# Travel at a fraction of light speed. Subjective time question.

This topic is 2427 days old which is more than the 365 day threshold we allow for new replies. Please post a new topic.

## Recommended Posts

[quote name='owl' timestamp='1305053252' post='4809078']
Last night I kept reading a little more and learned that for a spaceship traveling almost at the speed of light, all the radiations that arrives to the spaceship would be so shifted to the extremes (X-rays, ultra-infra-red) of the spectrum that nothing from the outside would be visible from the inside.
[/quote]

Not that I really understand these things, but I find that a bit strange. Wouldn't this effect depend on direction? So radiation travelling in the same direction as the space-ship will be shifted past infra-red, and radiation travelling the other way past X-rays. But won't light perpendicular to the direction of travel still be observable?

##### Share on other sites
[quote name='szecs' timestamp='1305012264' post='4808863']
A more interesting question:
How would we relativistically simulate a space-fighting game with all these time dilatations in a multiplayer game? All controls/the behavior of the spaceship would slow down compared to the outside scene? That is an interesting game design question....
[/quote]

See the indie RTS [url="http://www.achrongame.com/site/"]Achron[/url]. It's not a space-fighting game, but it involves time-travel. You're asking about how to design a multiplayer game in which the players can be in different temporal frames. This RTS deals with a game design problem that seems similar: how to design a multiplayer game that allows each player to travel back in time and affect the course of game events. It's a very interesting design they came up with.

Sorry to throw the thread off-topic, back to talking relativity BTW, an observer traveling at near the speed of light would be able to see almost nothing. "light perpendicular to the direction of travel", from the frame of reference of the traveler, would necessarily have to emanate from a source that was traveling at near the same speed in near the same direction (i.e. an object that appeared, to the traveler, to be almost at rest). Since this is unlikely, you would see almost nothing.

##### Share on other sites
An alternative way to calculate this is from the traveler's reference frame. He will see the destination as nearing at speed of 0.5c, but the distance to the destination is not 10 light years to him. Rather distance contraction (just as time dilation) plays a role here. Lorentz contractions on distance say the distance is now (10light years)/(gamma), where gamma is 1/sqrt(1-v^2/c^2)=1.15. So d=8.65 and we get the time to traverse this distance is 17 years again.

I played with the idea of incorporating relativistic effects into a video game a while back and was unable to come up with anything particularly compelling. The simulation is non-trivial. At relativistic speeds it becomes necessary to not have information transmitted instantly - you must be careful that everything is "seen" after a speed-of-light delay (at least), which makes simulation considerably more complicated. This is necessary, for otherwise you break causality and let things "see" into their future. So it is necessary to keep track not only of the current state, but of previous states. For a suitably simplistic game, this might be practical. I am also interested in the possibilities for a puzzle game to be made where you combine relativity with something like wormholes to create time travel if properly moved. (One of Brian Greene's books has an interesting section on this possibility: take one end of a wormhole on a long, fast spaceship ride and now you have temporally (as well as spatially) separated the ends.)

##### Share on other sites
I've thought about making a game or just a toy where Physics are modified so c is something like 100Km/h, and relativistic effects are taken into account. I don't understand relativity well enough to really know how to make it or what it would look like, and that's precisely why I think it would be an interesting teaching tool to help people gain an intuition for how a relativistic world works.

##### Share on other sites
[quote name='__sprite' timestamp='1305116294' post='4809382']
[quote name='owl' timestamp='1305053252' post='4809078']
Last night I kept reading a little more and learned that for a spaceship traveling almost at the speed of light, all the radiations that arrives to the spaceship would be so shifted to the extremes (X-rays, ultra-infra-red) of the spectrum that nothing from the outside would be visible from the inside.
[/quote]

Not that I really understand these things, but I find that a bit strange. Wouldn't this effect depend on direction? So radiation travelling in the same direction as the space-ship will be shifted past infra-red, and radiation travelling the other way past X-rays. But won't light perpendicular to the direction of travel still be observable?
[/quote]

I'm not really sure. My guess is that light perpendicular to the direction of travel should arrive at the speed of light and it's wavelength shouldn't "suffer" any change in relation to the velocity of the ship. Maybe some sort of lens effect due to space contraction inside the vessel?

