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# A serious issue with space exploration

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Hello!

I have been thinking about space exploration and have come across an interesting problem. I haven't seen anyone mention this before, bu here it goes:

The biggest problem right now with traveling to other stars/galaxies is the light speed gap. We cannot even come close to light speed at this time. An alternative that is frequently proposed is using wormholes or shortcuts in space-time.

Ok, lets assume that we have the technology to create wormholes and we could use them to transport ships to other galaxies in an instant. Here is were I came across jet another problem. If we send a ship to another galaxy, lets not forget that other galaxies are moving away from is (the expanding universe) and may of them are doing so at great velocities in reference to us. Wouldn't this cause time dilation between the ship and home and as a result one would age more than the other or something to that regard?

Time is not constant, so this would make establishing a universal "empire" impossible it seems. Or what would you say?

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I think we get stuck with the idea of physical travel, when it seems unlikly and unnecessary to travel to other worlds. Biologically its best we stay separate . All that needs to travel though space to connect and advance mankind is information.

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I think it would be wise to travel to Mars before we attempt going to another galaxy. But hey, that's just me. ;)

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I am in agreement with Tormod. However, if we were advanced enough to successfully create wormholes that don't collapse allowing one to circumvent the vastness of space.

Then I would think you might be able to direct where/when in spacetime you end up.

Though I admit that without any specific examples, it would be conjecture without facility

for proof.

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If we send a ship to another galaxy, lets not forget that other galaxies are moving away from is (the expanding universe) and may of them are doing so at great velocities in reference to us. Wouldn't this cause time dilation between the ship and home and as a result one would age more than the other or something to that regard?
For travel within our Milky Way galaxies and its neighbors, the time dilation would be slight.

Within a typical galaxy, the maximum relative velocity of stars is on the order of $10^5 \,\mbox{m/s}$, or 0.0003 c. The fastest moving stars, known as hypervelocity stars, have speeds on the order of 10 times as fast, 0.003 c. Our neighboring galaxies, the Local Group, are within about 1.5 Mpc of us, so, per Hubble’s law, have velocities away from us on the order of $10^5 \,\mbox{m/s}$, about the same as for stars within a galaxy. Per special relativity, this means that the maximum time dilation “tau” factor is about

$\tau = \sqrt{1-\left( \frac{v}{c}\right)^2} \, = \, \sqrt{1-0.003^2} \, \dot= \, 0.9999955$

Were we to travel between stars, or even just between planets in our solar system, time dilation factors of similar size can be had by changing ones distance from massive bodies such as galactic centers, stars, or giant planets, via gravitational time dilation. For example, moving from the Solar system to a star 1 parsec from galactic center dilates time by about 0.99999975, from Earth to Mercury about 0.999999978, to the atmosphere top of Jupiter, about .999999988.

Though significant in terms such as synchronizing clocks – a time dilation factor of 0.99999975 equates to about 8 sec/year – it wouldn’t be very noticeable in terms of ordinary human experience.

Getting near actual black holes, on the other hand can produce time dilation factors of nearly zero, effectively freezing time. Various folk, from SF authors to physicists, have suggested that, if you’re able to build traversable wormholes with movable mouths, parking a mouth near the event horizon of a black hole is a good way to make a practical time machine.

Sources: wikipedia articles “stellar kinematics”, “Hubble’s law”, “Local Group”, “supermassive black hole”, “time dilation”, and “time travel”.

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Interesting. So the time dilation effect in our local group of galaxies is not that notable.

How about distant galaxies then? Those, which have a relative velocity of near c to us. Wouldn't exploring those be a problem?

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How about distant galaxies then? Those, which have a relative velocity of near c to us.
The time dilation for objects at nearly opposite sides of the visible universe would be pretty dramatic, though still not, I think, a big problem.

The greatest observed redshift are on the order of z=7, which corresponds to a distance of about 4 billion parsecs, a few thousand times the radius of the Local Group. Hubble’s law, which is widely believed to give accurate estimates at all distances in the observed universe, gives a recessional velocity for such a very distant body of about

$v = D H_0 = 3.9 \,\mbox{Gpc} \cdot 70 \,\mbox{km/s/Mpc} = 273000000 \,\mbox{m/s} = 0 .91 \,\mbox{c}$

, corresponding to a time dilation factor of about $\sqrt{1-0.91^2} \dot= 0.41$.

