My wife always used to tell at me for overanalyzing the shows so I just grin and enjoy them these days.

 

 

At the velocities you are considering any matter that gets in the way will tend to make a nasty mess, simple e=½mv² kinetic energy calculations show how much energy is involved. Also the same formula tells you how difficult it is to decelerate from those velocities (which affects manoeuvrability)

 

If we assume that all the impossibles have been overcome (by magic) and we can actually accelerated to these velocities and maintain them, then the best method of collision avoidance would be a deflector system, and this principle is used in Star Trek, (the "shields" are primarily particle deflectors for use at warp speed that were later used in defensive role)... and in Casey Jones (where deflecting cows was easier that trying to manoeuvre around them)

 

 

 

Of course the other impossible thing in Star Trek is the long range sensor array that is used in conjunction with the deflector system at warp speeds... when travelling at warp the sensor scans ahead of the ship, which means its beam is travelling faster than the ship. For communication at warp Star Trek invented Transwarp and Subspace, neither of which would detect objects in the path of the ship at ftl speeds as they operate outside standard space-time, so quite how the long range sensors work is anyone's guess

By the way, Star Trek made the first cell phone ever. 

 

http://upload.wikimedia.org/wikipedia/en/4/47/Dt2wrr.jpg" rel="nofollow">

 

Part of the enjoyment of SF novels for me is how the author can introduce something that is impossible or implausible, (such as superluminal speeds, teleporation and time travel), and then explain it in such a way to make the reader suspend just enough belief (in the physical world) to accept the premise. This does not occur so much film or TV where reasonable explanations are seldom given, or they are too vague to be convincing, so we just accept them at face value with none of the scientific/technical window dressing - a replicator can make Tea, Earl Grey, Hot - we don't need to know how, on screen it just works like a voice activated vending machine, and we generally don't need to know how a vending machine works, nor do we have to suspend belief to accept that one could exist. (" http://www.guardian.co.uk/technology/2013/jun/04/nasa-3d-printer-space-food" rel="nofollow - Home-baked idea? Nasa mulls 3D printers for food replication "). Similarly in film or tv a space ship can jump to warp with no explanation, we just accept that it can, but in the written text some conciliatory explanation is required. In the Dune novels of Frank Herbert - faster than light space travel was made possible due to folded space, which is a result of the Holzman effect (Herbert's general purpose get-out-of-jail free card - like Dr Who's sonic screwdriver or Star Trek's tachyon field) - plotting a path through folded space, as http://www.progarchives.com/forum/forum_posts.asp?TID=94551&PID=4823317#4823317" rel="nofollow - Pat said for FTL travel, would require an impossible amount of computation, so Herbert uses prescience to predict a route, and since that is also impossible, he enhances the abilities of the Guild Navigators with Spice (the value of which is the central theme of the books).

http://www.amazon.co.uk/Physics-Impossible-Scientific-Exploration-Teleportation/dp/0141030909/ref=sr_1_1?ie=UTF8&qid=1374666010&sr=8-1&keywords=physics+of+the+impossible" rel="nofollow - Physics of the impossible

in it he talks about many of the technologies frequently used in fiction and classifies them in 3 categories:

Class I 'impossibilities': technologies which are impossible today but do not violate any known laws of physics so there's no fundamental reason why they can't eventually become reality. These include (quantum-based) teleportation, antimatter engines, (certain forms of) telepathy, invisibility...

Class II 'impossibilities': technologies that lie at the very edge of our current understanding of the universe, they might be feasible but surely not within a few centuries, perhaps millenia, such as time machines or hyperspace travel.

Class III impossibilities: technologies that violate currently 'known' laws of physics. Only a radical shift in our understanding of the universe could ever make them possible. Perhaps surprisingly, he can only think of 2: perpetual motion machines and precognition.

No, I said that his Class I and II hypothetical technologies are those which do not violate any known laws pf physics.

and then you listed teleportation, antimatter engines, (certain forms of) telepathy, invisibility... time travel, ftl travel...

 

Read the book and you will understand in which sense none of these violate any known laws of physics (some may have a 'catch', such as what is meant by 'teleportation' in the quantum sense). I'm afraid that replicating the whole book here would take a bit too long, but I assure you that he has his point in all of these (he may be a popularizing scientist but he is not a crackpot, he is a respected physicist, a bit like what Brian Greene is regarding String Theory) .

It's quite some time ago that I read it but I still have it, if you want to get deeper into any of these let me know, I can re-read the relevant chapter (if I don't remember enough of it) and make a summary of his points.

http://en.wikipedia.org/wiki/Quantum_teleportation" rel="nofollow - Quantum teleportation

http://prl.aps.org/abstract/PRL/v110/i14/e140403" rel="nofollow - http://prl.aps.org/abstract/PRL/v110/i14/e140403

Except that it does not permit "travel" as we know and accept it (i.e. the transference of matter from one place to another unaltered and unaffected) - we can make a camel pass through the eye of a needle if we so desire, all it takes is a huge liquidiser and a hypodermic syringe, reconstituting the camel on the other side is a tad more complicated - and that is the essential problem with any travel that warps space or time ... what arrives the other end would bear little semblence to what departed from the jump-point... this we can predict, for example in crossing the event horizon of a black hole where matter (and thus distance) is compressed but time is stretched.