##### Share on other sites
Look at the link I posted. I don't completely get the details, but it seems that light coming from behind the vessel will have infinite wavelength and light coming from the front has zero wavelength. So it's sure it has that red -blue rainbow-halo effect, and I think light perpendicular to the vessel WILL have longer wavelength. I think the more you approach c, the more "compressed" will the visible spot be in front of the vessel. I think... I don't think the perpendicular direction is any special.

##### Share on other sites
[quote name='szecs' timestamp='1305181762' post='4809677']
I don't think the perpendicular direction is any special.
[/quote]

I've been thinking about this and I remembered the famous "elevator" analogy used by Einstein. The light arriving perpendicular will describe a curved path inside the vessel, just as it would do if it passed nearby a very massive body. Traveling at c (or almost), from the perspective of the vessel it'd be like if the light coming from the outside bended downwards following the shape of the container (more or less I think...).

[code]

BBBBBBBBBBBB
B B ^
perpendicular light >----------\ O WTF? B |
| B / \ B |
| B | B |
| B / \ B |
| BBBBBBBBBBBB
\------------------------------------>
[/code]

##### Share on other sites
Yes, that's true. So you'll have your lens effect, and I guess at c the vessel becomes a black hole from it's own point of view. Or something.... Shit, that's exciting stuff!

##### Share on other sites
[quote name='szecs' timestamp='1305184200' post='4809690']
and I guess at c the vessel becomes a black hole from it's own point of view. Or something.... Shit, that's exciting stuff!
[/quote]

It is exciting indeed!

If I'm not wrong, if a body with mass get's accelerated to c, then it acquires infinite mass. Also, for that to happen it'd be required infinite energy. Now, if that's true, either black holes cannot exist or acceleration and gravity aren't completely analogous. (I'm shooting kind of blind here though)

##### Share on other sites
You don't need infinite mass to make a black hole. I don't know where you got that idea.

##### Share on other sites
when you reach 1/2 speed of light you wont speed up?

if not s = v * t

t = s/v

remember to calcuale meters per second with meters and seconds

note that destination point and start point dont change

i wonder is this foruma correct for this kind of speed does anyone can describe the problem if not lol ;o

##### Share on other sites
[quote name='owl' timestamp='1305183261' post='4809686']
[quote name='szecs' timestamp='1305181762' post='4809677']
I don't think the perpendicular direction is any special.
[/quote]

I've been thinking about this and I remembered the famous "elevator" analogy used by Einstein. The light arriving perpendicular will describe a curved path inside the vessel, just as it would do if it passed nearby a very massive body. Traveling at c (or almost), from the perspective of the vessel it'd be like if the light coming from the outside bended downwards following the shape of the container (more or less I think...).

[code]

BBBBBBBBBBBB
B B ^
perpendicular light >----------\ O WTF? B |
| B / \ B |
| B | B |
| B / \ B |
| BBBBBBBBBBBB
\------------------------------------>
[/code]
[/quote]

What are you guys talking about? Light would not follow a curved path. He described an [b]instantly accelerating object[/b]. The light would only appear to follow a curved path if the observer was accelerating.

As I said before. Just think about it. In all reference frames, the only incoming light that can strike you at an angle orthogonal to your direction of travel is light that was emitted from a reference frame similar to yours. If you are traveling near [i]c[/i] in direction [i]v[/i], only objects traveling near [i]c[/i] in direction [i]v[/i] could possibly shoot a photon at you in a path that would appear to you to be horizontal.

Edit: snipped out some stuff I had here that on second thought was incorrect.

