Nothing can go faster than twice the speed of light.

[Garnier Fructis]

That's the problem. 2nd principle of special relativity doesn't simply mean "speed of light is 300000 km/s" but:

The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

This mean that for all reference system (particles included) the speed of the other particle is always c. Hypotetically if you sit on one photon and you look to the other photon you'll see it moving away from you at c and not 2c, because you're in an inertial frame with your photon and you're still.

Doesn't exist a reference system which they can move away from each other at speed faster than c because this would go against the principle. Principle is an experimental fact!
I don't know what's your physics (and LaTex) skills but this is the Velocity composition for Special Relativity, where u' is the speed of photon moving away from you (in his system) , u is the speed you are going to calculate (the speed of the photon moving away in your system) and v is the speed between both systems:
u=\frac{u'+v}{1+u'v/c^2}

assuming the photon is moving at speed of light (lol really?), you can replace u' with c. Now you're going to see that u=c (the photon is moving away from you regardless your speed, in fact v is simplified).
ps. sorry for my bad english :/

The distance between two objects can increase faster than c for the same reason shadows can move faster than c - it's not a physical object. From any observer, both particles will be moving c.

Also, you can't 'sit' on one photon. Such a reference frame is self contradictory, since photons must travel at c in vacuum in every inertial frame, so an inertial frame with a still photon is a contradiction.

[May90]

That's the problem. 2nd principle of special relativity doesn't simply mean "speed of light is 300000 km/s" but:

The speed of light in a vacuum is the same for all observers, regardless of the motion of the light source.

This mean that for all reference system (particles included) the speed of the other particle is always c. Hypotetically if you sit on one photon and you look to the other photon you'll see it moving away from you at c and not 2c, because you're in an inertial frame with your photon and you're still.

Doesn't exist a reference system which they can move away from each other at speed faster than c because this would go against the principle. Principle is an experimental fact!
I don't know what's your physics (and LaTex) skills but this is the Velocity composition for Special Relativity, where u' is the speed of photon moving away from you (in his system) , u is the speed you are going to calculate (the speed of the photon moving away in your system) and v is the speed between both systems:
u=\frac{u'+v}{1+u'v/c^2}

assuming the photon is moving at speed of light (lol really?), you can replace u' with c. Now you're going to see that u=c (the photon is moving away from you regardless your speed, in fact v is simplified).

First, there is no inertial reference system sitting on a photon.

Second, I was talking about the speed at which the distance between two objects increases, not the speed of one object in other object's reference frame. Like I said (not sure why this causes so much confusion), if I look at two photons going away from each other, then from my perspective they move away from each other at 2c.

[Adam Jensen]

by taking a wormhole you can effectively travel distances faster than light.

This is false.

You merely appear to travel faster than light. You're actual speed never passed C. You simply moved through a short cut.

[axlpose]

don't we kind of know this already? i remember being taught this in seventh grade.

[Forogil]

Well, that's like saying sum of two numbers can be bigger than C.

Here are a few problems to solve:

Two trains traveling towards each other start from town A and B - that are 180 miles apart at exactly noon January 1st 2020, one is doing 40mph, the other 20mph. When and where will they collide?

At the same time two photons leave town A and B - the towns are 1 light-milli-second apart and the photons travel at about light-speed, when and where will those photons "meet"?
The same for two hyper-trains doing 0.6c and 0.4c.

They show that the "relative velocity" can be used to solve a class of problems - and isn't just randomly adding numbers together.
There are some issues with the relativistic case - but they are not directly related to the question itself. However, it isn't that useful, fairly trivial, and can cause confusion, so no-one focuses on it.

[HumbleDuck]

Here are a few problems to solve:

Two trains traveling towards each other start from town A and B - that are 180 miles apart at exactly noon January 1st 2020, one is doing 40mph, the other 20mph. When and where will they collide?

At the same time two photons leave town A and B - the towns are 1 light-milli-second apart and the photons travel at about light-speed, when and where will those photons "meet"?
The same for two hyper-trains doing 0.6c and 0.4c.

They show that the "relative velocity" can be used to solve a class of problems - and isn't just randomly adding numbers together.
There are some issues with the relativistic case - but they are not directly related to the question itself. However, it isn't that useful, fairly trivial, and can cause confusion, so no-one focuses on it.

That is not velocity of any object, you are simply deviding some distance by some velocity.
Sure you can sum two numbers or multiply two numbers to generate a bigger number than C, but even in your example, no object us moving faster than C. ( well, maybe the shadow of the trains do, but again, they are not objects.)

[Deleted]

If movement is relative, then if you have one thing moving the speed of light one way, and another thing moving the speed of light the exact opposite way, than the truth is relativity means that nothing can go faster than TWICE the speed of light, since the gap between those two would be created at twice the speed of light.

