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Why can’t I go faster than the speed of light?

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Re: Why can’t I go faster than the speed of light?

#211

Post by sparkplug94 »

raattgift wrote: post #164
pfalafel wrote: post #140

Earlier quoted context omitted.

Why are gravitational waves limited by the same speed limit? There is no mass traveling in that case, I suppose.

This is an excellent question, which predates the theory of General Relativity by a few years, and has only been astrophysically verified after 1974 (Hulse-Taylor) or even later. As far as I am aware, a complete theoretical answer in General Relativity itself is still elusive. I'll largely stick with the theory, which I guess is what you are interested in.[1] The topic is in Part VIII (Chapter 35) of Misner, Thorne &…

This is fascinating, thank you! You’re absolutely correct, the single GR course i took linearized the EFE for the section on gravity waves and the concept that gravitation self-gravitates did not really come through. In hindsight maybe that should have been obvious though, since all waves have this property.

Re: Why can’t I go faster than the speed of light?

#212

Post by hyperman1 »

mewse wrote: post #195
hyperman1 wrote: post #63

Earlier quoted context omitted.

AFAIK 'nothing goes faster than c' is wrong in the 'lies to children category'. The devil is in the details. In fact, plenty of things go faster than c. As a standard example, take a wall at 1m distance and a flashlight. move the light spot at 1 m/s. If you put the wall at 2m, the same spot will now go 2 m/s. If you put it at 300 000 km, the spot will go at 300 000 km/s, slightly over light speed. The problem is more…

That's not the "same" spot, though. It doesn't consist of the same photons; the spot in the "new" position consists of photons which have been traveling in a straight line from your flash light at (assuming vacuum) a constant rate of C (and then reflected back to reach your eye again, also at a constant rate of C); they haven't traveled to or from the previous "spot" position at all, much less exceeded the speed of l…

That's correct in both the superluminal and the subluminal case, and more or less the point of the example. The spot moves superluminal, but the particles creating it don't. This makes it impossible to use it for superluminal information transfer.

AFAIK Expansion of the universe is comparable. It pushes things away not by speed but by putting extra 'new' space between things.

Your sibling comment makes a variant of the same point:c is the maximal speed of information or causality. Light in a vacuum is just one of the things capable of reaching it.

Re: Why can’t I go faster than the speed of light?

#213

Post by seanmcdirmid »

dotancohen wrote: post #171
seanmcdirmid wrote: post #152

Earlier quoted context omitted.

You need to spend equal amounts of energy and time on both acceleration and deceleration. It doesn’t help to go fast if you can’t stop at where you are going.

Depends what the cargo is.

Well, if you just want to destroy a planet at near light speed, they would never see it coming and deceleration is unnecessary. Scary thought though.

Re: Why can’t I go faster than the speed of light?

#214

Post by dhosek »

juped wrote: post #180
dhosek wrote: post #97

My favorite general relativity paradox: A runner is running with a 20m pole at a sufficient velocity so that a stationary observer sees the pole having length 10m. The runner runs through a barn which is 10m long. And two people outside shut the doors simultaneously so that the runner is completely enclosed in the barn for a moment. But from the runner's perspective, the barn has length 5m long. What does she see hap…

This is a special relativity paradox.

D'oh! I know better than to make that mistake and yet "their" it is.

Re: Why can’t I go faster than the speed of light?

#215

Post by reddiky »

tenuousemphasis wrote: post #172
jandrese wrote: post #147

Earlier quoted context omitted.

It's a fantasy as long as you are still living under the tyranny of the rocket equation.

I don't think anybody thinks such a ship is possible with chemical rockets.

Just need Astrophage /s

Re: Why can’t I go faster than the speed of light?

#216

Post by jvanderbot »

eafkuor wrote: post #202
paulrouget wrote: post #167

Earlier quoted context omitted.

Yep. At 1g you could literally go to the edge of the visible universe in less than 50 years (assuming you are targeting the edge as defined at time of departure).

If this is true (I'm too stupid to do the math), then consider my mind blown!

It is technically true, but requires all kinds of miracles, not the least of which is avoiding every trace of matter/antimatter between here and there.

Oh, and all light from the universe is now gamma radiation focused intensely ahead of the spacecraft cooking the whole thing.

An atomic nucleus sitting in deep space becomes an apocalyptic collision at 99.99% light speed.

Re: Why can’t I go faster than the speed of light?

#217

Post by caf »

midrus wrote: post #94

I have a probably very dumb question. If speed is relative, when we say something travels at... I don't know... 50% the speed of light, that speed is relative to what? how do you know it is 50% and not 53%? How do we know we're not already moving at 99% the speed of light (like our observable universe as a whole having that speed )? I love this stuff, but it is so counter intuitive for the average human.

When we say something travels at 50% the speed of light, it's always relative to some inertial, approximately inertial, reference frame, often implied.

So eg. if you hear about the particles at the LHC travelling at near the speed of light, it's implied that's relative to the Earth.

The speed limit comes in here: no-matter what nearby object you look at, its speed relative to you always be less than the speed of light. (Here, nearby means something like "within the local group of galaxies" - objects a long way away can be receding faster than the speed of light due to the expansion of space in between you and the object). You can accelerate as much as you like for as long as you like and when you stop accelerating and check, that'll still be the case.

Re: Why can’t I go faster than the speed of light?

#218

Post by smegger001 »

wruza wrote: post #123
eesmith wrote: post #106

Earlier quoted context omitted.

It looks like about 57 years. Assuming constant acceleration to the 1/2-way point, flip, deceleration, and using http://www.projectrho.com/public_html/rocket/slowerlight3.ph... : Time elapsed (in starship's frame of reference, "Proper time") T = (c/a) * ArcCosh[a*d/(c^2) + 1] (given acceleration and distance) year = 365.25*24*3600; c = 3E8; a=9.8; d=1.25*1_000_000*(c * year) from math import acosh T = (c/a) * acosh(a…

Bad news CMB shifts into infrared, visible, uv, xray and then hard gamma.

since we are talking insane speeds and energies required to reach them why not just say you are in a bigger ship with more radiation shielding a dozen or so meters of water in the hull and a layer of lead a foot thick aught to stop most of it

Re: Why can’t I go faster than the speed of light?

#219

Post by vigormortis »

seanmcdirmid wrote: post #152
wruza wrote: post #114

Earlier quoted context omitted.

Also, it doesn’t take too much acceleration to make that trip. Comfortable earthlike 1g (~ 10m/s/s) is enough to build up a decent speed in a very reasonable time. Energy is the issue though.

You need to spend equal amounts of energy and time on both acceleration and deceleration. It doesn’t help to go fast if you can’t stop at where you are going.

Check out Pohl Anderson's "Tau Zero" which examines precisely this scenario. A starship using bussard ramjet needs to find an "empty" section of space free of interstellar hydrogen so they can fix their engines and decelerate without getting fried. This requires them to continue accelerating so that time dilation effects will allow the trip within the crew's lifetime. That's the setup, not the plot.

Re: Why can’t I go faster than the speed of light?

#220

Post by baja_blast »

lovegoblin wrote: post #89
xondono wrote: post #64

Earlier quoted context omitted.

The Alcubierre drive “beats” the speed of light, for the effects described 0.7-0.9c are more than good enough (if slightly unpractical). We can’t do that either for now, but is way easier on the feasibility scale.

> The Alcubierre drive “beats” the speed of light The Alcubierre drive is only a thought experiment that requires "exotic matter" (aka fairy dust) to work.

Papers have been published that have found ways to create warp drives without the need for exotic matter: https://arxiv.org/pdf/2006.07125.pdf
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