light speed and space exploration

When we say that increasing the velocity of a body increases it's mass, we're talking about "inertial mass". Inertial mass defines how much energy is required to move the body a certain distance over a certain length time.

Energy is not really a "material" thing -- it is the potential to do work. But it is manifested in material things. If you throw a rock up into the air, you've added energy to it's inertia. While it's falling back to the ground it doesn't seem to have any energy at all -- it's in freefall. But it does have "kinetic energy", meaning that it has inertia relative to everything else around it, and that energy will be revealed when it hits the ground and kicks up dirt.

Increasing the speed of a rocketship, say, requires energy being released from it's engines. This energy is added to the rocket's inertia and the inertial mass of the ship increases -- it'll take more energy to slow it down than it did before. So, when Einstein says that when a body loses energy it's mass decreases (E=mc^2), he's saying that energy itself adds "weight", and the closer the ship gets to c the heavier and heavier it becomes.

Energy that is not "attached" to any mass (radiation/light) travels at c because energy itself has no rest mass. But, once that energy is put to work ("attached" to a body), it adds to that body's mass.

The reason that adding velocities at these speeds is not simply cumulative is that empty space is not nothing, it's the relationship between - and a part of / created by - the matter in it. Gravity, though a weak force, has infinite range and cannot be escaped. The velocities of travelling bodies are constrained by the nature of the space they are travelling through, and we can't travel faster than light any more than we can flow a river uphill.

The time dilation of high velocity bodies is described like this:

t = to / sq.rt. of: 1 - v^2/c^2

t is the time dilation
to is the "rest time" ("normal" time, so to speak)

The closer that velocity (v) gets to c, the closer v^2/c^c equals 1, and as 1-1=0, time stops (and the body's inertial mass is infinate). Since it'd be a paradox to perform an event where time is stopped, the speed of light is unreachable.

ok i may be wrong about the gravity-light issue after reading about sir arthur eddington's findings during the solar eclipse of 1919 which showed the light from a star cluster being "bent" by our sun. BUT, i am not one to place all my beliefs on one observation which could have been influenced by many factors. the other evidence i've read so far didnt convince me either but i'm still reading

good post tuning spork

in space (a relatively frictionless environment), propulsion isnt really bound by time like it is on earth. when you drive 100 mph down the highway it requires the engine to maintain that speed. it doesnt work like that in space. speed remains constant until it is slowed by something. earth does not have big engines on it, yet it continues to move at increadible speeds around the sun because there is nothing to slow it down. if you fly straight off the earth in the direction that the planet is moving around the sun it doesnt take any more energy than it would if you flew straight off the other side of the earth in the opposite direction of the earth's orbit. movement in any specific direction takes no more or less energy than any other direction regardless even of current speed (walking to the back of a moving train takes no less energy than walking to the front)

"Increasing the speed of a rocketship, say, requires energy being released from it's engines. This energy is added to the rocket's inertia and the inertial mass of the ship increases -- it'll take more energy to slow it down than it did before"

i'm not saying this statement is false but i dont see how it is true, can you go into more detail? i dont see how it would take more energy to deccelerate than it did to accelerate

although i know that speed in space is not completely cumulative. i believe that near light speed could be reached and then easily maintained by almost any type of matter. as i said in a previous post though i now understand why an object could not remain an object if it were to go beyond light speed (although i still think the object could reach light speed and be desolved in doing so)

your statement below, while i dont feel that your numbers are inaccurate, i dont understand where this theory or proof of this idea comes from:

"t = to / sq.rt. of: 1 - v^2/c^2
t is the time dilation
to is the "rest time" ("normal" time, so to speak)
The closer that velocity (v) gets to c, the closer v^2/c^c equals 1, and as 1-1=0, time stops (and the body's inertial mass is infinate). Since it'd be a paradox to perform an event where time is stopped, the speed of light is unreachable."

Ooops. No, it doesn't take more energy to deccelerate than to accelerate. What I meant to say is that it takes more energy to slow an object when it's moving faster than when it's moving slower relative to a stationary reference point. The "stationary reference point", in this case, would be the universe as a whole. Sorry.



Here's a pretty good explanation of where it comes from:
www.johnstonsarchive.net...
While the author of that page wrote that the speed of light being the same to all observers is an "assumption", it is not. It was discovered by Mitchelson and Morley in, I believe, the 1890s.

As for proof, the theory has been pretty well established by particle accelerator experiments. Here a pretty good page: www.physlink.com...

One thing that's buggered me around with light speed...

It's relative. So that means if you're travelling in one direction at 0.6c, and another object to your side passes you in the opposite direction at 0.6c, the relative speed to you of that object would be 1.2c.

Assuming you know infinitely fast that an object is travelling past you at the aforementioned speeds, it would be possible.

As far as I know, this relativity is based off being able to actually see the event occur.

