Question

Is it possible to achieve speed of light? I was thinking about this situation. If a train is travelling at 70km/h, and if we install another smaller device which travels 30km/h inside the train. so does it mean that the device is actually travelling at 100km/h(vector addition) with respect to the people looking from outside the train? So can we have a series of combinations to achieve the speed of light?

Answer 1

no, we can't.
the speed of light can only be achieved by massless entity.
beside, the velocity addition that you mention above works only for classical physics, for relativistic physics we have to use Einstein's velocity addition.

Answer 2

i think yoy can achieve it but not pass

Answer 3

Velocity does not add linearly under special relativity. Rather, \(v_1\) and \(v_2\) directed in the same direction add as follows.\[\Huge v_{1+2}=\frac{v_1+v_2}{1+\frac{v_1v_2}{c^2}}\] See the second section here. http://en.wikipedia.org/wiki/Velocity_addition_formula

Answer 4

Ok i will explain it in my own way.

An object with certain kinetic energy, has greater value of mass than a similar body at rest.. Its just that the mass equivalent to Kinetic energy is added to it.
Now, as the object approaches a velocity near to the velocity of light, it's Kinetic energy is tremendous.. so, it's mass is also tremendous.. Now to accelerate it, you need a force given by the relation, F=ma.. Here mass is of a much greater magnitude and so would be the force required to accelerate it.. Ok, somehow you manage to provide that force and accelerate it.. What then, the velocity will increase again and the KE will increase and mass will increase.. This process continues... velocity increases, mass increases.....and ultimately as the speed approaches velocity of light, its mass finally tends to "infinity".. (yet the speed is not equal to that of light).. So, to achieve a velocity of light you need to accelerate it further.. But now by the relation F=ma, you need an infinite force (the mass already tends to infinity)... So, can you generate infinite force?

Answer 5

I prefer @yakeyglee's explanation, which is purely kinematic.

Actually @ujjwal's statement explains what happens in particle accelerators but does not involve adding velocities because one frame is moving relative to another frame.

BTW: in lots of countries, mass does not increase with speed; we use a different expression for relativistic momentum instead.

Answer 6

Here's a very physical description of @yakeyglee's equation as told by Prof Stephen Hawking--Suppose a train is moving at a speed very, very close to that of light with respect to a person standing on the platform. Now suppose that a little girl from the back of the train comes running to her mom who is sitting near the front. Does this girl break the speed of light?
The answer is NO, she doesn't. It is because, even though she *thinks* everything is normal and that she ran 5 metres in one second (making her speed 5m/s), her time will be so slowed down that (according to the observer on the platform), although her speed is greater than that of the train, it is still well within the speed of light.

Answer 7

One of einstein's fundamental postulates says that even if you run in the opposite direction of light at some velocity, it will still pass you at the speed of light, so u cant create these combinations like u say

Answer 8

Thanks a lot for the opinions. Anyway I might need to check on the web about what you guys are saying as I did not study so thoroughly about special relativity. :O