Gah, this one's been done to death on this forum. It's not teleportation. There is no " at the moment " ... it will never develop into matter teleportation (or even simple information teleportation) because it's simply not applicable (this is like extrapolating the physics of the pond-skater to predict a hydrofoil )

Again, the definition of his 3 classes is not whether they may ever be achievable in practice or not, just whether they violate currently know laws of physics or not, regardless of the technological or practical challenges involved. I'm not defending that any of these technologies will ever be realised, I just pointed Finnforest to an interesting book if he is interested in this kind of stuff, don't shoot at me for that.

Of course not, only information is being transported. What's the problem about that? I already said that this has little if anything to do with teletransporting macroscopic objects as in sci-fi books or movies, only that the possibility of teletransporting information has been experimentally confirmed, and in the view of some (take highly respected physicist John Wheeler) our universe is little more than information. The known laws of physics allow for teletransportation of information, that's all I meant. I also said clearly 'there's a catch on this one, being what is meant by 'teleportation' in the quantum sense'. I don't know what I can be accused about.

How can you say so? some fictional uses of a theoretical concept may, some others may not. What fictional scenarios make out of it does not undermine the underlying scientific theoretical validity of the concept.

A warp drive does what it says on the tin - it warps space, it is not using wormholes, which (incidentally) we may not call hyperspace. If you include folded space, (which is not the same as warped space), then they represent the four basic fictional methodologies seeking to overcome the vast distances in space and the problems associated with crossing them. None of these rely on any theoretical science to say that the method is practical or even possible or whether any real-space matter could survive the journey at all (which in all of them is highly unlikely).

 

[there is a fifth method that relies on a substance such as antimatter, (or counter-terrestrial matter or negative matter, as it was known before the 1920s), to affect time and space, though now-a-days that usually used to facilitate one of the other four, erm... much like the Alcubierre drive (which is as much SF as any other), which relies on the hypothetical concept of negative mass].

 

Since warping, folding and wormholes are not hyperspace, they would occur in natural space, with all the matter present between the two points being traversed still present in the warp, fold or hole, it is just the distance between them that is compressed (or the time taken to travel that distance is compressed, depending on how you look at it).

 

The collision of two galaxies will result in some physical damage to both systems - stars (and other celestial objects) will die in the process, the speed at which this occurs is immaterial, it will occur and the stars involved will be unable to avoid it. Also, current thinking is that the space between galaxies is far from empty, it's just that we don't know what it is filled with (dark energy being a strong contender). As is frequently pointed out on this forum by Friede, solid objects are far from solid with the distances between subatomic particles being far greater than their physical size, yet solid objects cannot pass through each other unscathed.

 

Warp speed (keeping this in the Star Trek universe for a moment) is significantly faster than the speed of light (for example Warp-5 is approximately 214 times the speed of light) so the energy release in colliding with an atom of hydrogen at that velocity is measurable (energy = atomic weight/Avogadro constant * velocity squared), in fact at warp 5 it has 45,688 times more energy than at light speed. Sure a single atom of hydrogen is still not going to cause any noticeable damage to a star ship, but a cloud of many hydrogen atoms will and space is far from empty - remember that stars are formed from within dust clouds where hydrogen is the most abundant element and if we take the mass of our insignificant little star as an example, the mass of hydrogen involved is (erm) astronomical. Also if every star system has an equivalent of a Kuiper belt and/or an Oort cloud (and given how stars are formed there is no reason to assume otherwise) then the space between stars is a minefield of debris significantly larger than a single hydrogen atom and much denser than a cloud of hydrogen atoms; couple that with the simplest observation that if you are warping or folding space then you are reducing the distance between every object within that space which in turn increases the density of space considerably, then the chances of hitting something are incredibly low to the point of being inevitable.

 

 

Could you please elaborate about the wormhole case? IMHO wormholes do not go through 3-D space so they may be legitimately referred to as hyperspace.

Dean wrote: The collision of two galaxies will result in some physical damage to both systems - stars (and other celestial objects) will die in the process, the speed at which this occurs is immaterial, it will occur and the stars involved will be unable to avoid it.

It is not immaterial in the subject of the present discussion, but with the gravitational effects it is entirely likely that an artificially powered spaceship could cross through 'colliding' galaxies without its path being significantly affected, regardless if the dynamics of the stars in those two galaxies were changing because of the 'collision'.

Dean wrote: Also, current thinking is that the space between galaxies is far from empty, it's just that we don't know what it is filled with (dark energy being a strong contender).

