In another threadI said that [nothing is really moving in space](I’m curious about travelling at the speed of light. What’s the reference point?), unless it’s compared to a reference point.
This is obviously true. So, my question is, how can anyone measure the speed they’re travelling in space?
When the Apollo missions went to the moon I believe they were travelling at about 10,000 miles per hour, but that was relative to Earth. For all intents they were standing still and the Earth was dropping away at 10,000 MPH.
So how do you travel at C? Relative to what exactly? What if I was travelling at 0.5 C and passed a planet travelling in the other direction at -0.5C? I would be travelling at the speed of light, no?
Hey, look I know I’m not the first to think of this extremely basic concept, but could someone please elaborate?
In essence you can’t, but in a practical sense there are reference points you can use; the sun, the local bubble, the galaxy, whatever.
But also, importantly, you can know how much you are accelerating in a particular direction.
Distances are relative also. So if, from the perspective of earth a ship was moving towards Mars at 0.5c, and Mars was moving towards the ship at 0.5c, from the perspective of the ship, its distance to Mars is much shorter, and it’s closing that distance at less than c.
Hopefully a physicist will be along to confirm everything I’ve said
You are correct, it is only possible to measure relative velocities. There is no such thing as an absolute speed, unless that speed is the speed of light, which will be measured the same in any frame of reference.
However, you are wrong about your speed being c relative to the planet in your example. In relativity, you cannot simply add velocities unless they are very small compared to the speed of light. If you are moving at a speed u in one reference system and the planet is moving at speed v in the opposite direction in same reference system, then the relative speed between you and the planet is (u+v)/(1+ u*v/c^2). If u and v are each 0.5c, as in your example, then the relative speed is c/(1+0.25)= 0.8c or 80% of the speed of light.
I just came to second what JWT Kottekoe said: you can’t just add speeds in a linear fashion once you get up into appreciable fractions of the speed of light.
The mathematics is weird…but not too awfully hard. All you need are square roots. At this level, anyway, it’s junior high school math.
As for reference points, Mijin is right. It’s a little like navigating on earth: no one has ever gone to the effort of painting latitude and longitude lines on the surface (tricky, at sea!) but you can create an artificial reference point – Greenwich, or Paris, or a GPS network – and use that to define your location.
In space, you could use the Sun, or Rigel, or the center of the Milky Way Galaxy, or some reference Cepheids in Andromeda, or just dump a signal buoy from your ship and use that.
The speed of light is constant, no matter what your reference frame. No matter how fast you’re moving relative to whatever other thing you want to choose, you will always measure the exact same speed of light. That is the one (rather bizarre) observation that leads to all of the weirdness of relativity.
Let’s say you’re sitting here motionless relative to Earth. You measure the speed of light arriving from the sun as c. Now you fly away from the sun at 1000 mph relative to the Earth. You’d logically expect that light from the sun would seem to be going at c - 1000 mph. But it’s not. It’s still c. All that has changed is that the color of the light has shifted slightly toward the red.
That’s what is behind all of relativity. Einstein’s insight was that since the speed of light can’t change, other things, like mass, size, and even time itself, must be flexible to make all the math come out correctly.
The point is that c is a limit for the relative velocities between things. By “thing” we mean stuff with mass or energy or information (all three of which are different versions of one another).
You can have massless phenomena move faster than c relative to other stuff. For example if you are in a big room and put a laser on a turntable and spin the turntable faster and faster, the spot where the laser hits the wall can go faster than c. However, depending on how you move, from your point of view that spot might become two spots or zero spots, or get to its destination before it leaves its origin, or other such things.
