Where am I wrong in this travelling faster than light Idea I've had?

[ulrichburke]

TL;DR Summary

Just an idea about travelling faster than light and I'm too ignorant to know why it wouldn't work!

Dear Anyone.

Please be nice and let me down gently, I know more about tapdancing than physics and I can't tapdance! It's just...

Kept reading nothing could go faster than light, in a vacuum (space.) So I Googled 'why' and discovered it's because things get heavier and heavier as they get closer to the speed of light. So I Googled 'what's the extra weight' and discovered it was energy, but energy that wasn't doing anything. Assuming I'm right thus far...

Imagine you're in a Viking boat, powered by rows of rowers, one row each side. They're very good rowers, the boat's going faster and faster past islands. But every time it passes an island, a bunch of passengers jump on board. Being passengers, they just sit at tables not doing much except adding to the weight of the boat. So after awhile the boat gets so heavy with the weight of all the passengers, the rowers can't make it go any faster and it sticks at its max speed.

Am I doing OK with the allusion thus far? If 'yes', here's the kicker!

Why can't all the passengers be conscripted as extra rowers? Translated, why can't all this extra energy that's mysteriously built up around the spaceship as it gets close to lightspeed - I can't find anywhere that says where the energy COMES from, just that 'it builds up' - be 'ignited' to provide extra thrust to help get the spaceship past the speed of light? I mean it's been using energy to get it CLOSE to lightspeed, it's now got a load of extra energy built up around it from somewheres, why can't it use that too? Here's another ghastly allusion - I have to use allusions because I don't know anything else TO use, I'm just using what I've seen in Real Life.

Think of an electric kettle. When you turn it on, you've got one tiny central thread of energy - the spaceship - surrounded by a LOAD of inert stuff - the water. But it uses convection currents to 'convert' more of the water to being energetic which, to me, is like getting more of the passengers to join in with the rowing, in the boat analogy above. Now I know we can convert potential energy - which is what the spaceship is surrounded by as though it was in the middle of a snowball, am I right? - to force by the law of kinetics - when something isn't moving it's got stored energy, when it starts moving it changes to kinetic energy. Just showing I know that much. So we've got this spaceship inside a massive snowball of potential energy that's come from some source I don't understand - but you're welcome to tell me! - as it's gotten closer to lightspeed.

Why can't it convert all that potential energy to kinetic energy and use it to push itself through the lightspeed barrier?

Yours puzzledly

Chris, who knows he isn't a physicist in any way, shape or form, which is why he's here!

 

[Baluncore]

Welcome to PF.

Why can't it convert all that potential energy to kinetic energy and use it to push itself through the lightspeed barrier?

General relativity shows that, to cross the light speed barrier requires infinite energy.

 

[Ibix]

Unfortunately, although the "mass increases as you approach lightspeed" is a popular explanation of why you can't exceed the speed of light, it's a symptom not a cause. The underlying cause is Einstein's second postulate: the speed of light is the same in all inertial frames. That means that you initially measure the speed at which light is passing you and get 300,000,000ms^-1, so you know how much you have to accelerate. So you accelerate for a bit and measure again, and get the exact same 300,000,000ms^-1, because the speed of light is the same in all frames. So if you want to reach the speed of light you still have the entire job to do, and you always will. Sure, Earth will see you moving and you will see Earth moving, but you will be no closer to the speed of light by your own measures than you were when you started.

Things like "mass increases as you go faster" (which uses a concept called "relativistic mass", which was abandoned decades ago by working physicists as being a stupid way to define mass but persists in pop science presentations because it sounds cool) are derived from that postulate. Einstein didn't know it at the time, but he was essentially saying something about the structure of spacetime, the background on which physics happens. He had found that the speed of light limit is baked into the structure of the universe. You can't sneak round it (there are a number of proposed schemes to cheat it, but all require us to be able to do one thing that's impossible as far as we know).

