# I'm confused as to why you can't go faster than light, if you compound velocities

1. Mar 25, 2009

### Somedude99

You have 2 identical spaceships, each that can go .90 the speed of light relative to Earth. One spaceship tows the other, and once reaching the maximal speed (.90c relative to earth), the second ship that is currently being towed, is released, which then proceeds to accelerate toward its maximal speed (again, .90c relative to earth). Obviously the second ship, the one that is accelerating after being towed will leave the first ship behind, so why is it not possible to go faster than light?

2. Mar 25, 2009

### Somedude99

Could the "light is relative to the observer" be wrong, and there is some hole in which you can travel faster? How do we know for certain that this is the truth, and that we are not mistaken?

3. Mar 25, 2009

### Dmitry67

4. Mar 25, 2009

### phyti

You are confused, because both ships are currently moving at .9c relative to earth!
Rethink or reword your problem.

5. Mar 25, 2009

### Somedude99

I guess I should have said relative to the spaceship towing it. I just don't understand how you can have something going (say for example) .999999c, and have something else go .999999c relative to that, and have it not go over.

What really troubles me is if someone standing still on earth and someone in a space ship going .99999c view the same beam of light it will appear the same for both viewers. Light must not be an actual thing then, but an illusion.

It would be like watching a train go from point a to point b on earth, and watching the same thing from the ship, only on the ship more time would have passed on earth, thereby making the train late but having the ship view it in real time. How can the train be in two different positions at once?

I think light is an illusion of some kind.

6. Mar 25, 2009

### DaveC426913

The definition of velocity is a measurement of distance over time. It's not the distance that's changing, it's the time. At .999999c, the craft experiences time dilation of a factor of 700. At that factor, another ship moving relative to it, only needs to go from .999999 up to ~.99999999 in order for them to see a doubling of their relative speeds.

7. Mar 25, 2009

### Somedude99

So theoretically you could go infinitely fast and never reach the speed of light? This is some fked up ****.

8. Mar 25, 2009

### Staff: Mentor

That's a contradiction. You can't go infinitely fast....you can accelerate forever and not reach the speed of light, though.

9. Mar 25, 2009

### neopolitan

Infinitely fast relative to what?

If you don't put a "relative to" or "in relation to" in your sentence, you are going to get in all sorts of trouble. And that is not some "post-Einstein" peculiarity. We always move relative to something else - we just normally make an unstated assumption about what that something else is, the Earth's surface passing below or the air through which a plane is moving, the road we are driving or walking on, the train tracks on which a train is moving, the train body in which we are walking.

Put something into deep space and you no longer have anything obvious to move in relation to.

Before you head down that path, no, not having anything obvious to move in relation to has no effect whatsoever on relativity. Relativity works equally well for a snail as it does for a rocket travelling at 0.9c, our inability to notice the effect notwithstanding.

cheers,

neopolitan

10. Mar 26, 2009

### DaveC426913

You could accelerate forever and never reach the speed of light.

11. Mar 26, 2009

### Somedude99

Thanks

12. Mar 26, 2009

### phyti

If the ship moves at near light speed, time dilation slows all activity on the ship, which includes any rocket propulsion, thus it takes longer to accelerate. This would be observable by earth viewers but not by the ships crew.

They do not view the same light!

The train path is the same for both, they will not agree on the time it took for the trip.

Read about time dilation, if you haven't yet.

13. Mar 26, 2009

### Somedude99

I just don't understand how things can look different if physical matter is taking up space. It would mean that everything we can see is just an illusion with reference to who is seeing it.

Does that mean that there could be infinite dimensions, since every speed can view its own light relative to it, which means that there are an infinite number of ways light can co-exist at the same time?

14. Mar 26, 2009

### Staff: Mentor

Just because things are reference frame dependent, that does not mean they are illusions.

Can you play billiards on a train moving at constant speed (on very smooth tracks!)? Of course. A billiards ball that is stationary on the table might be moving at 100 miles an hour relative to the ground, but it doesn't affect the game at all. That's frame dependent velocity, just like with Einstein's version of Relativity (well....almost). And Newton/Galileo would agree. That's the principle of relativity in action, it is just that those guys didn't know how to apply it correctly when having to do with the speed of light.

15. Apr 9, 2009

### gravityandlev

16. Apr 14, 2009

### scibuff

actually, time dilation and length contraction happen at the same "time", it really depends on the way you look at it (distance = velocity * time so if time is shortened, so is the distance)

17. Apr 14, 2009

### JesseM

The distance between two markers (say, Earth and a distant star) is only shortened in a frame moving relative to those markers. So if we consider a ship moving at 0.999999c in the frame of the Earth, in this frame there is no length contraction of the distance the ship has to travel (though the ship itself will be shortened in this frame), while there is time dilation of the ship's clock. On the other hand, in the rest frame of the ship the distance from Earth to the star is contracted relative to the distance in the Earth's rest frame, but the ship's clock is not dilated (although a clock on Earth would be in this frame).

