Faster Than Light Travel

[Dave1001]

AT this point, let's just try to get travel UP to light speed. Then I'm sure someone will learn how to break the barrier, just like they did with the speed of sound

There's no point. Travelling at light speed requires infininte energy. Travelling at near light speed requires unfeasibly vast amounts of energy.

I disagree that there's no point. I agree with Godmode's general idea that our focus as a species should be on increasing traveling speed records, rather than celebrating what we currently perceive to be inherent limitations.

By the way, I recall reading about speculation that some particles (neutrinos?) may travel faster than the speed of light. Have these speculations been discredited?

 

[Dave1001]

Wrong. C is a constant, so the speed of the ship firing the laser doesn't enter into it. The laser beam would be redshifted by the recession of the light source, but it would pass the other ship.

Can a black hole slow down light rather than trap it completely. How about a black hole growing in mass at a particular rate?

 

[Dave1001]

Hmm... I figured as much.

So, two ships moving away from each other, each travelling at at .9c, would be travelling at a velocity <c relative to each other, meaning that a laser beam fired from one toward the other would overtake the fired-upon ship as if the firing ship were at rest (relative to the fired-upon). This despite the fact that the distance between the ships would be increasing at a rate greater than 186,000 miles/sec (miles and seconds being my units of choice)-- I think I get it ("think" being the operative word-- not my strong suit). Still very weird--

If I've managed to grossly misunderstand you in your attempts to enlighten, please feel free to lay the smackdown on me. Your feedback is appreciated.

I think we may still have a way for a ship to outrace a laser yet.


How about if the ship is going .51c in a straight path, but along the path we've set up floating mirrors parallel to each other along the route, and bounce the laser such that it's bouncing along the same route as the ship but covering twice the distance. The laser will never reach the ship. Falsify that!

 

[DeviousB]

Actually, stuff travels faster than light all the time. I've seen a few things do it with my own eyes (sorta).


<=== Clue.

 

[CriticalThanking]

Actually, stuff travels faster than light all the time. I've seen a few things do it with my own eyes (sorta).

Yep - office rumors. Happens all the time.

CT

 

[DeviousB]

Yep - office rumors. Happens all the time.

You can see rumours?! Is this like synethsesia or something?

 

[Ziggurat]

So, two ships moving away from each other, each travelling at at .9c, would be travelling at a velocity <c relative to each other,

Yes

meaning that a laser beam fired from one toward the other would overtake the fired-upon ship as if the firing ship were at rest (relative to the fired-upon).

Let's talk about a laser pulse (since it's the leading edge that we're really interested in. The relative speed of the ship firing the pulse will affect the frequency of the pulse (doppler shift) as measured by the target ship, but it will not affect the propagation speed of the pulse in any reference frame.

This despite the fact that the distance between the ships would be increasing at a rate greater than 186,000 miles/sec (miles and seconds being my units of choice)--

Here's where you need to be careful. The distance between the ships depends on your frame of reference. It is growing faster than 186,000 miles/sec in SOME reference frames where both ships are moving, but it is NOT growing faster than that in a reference frame where either ship is stationary.

And it is only in a frame where the firing ship is not moving that it makes sense to use this distance as a measure of how far the light needs to travel to get to the other ship - in frames where both ships are moving, the distance the firing ship moved after firing the pulse doesn't matter for that calculation.

 

[Hellbound]

I think we may still have a way for a ship to outrace a laser yet.


How about if the ship is going .51c in a straight path, but along the path we've set up floating mirrors parallel to each other along the route, and bounce the laser such that it's bouncing along the same route as the ship but covering twice the distance. The laser will never reach the ship. Falsify that!

Actually, this is reminiscent of the "light clock" idea that Einstein used to develop relativity theory and it's effects on time and distance.

Yes, the laser would catch up. Even if your ship was moving .99999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999999c, the laser would catch up (eventually to an external observer "stationary" with respect to the velocities listed...almost immediately from the POV of those in the fast-moving ship).

 

[CriticalThanking]

[sound of rust flaking off some very old brain cells]
I seem to recall a science piece on tv (Nova?) about the FTL question. One scientist was quoted as saying that perhaps some energy could go FTL, but no information could be sent FTL. The show cut to another group of scientists who claimed to be sending and receiving (radio?) FTL. Their quote was something like "If a Mozart symphony is not information, I don't know what is." Does anyone else remember this, or is this a private hallucination?

CT

 

[CriticalThanking]

You can see rumours?! Is this like synethsesia or something?

Like many powerful forces, you cannot observe them directly, but extrapolate based upon the toxic radioactive decay components left in their wake.

