Why can't we reach to Speed of light at Space?

[Drakkith]

Mass and velocity are two totally unrelated physical parameters.
They shouldn't even be remotely related, but special relativity somehow made mass dependent on velocity.
Is it something wrong in our very definition of mass and speed?

No, why would there be?

 

[phinds]

Mass and velocity are two totally unrelated physical parameters.
They shouldn't even be remotely related, but special relativity somehow made mass dependent on velocity.
Is it something wrong in our very definition of mass and speed?

No, you have simply bought into a pop-science misconception called "relativistic mass" which has been deprecated for decades.

Think about it this way: if mass were actually dependent on speed, then an object would have to have an infinite number of different values for its mass, all at the same time, because it has an infinite number of different speeds, depending on the infinite number of reference frames that one can choose to look at it from. Clearly doesn't work.

 

[uumlau]

Mass and velocity are two totally unrelated physical parameters.
They shouldn't even be remotely related, but special relativity somehow made mass dependent on velocity.
Is it something wrong in our very definition of mass and speed?

No, the "mass" isn't dependent on velocity. The "mass-energy" is dependent on velocity.

The easiest way to look at it is with the equation E^2 = p^2 + m^2 ($c$ is set to "1" here, for mathematical convenience.)

The $m$ in the equation above is the actual mass of whatever object you are dealing with. $p$ is its momentum, and $E$ is its energy.

If the object is sitting still, then $$E = m,$$ or, adding the $c$ back in, $$E = mc^2.$$ (This should be a very familiar equation to you! )

Photons, however, have no mass. Photons and all massless objects instead have $$E = p,$$ or $$E = pc$$

If you have a system of objects and/or particles, the total effective mass of the system (how hard is it to push, how much it bends space-time to generate gravity) is a sum of all the masses, $m$, plus the contributions from the momentum of each. This includes all of the electrons and protons and neutrons and such, along with any photons or other massless particles that are part of the system. This is a simplistic, cartoonish version of the actual physics, of course. The real version involves stress energy tensors and general relativity.

However, since it's kind of silly to bring in full-fledged general relativity in for just determining how forces will interact with a single particle, you can use the short cut of saying that the "effective mass" of the particle is $m\gamma$, because the momentum of the particle is $m\gamma v$, where $\gamma$ is the the Lorentz factor from special relativity.

Read here for more detail, including the minor controversy on whether "relativistic mass" is a useful concept: http://en.wikipedia.org/wiki/Mass_in_special_relativity

The main problem with the concept of relativistic mass is that it leads to misunderstandings such as exemplified by your post. No, the "mass of the object itself" is not changing, but it seems to be implied that it is. It's just a convenient way of doing calculations in very simple systems.

 

[AgentSmith]

So, let's say I, in my close to c spaceship, pass three observers. One is traveling close to my speed in my direction with a difference of 100 mph. I pass another standing on a planet in a relatively...there's that word again...stationary position. And a third in a spaceship going the other direction at near c.

Now, as I understand it two things are happening.

As to actual relative speeds...

I pass the first with an actual difference in speed of about 100 mph. I actually pass the observer on the planet at near c. As to the third, I actually pass him at near 2c, actual theoretical velocity.

Now to the second thing.

The first observer observes me pass him at about 100 mph since relativistic effects are minimal at those speed differences. The second has considerable relativistic observational warpages but thinks I'm going near c. The third doesn't observe the actual near 2c velocity difference. To both me and the other guy, we both will never observe the other going greater than c. To each of us the most we can hope to observe is each other traveling away from each other at no more than c.

Is that close to correct?

tex

Sorta kinda. You have to use the Lorenzt equations to find your velocity from different observers view. You can find tutorials on this at World Science University. You will never observe anyone traveling at c, not just faster than c.

 

[Soul Intent]

I will not pretend that I am an expert, but I do know that the laws of physics break down at that speed correct?

Only if the object has a predetermined mass.

 

[phinds]

Only if the object has a predetermined mass.