For example, an experiment about speeding light up in a tube. You see the beam of light exit before it even enters. Hard to explain what I mean, but... yeah.

great info tuning spork but i still have some problems with what i'm reading now

"First, for an object with non-zero mass as its velocity approaches the speed of light its momentum will approach infinity. This implies that that it would take an infinite amount of energy to accelerate an object with mass to the speed of light. Since this amount of energy is unavailable, objects with mass cannot travel at the speed of light (in vaccum) or faster"

let me break this down a little

"an object with non-zero mass as its velocity approaches the speed of light its momentum will approach infinity" based on e=mc^2

"This implies that that it would take an infinite amount of energy to accelerate an object with mass to the speed of light"

THIS statement (and e=mc^2) implies that acceleration and velocity are completely independant in vacuum. but i dont see how they are, once acceleration stops, then velocity remains constant. so how can further acceleration require any more energy than prior acceleration did???

its like this: IF more energy is required to accelerate a non-zero mass depending on how close it is to c then stopping all acceleration would result in a REDUCTION in speed, but thats not possible because we can see clearly that planets have no acceleration at all yet they maintain a constant speed

"One thing that's buggered me around with light speed...

It's relative. So that means if you're travelling in one direction at 0.6c, and another object to your side passes you in the opposite direction at 0.6c, the relative speed to you of that object would be 1.2c."

good point

i'm having trouble with this one now as well

if:
"Einstein's theory of special relativity is based on two assumptions:

1. All inertial (i.e. non-accelerating) frames of reference are equally valid
2. The speed of light is constant for all inertial frames of reference."

then if an inertial frame of reference was traveling at 0.6c and a non-zero mass passed by in the opposite direction at 0.6c it would appear to travel at 0.6c

can anyone help explain how that is possible?

Because the velocity at these great speeds is relative, though not exactly in the same way that Newton described velocity as relative in the slow-moving reference frame.

F'rinstance, let's say we're "standing still" somewhere. (Regardless of our relative motion to the "fixed" stars, we'll consider ourselves to be at rest.)

One ship is travelling away from us, eastward, at what we measure as .9c. Another ship is travelling westward at what we also measure as .9c. Classical mechanics would suggest that the two ships are moving away from each other at 1.8c. But, weirdly, they are not.
In the equation, if you punch in a value for v that is greater than c, the result will be a negative number. If v = c, the result will be infinity. (Or, E for error on your calculator.) So we can't simply add .9c and .9c and come up with 1.8c, because that's not what actually happens.

The theory and the evidence show that one body cannot move away from another body at an observable velocity greater than c because then those two bodies will, in essence, escape each other's realm of influence... they will cease to exist in each other's universe. The universe as a whole, it seems, doesn't allow parts of it to be cut off from any other part of it. So, at these high velocities, the velocities become more and more relative as far as observation goes. The two ships must always be able to communicate with each other.

Does that make sense? Of course not! It's completely counter-intuitive since our minds did not evolve in a world where these high velocities are ever encountered. But, just as we can learn how in the heck a fish can breathe water, we can understand the weirdness of how the universe has its ways of remaining whole in the superduper macroscopic world.

It's weird but, believe it or not, it works!

side topic:

"The fastest speed ever attained by humans (relative to the Earth) was 11.1 kilometers/second, by Apollo 10 astronauts returning from the Moon in 1969"

imo this should be a record that we try to break every chance we get. it would be much easier to accept e=mc^2 and time dilation if it were occuring on something larger than a muon (big electron)

all the evidence to time dilation and mass increasing based on velocity are from vague experiments on molecular particles. little hard to accept imo

heres why e=mc^2 is hard for me to accept:

a small satelite flying in space using an infinite fuel 10 lb thrust engine. e=mc^2 says that at some point as it gets closer and closer to c the engine will not accelerate it anymore because it's mass has become too great for the thrust of the engine to move. if the engine was on and not moving the satelite then wouldnt turning the engine off result in a decrease in speed? how is that possible in the vacuum of space?

ok here:

"Each year, the Earth travels a tremendous distance in its orbit around the sun, at a speed of around 30 km/second, over 100,000 km per hour. (The sun itself is traveling about the galactic center at speeds estimated to be between 217 to 250 km/s, and the galaxy itself is at motion relative to other galaxies"

so at full momentum we are traveling at least 217+30 km/s to 250+30 km/s which is definitely enough to affect our e=mc^2 because we are MUCH closer to c than if we were at absolute rest. so, if e=mc^2 is true then if we decrease our speed we will reduce our mass. since we are already traveling at 200+ km/s then it should be easy to drastically decrease our mass by just slowing down (which would look like speeding up relative to earth). i'm not very math capable so if someone could find this out it would be great. if we were to head out into space in the opposite direction of the sun's orbit around the galactic center (esentially bringing us closer to absolute rest and thus, lower mass) how much mass could we lose hypothetically?

now to apply this to space travel/exploration. instead of launching in a ship and heading for a nearby star, we leave our solar system and let the star come to us (because we are traveling in the opposite direction of the galaxy's rotation). if e=mc^2 is true then it would take SIGNIFICANTLY less energy to go a much greater distance since our ship would have less mass and would be easier to propel

An inertial frame of reference is, by definition, at rest. If a "frame of reference" is travelling at .6c then we are saying that it is travelling at .6c relative to an outside frame of reference.

