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Why can't matter reach the speed of light?

F

Fox5

Diamond Member
Just wondering, if something was in a vacuum, why couldn't it accelerate forever until it reached the speed of light? What forces would counter the acceleration?
 
Heres one take, someone correct me if Im wrong. To accelerate you need to add energy to the object youre accelerating. Adding energy to it adds to its mass, via the e=mc^2 bit, it is now harder to accelerate. So you add more energy to go faster, again adding to its mass, etc etc. Eventually you have added so much energy, the mass of the object approaches infinite, and you need infinite energy to go any faster. That limit is the speed of light.
 
Originally posted by: Fox5
Just wondering, if something was in a vacuum, why couldn't it accelerate forever until it reached the speed of light? What forces would counter the acceleration?
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Gravity for one I think. Even in a vaccuum can't there still be gravity?
 
Originally posted by: Fox5
Just wondering, if something was in a vacuum, why couldn't it accelerate forever until it reached the speed of light? What forces would counter the acceleration?
Click to expand...

You need to add energy (e.g. rocket fuel) in order to accerlerate an object, vacuum or air really doesn't matter that much.
The higher the speed the more energy you need to add in order to keep accelerating but no matter how much energy you add you can never reach the speed of light.

Gravity has nothing to do with it.
 
Originally posted by: Gamingphreek
Yes, to reach the speed of light matter turns into pure energy requiring infinite energy and having infinite mass.

-Kevin
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Hmm, it's not all relative?
Like someone traveling at the speed of light would appear to us to have infinite mass, but to them they'd think they were unchanged in mass?
 
Seems to me that you folks have it backward a bit. Its not the speed or mass that keeps you from going faster than light, its the time dilation involved. The faster you go, time compresses, to the point where there isnt enough time to burn enough fuel to go any faster...you have to remember time and space (as in distance) are inseperable.
 
There is also the bit about length decreasing as you approach the speed of light, and you cant be shorter than zero length. I think all three explanations are correct, just different aspects of it.

 
The mass of a moving object varies with its speed as such:
Mass(moving) = mass(zero speed) / (square root of (1 - (current speed/light speed)^2))

As you try to add speed, the mass of the object became greater and greater.
(well, it might be a simplistic explanation, but for me it's good enough)
 
Originally posted by: Calin
The mass of a moving object varies with its speed as such:
Mass(moving) = mass(zero speed) / (square root of (1 - (current speed/light speed)^2))

As you try to add speed, the mass of the object became greater and greater.
(well, it might be a simplistic explanation, but for me it's good enough)
Click to expand...

You can use the concept of relativistic mass to explain that, but it will mislead you in reasoning about special relativity in many ways, as Taylor and Wheeler point out in the classic text on relativity, Spacetime Physics:
"Ouch! The concept of `relativistic mass' is subject to misunderstanding. That's why we don't use it. First, it applies the name mass--belonging to the magnitude of a four-vector--to a very different concept, the time component of a four-vector. Second, it makes increase of energy of an object with velocity or momentum appear to be connected with some change in internal structure of the object. In reality, the increase of energy with velocity originates not in the object but in the geometric properties of space-time itself."
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or if you want an older authority, Einstein also made the same point:
"It is not good to introduce the concept of the mass M = m/(1-v2/c2)^1/2 of a body for which no clear definition can be given. It is better to introduce no other mass than `the rest mass' m. Instead of introducing M, it is better to mention the expression for the momentum and energy of a body in motion."
Click to expand...
 
Originally posted by: Fox5
Just wondering, if something was in a vacuum, why couldn't it accelerate forever until it reached the speed of light? What forces would counter the acceleration?
Click to expand...

The relativistic equation for momentum is p = mv / sqrt(1 - v^2/c^2), which mathematically shows that it requires infinite momentum to reach light speed. While that's well experimentally verified in particle accelerators, where objects are accelerated to faster than 99.999999% the speed of light on a daily basis, it may not be the most satisfactory explanation.

