[greenslime (OP)]

I mean, yeah it would be awesome if space craft could travel at the speed of light but if you are travelling at such speed, 299,792,458 metres per second, how much damage would be caused to the craft if you hit a rock the size of a fist or something?

Would your reactions need to be that of a fighter pilot or Formula 1 driver or even better than that?

Presumably a defence mechanism would need to be introduced to deter space debris right?

[JP]

Actually the bigger worry is the few hydrogen atoms floating around in interstellar space.  If you're going near the speed of light, they would come at you so fast as to actually be dangerous.  Even though there aren't many of them, you're traveling so fast that you hit a lot of them each second--enough to kill you quickly and damage your ship.

(See, for example http://www.newscientist.com/article/dn18532-starship-pilots-speed-kills-especially-warp-speed.html)

[greenslime (OP)]

Blimey. Not such a gopd idea then. What kind of speed would be deemed "safe" in space?

[lightarrow]

I mean, yeah it would be awesome if space craft could travel at the speed of light but if you are travelling at such speed, 299,792,458 metres per second, how much damage would be caused to the craft if you hit a rock the size of a fist or something?

Would your reactions need to be that of a fighter pilot or Formula 1 driver or even better than that?

Presumably a defence mechanism would need to be introduced to deter space debris right?

Well, it all depends on what do you exactly mean with "299,792,458".
Do you mean that speed with an uncertainty on the last figure, as we do in physics with measured values? Or do you mean "exactly light speed"? In the second case, even a subatomic particle, even a photon of the everywhere present CMBR, has *infinite* energy. What a subatomic particle or a photon with infinite energy would do to a starship, it's beyond my knowledge; if you find someone able to answer this question I'd be grateful to you []

In the first case, let's see. You would admit that the starship has a speed wich differ from light speed for 1 m/s, so γ = Sqrt[1 - (v/c)²] ≈ 1.7*10⁴ and so only a grain of dust would have a devastating effect.

[Bored chemist]

How fast do you have to travel before the cosmic background radiation gets shifted from microwaves to gamma rays and needs silly amounts of shielding?

[JP]

Unless I'm doing the calculation wrong, the Doppler shift factor is Sqrt(1+v²)/Sqrt(1-v²) and the frequency of the CMBR (from wikipedia) is ~10¹¹ Hz.  In order to get up to ~10¹⁹ Hz, which is the frequency of gamma rays, you'd need to be going at roughly 0.9999999999999999c. 

[Geezer]

Assuming we could overcome minor obstacles like collisions with small particles punching holes in our craft, and, of course, finding a source for the unbelievable amount of energy required to accelerate us to, say 0.9c, how long would it take us to travel one light year from our perspective?

Or should I start a new topic?

[JP]

I believe it would be ~0.5 years from the point of view of the observer on board the ship.  (I multiplied 1.1 by the Lorentz factor, Sqrt[1-(v/c)²].)

Edit: You might want to check my math.  I found a mistake that I just fixed.

[greenslime (OP)]

I mean, yeah it would be awesome if space craft could travel at the speed of light but if you are travelling at such speed, 299,792,458 metres per second, how much damage would be caused to the craft if you hit a rock the size of a fist or something?

Would your reactions need to be that of a fighter pilot or Formula 1 driver or even better than that?

Presumably a defence mechanism would need to be introduced to deter space debris right?

Well, it all depends on what do you exactly mean with "299,792,458".
Do you mean that speed with an uncertainty on the last figure, as we do in physics with measured values? Or do you mean "exactly light speed"?

Erm, the number I got was from wikipedia  [] it said lightspeed was 299,792,458 metres per second.

All the other posts I'm afraid flew right over my head. Sorry.

So on really really basic terms like primary school terms. If it were possible to travel at the speed of light without suffering any damage at all to the space craft, when you look out the window of the space craft would it all be just lines and lines of light everywhere and if you were travelling parallel to a beam of light would it appear as though it is in a static position?

[Soul Surfer]

NO  the main point is that objects with mass just cannot travel AT the speed of light. they can only travel at just below the speed of light and whatever speed you are doing light appears to pass by in any direction AT the speed of light.

