Max Sublight?

[oniel193]

The title is sartastic
In the atmosphere there wouldbe a maximum speed due to gravity and more.

But in space there arent those limitations the only limit is the acceleration.(Except for light speed although i dont know why)

[Quadhelix]

But in space there arent those limitations the only limit is the acceleration.

I would imagine that ships in Stargate have some sort of magnetic scoop to collect interstellar/intergalactic hydrogen to use as remass (i.e., "reaction mass" or "propellant"). If this is indeed the case, the scoop could create drag, preventing the ship from exceeding a certain speed relative to the material through which it is traveling.

(Except for light speed although i dont know why)

The light speed limitation is "explained" by Special Relativity, which has two postulates:

1. All frames of reference have an equally valid claim of being the "correct" frame of reference.
2. All observers will measure a beam of light to have the same speed relative to themselves. This speed is c, or 180,000 miles per second (300 million meters per second).

Let's just deal with the second postulate for now. Let us say that we have a ship traveling faster than light, relative the Earth, and that this ship is flying from Earth to a planet near Alpha Centauri; let us call this planet Chiron. On Earth, there is a laser firing pulses of laser-light along the ship's path, and on Chiron, there is another laser firing pulses back at the Earth. Let us say that the pulses from Earth and the pulses from Chiron are polarized differently, so that we can tell the difference between them.

Now, observers on Earth and on Chiron will agree that we have two different sets of pulses headed in opposite directions.

However, the ship is a slightly different issue. The ship is overtaking the series of pulses sent from Earth. This means that they see the pulses traveling toward Earth; same as the pulses from Chiron. Therefore, the crew of the ship will see two sets of pulses headed in the same direction. More importantly, they will see the pulses traveling in the same direction at the same speed. "In front of" and "behind" become the same thing.

This, however, is a contradiction: the ship's frame of reference is distinctly different from the Earth's frame of reference, in violation of the first postulate. Therefore, it is impossible to have a ship travel faster than light.



And, by the way, if you think that the postulates are wrong, then you should know that there are experiments that we can do where, if Special Relativity were wrong, it would be immediately obvious.

The best example of such an experiment is the Large Electron-Positron Collider, which accelerated electrons (and positrons) up to roughly 99.9995% of the speed of light. If the speed of light were not the absolute speed limit, then the electrons would have been traveling at over 400 times the speed of light.

[DigiFluid]

However, the ship is a slightly different issue. The ship is overtaking the series of pulses sent from Earth. This means that they see the pulses traveling toward Earth; same as the pulses from Chiron. Therefore, the crew of the ship will see two sets of pulses headed in the same direction. More importantly, they will see the pulses traveling in the same direction at the same speed. "In front of" and "behind" become the same thing.

I don't often venture into the Sci/Tech board so maybe I've brought this on myself, but that completely lost me.

[locutes]

sup light speeds are 0-<lightspeed

[thekillman]

max sublight is infinitely close to but not equal to C, or light speed

[Quadhelix]

I don't often venture into the Sci/Tech board so maybe I've brought this on myself, but that completely lost me.

Well, it is a bizarre scenario, so I'm not surprised that it would be confusing. That is almost, in a way, the point: a ship traveling faster than light is nonsense.

The point is that, if the ship traveling from Earth to "Chiron" faster than light, then the ship will overtake the laser pulses emitted from Earth. This means that the pulses will travel from the "front" of the ship to the "back" of the ship. Moreover, because ship is "flying into" the laser pulses from Chiron, they too will travel from the "front" of the ship to the "back" of the ship.

However, were things really start to get weird is when we consider the second postulate of Special Relativity: light always travels at the exact same speed relative to every observer. This means that the crew of this hypothetical ship see the laser light from Earth and the laser light from Chiron going from the "front" of the ship to the "back" of the ship at the same speed. In plainer terms, you could wind up with two pulses of light, one from Earth and one from Chiron, traveling along side one another.

However, an observer on Earth or Chiron would say, "But wait a second, those pulses should be traveling in opposite directions, not the same direction." And that observer would be right. Hence, it is a rather absurd prospect to have a ship travel through "normal space" faster than light.

sup light speeds are 0-<lightspeed

He knows that: he is asking why, in Stargate, ships have a maximum sublight speed other than the speed of light (e.g., in "The Return," they said that there was no way that they could get the Daedalus near the speed of light).

Again, my best guess is that ships are equipped with magnetic collection scoops, which create a sort of drag effect.

max sublight is infinitely close to but not equal to C, or light speed

Not only does this not even address the question, but it also contradicts evidence from the show.

[Splitsecond]

The reason for the discrepancy is due to writer ignorance. Each ship has an on-board generator that powers the engines. If they used a scoop how would they start from rest? Also how would they keep up sublight speeds in relatively near empty regions of space?

[Crazy Tom]

The reason for the discrepancy is due to writer ignorance. Each ship has an on-board generator that powers the engines. If they used a scoop how would they start from rest? Also how would they keep up sublight speeds in relatively near empty regions of space?

The reason for the high speeds n relatively small on-board reserves of fuel is the following: inertial dampeners.

They effectively make the ship lighter, drastically reducing the amount of propellant required to accelerate it to a certain speed, and that doesn't take into account inertia-less drives which need no propellant.

