# I have a question about relativity theory

SniperWish

I have a question about relativity theory.

As we all know, the theory assumes speed of light is a constant that can not be surpassed.

Now assume you have build a spaceship that is flying exactly at the speed of light. There is now an additional amount of energy used to further accelerate the spaceship. According to relativity, nothing happens. The spaceship keeps flying with the speed of light, and doesn't accelerate anymore. But what happens to the energy? Where did it go? Would that not mean that you have destroyed energy, and therefore broke the first law of thermodynamics?

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TurtleCat

your additional bit of energy means fuckall at the point where you needed infinite energy to get to

eGremlin

Assume a massless spaceship then. Practical engineering difficulties is not what this question is about.

Stupidasole

assume magic
I would rather not
Relativity deals with things which gain mass with acceleration
even light does
we don't know what happens to your magic asspull spaceship

Spazyfool

Another question:

How does photons pushing a solar sail not decrease their momentum?

Supergrass

a spaceship that is flying exactly at the speed of light
Mass. So no c.

Assume a massless spaceship
No mass. Nothing but c.

Dreamworx

Not photons but charged particles iirc

Raving_Cute

Additional to that, if Photons don't lose momentum, what stops you from building a photon-based perpetuum mobile, and once again break the first law of thermodynamics?

Lunatick

photons lose energy when they interact with matter
their wavelength increases until they completely dissipate

Harmless_Venom

They don't lose momentum though. Different wave length does not mean they move slower.

Sir_Gallonhead

Assume a massless spaceship then
A spaceship made of photons will blueshift when energy is added.

That's the only sensible answer to your question. You're fundamentally misunderstanding what the "speed limit" really means. You can ALWAYS accelerate and go faster if you have mass. If you had an efficient enough method of space travel you can even reach another galaxy within your lifetime. But an observer on Earth or your destination would still see you taking millions of years at a minimum. This is due to the effects of length contraction and time dilation. The closer you get to the speed of light, the flatter the universe becomes in your frame of reference. So even though you measure only a slight increase in your speed relative to your destination, you will observe that your destination is suddenly much closer due to the contraction of space. At the speed of light, the universe is literally flat and no time passes for you whatsoever until you collide with your destination. But this is, of course, impossible for anything with mass. Likewise, it's impossible for anything WITHOUT mass to experience time or move any slower or faster than lightspeed.

TL;DR the question is a meaningless one not because of engineering difficulties but because of how space and time actually work at relativistic speeds.

Boy_vs_Girl

they have less energy to give off though

King_Martha

If you had an efficient enough method of space travel you can even reach another galaxy within your lifetime. But an observer on Earth or your destination would still see you taking millions of years at a minimum.
This is due to the effects of length contraction and time dilation.
Yeah, nothing to do with the fact that the light that reflects off the ship as it hurls towards Andromeda has to cross 2 million light years to get to your retina, it's length contraction and time dilation.

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haveahappyday

Yeah, nothing to do with the fact that the light that reflects off the ship as it hurls towards Andromeda has to cross 2 million light years to get to your retina
It has everything to do with that. And length contraction and time dilation. All of these effects are linked by the constant speed of light in both reference frames. Describing it that way is less relevant to the OP's question because the reader might be mistakenly led to believe that we're just talking about a visual effect or illusion, but it is neither. In our time that trip takes millions of years and we observe the ship moving at some high percentage of the speed of light while also observing that all systems on it evolve much slower than our own.

Inmate

If you move at the speed of light, you are instantly arrive at your destination.

lostmypassword

they don't lose momentum with change of wavelength
t. brainlet

$p = \hbar k = \frac{h}{\lambda }$

Seriously, you shouldn't discuss matters you don't have even basic knowledge of.

Carnalpleasure

In our time that trip takes millions of years
it doesn't, the ship could turn back and come back to earth before its afterimage caught up

AwesomeTucker

What? That would require the ship going faster than the speed of light.

Lord_Tryzalot

relativity theory
it's bullshit

King_Martha

A lot of bullshit answers here by a lot of wannabes. In that scenario the energy added would turn into mass, and actually slow the ship down. That's the theory. But of course nobody knows what would actually happen.

haveahappyday

two particles of light passing at opposite vectors are traveling ftl relativly.

einstein was a fuckhead jew press puppet.

takes2long

People on spaceship age less, relative to people on planet

Doesn't this fuck with the idea that there is no preferred frame of reference?

BunnyJinx

Its the acceleration to get to another reference frame that causes that.

happy_sad

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Inmate

A spaceship flying exactly at the speed of light is impossible.
Period. Contradiction in terms. NOTHING with rest-mass can attain lightspeed. Nothing without rest-mass can move any slower.

You can accelerate to lightspeed minus a snail's pace, but that's it.
"Accellerating to just short of lightspeed" is really meaningless too. Photons still overtake you at the usual 300,000 km/sec. But I know what you're asking.

If you turn on the motors, you edge a little closer to lightspeed, that's all. The energy goes into the kinetic energy of the ship, as you'd expect, but the velocity hardly changes. Instead, your mass increases, making it even harder for the NEXT burst of rocket power to do you any good.

When photons reflect off a solar sail, it's analogous to tossing a superball against the rear bumper of a truck. The truck gets a little "kick" and the photon comes back to you with reduced energy. The analogy isn't perfect. The ball comes back slower. The photon is still moving at lightspeed, but it comes back somewhat red-shifted.

If you're trying to brake an approaching solar said, the light reflects back to you blue-shifted. The sail is losing energy and it's doing work on the light.

Burnblaze

It would not slow the ship, See The added mass just keeps making further acceleration more difficult.

All inertial frames of reference are equally valid. "Inertial" mean non-accelerating.
Time dilation and length contraction kick in when you change frames; i.e. when you accelerate.
This is the source of the "twin paradox". The traveling twin turns around and returns to Earth to compare clocks. The acceleration during the turn is what introduces the asymmetry causing him to age less.

Flameblow

Troll somewhere else, stupid!

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