why can't a rocket go faster than the speed of light?

[roid]

my simple explanation for why you can't go faster than C was that as you approach C your mass increases, so you're harder to move around. (i think i learned this from someone here on dbb.net )

But, rockets carry their own reaction mass. And it's this mass - expelled out as \"exhaust\" - that gives the rocket it's acceleration. As the rocket approaches C, would not it's reaction mass increase at the same rate as the rocket's mass? The rocket's mass would be increasing, but the mass of it's exhaust would be increasing at the same rate. Wouldn't the 2 cancel eachother out?
So, what is stopping that rocket from punching through C?

-----
I'm trying to work the numbers to see the theoretical max limits for a rocket journey, that's why i need to understand how C effects things.

If a theoretical rocket had an exhaust velocity of C (ie: a photonic rocket), so it's reaction mass was traveling at C (ie: the theoretical max efficiency for a rocket).

If that entire theoretical rocket was reaction mass, in otherwords 100% fuel.
Then that rocket would be able to accelerate at 1G nonstop for 354 days before it runs out. That seems to be the theoretical limit.

But i'm sure C gets in the way somehow as well, how do i factor that in? Say i wanted to goto Alpha Cenauri (4.39light years away) with this theoretical rocket, what will C do to this situation? Will it ever slow down my acceleration to somethign lower than 1G?

[Isaac]

Well everything you SEE is going at the speed of light

im a jackass

[Richard Cranium]

E=MC^2

[Topher]

E=MC^2

Right, so, as you add more energy to the rocket to make it go faster, it gains in mass so it gets harder and harder to make it go faster. Eventually you hit a limit where no matter how much energy you add, it makes a neglidgeable change in speed and only appears as mass in the object. The speed you're approaching is the speed of light.

However, you are always moving at C, whether you like it or not, through time. When you approach the speed of light, you change your velocity and you're moving faster through space than you are through time. Eventually you are moving completely in space and not in time at all and you're at the speed of light.

[Mobius]

No Topher, that's incorrect.

You can't STOP time. The \"arrow of time\" is not so easy to defeat. You can step sideways to avoid the effects of time, by travelling at C, but that doesn't STOP time, merely your perception of it, and how quickly you experience it. IE, at light speed, things outside your craft appear to happen impossibly fast, and to someone looking in on your ship, you are in suspended animation.

Remember - time is RELATIVE to your perception of it, and not all places experience the same time, yet they all exist in the same universe, and not in different \"time zones\". i.e. an astronaut loses some time while he is in orbit because his velocity RELATIVE TO YOU is faster; he therefore epxeriences less time than you do.

Even clocks placed at the bottom and top of a sky scraper record different rates of time, because the top of the building has more angular momentum than the bottom, due to the rotation of the Earth. (This has been done too, BTW).

Relativity is probably only understood by a handful of people in the world, but the effects of it are easy enough to understand.

[Topher]

If you are traveling at C, things outside appear to be moving infinitely fast, someone looking at you would see that you are no longer aging at all. It's impossible to get to C, but we're just doing a thought experiment, I don't think I said anything incorrect.

[fliptw]

So, what is stopping that rocket from punching through C

its c, btw.

It simple really. the rocket explodes when it hits the speed of light.

Photons can travel at the speed of light, because it has no mass - its just pure energy.

You'd need to find a way for the rocket's mass to reach zero as it accelerates, and well...

[Richard Cranium]

its just pure energy.

Reminds me of a song in the 80s.

[The Lion]

And I think in practice any rocket would explode well before it hits the speed of light. :)

[TIGERassault]

Well, why couldn't a rocket go faster than light? I never heard anyone e;se say it couldn't due to mass reasons.

[Ziqidel]

THe point is, you can go as fast as you like but everyone else would think you are inly going as fast as light, and BTW, light always goes c faster than you whatever speed you are really going at

[Sirius]

It is a fundamental rule that nothing can exceed c, no matter what.

Someone probably knows the why, but I don't (at least not once you get down to the hard details). What happens as you get closer to c is fairly well-known, though.

Edit: Roid - the important thing that happens there is conservation of momentum. The loss in momentum of the fuel would increase the momentum of the rocket at a fairly constant rate, yes - but since momentum is proportial to mass x velocity... well... guess which increases.

[roid]

uh, i dunno.
which of my questions is this an answer to?

