# Why Light?

1. Jun 6, 2007

### LurkingEyes

I'm sorry if this doesn't exactly fit in the topic, but it applies to the general subject I guess.

I have been pondering on this question for about a week, and now I want to ask it.

Why do we have it so ingrained in us that we HAVE to go the speed of light (or faster) to get anywhere? Assuming light travels 186,000mps in a vacuum, and taking into account a previous threads and observations where light is alway moving c faster than you in any frame of reference, then it is and has been blatantly obvious that we'll never reach that speed, because, as said a million times, that becomes infinite. So why not just go 186,001mps? Is there anything saying that we can't go faster than light's speed (with us at a standstill, I'm assuming)? Or twice or ten times that speed? Common sense tells that if you just keep on the accelerator that you'll eventually go faster than that, but what I'm not sure of, is if general physics breaks down at that speed, even though relatively you're not even near c.

Or is it just pop-culture that we assume we have to achieve something that we think is impossible?

Last edited: Jun 7, 2007
2. Jun 7, 2007

### chroot

Staff Emeritus
Special relativity does not forbid velocities greater than c; particles with velocities in excess of c are called tachyons (or would be, if they existed).

However, special relativity does forbid a body moving at a velocity less than c (like us, or our spacecrafts) from ever crossing the boundary and moving faster than c. Similarly, it prevents anything moving faster than c (like those tachyons) from ever being able to slow down to anything less than or equal to c.

Nothing with mass can ever travel exactly c. Conversely, a particle with zero mass cannot travel at any velocity other than c.

- Warren

3. Jun 7, 2007

### LurkingEyes

You're using C here as light + your speed, or light in a vacuum?

4. Jun 7, 2007

### chroot

Staff Emeritus
The constant c (always lower-case) always means the speed of light in vacuum.

- Warren

5. Jun 7, 2007

### LurkingEyes

Gotcha, thanks, that clears up a Lot. I guess I just haven't come to terms with why relative speed doesn't up the limit. I'm going to read around more.

6. Jun 7, 2007

### chroot

Staff Emeritus
Special relativity is based entirely on two postulates:

1) The laws of physics are the same in all inertial frames.
2) The speed of light is constant, for all observers.

All of the rest of special relativity (time slowing down, particles not being able to accelerate to or past c, etc.) are simply conclusions drawn from these two postulates.

- Warren

7. Jun 7, 2007

### pervect

Staff Emeritus
The original poster (OP) might be interested in http://math.ucr.edu/home/baez/physics/Relativity/SR/rocket.html [Broken]

which describes the equations of motion of a rocket accelerating a constant proper acceleration.

"Proper acceleration" is basically the acceleration one would measure on a rocket with an accelerometer.

It will be seen that the increase in velocity is monotonic, as one would expect - i.e. there are no sudden "jumps" in velocity.

Because "jumps" are impossible, it is not possible to jump from below 'c' to above 'c' and the velocity of the continuously accelerating rocket with a constant proper acceleration will always be below c.

The behavior of the relativistic rocket can be somewhat intuitively understood in terms of the velocity addition formula with enough effort:

v' = (v + delta) / (1+ v*delta/c^2)

One can approximate the continuous acceleration process by making delta very small, and performing repeated iterations of the addition formula. The result will be a function that approaches 'c' but never reaches it.

Alternatively, one can just study the formula given for the continuous case.

On a "practical" level, one will note that as far as ship time is concerned, being able to reach 'c' doesn't matter - one can still cover enormous distances in short amounts of ship time (proper time) if one could build a rocket that could accelerate at 1g for multi-decade periods.

Examining the details of what is required to build such a rocket will illustrate why this isn't really all that practical even making wildly optimistic theoretical assumptions. For instance, with a matter antimatter rocket, one would need enormous payload/mass ratios. Lightsail type rockets, using an external laser for power, would seem more practical, but a more detailed analysis including issues like focussing distances and doppler shifts illustrate that this idea has severe problems as well.

Last edited by a moderator: May 2, 2017
8. Jun 7, 2007

### LurkingEyes

That's exactly the type of answer I was looking for. Not how I was approaching the problem, but that's the answer I needed. So really, to get that far you wouldn't need to go FTL, just find a way to keep accelerating, for as long as possible. Where .999c is a lot faster than .99c. I had looked at it this way once, but went in favor of what I originally explained.

9. Jun 10, 2007

### jiufer

Wait, so if we are to agree that slowing any mass moving at the speed of light would require an infinite amount of force to speed up or slow down that mass as Einstein stated, how is it possible that experimentation on slowing the speed of light through certain materials has been progressing such as in this article which states they have managed to completely stop light? Are they addressing something different or is it that they aren't actually slowing light but "capturing" it in a mirrored cube and releasing it later?

10. Jun 10, 2007

### chroot

Staff Emeritus
The theory of relativity says nothing about the speed of light in materials; it only involves the speed of light in vacuum.

- Warren

11. Jun 10, 2007

### rbj

it's not just the speed of light, which is the speed of the EM action, which is the speed of the effect of me moving a charge (that i am holding) on another charge (that you are holding).

$c$ is the speed of all things ostensibly instantaneous. gravitation, too.

that's my canned answer to the "why light?" or "what should light, and the speed inherent to light, be this critical scaling parameter in relativity? why not the speed of the electron in the Bohr atom (which happens to be $\alpha c$)?"

Last edited: Jun 10, 2007