CT

 

[Captain Trips]

Here's my take on it:

There are three possible velocity ranges: "below c," "at c," and "above c." If an object has mass it cannot go "at c." If an object has no mass (such as a photon) then it can ONLY go "at c."

Now, given that, we know it is impossible for any object with mass to attain light speed. Why does that preclude going ftl? IF there were some way to instantaneously jump in velocity from .9999...9c to 1.000...1c, then the light-speed barrier would be "broken" without having to actually go "at c."

Of course, this would effectively involve a jump from a universe with the laws we know to one with different laws, as going "above c" involves what we would think of as "negative mass." In such a universe, it would be possible to go infinitely fast by using no energy, and putting energy into your system would cause a slowing down approaching c. All we now need is a way to jump from our universe into that one, and we would be circumventing the lightspeed barrier. Then, once we arrive at our destination, we would jump back into our universe. This is the basis of many versions of science-fiction ftl travel, such as Babylon 5's "hyperspace." (Larry Niven has postulated versions of hyperspace where only one velocity is allowable, but that velocity is significantly greater than c in our universe.)

Now, all we need is a theory postulating such alternate universes, and then another theory as to how to jump between those and ours. Then we could postulate "above c" velocities.

But with the laws of physics as we know them today, matter in our universe is limited to "below c" velocities, as all matter is made of objects with mass. "Negative mass" is purely speculation. And "at c" is unobtainable for non-massless objects.

 

[DeviousB]

Here's my take on it: [...]

Whether this is possible would depend entirely on how you combine quantum physics and relativity.

Show us your workings and we'll tell you if it's possible!

 

[Captain Trips]

Whether this is possible would depend entirely on how you combine quantum physics and relativity.

Show us your workings and we'll tell you if it's possible!

I'm sorry if I misled you -- I never meant to imply it was even possible. Just that this is one way of circumventing the "light speed barrier." Personally, I don't think it is possible -- we are made of normal matter, and even if we were able to jump to an alternate universe (should it exist) then our "normal matter" wouldn't even be able to exist under that universe's laws of physics. We would stand as much chance of survival as we would on a journey into a black hole.

So, let's go back to the original posting -- ftl travel IS fantasy. Therefore, the original poster should reject any science fiction that includes it just as he rejects any other form of fantasy. If you want sf based purely on actual theories of physics, stick to early Clarke or Asimov, or a very few modern authors. (Some of David Brin's writings come to mind, but only because he's an old friend of mine. And even he will push the limit in favor of a good story!) But if you are willing to apply the "willing suspension of disbelief" then there is a world full of great literature open to you.

 

[Yllanes]

I had written a better answer before, but I lost the connection and the post...

By the way, I recall reading about speculation that some particles (neutrinos?) may travel faster than the speed of light. Have these speculations been discredited?

Neutrinos travel at almost c and have very little mass. Until not long ago we thought they were massless, and thus travelled at exactly c.

Here's my take on it:

There are three possible velocity ranges: "below c," "at c," and "above c." If an object has mass it cannot go "at c." If an object has no mass (such as a photon) then it can ONLY go "at c."

Now, given that, we know it is impossible for any object with mass to attain light speed. Why does that preclude going ftl? IF there were some way to instantaneously jump in velocity from .9999...9c to 1.000...1c, then the light-speed barrier would be "broken" without having to actually go "at c."

Negative mass is not enough. You need inaginary mass to travel at superluminal speeds.

Within quantum field theory, particles travelling faster than c would be perfectly consistent. Mathematically, QFT allows for particles with m² > 0, which travel at v < c; particles with m² = 0, wich travel at v = c; and particles with m² < 0, which travel at v > c. But it is impossible to cross the c barrier. There is no way a massive particle can be accelerated and attain a speed greater than c. So it is possible to have a particle that is always travelling faster than c and would have m² < 0, but it is impossible for ordinary matter to go FTL. The existence of these so called tachyons would not upset our current understanding of physics. They wouldn't violate causality, for example. The important thing is that, while they are mathematically sound, none have ever been found, so we consider their existence very unlikely and drop them from our theories.

For the rare reader who knows about Group Theory but not about QFT, particles are the inequivalent irreducible unitary representations (irreps) of the Poincaré group (which includes the most general kind of Lorentz transformation + traslations). Wigner classified these representations. It was found that, among all the mathematically possible representations, only some correspond to physical objects. Some representations have m² < 0, others have negative energy, other some kind of continuous spin. So we add an axiom that says that all the eigenvalues of P² are in the future light cone. This gets rid of all the non-physical representations. But there's room in the theory for tachyons, should we ever find they exist.

