Flashlights on an FTL ship

[Tankbuster32]

So I've been thinking lately, what if you were onboard a fictional spacecraft that was capable of FTL travel, and lit a flashlight. Wouldn't the photons inherit the velocity of the craft and exceed the speed of light themselves? Now FTL travel is of course completely fictional and we don't know how matter behaves even at nearlight speeds, or how space behaves for that matter. Who says that space is the frictionless vacuum we know of at those speeds. Anyway someone who is better at physics than I am could probably answer this and that's why I'm writing this.

[maltesh]

Unless and until you define your fictional method of FTL travel, the question is impossible to answer. It's like asking "Could you shoot down a dragon with an arrow?" without explaining which of the thousands of different types of dragons in the thousands of myths, books, legends, films, and tv shows that have dragons in them that you're talking about.

[Albert VDS]

If you want your fictional spacecraft to abide to the law of nature then it couldn't go faster than light.

If it uses an Alcubierre drive then timespace in the craft would work the same as if you weren't moving.

This is because the craft would be inside a "bubble" with spacetime being contracted and expanded in the front and back,

which wouldn't have effect on the spacetime inside the bubble.

So light would work the same in side your craft.

[K^2]

There are some papers out there that describe null geodesics of the Alcubierre metric. (Clark et al., Muller et al.) These are very technical, but the conclusion is pretty straight forward. Standing on the bridge of a warp ship, you will only see a fraction of the universe. There is going to be a blind spot behind the ship from which the light cannot reach the bridge. However, you won't actually see a blind spot, because the portion of the universe you can see will be stretched to fill the 360 degree view. Similarly, if you turn on a lamp on the ship, there will be a dark spot ahead of the ship where the light from the lamp will not reach. So even if you shine light straight ahead, it won't come out from the warp bubble going straight ahead, but rather at an angle.

[Tankbuster32]

I'm thinking of something different than a warp bubble, because inside that bubble you wouldn't be traviling faster than light but instead be "moving the universe around you". I'm thinking more like something capable of straight up pushing past the speed of light (completely theoretical of course). Nearlight speeds would also suffice, but thanks for the answers guys, I should get to reading those papers though.

[K^2]

something capable of straight up pushing past the speed of light (completely theoretical of course).

You are asking a theoretical question based on assumption that violates core assumptions of said theory. Therefore, the correct answer to your question is pink elephants.

[Smidge204]

Wouldn't the photons inherit the velocity of the craft and exceed the speed of light themselves?

No, because the speed of light is always the same relative to the observer. Even if you are going 99.999% the speed of light (since you can't actually get to 100%), light still appears, to you, to be traveling at the speed of light relative to you. If you hypothetically travel at the speed of light, the math says you literally divide by zero.

=Smidge=

[Tankbuster32]

my question has been answered, thanks guys! Also, wait... the speed of light is relative to the observer!? I should read more physics...

[Albert VDS]

my question has been answered, thanks guys! Also, wait... the speed of light is relative to the observer!? I should read more physics...

Note: Observer should be everything.

Light travels at the same speed relative to everything.

Imagine you are going almost the speed of light, in your spacecraft, and you shine a flash light at the wall.

Light would still reach the wall with the same speed relative to the spacecraft.

That does sound like the flashlight light is traveling faster than the speed of light, but it doesn't.

It's the spacecraft which changes, it contracts in the direction it's moving and time slows down.

So the light from the flashlight only travels a shorter distance.

Now what would happen if the space craft actually traveled at the speed of light?

It's size would contract to 0, time would stop and it's mass would become infinite.

[Monkeh]

Your question about photons inheriting the speed from the vessel could apply to a car as well. You don't need to going a significant fraction of c to realise it can't work that way.

A car may only be travelling at 30 mph but c+30 mph is still >c. Even walking forward with a torch would achieve a similar result.

[jwenting]

You are asking a theoretical question based on assumption that violates core assumptions of said theory. Therefore, the correct answer to your question is pink elephants.

no! purple with yellow stripes.

[Sternface]

There is another large problem with going the speed of light. Time and distance would no longer be in effect, or would no longer be what we think of as distance and time. Time dilation and Lorentz contraction in Special relativity is based on massive particle traveling at < C. Extending these SR's equations to C is probably not going to be correct. Such a frame of reference is incomprehensible, because such a frame of reference cannot exist. At least under special relativity.

[Traches]

OP, you should read up on special relativity. It's wonderfully interesting, and there are a lot of good explanations and thought experiments out there, including ones like yours.

[Nuke]

i sometimes wonder if on a relativistic (not ftl) ship going in the neibothood of 99.999999 the speed of light if it would be like walking uphil to walk towards the front of the ship (assuming said ship had a mans of gravity, like a centrifuge) because of the mass inherited from trying to go faster.

[ZetaX]

No, one of the main properties of special relativity is that only relative speeds matters, or more generally that there is no special distinguished speed like "standing still". It is therefore completely indistinguishable from the inside whether the space ship is "moving" (which, without another object to compare it to, doesn't even make sense in the context of special relativity as said in the first sentence). What you can distinguish is when it accelerates (but then, general relativity is mostly equivalent to the claim that this is indistinguishable from gravity).

