Time travel

Factual Questions
21

Well, you make a very good point, Soup Nazi.

You could be right. The difference between the cases you cited and the speed of light is experimental evidence.

The Earth and Sun centered universes were descriptions of the way we saw things, true. They turned out to be wrong. I propose though that they were originally what we ‘knew’ based on ‘common belief’ and religious doctrine rather than experimental evidence.

Much as my grip on history is tenuous, I seem to recall that the belief that Earth was at the center of the universe persisted for a fair time after evidence was starting to mount to the contrary (a quick perusal of the history of astronomy, specifically Galileo, should bear this out some). The original belief was upheld by powerful religious figures for quite some time, and hence in the public mind.

A belief that the Earth was flat, or that washing with water was unhealthy, was similarly borne by ‘public consensus’ which is not a terribly good way to determine the truth.

However, and this depends on whether or not you have more faith in the scientific method than in other forms of truth-determination, we have considerable scientific evidence, measured and calculated and statistically analyzed evidence, that there really is a speed of light limit. We can make good predictions about what will happen in the future based on experments we start now, which is a good basis for something being true.

There’s also some minor evidence lately that this limit has been changing slightly over the history of the universe, true, but this doesn’t change the idea that at any given point in the history of the universe some such limit evidently seems to exist.

We regularly test this limit by way of particle accelerators (where we need to know the properties of near light speed travel to properly time the acdelerating elements), or with GPS (the Global Positioning System that depends on this hard limit in its calculations of position), or in celestial mechanics (determining the true orbits of planets despite the propagation time of the light coming from them).

Does this say we can’t exceed the speed of light somehow?

Well, no, of course it doesn’t. We’re a notoriously clever and stubborn race, and if there’s a way to get around it, we’ll probably find it.

But the limit is not just a fictionlike the ‘edge of the world’ that people believed because it seemed like a good idea at the time. The limit, at least locally, is very real, so there is a very real ‘problem’ to get around. The limit is a very real phenomenon that we not only expect, but depend on.

22

The speed of light looks pretty rock solid as a true barrier. The math suggests it is and numerous experiments have verified the theory. For instance, if you hit light speed you would have an infinite mass so it would take an infinite amount of energy for you to achieve light speed…more energy than the entire Universe has. That’s just one problem with it.

There may be things we can do to circumvent the light speed limit such as travelling in a hypothetical hyperspace or through wormholes that effectively shorten your trip (as does Warp Speed in Star Trek) but you will never ever get your velocity actually to or beyond the speed of light. This is about as proven as such a thing could be.

I’m looking for a cite for you as to the other bit.

23

Here is a thread which talks about using wormholes to bypass “C” as a speed limit and hence travel time.

24

Another thing to note about light speed…

Even if it were possible to achieve light speed you wouldn’t want to. As you approach light speed time slows down (this has been proven experimentally numerous times). Once you hit light speed time stops. If you hit light speed you would never ever come out of it till the Universe itself ceased to exist (if it ever would cease to exist I don’t know). If you plan on travelling faster than light speed to go backwards in time you still have this problem…you need to cross the light speed barrier on your way to faster speeds and time will stop at that instant for you so you will NEVER accelerate any faster.

25

I’ve read a ‘relativity made simple’ book and here is the most simple explanation I have in my mind as to why FTL travel is impossible:
Let’s presume you build a ship that could go at any speed, any speed at all. In fact, this ship was SOOO cool you could go INFINITELY FAST and would end up INFINITELY far away from your starting point even if you traveled for one thousandth of a microsecond. Nothing would be able to surpass the speed of your ship obviously. Now, let’s say that you got some big huevos and nothing to lose and take this ship on the ultimate ride. As
the ship accellerated, the rest of the universe would accellerate temporally as well, or time for you would slow down. As soon as your ship gauges indicate you’ve reached 299,792,458 m/s , or the speed of light, you would then theoretically experience one hell of a let down because time for the rest of the universe has temporally accelerated relative to you. (hijack- does anybody know what this speed is? can it be calculated?) Only when your ship finally reaches infinite speed would you appear to be going the speed of light to the rest of the universe.
Now how would you make your kick ass ship go any faster than infinitely fast? Answer that question and you can then achieve FTL travel.

26

Uh, this really isn’t quite right.

Relative to some origin point, say the Earth, no matter how much energy you dump out of your engines, you’ll never be travelling faster than the speed of light.

Look back at the Earth and you’ll see it receding faster and faster, approaching the speed of light.

