Traveling at the speed of light and time dilation

[phinds]

A thing will be always greater than a thing(not infinity).
It is possible.

That is very awkwardly stated, but I think you mean that if X is not infinite there is always something bigger than X, yes? If so, then yes, that's pretty much self-evident. Am I missing your point?

 

[micromass]

That is very awkwardly stated, but I think you mean that if X is not infinite there is always something bigger than X, yes? If so, then yes, that's pretty much self-evident. Am I missing your point?

I think he suggests that if $X$ is a finite speed, then there must be a speed bigger than $X$. So he suggests that since $c$ is finite, there must be some object that goes faster than $c$. This is wrong, of course.

 

[phinds]

I think he suggests that if $X$ is a finite speed, then there must be a speed bigger than $X$. So he suggests that since $c$ is finite, there must be some object that goes faster than $c$. This is wrong, of course.

Ah ... good point. It did not occur to me that he might be that far out of touch with physics.

 

[DHF]

Could you give examples of what you mean by this? No matter where you put the engines the mass, and therefore the force needed, will be exactly the same
.

I was referring to Dr. Charles Pellegrino's Valkyrie design. He claims that putting the engine in front of the ship will allow the ship to be less massive. He claims that it would be easier for the engines to pull a mass rather then push it.

 

[Bandersnatch]

@ Ryan:
The mass reduction is due to structural differences in ship design(i.e., shielding, beams), rather than some new physics.
http://en.wikipedia.org/wiki/Project_Valkyrie

 

[Ryan_m_b]

I was referring to Dr. Charles Pellegrino's Valkyrie design. He claims that putting the engine in front of the ship will allow the ship to be less massive. He claims that it would be easier for the engines to pull a mass rather then push it.

From reading the wiki it seems clear that the proposed advantage is that by dragging the crew compartment on a long tether large amounts of radiation shielding can be left out (which would be needed in any antimatter rocket). This doesn't mean that engine placement matters in terms of how good the engine is, it's just an approach to minimising what other things you have to take with you.

 

[DHF]

From reading the wiki it seems clear that the proposed advantage is that by dragging the crew compartment on a long tether large amounts of radiation shielding can be left out (which would be needed in any antimatter rocket). This doesn't mean that engine placement matters in terms of how good the engine is, it's just an approach to minimizing what other things you have to take with you.

ok fair enough. That being the case I don't think it will make a difference for me. Since I am using a non living crew I shouldn't have to worry about radiation as much. And to keep the mass of the ship down I will be waving my hands a little and will be using pollyunobtanium. :D

 

[Ryan_m_b]

ok fair enough. That being the case I don't think it will make a difference for me. Since I am using a non living crew I shouldn't have to worry about radiation as much. And to keep the mass of the ship down I will be waving my hands a little and will be using pollyunobtanium. :D

I wouldn't be so sure about not worrying about radiation. Radiation induced bit rot would probably damage your machines as well.

 

[DHF]

Fair enough so they would be radiation tolerant but not radiation proof. If I am waving my hands for a as of yet un-invented super light material for the ship hull then I can just take the Valkyrie design in reverse, instead of having a long tether trailing behind the engines, I could protect the crew from radiation by distancing them rather then packing many tones of shielding. if for example the ship were 2 kilometers long and the crew compartment was near the front, that would keep them away from the engines and radiation with minimal shielding. with the story taking place over two hundred years from now I don't think a lot of hand waving is needed to assumed the ship is constructed of some advanced decedent of Graphine or a type of the like. Thus letting the ship be incredibly long yet light. I didn't picture a thick battle ship design either, I was imagining the ship would have a honeycomb latticework structure with only the crew an cargo compartments being solid.

 

[Infinite/Zero]

Am I missing your point?

No, you are right

 

[Ryan_m_b]

No, you are right

Velocity doesn't work like that, sure there might be a number higher but it is not possible for objects with mass to reach the speed of light, nor for anything to exceed it.

 

[DHF]

Ok I did some calculations on how much fuel I would need but I have never done these calculations before so I would be appreciative if anyone could tell me if I am on the right track or if I am totally missing something.

Dry Mass =1000 Metric Tons
Engine output = 644.93 TJ/kg
Desired speed = 11.91% c
Fuel needed = 1000 Metric tons

This is assuming the ship will accelerate at .10G until it reaches 11.9c then it will coast for several decades then decelerate at .10G

The Engine output is assuming a proton to proton Fusion reactor. It will require a small Anti-matter catalyst but I am not adding those figures in at this time. I just wanted to make sure I was on the right track for my mass ratio calculations.

 

[Ryan_m_b]

What are you using to work that out? When you say "644.93 TJ/kg" are you referring to the kinetic energy you'll be imparting per kg? If so I make that an exhaust velocity of ~0.12C. Using the ideal rocket equation with:

1e6kg payload
3.6e7mps exhaust velocity
3.6e7mps desired velocity

I get 1.77e6kg of fuel. To slow down again I get 6.7e6kg. In other words your craft would have to carry 870 grams of fuel for every 1kg of payload. For an initial acceleration of .1g you would have to expend 25 grams of fuel per second.

