# I Why would we need faster than light travel?

#### PAllen

So with respect to what Chris and jbriggs44 were saying; Chris didn't mention which of the two frames were making the measurement so the statement "distance separating two frames of reference" is non sensible ?

#### rootone

Yes, because the distance is an outcome of the relative velocity of the frames being considered.
An observer in frame A will draw the same conclusion as an observer in frame B.
Other observers in frame C and D and so on would disagree that the speed of A and B are the same.
If observer C is on a moon close to observer A, then A isn't moving much, B is doing all the moving.

Last edited:

#### nitsuj

oh...it was so obvious what the op meant i thought it must be something im missing....op didnt even say how this measurement would be done.

#### nitsuj

I've gained a lot of better understanding here, but it still seems that if I accelerate at 1g, I'll reach (and maybe exceed) c, relative to earth's frame of ref, in under one of my years. And, in my frame of ref, I'll expend no unusual amount of e. Of course no one on earth will/can ever observe me doing this this, just as they could not see me fall into a black hole, even though, in my frame of ref, I will have.

Just a point about accelerating and c being an invariant speed....while the phrase "The maximum speed is c" implies it, it is not a "physical barrier" that prohibits you from motion. Geometry is used to determine speed, the geometry changes comparatively for relative motion....emergent from relative motion is a MEASURED and CALCULATED speed.

From your perspective you can continuously accelerate towards c. There will be no fancy or weird physics happening when accelerating 1g at 1,000km/s, 200,000km/s or even 299,785km/s...you will merrily be on your way to approaching c (just as you are now in some FOR)...but for us watching we see that comparatively your proper time is ticking more and more slowly and your meter ruler is contracting as you approach c. Converging to a point where time and length will be null at the exact point you reach c....we observers say you'll never reach c....you say just gimme more time and space I'm making progress.

While just a preference, I personally don't feel as though speed is the constraint, it's the geometry of time and length that prohibit FTL speeds. Just a bit more down that road....the common concept being c is invariant as a consequence of causality.

#### phinds

Gold Member
So with respect to what Chris and jbriggs44 were saying; Chris didn't mention which of the two frames were making the measurement so the statement "distance separating two frames of reference" is non sensible ?
I disagree. The reason it is nonsensical it NOT because he didn't mention in which of the two frames a measurement was made, it is nonsensical because it describes an activity (measuring the distance between two frames) that just flat does not make any sense. For example, Let's say that you have a frame of reference that is taken as being measured with an origin at a particular point on the moon, with X, Y, and Z axes defined in some understandable way and you have have another FOR that is defined as being at a particular point on the Earth, again with well defined X, Y, and Z axes. Now you could measure the distance between those points of origin but that would not in any real or helpful sense be the "distance between the two FORs" because in nether case is the FOR its point of origin. The FOR encompasses all of space. The FOR on the moon includes the origin of the Earth FOR (and all the rest of space) and the FOR on the Earth encompasses the origin of the moon's FOR. There IS no "distance" between the two FORs, they are just different ways of measuring points throughout space.

#### Chris Miller

From your perspective you can continuously accelerate towards c. There will be no fancy or weird physics happening when accelerating 1g at 1,000km/s, 200,000km/s or even 299,785km/s...you will merrily be on your way to approaching c (just as you are now in some FOR)...but for us watching we see that comparatively your proper time is ticking more and more slowly and your meter ruler is contracting as you approach c. Converging to a point where time and length will be null at the exact point you reach c....we observers say you'll never reach c....you say just gimme more time and space I'm making progress.
Thanks so much nitsuj. Though maybe it should worry you that this is exactly what I've been trying and mostly failing to say. I guess it's not (yet) possible to say what happens in that (i.e., my) final second, any more than what happened "before" the big bang.

#### jbriggs444

Homework Helper
Thanks so much nitsuj. Though maybe it should worry you that this is exactly what I've been trying and mostly failing to say. I guess it's not (yet) possible to say what happens in that (i.e., my) final second, any more than what happened "before" the big bang.
What "final second"?