##### Share on other sites
Black holes are actually quite easy to construct in special relativity. We know the famous E=mc^2 derived from special relativity, which gives energy as a function of mass. The gravitational potential energy of a point mass of mass m in a gravitational field made by another mass of mass M at a distance of r is -GMm/r. (G being the gravitational constant.) The total energy stored in mass m is mc^2, so it cannot possibly escape the gravitational field of mass M if GMm/r > mc^2 by conservation of energy. Solving for this, we get that r < GM/c^2 is sufficient to "trap" the mass m in M's gravitational field.

Although we assumed M and m were point masses, the equations hold so long as M is a sphere of uniform density of radius less than r. So if we take a mass M, it must be smaller than GM/c^2 for something to be sufficiently close to it to be trapped within its gravitational field. This gives the event horizon of the black hole. The actual results have general relativity considerations to be taken into account, but IIRC this simple calculation is accurate to about a factor of 2. It should be noted that [i]any[/i] mass can make a black hole so long as it's compressed small enough ( by this simple model - at some point quantum mechanics plays a role and I know nothing about that).

@ ___: The correct equations for special relativity have already been given. Yours do not take into account any special relativity: time dilation or space contraction (depending on reference frame).

##### Share on other sites
[quote name='Ezbez' timestamp='1305211212' post='4809796']
Black holes are actually quite easy to construct in special relativity. [...] we get that r < GM/c^2 is sufficient to "trap" the mass m in M's gravitational field.
[/quote]

Yes! I really like this; it's clean, simple, and gets the idea across -- which is important. However, IIRC, this "classical" derivation of the Schwarzschild radius is off by a factor of two... (I have not done the derivation to see why; my fuzzy understanding is that you need to get down into the differential geometry to see it.)

##### Share on other sites
[font=arial, verdana, tahoma, sans-serif][size=2][quote name='A Brain in a Vat' timestamp='1305210308' post='4809792']
[quote name='owl' timestamp='1305183261' post='4809686']
[quote name='szecs' timestamp='1305181762' post='4809677']
I don't think the perpendicular direction is any special.
[/quote]

I've been thinking about this and I remembered the famous "elevator" analogy used by Einstein. The light arriving perpendicular will describe a curved path inside the vessel, just as it would do if it passed nearby a very massive body. Traveling at c (or almost), from the perspective of the vessel it'd be like if the light coming from the outside bended downwards following the shape of the container (more or less I think...).

[code]

BBBBBBBBBBBB
B B ^
perpendicular light >----------\ O WTF? B |
| B / \ B |
| B | B |
| B / \ B |
| BBBBBBBBBBBB
\------------------------------------>
[/code]
[/quote]

What are you guys talking about? Light would not follow a curved path. He described an [b]instantly accelerating object[/b]. The light would only appear to follow a curved path if the observer was accelerating.[/quote]

You're right there. That drawing doesn't apply for an object that isn't accelerating.

[quote name='A Brain in a Vat' timestamp='1305210308' post='4809792']
In all reference frames, the only incoming light that can strike you at an angle orthogonal to your direction of travel is light that was emitted from a reference frame similar to yours. If you are traveling near [i]c[/i] in direction [i]v[/i], only objects traveling near [i]c[/i] in direction [i]v[/i] could possibly shoot a photon at you in a path that would appear to you to be horizontal.[/quote]
[/size][/font]

How would light emitted, say, by stars behave in relation to the moving object? For some reason (my fault) I'm having trouble picturing that.

##### Share on other sites
[font="arial, verdana, tahoma, sans-serif"][size="2"][quote name='A Brain in a Vat' timestamp='1305210308' post='4809792']
In all reference frames, the only incoming light that can strike you at an angle orthogonal to your direction of travel is light that was emitted from a reference frame similar to yours. If you are traveling near [i]c[/i] in direction [i]v[/i], only objects traveling near [i]c[/i] in direction [i]v[/i] could possibly shoot a photon at you in a path that would appear to you to be horizontal.[/quote][/size][/font]

That isn't right. You can get photons in any direction from a source that is moving in any way.

