacuum energy is the Casimir effect. Get two metal plates close enough together and this vacuum energy will push them together. This is because the plates block out the light waves that are too big to fit between the plates. Eventually you have more waves bouncing on the outside than from the inside, the plates will get pushed together from this difference in light pressure. This effect has been experimentally demonstrated.

Can we tap into this energy?

It is doubtful that this can be tapped, and if it could be tapped, it is unknown what the secondary consequences would be. Remember that this is our lowest energy point. To get energy out, you presumably need to be at a lower energy state. Theoretical methods have been suggested to take advantage of the Casimir effect to extract energy (let the plates collapse and do work in the process) since the region inside the Casimir cavity can be interpreted as being at a lower energy state. Such concepts are only at the point of theoretical exercises at this point.

With such large amount of energy, why is it so hard to notice?

Imagine, for example, if you lived on a large plateau, so large that you didn’t know you were 1000 ft up. From your point of view, your ground is at zero height. As long as your not near the edge of your 1000 ft plateau, you won’t fall off, and you will never know that your zero is really 1000. It’s kind of the same way with this vacuum energy. It is essentially our zero reference point.

What about propulsion implications?

The vacuum fluctuations have also been theorized by Haisch, Rueda, and Puthoff to cause gravity and inertia. Those particular gravity theories are still up for debate. Even if the theories are correct, in their present form they do not provide a means to use electromagnetic means to induce propulsive forces. It has also been suggested by Millis that any asymmetric interactions with the vacuum energy might provide a propulsion effect.

 

[ohwilleke]

Against wormholes:

http://www.arxiv.org/abs/gr-qc/0503097

Date (revised v3): Fri, 8 Apr 2005 19:54:25 GMT (6kb)

Wormholes and Time Travel? Not Likely
Authors: Leonard Susskind
Comments: 5 pages, remark added about time delay in identification. Reference added

Wormholes have been advanced as both a method for circumventing the limitations of the speed of light as well as a means for building a time machine (to travel to the past). Thus it is argued that General Relativity may allow both of these possibilities. In this note I argue that traversable wormholes connecting otherwise causally disconnected regions, violate two of the most fundamental principles physics, namely local energy conservation and the energy-time uncertainty principle.

 

[ohwilleke]

[W]e find that $$\gamma \sim 500$$ is needed to allow a person to live for the entire trip. For shorter trips, you just multiply this number by the factor by which you want it to be shortened (e.g. $$\gamma\sim 2000$$ for a 25 year trip).

To explore the practical issues surrounding this problem, however, we want to look at it from a different point of view -- that is, we'd like to know how much energy we need to give the ship in order to make the trip. The relativistic equation for kinetic energy is:

$$E_k=(\gamma-1)mc^2 \sim \gamma mc^2$$

if $$\gamma >> 1$$. For a 1000 kg ship and the factor calculated above, this gives an energy of $$\sim 5 \times 10^{22}\ Joules$$. This is about a hundred times the total annual energy use of the United States. A steep cost, but certainly not impossible.

There is one more step you have to take to really get a good handle on the energy costs. This is the problem of how you get your ship to go so fast.

If you accellerate with carried fuel, then $$E_k$$ is proportional to your fuel requirements or worse, and twice that much if you want to both speed up and then slow down when you arrive at your destination. The theoretical limit on the amount of energy that can be extracted from carried fuel is $$E=mc^2$$ (i.e. an anti-matter drive). But, of course, no system can be 100% efficient in converting matter to energy.

Of course, the problem is that $$E_k$$ is also proportional to the mass of the object you are trying to move, and if you want to travel at $$\gamma \sim 500$$, you are going to find that a very large proportion of your ship's entire mass needs to be fuel.

In particle accellerators, we can get particles to close to the velocity c, by using a device to accellerate the particle which it outside and hence doesn't add mass to the particle itself. But, if you are to have a viable, conventional relativity starship, you need to either have massively huge ships with very small payloads, or find some way to not carry your own fuel, or to carry your fuel it in a form that has energy and not matter (e.g. photons swirling at high density in a nearly perfectly optically transparent ring).