Also, to actually answer this post, this isn't true because space doesn't move, the space between the objects is neither created, nor expanding or anything else. Space is the fabric through which they move, the only way to move space-time is to put a giant fuck-off rock in it and force space to move around it, like a planet, or star.

[Juvencus]

You've got more than mathematically theory to back that up right?

Exactly, the math sooner or later falls apart. Much like when entering into a black hole. Math in itself is a man made construct that we developed to better understand the universe, but like all things it's proven to have it's limits. We just gotta wait another hundred or thousand years for someone like Newton or Einstein to come along and point out what morons the experts had been for the past few hundred.

Mathematics is the visualization of the physical world. You can say it's the language of the universe.
Do we make mistakes in the translation? Sure. But that's no excuse to just give up and wait for a brilliant person to show up.

Newton and Einstein themselves stood on the shoulders of previous metaphorical giants.

[Deleted]

they dont go FTL in star trek, they warp space, so apparently you didnt watch star trek

and actually things can go faster than the speed of light, they just cant accelerate to it, so if something already is faster than light and it exists above it then it can exists

Seems to be an FTL drive via sub space.

Warp drive is a faster-than-light (FTL) spacecraft propulsion system in many science fiction works, most notably Star Trek. A spacecraft equipped with a warp drive may travel at speeds greater than that of light by many orders of magnitude.

[Noradin]

For example, near the very edge of the observable universe the space expansion makes the space move away from you at the speed 0.99c. If an object there moves at 0.99c relative to some object, away from you, then the distance between you and it will be increasing at the rate 1.98c (normal velocity summation is applicable in this case). You won't see it for that reason, but its gravitational effects will be observable, once the signal reaches you.

An increase of distance is not the same as a velocity, though.

[Dundebuns]

Einstein very famously postulated that the speed limit of the universe as currently understood is ~3x10^8 m/s. That's the fastest speed in which information can travel in any reference frame, and so far can only be obtained by massless particles.

If movement is relative, then if you have one thing moving the speed of light one way, and another thing moving the speed of light the exact opposite way, than the truth is relativity means that nothing can go faster than TWICE the speed of light, since the gap between those two would be created at twice the speed of light.

In this hypothetical situation, we have Observer A and B. Observer A travels at c to the right, Oberserver B travels at c to the left. To Observer A, Observer B is receding from it at the speed of light, not twice the speed of light. (And to Observer B, A is also receding from it at the speed of light) Why you may ask? Because things get weird when you travel at or near the speed of light.

[heerobya]

Wait...

So what you are saying is, if two cars are heading opposite directions on the freeway at 70mph each, they aren't actually going 70mph, they are REALLY going 140 mph?

Doesn't make any sense.

They may be heading 140mph away from each other, but each object is still only traveling 70mph so... the speeds remain constant.

Relative speed to each other doesn't = actual speed. Right?

[Dundebuns]

Wait...

So what you are saying is, if two cars are heading opposite directions on the freeway at 70mph each, they aren't actually going 70mph, they are REALLY going 140 mph?

Doesn't make any sense.

They may be heading 140mph away from each other, but each object is still only traveling 70mph so... the speeds remain constant.

Relative speed to each other doesn't = actual speed. Right?

It actually all depends on your frame of reference. If you are in one car, then to you you are actually stationary and the environment is moving past you at 70 mph. Hence, the other car is actually moving away from you at 140 mph.

[caervek]

Seems to be an FTL drive via sub space.

This is something of a odd concept but basically, most ships capable of FTL travel in sci-fi cannot actually move faster than light. It sounds strange but a basic example is the Event Horizon's gravity drive, the ship creates a portal at point A that comes out at point B, the ship then travels through the portal under conventional power (so not much faster than a moon rocket). Theoretically* this results in the ship covering a huge distance much much faster than light could, despite the ship itself being significantly slower than light. Almost all Sci-fi ships capable of FTL use these type of "cheats".


*Assuming the ship doesn't go to hell instead and the crew get flayed/raped/etc to death.

[Dundebuns]

This is something of a odd concept but basically, most ships capable of FTL travel in sci-fi cannot actually move faster than light. It sounds strange but a basic example is the Event Horizon's gravity drive, the ship creates a portal at point A that comes out at point B, the ship then travels through the portal under conventional power (so not much faster than a moon rocket). Theoretically* this results in the ship covering a huge distance much much faster than light could, despite the ship itself being significantly slower than light. Almost all Sci-fi ships capable of FTL use these type of "cheats".

