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

• keeper blue
In summary: 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.
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

keeper blue said:
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.
keeper blue said:
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.

Ibix said:
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.)

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

russ_watters
hutchphd said:
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?

I am saying it is a foolish question.

hutchphd said:
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.

keeper blue said:
None of the FTL posts I have read offer any imaginary advantages to FTL,

hutchphd said:
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.

hutchphd and bob012345
Better than that restaurant on the moon. The food is good, but there's no atmosphere.
Snickers

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'.

Lren Zvsm, PeroK and hutchphd
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.

keeper blue said:
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.)

Lren Zvsm and PeroK
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?

Dale
keeper blue said:
...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.

russ_watters, phinds and bob012345
keeper blue said:
So nobody has yet offered any explaination of any difference
There is a difference between one person not reading and other people not writing.

PeroK, russ_watters and phinds
DaveC426913 said:
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.

keeper blue said:
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.

keeper blue said:
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.

Lren Zvsm and PeroK
This thread is now closed. Thanks to everyone who participated.

## 1. What is FTL and why is it important?

FTL stands for "faster-than-light" and it refers to the concept of traveling faster than the speed of light. This is important because according to Einstein's theory of relativity, the speed of light is the fastest possible speed in the universe. Therefore, the idea of FTL travel challenges our understanding of physics and has been a topic of scientific research and speculation for decades.

## 2. Is FTL travel possible?

Currently, there is no scientific evidence or technology that supports the possibility of FTL travel. While there have been theoretical proposals and experiments conducted, none have been successful in achieving FTL speeds. However, as technology and our understanding of physics advances, it is possible that FTL travel may become a reality in the future.

## 3. How does FTL travel work?

There are several proposed theories for how FTL travel could work, such as using wormholes or manipulating spacetime. However, these are all still theoretical and have not been proven to be possible. The concept of FTL travel also raises questions about causality and the possibility of time travel, which adds to the complexity of understanding how it could work.

## 4. Are there any potential consequences of FTL travel?

One potential consequence of FTL travel is the violation of causality, which is the principle that an effect cannot occur before its cause. This could lead to paradoxes and disrupt the laws of physics as we know them. Additionally, if FTL travel were to become a reality, it could have significant impacts on society, such as changing the way we view space and time, and potentially even leading to conflicts over resources in space.

## 5. How does the concept of FTL travel relate to science fiction?

FTL travel has been a popular topic in science fiction for many years, with many stories and movies featuring spaceships traveling faster than the speed of light. However, it is important to remember that these depictions are purely fictional and not based on scientific fact. While FTL travel may inspire creative storytelling, it is still an unproven concept in the realm of science.

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