No the laws don't break down for massive objects at that speed, they show that you can never GET to that speed, as has already been pointed out.

 

[Soul Intent]

No the laws don't break down for massive objects at that speed, they show that you can never GET to that speed, as has already been pointed out.

Yeah but something like a photon could get to that speed. The only reason a massive object can't accelerate to the speed of light is being it has mass.

 

[phinds]

Yeah but something like a photon could get to that speed. The only reason a massive object can't accelerate to the speed of light is being it has mass.

Yes. Your point being?

And by the way, it is not "a photon could get to that speed", it's "a photon has to travel at that speed and only that speed (in a vacuum)"

 

[Drakkith]

There is a subtle but important difference between a photon getting to c and being at c. A photon doesn't accelerate up to c. From the moment it is created to the moment it is absorbed it travels at c.

 

[PWiz]

After reading many (confused?) posts in this thread, I'm finally beginning to appreciate the decision of discarding "relativistic mass" from common scientific literature nowadays. It tends to make people think that the fact that an object has any mass at all is a result of choosing a particular coordinate system.

 

[Monsterboy]

I think you are right , what the OP asked is that , if any object( spacecraft in this case) is accelerated continuously in space without running out of fuel , why can't that object reach or exceed the speed of light ?

I found this when I googled http://physics.about.com/od/relativisticmechanics/f/SpeedofLight.htm
So, according to this link , you can travel at the speed of light but you will need infinite amount of energy to do so.

The universe itself has finite amount of energy (first law of thermodynamics) hence you can't travel at the speed of light.
I hope someone with a background in this subject validates the information in this link.

That would better be interpreted as "because there is no such thing as an infinite amount of energy, objects with mass cannot travel at the speed of light".

Agent Smith, this is a misleading argument. It implies that the reason we cannot reach the speed of light is a limitation of the propulsion. This is not so.

Even a "magical" propulsion system that had unlimited thrust, unlimited fuel and unlimited time will never reach c.

In fact, it is the laws of the universe itself that prevent us from reaching the speed of light. The nature of the limit is time dilation.

So the article i provided is wrong ?

 

[DaveC426913]

So the article i provided is wrong ?

From post #33?

Er... Any specific part?

 

[rootone]

So the article i provided is wrong ?

If you mean the article posted just above, the article is right, but you have misunderstood it.
Since an infinite amount of energy is something which cannot physically exist then it's impossible for an object with mass to travel at the speed of light..
Don't be thinking 'Well an objectt could travel at light speed if it had that energy available' - That amount of energy is not available, infinity i not a normal number.
Infinite energy is impossible, so therefore travel at light speed is impossible.

 

[Monsterboy]

From post #33?

Er... Any specific part?

Yes ... http://physics.about.com/od/relativisticmechanics/f/SpeedofLight.htm

Slower Than the Speed of Light
The next major set of particles (so far as we know, all of the ones that aren't bosons) move slower than the speed of light. Relativity tells us that it is physically impossible to ever accelerate these particles fast enough to reach the speed of light.

Why is this? It actually amounts to some basic mathematical concepts

Since these objects contain mass, relativity tells us that the equation on the top right of this page determines the kinetic energy of the object, based upon its velocity. Notice the denominator which contains the variablev (for velocity). As the velocity gets closer and closer to the speed of light (c), that v2/C2term will get closer and closer to 1 ... which means that the value of the denominator ("the square root of 1 - v2/C2") will get closer and closer to 0.

As the denominator gets smaller, the energy itself gets larger and larger, approaching infinity. Therefore, when you try to accelerate a particle nearly to the speed of light, it takes more and more energy to do it. Actually accelerating to the speed of light itself would take an infinite amount of energy, which is impossible.

 

[phinds]

Yes, this says exactly what WE have been saying. If you think otherwise then you are somehow misinterpreting it.

 

[DaveC426913]

I was going to say that the idea of 'not being able to approach c because of mass increase' is an outdated explanation.