An interesting thing they miss out in Sci Fi is deceleration of something that is traveling at or greater than the speed of light. Take for example a ship that has spent a long time accelerating and is now moving at 0.99c. If it wants to stop, it has to take the same amount of time decelerating as it did to accelerate if it uses the same force. If it decelerates rapidly, like coming to a complete stop, it has all that kinetic energy that needs to go somewhere. Since you're in space, the best way to get rid of extra energy is to emit it as photons. So if you stopped anywhere near a habitable planet, you'd irradiate it and everything on it.

This is probably why the whole "folding/warping of space" is so popular as you don't have to worry about it.

[heerobya]

It actually all depends on your frame of reference. If you are in one car, then to you you are actually stationary and the environment is moving past you at 70 mph. Hence, the other car is actually moving away from you at 140 mph.

But moving 140 mph away from each other does not = each object is actually traveling at 140 mph, each object is still traveling at a speed of 70mph.

There has to be some kind of "observational bias" or something.

I'm pretty sure even at light speed, you wouldn't observe another object travelling any faster than light speed no matter the observational factor. I don't think it's that simple of math (1+1=2) at those speeds.

[Barael]

Wait...

So what you are saying is, if two cars are heading opposite directions on the freeway at 70mph each, they aren't actually going 70mph, they are REALLY going 140 mph?

Doesn't make any sense.

They may be heading 140mph away from each other, but each object is still only traveling 70mph so... the speeds remain constant.

Relative speed to each other doesn't = actual speed. Right?

All speed (or rather velocity) is relative. The whole point of relativity is that there is no "authority" (preferred frame) that decides what the "actual" speed is. If there was, things would be actually be much, MUCH weirder than than any relativity "weirdness" (such as time slowing down, length contraction etc).

So yeah the situation with the cars vs light-speed rockets isn't perfectly comparable. A car can easily recede from you at 140 mph; nothing can recede from you faster than c (astronomically distant objects non-withstanding).

[Dundebuns]

But moving 140 mph away from each other does not = each object is actually traveling at 140 mph, each object is still traveling at a speed of 70mph.

There has to be some kind of "observational bias" or something.

I'm pretty sure even at light speed, you wouldn't observe another object travelling any faster than light speed no matter the observational factor. I don't think it's that simple of math (1+1=2) at those speeds.

But it does. This is why relativity can be a challenge to get your head around. It all depends on where the observers are and how fast they're moving. These frames of reference can influence what you see in the universe about you, and it means that what you see isn't the same as what someone else sees. Usually we have a rest frame where we can take stock of what is actually going on, but even then the rest frame is still another inertial frame.

I found the easiest way to get my head around it was to consider that when I am moving at constant velocity, you can't actually feel that you're moving (we detect acceleration as a biological organism, not constant velocity) but it looks like the world is moving instead. If you want a good Sci Fi example of this, look up the Planet Express ships drive from Futurama, in which the ship never moves, the drive moves the universe around it instead.

When you get to near light speed velocities the equations change to take this into account. In fact, classical equations such as s = d/t are derivable from relativistic equations, and it's another example of deriving classical (and hence easier to understand) physics from quantum and relativistic physics.

(astronomically distant objects non-withstanding).

Even then it's still c, but then we have crazy shit going on with the expansion of space :S

[Forogil]

That is not velocity of any object, you are simply deviding some distance by some velocity.

It's the "relative velocity" between two objects, which can be found by differentiating the relative distance between them - or by adding the absolute velocity-vectors (in one reference frame).

It's useful for solving this exact class of problems, and works the same in classic mechanics as in special relativity, but this "relative velocity" would be an awkward object in special relativity, not solve that much, and is thus skipped.

Sure you can sum two numbers or multiply two numbers to generate a bigger number than C, but even in your example, no object us moving faster than C. ( well, maybe the shadow of the trains do, but again, they are not objects.)

I never claimed that any object was moving faster than c = 299 792 458 m/s - just that you could use this "relative velocity" as someone else had already stated.

[HumbleDuck]

It's the "relative velocity" between two objects, which can be found by differentiating the relative distance between them - or by adding the absolute velocity-vectors (in one reference frame).

It's useful for solving this exact class of problems, and works the same in classic mechanics as in special relativity, but this "relative velocity" would be an awkward object in special relativity, not solve that much, and is thus skipped.


I never claimed that any object was moving faster than c = 299 792 458 m/s - just that you could use this "relative velocity" as someone else had already stated.

That's bad terminology.
The relative velocity is the velocity of an object or observer in the rest frame of another object or observer.

https://en.m.wikipedia.org/wiki/Relative_velocity

Not all distances divided by time are usually called velocity, but I guess you can define it that way, its just not common to use it that way.