Unfortunately, it also means that there isn't a really satisfactory explanation for why you can't exceed the speed of light (which is why "explanations" of the kind you found persist). When we use theories derived from (or requiring) Einstein's postulates we make accurate predictions and can build technology like nuclear power, GPS, and even radio (which predates Einstein, but oddities in the maths describing them is what lead Einstein to his theory). So we're confident it's right. But at our current level of understanding there's no explanation beyond that's just the way it is.

 

[Ibix]

Welcome to PF.

General relativity shows that, to cross the light speed barrier requires infinite energy.

I wouldn't say that. Rather, it says that an object with mass traveling at lightspeed is a contradiction in terms. Thus, using the formula for kinetic energy of a massive body moving at the speed of light (which does give infinity) is self-contradictory and has no meaning.

 

[PeroK]

Chris, who knows he isn't a physicist in any way, shape or form, which is why he's here!

The energy of an object of mass $m$ moving at speed $v$ in some inertial reference frame is given by:
$$E = \frac{mc^2}{\sqrt{1 - v^2/c^2}}$$We can rearrange this equation to give the speed of the object if its total energy is $E$:$$v = c\big (\frac{\sqrt{E^2 - m^2c^4}}{E} \big) < c$$Note that the term in brackets is always less than $1$, as $E^2 - m^2c^4 < E^2$.

In conclusion, however much energy an object has, its speed is always less than $c$.

 

[Dale]

Why can't it convert all that potential energy to kinetic energy and use it to push itself through the lightspeed barrier?

It isn’t potential energy. It is already kinetic energy. And you don’t need to convert it, you just need ever more of it to go faster. Infinitely more.

I can't find anywhere that says where the energy COMES from

It comes from whatever the energy source is. If it is a rocket then it comes from the rocket fuel. If it is a light-sail then it comes from the light hitting the sail, etc. This energy is the kinetic energy provided by the propulsion system. It is not additional energy, it is the propulsion energy.

Why can't all the passengers be conscripted as extra rowers?

They are already rowing as soon as they get on the boat. They are never not rowing to begin with.

things get heavier and heavier as they get closer to the speed of light

Relativistic mass is a discarded concept. If you are reading a source that uses that as an explanation you should be aware that the source is feeding you information that is about 100 years out of date

 

[topsquark]

TL;DR Summary: Just an idea about traveling faster than light and I'm too ignorant to know why it wouldn't work!

Dear Anyone.

Please be nice and let me down gently, I know more about tapdancing than physics and I can't tapdance! It's just...

Kept reading nothing could go faster than light, in a vacuum (space.) So I Googled 'why' and discovered it's because things get heavier and heavier as they get closer to the speed of light. So I Googled 'what's the extra weight' and discovered it was energy, but energy that wasn't doing anything. Assuming I'm right thus far...

Imagine you're in a Viking boat, powered by rows of rowers, one row each side. They're very good rowers, the boat's going faster and faster past islands. But every time it passes an island, a bunch of passengers jump on board. Being passengers, they just sit at tables not doing much except adding to the weight of the boat. So after awhile the boat gets so heavy with the weight of all the passengers, the rowers can't make it go any faster and it sticks at its max speed.
...

Yours puzzledly

Chris, who knows he isn't a physicist in any way, shape or form, which is why he's here!

I agree with the comments above about Relativistic mass... It's better to talk about momentum. (That means the following comments are a bit off, but look at the spirit of the argument.) However, if you want to think of it in terms of mass, the faster you go the greater your mass. So it takes more and more effort to increase your speed. Your mass tends toward the infinite as you get close to the speed of light, so you'd need an infinite number of rowers to even approach it.

-Dan

 

[PeroK]

so you'd need an infinite number of rowers to even approach it.

It's not that easy to find an infinite number of rowers!

 

[anorlunda]

It's not that easy to find an infinite number of rowers!

Just put up a sign, "Free Beer For Rowers"

 

[topsquark]

It's not that easy to find an infinite number of rowers!

