# Silly questions

1. Apr 4, 2005

### uk9999

ok two question that kinda just have bugged me (im only 17 so this may all sound a bit basic!)

ok traveling at the speed of light time is 0... as all the 'speed' is used up in 1 of the 3 space dimensions
what would happen to someone travelling where the is no time???
would they simply be frozen or would it be (from there prespective instantanoues travel to the point where they stop travelling at the speed of light in that dimension?

my second question is a we are on the earth we are moving so how far out would a persons 'clock' be on earth compared to someone in no movement what so ever

2. Apr 4, 2005

### chroot

Staff Emeritus
Nothing with mass can do the speed of light, so there is no answer to this question. It can't happen.
There is no such thing as a person with "no movement what so ever." The motion of one object can only be defined relative to some other object. There is no absolute standard of rest.

- Warren

3. Apr 4, 2005

### uk9999

well ok what i meant by no movement whatsoever was having (theorical) no gravitaional fields infact no forces acting on a person with a clock

thanks for ur replys neways just me wondering what would happen IF you could travel at the speed of light (with the infinite amount of energy required) i suppose its not worth thinking about

4. Apr 4, 2005

### uk9999

another question
say you were travelling at 5 miles less than light
and then you through a ball out of the side of your spaceship
what will happen to the ball?

would it just get heavier and larger n need more engery n infantly go up

5. Apr 4, 2005

### chroot

Staff Emeritus
An object with no forces applied to it is a quite different thing than an object with "no movement whatsoever." Forces produce accelerations, and they are directly measurable. Velocity itself, however, cannot be measured without reference to some other object.
If you threw it in the direction of travel, the ball would be going faster than you, a little closer to the speed of light. From your perspective (in which the ball's velocity is not much different from your own), the ball will appear perfectly normal, and the physics it will seem to obey will be just like the familiar Newtonian physics you see on a baseball diamond.

Remember, there is no way to tell you're going "5 miles [per hour] less than the speed of light" without seeing something go past you.

- Warren

6. Apr 4, 2005

### DaveC426913

However, you can get arbitrarily close to the speed of light. As you do so, time slows to a crawl. If you were to extrapolate what you might see at the speed of light (say, if you were a photon), you would see the entire universe age and die in the blink of an eye.

7. Apr 4, 2005

### chroot

Staff Emeritus
Such extrapolations are dangerous. Since it isn't possible, it's best just to say it isn't possible. Too many people in the world already have misconceptions about the speed of light.

- Warren

8. Apr 4, 2005

### DaveC426913

Gravity on the scale of the Earth doesn't have a dramatic effect on time dilation. But it's not negligible. They were able to detect the difference in time between two clocks at different altitudes - but it's, like, nanoseconds. The effect decreases with distance, so out where there's very little gravitational pull, the effect will be even smaller.

9. Apr 4, 2005

### DaveC426913

I'm not here to manage the poster's use of the info.

Anyway, in this case, it's possible to answer his question without violating anything. We can examine speeds arbitrarily close to light, and see the effect. A 'short' blink of and eye at .99999c is pretty much the same as an almost zero-time blink of an eye at .99999999999999c. So the universe ages and dies in a nanosecond - or two.

10. Apr 5, 2005

### TheUnknown

i really like this... so if you were traveling at 5 mph less than the speed of light (all theoritcal... hypothetically speaking... saying we know how fast we are going) and then threw the ball in front of you at 10 mph...? the ball is now traveling.. faster than the speed of light? or is the factor of speed of light now dismissed because you are inside of a space ship with a controlled environment and you don't feel many G's... well not enough to make the ball go straight backwards when you threw it.

Last edited: Apr 5, 2005
11. Apr 5, 2005

### chroot

Staff Emeritus
False. Velocities do not add linearly in special relativity.

You must use this relationship:

http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel.html

- Warren

12. Apr 5, 2005

### TheUnknown

i read it... a couple times... But what you are saying is that velocity can exceed the speed of light.. but speed cannot? I am new to this area... sorry if my questions sound silly... but something is then exceeding the speed of light after the ball is thrown, what is it?

Last edited: Apr 5, 2005
13. Apr 5, 2005

### JesseM

If I am moving at velocity u relative to you, and I throw a ball at velocity v relative to me (in the same direction that I am moving relative to you), then the velocity of the ball relative to you is $$(u + v)/(1 + uv/c^2)$$. For example, if you see me go by at 0.8c to the right, and I throw a ball to the right which moves at 0.5c relative to me, you will see the ball move at (0.8c + 0.5c)/(1 + 0.8*0.5) = 1.3c/1.4 = about 0.93c.