[TheGatesofLogic]

best thing about light? if it could coalesce into something sentient it would not experience time

[ZetaX]

That's like saying "green protons are dirty". You are applying properties to things they are not defined nor made for.

[K^2]

That's like saying "green protons are dirty".

*Cough cough*

[ZetaX]

Good look in defining "dirty" after you create the first non-color-neutral proton ;-)

[K^2]

This is kind of up to interpretation, but if you describe strong interaction between nucleons as virtual gluon exchange, then the nucleons can be non-color-neutral during the exchange. So a green proton is easy. I know that's not what you meant, but I found it humorous.

No ideas on the "dirty" part, though. Sorry.

[Savage117]

Iv always thought it was funny that people who say, "truly traveling faster than light, as in speeds greater than C, is impossible." How can people say that? It can't be tested or proved as of yet. I'm aware a very very intelligent man came up with the theory, but even he said it was just that, a theory. Until we prove it beyond a doubt, I will still say that faster than light travel is not impossible it's just improbable. But evidence does show that that very very smart man was probably right, so its more likely we find a way around the barrier instead of through it.

Back on topic, I believe what the other posters are saying is true, you would see no difference, what fascinates me is what would be happening outside. Say you had a ship that could some how break the light barrier, not go around it but through it, what would you see outside, darkness? I know you would see darkness ahead of you as you approached light speed, but what would you see after breaking through it? Also there is a hitch in breaking light speed other than the theory of special relativity. The craft in question, even if it was only going over the speed of light by 1%, would be going ridiculously fast, there is no computer on this planet that could keep up with that kind of speed, you would blink and then be outside the solar system. You would have to figure everything before entering FTL or you would slam into a star or an asteroid before you even registered you had let your finger off the button. It would be like sailors plotting courses through unknown waters all over again, but this time if you mess up you don't sink, you get atomized.

[Jim DiGriz]

So I've been thinking lately, what if you were onboard a fictional spacecraft that was capable of FTL travel, and lit a flashlight. Wouldn't the photons inherit the velocity of the craft and exceed the speed of light themselves? Now FTL travel is of course completely fictional and we don't know how matter behaves even at nearlight speeds, or how space behaves for that matter. Who says that space is the frictionless vacuum we know of at those speeds. Anyway someone who is better at physics than I am could probably answer this and that's why I'm writing this.

If you set aside FTL or light-speed travel and just consider large fractions of the speed of light, then you've arrived at the thought experiment that special relativity solves.

Instead think about what happens when you're moving at 90% of c. If you turn on a strobe do the photons going in front of you seem to travel at 10% of c, while those behind you travel at 190% of c?

The answer is that you observe light to travel at c no matter what your reference frame is. You will measure the light beam out of the front of your space ship as being 100% of c and the same with the tail lights.

An observer back on Earth will also measure both your headlights and taillights to be traveling at c as well. The speed of light is invariant under transformations between non-accelerating reference frames. Work out that statement in math and you'll get the time dilation and length contraction of special relativity.

[Jim DiGriz]

Iv always thought it was funny that people who say, "truly traveling faster than light, as in speeds greater than C, is impossible." How can people say that? It can't be tested or proved as of yet. I'm aware a very very intelligent man came up with the theory, but even he said it was just that, a theory. Until we prove it beyond a doubt, I will still say that faster than light travel is not impossible it's just improbable. But evidence does show that that very very smart man was probably right, so its more likely we find a way around the barrier instead of through it.

FTL travel in one reference frame is also traveling backwards in time in another reference frame, so if you can travel FTL you can go back in time, then you wind up killing your grandfather or hitler or whatever...

[Depth]

Iv always thought it was funny that people who say, "truly traveling faster than light, as in speeds greater than C, is impossible." How can people say that? It can't be tested or proved as of yet.

I will still say that faster than light travel is not impossible it's just improbable.

The faster an object goes the more mass it has. Closer to the speed of light the mass increases exponentially until approaching infinity. Since E=Mc², the amount of energy would also approach infinity.

Even if we had infinite energy, the mass of the object would also be infinite, so what happens when an unstoppable force meets an immovable object? Is it either impossible or is it just improbable that the object would move?

[Seret]

Iv always thought it was funny that people who say, "truly traveling faster than light, as in speeds greater than C, is impossible." How can people say that? It can't be tested or proved as of yet. I'm aware a very very intelligent man came up with the theory, but even he said it was just that, a theory. Until we prove it beyond a doubt, I will still say that faster than light travel is not impossible it's just improbable. But evidence does show that that very very smart man was probably right, so its more likely we find a way around the barrier instead of through it.

Einstein's theory of relativity does explain some observations better than preceding theories, and it made predictions which have subsequently been confirmed by new observations. So it has in fact been proven to a high standard. Denoting something a "theory" in science actually refers to a fairly complete idea with a large body of supporting evidence.

No, it's not a complete description of all phenomena in the universe, but it has been shown to be highly accurate. So if the most accurate theoretical framework we have says that light speed does present certain insurmountable practical barriers, then we'd be foolish to say otherwise.