"Speed of light question" Topic

Imagine 2 spaceships moving away from each other. Both moving at the speed of light. 2 questions:

1) Does that mean that in relation to each other 2 ships move faster than speed of light?

2) Is it ever possible for 2 ships to see each other since they move in opposite direction at the speed of light?

Obviously you were not paying attention during General Relativity 101.
First, the space ships can NEVER attain the speed of light.
Two space ships moving away from each other at .99c are … In a fair universe, you would be burntat the satke for that.
But note that the SHIPS are not moving at 2c but still at .99c, as they always were RELATIVE to the universe.
relative to each other does not count, because it confuses people.

I think a far better question is "How does inflation immediately after the Big Bang not violate the speed of light?"
And "How does man made global warming affect General Relativity?"

How does man made global warming affect General Relativity

Wrong forum John.

As for an answer to your second question, I would say accept the null hypothesis.

Space ships are made out of light particles and have no mass. Therefore they could move at the speed of light. I know my 101's. So, let's stay on the topic.

1) Yes, but in terms of physics it's irrelevant to the question of whether an object can travel FTL. At most it's just a "neat trick you can do with math," as the relevant factor is only the ship's speed relative to the Universe (for lack of a better term). Think of the classic two trains on opposite tracks. Both may be doing 100mph, and thus separating at a relative 200mph, but neither is getting to either Boston or Chicago any quicker because of that.

2) Interesting question. Kicking Einstein aside to make this situation possible, the last image they see of each other will be from the initial departure. After that, they each outdistance any light coming from the sister ship. However, this brings up the issue of what you can "see" if you travel at the speed of light, as presumably you're travelling at a speed that would prevent any light behind you from "catching up." If you're always facing backwards, then it seems to me that light remains in contact with your retina, and I have no idea how that affects the working of the eye. In order for you to see something, the light from it must strike your retina, triggering the electrochemical signals that the brain interprets as vision. If the light hits and then you move away instantly at the speed of light, without changing your direction of vision, what does that do to the light you are receiving when it all happens? Does it sit there against the retina? Since I assume the impact of the photon is necessary for the retina to function, it would seem to me that perhaps you would see the ship and then instantly it would disappear from your vision, as the photons from it can't strike your retina any more to trigger the vision process.
Might there be a red shift affect initially? I don't know.
Certainly if you turn around and face forward, you'd see objects approaching as blue shifted (as well as possibly other bizarre effects as you keep ramming into photons moving twice the relative speed your retinas are used to). If you then turn back around to face the light travelling with you, I think you wouldn't see it (because it's not hitting your retina). I'm not even gonna try to think about what happens if you then take a step closer to the rear window, thus entering into light previously unseen (and reducing your own speed below the speed of light? Yipes… That one makes my head go all wobbly…)

1) Does that mean that in relation to each other 2 ships move faster than speed of light?

In a sense, yes, but not really. ;->=

The problem with your questions is you still assume a fixed point of reference in the universe. Alfred claims 'no such thing'.

From our point of view, they have a combined velocity of greater than light, but the point is, to us, they are each slower than. While, they're own sense of time distorts such that NEITHER is receding at greater than lightspeed to each other. Remember it's called 'relativity' for a reason.

As for for seeing something else at lightspeed, no, but, as is said, you can never actually attain the speed of light. As your speed increases, a) your mass increases approaching infinity as a limit, b) your time dilation warps your perceptions.

And if you think I actually have a clue, sorry, wrong again.

Doug

PS And, no, not light particles: wavicles.

Even better is if you manage to get to the speed of light you can never slow down again since time has stopped for you. If you get around that by using a stutterwarp then the ships do not see each other since light never catches up.

The Traveller 2300 setting is neat like that, ships got FTL till they get in a system and then slow down. So you see a ship going in two directions at once the instant they cross that boundry (and the light from the two events reaches you).

Crickies, I actually typed 'Alfred' when I meant 'Albert"?!?

Will I ever live through the shame?!?

Doug

Edit: Of course, we've not even touched on the whole 'mass warps space, so as your mass approaches infinite…'

Has anyone heard lately about trans-light-speed? I believe superluminal was possible IF you could get to FTL without first being near lightspeed?

I think a far better question is "How does inflation immediately after the Big Bang not violate the speed of light?"

IIRC Spacetime was expanding faster than the speed of light, not physical matter. This is why warp engines could (theoretically) work.

Damon.

1) Does that mean that in relation to each other 2 ships move faster than speed of light?

