Why can’t I go faster than the speed of light? • Hacker News Board

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

#201

Post by RogerL » Mon, Sep 13, 2021, 8:02 AM UTC

stareatgoats wrote: post #52

otterley wrote: post #21

My favorite way to think of it is that the speed of anything is measured in distance per unit time. You can’t change the constant that is the speed of light, but what if you could change what “time” means? The constant holds, but time doesn’t; it varies among observers. A second from the perspective of observer A is not identical to a second from the perspective of observer B. Freaky, but true.

I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source . It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective obs…

If you don't know, every experiment we have ever carried out to test this tells us it is true.

I wrote a long answer to someone else; I'm not going to retype it. But what is invariant in the 3D space you think you live in is distances. The distance of you from the kitchen is computed to be 8 meters no matter what we choose to say the x and y directions are, right? If we choose different directions for y, you may say the kitchen is 6 meters from you in the y direction, I say it is 3.7 meters in the y direction. No mystery, our frames are rotated. But we both get the same value for distances. Rotating a map doesn't change the reality of how far you are from the kitchen.

Well, you don't actually live in 3D space, you live in 4D space, where time is a dimension. So what is invariant is not distance (3 of the 4 dimensions), but intervals (time and distance between two events). when you travel at a significant fraction of light speed, you are rotating that 4D 'map'. If you rotate frames, you get different values for x, y, z, and t. But the intervals are still constant.

The intervals are the same for all observers. It is just the individual coordinates (x,y,z,t) that vary for different observers, just like (x,y) are different for you and me wrt your kitchen, but the distance we compute are the same.

edit: in summary, wouldn't you find it bizarre if I rotated the floor plan to your house and suddenly got a different distance from your couch to the kitchen? That would be absurd! Well, when you rotate in space time intervals are unchanged. IOW, the speed of light is unchanged. It would be bizarre if by a simple rotation you got a different value! You just don't 'see' (literally and figuratively) that time is a dimension, and the map rotation as non-relativistic speeds is so tiny that you don't realize that t changes along with x, y, z. But it does. It would be bizarre if it didn't.

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

#202

Post by eafkuor » Mon, Sep 13, 2021, 8:29 AM UTC

paulrouget wrote: post #167

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.

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!

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

#203

Post by RogerL » Mon, Sep 13, 2021, 8:33 AM UTC

ncallaway wrote: post #82

stareatgoats wrote: post #52

Earlier quoted context omitted.

I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source . It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective obs…

> but to me this is simply not logical. It is not logical if you really believe that speeds add linearly (that is, if you are going 5mph past an observer and throw a ball 5mph, that the result is the ball moving 10mph). Speeds don’t actually add linearly like that, but they come very very very very close to doing so for all speeds humans are used to dealing with. So, we all have very deeply held gut feelings that spe…

But we don't really think that speeds add linearly. if you drive 80 kph North, and I drive 60 kph East, no one would say we are separating at 140 kph. we would use Pythagoras to get the right number.

We just don't realize we live in a 4D space where time is one dimension, so we think driving in opposite directions is a special case where we can add the speeds. but there is the fourth dimension, and we have to use an equation very similar to Pythagoras to get the right value.

I know you must know this, this is more for the benefit of the reader.

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

#204

Post by caf » Mon, Sep 13, 2021, 8:41 AM UTC

awb wrote: post #117

subroutine wrote: post #75

Q1. Does the future already exist? I realize this might be purely philosophical, but if I leave Earth, and fly through space near c kph, and return home a few hours later to find everyone aged 50 years... are they the "same" people, or are they a future-instance of the people left? To clarify, theoretically I could leave earth, and return home in exactly 1 hour (from my ref frame), and basically make people whatever…

Q1. > basically make people whatever age I want by varying my speed. No more so than making the Earth change shape by moving your physical location. > Since the same 1 hour passes for me no matter the age for them I chose, it seems like I'm selecting an already existent future from a stack, not fast-forwarding the present I left. Yeah these are more philosophical questions rather than a testable hypothesis. It’s an i…

You can even make this clear in the "leave Earth in a fast ship and return" scenario - have the people on Earth send you a birthday card by radio every year that you're away, on your birthday in their reference frame. You'll receive those ticks of time as you travel (most of them on your way back).

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

#205

Post by snak » Mon, Sep 13, 2021, 8:53 AM UTC

joombaga wrote: post #154

agumonkey wrote: post #136

Earlier quoted context omitted.

