"Speed of Light" travel

I have always heard that one of the barriers to light speed travel is that as a body(ship) approaches the speed of light it increases in mass until finally, at the speed of light, it's mass is infinite. Therefore, as the ship increases in mass, it requires more and more energy(fuel) to propel it.
Excuse me, but doesn't the fuel also increase in mass and therefore provide more energy?
Maybe I'm missing something here.
Can any of you physicists shed some light(pardon the pun) on this for me?

The mass might increase but the number of moles don't.

The mass is realtive. In the reference frame of the space ship, the mass of fuel does not increase. There are 2 kinds of mass, rest mass, and relativistic mass. The rest mass never changes, it is the mass as measured in a reference frame at rest realtive to the object being measured. Realtivistic mass is the mass of an object moving relative to an observer.

Great explanation RL.

Here's a great summary of some interesting experiments regarding "faster that light."

I predict that when we fully understand what empty space is composed of, we'll learn to subtract it. :wink:

Nobody alive has ever moved at a rate anywhere near the speed of light, and if I know anything, it's that theories have a way of not being entirely accurate when you try them out in practice.

What do subterranean mammals have to do with this?? (Just kidding) Please explain further, you are not talking(typing) to a scientist here. I barely remember moles from school.

R. Lewis,
My point exactly! It's all relative.


the fuel is on board, and it is used up by a chemical process that releases energy that in turn creates momentum that is expelled out the back of the rocket.

The amount of energy released is based on the moles consumed in the reaction. The moles are the number of molecules and that number does not change with velocity.

The energy yield of a nuclear reaction still depends on the number. So the above reasoning should still apply.

Raven, doesn't the amount of energy released also increase with mass? Isn't that what Einstein said about the relation ship between mass & energy? If I am thinking correctly, the energy released has something to do with the forces that bind the molecules together. Isn't it possible that those forces increase as the mass of the molecules increases?

Eddie, no...reply 2 from r lewis is correct. For the engineers on board your relativistic ship, the mass of the fusion fuel they fuse, or the coal they heap into the boilers, is the same as the rest mass of that fuel. It provides precisely no more energy than it did when "stationary". That that energy is being used to accelerate a relativistic-mass-increased-ship means that their acceleration will decrease as their speed increases.

Your best bet for light-speed travel is to uncover something amazing with the Higgs field, and somehow decrease the apparent rest mass of the ship to as low as possible, thereby allowing you theoretically any acceleration with any (even tiny) thrust.



thinking about the practicallity of it all,
if I wanted to travel to the nearest star to our sun, and say I could travel on a space ship at say 99.99% the speed of light, how long would the journey take measured by a clock travelling on the spaceship (and neglecting accel and decell.)?

the reason i ask is that i read somewhere that a photon can be viewed as a electomagnetic wavelet frozen in time.

Andy G. If the mass of the ship increases, the mass of everything on it will increase at the same rate, including the fuel. I agree that the engineers on the ship will not see the fuel's mass increase, but they will also not see the increase of the ship's mass, just as they do not see the time dialation. These increases are only realative to a stationary observer( Who, by definition, cannot be on the ship). That is why I think the idea of the ship getting too heavy to push could be wrong. Now I don't know enough about moles and such, but I assume that the energy released in a reaction is relative to the mass of the material used in the reaction.
So either the fuel puts out more energy as it gets heavier or it is relative in the same way as the time dialation theory, in that the mass differences at velocity are only apparent to a stationary observer. Since the stationary observer has nothing to do with the propulsion of the ship, it is all nonsense. The ship would just keep on accelerating as long as the fuel held out.

Raven, re the end of your reply 9. Quite correct. At the speed of light, time dilation is infinite. That is, to the photon, its creation (in a star maybe a billion light years away) is at precisely the same photon-moment that it splutters out on your retina. Which could be said to mean:

1/ that it doesn't really exist, from its point of view. Created and destroyed in the same instant.

2/ that the entire space-time universe we think we dwell in is literally filled with a network of crisscrossing photons (and other s-o-l particles) whose links between past, present and future make a complete mockery of our perverted sense of the here-&-now.

There you go...two hard things to conceptualise before brekkie. ;-)

Re: your 99.99% ship. Time dilation is 70.7 times. So, ignoring accel and decel (which you can't really) a 10 light year trip will take an apparent 52 days. And each 1/4pound hamburger eaten by the crew during the flight will mass (from an external observer's viewpoint) 8kg and be about 1mm in length. ;-)

Regarding Eddie's reply 10: The engineers on board will certainly not be aware of the time dilation or mass increase effects within the ship, but they will be able to see the universe contract as their speed increases, along with other relativistic effects of their travel on the external universe. Their power output won't change, assuming fuel is burnt/fused/whatever at the same rate as when they started, but its effects on their vessel (due to increasing mass-through-relativistic-motion) will diminish.

Andy G

Andy G, Again, the fuel is moving at the same velocity as the ship. There is no relativistic motion.
As far as light is concerned, if the accepted theories are correct. A photon of light could exist in every place at once.

I'm not going to pretend I have a comprehension of the theory relativity . I seriously doubt that even Einstein could understand what his calculations were showing.
I think human mind just can not grasp speed of light being constant for all observers.
IMO Eddie's point is valid. To put it in physics 101 style, imagine a spaceship in space filled with a great number of non-regulation basketballs each having a mass of 1 kg. A robotic arm picks up the balls and ejects them from the ship at regular intervals. Each ejection action will have a reactional force. According to the Newtonian model the ship will accelerate linearly and given enough basketballs will eventually surpass the speed of light. According to the theory of relativity however the increased speed will result in an increased mass reaching infinity at the speed of light. However each ball will also increase in mass negating the increased mass of the ship. This really translates to saying 2/0 = 1/0. The real problem is infinity is undefined and mathematics fails to yield an understanding. The ejection of basketballs is completely analogous to a rocket powered propulsion.

"According to the Newtonian model the ship will accelerate linearly"


How do you prove linear acceleration? How would you measure it?

My basic understanding of relativity tells me that that the 'energy equation' E=MxCxC squared should really be M0 where M0 is the rest mass of the object or fuel, hence the energy that could be obtained from the fuel is the same at any speed up to C as if the mass is at rest.

I am not qualified to give a lecture on the subject but relativity theory comes from the simple idea that the basic laws of physics dont actually change in a moving frame of reference. Any measurements conducted in two frames each moving at different speeds give the same results. ( Hence an observer in each frame has no way of determining his absolute motion with respect to an immobile frame).

Special Relativity which was published first deals with effects arising from uniform non accellerating motion.

General Relativity deals with effects from motions which are accellerating relative to one another.

Hi raven,
Since the basketballs are of equal weight (1 kg) and are ejected at regular intervals. Each ejection will add a constant amount to the current velocity over time. The reduced mass due to each missing ball is also a constant amount. So It will accelerate linearly over time but not constantly. This as opposed to for example exponentially. If you are suggesting that intermittency can not be linear, you are technically right.

I was wrong :blush:
If you consider missing weight the accelerating won't be linear.

Hi UR,
The comprehension difficulty arises when you consider that two observers at two different relative frames measure the same beam of light to be of a constant speed C. In regular everyday experience two such observers would measure any other moving objects having a different relative velocity. But speed of light is always constant in vacuum regardless of your motion when you measure it.