Not if you use Lorentz transformations …. they still move at the speed of light. Remember the Michelson Morley experiment.

This is why space combat games should stick to worm holes and jump drives and just say no to FTL.

Spacetime was expanding faster than the speed of light,

But there was matter in that universe; I know, I've heard it was space that was expanding, but it does seem things were being separated superliminally. They just weren't 'moving'.

Like I said, I plead clueless. I just watch the specials for the pretty pictures…

Doug

Relativity creates a bizarre image.

At nearing light speed the person in such a ship first sees the universe start to compress into the forward part of their view until it becomes a fuzzy orb in front of them. Basically, things behind them where light cannot catch up does not exist in their universe. What they would be left with just before they vanished from our universe (and achieve light speed) would be the cosmic radiation initial glow of the big bang in front of them shifted into visible light. As they go faster the light would also compress into lower wave length x-rays. Unless they were well shielded, they wouldn't live long. A grain of dust sitting in their path while they were at 99% light speed would be all it takes.

link

What they do not see is the ridiculousness of rushing stars seen in Star wars.

Mostly what happens is you look at the other ship, don't see anything, then go crazy. So even if you did see it, you wouldn't care.

Crickies, I actually typed 'Alfred' when I meant 'Albert"?!?

Will I ever live through the shame?!?

Just blame autocorrect.

Thanks to everyone with thoughtful answers. Excellent link too, EJNashIII. Keep it up. Except for the troll who had nothing smart to say.

Answer:

The ships from a third observer's perspective would both be going just under the speed of light. From an observer on either ship, the other ship would be moving away at just under the speed of light. Part of the reason why this is true is that time on the two ships slows down as the ships speed up.

Note the "just under the speed of light." The math works in such a way that light speed is a barrier — an object that starts out going the speed of light will never go the speed of light or faster, while an object that's somehow already going faster than the speed of light can't drop below light speed.

If I understand Relativity correctly, as you get closer to the speed of light, time slows until you reach the speed of light at which point time would stop. It would also take an infinite amount of energy to get you there.

I don't think you will be able to "see" the other ship. At the speed of light both ships are not only moving apart in space, but in time too, relative to their starting point. At the speed of light the passengers would stop aging altogether, while anyone at their point of origin would have aged a gazillion years or something…Argh. Here. This link explains it…I think. I read it until my head exploded.

link

I think this conversation made me snap a hamstring in my brain

I would think that saying that space ships are made out of light waves would also come under the heading of "having nothing smart to say ".
Iron? Steel? Titanium? The people inside? Made out of light waves?
What strange sci fi have you been reading?

Light particles. Excuse me.

Gets worse; may be, not 'particles', but 'particle'.

One. Across the entire spacetime.

T'was an early theory in the search that led to string theory, but, while less attractive than before, I don't think it's ever been totally abandoned.

And, of course, the term wavicle implies that it's both a particle, and neither. Or maybe. There's always the indeterminate of quantum mechanics.

Get a tarp.

Doug

link

Awesome link!

Been there; it has it's own problems. Slowness in free fall is half explained by the article, and the other half, in the case of ST advanced tech, the handwavium 'inertial dampers', without which, everybody would be jelly.

Doug

Seriously, worm holes and jump drives are the way to go in space combat games. Much of what we think we know about physics tells us that FTL is probably impossible. Even if it were possible, combat at FTL speeds would be impractical at best if not impossible.

Also, FTL combat isn't necessary for the genre. FTL travel need only exist for the purpose of allowing the opposing galactic empires to come into conflict with each other. To that end, worm holes provide a far more interesting means of explaining away the FTL travel as they provide a nice fixed strategic objective for the opposing galactic empires to fight over and a reason for the ships to engage in combat rather than just use their FTL ability to evade the enemy.

If we assume that Alcubierre is right, and that we can somehow produce exotic matter, then actual FTL combat would be fought using FTL missiles.

And as big as those first two "ifs" are, the challenge that might be hardest is target detection.

Detect by gravity? How fast does gravity "propagate"?

Also, combat in an FTL universe would be Mahanian in nature.

@Daricles: "FTL is probably impossible."