 

Dean wrote: Warp speed (keeping this in the Star Trek universe for a moment) is significantly faster than the speed of light (for example Warp-5 is approximately 214 times the speed of light) so the energy release in colliding with an atom of hydrogen at that velocity is measurable (energy = atomic weight/Avogadro constant * velocity squared), in fact at warp 5 it has 45,688 times more energy than at light speed. Sure a single atom of hydrogen is still not going to cause any noticeable damage to a star ship, but a cloud of many hydrogen atoms will and space is far from empty - remember that stars are formed from within dust clouds where hydrogen is the most abundant element and if we take the mass of our insignificant little star as an example, the mass of hydrogen involved is (erm) astronomical. Also if every star system has an equivalent of a Kuiper belt and/or an Oort cloud (and given how stars are formed there is no reason to assume otherwise) then the space between stars is a minefield of debris significantly larger than a single hydrogen atom and much denser than a cloud of hydrogen atoms; couple that with the simplest observation that if you are warping or folding space then you are reducing the distance between every object within that space which in turn increases the density of space considerably, then the chances of hitting something are incredibly low to the point of being inevitable.

Thanks but you didn't say anything I didn't know or which contributed to the discussion (although it may certainly have been useful for others, not saying you shouldn't have posted at all).

You're welcome, I hope I can understand that you agree with me in that a path through 'hyperspace' between event A and event B, through a wormhole, 4-D or whatever you want to call it, would not hit any material object lying in the actual 3-D space between A and B.

Mmmmm, not so sure about it, a wormhole is by definition a tear in our spacetime, connecting two space-time events directly even if they are not in a relativistic causally connected timeline cone.

Why should there be any mass in that path? The fact that hypothetically we (ordinary matter) could travel through that wormhole does not (at all) necessarily mean that ordinary matter would be lying there in the path.

Hyperspace (that makes FTL travel possble in fiction) does not actually exist. In SF hyperspace travel you enter into another dimention (through a portal) and then travel through that dimension (which has different time-space geometries to our dimension ... one of which is hopefully shorter than ours) at normal (subluminal) speeds and exit via another portal back into our dimension. In doing that the distance travelled (or time spent) in one dimension (hyperspace) is different (and hopefully shorter or the whole endeavour is pointless). This is the method of FTL travel we see in TV SF such as Stargate and Babylon 5. It is not a wormhole, nor is it warped or folded space.

 

Wormholes are not conduits into (and out of) the so called hyperspace of superstring theory and other multi-dimensional theories, but are bridges over the time-space dimension we occupy (or tunnels through it), the space within the wormhole is of the same dimension you are travelling to and from and will contain matter... in travelling through a wormhole you do not leave our 3D dimension just as in travelling through the http://en.wikipedia.org/wiki/Channel_Tunnel" rel="nofollow - Channel Tunnel you do not leave Earth.

 

(it's a 4D topology projected into a 3D universe that is difficult to visualise because we are used to thinking in 3D space).

 

 

Gerinski wrote: Dean wrote: The collision of two galaxies will result in some physical damage to both systems - stars (and other celestial objects) will die in the process, the speed at which this occurs is immaterial, it will occur and the stars involved will be unable to avoid it.  It is not immaterial in the subject of the present discussion, but with the gravitational effects it is entirely likely that an artificially powered spaceship could cross through 'colliding' galaxies without its path being significantly affected, regardless if the dynamics of the stars in those two galaxies were changing because of the 'collision'.

What? The point you were making was that two glaxies could "collide" without any celestial body within them ever phsycially colliding, not that an artificially powered spaceship could cross through such an event at low speeds. Of course a spaceship could manoeuver around any object if its velocity was sufficiently slow enough and its scanners sufficiently sensitive, just as an ocean supertanker can navigate through busy shipping channels today (bearing in mind that a supertanker travelling at full speed requires three kilometers to stop) but when galaxies collide  stars cannot brake or take avoiding action.

 

Gerinski wrote: Dean wrote: Also, current thinking is that the space between galaxies is far from empty, it's just that we don't know what it is filled with (dark energy being a strong contender).  Agreed, but as far as we now know there's no reason to believe that it would impact the way known matter and spacetime interact (i.e. impact on space travel physics).