Look from the Earth at the spaceship, and it will be seen receding faster and faster, approaching the speed of light.

None of this qualifies as ‘infinitely fast’ in any manner whatsoever.

You will never see your velocity gauge, measured relative to the Earth, hit 299,792,458 m/s, so you will never experience some sort of ‘let down’. You don’t suddenly stop getting faster. It simply takes more and more effort to get closer and closer.

This effect is smooth and continuous (though not linear)… it takes more energy to go from 10 kilometers meters an hour to 20 kilometers an hour as it does from 20 to 30, but the effect is so tiny at small speeds that it is effectively not there for any useful reason. But up near 299,792,458 m/s, an increase of 10km/h is enormously difficult, and the next 10km/h is even tougher. All of this is measured relative to Earth of course.

The time-dilation or expansion is continuous too, but again, at slow velocities, we just don’t measure it, and the effect gets ‘steeper’ closer to C. In fact, if you were happily sitting at a mere fraction away from the speed of light, you would see half the universe temporally accelerated (ahead of you) and half the universe temporally decelerated (behind you). You’d measure neither half as moving faster than light, and neither half would measure you as travelling faster than light. You would be measured as having, however, different relative times and positions.

You cannot measure your speed relative to a starting place that you were once at or could return to in any meaningful way that would give ‘infinite speed’ in the physical universe.

S’far as I know. I’m not a physicist, but I play one on weekends.

27

Ummm. William_Ashbless, this is all from a relative stand point. Your not lookingat this from the relative viewpoint of the ship. I know this is a hard concept to grasp, but the realities experienced diverge the faster that you travel. From the standpoint of a photon,(assuming it could experience things) It does not have a timeline of existence. A photon leaving the sun would instantly strike the earth , jupiter, or pluto from it’s viewpoint. Our witnessing of it is a different matter altogether.

28

Yes, but you’re not a photon, and neither is your spacecraft. Time passes finitely for you. You cannot move so fast away from the Earth that the Earth would appear to be receding away from you faster than light. You must by definition be moving slower than light, if only by a hair. So while Earth appears to be moving through its time at a snail’s pace relative to you, it’s still receding at a rate slower than light.

Now you can enjoy all sorts of interesting side effects like taking only a few hours subjective time to arrive at Proxima Centauri, but this is a function of time dilation. Neither Proxima Centauri nor Earth will appear to be moving faster than light.

In fact, I totally agree that different viewpoints show different event sequences, but you cannot escape the light cone of any point in space that you start at, whether your viewpoint is at the spacecraft or the launch pad.

29

No. Once you accounted for the time delay for the light to reach you, you would observe the “stationary” universe’s clocks as ticking more slowly than your clock whether the clock is ahead of you or behind you or along side of you. Also, since velocity is relative, a “stationary” observer would observe your clock as ticking more slowly than a"stationary" clock.

Mangetout

The reference frames I was talking about are not imaginary. If you have an array of lights and flash them all at once, there are real reference frames where the lights turn on in sequence right to left (faster than the speed of light) and other real reference frames where the sequence is reversed. Simultaneity is relative.

30

Mm, let me ponder this.

Assuming you were initially at point A and travelling to point B:

A--------------------------B
O

and assuming A and B were transmitting one second pulses and that A and B were stationary relative to one another, then you’d receive pulses both from A and from B at a rate of one second per second.

Let’s assume you accelerated very very quickly such that not long after takeoff you were at your maximum speed:

A--------------------------B
…O

At this point, being at observer point O, would you not appear to be receiving pulses at a rate of faster than one second per second from B, and slower than one second per second from A?

31

Hmm. Now I’m not so sure about this. I’m convinced there’s no way to achieve any sort of ‘infinite speed’ relative to something you’re able to detect throughout your trip, but I’m not convinced about the relative operation of detectable clocks from the point of view of the spacecraft. Fudge.

Time to dust off the relativity FAQ and relevant tutorials. I detest not knowing.

32

Oh, how embarrassing, I’m falling into the trap of fixed reference frames. Perhaps I’ll let the physicists tackle this one. I’ve got some event diagrams to bone up on. Sigh.

33

One time, I bumped into myself and my other me told me that one day I would figure out how to travel through time, but I couldn’t reveal to myself how I figured it out since it wasn’t yet time for me to know how, except that I had gotten the idea that it would be cool to travel back in time to tell me about how I could travel in time. I agreed with myself that it was cool to hear this, especially from the future me, and I’m glad that I didn’t reveal any hints about how to travel in time to myself, so now I will have to figure it out for myself. When I figure it out, and I know that I will, I try to let you all know how I did it.