[Flabby Chick]

DOCTOR POWELL:What if I were to tell you that according to a man who lived on our planet, named Einstein, that nothing can travel faster than the speed of light?


PROT: I would say that you misread Einstein, Dr. Powell. May I call you Mark? You see Mark, what Einstein actually said was that nothing can accelerate to the speed of light because its mass would become infinite. Einstein said nothing about entities already traveling at the speed of light or faster.

Yawn!!!..will y'all just listen to yer man PROT and get on with yer everyday lives....Humans are so ...limited....

[Isaac]

DOCTOR POWELL:What if I were to tell you that according to a man who lived on our planet, named Einstein, that nothing can travel faster than the speed of light?


PROT: I would say that you misread Einstein, Dr. Powell. May I call you Mark? You see Mark, what Einstein actually said was that nothing can accelerate to the speed of light because its mass would become infinite. Einstein said nothing about entities already traveling at the speed of light or faster.

Yawn!!!..will y'all just listen to yer man PROT and get on with yer everyday lives....Humans are so ...limited....

chuck it into a black hole and see how fast it gose

[Testiculese]

Nowhere near the speed of light, is the fastest it GOES

[Foil]

Let's get back to the original question.

Roid, if I understand your question correctly, you're posing the question something like this:

\"1.) I've been told the reason it's impossible to get to c (the speed of light) is that a force cannot accelerate a rocket enough because mass increases dramatically near c.

2.) The force which accelerates a rocket is proportional to the mass of its fuel load. According to 1. above, the mass of the fuel would also increase dramatically (and at the same rate as the rocket) near c.

3.) So... why don't 1 and 2 cancel each other out? (i.e. why doesn't the increased force from the increased fuel mass compensate for the increased mass of the rocket)?\"

Hmmm, I don't know to give a non-technical answer to your question. As Mobius and Topher alluded to, it involves the way speed and time relate in different \"frames of reference\".

...does anyone know of a good analogy to demonstrate why 3. (above) doesn't work?

[Topher]

The rocket never has zero mass, so the more energy you add to it to make it go faster, the more massive that rocket becomes, whether it's losing fuel or not. Even if the rocket, without any fuel was to weigh as much as a feather (on Earth), it would still gain mass as it goes faster and faster to reach the same speed limit as a rocket that weighs as much as an elephant, a house, a planet, etc. Because the rocket never loses all of its mass, the rocket will always gain in mass as it goes faster.

[Foil]

No, no, I don't think that's what Roid is asking at all.

Roid is saying: \"Say a rocket's speed is close enough to c that its mass has increased by a factor of 10. Well, the mass of the rocket's fuel (inside the rocket) has also increased by a factor of 10, right?! So why don't these cancel out, because the extra fuel mass makes up for the extra rocket mass?\"

It's a good question, and I think it deserves a good analogy to explain the answer, although I can't think of one.

[Topher]

Ah ok, because the gain in mass is in energy moving the rocket and fuel, not from added fuel. While the fuel gains in mass, it doesn't gain in potential energy, you don't gain any more fuel molecules. The energy stored in the chemical bonds hasn't changed at all, just the mass of the atoms together with the energy moving them through space. You'll get the same amount of push out of the fuel whether it's not moving or moving near light speed.

[edit: my analogy sucks, someone think of a better one]

[TIGERassault]

The energy of a rocket going through space isn't related to how heavy the fuel is, it's how much potential energy is stored there.

Umm...
I don't think you know how space travel works...

The point on having fuel in space is that it's ejected out the back. And, according to newton's third law (It might have been his second, I can't remember too good), this causes an opposite effect on the rocket, thus accelerating it forwards.
And it's all dependant on mass. The potential energy is only there to throw out the fuel.

[Topher]

Right, the rocket increases in speed because you throw material out the back. When I combust fuel it creates gas that is ejected out the back and that then pushes the rocket forward. The rocket will move faster if there is more enegry in the gas that gets ejected.

The transfer of momentum is mass times velocity. p=m*v
The speed you get from kinetic energy is E=1/2*m*v*v

So, as my rocket reaches the speed of light, the energy that the fuel can produce remains the same, but its mass increases as it gets faster. So, the velocity that the heavier gas gets ejected at is slower:
v=sqrt( 2*E/m )
As m increases, v decreases.

Now look at the fuel's momentum as it gets ejected:
p=m*v

Plug in for energy of the fuel:
p=m*sqrt( 2*E/m )
p=sqrt(m)*sqrt( 2*E )

As velocity of the rocket increases, mass of the fuel increases. As mass of the fuel increase, the momentum it produces becomes less and less effective.

