Racecar on a Train

[Bullgrit]

xkcd: Revolutionary

Someone explain the "racecar on a train" idea, and how it's "not even wrong," please. I think I know what it means, but I can't find any insight with a Google search.

Bullgrit

 

[jonesy]

How about their forums:

xkcd • View topic - 0675: "Revolutionary"

 

[Stumblewyk]

xkcd • View topic - 0675: "Revolutionary"

Basically - if you're seated in a racecar, on a train going the speed of light, and you stomp on the gas pedal, you're *NOT* going faster than the speed of light.

"Not even wrong" means that you're so far from anything resembling valid logic, that you're not right, you're not wrong, you're not *even* wrong.

Not even wrong - RationalWiki

 

[Bullgrit]

Yeah, I know/understand "not even wrong."

The xkcd thread on this, (at least the first page), talks about the train and racecar going opposite directions, but my first thought was that the train is going .9c and the racecar on it then goes .9c. How does physics keep this from becoming 1.8c?

I'm not believing this thought experiment breaks physics. I just want to understand how it doesn't. I like this kind of stuff, (see: http://www.enworld.org/forum/media-...geek-stuff/291824-crossing-event-horizon.html), but I'm a total layman on the subject.

Bullgrit

 

[Umbran]

Except, that particular example isn't "not even wrong". It is perfectly valid logic for a layman - it is just ignorant of how Relativity works. "Not even wrong" is for things that are really far out in left field, that make no sense whatsoever, even in the speaker's context. Someone thinking in Newtonian terms makes perfect sense, as that's what they experience.

 

[Morrus]

Of course, if the racecar is a DeLorean then of course the situation changes completely.

 

[Janx]

Maybe the problem is the involvement of light speed and its special rules.

I got a really long train on tracks, with a racecar resting on the caboose facing forward.

The train is rolling at 60MPH.
The car then takes off, also at 60MPH.

Relative to the train, the car is only going 60MPH (as it moves away from the caboose)

Relative to police car sitting at the rail crossing, the racecar is 120MPH as measured from his fixed point in 2 readings when the radar bounces off the race car.

i assume the statements and math to be correct. Obviously, a physicist could prove me wrong.

Now make it a space train with a spacecar on top.
the train is going .4c to Alpha Centauri from eath
the car takes off again. the caboose is receding from us at .4c
the car should be receding from us at .8c

Make the train's speed be faster still, and the concept is meant to scale.

Somehow, the magic of relativity and light speed however, make the math all funky. From the concept of the car driver, time slows down, etc.

But nobody gives a rats ass about the driver. The point is, does the car go faster because it's starting speed is = train's speed when it takes off?

Therefore, can it, from any vantage point, be going faster than the speed of light?

never mind that it's a short term experiment, as the car will run out of train to race along. Or that its forward movement will apply some negative movement to the train.

 

[jonesy]

I'll try. I'll probably get corrected too.

It's time dilation and length contraction. The train is moving at .9c from the point of view of someone standing still from the point of view of the train. The racecar is moving at .9c from the point of view of the train, but from the point of view of the person standing still from the point of view of the train time dilation occurs and the racecar on the train wouldn't actually seem to be moving as fast as someone on the train would think it was compared to the train, and both would be contracted in length (not just seemingly, but actually) making them almost the same object (a very very very short train) from the point of view of the person standing still from the point of view of the train making the measurements from the person standing still give both the train and racecar a speed of .9c because of the time dilation affecting all parties in the scenario in a different manner depending on where they are from the point of view of everyone.

In other words, from the pov of the person standing still, they are moving at the same speed. The whole thing is relative to where you are.

 

[Janx]

Yeah, I know/understand "not even wrong."

The xkcd thread on this, (at least the first page), talks about the train and racecar going opposite directions, but my first thought was that the train is going .9c and the racecar on it then goes .9c. How does physics keep this from becoming 1.8c?

I'm not believing this thought experiment breaks physics. I just want to understand how it doesn't. I like this kind of stuff, (see: http://www.enworld.org/forum/media-...geek-stuff/291824-crossing-event-horizon.html), but I'm a total layman on the subject.

Bullgrit

one question to ponder:
if I am in fast car and I shoot a gun at the guy in front of me, does my bullet go faster (muzzle velocity + car velocity) or just muzzle velocity?

conceptually, if the motionless car on the train wasn't bolted down, when the train started moving, the car could actually roll off the back of the train.

So steps have to be taken to prevent that (bolt the car down, hold down the brakes, actually acccelerate the car?).

The first layman physics question then, is whether velocity is additive when one object carries another object that is also moving?

 

[jonesy]

The train is moving at .9c from the point of view of someone standing still from the point of view of the train.

Actually, I'll be the first to correct me. That should say:

The train is moving at .9c from the point of view of someone moving away from the train at .9c from the pov of the train.

There is no such thing as a stable observation location when you are talking about relativity. Everything is moving.