34

I think I might understand what mangomerlot is getting at. In a sense there is an ‘infinite speed’ and that speed would be the speed of light.

To a ‘stationary’ observer watching you it would seem as though you were moving at a finite speed…the speed of light. However, for the person travelling light speed your speed would be infinite.

Our light speed traveller would see the universe shrink to a point and for all intents and purposes our light speed traveller would be in all locations in the universe simultaneously. Effectively, at light speed, the traveller could choose any point in the universe and be there instantly (from the perspective of the traveller it takes zero time to reach any point they like). To me that effectively equates to infinite speed.

Mind you I say all of the above knowing the impossibility of doing this.

35

OK, but in that sense, is it OK to suggest that in the reference frame of me walking from home to work, I am travelling forward in time at one second per second, but in the reference frame of me walking backwards from work to home (on the very same journey), I am travelling backwards in time at one second per second?

36

Thank you, Whack-a-Mole, I’m glad that the corallary of relativity is not lost on everyone. It is much simpler to see things from that side of it. I do think i have a problem using extremes when giving an example tho. Let me give some better examples of relativity and the speed of light (as I understand it )
Relativity does not put a speed limit on objects travelling the universe per se. It is theoretically possible to build a vessel to reach anywhere in one second. However, if you traveled 10 light-years in one second, then the rest of the universe will have lapsed ~10 years in that brief time. By the time you made it back, everybody would be ~20 years older in what would seem to be a matter of seconds. If there was a homing device on the ship, it would show that you traveled at speeds very close to the speed of light. The gauges on the ship would show you traveling at ~23,651,826,181,452,000 m/s. This figure much, MUCH higher than the speed of light.
So you can bulid a ship to theoretically go as fast as you want, but it will have to go infinite m/s just to match the speed of a photon. This should make clear why traveling at or faster than the speed of light is more than likely to be impossible.
This is only taking into account one relevant aspect of relativity, by the way.

37

Oh and by the way, I don’t think the theoretical ‘light speed traveler’ would be in all locations at once as Whack-a-Mole stated. The traveler would only be in one spot in his trajectory at any time like anyone else travelling. However, the traveler would arrive at a point infinitely distant from his starting point at the end of eternity the moment he cut the engine. (now wrap your noodle around that one!)

38

This makes no sense. Much as I’ve proven my knowledge of relativity to be somewhat flawed, you’re suggesting that somehow you will measure this 23,651,826,181,452,000 m/s relative to, say, Earth, having left it for parts unknown.

This would imply that being on the spacecraft you’re measuring something from Earth to do this, such as… well, light. Unless you’re suggesting you’re overtaking light leaving the Earth, you could not measure your velocity at this goofy high speed. If you can’t see any photons leaving the Earth, then you’re not measuring your distance relative to the Earth. Since you must be seeing the Earth, any attempt to measure your velocity relative to Earth must measure your velocity as below C. You can bounce radar off the Earth to determine this.

There’s no question Earth would measure you on the spacecraft as going sub-light either. They can bounce radar off of you to determine this.

This is a fundamental tenet of relativity, is it not? No matter if you’re heading towards or away from Earth, or its heading towards or away from you, the measured speed of light is constant.

Put it another way: If you went chasing a photon heading away from Earth, and wanted to catch up with it, you could try… but it would forever elude you. Once you got to half the speed of light heading away from Earth, relativity demands that the photon still appear to be heading away from you at the speed of light. You can’t ever catch up to it. Your relative time must simply slow down so that to you, the photon is still rocketing away at the speed of light. Er, assuming you could ‘see’ a photon that wasn’t hitting you. :slight_smile:

39

Slight Hijack:

If you were traveling at near-light speed, wouldn’t light be blue-shifted to your perspective? Anyone traveling at those speeds would be exposing themselves to high-intensity radiation, no?

40

Nope…at light speed you are effectively everywhere at once. As you approach the speed of light the Universe will contract. Once you reach light speed the Universe will be a point to you. You also have an infinite mass which I think would have to take infinite space to hold you (another problem with travelling light speed). Finally, time stops for you so it takes ZERO time for you to be anywhere which flipped around makes you effectively everywhere simultaneously. Assuming you could actually turn off your light speed drive (which you can’t because time has stoppped for you) you could theoretically pick your ‘re-entry’ point in the Universe thus getting you effective instantaneous travel…infinite speed as far as you are concerned.