Why can't we go faster than light?

[kirbsrob]

Why can't we just go faster than the speed of light? I know its supposed to be a big law in physics but it doesn't really make sense to me. I mean light isn't the fastest thing in the universe. There are other various particles that travel WAY faster than light would. So why can't we? How is adding a few more kmps from a speed almost the speed of light to go faster than light such a problem?

[Janus]

Why can't we just go faster than the speed of light? I know its supposed to be a big law in physics but it doesn't really make sense to me. I mean light isn't the fastest thing in the universe. There are other various particles that travel WAY faster than light would. So why can't we? How is adding a few more kmps from a speed almost the speed of light to go faster than light such a problem?

 

Just what particles are you talking about? We know of no particles that travel faster than c. The amount of energy needed to increase your speed increases to infinity as you approach the speed of light. It takes roughly three times as much energy to go from 0.99 c to 0.999c as it did to go from 0.9c to .99c, and it takes another three times as much to go from 0.999c to .9999c. In essence, it just keeps taking more and more energy to add each additional "9", and no finite amount of energy will ever get you to 1c.

[ajb]

Why can't we just go faster than the speed of light? I know its supposed to be a big law in physics but it doesn't really make sense to me.

It is written into the structure of space-time that locally the speed of light is barrier for which nothing can pass.

 

I mean light isn't the fastest thing in the universe.

Massless things, like light are the fastest objects that we know of. However the speed of light law is a local thing and it is possible to have apparent faster than light speed, such as using a wormhole. Also this law does not apply to expansion of space-time itself, so objects can have a separation speed greater than the speed of light, but as these objects don't come close to each other there is no violation of the laws of physics.

 

There are other various particles that travel WAY faster than light would. So why can't we?

Yes, we have the possibility of tachyons which are particles that can never be slowed down to speed slower than the speed of light. However, we have no evidence of such particles and theoretically standard tachyons are unstable and would very quickly decay into standard particles.

Edited March 16, 2014 by ajb

[Endercreeper01]

We can't travel faster then light because of special relativity.

The energy required to accelerate an object to a velocity v is given by

[latex]E=\frac{1}{2}mv^2[/latex]

In classical mechanics. However, this is only an approximation, and one term in an infinite series.

In non-expanded form, the equation for kinetic energy in special relativity is given by

[latex]E=mc^2 \left(\frac {1}{\sqrt{1-\frac {v^2}{c^2}}} - 1 \right)[/latex]

Where c is the speed of light.

As v approaches c, the kinetic energy approaches infinity for something with mass, so we can assume that it takes infinite energy to accelerate a mass to the speed of light.

Because it is impossible to have infinite energy, we cannot travel at the speed of light.

When you try to plug in a velocity greater then the speed of light into the equation, you end up with a complex number. We do not know what it means to have an energy with an imaginary value, but we are unable to obtain imaginary energy values in other ways, meaning it is impossible to accelerate a non-imaginary mass to the speed of light or beyond.

However, it is only impossible to travel at the speed of light in a vacuum. In a medium, light travels slower, although the propagation speed is alway c.

Because light is slower in a medium, it is possible to travel faster then it, if you are in a medium.

Edited March 16, 2014 by Endercreeper01

[Cosmobrain]

The energy of motion turns into m, your mass. The faster you move, the heavier you get

[ajb]

The energy of motion turns into m, your mass. The faster you move, the heavier you get

This is a not so great interpretation of what is going on, it is of course a common one. The problem is two-fold.

 

i) The object itself does not measure its mass to change.

ii) To whom is this motion with respect to?

 

Usually it is best just to keep mass to mean the rest or invariant mass.

[slyrat]

Based on our current understanding of the physics of our universe, nothing can go faster than the speed of light, and only massless objects can travel at lightspeed. My comprehension could be faulty, but I understand E=mc squared means that adding energy to an object also adds to its mass ( mass equals energy divided by the speed of light squared). This includes kinetic energy, thus increased speed means increased mass, which means more energy must be added to the traveller to maintain acceleration, and the increased speed adds to the mass, which means even more energy needs to be added.....

this boils down to the idea that as your speed increases, so does your mass, and to increase your speed (to accelerate) you need to use a percentage of the energy already used to get you to your current speed, added to your current energy usage, and the percentage increases as your speed increases, because your speed increases your mass. this doesn't mean much at the speeds we travel, but when you get to the speeds light travels, the numbers become ridiculous. If I could travel at .999 lightspeed, my mass would be enormous. I believe I was told it would be greater than the mass of the universe. [edit: maybe it was equal to the mass of the universe.]