The title of Einstein's 1905 paper on special relativity--"On the Electrodynamics of Moving Bodies"--gives us an idea of where to look for the source of this speed limit: Maxwell's theory of electromagnetism. We know that a changing electric field produces a changing magnetic field, which will in turn produce a changing electric field, leading to electromagnetic waves.

Using Maxwell's equations, you can compute the velocity of those waves and the answer is c, the speed of light. Note that the answer is a constant: there's no dependency on the velocity of the electric charge that produced the initial electric field. Maxwell is where the constant velocity of light came from. Einstein used Maxwell's result as the basic of his special relativity.

Now Maxwell didn't say that objects couldn't travel at the speed of light, but let's think aobut what the consequences would be. If you're travelling at the speed of light, then any electromagnetic waves (i.e., light) will be motionless in your reference frame and thus you'll experience a constant electromagnetic field. However, such a field is only produced by stationary charged particles, but there aren't any such sources here. That's a paradox.

Alternatively, we can treat light in terms of particles: photons. Photons are massless particles with an energy of E=hf, where h is Planck's constant and f is the frequency of the light. If you're travelling at the speed of light, then any electromagnetic waves (i.e. light) will simply not exist your frame of reference. As photons would be stationary in your frame of reference, they would have zero energy and zero mass, and thus don't exist. That's another paradox.

Those paradoxes from attempting to reconcile electromagnetism with travelling at the speed of light are what led Einstein to develop his theory of special relativity, as we can read in his own words:
If I pursue a beam of light with the velocity c (velocity of light in a vacuum), I should observe such a beam of light as a spatially oscillatory electromagnetic field at rest. However, there seems to be no such thing, whether on the basis of experience or according to Maxwell's equations. From the very beginning it appeared to me intuitively clear that, judged from the standpoint of such an observer, everything would have to happen according to the same laws as for an observer who, relative to the earth, was at rest. For how, otherwise, should the first observer know, i.e., be able to determine, that he is in a state of fast uniform motion? One sees that in this paradox the germ of the special relativity theory is already contained. Today everyone knows, of course, that all attempts to clarify this paradox satisfactorily were condemned to failure as long as the axiom of the absolute character of time, viz., of a simultaneous, unrecognizedly was anchored in the unconscious. Clearly to recognize this axiom and its arbitrary character really implies already the solution to the problem.
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as cquark mentioned, it's better to say momentum approaches infinity, rather than mass.
 
Hmm, can I propose one more thing...
The speed of light is a constant from any frame of reference, right?
In that case, could it be that things can exceed the speed of light, but we just can't observe them because they're traveling faster than light?
 
Originally posted by: brentkiosk
A photon is light. Not surprisingly, it travels at the speed of light. Photons don't start from rest and accelerate.
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I would argue they do. For example, when you turn on the light switch.
 
Originally posted by: her209
Originally posted by: brentkiosk
A photon is light. Not surprisingly, it travels at the speed of light. Photons don't start from rest and accelerate.
Click to expand...
I would argue they do. For example, when you turn on the light switch.
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Wouldn't that only apply if photons were matter? If they're just waves, then I don't think they'd need to accelerate, though the quantity emitted would increase as the light bulb powers on.

BTW, why can't things travel faster than the speed of light, just we can't observe anything faster than the speed of light....maybe that'd explain quantum physics!(how can something be in multiple states/places at the same time? it isn't, we just observe it to be because it went faster than light...)
 
Originally posted by: Chode Messiah
could a black hole somehow be accelerated to the speed of light?
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I don't understand what you mean... how can a black hole even be accelerated to ANY speed?
 
Originally posted by: her209
Originally posted by: brentkiosk
A photon is light. Not surprisingly, it travels at the speed of light. Photons don't start from rest and accelerate.
Click to expand...
I would argue they do. For example, when you turn on the light switch.
Click to expand...

No, when you turn on a light switch photons are created (the enery of the electric current of converted into radiation=photons); each photon carries an energy (Planck's constant)x(frequency) and travels at a speed c.



 
Originally posted by: Chode Messiah
could a black hole somehow be accelerated to the speed of light?
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No, they are limited to speed below c just as everything else; from that point of they just behave as ordinary stars.


 
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