All seems reasonable until you consider that your craft is producing as much energy per second as the entire Earth.

 

[DHF]

Yes the 644.93 TJ/kg refers to the kinetic energy imparted per kg. and yes it is a mind numbing amount of energy, clearly ridiculous by today's standards. Even still, I am trying to keep the specs of the ship fairly modest and we still are looking at insane amounts of energy.

To get the figures I posed I used The figures posted for fusion reactions on this site: http://www.projectrho.com/public_html/rocket/fusionfuel.php and then I used the Relativistic Star Ship Calculator to calculate the mount of energy I would need to move the ship to the desired speed.

I added a shield in front of the ship to take care of dust and micrometeorite impacts. however in order to decelerate the ship needs to flip around and fire its engines for over a year before it enters the Alpha Centuri system. There is no shield on the back of the ship so how does the ship protect itself from micro impacts for the year during it's deceleration? will the thrust from the engines clear the way? will debris still get through?

Another aspect of the ships's designs are the radiators. instead of simple heat sinks that dumped waste heat into space I wanted to turn them into generators that converted the heat into electrical energy to power the ships systems this way the fusion reactors could devote all their energy to propulsion. Is it conceivable to convert all the heat into electrical energy or would the generators still need to dump waste heat once the electrical energy was produced?

 

[DHF]

In my last post I asked if the thrust from the engines would clear the way, I meant exhaust not thrust. duh.

 

[LitleBang]

There is no speed limit. As far as the crew of a ship with an unlimited supply of fuel is concerned they can accelerate up to c and beyond. A trip from Earth to Andromeda for instance. They can accelerate up to c, as viewed by a stationary observer, for one month and decelerate for one month for a total travel time of two months. Remember that their clocks stop at c. Obviously they can never reach c as viewed by a stationary observer but as far as the crew is concerned they did reach c and beyond.

 

[phinds]

There is no speed limit. As far as the crew of a ship with an unlimited supply of fuel is concerned they can accelerate up to c and beyond. A trip from Earth to Andromeda for instance. They can accelerate up to c, as viewed by a stationary observer, for one month and decelerate for one month for a total travel time of two months. Remember that their clocks stop at c. Obviously they can never reach c as viewed by a stationary observer but as far as the crew is concerned they did reach c and beyond.

Uh, relative to WHAT is it that you think they reached c? There is no absolute motion so it has to be relative to SOMETHING and since you even realize yourself that they won't reach c according to a "starionary observer" (meaningless concept though that is), then what DO they reach c relative to?

Also, why do you think their clocks stop? That certainly isn't true. Their clocks just keep right on ticking at one second per second.

 

[DHF]

Phinds has already mentioned why that isn't accurate but its a moot point really because it doesn't have any real world applications, even in a fictional story. You would never have an unlimited amount of fuel and fuel alone is not the sole determining factor, you would need engines with enough power to accelerate you to that speed. In this thread we touched on the absolute ludicrous amounts of power you would need to reach even 12% of c and at a crawl of an acceleration.

This thread has given me a real appreciation for just how challenging interstellar travel can be when you play within the laws of physics. Its why nearly all Sci Fi tales with space travel jettison said laws. Otherwise the Enterprise wouldn't even have made it to Alpha Centuri by the time the series was over and they would have spent several Jupiters in fuel mass to get that far.

 

[Drakkith]

There is no speed limit. As far as the crew of a ship with an unlimited supply of fuel is concerned they can accelerate up to c and beyond. A trip from Earth to Andromeda for instance. They can accelerate up to c, as viewed by a stationary observer, for one month and decelerate for one month for a total travel time of two months. Remember that their clocks stop at c. Obviously they can never reach c as viewed by a stationary observer but as far as the crew is concerned they did reach c and beyond.

Not true. You have forgotten the effects of length contraction. To the observers on board the spaceship, the distance between them and the Andromeda Galaxy contracts so that they can reach their destination without ever reaching c or beyond.

 

[Paintjunkie]

gamma...?

 

[Drakkith]

gamma...?

Can you elaborate on your question? And possibly quote the section of whatever post it's related to?

 

[Paintjunkie]

sorry I don't have a question its more a GAMMA! but maybe my question is... mathematically isn't that formula that gives us gamma the reason why we can go faster than the speed of light?

 

[Drakkith]

sorry I don't have a question its more a GAMMA! but maybe my question is... mathematically isn't that formula that gives us gamma the reason why we can go faster than the speed of light?

I believe so. Also, please don't post "noise". Make sure your posts are relevant to the thread.

 

[frish]

Hello DHFPeople read about Relativity in the newspapers, memorize the 'mc square', hear strange tales about objects getting heavier when they move at close to the speed of light, then bring themselves to ask on Internet forums:
"What does it look like when one is moving with a speed close to the speed of light?" in the same way they might ask in long distance global phone call,
What's the weather down there? Is it hot?;"

Well, it is not so, as explained in more details here:http://www.iei.info/knol/pf/3trm10yysobsi-15-relativity-str-and-gtr-in-three-k.php