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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?

#41

Post by cletus »

The best analogy for this I've read is that your motion through spacetime is constant. Think of a quarter circle dial with an arrow that goes all the way from 0 degrees (no relative movement in space) to 90 degrees (relative speed of light). The faster you go through space, the slower you go through time. The effects of this are barely noticeable until you get near the speed of light. This effect is time dilation.

This, I believe, is the general relativity view of spacetime.

What I like most about this is that it highlights the importance of understanding the domain of a function. The above shows how any object with rest mass has a space velocity domain of [0, c).

People really don't want this to be true so latch on to any wild theory that would seem to bypass this cosmic speed limit, be it wormholes, FTL drives or whatever. Pretty much all of these theories rely on taking an equation with a domain over real numbers and plugging in negative values for things like mass.

Garbage in, garbage out.

I mean if mass can be negative, why limit yourself to real numbers? Why not a complex number for mass?

As to why this is the case and how to reconcile it with a quantized view of spacetime... is beyond my pay grade.

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

#42

Post by kadoban »

nimish wrote: post #37
xyzzyz wrote: post #26

Earlier quoted context omitted.

To put it another way, time travel machines are very much theoretically possible, and in fact are “only” an engineering problem (an extremely hard one, however). But, only forward travel is allowed: time travel machine could take you as far forward as its engineering would allow it, but there is no going back.

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.

Gp is talking about forward-only time travel though, you don't need anything exotic for that at all, just lots of energy and an efficient way to turn it into thrust.

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

#43

Post by yawaworht1978 »

redwood wrote: post #23

I think a lot of people assume that this means you couldn't go more than a few (~100) light-years in your lifetime... But this is not actually correct. Counter-intuitively you can theorically go any number of light-years (essentially) in your lifetime, as long as you are able to approach the speed of light because when you do so the distance is dilated and hence you're covering far more ground within your reference f…

I know this is true, but I never got my head around this, maybe someone can help. So if a photon is emitted from the sun, it's passing earth immediately? Then why does sunlight need 7 or 8 minutes to get to earth? And let's say, i travel one light-year at the speed of light, that should be instantaneous, right? The odometer would show 1 light-year, my watch would should 0 seconds and some decimals. How much would I have aged by the end of the journey?

If I travelled at one percent of speed of light, same distance, i suppose 100 years would elapse for me, how much would elapse on earth? Odometer would still show one light year?

And what if I left earth at 50percent of light speed , traveled 1 light-year away and did a turn and came back to earth at same speed. For me, it would be 1 year, but if I had a twin brother who was waiting on earth, would i now be a year younger than him? And how is this possible?

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

#44

Post by zamadatix »

xyzzyz wrote: post #26
redwood wrote: post #23

I think a lot of people assume that this means you couldn't go more than a few (~100) light-years in your lifetime... But this is not actually correct. Counter-intuitively you can theorically go any number of light-years (essentially) in your lifetime, as long as you are able to approach the speed of light because when you do so the distance is dilated and hence you're covering far more ground within your reference f…

To put it another way, time travel machines are very much theoretically possible, and in fact are “only” an engineering problem (an extremely hard one, however). But, only forward travel is allowed: time travel machine could take you as far forward as its engineering would allow it, but there is no going back.

At 1G constant acceleration (both speeding up and slowing down) you can make it on a short vacation to the Andromeda galaxy and back in a lifetime (or 5 million years from the perspective or Earth).

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

#46

Post by 8note »

gizmo686 wrote: post #27
chacham15 wrote: post #9

Earlier quoted context omitted.

If you could move in space but not in time, then you would be in two places at the same time which would violate conservation of matter, right?

That depends on how you define 'conservation of energy'. Consider an arbitrary bounded volume of spacetime. Conservation of energy says that the net flow across the boundary of any such volume is 0. Under this definition moving in space but not time is not a violation, as the same amount of mass enters the volume as exits it. The only odity is that both events happen at the same time

That definition sounds broken, since energy is still conserved if I move something into or out of the bounds.

It stops being conserved if there's suddenly more or less energy inside the volume without the same amount crossing the boundary

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

#47

Post by formerly_proven »

moron4hire wrote: post #28
gizmo686 wrote: post #19

Earlier quoted context omitted.

You have to be careful when talking about 'speed' when you make time a dimension in your geometry. Traditionally, speed is a measure of how much of your space-time curve is in along the time axis. For any space-time path, you can consider the coordinate system as an observer traveling that path would see it, in which that observer would see itself traveling through time at a rate of one second per second. Geometrical…

To me, this was why the statement that "B would see the objects growing further apart while A would see them getting closer together", if FTL was allowed, was not a very satisfying answer. So given what you've described, that means that forces applied to bodies need to have a time component equal in magnitude to the spatial components. Forces must always exist along that 45° line. The limit of the force required to c…

> Addendum: I don't understand why you said 45° instead of 90°. I thought objects traveling at the speed of light would experience infinite time dilation, and thus be observed as having 0 passage of time.

Think about what the diagram shows: Every (s[pace], t[ime]) coordinate pair on the spacetime diagram shows an observation of a particle. So in natural units, a photon's wordline is given by s=t or s=-t (traveling in one or the other direction). If you draw that, it's a 45° line. It also gives you the light cone of the observer at (0, 0).

A horizontal wordline would be something moving at infinite speed, not the speed of light, as it is observed at every place at the same time.

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

#48

Post by SnowProblem »

soneil wrote: post #3

The simplest way I could ever wrap my head around this, is to understand that everything travels through spacetime at the same speed. There is no travelling faster or slower, just crossing the graph at a different angle.

And the reason for this, as we all know, is because the universe runs on distributed computers with no global state, where each computer simulates its own local physics and connects only to other nearby computers, and thus the so-called speed of light is simply the emergent rate that data may travel across the network.

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

#49

Post by ergocoder »

Choc13 wrote: post #38
ergocoder wrote: post #35

"Nothing can go faster than light" is just a convention and is not experimentally confirmed. IIRC there are a few things that go faster than speed of light (e.g. universe expanding). "Spooky action at a distance" is also not known very well. That could also break the speed of causality law. While Bell's theorem hints that this is not the case, there are some exceptions to Bell's theorem. Right now a lot in physics ar…

Hmm not sure if troll, but energy conservation is definitely not just some convention. It’s a fundamental thermodynamic law. Spooky action at a distance does not violate speed of light information propagation either. The particles have to be entangled before they’re set off in opposite directions. Only once one is observed does the other also collapse, but it doesn’t mean you can communicate faster than the speed of…

Why do you feel the need to say somebody is a troll?

Here: https://phys.org/news/2017-01-violations-energy-early-univer...

Here: https://phys.org/news/2015-02-space-faster.html - we are trying hard to reconcile this and categorize universe expanding as something else (e.g. not a movement that has speed because time itself is a dimension or something). But this is still up for debate, tbh.

Another example was one where we said CP symmetry was true (it was a law like a lot of things in physics) until it was violated by a weak nuclear force experiment.

And now we are holding the fort at CPT symmetry as the law.

In Physics, the evidence of anything is kinda light. A lot of reasonable extrapolations has been made. Still they are extrapolations (e.g. intelligent guess).

Even the big bang itself is just an extrapolation from the "theory" that the universe expanding.

To be fair, it is difficult to find good evidence because we can't dial back time, can't go observe things on Neptune, can't measure gravity at the subatomic scale, and etc. So, we have to work with what we can experimentally observe.

Our tools are getting better, and this is where the physics paradigm shift will come from.

You say like these are 100%. It is just a theory that we currently hold according to the little evidence that we have.

Failing to recognize that is straight up unscientific.

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