Well, yes and no. No, it's impossible to accelerate matter to or beyond the speed of light. But it's not impossible to move a portion of spacetime faster than the speed of light, and thereby transport matter within that portion at the same speeds. So the matter itself never achieves FTL within its frame of reference (as I understand it, it never accelerates or gains any velocity at all), but its frame of reference changes position within the universe at FTL velocity relative to the universe itself. Or something like that. Anyway, NASA is very much studying this concept, and the current TMP discussion is here: TMP link

But in short, if such a system works, then the warp vessel doesn't need "inertial dampeners" or other handwavium solutions because the vessel never gains inertia from the process. It would simply have whatever velocity and vector it had when the warp drive was engaged.
I doubt that the system would allow for combat at warp speeds, if it even allowed any contact with the universe outside the warp bubble at all. That's way beyond my pay grade, is all I can say.

Also, combat in an FTL universe would be Mahanian in nature.

I assume by this you mean that it will be centered around destinations and "ports," that is within a planetary system, rather than ever in the interstellar void. I agree. I can't think of how one would detect or intercept a warp vessel until it appeared, so arrival and departure points are the only viable sites for combat, which would necessarily be at sublight speeds.

If alcubierre is right and if the exotic matter exists and if we can produce it in sufficient quantities we would still be very long way from fitting the drive system into a missile and having enough exotic matter fuel to waste on munitions.

I agree that if FTL travel is achieved that threat detection and tracking becomes the next difficult obstacle to overcome.

I believe space combat would definitely center around strategic points.

I assume by this you mean that it will be centered around destinations and "ports," that is within a planetary system, rather than ever in the interstellar void.

You are correct.

(I love that folks on this board are knowledgeable enough that I don't always need 100 words to explain something.)

… having enough exotic matter fuel to waste on munitions.

This is true, but the alternative would require you to be in front of your opponent, and fire into their path.

…

(Scribbles notes furiously for a Starships scenario special rule.)

Or wait for them to drop out of FTL to engage.

If we're positing an Alcubierre drive, waiting might not be the smartest thing. There's possibly a nasty gamma ray burst that would precede a ship leaving warp, which means that the ship waiting in normal space could be targeted.

FTL tactics in an Alcubierre universe are potentially quite interesting.

I think this conversation made me snap a hamstring in my brain

Yeah, getting anywhere *near* relativity-level math does the same thing to me.

…then the warp vessel doesn't need "inertial dampeners" or other handwavium solutions…

Just to be clear, ST tech includes FTL and STL 'impulse', with an implied acceleration with high likelihood of a jelly factor. My comment was about the website comments about freefall, not FTL.

Thanks, Parzival! Given the conversation of most of the folks I hang with, 'Mahan' brings 'manifest destiny' and imperialism to mind, rather than his points derived from Jomini.

Doug

I've read SF settings that have such high accelerations, however hard the setting, without mashing the occupants. It simply takes a highly engineered device to protect the bodies. This is usually in the form of a couch, or perhaps coccoon. Having a leg over the side at that point usually results in breaking it. Being completely outside the unit means the ship should stock some mops and buckets.

Last I read, you don't need to make FTL warp missiles and carry them on the ship. The theory went that when the craft entered a system and left warp it would have an effect comparable to the Sun Crusher. This was only last years theory, BTW. And the exotic matter was, according to the most reputable place, dark matter. Which, granted, is still rather exotic!

Lucky caesar isn't here (yet), you know how he keeps confusing "under scientific conjecture" for "in development". Like last year when he mistook a heat ray for a tractor beam, just because it had that effect at the tiniest level most universities can observe, and the news article reporting this discovery ended up somehow full of Star Trek videos.

An Alcubierre drive relies on so much that we haven't found, being found, that in fact we may as well be waiting for the discovery of magic and development of techno-wizardry to the point of rift-drives. I.e. it's so impractical it may as well be impossible.

"What strange sci fi have you been reading?"
Red Dwarf?

Sorry, the bench or cocoon simply isn't enough at high enough accels, not even Haldeman's fluid-filled cocoons and residents.

However, apologies to all, as this is rather OT.

Doug

The theory went that when the craft entered a system and left warp it would have an effect comparable to the Sun Crusher

I'm taking that claim with a grain of salt greater than the grain of salt that already is required for the warp drive.

Maybe that theory just aimed it too close?
"Sorry, the bench or cocoon simply isn't enough at high enough accels, not even Haldeman's fluid-filled cocoons and residents."
Best to drive slower.

I'm taking that claim with a grain of salt greater than the grain of salt that already is required for the warp drive.

The trick is the so-called exotic matter. There's apparently some math that indicates that it is possible, and would be a particle made from a combination of four specific quarks.

So, yeah, we're really in science fiction here — humans can create previously unseen sub-atomic particles, and mass production of a specific one might be the key to FTL travel.