That is what relativity is all about

[lightarrow]

Unless I'm doing the calculation wrong, the Doppler shift factor is Sqrt(1+v²)/Sqrt(1-v²)

Shouldn't it be Sqrt(1+v/c)/Sqrt(1-v/c)?
Which means v/c ≈ 1-2*10^-16 or: v = 0,9999999999999998 c
almost the same result you got, anyway.

[JP]

Ah.  I squared the v/c and I shouldn't have.

[lightarrow]

Assuming we could overcome minor obstacles like collisions with small particles punching holes in our craft, and, of course, finding a source for the unbelievable amount of energy required to accelerate us to, say 0.9c, how long would it take us to travel one light year from our perspective?

If your motion is a uniform "proper" acceleration α, indicating with U your final speed and with T the total time seen from an observer on Earth, you have:

T = U/{α*Sqrt[1 - (U/c)²]}

The proper time (time in the starship frame of reference) is:

τ = {c/α}*arsin(U/c).

You can choose the proper acceleration α so that it's sustainable for you and then you compute the total time. If I have made the right computations, with α = 1m/s², τ should be of the order of 10 years.

Edit: I added curly brackets to the formulae.

[latebind]

I don't see a problem with it at all.

By the time we are able to travel at the speed of light, we will probably have some sort of force-field technology, because the latter is much more technically easy than the former.

[greenslime (OP)]

NO  the main point is that objects with mass just cannot travel AT the speed of light. they can only travel at just below the speed of light and whatever speed you are doing light appears to pass by in any direction AT the speed of light.

That is what relativity is all about

Well unfortunately, I'm not that well educated in the laws of physics so I don't know what relativity is all about. I still don't understand though. If you are travelling at 60 mph in a car and a car over takes you going at 70 mph would it not seem the same as if you are standing still and a car travels past you at 10 mph? Why does this not work if you were in space and travelling just below the speed of light and a light ray went past you?

[Soul Surfer]

because that is the way the laws of physics work!  Too many people these days are completely saturated with fantasy stories and seem to have the idea that the fundamental rules of the universe gan be easily bent and broken.  The laws of nature are fantastic as they stand and the most important thing in life is to understand them and use them to best effect.

[Soul Surfer]

To give you a more serious answer it all works out based on the way we express things that happen at the same time for different observer moving at different speeds based on the velocity of light communicating the information. AND IT WORKS AND HAS BEEN PROVED BY MANY EXPERIMENTS.  If it didn't the universe would not function and would not be understandable.

[Tintin_Triton]

well i don't understand much about relativity either. Not at the most complete level that is(I wonder who can), but it is as simple as considering one postulate, that you can't travel faster than the speed of light. Anything, whether it be an information or a n object, it can't travel beyond the speed of light.

Like you have two charges separated by a great distance, and if one charge moves, the other would not come to realize it, because it won't be able to 'see' and so not 'know' when the charge actually moved.

Because we understand the universe as we see it, we won't be able to tell anything about the movement until the information that it has moved hasn't reached us. When it does, well we say it has moved.
Thus the essence of time is related with speed of light 

[JP]

because that is the way the laws of physics work!  Too many people these days are completely saturated with fantasy stories and seem to have the idea that the fundamental rules of the universe gan be easily bent and broken.  The laws of nature are fantastic as they stand and the most important thing in life is to understand them and use them to best effect.

Exactly.  And special relativity, which is the theory that you need to use to answer this question, is based on two beautifully simple postulates:

1) No matter how fast you're moving (with constant velocity), light always seems to be moving with the same constant speed relative to you.

2) The laws of physics are the same no matter how fast you're moving (again with constant velocity). 
 
Even if the rest of the theory looks like mathematical gibberish, it all comes from these two beautifully simple postulates. 

[yor_on]

Take a look at velocity/distance. The way Fontwell defines it (as I understood it:) is that with two differentiating velocities versus your origin (Earth) you will get two different readings on your distance, as observed from the rockets. And the distance measured will be such that the lights speed always will be at 'C', whichever rocket you measure it from.