 

[DeviousB]

I thought tachyons couldn't interact with ordinary matter (hence could be ignored, didn't violate causality, etc.), therefore not only have we never found any, we're never going to.

Or, from the tachyon's perspective, "Not only are we never going to find this theoretical STL matter, we never have!"

 

[Yllanes]

I thought tachyons couldn't interact with ordinary matter (hence could be ignored, didn't violate causality, etc.), therefore not only have we never found any, we're never going to.

There is no hard law that says tachyons cannot interact with ordinary matter. In principle it could be possible to detect them as shadows, Cerenkov radiation, etc. In the same way photons, with m² = 0 and v = c can interact with massive particles there is no strict fundamental reason to forbid the hypothetical tachyons from doing the same thing. Free tachyons have been studied in some detail. The reason, from a QFT point of view, is simple: many of the representations in Wigner's classification correspond to physical particles, so it makes sense to study the rest to see if we find some new interesting phenomenon.

Of course, if they did exist and did interact with ordinary matter, we would have already detected them. If you assume they can interact with matter, they still don't necessarily violate causality. I forgot to link to the Physics FAQ. The last time I looked the Wikipedia article on them had some information on the causality issue; but it also had some strange sentences, a mini POV dispute about G. Sudarshan (a theoretical physicist) and a nutjob in the talk page saying human precognition was proof that tachyons exist, so the article should be handled with some care.

 

[ponderingturtle]

Hmm... I figured as much.

So, two ships moving away from each other, each travelling at at .9c, would be travelling at a velocity <c relative to each other, meaning that a laser beam fired from one toward the other would overtake the fired-upon ship as if the firing ship were at rest (relative to the fired-upon). This despite the fact that the distance between the ships would be increasing at a rate greater than 186,000 miles/sec (miles and seconds being my units of choice)-- I think I get it ("think" being the operative word-- not my strong suit). Still very weird--

If I've managed to grossly misunderstand you in your attempts to enlighten, please feel free to lay the smackdown on me. Your feedback is appreciated.

Sort of, you have to remember that you are dealing with three different reference frames in this question. A(the outside observer), B and C(the two starships).

A sees ship B that is going .9c away from him emit a light moving at C, and aproches the ship moving away from him in the other dirrection at .9c at C, so it is gainin by .1C on ship C at all times from A's point of view.

The thing is that everyone measures the speed of the light as C, it does not matter how they are moving, that is where you are getting caught up, by thinking that the speed of the light changes in different reference frames.

From B's point of view A is going a .9 c and C is goin at .994c.

Rember Velocities do not add in the normal ways

 

[ponderingturtle]

I disagree that there's no point. I agree with Godmode's general idea that our focus as a species should be on increasing traveling speed records, rather than celebrating what we currently perceive to be inherent limitations.

By the way, I recall reading about speculation that some particles (neutrinos?) may travel faster than the speed of light. Have these speculations been discredited?

Yes, neutrinos have mass experaince time and move at less than C.

 

[ponderingturtle]

Here's my take on it:

There are three possible velocity ranges: "below c," "at c," and "above c." If an object has mass it cannot go "at c." If an object has no mass (such as a photon) then it can ONLY go "at c."

Or you can say that everything is going at c thought 4d space time, if you factor in your rate of travel through time, everything goes at c all the time.


This is not a common definition of speed, but the math works(I really need to find the proof though)

 

[ponderingturtle]

There is no hard law that says tachyons cannot interact with ordinary matter. In principle it could be possible to detect them as shadows, Cerenkov radiation, etc. In the same way photons, with m² = 0 and v = c can interact with massive particles there is no strict fundamental reason to forbid the hypothetical tachyons from doing the same thing. Free tachyons have been studied in some detail. The reason, from a QFT point of view, is simple: many of the representations in Wigner's classification correspond to physical particles, so it makes sense to study the rest to see if we find some new interesting phenomenon.

Of course, if they did exist and did interact with ordinary matter, we would have already detected them. If you assume they can interact with matter, they still don't necessarily violate causality. I forgot to link to the Physics FAQ. The last time I looked the Wikipedia article on them had some information on the causality issue; but it also had some strange sentences, a mini POV dispute about G. Sudarshan (a theoretical physicist) and a nutjob in the talk page saying human precognition was proof that tachyons exist, so the article should be handled with some care.

So do they go back in time, or is there a privileged refernce frame? Or what about Inertial reference frame translations of Special Reletivity is wrong?