 

 

Gerinski wrote:   Dean wrote: Warp speed (keeping this in the Star Trek universe for a moment) is significantly faster than the speed of light (for example Warp-5 is approximately 214 times the speed of light) so the energy release in colliding with an atom of hydrogen at that velocity is measurable (energy = atomic weight/Avogadro constant * velocity squared), in fact at warp 5 it has 45,688 times more energy than at light speed. Sure a single atom of hydrogen is still not going to cause any noticeable damage to a star ship, but a cloud of many hydrogen atoms will and space is far from empty - remember that stars are formed from within dust clouds where hydrogen is the most abundant element and if we take the mass of our insignificant little star as an example, the mass of hydrogen involved is (erm) astronomical. Also if every star system has an equivalent of a Kuiper belt and/or an Oort cloud (and given how stars are formed there is no reason to assume otherwise) then the space between stars is a minefield of debris significantly larger than a single hydrogen atom and much denser than a cloud of hydrogen atoms; couple that with the simplest observation that if you are warping or folding space then you are reducing the distance between every object within that space which in turn increases the density of space considerably, then the chances of hitting something are incredibly low to the point of being inevitable.   I assume that space travelers would slow down before getting into an area where matter density starts to be a concern.

How do they see where those areas of concern would be in order to slow down? (this is not a trivial question).

-------------
What?

  a wormhole definition is precisely that which connects spacetime events which lie in different light cones.

Sorry but I don't agree, in the traditional scientific sense, wormholes 'make a short cut' through the normal 3-spatial dimensions, it's quite different from just a 'faster than light straight line travel'. You will find no matter through the wormhole (unless you think that because you travel through the wormhole, you may find others traveling through it at the same time).

 

 

Dean wrote: What? The point you were making was that two glaxies could "collide" without any celestial body within them ever phsycially colliding, not that an artificially powered spaceship could cross through such an event at low speeds. Of course a spaceship could manoeuver around any object if its velocity was sufficiently slow enough and its scanners sufficiently sensitive, just as an ocean supertanker can navigate through busy shipping channels today (bearing in mind that a supertanker travelling at full speed requires three kilometers to stop) but when galaxies collide  stars cannot brake or take avoiding action.

I maintain, when two galaxies 'collide', the chance of any actual body to body collisions is extremely small. 

Dean wrote: You said intergalactic space is empty. It is not. If it is dark matter or dark energy we do know that it would impact on spacetime (and thusly matter and space travel physics) because that's why we have predicted that it exists. It does not necessarily have to be entirely exotic dark matter or dark energy, it can be ordinary everyday common or garden lumps of matter such as a dead star, an ejected planet or asteroid, comets - the only celestial objects we can see are those that emit light and those local objects that reflect light, in the space between star systems and galaxies we cannot see those reflective (and non-reflective) objects.

It is clear that dark matter can't be 'invisible, non-light-radiating normal matter', that possibility was outruled long ago.

At any rate, for what we now know, there is nothing bigger than an helium atom which could likely get in the way of any intergalactic ship in normal circumstances (unless the extremely improbable fact of hitting a bigger solid body). Dark matter seems to exist, but on the scale of conceivable space travel it is probably neglectable. We have launched the Voyager probes to the edge of the Solar system and without any corrections for dark matter they are still behaving as we think they should. Dark matter has only influence on very large structures, most probably our first space travels would be within regions not affected at all by dark matter anyway?

 

Dean wrote: Warp speed (keeping this in the Star Trek universe for a moment) is significantly faster than the speed of light (for example Warp-5 is approximately 214 times the speed of light) so the energy release in colliding with an atom of hydrogen at that velocity is measurable (energy = atomic weight/Avogadro constant * velocity squared), in fact at warp 5 it has 45,688 times more energy than at light speed. Sure a single atom of hydrogen is still not going to cause any noticeable damage to a star ship, but a cloud of many hydrogen atoms will and space is far from empty - remember that stars are formed from within dust clouds where hydrogen is the most abundant element and if we take the mass of our insignificant little star as an example, the mass of hydrogen involved is (erm) astronomical. Also if every star system has an equivalent of a Kuiper belt and/or an Oort cloud (and given how stars are formed there is no reason to assume otherwise) then the space between stars is a minefield of debris significantly larger than a single hydrogen atom and much denser than a cloud of hydrogen atoms; couple that with the simplest observation that if you are warping or folding space then you are reducing the distance between every object within that space which in turn increases the density of space considerably, then the chances of hitting something are incredibly low to the point of being inevitable.

http://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision" rel="nofollow - http://en.wikipedia.org/wiki/Andromeda%E2%80%93Milky_Way_collision

"While the Andromeda Galaxy contains about 1 trillion (10¹²)  http://en.wikipedia.org/wiki/Star" rel="nofollow - stars  and the Milky Way contains about 300 billion (3×10¹¹), the chance of even two stars colliding is negligible because of the huge distances between the stars"

More generally

http://en.wikipedia.org/wiki/Interacting_galaxy" rel="nofollow - http://en.wikipedia.org/wiki/Interacting_galaxy

OK in that sense I agree, but you can't 'hit any matter in the way through the wormhole' since by definition the distance between the two mouths of the wormhole is zero. There is no space in between the two mouths so where there's no space there can't be any matter.

Thanks for such an elaborated reply.

Okay, we will continue to contradict each other.