 

[Monsterboy]

Even a "magical" propulsion system that had unlimited thrust, unlimited fuel and unlimited time will never reach c.

As the denominator gets smaller, the energy itself gets larger and larger, approaching infinity. Therefore, when you try to accelerate a particle nearly to the speed of light, it takes more and more energy to do it. Actually accelerating to the speed of light itself would take an infinite amount of energy, which is impossible.

Yes, this says exactly what WE have been saying. If you think otherwise then you are somehow misinterpreting it.

I just equated "unlimited fuel" and "infinite amount of energy" ,then Dave's statement goes like this " Even a "magical" propulsion system that had unlimited thrust, infinite amount energy and unlimited time will never reach c.

If you meant what rootone said, then okay i got it.

 

[uumlau]

I think that while the article is technically correct in saying that it would take an infinite amount of energy to accelerate to the speed of light, it's a poor phrasing subject to misinterpretation. You don't even need math to understand why. All you need is the principle of equivalence as applied to electromagnetic phenomena. The math is just a side effect and as I've mentioned elsewhere, the math is not the physics.

Due to the principle of equivalence, no matter how "fast" you go, the speed of light, to you, will still be $c$. If you go even faster, it's still $c$. If you stop, it's still $c$, if you start running backwards, it's still $c$. So saying that it would take"an infinite amount of energy" is misleading, because it suggests that you'd even make a dent in catching up with a beam of light. Yes, other objects might seem to be getting close to $c$, that some kind of extra push might get them faster than $c$, but that's mostly an illusion. In those other objects' frames, photons still travel away from them at $c$, with no chance to catch up at all.

 

[Neandethal00]

Think about it this way: if mass were actually dependent on speed, then an object would have to have an infinite number of different values for its mass, all at the same time, because it has an infinite number of different speeds, depending on the infinite number of reference frames that one can choose to look at it from. Clearly doesn't work.

Shouldn't this be the reason for doubt about interpretations of special relativity? At one point we are saying mass is not dependent on velocity because that would make all objects to have no definite mass. Then we turn around and say mass increases with velocity.

@uumlau: You are using relativistic mechanics to prove certain outcome of relativistic mechanics are wrong. This will never happen.

 

[rootone]

All objects excepting photons (there may be a few others theoretically) have a rest mass and that mass is the same for all identical objects.
Relativistic mass when the object is moving in relation to some frame other than itself is a consequence of mass energy equivalence.
In that case, the moving object has kinetic energy added, the object has greater energy than the non moving object.
Since energy and mass are equivalent in relativity, the object's greater energy can be considered as greater mass.

 

[phinds]

. Then we turn around and say mass increases with velocity.

I don't have any idea who that "we" is that you are talking about. I'm not aware of any knowledgeable physicists who says any such thing and you will not find any such members of this forum saying so. There are HUNDREDS of threads on this forum pointing out that objects do NOT gain mass in that manner, they gain energy.

 

[Neandethal00]

Since energy and mass are equivalent in relativity, the object's greater energy can be considered as greater mass.

Good, this can be one interpretation of effects of velocity on special relativity.
My thinking is it is not mass that increases, it is the inertia that increases requiring larger energy to move through space.
We know very little about interaction between 'empty space' and matter.

If there are 'fish scientists' in the ocean, the fish scientists formulate theories of physics to explain everything to other 'fish' totally ignoring the 'water'.
That's what we (Yes, Phind, I consider myself a scientist, it is called self criticism) are doing now.

 

[PWiz]

there are 'fish scientists' in the ocean, the fish scientists formulate theories of physics to explain everything to other 'fish' totally ignoring the 'water'.

We do that with light because there is no "medium" required for it to propagate through - no "ether". This might not always be the case with other phenomenona.

My thinking is it is not mass that increases, it is the inertia that increases requiring larger energy to move through space.

That's right. This is best demonstrated using a force 4-vector: $F_μ = γ\frac{∂p_μ}{∂t}$. This implies that the force required to accelerate an object at a constant value tends to infinity over time (for rectilinear motion).