Why is the speed of light our "speed limit"?

[Shadow Conception]

Why LIGHT? Why couldn't it be something else? What is so special about photons that allows them to travel so fast? Is it their lack of mass? How can anything be massless?

Also, if I were a photon, and my name were Mr. Photon fathering five children and loving a lovely Mrs. Photon... would time be frozen for us all? Photons don't experience time because they travel at the speed of light?

This is assuming that FTL travel is impossible.

 

[Cogman]

1. Photons aren't massless.
2. Photons aren't things either, they are waves, and particles. If you think of a photon as a ball or whatever then you are off. If you think of it as a wave you are also off. It is a wave-particle.
3. If you where traveling near the speed of light (you and your environment) then you wouldn't notice anything, time would appear to be stopped to you, but every thing you are passing would look like it wasn't moving.

As for the "Why can't things go faster then light" I believe the answer is something to the effect that as something nears the speed of light its mass increases and nears infinity. Something like that, so the force required to propel the object past the speed of light would be infinite.

 

[Shadow Conception]

But to travel at exactly the speed of light... if photons aren't massless, how do they pull it off? To travel at the speed of light, your mass would have to be infinity, and the force needed to propel you would be infinite. Is it 'cause they're "wave-particles"?

Or maybe my facts are just wrong, that's more likely.

 

[DrPizza]

And, on the opposite side of the speed of light, the speed of light also serves as a boundary. The concept of tachyons boggles my mind though, when I think about the size of the universe, etc. considerations. But, I've done very little reading on them - just enough to know that they don't interact with "our universe" & that people in higher education actually do write papers with different hypotheses.

 

[firewolfsm]

Tachyons are particles that have imaginary mass, that is the only way to travel faster than light and satisfy Einstein's equations. Particles with normal mass would require infinite energy to reach the speed of light. Photons are massless in the sense that they have no rest mass. Their mass comes from their energy (E=mc^2.) If it were to stop, it would cease to exist, and therefore doesn't. Particles with imaginary mass, if real, can only travel faster than light and never reach C for the same reason normal particles can't.

But that's not really your question.

C is defined because photons are in fact massless, it just depends on your definition.

A way to understand the change in time with speed is to imagine "spacetime" as something we are all traveling though . The faster you move through space relative to others, the slower you move through time relative to others. If someone could reach .999C in a spaceship they would fly around and watch the universe evolve, crunch, expand, cool, whatever would happen to it, would happen in almost an instant for their eyes because while they are slowed a new point of view is set, and everything else seems to speed up.

 

[KIAman]

The simplest answer to the OP's questions is that light is the fastest form of information transfer. There could very well be things beyond our wildest imagination that are faster than light but there is no way for us to know it, as of now, because we cannot detect it using light.

It's kinda like a microscope using optical light and one using electron scanning. The resolving power (how small you can see) depends on the wavelength of light and the best case for a light microscope is resolving around 400nm in size. An electron scanning device can resolve down to .1nm. So using an optical microscope, that 10nm object will be invisible (the wavelength completely passes over the length of the object) while the electron scanning can see it.

So, until we have a faster medium of information transfer, light, simply by our definition of information, is the cosmic speed limit.

Also, photons do have a mass, just no rest mass.

 

[lousydood]

Flip your question around: why do massive particles not travel at the speed of light? Every "mass-less" particle travels at the speed of light: photons are simply the bosons which transmit the electromagnetic force. Bosons are a general category of particle -- with integral spin and not subject to the Pauli exclusion principle. There are plenty of other such mass-less particles which travel at the "speed of light" which should be renamed "the speed of mass-less particles."

So one might say the distinguishing characteristic of massive particles is that they travel slower [1]. The popular explanation of mass is the Higgs mechanism: a Weak-force field permeates space and particles with mass interact with this field via the Higgs boson. (I hope I didn't screw that explanation up too much). My interpretation of this idea is that massive particles are prevented from traveling the speed of light because of this mechanism, and the effect is observed as "mass." The new particle accelerator hopes to find evidence of the Higgs boson, which would put this on a solid experimental foundation.

As for relativistic effects, there are 3:

1) Time Dilation. If a spaceship whizzes by you at high speed, you observe time to move more slowly aboard the spaceship.

2) Length Contraction. The spaceship also appears to become shorter in the direction of travel. If you are on-board the spaceship, then everything else seems to be shorter in the direction of travel, including the distance you have left to go. Approaching the speed of light, the distance remaining to your destination would approach zero asymptotically.