14. Apr 5, 2005

### TheUnknown

ah... okay.. i'm not sure i totally understand this then... because to me, if i am traveling through space at .9999999999999999999% the speed of light... and i throw a ball at 30 mph... and it exceeds the speed of light... why does it "not" exceed it? something is exceeding the speed of light right? okay here's something that will make it a little bit different... thanks for the help by the way, i am just interested in learning more about what i don't know, and you guys know a lot. so say i am traveling in a huge space ship (this is all extremely theoretical, and would probably never be possible... but isntead of dismissing it, i'd just like to hear sme opinions, i know most people don't like to talk about it if it can't be done with todays science) 5 mph slower than the speed of light, inside the space craft, the environment is "controlled" meaning there are no G forces, and no outside forces are felt inside the craft. A rocket is then launched from the back of the ship to the front, of course before exiting it would be pushed back inside since once the ship opens and the rocket or craft being propulsed tries to exit it would not be traveling faster than the environment passing it outside... so if this rocket reaches a speed of 7,000 MPH... it's obviously traveling 6,995 mph faster than the speed of light, or something is while it is still in the ship... so no matter what, no matter can travel faster than the speed of light in any circumstance? I've also heard some speculation.. that gravity can speed up the rate of travel for matter (black holes) and i've heard that supernova can cause matter to travel at .999999999% the seepd of light... what if a supernova exploded near a black hole? is the matter then alreday traveling to fast to be effected by the gravitational pull of the black hole? or is all of this jut impossible because it would seem unlikely to created a simulated environment inside a ship traveling so fast.

Last edited: Apr 5, 2005
15. Apr 5, 2005

### chroot

Staff Emeritus
It doesn't exceed it. In special relativity, velocities do not add the same way they do in normal Galilean relativity. It's a consequence of two postulates of special relativity: the speed of light is the same for all observers, and the physics of the universe is the same in all inertial reference frames.
To an observer inside the ship, the rocket's going 7,000 mph. To an observer outside the ship, you have to use the special relativistic velocity addition formula, and, again, the rocket will not be going faster than the speed of light. Keep in mind that, to an outside observer, events inside the ship, including the flight of the rocket, will appear to be happening in "slow motion."
No.
No matter what explodes next to what, the physics of this universe appears to preclude anything with matter from going the speed of light or faster.

- Warren

16. Apr 5, 2005

### JesseM

Because there is no reference frame where it is traveling faster than light. Keep in mind there is no such thing as absolute velocity in relativity, all velocities are measured relative to a particular reference frame. There may be some reference frame where you are traveling at .9999999999999999999c, but in your reference frame, you are at rest, by definition. So in your reference frame, the ball is only going at 30 mph. On the other hand, in this other reference frame that sees you going at .9999999999999999999c, the ball is not going 30 mph faster than you. This has to do with the fact that "speed" really means the distance an object travels in a given time, but each frame sees rulers in other frames shorter than their own, and each frame sees clocks in other frames ticking slower than their own, so different frames can disagree about how fast one object is moving away from another object.
Again, there's no absolute truth about how much slower than the speed of light you're moving, all velocities are relative. There may be some frame where you are moving at the speed of light minus 5 mph (about 186282.397 miles per second - 0.0014 miles per second), but in your frame light is moving at the same speed it moves in every frame, about 186282.397 miles per second.
No it isn't. In the ship's frame, the rocket may be traveling at 7000 MPH = 1.94 miles per second, but light is still traveling at 186282.397 miles per second in the ship's frame, so the rocket is moving much slower than the speed of light.
According to the theory of general relativity, which is the theory that predicts black holes, every observer will see light moving locally at the same speed, where "locally" basically means in an arbitrarily small region of spacetime near him. This is just as true in the vicinity of black holes as anywhere else.

Last edited: Apr 5, 2005
17. Apr 5, 2005

### TheUnknown

so to an outside observer it would be happening in slow motion? so light IS time... and time is light? and the speed of light controls time and the way we percieve it? is this true? and is there anywhere where i can read more about this. sorry if i missed a link, i am just really excited, i have never been to a site like this, and i am fascinated with the universe and all physics, and i am goign crazy right now with exhilaration! lol this is great. I'm still very interested now... in why inside the ship the observer sees the rocket traveling at 7,000 mph, but outside it is not... what would happen if the outside observer could walk through the wall of the ship and witness the rocket in mid flight... would it go from slow motion to 7000 mph instantly? and is there any equation that could tell us how fast "slow motion" the rocket is moving from the outsiders perspective while it is in the ship?