18. Apr 14, 2009

### scibuff

I guess i should have been clearer ... whenever two observes disagree on the time measured, they will always disagree on the length as well. That's what I meant by saying that length contraction and time dilation happen at the same time - which one of the two you see depends on your frame of reference.

19. Apr 14, 2009

### JesseM

Yes, that's true, although I don't see where DaveC426913's post was contradicting this.

20. Apr 30, 2009

### Idjot

Illusion? Consider this: The majority of today's "relativistic" scientific community is conditioned to(and obscenely comfortable with) the idea that nothing qualifies as an absolute frame of reference. This same community is also comfortable studying complex diagrams illustrating the intricacies of the various stages of imaginary twins in an imaginary experiment that proves the existence of an imaginary paradox that is in reality not a paradox at all, but a mere illusion. Although c may be constant, it takes little to distort our perception of it. If you were to take all variables into account to the smallest degree at any given time in your observation of any given event you could prove that most of what you see is indeed, an illusion. However light does not experience illusions like you do. It does exactly as it should. You just see it differently from different reference frames.

21. Apr 30, 2009

### JesseM

No one in the physics community thinks it's a "paradox", it's only called the "twin paradox" because it superficially looks like a paradox if you don't relativity very well. And there is plenty of experimental evidence that clocks experience time dilation in just the way predicted by relativity--for example, the clocks of GPS satellites are designed to adjust to compensate for time dilation, if time dilation didn't occur in the predicted manner the GPS system wouldn't work.

22. Apr 30, 2009

### Idjot

When people imagine space ships traveling at light speed they are imagining something that is truly impossible. Period. Light is radiation not matter. That's why it can travel as fast as it does. And whether or not people want to admit it, noone truly knows why it tops out at 299,792.458 km/s. Maybe if we didn't divide our time into increments to fit our orbit, we'd find a connection somewhere by making c a whole number. Anyway, imagining a space ship moving even remotely close to the speed of light is like hoping that water is actually made of nitrogen. It will never happen in the life of the universe. Never ever forever never. If something moves faster than light, it's certainly not visible and it certainly isn't matter, and most likely it's light itself in a specialized circumstance somewhere beyond the event horizon of a black hole, and even then if we could observe anything of it, it would probably be an illusion.

23. May 1, 2009

### Idjot

Of course. Gravitational time dilation. No paradox. However the paradox scenario has been the subject of much discussion and debate for many years and still is and you know it.
I was goofing on the irony of so many scientific minds playing with an imaginary idea that was born from the fear of actively choosing a frame of reference, because a witty man with funny hair said they couldn't so many years ago. Sociologically speaking, sometimes Relativity reminds me of Political Correctness.

24. May 1, 2009

### JesseM

The classic twin paradox doesn't involve any gravitational time dilation, not in inertial frames anyway.
No debate among physicists! It's discussed only because it's an interesting pedagogical example, and because a lot of people who don't understand relativity very well think there's a genuine paradox there.
You don't need to choose a frame of reference, all inertial frames make the same prediction about how old the two twins are when they reunite.
It's a basic principle of the theory that all inertial frames are on equal footing, and the theory has abundant experimental support. Nothing to do with taking Einstein's word as gospel.
Your comments lead me to doubt you have taken the time to understand the theory before casting these sorts of judgments on it.

25. May 1, 2009

### Idjot

Exactly. But GPS satellites?

It's imaginary. People who do understand relativity should dispose of it as such. It's not logically presented. It's contradictory and confusing to people trying to learn the truth. Noone observes someone else's clock from a distance at impossible velocities. It's an imaginary scenario akin to an Escher painting. It's puzzling entertainment, nothing more.

An inaccurate prediction based on illusion.

I don't dispute that all inertial frames are on equal footing but I do think Relativity hindered the search for the absolute. Relativity convinced everyone that an absolute can not be found, when an absolute is exactly what we should be looking for. If you're going to imagine something, imagine that the absolute frame does exist out there and that Relativity is only here to get us through until we find it (and choose to accept it). Then we'll be able to figure out how much time has truly dilated for the inertial frames and how old everything truly is in the universe. But maybe that's not important to you. Maybe you're happy just knowing that clocks look different when you're passing them in space at a truly undefined velocity.

I do not make judgments on the theory. It's the most brilliant theory of all time and I absolutely love it. I only make judgments on the people who think it answers everything.

Last edited: May 1, 2009
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