# Confused on how nothing can be faster then light

1. Oct 7, 2012

### Cyclopsicle

I'm confused on how it would be impossible to be faster then light, is it your acceleration? or is it how fast you are going in comparison to some other frame of reference (I do see that phrase being thrown around here)? Because if 2 objects are moving towards eachother at more than half the speed of light, doesn't it mean that in respect to eachother it is moving faster then c?

2. Oct 7, 2012

### StukaJU87c

The problem with any type of matter travelling at or near the speed of light is that as speed increases, so does mass. At the speed of light mass would become infinite and thus require infinite propulsion and there are just all sorts of problems with that.

Not to mention faster than light travel would in theory cause one to travel back in time which violates causality. Explanation as to why traveling into the past doesn't work is rather obvious and circular to explain.

3. Oct 7, 2012

### Cyclopsicle

ok, but what if two objects are moving towards eachother at more than half c, in reference to a body at rest, wouldn't the objects be travelling more than the speed of light in reference to eachother?

4. Oct 7, 2012

### StukaJU87c

This I'm not entirely sure about here's how it seems to me:

Yes, say they are both travelling at 60% of c. In reference to object A, object B would be travelling towards it at 120% of c and vice versa, however neither has actually exceeded the speed of light. They would both be experiencing the same amount of time dilation and length contraction. In reference to the speed of light being a cosmological speed limit, I believe the only reference frame that matters is that of a body at rest.

5. Oct 7, 2012

### Cyclopsicle

Yes but couldn't you pick one of the objects as the reference frame and see the other object exceeding the speed of light?

6. Oct 7, 2012

### StukaJU87c

It would be more or less an illusion. What matters here is that nothing with mass can travel at or faster than 299,792,458 m/s. Both object A and B (at 60% of c) are travelling at 179,875,474 m/s. Neither has truly exceeded the speed of light, even if it seems as such from either perspective.

7. Oct 7, 2012

### Staff: Mentor

Velocities don't "add" in relativity the same way that they do in non-relativistic mechanics.

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

Relative velocity of two objects moving towards each other:
http://hyperphysics.phy-astr.gsu.edu/hbase/relativ/einvel2.html#c2

8. Oct 7, 2012

### algebrat

If two objects are moving at each other, their relative velocity is not computed the way you are thinking. You have to use the formula for adding velocities. You're instinct to simply combine the velocities is a result of our everyday experience with nonrelativistic speeds.

http://math.ucr.edu/home/baez/physics/Relativity/SR/velocity.html

9. Oct 8, 2012

### HallsofIvy

Staff Emeritus
Specifically, if you observe two objects moving directly toward you at 60% the speed of light, then each, relative to the other, has speed
$$\frac{.6c+ .6c}{1+ \frac{(.6c)^2}{c^2}}= \frac{1.2c}{1.36}= .88$$

So each would see the other as approaching at 88% the speed of light, not 120% the speed of light.

10. Oct 8, 2012

### phinds

Halls clearly meant towards EACH OTHER

11. Oct 8, 2012

### harrylin

Most of the answers were probably given quite recently on this forum, for example here:

and here (ongoing discussion):

In what way are your questions different from those?

12. Oct 8, 2012

### HallsofIvy

Staff Emeritus
Actually, in order not to worry about angles, I was thinking of the situation in which "you" are standing directly between two objects, each moving toward "you" with speed, relative to "you", of .6c.

13. Oct 20, 2012

### EBENEZR

If, for whatever reason, you travelled faster than the speed of light relative to another individual, would you not merely experience situations before the other individual has had a chance to catch up? Much like that early episode of Red Dwarf, you wouldn't be travelling back in time, but in fact in front of other people's time?

For that matter, does the fact light is limited to a certain velocity, time travels at a particular velocity also? Time is not in fact instantaneous, rather time travels at ~299x10^6m/s?

14. Oct 20, 2012

### Staff: Mentor

Yes, according to FICTION like Red Dwarf that is exactly what happens. Different fiction authors may do it differently.

However, this forum is for discussing SCIENCE. The premise of your question is wrong; no massive object can travel at or faster than light locally. That is all that science can say about it.

15. Oct 20, 2012

### harrylin

16. Oct 20, 2012

### HallsofIvy

Staff Emeritus
The problem is that you are effectively saying "If relativity we not true, what would relativity say about this situation?" If relativity is not true (if you CAN travel faster than light), then relativity wouldn't say anything about! You have to fall back to Newtonian physics which says that it is possible to travel faster than light but nothing can travel back in time.

17. Oct 21, 2012

### EBENEZR

Sorry! I'm new to all this, I know the rules of the forum but I don't know physics that well. In my defence, everything I said was a question.

Last edited: Oct 21, 2012