Traveling at Speed of Light?

[indepth]

I am just curious to know if such technology ever existed, would we even be able to travel at the speed of light? If not, how would we ever visit other inhabitable planets that are 100's or 1000's of LY away?

 

[SpeedFreek]

Theoretically, there is no possible technology that would allow us to travel at the speed of light, as the laws of physics tell us that only massless particles can travel at c. For anything with mass, you would require infinite energy to accelerate to the speed of light, whatever your method of propulsion.

But, there is nothing to theoretically stop us travelling near the speed of light, with the right technology. There are a few obstacles to overcome, but travelling near the speed of light is theoretically possible, whereas travelling at the speed light is not.

 

[indepth]

Thanks speed. So how do we plan to explore other planets that we believe could contain life even with the technology of traveling at the speed of light? They just seem so far away it seems unlikely we would ever be able to actually visit such planets.

 

[Jerromy]

Keep in mind that even at close to the speed of light a trip to a planet 100 light years away would take more than 100 years and with the expansion of space as one would travel the destination would become further away. I don't have the patience right now to calculate a rough estimate of the Hubble constant of expansion but even a message sent to an alien race 100 light years away at a constant rate of c would take over 100 years to get to them. The only hope anyone could have of reaching such a distant planet in their lifetime is that time may slow down travelling that fast and the trip could seem to take much less time. Nothing to write home about though because here on Earth you would still have travelled for over 100 years by our clocks. The only option without faster than light travel is generations of families choosing their children to be born into a duty to continue the trip they had no ability to choose.

 

[SpeedFreek]

Keep in mind that even at close to the speed of light a trip to a planet 100 light years away would take more than 100 years and with the expansion of space as one would travel the destination would become further away. I don't have the patience right now to calculate a rough estimate of the Hubble constant of expansion but even a message sent to an alien race 100 light years away at a constant rate of c would take over 100 years to get to them.

The expansion of space is not relevant at such small scales, nor is it relevant in our galaxy at all due to the whole galaxy being part of a gravitationally bound system.

And at speeds close to c, we have time-dilation to consider. On a spaceship at 99% of the speed of light, time would be passing 7 times slower than it is back on Earth. From our point of view back home, a 100 ly journey would take a little over 100 years, but it would take only a little over 14 years for the occupants of the spaceship.

 

[Solifugae]

And at speeds close to c, we have time-dilation to consider. On a spaceship at 99% of the speed of light, time would be passing 7 times slower than it is back on Earth. From our point of view back home, a 100 ly journey would take a little over 100 years, but it would take only a little over 14 years for the occupants of the spaceship.

This is actually extremely handy. Time dilation is our friend. At least if we're not worrying about returning to earth it is. That said, accelerating up to such speeds at a tolerable 1G will take a LONG time. Best have a generation ship/longer lifespans before this is attempted.

 

[SpeedFreek]

That said, accelerating up to such speeds at a tolerable 1G will take a LONG time. Best have a generation ship/longer lifespans before this is attempted.

This will surprise you... If you could build a method of propulsion that could maintain a constant acceleration at 1g, it doesn't actually take very long to achieve relativistic speeds!

With a constant acceleration of 1g:

T______t________d_________v_____________γ
1 year__1.19 yrs__0.56 lyrs__0.77c__________1.58
2______3.75_____2.90______0.97___________3.99
5______83.7_____82.7______0.99993________86.2
8______1,840____1,839_____0.9999998______1,895
12_____113,243__113,242___0.99999999996__116,641

T = proper time on board your spaceship
t = time measured in non-accelerating frame (i.e back on Earth)
d = distance measured in non-accelerating frame (i.e back on Earth)
v = final velocity
γ = gamma: time-dilation/length contraction

So, we can see that, with a constant acceleration of 1g, after 1 year on the spaceship, 1.19 years will have passed back on Earth, you will be just over half a light-year away from Earth and you will be travelling at 77% of the speed of light, relative to Earth. For every 1 second that passes on the spaceship, 1.58 seconds pass on Earth.

But after another year on board your spaceship, 3.75 years will have passed on Earth, you are nearly 3 light-years away from Earth and travelling at 97% the speed of light! For every second that passes on the spaceship, nearly 4 seconds are passing back on Earth!

After 5 years, nearly 84 years would have passed on Earth and you would be almost 83 light-years from home, travelling at 99.993% of light speed!

These figures are for constant acceleration, passing your target destination at high speed. If you want to arrive at your destination and stop there, you have to turn round at the halfway point and accelerate in the opposite direction. As you slow down, you lose the benefits of time-dilation, so the overall journey takes longer if you want to slow down when you get there.

