Can someone explain approaching the speed of light?

Question

There has to be a reference point to determine the speed of an object. Supposedly as you approach the speed of light mass and the energy required approach infinity. My question is how can you approach the speed of light without a reference point? Since everything is relative why would not the Earth approach infinite mass as it is backing away from me near the speed of light rather than my ship moving forward? I am trying to read "A Short History of Time" and that is where I get confused.

Answer 1

You are missing something. If you start on the earth and move away from it at approaching the speed of light then yes it would appear to you that the earth is moving away from you. But you are forgetting that you - and not the earth - has gone through and experienced a period of acceleration to get to the speed you are moving at. That is the difference and that is why if you later returned to the earth you would have aged less than the people on the earth.

In other words you know that your speed is increasing because you are accelerating and that acceleration causes physical effects that you can feel and measure. Suppose you are on a spaceship that is accelerating at 32 feet per second squared (the acceleration of gravity on earth) then you would feel a force apparently pulling you down against the floor of the spaceship and it would feel like you were on the surface of the earth. In books and stuff that talk about special relativity they usually skip pointing out that an object must be accelerated to get to the speed it is moving at and it is that period of acceleration that introduces an asymmetry between say me standing in a lab and you in a rocketship flying by.

I can't help but comment on what someone else said: "The same for the speed of light, when traveling at the speed of light you wouldn’t see anything except for any other objects traveling at that speed. Which would only be light, of course."
Well for one thing you can't travel at the speed of light but let us assume that they mean you are very very close to it. Now what happens as you go faster and faster on a spaceship say traveling through the galaxy? What would you see? Everyone I think knows that if an object is moving away from you then it's light is shifted to the red and if moving towards you then it is shifted to the blue. Now at small speeds this effect would not be noticed but as you move faster and faster the starlight from in front of you would start shifting to the blue and beyond that to the ultraviolet and further. That from behind you would shift into the red and then disappear into the infrared. Locations of stars would also be distorted. Anyway, I think I am going to have to look into this.

Answer 2

> There has to be a reference point to determine the speed of an object.

You've made an excellent observation, and you are exactly right. And, because velocity is relative to the observer, every quantity which depends on velocity, is also relative. This includes such things as energy and mass.

To make your example simpler, say it's two identical spaceships receding from each other (rather than you & the earth receding from each other). There is no "absolute" amount or energy "contained" in either spaceship. Ship "A" perceives that Ship "B" contains more energy, while Ship "B" perceives that Ship "A" contains more energy. Energy is relative; this is true even in "pre-Einstein" physics.

Likewise, the mass is relative. Each of the ships perceives that the _other_ is the more massive.

Now, let's say the ships double their speed of recession. From the perspective of Ship "A", it looks like more energy has been added to "B". But from the perspective of Ship "B", it looks like Ship "A" is the one that's increased its energy. But both ships agree that some energy was added to "the system" from somewhere--they simply disagree about who "received" the extra energy. But from either perspective, it takes an infinite amount of added energy for the two ships to reach a recession velocity of "c".

Answer 3

The earth would not reach an infinite mass because its not moving ''away'' from you, you would be moving away from it. The reference point is like a ''marker'' and can be used for measurement. As speed=distance/time you can calculate your speed as you approach your frame of reference.

Not everything is relative. Relativity is only from your perspective. When you watch a train zip bye, it looks like a streak. When your on the train and another one is traveling at the same speed when you look over at the other one it seams as if it is standing still, but its not, that’s relativity. The same for the speed of light, when traveling at the speed of light you wouldn’t see anything except for any other objects traveling at that speed. Which would only be light, of course.

Answer 4

you dont really need a reference point since we've figure out how fast the speed of light is (186,000 mile/s).

you're right though that everything is relative--so how fast object "A" is going depends on object "B." but if object "A" object "B" are both moving at the speed of light, neither object A or B would know this--except you.

as for when an object approaches the speed of light, it takes an infinity amount of mass/energy to actually propell it to the speed of light--that's still a theory, after all.

Answer 5

i thinks its brief history of time
and you need a fixed reference for calculation not for thought
you can conceive of the idea that all things are approaching the speed of light but moving in different directions but to get precise math on that you need frames of reference.