Now, in real space, i imagine max sublight would be the speed at which the ship's shielding can no longer protect it from the impact of the small amounts of matter that permeate space.

[locutes]

i imagine max sublight would be the speed at which the ship's shielding can no longer protect it from the impact of the small amounts of matter that permeate space.

that is the max effective sublight speed, after that it will pose problems to the ship. Though if spoken literally the max sublight speed is the highest speed you can obtain before you get to lightspeed. which is <lightspeed

[K^2]

The reason for the discrepancy is due to writer ignorance. Each ship has an on-board generator that powers the engines. If they used a scoop how would they start from rest? Also how would they keep up sublight speeds in relatively near empty regions of space?

You still need reaction matter. You aren't going to get far on photon drives.

Space is never empty. A good Buzzard Scoop will manage to pick up a good amount of matter even in interstellar space.

As for starting up, I would imagine there would be some reserve of reaction matter that can be replenished every time the ship is in motion. Also, keep in mind that the motion needs to be relative to either interstellar medium or solar wind. Near Earth orbit, for example, the solar wind is approximately 500km/s. So you need only to go beyond Earth's magnetosphere, and you have a good quantity of reaction matter to trap with your scoops and use with ion drives or similar systems.

[AVFan]

Well, it is a bizarre scenario, so I'm not surprised that it would be confusing. That is almost, in a way, the point: a ship traveling faster than light is nonsense.

The point is that, if the ship traveling from Earth to "Chiron" faster than light, then the ship will overtake the laser pulses emitted from Earth. This means that the pulses will travel from the "front" of the ship to the "back" of the ship. Moreover, because ship is "flying into" the laser pulses from Chiron, they too will travel from the "front" of the ship to the "back" of the ship.

However, were things really start to get weird is when we consider the second postulate of Special Relativity: light always travels at the exact same speed relative to every observer. This means that the crew of this hypothetical ship see the laser light from Earth and the laser light from Chiron going from the "front" of the ship to the "back" of the ship at the same speed. In plainer terms, you could wind up with two pulses of light, one from Earth and one from Chiron, traveling along side one another.

However, an observer on Earth or Chiron would say, "But wait a second, those pulses should be traveling in opposite directions, not the same direction." And that observer would be right. Hence, it is a rather absurd prospect to have a ship travel through "normal space" faster than light.

I'm still having problems with this...

Using a real life example, say a car(#1) was driving on a highway from L.A. to New York at 70 mph, and another car(#2) was traveling the exact opposite way (NY to LA) at 70 mph. A third car(obviously #3) is driving along Car 1's route, though at 300 mph (don't ask me how, it just hypothetical) in the same direction, but faster. Car 3 then passes #1 and #2.

If someone were to tape that and play it back, it would appear as though #1 and #2 were moving towards LA- though any observer anywhere in a fixed location see it as it actually was: #1 moving East and #2 moving West.

But I don't see that as absurd. Why would the same experiment involving light be any different?

[Quadhelix]

But I don't see that as absurd. Why would the same experiment involving light be any different?

Because of the second postulate of Special Relativity: all observers will measure all beams of light to have the same speed relative to themselves.

In your example, Car 3 would measure Car 2 as traveling West at 370mph and Car 1 as traveling in the same direction at 230mph. All observers agree that the two cars do not have the same velocity.

However, because of the second postulate of Special Relativity, the crew of our hypothetical starship have to measure the light from Earth and the light from Chiron as traveling in the same direction at the same speed. That is were the real paradox occurs.

[K^2]

Since we are talking relativity, it won't be exactly 230 and 370 mph, either.

[Splitsecond]

Here is a nice website with a load of special relativity thought experiments which are easy to understand and explain most phenomena

[51 Pegasus]

The key thing to understand is that you can't understand relativity through analogies with lower speed (ie familiar) objects; the entire paradigm of relativity (general relativity in particular) is that our intuitive 'everyday' understanding of the world does not hold up in regimes of high mass and energy. Similarly, quantum mechanics leads to apparently counter-intuitive and seemingly odd behaviour because on *very* small scales, reality does not match our perception of it.

I personally always regarded space flight in most sci fi as being classified into two distinct types;
1. faster than light, theoretically impossible in reality (as far as we currently know) by SR, achieved through some hand-wavey roundabout manipulation of GR (eg hyperdrive in Stargate, warp drive in Star Trek etc).
2. sublight, theoretically possible (technically all speeds of our space craft are less than lightspeed, but its only at significan fractions of the speed of light that relativistic effects become really noticable).

Another way to think of why we can't *accelerate* past lightspeed is that SR tells us (this is a consequence of the famous E = mc^2) that the faster you go the heavier you get. As more massive objects require more energy to accelerate (due to good old Newtonian F = ma), the faster you go the more energy you would need to go even faster and so the *rate* of acceleration will decrease. This occurs to the point where to accelerate to lightspeed would require an infinite amount of energy as the mass became infinite! Hence, if a ship was using their sublight engines (and reasonably assuming they don't have an infinite supply of fuel!) then their maximum achievable speed will be restricted depending on the amount of fuel.

Hope that helps, and hasn't confused you further!