On the river Styx? :)

-Dan

 

[russ_watters]

But every time it passes an island, a bunch of passengers jump on board...

Why can't all the passengers be conscripted as extra rowers?

That's probably a good reason why the concept of relativistic mass was discarded: there are no extra rowers. For those on the boat, nothing about the boat ever changes.

Relativistic mass is like people watching from shore seeing the boat's acceleration decreasing, and concluding it must be getting heavier. But it isn't.

 

[Borg]

So I Googled 'why' and discovered it's because things get heavier and heavier as they get closer to the speed of light.

Something that often gets dropped in people's understanding is that this is defined in the Theory of Relativity.
The word relativity isn't just a coincidence, it actually refers to what's going on. Everything is measured according to its frame of reference relative to other frames of reference. If you have a person in a spaceship that sees another ship go past at nearly the speed of light, that person can say that the other ship is traveling near the speed of light and he is stationary. However, the other person can say the same thing - as far as he is concerned, he isn't moving but the other one is. There is no absolute frame of reference in the universe. If that doesn't make sense, remember that you are on a planet that is traveling around the center galaxy a many kilometers per second. Our galaxy is also traveling towards other galaxies and away from other galaxies at the same time while taking you along for the ride. And, people on those galaxies see things from a completely different perspective. Who gets to be the center of the universe and claim that everything moves with respect to them? The answer is nobody. However, that doesn't stop everyone from being able to claim that they aren't moving but everything else is while doing their calculations.

To get back to what you Googled, each sees the other as becoming more massive as their speed differential increases. Neither of them thinks they are the one getting heavier becoming more massive (referring to weight is technically incorrect). The increase in mass is only what they each perceive about the other with respect to their own frame of reference.

 

[phinds]

things get heavier and heavier as they get closer to the speed of light

Uh ... but YOU are going at almost the speed of light (relative to an accelerated particle in some particle accelerator on Earth). Do you fell any heavier?

 

[jbriggs444]

Uh ... but YOU are going at almost the speed of light (relative to an accelerated particle in some particle accelerator on Earth). Do you fell any heavier?

Let us take this a step farther. A quick trip to Google says that protons at CERN can travel at 99.9999991 percent of the speed of light. So that means that the delta between your speed (relative to the proton) and light speed is $300000000 \text{m/s} \times .000000009 = 2.7 \text{m/s}$. That is an achievable running speed. [Assuming I have not slipped a digit in the math]

Suppose that you run fast or hop into your car and start moving at 2.7 m/s. Does this mean that you have now exceeded the speed of light relative to that proton in CERN? Nope. There is the pesky problem of relativistic velocity addition.

Your 2.7 m/s change in velocity was measured in your frame of reference. Due to length contraction, time dilation and the relativity of simultaneity, your change in velocity measured according to the rest frame of that proton will be much less.

From the Wiki article, the formula is:$$u = \frac{v + u'}{1 + ( vu'/c^2)}$$Here $u'$ is your starting proton-relative velocity. $v$ is your personally measured change in velocity and $u$ is your final velocity measured by the proton.

If we put numbers to this then $u' = .999999991c$, $v = .000000009c$ so $$u = \frac{.000000009c + .999999991c}{1 + .000000009 \times .999999991} \approx \frac{c}{1+ .0000000009} \approx .999999991c$$
Your 2.7 m/s change in velocity made (as might have been expected) no significant difference to your proton-relative speed.

 

[Borg]

The OP hasn't returned since submitting the original question. Perhaps he is traveling at relativistic speeds w.r.t. us and won't be back for a long time as we experience it.

 

[berkeman]

"Free Beer For Rowers"

I think this is where we lost the OP. He's been out on this part of his quest since then...

 

[PeroK]

I think this is where we lost the OP. He's been out on this part of his quest since then...

... or been detained at the Viking museum.

 

[DaveC426913]

"Free Beer For Rowers"

That's an interesting phrase. It's 9/17ths (53%) palindromic.
"Sreworro free beer f"