##### Share on other sites
[quote]¨A more interesting question:
How would we relativistically simulate a space-fighting game with all these time dilatations in a multiplayer game? All controls/the behavior of the spaceship would slow down compared to the outside scene? That is an interesting game design question.... [/quote]
in despite that i am a nube here, i guess that's the best answer of all...

forget all the realistic speed/time relations, it makes space games impossible, multiplayer or not.

this are at least the guessing of a frontier addicted.
1st not important for gameplay
2nd it makes them impossible from my point of view

if you're really up to a space sim (i would love to see it) concentrate on other things rather than especially something that is not to solve in a game.

almost realistic newtonian physics are more of interest to me then speed/time relations, else you can forget all the sci-fi stories and games.
there is really no proper solution for that problem, theoretically maybe but in the end it doesn't matters.

it depends a bit on the goal you have, a space flight sim like frontier don't deserves such and it's apart from that impossible to realize imo.

[b]but[/b] if it's a more strategic type of game where colonization and such plays the main part then you could think about such, i guess.
it will be then a interesting limitation of the power one has and calculations for such could be a task of the game, i think so.
means how much time will my colonization ships be on their way from the view of the "static" home world and how this will influence my progress compared to my opponent (humans or AI).
i guess if then at all one could limit space battles to a range where time dilatation plays no big role, e.g. "close" to orbit of a body.
it's to imagine that you enter (with what technology ever, gravitation lens effect or hyperspace) in a close range to the target planet.
so speed for your ships will be into a range where dilatation plays no big role and you can leave it aside.

all the space flight sims i know disrespect that fact, that's true, but i guess it's not that the devs wasn't clever enough to think about such, only that a fighting game didn't deserves it.
it's already a nearly not to solve problem to make a multiplayer game in the style of elite/frontier, we discussed that often on frontier forums and SSC.
some suggest that there is no proper way except you limit the space like in X-beyond, while others like me think that this plays no role as long as you leave the factor realistic time/speed relations aside.
i mean it should be possible to calculate the course of two space ships in a open space, doesn't matter if it controlled by a player or AI.
another problem is that such travels still take a long time in reality and who likes to wait half a day for his opponent, leave the computer until you arrived?
certainly not, you like to play and the player has to be entertained else he loses interest in the game, if he loves realism or not.
so you will need something like a "star dreamer" or to say in other words, speed up time for the flight, but there starts the real problem for the game.
one can select his own time? not possible time has to be synchronized* else you will never meet your opponent.
but i let that up to you, especially because i don't know in which direction you plan goes.

anyways this sounds interesting to me and as i am interested in any space sim game, let me know your progress.
all of us at Space Sim Central will be praise you for a new space game, flight sim or strategic.

* if you ever played frontier you will know this situation;

i am into a fight with a "imperial courier" this ship has a very special behave, as soon as it fires a missile at you, it will leave the close space using maximum time speed up, since it takes a little time until you noticed that, there is nearly no way to catch it. you see that's a serious problem and time dilatation is left aside else...

##### Share on other sites
*too lazy to read the 50 posts*

The 'speed of light' is a limit to how big the magnitude of the velocity of a object can be.

Lets say we have 2 particles, A and B

theyre next to each other, going up.

A_B

lets say theyre communicating by vibrating or something. If theyre not moving, they receive the signal at the speed of light.

If theyre going half the speed of light, the signals can only travel at half the speed of light. If you somehow manage to get them travel at the speed of light, the signals they send will be traveling at the same speed as the particle, and thus they cannot gain velocity to any other direction (unless being pulled by something, as that will redirect the force) so a particle going at light speed cannot interact with other particles unless they collide with each other or something like that...

i think you would just multiply the velocity by 1-Velocity_Magnitude / Max_Speed

Also, if the object instantly accelerates to speed of light and then stops somewhere else, it will also feel like it travelled in an instant, even if the journey took years. Basically for the object travelling, it makes it look like there was no speed limit, and that speed of light equals to infinite speed.

Thats also why it wouldnt really work for a game, because in the game the player will notice that infinite speed didnt take him there in 0 time.