3) Relativistic Momentum. The momentum of the spaceship goes up very fast as you approach the speed of light.



[1] In a vacuum. On a side note: Feynman's QED theory permits photons to travel slower and faster than the speed of light, even in a vacuum, but the sum of amplitudes (normalized probability vectors) will result in the observed effect of a photon traveling at the speed of light.

 

[firewolfsm]

The Higgs Boson would give particles the property of mass and slow them down. That's the simplest explanation available.

 

[silverpig]

Originally posted by: Cogman
1. Photons aren't massless.
2. Photons aren't things either, they are waves, and particles. If you think of a photon as a ball or whatever then you are off. If you think of it as a wave you are also off. It is a wave-particle.
3. If you where traveling near the speed of light (you and your environment) then you wouldn't notice anything, time would appear to be stopped to you, but every thing you are passing would look like it wasn't moving.

As for the "Why can't things go faster then light" I believe the answer is something to the effect that as something nears the speed of light its mass increases and nears infinity. Something like that, so the force required to propel the object past the speed of light would be infinite.

3. is backwards. If you got to c the universe would end in the blink of an eye

 

[Biftheunderstudy]

As a distinction, photons are massless. What they do have is momentum, but this does not mean they have mass.
That being said there are parts of BEC theory which have the photon gain mass (real mass not just momentum) inside a superconductor. As an interesting digression, an off shoot of the same theory shows the graviton with a real mass too...
The finite propagation speed of a wave can be proven mathematically, the details escape me but I've seen it done.
The really deep question is: why is c = 3x10^8?
I mean it doesn't have to be, its just defined that way because of the measurable fundamental constants that give rise to it. Namely the permeativity and the permeability of free space.

 

[Cogman]

Originally posted by: silverpig
3. is backwards. If you got to c the universe would end in the blink of an eye

Crap, I always get that backwards because Im thinking "Go faster then the speed of light, go back in time" But you are right, I remember the picture from my physics book talking about hans and frans and the millennium falcon (hmm, thats a bit ambiguous, oh well)

As a distinction, photons are massless. What they do have is momentum, but this does not mean they have mass.
That being said there are parts of BEC theory which have the photon gain mass (real mass not just momentum) inside a superconductor. As an interesting digression, an off shoot of the same theory shows the graviton with a real mass too...
The finite propagation speed of a wave can be proven mathematically, the details escape me but I've seen it done.
The really deep question is: why is c = 3x10^8?
I mean it doesn't have to be, its just defined that way because of the measurable fundamental constants that give rise to it. Namely the permeativity and the permeability of free space.

Unless I'm mistaken (and I may very well be) they have yet to prove the existence of a graviton. is this just a theroretical thing predicting the existence of the graviton?

 

[PolymerTim]

Originally posted by: Biftheunderstudy
The really deep question is: why is c = 3x10^8?
I mean it doesn't have to be, its just defined that way because of the measurable fundamental constants that give rise to it. Namely the permeativity and the permeability of free space.

Interesting question, but I wouldn't put much into the actual number since it depends on units that researchers contrived out of convenience. It is important to remember what is a real correlation versus one with no meaning. It's kind of like all those doomsayers that predict the end of the world at every millenium/century/decade... You might argue that the year is a natural unit (has a physical basis), but the difference between 1999 years and 2000 years only seems tobe important because of our base ten counting system which, as far as I know, has no natural basis and therefor can not have a causal effect on the outcome.

So I agree with the question, but would reword it to ask how is this constant related to other observables and then learn about the connection between them. The number 3X10^8 really doesn't have any meaning itself since it changes with the units applied to it.

 

[silverpig]

Originally posted by: PolymerTim

Originally posted by: Biftheunderstudy
The really deep question is: why is c = 3x10^8?
I mean it doesn't have to be, its just defined that way because of the measurable fundamental constants that give rise to it. Namely the permeativity and the permeability of free space.

Interesting question, but I wouldn't put much into the actual number since it depends on units that researchers contrived out of convenience. It is important to remember what is a real correlation versus one with no meaning. It's kind of like all those doomsayers that predict the end of the world at every millenium/century/decade... You might argue that the year is a natural unit (has a physical basis), but the difference between 1999 years and 2000 years only seems tobe important because of our base ten counting system which, as far as I know, has no natural basis and therefor can not have a causal effect on the outcome.

So I agree with the question, but would reword it to ask how is this constant related to other observables and then learn about the connection between them. The number 3X10^8 really doesn't have any meaning itself since it changes with the units applied to it.