Wouldn't exploring those be a problem?
Exploring even nearby stars certainly pose huge problems, doing so via wormholes even more so, because while we’re confident that propelling spacecraft to distant stars is theoretically possible, and have some practical engineering experience with how to go about doing it, we’ve no theoretical confidence that traversable wormholes can exist, and only the vaguest hunches about how one would create and/or traverse them.

Much of what we know, we know because of the efforts of Kip Thorne and friends to help the late Carl Sagan write as scientifically plausible a novel as possible in which characters travel quickly to and back from a place near our galaxy’s center. While the result, 1985’s “Contact”, is arguably the best scientifically researched science fiction novel written, the theoretical work that went in to it is very speculative, and far from any claim of scientific plausibility. This is not to say that such a thing isn’t possible, only that a lot of scientific work is needed before we can even say with much confidence whether it is or not.

If space travel via wormholes is possible, best theory strongly suggests that any wormhole couldn’t be “dug” faster than the speed of light. Like the subway train system SF stories commonly describe them as like, if possible, such systems would likely take a very long time to build – the reason why much good SF, such as “Contact”, tells tales of humans finding such systems already built by ancient civilizations, rather than building them ourselves.

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– the reason why much good SF, such as “Contact”, tells tales of humans finding such systems already built by ancient civilizations, rather than building them ourselves

Just as we discovered radio waves then invented a radio receiver -transmitter, or discovered other properties of quantum mechanics that advance communication, we may just find a vast network of communication system that been in place for thousands of years. To be connected to the universal web of information. That's all you would need. Any ideas on this Craig ? it would have to be faster than light communication for it to be of practice use, unless it was in the form of a embedded information a “Basic Guide to the universe.”

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Just as we discovered radio waves then invented a radio receiver -transmitter, or discovered other properties of quantum mechanics that advance communication, we may just find a vast network of communication system that been in place for thousands of years. To be connected to the universal web of information. That's all you would need. Any ideas on this Craig ?
That’s along the lines of what Sagan wrote in “Contact”, except rather than an internet-like communication system, what was discovered was a physical transportation system, and rather than everybody getting connected to it, it’s administrators went to lengths to make sure only a few people learned about it, and gave them only a few hints about what to do next to lead humankind to eventual galactic citizenship.

It’s just a SF novel, but I consider “Contact” required reading for enthusiasts of ideas like these (a 1997 major movie was made of it, but alters and omits many of the book’s major ideas). It’s important to remember, still, that this and similar stories are fiction, and there’s no such evidence that ideas about existing or preceding great space civilizations are anything but fiction.

it would have to be faster than light communication for it to be of practice use, unless it was in the form of a embedded information a “Basic Guide to the universe.”
The basic idea of traversable wormhole transportation systems is that they’re very long on the outside, and much shorter on the inside, so any EM signal sent through one appears faster than light relative to the outside universe.

If long traversable wormholes exist, it’s difficult to come up with an explanation in which they can’t be used to build paradox-creating time machines, a vast, fun, wiggly can of physics and philosophy worms. In his 1994 “Black Holes and Time Warps: Einstein's Outrageous Legacy”, Thorne discusses this, and the connection of his research with Sagan’s novel. I highly recommend it as a sort of non-fiction companion to “Contact”.

Though all this, it’s important, IMHO, to keep firmly in mind the very tentative nature of the scientific understanding of wormholes. It’s very possible, arguably likely, that they simply don’t and can’t exist.

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The greatest observed redshift are on the order of z=7, which corresponds to a distance of about 4 billion parsecs, a few thousand times the radius of the Local Group. Hubble’s law, which is widely believed to give accurate estimates at all distances in the observed universe, gives a recessional velocity for such a very distant body of about

$v = D H_0 = 3.9 \,\mbox{Gpc} \cdot 70 \,\mbox{km/s/Mpc} = 273000000 \,\mbox{m/s} = 0 .91 \,\mbox{c}$

, corresponding to a time dilation factor of about $\sqrt{1-0.91^2} \dot= 0.41$.

Thanks! That was pretty much the answer I was looking for.

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Hello!