We may be able to see a ship coming at us from the east at what we determine is .6c, and another coming from the west at .6c. To us they would seem to pass each other at 1.2c, but, to each other, they are passing at less than c. They will still be able to see each other coming.

Even by adding those velocities together, it's still a value of about 0.001c. 280 km/s is about 200 miles/sec. c is 186,000 miles/sec.

"c is 186,000 miles/sec"

and infinite mass

i'm just talking about reducing mass to a more managable amount for the purpose of space travel

And here's something to think about. Ignore relativity for a second.

Let's say you have a craft that weights about what the space shuttle weighs (100,000 kg). Now let's say you want to accelerate it up to light speed (300,000,000 m/s), while maintaining 1 g (9.8 m/s^2) of acceleration, so you don't crush the occupants.

That would take 980,000 newtons of force. That's about 18% of the thrust the space shuttle main engines can generate. But you'd need to maintain it for over 355 days. And that doesn't take into account the fuel you'd have to carry to generate the thrust. We're talking an (excuse the pun) astronomical amount of energy.

Again, that's totally disregarding relativity and the ensuing increase in energy requirements that brings.

No, we're no where close to being able to develop near-light speed craft.

"In 1887 A. A. Michelson and E. W. Morley measured the speed of light in the direction of the earth's travel through the universe, in the opposite direction and in other directions. They determined with great accuracy that the measured speed was constant in eVery direction even though it was known that the earth on which the measurements were made was traveling through the universe at speeds of at least 30,000 meters per second"

hold on doesnt that prove that the speed of light IS relative rather than isnt relative?

if they measured light traveling in various directions to be constant than in one of those measurements wasnt light actually traveling at least c+30,000 m/s?

So far, seems like everyone's given excellent answers....

Though yes, you'd require more and more energy...this may be possible (or at least get close to that speed) by constantly using orbits to "slingshot" you faster and faster (like we do with probes), and/or new ways to create/use energy in space could be developed.

Still one big problem though... Would still take a LONG time to get around. I mean even at light speed, it'll take a little over 4 years to get to the next closest star!!! So to get to the next closest star we think may have life...well, could be waiting a while....even at light speed.

hmmm

if thats true then perhaps the forces that keep an object together could maintain the object even beyond light speed

so the problem with reaching light speed is having the energy to propel the mass to that speed. since its very fast it would take much fuel and since fuel would increase the mass it would continue to be more difficult to propel the mass

whats the most efficient type of space propulsion that we are capable of at this point? (most energy from least amount of fuel. preferably involving electricity since that could be harvested from solar)

The speed of light is a constant within any frame of reference. If we could observe a spaceship travelling away from us at .999999c, and an astronaut turned on a flashlight, it would appear to us that the light would travel slooooowly across the room on that ship. But, to the astronaut, it's still travelling wicked fast.

What Michelson and Morley discovered was that there is no fixed medium that light travels through (what they called "the ether"), that space is not to light as air is to sound.

If we were on an open-bed platform on a train and travelling at 1/2 the speed of sound, and I were at the lead end and you were at the trailing end, you shouts to me would sound very low-pitched and my shouts to you would sound very high-pitched. The Doppler effect. That's because sound travels at a certain speed through the air, and we are moving through the air at a great speed.

But space is not an "ether". There is no such thing as "absolute rest" in space. The location and velocity of any body exists only as relative to other bodies.

Light also has a Doppler effect, though. Light from a star that moving toward us at great speed will have a blueshift, and light from a star moving away at great speed will have a redshift. The relative motion of the light source will effect the light's frequency, as we measure it. Blueshifted light will be more intense, have a higher frequency and thus more energy, but we'll still measure it's velocity as c.

The interesting possibilty arising from that is that different frequencies of light may actually be equal in energy, but somehow propogating at different rates of time.

"Still one big problem though... Would still take a LONG time to get around. I mean even at light speed, it'll take a little over 4 years to get to the next closest star!!! So to get to the next closest star we think may have life...well, could be waiting a while....even at light speed."

well worth the wait imo

can anyone explain the mechanics of increasing speed using the gravity of a planetary body like a "sling shot"? or provide a link where i can read up on it?

One thing about travelling at close to the speed of light toward another star is that, with the blueshifting that would occur, the light from that star would be of such a high frequency that it could fry our spaceship.

wait wait wait

"The speed of light is a constant within any frame of reference. If we could observe a spaceship travelling away from us at .999999c, and an astronaut turned on a flashlight, it would appear to us that the light would travel slooooowly across the room on that ship. But, to the astronaut, it's still travelling wicked fast."

no, it would travel across the room of that ship wicked fast to us too but it would take a long time for the light of the flashlight, room, ship, everything, to GET to us depending on how far the ship traveled in that time. the speed of the flashlight's light compared to us would be VERY slow BUT the ship is at near light speed so it's room would make up for the speed and it would equal out at c to us same as it would to the astronaut