Here are some of the times you will age when journeying to a few well known space marks, arriving at low speed:

4.3 ly________nearest star__________3.6 years
27 ly_________Vega_______________6.6 years
30,000 ly_____Center of our galaxy___20 years
2,000,000 ly__Andromeda galaxy_____28 years

http://math.ucr.edu/home/baez/physics/R ... ocket.html

 

[Jerromy]

Excellent link, Speedfreek. Crazily complex to factor in the resistance one would encounter travelling near c, each individual photon would have an apparent effect unless the ship had a "black hole" in front of it to absorb the energy and channel it into propulsion. That would be the only possible shield I could imagine working when every atom we know of would break down at such high temperatures, and the plus side is every atom we encounter on this trip would be potential fuel.

 

[indepth]

Here are some of the times you will age when journeying to a few well known space marks, arriving at low speed:

4.3 ly________nearest star__________3.6 years
27 ly_________Vega_______________6.6 years
30,000 ly_____Center of our galaxy___20 years
2,000,000 ly__Andromeda galaxy_____28 years

http://math.ucr.edu/home/baez/physics/R ... ocket.html

[/quote]

This is really cool.

 

[Solifugae]

Excellent link, Speedfreek. Crazily complex to factor in the resistance one would encounter travelling near c...

That's the worst part about relativity. If I understand this correctly, if you were traveling at many .9 decimal places ahead of 99%c, and the outside time is faster in relation to you. Then as you take a year to cross the galaxy, besides from photons and cosmic rays, you will hit atoms every now and then. If you hit 1000 Hydrogen atoms per year (in reality it would be a lot more I'd think), but as your year passes, thousands do for the frames of the atoms, then don't you actually up the rate at which you are getting hit, even more than just caused by the speed increase itself? You are being hit by millions of atoms instead of 1000s. Never mind all those horrible subatomic particles.

Besides the energy needed to power it, the craft would need to be made out of some impossible material which passes through everything like a ghost, but that's a story for the unexplained.

Seems like we'll be stuck to taking 10s of thousands of years to colonize the local stars.

 

[origin]

Excellent link, Speedfreek. Crazily complex to factor in the resistance one would encounter travelling near c...

That's the worst part about relativity. If I understand this correctly, if you were traveling at many .9 decimal places ahead of 99%c, and the outside time is faster in relation to you. Then as you take a year to cross the galaxy, besides from photons and cosmic rays, you will hit atoms every now and then. If you hit 1000 Hydrogen atoms per year (in reality it would be a lot more I'd think), but as your year passes, thousands do for the frames of the atoms, then don't you actually up the rate at which you are getting hit, even more than just caused by the speed increase itself? You are being hit by millions of atoms instead of 1000s. Never mind all those horrible subatomic particles.

Besides the energy needed to power it, the craft would need to be made out of some impossible material which passes through everything like a ghost, but that's a story for the unexplained.

Seems like we'll be stuck to taking 10s of thousands of years to colonize the local stars.

Yeah, there are all sorts of implications if you travel close to the speed of light. Even though the speed of light is constant no matter how fast you are going, the energy of the photons does change. If you are traveling towards a star very close to the speed of light the photons in the visible range of light can be blue shifted all the way to hard gamma rays. Luckily, I guess, gamma rays that are 'chasing' you will be red shifted to a less dangerous energies.

 

[XairstrikeXD]

i dont understand. how is it possible for time to pass differently on earth. if the universe is expanding at a constant rate i can see how the trip would take a little less than 100 light if it is expanding towards you but i still dont understand how time flows differently?

 

[origin]

i dont understand. how is it possible for time to pass differently on earth. if the universe is expanding at a constant rate i can see how the trip would take a little less than 100 light if it is expanding towards you but i still dont understand how time flows differently?

It is possible because time is not absolute. Our everyday experiences tell us that time is constant but it is not. Time dialation does not really have anything to do with the expansion of the universe.

What is at the heart of time dialation is that the speed of light is constant relative to any observer no matter what their velocity is. This site has a little graphic that shows why time is measured differently by individuals moving at different velocities. Keep in mind that everyone will always measure the speed of light as having the same velocity.

 

[XairstrikeXD]

i understand better now but wouldnt that make it to were every piece of matter has its own seperate time flow?

 

[Mee_n_Mac]

i understand better now but wouldnt that make it to were every piece of matter has its own seperate time flow?

I a word ... yes that's true. Take the case of muons generated in the upper atmosphere when a cosmic ray smacks into something. A muon is generated and decays in a couple of millionths of a second ... of it's time. Since they experience their own time, their 2 uSecs is long enough for them to reach the Earth's surface and be detected. Where time dilation not to occur they wouldn't last long enough to go even a mile. Another example are the satellites which we use for our GPS system.

http://www.astronomy.ohio-state.edu/~po ... 5/gps.html

For the most part all the stuff around us experiences about the same rate of time flow since we all moving so incredibly slowly relative to c thus the differences are too small to notice in (most of) everyday life.

 

[Solifugae]

It's best to think of time as processes happening in relation to each other. Processes like the movement of atoms (and even further down) in your bodies will occur at a particular rate when your mass is undergoing a particular acceleration. Compared to other accelerations it will occur differently. If every process were to slow down within a ship moving near the speed of light, things at rest on earth, will be racing along in comparison.