Why do we need FTL if traveling at the speed of light is instantaneous?

[keeper blue]

I notice that quite a number of threads revolve around FTL matters. My understanding is that if a body is traveling at the speed of light, then according to its on-board clock, departure and arrival are instantaneous irrespective of the distances involved. Why then does one need to posit FTL? What perceived or imaginary advantages does FTL provide? O_O

 

[Ibix]

My understanding is that if a body is traveling at the speed of light, then according to its on-board clock, departure and arrival are instantaneous irrespective of the distances involved.

No, although it's a common misconception. Something that can have a clock and is travelling at light speed turns out to be a contradiction in terms. Asking how much time elapses for something travelling at the speed of light is like asking what green smells like - it makes no sense.

And the fuel cost of accelerating to near light speed is stupid anyway - billions of tons of antimatter to move a 1kg mass.

Why then does one need to posit FTL? What perceived or imaginary advantages does FTL provide?

You can get to other stars and back in an arbitrarily short time by your clock if you travel arbitrarily close to $c$, but the loved ones you left at home will be dead and gone by the time you get back.

 

[keeper blue]

No, although it's a common misconception. Something that can have a clock and is travelling at light speed turns out to be a contradiction in terms. Asking how much time elapses for something travelling at the speed of light is like asking what green smells like - it makes no sense.

And the fuel cost of accelerating to near light speed is stupid anyway - billions of tons of antimatter to move a 1kg mass.

You can get to other stars and back in an arbitrarily short time by your clock if you travel arbitrarily close to $c$, but the loved ones you left at home will be dead and gone by the time you get back.

Hi again Ibix, everything you have said is a given for $C$ or close to it, but it still doesn't explain the fascination and benefits of FTL. (Oh and green smells like anchovies... trust me.)

 

[hutchphd]

Because if you wish to travel to Aldebaran IX for an exquisite dinner it would be more fun if your children were not in their graves upon your return.
That would be one benefit

 

[keeper blue]

Because if you wish to travel to Aldebaran IX for an exquisite dinner it would be more fun if your children were not in their graves upon your return.
That would be one benefit

So are you saying that relativity does not apply to FTL?

 

[hutchphd]

I am saying it is a foolish question.

 

[keeper blue]

I am saying it is a foolish question.

"Why then does one need to posit FTL? What perceived or imaginary advantages does FTL provide?"

Why are they foolish questions? None of the FTL posts I have read offer any imaginary advantages to FTL, so I am asking why people posit it in the first place.

 

[Vanadium 50]

None of the FTL posts I have read offer any imaginary advantages to FTL,

Then you need to read them again. Start with #2.

 

[Vanadium 50]

if you wish to travel to Aldebaran IX for an exquisite dinner

Better than that restaurant on the moon. The food is good, but there's no atmosphere.

 

[keeper blue]

Better than that restaurant on the moon. The food is good, but there's no atmosphere.

Snickers

 

[DaveC426913]

You're barking up two separate trees.

STL travel is based in known physics. It's required for a realistic story, but the imposes colossal limits on the story - namely, that interstellar travel is extremely time-consuming and awkward.

FTL travel requires some aspect of fantasy - whatever your preferred flavour of transport is, it's not part of the science we know today (and that includes the Alcubierre drive). But it opens up the whole galaxy (and occasionally beyond) for stories. In particular, it opens up the possibility of first contact with alien species on their home turf.

Larry Niven's has a rule that is essentially: If you have to lie, lie early. The bigger the lie the earlier you have to tell it. So, if you choose to ignore the relativistic effects of STL then get comfortable with the idea that your story has at least that component of 'fiction' in your 'science fiction'.

 

[keeper blue]

Hi Dave, I understand that. The crux of my question revolves around what does FTL offer that $C$ does not? If traveling at 99.99r% of $C$ gets me on my ship from A to B in an instant (irrespective of the distance involved), what imaginary benefit does FTL give over and above this? I just cannot see the need to posit FTL in a SciFi novel.

 

[DaveC426913]

Hi Dave, I understand that. The crux of my question revolves around what does FTL offer that $C$ does not? If traveling at 99.99r% of $C$ gets me on my ship from A to B in an instant (irrespective of the distance involved), what imaginary benefit does FTL give over and above this? I just cannot see the need to posit FTL in a SciFi novel.

Because - as several people have pointed out - its a one-way trip.

You can get there very fast subjectively, but thousands - or millions - of years will have passed both back home and at your destination. (Heck your target star may have exploded by the time you arrive!)

That severely restricts your story ideas to colony ships. They better have brought a working colony with them or they're space toast.

(Unless you're Larry Niven. In A World Out of Time, Jaybee Corbell came back to an Earth 3 million years older than the one he left.)

 

[keeper blue]

So nobody has yet offered any explaination of any difference between FTL and Light Speed in terms of the relativistic effects... so I ask my question again, why posit FTL if there are no imaginary benefits and it behaves exactly the same as traveling at the speed of light?

 

[DaveC426913]

...it behaves exactly the same as traveling at the speed of light?

IT DOESN'T.

PLEASE SEE POST 13. Going to request that OP actually read the copious responses he has gotten before responding.

 

[Vanadium 50]

So nobody has yet offered any explaination of any difference

There is a difference between one person not reading and other people not writing.

 

[keeper blue]

IT DOESN'T.

PLEASE SEE POST 13. Going to request that OP actually read the copious responses he has gotten before responding.

So what you are saying is that if we posit FTL we can ignore the relativistic effects? In other words, don't let science stand in the way of a good story. That is ok.

 

[PeterDonis]

My understanding is that if a body is traveling at the speed of light, then according to its on-board clock, departure and arrival are instantaneous irrespective of the distances involved.

Your understanding is wrong. The correct understanding is that a lightlike object, like a light ray, cannot even have an "onboard clock" at all; the whole concept of "clock time" (the correct term is "proper time") does not even apply to it.

You will find many, many, many previous PF threads in the relativity forum on this topic. Please go read them.

 

[PeterDonis]

Why then does one need to posit FTL?

SF story tellers generally posit FTL without even developing an actual theory of how it works; they do it because it makes it much easier to construct a story that covers interstellar distances. So it is pointless to try to figure out what physical implications of FTL based on actual physical theories drove their decisions. There weren't any.

 

[PeterDonis]

This thread is now closed. Thanks to everyone who participated.