You may be groping toward the notion of a Rindler horizon, but a traveler who undergoes a finite proper acceleration for a finite proper time never steps across such a horizon

#### Chris Miller

Wikipedia: "For the moment, we simply consider the Rindler horizon as the boundary of the Rindler coordinates. Later we will see that it is in fact analogous in some important respects, to the event horizon of a black hole." In Liu Cixin's "Death's End" a character jumps into a man made black hole. Observers never see him fall through its event horizon, but rather frozen forever in time. So, even though it's alleged that in his frame of reference he has fallen through, his insurance company refuses to pay death benefits claiming the policy should be bound to the frame in which it was purchased. Anyway, I'm wondering if it's like that with c. My "final second" is the second over which my additional 10 m/sec would take me over 299792458 m/sec (were there the time and space).

#### jbriggs444

Homework Helper
On these forums, Wikipedia is not an acceptable reference. It is certainly not good enough to serve as support for speculations of the sort that you are stubbornly clinging to here. Skipping past that concern...

My "final second" is the second over which my additional 10 m/sec would take me over 299792458 m/sec
There is no such second. No matter how long you thrust at 1g and no matter how far you succeed in getting as a result, the next 10 meters per second of self-relative delta V you gain will still leave you moving at less than the speed of light.

Last edited:

#### Chris Miller

True from your frame of reference.

#### PeroK

Homework Helper
Gold Member
2018 Award
True from your frame of reference.
In your own frame of reference you are always at rest, proper acceleration or not. So, reaching $c$ in your own reference frame is an even more forlorn hope!

#### nitsuj

True from your frame of reference.
I think what is being so elusive is the concept of a "finite" infinity. I think if you learn about infinities it'll be clear that there is ZERO possibility to accelerate to c, you go for ever and never reach it...given your persistence, I have concern that you would use up all the "workable" energy in the universe trying to succeed :D

#### Mister T

Gold Member
Thanks so much nitsuj. Though maybe it should worry you that this is exactly what I've been trying and mostly failing to say. I guess it's not (yet) possible to say what happens in that (i.e., my) final second, any more than what happened "before" the big bang.
But what he said is just as wrong as what you've been saying! Here is the corrected version of the portion of the @nitsuj post that you quoted:

From your perspective you can continuously accelerate towards c. There will be no fancy or weird physics happening when accelerating 1g at 1,000km/s, 200,000km/s or even 299,785km/s...you will merrily be on your way to approaching c (just as you are now in some FOR)...but for us watching we see that comparatively your proper time is ticking more and more slowly and your meter ruler is contracting as you approach c. Converging to a point where time and length will be null at the exact point you reach c....we observers say you'll never reach c....you say just gimme more time and space I'm making progress.
You indeed can continue to make progress. You cannot reach a speed of $c$ according to that observer or indeed according to any observer. What you are increasing is the Lorentz factor $\gamma$. That will continue to increase as you continue to apply the force with your rocket engines. There is no limit on how large you can make $\gamma$. This is a fact of life for all of the scientists, engineers, and technicians who accelerate particles for a living.

Let $\epsilon$ be a small positive number equal to the difference between your speed relative to Earth and the speed $c$. There is no limit on how small you can make $\epsilon$, but it will always be nonzero.

#### nitsuj

But what he said is just as wrong as what you've been saying! Here is the corrected version of the portion of the @nitsuj post that you quoted:

You indeed can continue to make progress. You cannot reach a speed of $c$ according to that observer or indeed according to any observer. What you are increasing is the Lorentz factor $\gamma$. That will continue to increase as you continue to apply the force with your rocket engines. There is no limit on how large you can make $\gamma$. This is a fact of life for all of the scientists, engineers, and technicians who accelerate particles for a living.

Let $\epsilon$ be a small positive number equal to the difference between your speed relative to Earth and the speed $c$. There is no limit on how small you can make $\epsilon$, but it will always be nonzero.
Huh? time and length do comparatively reduce to null at c, to your point of gamma. I specifically said distant observers would clearly see he wont reach c, but that his perspective he will always make progress...as easily as he can accelerate right now. I guess my point was its no different than the physics he experiences right now...that c is postulated to be invariant and that motion is relative.

Chris Miller what I said is right, but ah to satisfy the issue grammatically "Converging to a point where time and length are null at c."