It's funny, as a programmer i had similar interpretations. Universe is like a processor and the c is the frequency limit. There was a talk with lawrence krauss I think, where he explained that at galaxy scales, everything is distributed, there's not one reality but an infinity since no point in space can be aware of far points in the universe. I also wonder if there would be ways to tweak C.

This short story might tickle your fancy. https://qntm.org/admin (I also highly recommend their book "There Is No Antimemetics Division" if you're at all interested in SCP Foundation)

Nice one, thanks for sharing.

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

#206

Post by xondono » Mon, Sep 13, 2021, 1:37 PM UTC

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.

I know, I was making reference to this

> They are only theoretically possible if you allow for negative mass and energy--not an engineering problem so much as a "need to find exotic matter"

Basically people ran the EFE "backwards" to see what matter distribution makes the wanted curvature. You get either negative mass-energy or the bubble doesn't travel ftl iirc.

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

#207

Post by pfalafel » Mon, Sep 13, 2021, 1:46 PM UTC

jandrese wrote: post #149

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.

What we think of as the speed of light could also be thought of as the rate at which change propagates throughout the universe. The rate at which cause and effect travel.

Two runners overtake the field at incredible speed, only to cross the finish line at exactly the same time. Would also raise eyebrows.

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

#208

Post by stareatgoats » Mon, Sep 13, 2021, 1:50 PM UTC

RogerL wrote: post #201

stareatgoats wrote: post #52

Earlier quoted context omitted.

I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source . It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective obs…

If you don't know, every experiment we have ever carried out to test this tells us it is true. I wrote a long answer to someone else; I'm not going to retype it. But what is invariant in the 3D space you think you live in is distances. The distance of you from the kitchen is computed to be 8 meters no matter what we choose to say the x and y directions are, right? If we choose different directions for y, you may say…

well, sure looks like you've grasped it. I'll certainly ponder it. But for the record it did not immediately help me in trying to solve the central riddle of the axiom: that the speed of light from a single light source is supposedly the same for all its observers irrespective of their respective speeds relative to the source. I.e. that the observed speed of light coming from our sun would be observed to be e, and an extremely fast spaceship travelling away from the sun would observe the speed of the suns light to be e, too.

Here is one aspect of the conundrum: a certain photon travelling at the speed of light from the sun reaches the earth, which is relatively stationary vs the sun, in approx 8 minutes. It will obviously reach our rocket (which at the moment the photon was fired from the sun was at the same distance as the earth from the sun) some time later, depending on its relative speed to the sun. However, once the photon hits the spaceship it supposedly has the same speed as the speed of light hitting the earth.

I realize this axiom seems to have been experimentally proven, and that I probably just have not found the right key for me towards the understanding of it. Looking forward to that day, which might also lead me to understand how to fit the fact that the blueshift/redshift observed in light allows us to actually determine relative velocities. As well as understand how the expansion of space is a different kind of speed than normal speeds, allowing for the speeds greater than light that it does.

In the meantime I find some solace in the fact that I'm not the only one to find this bizarre - hence that whole linked article as I understand it. But thanks for your effort.

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

#209

Post by whatshisface » Mon, Sep 13, 2021, 2:01 PM UTC

thriftwy wrote: post #192

whatshisface wrote: post #166

Earlier quoted context omitted.

Two long charged rods moving with the same velocity relative to each other are two parallel wires carrying current, the second of which is a typical and easy-to-calculate example of electromagnetic attraction. In fact, that situation is so prototypical that it is used to define the Ampere in terms of what current is required to produce a certain force between parallel wires. [0] [0] http://www.physics.louisville.edu/…

In this scenario, rods are stationary but they have current running in them - a different situation. Can it be translated to the relativistic one? Maybe what they really wanted to say is "there is a speed c at which the charge of a moving rod is indistinguishable of current running through stationary rod"? Now that would make a lot of sense.

It's not different as far as electrodynamics is concerned. The motion of the charge is what matters; whether or not the neutral part is stationary or moving is of no concern.

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

#210

Post by stareatgoats » Mon, Sep 13, 2021, 2:15 PM UTC

l332mn wrote: post #81

stareatgoats wrote: post #52

Earlier quoted context omitted.

I still don't get this. I simply can not accept that the speed of light is that same for all observers, irrespective of their respective speeds vs the light source . It is obviously a cornerstone of Einsteins theory (I forget which; either the special or general relativity), but to me this is simply not logical. To state that the speed of light is the same only because the relative time is variable for respective obs…

> To state that the speed of light is the same only because the relative time is variable for respective observers is unconvincing Not sure what you mean by relative time?

You are right, 'relative' was not the correct term there.