Does that make sense? (Did I mess this up royally?)

[TIGERassault]

Oh right, yes. I forgot about speed.
Well then yes, I think Topher has got it! *takes off hat*

[Foil]

In other words, the fuel's mass increases, but it loses effectiveness (against the increased rocket mass) because it doesn't gain any potential energy.

Makes sense, although I think there's more to it... [scratching head, trying to remember my Relativistics course material]

[roid]

i think i get what you are saying.

although this rocket wouldn't be \"combusting\" it's fuel, the fuel is simply mass that gets accelerated via some kinda particle accelerator and then exhausted. But it seems the same thing.

So, as the mass of the fuelmass/exhaust increases it would take more energy for the particle accelerator to accelerate that (now heavier) exhaust to the desired exhaust speed. Interesting. Since fuel(mass) efficiency would be paramount, and the power output going into the particle accelerator would be fixed; the fuelmass would need to spend more time in the accelerator to reach the required exhaust exit speed. So the end result is that as c is approached, the effective thrust drops. So the acceleration would be comprimised and would drop below 1G. Hmm, unless: the craft is designed with all of the mass below the passengers feet - the increased mass of the whole craft/passengers as it approaches c could pull the passengers towards the floor anyway! With good designing perhaps the loss in acceleration could be perfectly counterbalanced by the increase in gravitational mass.

Still, i'm not sure how any of this relativistic stuff effects a photonic rocket.

[roid]

I'm trying to work the numbers to see the theoretical max limits for a rocket journey, that's why i need to understand how C effects things.

If a theoretical rocket had an exhaust velocity of C (ie: a photonic rocket), so it's reaction mass was traveling at C (ie: the theoretical max efficiency for a rocket).

If that entire theoretical rocket was reaction mass, in otherwords 100% fuel.
Then that rocket would be able to accelerate at 1G nonstop for 354 days before it runs out. That seems to be the theoretical limit.

here's an interesting thing i came across that answers a lot of questions i had.
http://math.ucr.edu/home/baez/physics/R ... ocket.html
according to this if i accelerate at 1G like the rocket i mentioned above, then i'd definitely attain relativistic speeds during the 354day burn. So space travel at relativistic speeds is theoretically attainable.

There is no mention of my 354day figure though, so i'm not sure if i got that right afterall. The webpage seems to indicate that the fuelmass/thrust ratio is larger than 1:1, so there is thus no upper limit to the length of time you can accelerate at 1G for. (assuming you kept well below c, maybe by just going in circles).

It's talking about matter/antimatter photonic engines though, maybe that changes things.

my calculation was simply
c = 299,792,458m/s
so expelling it's entire weight in fuel at exhaust speed c a rocket could accelerate at 30,570,323 Gs for 1 second,
or 1G for 354days.

*shrug* anyone here understand what i've done wrong?

edit: ah, i just realised that if the only fuel i use is onboard the rocket, my theoretical max final speed (ignoring relativity) could be no more than c. Factoring in relativity, to attain the nessesary fuel to accelerate to faster than approx 0.70c - i'd need to harvest reaction mass as i flew, perhaps by scooping up interstellar hydrogen.

[fliptw]

there are three things that an stationary observer sees of an object moving at high speeds:

1 the object gets longer in the direction of travel.

2 It's time slows down

3 It's mass increases.

#1 may mean that you might run out of space before you get anywhere near c... because you might be occupying all possible points in space when you hit c.

[Topher]

1 the object gets longer in the direction of travel.

Huh? That's the exact opposite of what happens. An observer observing an object moving near the speed of light will measure it shorter than an observer on the object.

http://en.wikipedia.org/wiki/Length_contraction

[fliptw]

ok, I got that backwards

[Testiculese]

Crossing the event horizon of a BH is where you 'lengthen'. Maybe that was your mixup.

[Aggressor Prime]

If a rocket reaches C, it stretches across the entire universe. You can't move anywhere if there is nowhere to move. Think of an explosion that changes matter into energy and that spreads that energy to the ends of the universe. An object, once it reaches the speed of light, turns into energy.

[Foil]

Hmmm, considering the fact that the our current model of Relativity is designed only for speeds approaching the speed of light, it makes sense that you get odd results (\"rocket stretches across the entire universe\"? ) from the equations if you plug in values at or beyond c.