#### nitsuj

I disagree. The reason it is nonsensical it NOT because he didn't mention in which of the two frames a measurement was made, it is nonsensical because it describes an activity (measuring the distance between two frames) that just flat does not make any sense. For example, Let's say that you have a frame of reference that is taken as being measured with an origin at a particular point on the moon, with X, Y, and Z axes defined in some understandable way and you have have another FOR that is defined as being at a particular point on the Earth, again with well defined X, Y, and Z axes. Now you could measure the distance between those points of origin but that would not in any real or helpful sense be the "distance between the two FORs" because in nether case is the FOR its point of origin. The FOR encompasses all of space. The FOR on the moon includes the origin of the Earth FOR (and all the rest of space) and the FOR on the Earth encompasses the origin of the moon's FOR. There IS no "distance" between the two FORs, they are just different ways of measuring points throughout space.

Cool thanks for the explanation phinds! I better understand what an FOR is.

#### PAllen

Huh? time and length do comparatively reduce to null at c, to your point of gamma. I specifically said distant observers would clearly see he wont reach c, but that his perspective he will always make progress...as easily as he can accelerate right now. I guess my point was its no different than the physics he experiences right now...that c is postulated to be invariant and that motion is relative.

Chris Miller what I said is right, but ah to satisfy the issue grammatically "Converging to a point where time and length are null at c."
Converging for who, how? For you, in you rocket, rulers and clocks seem perfectly normal even if you've been accelerating for a billion years. For earth, you are still moving less than c, and every event in your history can eventually be seen. Neither one ever gets closer to experiencing no time or distance.

Of interest is that the speed of earth per the rocket, using the most common convention for a noninertial frame, approaches zero, not c, with unbounded red shift being ever more due to pseudo gravitational shift.

#### nitsuj

Converging for who, how? For you, in you rocket, rulers and clocks seem perfectly normal even if you've been accelerating for a billion years. For earth, you are still moving less than c, and every event in your history can eventually be seen. Neither one ever gets closer to experiencing no time or distance.

Of interest is that the speed of earth per the rocket, using the most common convention for a noninertial frame, approaches zero, not c, with unbounded red shift being ever more due to pseudo gravitational shift.
Me the at rest guy making measurements of OP's rocket..seeing that as he speeds up his length measurements contract and time measurements dilate compared to my tools...I plot it and see his rulers and clocks won't..."exist at c" or however you preffer it to be said. I'm a strong believer in the invariance of c so would conclude he won't reach it. lol

#### Mister T

Gold Member
Huh? time and length do comparatively reduce to null at c, to your point of gamma.
No, time and length do not decrease to zero at speed $c$. Time dilates and lengths approach zero as the speed approaches $c$. The difference is more than grammatical. One statement describes a physically-possible process that's occurring every minute of every day at hundreds of facilities across the world. The other describes a physically-impossible event that has never occurred and never can occur.

$\gamma$ is defined only for speeds that are less than $c$. $\gamma$ is not defined for a speed of $c$. $\gamma$ is a function speed, and the speed $c$ is not in the domain of that function.

#### nitsuj

No, time and length do not decrease to zero at speed $c$. Time dilates and lengths approach zero as the speed approaches $c$. The difference is more than grammatical. One statement describes a physically-possible process that's occurring every minute of every day at hundreds of facilities across the world. The other describes a physically-impossible event that has never occurred and never can occur.

$\gamma$ is defined only for speeds that are less than $c$. $\gamma$ is not defined for a speed of $c$. $\gamma$ is a function speed, and the speed $c$ is not in the domain of that function.
Who said something massive was going c? I never said they decreased to zero. Zero is not the right word, and either is decreased, that implies they reach this unattainable value.....I said Converging to where time and length are null at c....JUST LIKE IN YOUR EQUATION Please dont put words into my mouth. I did mention and described a couple times that this "speed" is not reachable. What is a null geodesic anyways right?

Last edited:

#### Mister T

Gold Member
I said Converging to where time and length are null at c....
There is no such condition to converge to.

#### nitsuj

There is no such condition to converge to.
yea, that was mostly describe as "you can keep accelerating, but will never reach" many many times in this thread. that point was made a ton of times...unless you mean nothing can accelerate towards c...which doesn't make sense...things can accelerate towards c

to me calling going c a "condition" implies something can achieve that "speed" and not; which is impossible. Note that "where" is not a "condition".

Last edited: