# Is it possible for humans to travel at approximately the speed of light?

#### Spacecomputergeek

Hi, I am interested in space and write various articles in my school newspaper. I would like to learn more and be able to explain to others what it's all about . Can you help me to answer this question?

#### COLGeek

##### Cybernaut
Moderator
Is it possible based on the technology we have at our disposal today (near future)? No. Not even close.

If you search the forums for "speed of light" you'll find several threads addressing aspects of your topic. Feel free to sort through those and engage with other members in those threads.

Welcome to Space!!!

Spacecomputergeek

#### billslugg

The problem with trying to go near the speed of light is the amount of kinetic energy required. E= 1/2mV^2.

E = energy in Joules. One Joule is equal to one watt for one second. It is also equal to one Newton of force pushing for one meter. In Imperial terms it is equal to 1/1000 of a BTU.
m = mass in kilograms
V = velocity in meters per second

Suppose you have a 1000 kilogram automobile and you want it to go 100 kilometers per hour. This is 27.7 meters per second. (In Imperial this would be 2,205 pounds at 62 miles per hour).

E = 1/2 * 1,000 * (27.7^2)
= 386,000 Joules

There are 120,000 BTU in a gallon of gasoline, or about 120,000,000 Joules
With your motor efficieny of about 30% you would need about 1% of a gallon of gasoline to get you up to speed. This is easily achievable.

But suppose you want your car to go fast enough to reach the nearest star and return within a human lifetime. Proxima Centauri is 4.27 light years away or 4x10^16 meters, assuming a 30 year trip each way, you must average 44 million meters per second. To accelerate up to that speed as you leave the Earth:

E = 1/2 * 1000 * (44x10^6)^2
E = 1x10^18 Joules
E = 8 billion gallons of gasoline (or rocket fuel, they're about the same)

This amount of fuel would weigh 22 million times more than the car (or rocket)
This is not easily achievable. Like COLGeek says: "Not even close".

#### billslugg

Suppose you were to use nuclear fission to generate your energy? Fission converts about 0.1% of the mass of the fuel to energy. The energy in mass is equal to mc^2 or, for one kilogram it is equal to 9x10^16 Joules.
You need 1x10^18 joules for your rocket to reach 44x10^6 meters per second, or 11 kilograms of pure energy, or 11,000 kilograms of uranium. You need to do this four times. Accelerate to leave Earth, deccelerate when you get there, accelerate to come home and deccelerate once you reach Earth. You must also convert the heat to electriciy to power your ion thrusters, this is only about 30% efficient at best (Stirling engine). This means you need 11 * 4 * 3 times the mass of your vehicle, or 132 times its mass so this won't work.

How about fusion? Fusion of hydrogen to helium is within reason, anything above that requires the pressures of the center of a star thus is not reasonable. Hydrogen fusion gives 1% conversion to energy. Your fuel need would be 13 times the mass of your vehicle. This is not reasonable.

How about anti matter? Anti matter converts 100% of the mass of the reaction with matter into energy. Only 1/4 is available as gamma rays, which could be used to generate heat. The rest goes out as uncontrollable neutrinos. You would then only need about 25% of the mass of your vehicle as fuel. This is within reason. There are practical problems with anti matter though. One is that we don't know how to contain it, even held in a magnetic field in the vacuum of interstellar space, enough random atoms would degrade it over time, Another is the energy cost of production which is about a billion to one. Rough calculation shows we would need about 30,000 years of sunlight falling on Earth to generate enough antimatter for one trip. Again, not reasonable.

Spacecomputergeek

#### Helio

Hi, I am interested in space and write various articles in my school newspaper. I would like to learn more and be able to explain to others what it's all about . Can you help me to answer this question?
We would enjoy sharing what we know about space but can you be a little more specific since there are many topics about space? Is it space travel that you are interested in, as seems to be the assumption?

Since we are talking about travel, you should look at Special Relativity. It's not as tough as you might think.

The effects of a slower time rate can easily be calculated by...

t'=t* [1-(v/c)^2]^0.5 ; t' is that of the spacetravelers.

So, with a velocity of 0.8c, then t is 0.6 (60%) the amount of "normal" to get to their destination.

The mass and energy issues have been mentioned that explains why nothing has yet to go into deep space. But note that nuclear power cuts fuel consumption by over 1 million to 1. But something like an aircraft carrier has no big issue with mass, unlike the mass needed for a spacecraft using nuclear. This will change with technology, hopefully.

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