Another way to say it, if you take the opposite of what Relativity tells us, assuming that an object can actually accelerate to reach the speed of light, you can get some nonsensical and/or self-contradictory results.

In other words, when you go beyond the boundaries of the model, the results you get don't necessarily correspond to any reality. It works well for Science Fiction (ships \"jumping\" from one location to another by exceeding the speed of light, etc.), but not for real science.

[Bet51987]

Roid

The problem with the rocket had me going big time so I presented it here in a forum I go to.

http://www.sciencechatforum.com/bulleti ... php?t=3423

Look at what Lincoln said.(He's a Particle Physicist) Its interesting that Mass as we know it does NOT increase. I understand some of it but not all.

But the bottom line is you still can't exceed c and maybe the rest of you can teach me too...

Bettina

[Foil]

The critical part of his post is this:

The short answer is that the mass of the rocket does not increase.

I know, I know....most of you have been taught that or read it or something. It's not true.

...

Here's the deal.... The mass doesn't increase. Physicists teach students that it does, simply because it's a small untruth that vastly simplifies the teaching of the subject the first time around. ...

there is actually only one mass...the rest mass...the remainder (i.e. the so-called "relativistic mass") is some fancy mathematical slight of hand.

(/me starts to remember some of his Relativistics coursework from back in college)

Basically, saying "the mass increases" is a common way to explain the effect of the extra relativistic terms in the mathematics near c. It's not precisely true, but it helps many students start to grasp the basic concepts of Relativity.

A more precise statement would be, "the relativistic mass increases as the speed approaches c".

So what's the difference between "rest mass" and "relativistic mass"? They refer to two different things mathematically. The former refers to the measure we're familiar with, and the latter refers to a mathematical term which is essentially "mass plus some relativistic terms".



Of course, now that begs the educational question: Is it best to make the precise distinction from the beginning (which risks confusing students who are new to the concepts of Relativity), or to teach that "mass increases" (which initially helps students conceptually, but risks confusion later on if it isn't clarified)?

[roid]

Bett, that was quite interesting. I have one question that i'd like to make sure of:
could you please ask Lincoln if apparent (relative) gravitational mass (assuming this is the correct term) increases as you approach c?

ie: as i approach c will i be attracted to the side of my ship with greater force?

given what Lincoln said i'm assuming the answer is NO, but i'd like to make sure. He's explained how inertia increases and how this makes it seem that relative mass has increased, but hasn't mentioned whether this apparent relative mass increase translates (or not) into an increase in gravitational pull from the masses involved.

that would be handy to know

[Lothar]

Is it best to make the precise distinction from the beginning (which risks confusing students who are new to the concepts of Relativity), or to teach that "mass increases" (which initially helps students conceptually, but risks confusion later on if it isn't clarified)?

IMO the best way to teach it would be the most straightforward:

"The apparent mass increases." You don't get more actual stuff there, but it becomes harder to move.

[Bet51987]

Is it best to make the precise distinction from the beginning (which risks confusing students who are new to the concepts of Relativity), or to teach that "mass increases" (which initially helps students conceptually, but risks confusion later on if it isn't clarified)?

IMO the best way to teach it would be the most straightforward:

"The apparent mass increases." You don't get more actual stuff there, but it becomes harder to move.

This is the way I would teach it too. And, Roid, I posted your message to Lincoln. I'll let you know.

Bee

[Bet51987]

Ok, your answer is going to be here...

http://www.sciencechatforum.com/bulleti ... c&start=15

Pages 2 and 3. Scroll down where your question begins to be answered. I only recently met Lincoln, but I know Marshall from a couple of forums I go to and he always makes me work for an answer.

Bettina

[roid]

i dunno Bett, Marshal seems elusive - i'm suspicious.
Lets wait for Lincoln. Good ol Lincoln, mmm

[Bet51987]

i dunno Bett, Marshal seems elusive - i'm suspicious.
Lets wait for Lincoln. Good ol Lincoln, mmm

Your answer has been posted.

Like you, I kinda knew it but wanted verification. BTW, don't be suspicious of Marshal. I've known him for a few years at another forum under another name where he's a resident expert and well respected. Although he is very knowledgeable about physics, he once told me that sometimes he will make me work for an answer by guiding me to it instead of giving it out right away.

Lincoln is a particle physisist and explains things in great detail. I'm going to learn a lot from him.

Bettina