There may be ways to get from one place to another in an amount of time less than the amount of time light would take to travel there through normal space, but they will not involve flying a ship there through each and every mile of the distance that separates them.

Edited March 25, 2014 by slyrat

[swansont]

If I could travel at .999 lightspeed, my mass would be enormous. I believe I was told it would be greater than the mass of the universe.

 

 

You are traveling at 0.999c with respect to something. Notice any change in mass?

 

Your mass will never be greater than the mass of the universe, since you are part of the universe.

[slyrat]

 

You are traveling at 0.999c with respect to something. Notice any change in mass?

 

as the traveler, you would not notice the change in mass outright, just as you would not notice that time had slowed down for you, from the perspective of an outside observer. You could notice the mass change in the greater proportion of energy required to maintain acceleration for every unit of distance traveled.

 

And you are right, my mass could never be greater than that of the universe. It may have been "equal to the mass of the universe", but either way, it illustrates why speed of light travel would be impossible for anything with mass.

Edited March 25, 2014 by slyrat

[swansont]

That's not the point. All motion is relative to something. You don't need to accelerate to be moving at 0.999c with respect to something, you can accelerate that something.

 

I refer you to ajb's post above. Relativistic mass is a kludge. Invariant mass is much more elegant.

[slyrat]

 

 

I refer you to ajb's post above. Relativistic mass is a kludge. Invariant mass is much more elegant.

I looked up invariant mass and came across a few descriptions that, frankly, made my eyes glaze. I yelled at my dog, who was totally innocent, just to give me a reason to look away from the screen. Can you recommend a resource useful to an intelligent person who just happens to be a noob to that concept?

 

Edit: just to let everyone know, my dog forgave me. I gave her a treat in apology and we went and killed a couple of squeak toys.

Edited March 26, 2014 by slyrat

[ajb]

I looked up invariant mass and came across a few descriptions that, frankly, made my eyes glaze. I yelled at my dog, who was totally innocent, just to give me a reason to look away from the screen. Can you recommend a resource useful to an intelligent person who just happens to be a noob to that concept?

For a single isolated particle, the rest mass is the mass as measured in the rest frame of that particle. It is related to the energy and the momentum of the particle in any inertial frame of reference. That is in part why mass = rest mass is a better concept than the relativistic mass.

[swansont]

And in that way you basically limit yourself to mass and kinetic energy, and all of the frame-dependent variation is in the kinetic energy.

[I-try]

This is a not so great interpretation of what is going on, it is of course a common one. The problem is two-fold.

 

i) The object itself does not measure its mass to change.

ii) To whom is this motion with respect to?

 

Usually it is best just to keep mass to mean the rest or invariant mass.

ajb

The answer to your question 2) as I perceive it to be is; the accelerating ability referred to is relative to the equal and opposite law provided by Newton, and also a disregarding of the relativistic mass idea of GR by a substitution changing to relativistic momentum. If the mass of a body changed proportionally with increasing V, then we would be compelled to remove the directional component from V, because as V is now regarded, it would require an equal amount of energy to affect acceleration in any direction relative to the direction, speed and magnitude of the mass in motion.

 

The false idea regarding the ability to exceed the speed of C is indicated by our correct laws of thermodynamics, whereby we can only recover a given amount of work (in this case acceleration) from energy derived from fuel expended. If most of the mass of a spaceship consisted of the mass of the fuel, and disregarding all ideas derived from GR, the fuel would be completely consumed before C was reached.

If we allow the acceleration to be driven by an externally provided electromagnetic force, there is no possibility of an increase in acceleration beyond C, because the potential accelerating ability of the electromagnetic energy could only equal C. In this case, all relative velocities due to gravitational influence or universal expansion are disregarded.