He's asking a more fundamental question though. Why 2.9979(whatever)e8 and not half that number? Or twice that number? Why is it what it is?

One of the fundamental questions of particle physics right now is why do subatomic particles have the masses that they do? Why is a proton 1836 (I hope I got that right) times heavier than an electron? They hope to answer that question with the LHC and Higgs, but the speed of light one is still interesting.

 

[Eeezee]

The speed of light is the same in all reference frames. If you were accelerate to 0.9999* the speed of light and then you were to turn on a laser, you would still see light moving at the speed of light (relative to you, the light is moving at c while you are stationary even though you are actually on a spacecraft moving 0.9999c through space).

I'm not sure why the speed of light is so definitive. I don't think anyone has a good answer for that yet.

 

[Shadow Conception]

What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

 

[Biftheunderstudy]

Still c, always c according to special relativity. Special relativity states that light always is at c to all observers, through this you get all the consequences such as time dilation, length contraction etc.

 

[PlasmaBomb]

Originally posted by: Shadow Conception
What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

No.

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

 

[RossGr]

Originally posted by: Biftheunderstudy
Still c, always c according to special relativity. Special relativity states that light always is at c to all observers, through this you get all the consequences such as time dilation, length contraction etc.

No, not according to Special Relativity. The constancy of c is a POSTULATE of SR. SR is a result of of a constant c. Maxwel's equations are the source of a constant c. It was because of Maxwell's equations that Einstein was able to postulate c.

 

[RossGr]

Originally posted by: PlasmaBomb

Originally posted by: Shadow Conception
What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

No.

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

I think we need a definition of mass.

 

[Biftheunderstudy]

Good point. As I alluded to earlier, finite propagation speed can be proven mathematically from just the wave equation. Was it not Einstein that came up with constant c in all reference frames though? My history of that is a little fuzzy.

Originally posted by: PlasmaBomb

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

What kind of mass do they have?

 

[destey]

I might be way off here, but here's how i look at it:

There's no particles or anything that goes close to the speed of light. Think of a wave at a ballgame, no people actually move, just what we consider "the wave".

A wave isn't really a tangable thing, its more of an illusion. A way for people to view something. Speed of light is just how fast each neighboring particle can transfer its vibration to the next.

Note: there's no science behind this, mostly bong rips

 

[KIAman]

Originally posted by: destey
I might be way off here, but here's how i look at it:

There's no particles or anything that goes close to the speed of light. Think of a wave at a ballgame, no people actually move, just what we consider "the wave".

A wave isn't really a tangable thing, its more of an illusion. A way for people to view something. Speed of light is just how fast each neighboring particle can transfer its vibration to the next.

Note: there's no science behind this, mostly bong rips

Except that light does not need a medium (in your example, the crowd of people) to propagate. Something is definitely there, we just aren't sure exactly how to picture it in our human limited brains.

 

[Nathelion]

Originally posted by: PlasmaBomb

Originally posted by: Shadow Conception
What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

No.

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

Mass is often defined as what you refer to as inertial mass. Photons are commonly referred to as massless particles. It doesn't really matter which way you do it, but a convention should be established before this thread descends into nitpicking hell.

 

[QuantumPion]

Originally posted by: PlasmaBomb

Originally posted by: Shadow Conception
What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

No.

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

No.

Convention in the physics community is that "mass" means rest mass, inertial mass, invariant mass, etc. Relativistic mass is usually just called energy. You can be silly and say photons have mass all day long, referring to its relativistic mass, but in practice this is wrong.

In other words, the statement "photons aren't massless" is incorrect, because when talking about relativistic mass you have to explicitly say so, since just plain mass always refers to the invariant mass.

 

[silverpig]

Originally posted by: QuantumPion

Originally posted by: PlasmaBomb

Originally posted by: Shadow Conception
What if you turned on a laser on a spacecraft going 0.999c and shined it in the direction of the space shuttle's propulsion? Would this laser be going at (almost) 2c?

No.

PS. Photons aren't massless they just have zero inertial mass. There is a difference.

No.

Convention in the physics community is that "mass" means rest mass, inertial mass, invariant mass, etc. Relativistic mass is usually just called energy. You can be silly and say photons have mass all day long, referring to its relativistic mass, but in practice this is wrong.

In other words, the statement "photons aren't massless" is incorrect, because when talking about relativistic mass you have to explicitly say so, since just plain mass always refers to the invariant mass.

That's how I always learned it.