[tachyoff]

> The speed of light is not a speed limit on effective travel, but on observation.

It is absolutely a limit on effective travel (if we ignore things like wormholes for a second). Conventional motion through space is bounded by c for several reasons, not the least of which being that the closer you get to c, the more energy it takes to make smaller and smaller changes in velocity (the object acts as though it's gaining mass the faster it goes). In fact, it's asymptotic. Accelerating a particle to c would require an infinite amount of energy.

[TangoTrotFox]

This is a common misunderstanding, and was the point of the example. A stationary observer observing a particle being accelerated towards the speed of light would observe that particles apparent mass begin to approach infinity as its observed velocity approached the speed of light, thus requiring an amount of energy approaching infinity to continue to accelerate it further. However, this has nothing to do with the scenario with you being that particle.

You are currently moving at near the speed of light relative to many things at this very moment, yet your mass is certainly not approaching infinity, America notwithstanding! And if you accelerated enough in the opposite direction of your relative partner to exceed the speed of light it's not like you'd suddenly start finding it impossible. No, it's a matter of observation. From the other particle's perspective it would see your mass approaching infinity and your speed would slow, but the distances you covered would remain the same due to length contraction.

A similar effect explains why, for instance, particles in CERN's reactors travel distances that should be impossible for them to travel before decaying. E.g. if the speed of light was 10m/s and a particle decays after 3 seconds then it should be impossible to see that particle travel more than 30 meters. Yet we see it travel hundreds of meters. Isn't relativity fun?

[tachyoff]

Under relativity, you can't simply add speeds like that, unfortunately (I believe this is related to the fact that the speed of light is the same in all reference frames). Here's a link that can explain it better, though: http://curious.astro.cornell.edu/about-us/139-physics/the-th...

In short, you can't go faster than light in any reference frame, relative to any other observer.

Here's another source: https://physics.stackexchange.com/questions/75501/lorentz-an...

[TangoTrotFox]

Absolutely. You are not understanding what you are reading. Let's differentiate between 'mover' and 'observer' for clarity. In reality they're completely interchangeable as being at rest relative to something moving 10m/s is the same as you moving 10m/s if we consider the other particle to be at rest.

As you accelerate more and more you will never observe yourself or anything else exceeding the speed of light. Instead what will happen is that time will begin to slow down and distances will begin to contract. If in our frame of reference a distance is 100 meters, it would begin to seem to be, for instance, 10 meters.

And the people observing you will also never see you exceed the speed of light. Instead they will see you approach it, and then start to level off asymptotically. Both views are correct. Time itself is what changes. From the perspective of the observer, time will be moving more slowly for the observed. e.g. this is why particles at CERN travel vastly greater distances than they 'should' be able to before decaying. Imagine what it would be like to experience that travel from the particle's perspective.

This is why we see our particle last much longer than it should. And if humans could live for 1500 years, this is why we would see things like our ship make it's 700 light year journey, and then beam back a message that would only hit us 1400 years later - telling us that they're safe and sound and got there in 12 years. Quite fun stuff!

Put another way, this would be the timeline of our ship in years from its perspective and from earth's perspective:

    0 earth relative = ship leaves
    0 ship relative = ship leaves

    700 earth relative = ship arrives at planet
    12 ship relative = ship arrives at planet

    1400 earth relative = message from ship on planet arrives at earth
    712 ship relative = message from ship on planet arrives at earth

One final clarification, which might be unclear from the above. The ship obviously does not magically warp through time or anything like that. If there was a clock on the planet that started at T=0 when that ship left Earth, it would read T=700 years when that ship arrived since that planet is itself also roughly at rest relative to the ship.

This is the most counter intuitive thing about relativity, and time dilation. It's not some 'trick' or matter of perspective. Time itself does literally move at different rates for different people in different scenarios, even when we're all in the same universe. Our ship pilot could make the 12 year journey back and indeed 1400 Earth years would have passed in the interim, and if was somehow able to measure the age of the other planet - 1400 years would have also passed there as well. Even though he himself had only aged 24 years.

Should we achieve the ability to reach relativistic rates of travel - some people may get their wish. Expect huge chunks of the rich to up and leave 'to the future' (from our perspective we'd just see them constantly zipping around near the speed of light... for centuries) in hopes of discovering if humanity has overcome mortality by then! Reality is much stranger than fiction.

[coinerone]

" According to special relativity, c is the maximum speed at which all conventional matter and hence all known forms of information in the universe can travel. "

https://en.wikipedia.org/wiki/Speed_of_light

[saagarjha]

Sure, that’s all true, but you still haven’t shown that anything can go faster than the speed of light. Time dilation and length contraction conspire to cheat you from doing this.

[lodi]

Did you read what TangoTrotFox wrote? At no point did he invoke any faster-than-light travel. He's just saying that by traveling at relativistic speeds (e.g. 0.99c <= v < 1c), time dilation / length contraction work in your favor. You can travel to systems thousands of light years away from Earth in only a few years (as perceived by you, on your spaceship), all while traveling a bit slower than light itself. Everyone left on Earth will still perceive you taking >1000 years to arrive.

[saagarjha]

I did, but it was a long post on mobile so I misunderstood their intent. I though that TangoTrotFox was disagreeing with tachyoff, when in fact he was just providing a better explanation.

[tachyoff]

Except TangoTrotFox's original statement was:

> The speed of light is not a speed limit on effective travel, but on observation.

Which is absolutely factually false.