 

There is a very low probability that any one specific star would collide with any of the 1 trillion stars of the approaching galaxy, however there is a very high probability¹ that any one of the 300 billion stars would collide with any one of the 1 trillion stars of the approaching galaxy. Why is this high? Because stellar collisions are not rare events on a cosmological timescale. In our Milky Way galaxy stellar collisions occur once every 10,000 years. Got that? In a "static" galaxy where stars are separated by light-years, a star will collide with another star at some time in a 10,000 year window. This is because the density of the galaxy is not constant and the velocities and trajectories of the stars within it are not uniform. Even Sol and Proxima Centauri, which are located in a low density part of the outer spiral arm (the Orion Arm) that is orbiting the galactic centre at 200 km/s, are not moving at exactly the same velocity on exactly the same trajectory. In the time it takes Andromeda to collide with the Milky way there would have been 400,000 stellar collisions within our galaxy alone - it is safe to assume a greater number will have occurred in the Andromeda galaxy. So introduce another 1 trillion stars into that same volume of space and the number and frequency of those collisions will increase - it cannot decrease, so therefore there is a high probability that one of those stellar collisions will be between stars from each galaxy, I expect that the probability is so close to 1 that it is an inevitability. Also, even though (as I said) stars are unable to take evasive action to avoid a collision, gravitational attraction means they are inclined to do the opposite - if they get close enough to collide, they will collide.

 

This is not a wild guess based upon assumed densities (little stars far apart) but a reasonable prediction based upon what is happening now.

 

[¹if you write your name on a piece of paper and put it into a hat with nine other names then the probability that in a single draw your name will be picked is 0.1, however, the probability that someone's name will be picked it exactly 1]

 

And this is just taking stellar collisions into account. On smaller scales celestial collisions of non-stellar objects is even more common.

 

 

Also. The current explanations assume that the galaxies will mesh perfectly (because stars are small and the distance between them is great), but there is nothing to suggest that would be the case.

 

I never said that celestial bodies will never eventually collide. Gravitational attraction is likely to cause a lighter body to eventually fall into a heavier body. But we have been orbiting the Sun for quite a while and we did not yet fall into it. That takes a little while.

I said that if two galaxies 'collide', head to head collisions between their constituent stars are extremely unlikely, they will just pass through each other and gravitational effects will alter their structure, but body-to-body collisions will still be extremely unlikely, and I guess that the articles I provided support this claim.

You are over thinking.

 

I never said dark matter or dark energy would affect intergalactic travel, I said the reagion between galaxies was not empty, however I did imply that since the effects of dark matter and dark energy are gravitational there would be something would be affected.That said, I could care less whether dark matter affects intergalactic travel or not because there is nothing to suggest that exotic matter is the only substance to be found between galaxies.

 

Dark matter is a hyperthetical substance used as an explanation for the total mass of the universe and it is predicted that it is found in the space between galaxies. This prediction does not exclude the possibility that the space between galaxies also contains other matter. What was ruled out was the quantity of normal matter between galaxies was insufficient to account for the total mass of the Universe, not that it didn't exist at all.

 

We cannot see what is in the space between galaxies, not because it is invisible, or because it isn't there, or because it is not reflective but because it does not emit radiation (eg light) and there is no localised radiation source to reflect radiation from (to detect reflected light the source has to be local - it is not that it is not reflective, it is that it is not reflecting). The Earth, or Venus or any of the other solar bodies do not reflect starlight sufficiently to be detected at stellar distances - if we had no Sun then Venus (for example) would reflect nothing and so would be invisible and essentially undetectable from space. As it is we can see Venus as a very bright point of light at dawn and dusk, but if it was floating in the region of space midway between here and Proxima Centauri we would never see it - now transport it to the region of space between The Milky Way and Andromeda... at that distance even a lump or rock as big as Venus would not be big enough to be detected - we simply would not know it was there. No one can say there is no normal matter in the space between galaxies.

Wikipedia is not a supportive article - I have given extensive explanation as to  why I believe those articles are not the definitive answer. All you have done is contradicted me.

I did not learn that by Wiki but by several books I have read, several of them before Wiki existed, I just resorted to Wiki as an easy way to point these subjects to anyone interested in them. I could quote several other books by respected scientists saying the same, so don't take Wiki as my source.

Astronomers and Cosmologists are not so naive as to neglect that there must be invisible bodies made of baryonic matter floating around that we can't see.

Dark matter considerations arise more from the Big Bang models, which can not explain how so much ordinary matter as it seems it would be needed to produce the gravitational effects we observe could have been produced by the Big Bang, as well as why does that 'dark matter' not interact with ordinary matter neither electomagnetically, nor via weak force or via strong force, apparently it only interacts gravitationally. That's what makes it 'dark matter', not the fact that it does not emit or reflect light.

So? I'm not disputing any of that. The premis was you could fly at superluminal speeds between any two points in space and not hit anything. You cited the emptiness of intergalactic space, which I disputed, you cited the low density of galaxies, which I disputed. the how where or whyfore of dark matter is irrelevant. I never claimed that dark matter was so called because it does not emit or refect light, I simple state that all matter in the space between galaxies neither emits nor reflects light.

Misunderstanding cleared.

If you travel at a significant fraction of the speed of light (but still below it), the colour of the things you see will change. Looking ahead everything you see will turn more blue-ish and looking back everything will turn more reddish (due to Doppler effect).

A related curious but more intriguing fact is that looking ahead everything will look as in fast-forward motion, while looking back everything will look in slow-motion, the effect being more severe the faster you travel.

The extreme hypothetizations regarding what we might see at light speed were just an added amusing bonus thought experiment, the point was the scientifically sound facts that travelling fast, things ahead will look more blue-ish and in fast-motion, and the opposite when looking back where you come from. Nothing more and nothing less.

And btw, photons do travel through 3-D space at a certain speed, eyes exist, photons reach eyes, photons reach moving eyes... I don't see what is unscientific about wondering how would eyes moving at c perceive photons, even if we know that physical eyes can not travel at c. Schrodinger's Cat was neither a physical cat.

As I said, we can make an educated guess as to what we might "see" if travelling at c or even faster looking back to where we were coming from, the question becomes why can't we make a similarly educated guess for what we would "see" in the direction of our motion.

 

Light is an electromagnetic wave (EM wave), it behaves like a wave and it behaves like a particle, but it is still an electromagnetic wave so when you Doppler shift it it behaves like a wave and does all the things a wave can do. Doppler shifting at approaching light-speeds causes the EM wave to shift out of the visible portion of the spectrum - whether you have eyes or not, you cannot see it once that happens. The energy that a photon has is proportional to the frequecy of the EM wave - gamma rays have lots of energy, radio waves have not so much. [edit: my train of thought stopped here.. what I intended to say was as you Doppler shift the lightwaves the energy of the photon is increased or decreased depending on the direction of the shift]

 

 

 

 

As you approach the speed of light the light reflected off objects behind you will be stretched from optical wavelengths down into non-visible wavelengths. You will see nothing, but you could still detect them with suitable detectors - travelling at 99.9999% the speed of light you would be able receive 'light' from an object you are moving away from on a domestic radio receiver. The Doppler formula is very simple:

 

 

 

is the velocity of waves in the medium;

is the velocity of the receiver relative to the medium; positive if the receiver is moving towards the source (and negative in the other direction);

If  =  then the formula will yield f=2fo for light coming from the object you are moving towards and f=0 for objects you are moving away from. So if you were travelling at precisely the speed of light then the light reflected off objects behind you will be travelling at the same speed and will never reach you, light coming from objects in front of you will be compressed and shift up into the ultraviolet spectrum. Looking back all you would see would be total darkness (and here I mean absolute darkness - a black so black it would scare the crap out of an Goth) because it is not just the optical region of the spectrum that has been shifted to zero, all EM frequencies (radio, radar, microwaves, infrared, visible light, ultarviolet, x-rays, gamma ray) have been shifted to zero. However, looking forward you will see something because in that direction all the infrared frequencies will have been shifted into the optical region of the spectrum, you will see the heat profiles of objects. [This is an effect that many text books fail to mention].

 

So hyperthetically, if it were possible to travel faster than light what would happen would be that the lower EM frequecies would continue to be doppler shifted up the spectrum - visible light would shift through the ultraviolet into x and gamma rays (which is not a good thing for the observer) and lower frequencies such as heat and radio waves would be shifted into the visible region. We would see the Universe as a radio telescope sees it.

 

What we would not see would be the future, no matter how fast we travel we can only see things that have happened. We can only see a photon of EM radiation after it has left the source - the faster we go the sooner we see it but we will never see it before it has happened:

 

 

Thanks, that was really constructive. I would just perhaps argue that, travelling at precisely the speed of light and looking back at where you come from, you would still see a still picture rather than just black nothingness. You would see black nothingness if you exceeded the speed of light by any fraction. But you are welcome to argue that, I'm not 100% sure.

Regarding what we might "see" ahead of us, I get your points and yet I'm not sure of what a practical interpretation might be (regardless of the fact that we know that we can never achieve the speed of light in practice). I understand that we can not receive a photon before it has been emitted, in the hypothetical case of travelling at c towards the light source we would receive the photon just at the same time it was emitted.

to happen at normal speed (without any delay)?

No, you would see nothing. The photons of EM radiation would be travelling at exactly the same speed as you are so they would never reach your eyes.

 

/edit: You can never see a still image - that is impossible. Once the photons hit your retina to form an image they are "spent" - our persistence of vision will maintain that image for 1/25th of a second and then it to will be gone. There will be no more photons arriving because they cannot travel faster than light.

 

Gerinski wrote: Regarding what we might "see" ahead of us, I get your points and yet I'm not sure of what a practical interpretation might be (regardless of the fact that we know that we can never achieve the speed of light in practice). I understand that we can not receive a photon before it has been emitted, in the hypothetical case of travelling at c towards the light source we would receive the photon just at the same time it was emitted.

There is no practical interpretation over what I have stated -

So no. You will see it after it has been emitted even if that "after" is septillionth of a second after, it will still be after, it can never be just at the same time.

 

 

Gerinski wrote: But if we agree that as we travel faster and faster we will see the events in front of us unfolding quicker and quicker, what is the limit when we approach light speed? what does it mean "watching the events happening in front you extremely fast but still not before they happened"? I assume it means seeing events simply as they truly happen, without any time delay due to the distance they are being observed from, so actually they would end up not looking fast-forward at all but at the same speed we perceive them now, just without any delay. But since we said that events would appear to happen quicker and quicker the faster we travel, I still have some doubts as to what might that mean in practice. Things would just appear to happen without any delay at all. Would that have any practically appearance effect? How would events shift from appearing to happen fast-forward (with increasingly smaller delay) to appearing to happen at normal speed (without any delay)?

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What?

 

 

at v=0.1c,  (gamma) would be 1/sqrt(1-0.01)=1.005 so the 90 year journey to Sirius (as seen by observers on Earth and Sirius) would take 90 years, but for the passengers on the spaceship only 89½ years would have past. If the spaceship is travelling at 0.9c the dilation factor is 2.3 so the [10 year] journey time for the passengers would be 39¼ [edit] 4.36 years [10/2.3]. Because the speed of light (and their relative velocity) remained constant during the journey but the time taken to cross the distance between Earth and Sirius appeared to be shorter for the passengers the distance was contracted by the same proportion as specified by the Lorentz factor: time dilation shrinks space, so at  0.9c the relative distance between Earth and Sirius for the passengers would not be 9 light-years but 3.923 light years [edit: still true 4.36 x 0.9 = 3.923].

 

Imagine the scenario where Sirius is due to explode in 91 years so a spaceship is sent from Earth to observe the event. Travelling at 0.9c for passengers on the ship the total journey time would have taken 39¼  years, but Sirius would still explode 1 year after they arrive because the elapsed time on Sirius would still have been 90 years. [edit: this is wrong, I made a silly mistake - at 0.9c the journey time is 10 years not 90, so the correct scenario would be the Sirus exploding in 11 years the ship would take 10 years, 4.36 years would have elapsed for the passengers but Sirius would still explode 1 year after they arrived]

 

 

Star Trek does not solve this problem, it simply ignores it, their five year mission to meddle with strange new civilisations takes five years onboard ship and back on Earth. In reality their time spent at warp speeds during those five years would mean that hundreds, maybe thousands, of years would have elapsed back on Earth.

 

Unfortunately, the Lorentz factor also increases relative mass by the same proportion so at  0.9c their relative mass would have increased 2.3 times their actual mass, it does not make them 2.3 times heavier (a 70kg man will still weigh 70kg), their mass increases relative to a stationary mass, so it makes them harder to move. [edit: so since energy=½mv² says that more energy is needed the faster you go proportional to the square of velocity, it also means that because relative m also increases the faster you go even more energy is required]

 

 

[edit: I did say time dilation does my head in ]

 

Indeed. I should have been more careful and say that the trip would last 90 years, Earth or Sirius time. This would actually add to the fast-forward motion appearance I mentioned, since in my example of travelling a 0.1 c, the ship passengers would see 99 years of visual progress unfold in front of them during 89 ½ years of their time, and in your example travelling at 0.9 c they would see 99 years of progress happening during 39 ¼ years of their ship time.

 

At 0.9c the journey time is no longer 90 years relative to a static observer it is only 10 years, so the compressed time would be 19 years and elapsed time for the traveller would be 4.36 years.

Indeed, it's just an optical effect, my original extrapolations of the effect to 'what might you see traveling at c' were wrong. And certainly you can not use this to check the lucky lottery numbers for tomorrow 

In Stargate technology (and for Gerard's benefit, we are only talking about the fictional device from the Stargate SG1 franchise) two stargates are required to create a stable wormhole (agan, this is a fictional wormhole and not related to any hyperthetical astronomical wormhole) between them. Since astronomical wormholes are gravitational (created by a black hole for example) they would destroy any matter that passed through them the writers of SG1 overcame this by sending the energy signature of the traveller through the wormhole. The event horizion of the wormhole (the shimmering vertical puddle) performs the matter-energy transfer - in one of the early episodes someone (I forget who) tentively sticks their head through the open stargate and it instantly appears poking out of the other gate before they finalyt step all the way through - implying that not only is the transfer instantaneous, the conversion from matter to energy and back to matter is also instantaneous so no actual conversion takes place. You could think of this as sort of a wave-particle duality with the object being simultaneously matter and energy at the event horizon (as far as I know there is nothing in physics to support this). A few episodes have shown what happens to any object that is caught in the forming event horizon before the stable connection is made - they are destroyed (converted to energy and not converted back to matter).

 

O'Neill also commented in one early episode that travelling through the gate is instantaneous but seems to take forever. This is a little like the time dilation at the event horizion of a black hole seen by someone crossing the horizon (it takes forever) and by someone observing it from a distance (it is instantaneous), except here both are percieved by the traveller simultaneously while the observer only sees the instantaneous event). The other feature of the stargate wormhole is that it is a one way street - you can only travel in one direction, to return you need to redial in the opposite direction - you can do this from either end but the direction of travel is preset.

 

There isn't a "how exactly does that work?" explanation for any of this because it only works in fiction - this is one of the suspend-belief moments I mentioned previously - to enjoy the programme I accept the premise and suspend any disbelief I have in the actual physics of it.

 

[For example I'm not sure if how the ancients mamaged to distribute the stargates around the galaxy and beyond is ever explained since to do so would imply they either had another FTL transport system or they did it as using starships of subluminal speed.]

 

This is not the same as Star Trek's transporter where the conversion from matter to energy is stored in a pattern buffer before being transmitted to the destination where it is reconverted back into matter - the transfer and the conversion takes a finite amount of time but does not require a physical buffer and converter at the destination. How it does it is never explained since all analogous systems (fax, email, television etc.) require a reciever at the destination to make the conversion. So in Star Trek they can beam you anywhere in range of the transmitter, whereas in Stargate travel can only occur between two fixed gates but the range is theoretically unlimited.

Finnforest wrote: I assume neither are remotely possible for many hundreds of years.  If they were, I would travel much more than I do.

Neither are remotely possible. Time is not an issue, physics is. As Pat has already said - quantum teleportation is not physical matter transportation (and in the general sense there is no information transfer either) and I have already stated that what passes through a theoretical astronomical wormhole would not survive. Even if we could deconstruct, transmit and reconstruct matter, how we would reanimate the reconstituted matter with all its memories and knowledge intact is yet another mystery. Of course some SF writers (for example Heinlein, if I recall correctly) don't bother with the physics and simply transfer the knowledge and memories (spirit or soul if you believe in such things) between distant locations. (this is the method used in most fantasy fiction such as John Carter of Mars or The Thomas Covenant Chronicles).

 

In this respect the fictional versions are nothing more than "something wonderful happens", or "magic" as it is often known.

 

Finnforest wrote: By the way, I really enjoyed your responses to the last query.  Much of it was over my head but I read every word.  Thanks for posting them.

You know me - love to show-off. Thanks for the opportunity.

An average http://en.wikipedia.org/wiki/Composition_of_the_human_body" rel="nofollow - human is composed of  3 x 10²⁷ atoms (3 followed by 27 zeros)

 

(This is more than the total number of stars in the Universe (for example) and SI prefixes don't even have a name for this magnitude)

 

To encode this into a form that can be transmitted as data we would need to record the spacial location of each atom (x, y & z location) at a specific moment in time (ie we'd have to encode every atom simultaneously) together with information of what element it was (oxygen hydrogen carbon etc.) and then have some means of rebuilding the body from those elements at the destination this increases the "bit-count" of the data to several times the number of atoms we are converting. To convert the body to energy we would need to further decosntruct the body down to subatomic level and encode each subatomic particle, further increasing the amount of data required.

 

Having obtained our astromically large amount of data we now need to transmit it. If we assume that we could encode 3 x 10²⁷ atoms as 3 x 10²⁸ bits of data (and that's only ten bits per atom, which is inadequate IMO) and could transmit that at 1TBd (one tera baud) it would take almost 1 billion years to transmit the entire pattern. So even if it were possible (which it isn't) it would be impractical.

In wormholes the stuff really disappears at some point in spacetime and just appears out of nothing somewhere else.

Biology is outside my sphere of interest, I seldom ever watch medical dramas (except House) and never watch medical documentaries for example, but as far as i know there is no way of resuscitating a human once "brain death" occurs and resuscitation were the brain has been starved of oxygen results in memory and faculty loss, so it is fair to assume that a reconstituted brain would be impossible to reanimate with any memory or personality intact.

Regarding the information encoding problem, I guess that compression techniques could be developed, I mean you would probably not need to encode the precise position and type of every atom in the body. Our DNA is quite good at telling the body how to be and to behave with a limited amount of information.

Since I assume that memories and personalities arise from material atoms and processes allowed by the laws of physics, I don't see why it should be totally impossible. A completely identical reproduction of you right now should in principle think the same you do and have the same knowledge and memories as you do right now. The only thing possibly preventing it is if quantum processes are involved and relevant in the mind processes, quantum events are in principle impossible to know with absolute certainty so this might pose a fundamental difficulty to achieve the absolutely precise duplication of your mind state.