Ok, lets assume that we have the technology to create wormholes and we could use them to transport ships to other galaxies in an instant.?

Wouldn't constantly creating and closing wormholes cause some kind of disruption in space-time?

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• 4 weeks later...
Wouldn't constantly creating and closing wormholes cause some kind of disruption in space-time?

Considering that any such questions are highly speculative, to put it mildly, and that the wording of any question powerfully paths any answer(s) the first qualifier should be "Exactly what do you mean by 'disruption'?" Additionally, any question regarding wormholes, or any other means of sidestepping the speed of light problem (assuming the speed of light is in fact a real barrier) must first pose the speculation of how this is accomplished, how Space-Time is actually structured probably on the Planck Scale. We don't even know if Space-Time is discrete. All of the graphic examples I have seen assume a folding of Space so that the distance between Start and End is vastly reduced maintaining the sanctity of lightspeed. So assuming that mankind can actually cause a folding of Space we have already ssumed that Space can be folded. Considering that neutron stars are only characterized as causing mere dimples this in itself is a huge assumption.

Furthermore it is likely that if the energy requirements are so low as to allow them to be manipulated by humans anytime in the next thousand years or so (How does one manipulate, or even fund the manipulation of, the power of even a small star?), it may be likely that Space has been or is even regularly subject to folding, that it has occurred somewhat often, in cosmological time, and quite naturally. If that is true and if "disruption" implies "danger" to the Universe at large I'd have to answer , "No. No disruption" If instead it implies danger directly to us, well that goes without saying in that some people can't harness the energy of a lawnmower without mishap, let alone what surely must be more energy than that of our Sun on a good day. This is a very long way off since the engineering scale may even exceed recreating Ringworld, and that is beyond monumental. Disruption of Space-Time is maybe last on the long list of problems just getting there.

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Space opera, physics popularizations, and hard SF on the subject of disrupting space

Wouldn't constantly creating and closing wormholes cause some kind of disruption in space-time?

I agree with enorbet’s general point on how very speculative questions about of worm holes are.

The idea that various sorts of artificial fiddling with the vacuum can “weaken” or “break” it belong, I think, firmly in the real of space opera. It actually figures prominently in an episode of STTNG. I think it must be taken, however, as an allegorical cautionary tale (in it’s 40-some year run, Star Trek has been full of these) about wearing out the Earth’s ecosystem. Scientifically, though, it’s uncertain that wormholes are even possible, so speculation about their effect on the vacuum is … for lack of a better word, fantastic.

Thorne and Hawking are said to have an ongoing debate about the possibility of traversable wormholes, including some intimations that they might at least in some manner disrupt the space they constitute. It assumes

• A traversable – at least by very small particles, such as photons – wormhole can be found or created
• Its mouths are movable
• The usual laws of physics, including special and general relativity, apply to wormhole mouths
• The time required to traverse it is zero, or much smaller than the time needed for light to traverse the distance between the two mouths in normal space.

From these assumptions, it follows that one can build a time machine from a traversable wormhole. One way to do so is to place one mouth (B) in a strong gravitational field, subjecting it to gravitational time dilation, wait ‘til $\Delta t = t_A - t_B$, then return it to near the other (A). Anything entering A and emerging from B, then, will appear $\Delta t$ time units in its own past – that is, it will emerge from B before it enter A, existing near it’s own past self for a duration a bit less than $\Delta t$.

Some pretty profound paradoxes of causation arise from this, such as the grandfather paradox, ie the object of information emerging from B in its past self, preventing it from entering A, so that it never emerged from B – the core plot of countless SF stories and movies.

According to Thorne (eg: in Black Holes and Time Warps), one of the scenarios offered in support of a “cosmic censorship” principle that prevents such causation-violating setups as the above is that, were it built, at least one particle of some kind would pass into its own past, then, in its “second future”, into its past again, and so on, until such a large number of occurrences of the same particle existed in the same vicinity that, regardless of what particles are involved and how the wormhole is built, it would all just, for lack of a better phrase, get blown all to hell.

Back in the realm of SF – but in this case very hard SF – in his 1997 novel Diaspora, Greg Egan considered the possibility of traversable black holes being possible, but lacking all the qualities assumed above. Not wanting to spoil this excellent novel for folk who haven’t read it but will, I’ll say no more.

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