[ajb]

My question was rather rhetorical. Forgetting about acceleration for now, the motion of a body has to be described as being respect to some other body. Really here we mean with respect to specified inertial frames. A body is at rest with respect to itself and so it will see no change in its "mass". However with respect to some other inertial frame the "mass" will be larger. But this depends of the inertial frame employed.

 

Because of this we cannot really view the relativistic mass as having much intrinsic meaning. The rest mass however does and is invariant under changes of inertial frame.

[michel123456]

My question was rather rhetorical. Forgetting about acceleration for now, the motion of a body has to be described as being respect to some other body. Really here we mean with respect to specified inertial frames. A body is at rest with respect to itself and so it will see no change in its "mass". However with respect to some other inertial frame the "mass" will be larger. But this depends of the inertial frame employed.

 

Because of this we cannot really view the relativistic mass as having much intrinsic meaning. The rest mass however does and is invariant under changes of inertial frame.

Doesn't this fact create a "special" (preferred) Frame Of reference, the FOR that is at rest ? The only one FOR in which one can measure invariant mass.

Edited March 30, 2014 by michel123456

[Strange]

Doesn't this fact create a "special" (preferred) Frame Of reference, the FOR that is at rest ? The only one FOR in which one can measure invariant mass.

 

If you have 100 objects moving at different velocities, then they will each have a different frame of reference. The rest mass of each object will be the mass measured in its own frame of reference. So, clearly, no preferred frame.

[michel123456]

 

If you have 100 objects moving at different velocities, then they will each have a different frame of reference. The rest mass of each object will be the mass measured in its own frame of reference. So, clearly, no preferred frame.

Yes for the bold part

But

 

We are saying the same thing and make different conclusions.

Edited March 30, 2014 by michel123456

[swansont]

Doesn't this fact create a "special" (preferred) Frame Of reference, the FOR that is at rest ? The only one FOR in which one can measure invariant mass.

 

Each observer can measure the total energy and the kinetic energy. The difference (E^2-p^2c^2) will always be the rest energy. In any inertial frame.

[ajb]

The mass does not depend on the inertial frame you employ, you have to take into account energy and momentum in the inertial frame you use. (see above) The mass happens to be the "relativistic mass" when measured in the rest frame of the object.

Edited March 30, 2014 by ajb

[Strange]

Yes for the bold part

But

 

We are saying the same thing and make different conclusions.

 

You are concluding that there is one preferred frame from the fact there are an infinite number of frames of reference?

[michel123456]

 

You are concluding that there is one preferred frame from the fact there are an infinite number of frames of reference?

No. from the fact that there is only one value for the rest mass. And that this value is a minimum. All other observers from different FOR will measure (correctly) another value for this mass. And call this value "relativistic".

Which will always be more than the initial value. Never less.

Edited March 30, 2014 by michel123456

[Strange]

No. from the fact that there is only one value for the rest mass. And that this value is a minimum. All other observers from different FOR will measure (correctly) another value for this mass. And call this value "relativistic".

Which will always be more than the initial value. Never less.

 

None of which says there is a preferred frame of reference.

[SamBridge]

You can appear to be traveling faster than light in a non local frame where space is already moving faster than light, like with predicted redshift acceleration of the universe and the ergoshpere of rotating black holes, probably related to a higher dimensional curvature which is sometimes predicted in wormoles but not completely ironed out. Other than that, the space and time metric decreases as you approach light the conventional way meaning that as you try to travel more distance in a given amount of time, the amount of distance you say you have to travel to get to the next point and the amount of time it will take to get there keeps increasing in just the right way to keep you from going at or past the speed of light. Why does it do that? Who knows.

Edited March 31, 2014 by SamBridge

[michel123456]

 

None of which says there is a preferred frame of reference.

When you have a minimum value in graph, doesn't that point differ from all other values?

 

I would agree if there was a way for some observer to measure a mass less than the rest mass. A situation where rest mass of a random object can be measured by some observer in a range going from zero to infinite.

It is not the case.

What we observe is a range going from rest mass to infinite.

So IMHO rest mass is a very special value and the FOR in which you measure rest mass a very special FOR.