18. Apr 5, 2005

### DaveC426913

If the observer stepped outside the ship, he'd be floating in space next to it. Everything in the ship, such as clocks would remain moving at a normal speed. But the universe would be whizzing by him at .999c.

The only way he could put himself at rest with respect to the universe would be to decelerate from .999c to 0 (with respect to the universe, say a nearby planet). But it is equivalent to saying he is accelerating himself to .999c wrt the spaceship. If he could then peer at the clock in spaceship, it would be moving slowly.

19. Apr 5, 2005

### eNathan

1. If you were to violate relativity, and travel at the speed of light (v \ c) then from your frame of reference, time would stop around you. And from everybody else's frame of reference, time would stop for you. If you travel faster than light (v > c) then you will have negative time, length, and your mass will be beyond infinite. We cannot make mathematical sense out of this.

2. I cannot make sense out of your question. If you are talking about general relativity which says acceleration and gravity are the same, then this can be calculated. The escape velocity of the earth is 11000 meters per second per second. So time for us will slow down by a factor of 0.999999999. So if you were on earth for 1 hour, people in space would measure your time as 0.999999999 hours (and those are not significant numbers, it is less than that but google always rounds large numbers). Not much of a difference eh? But if your you are refereing to how the planets and galaxies are moving through the universe, those are great speeds so time would slow down significantly. But I do not know that velocity.

20. Apr 5, 2005

### JesseM

No, an outside observer also sees the light beam move at 186282.397 miles/sec in his frame. He would see clocks aboard the ship running slow, if that's what you mean.
No, or at least I can't think of any way that statement would make sense.
In a way I guess this is true. The way spacetime works in relativity can be derived uniquely from two postulates--the first says that the laws of physics should look the same in every reference frame, the second says light should be measured to move the same speed in every reference frame. If these are true, then each frame must see other frame's rulers shrink and clocks slowed down in the way relativity predicts.
Well, you probably want to start with learning about the "Lorentz transformation" which tells you how, if you know the coordinates of an event in one frame, you can figure out the coordinates of the same event in a different frame. You can play around with the Lorentz transformation equations and see for yourself that if the distance/time between two events is c in one frame, it will also be c in another frame. Once you are familiar with the Lorentz transformation then you can try to understand how it can be derived from Einstein's two postulates which I mentioned above. I'm not sure what good sites for these things would be, maybe someone else can offer a recommendation, or you can try looking around on google. This page is pretty good for learning the concepts of relativity, but it doesn't seem to talk about the details of the Lorentz transformation or how it is derived.

I can give you the equations of the Lorentz transform right now, though. Suppose you have two reference frames A and A', with A' moving at velocity v along the x-axis of A, and with the origins of each coordinate system matching at time zero in both frames (ie when t=0 in A and t'=0 in A', the point in space x=0, y=0, z=0 in A matches the point x'=0, y'=0, z'=0 in A'). In that case, if you know the space and time coordinates of some event x,y,z,t in A, and you want to know the coordinates x',y',z',t' of that same event in A', you'd use the Lorentz transformation:

$$x' = \gamma (x - vt)$$
$$y' = y$$
$$z' = z$$
$$t' = \gamma (t - vx/c^2)$$

where $$\gamma = 1/\sqrt{1 - v^2/c^2}$$

That's all there is to it, from this you can prove that each frame sees the other's clocks slowed down, that each sees the other's rulers shrunk, that if something is moving at velocity u along the x'-axis of A' then it will be moving at $$(u + v)/(1 + uv/c^2)$$ along the x-axis of A, and so on.
If the outside observer is at rest relative to the ship, then he will see the rocket moving at 7,000 mph, but even in Newtonian physics, if the outside observer saw the ship moving at 3,000 mph in his frame, then he wouldn't see the rocket moving at 7,000 mph, he'd see it moving at 10,000 mph. But like I've said, velocities don't add in this simple way in relativity. If you like we could analyze this problem in terms of the Lorentz transformation, finding the coordinates that the rocket is launced and the coordinates it reaches the front end, as well as the coordinates a light beam is emitted from the back and the coordinates it hits the front end, in both the rocket's frame and another frame where the rocket is moving.

Last edited: Apr 5, 2005