Speed of light makes space travel pointless

Factual Questions
21

Other’s have explained this already but the explanations seemed (to me) a bit difficult to follow without at least a few semesters of college physics.

To Astroboy14’s confusion you must remember that time is subjective to the person observing it. It is entirely possible for two people to get completely different results when timing the same event.

Now, if you are an astronaut flying at close to the speed of light then time slows down for you from the perspective of someone outside of your spaceship watching you (assume they can see through the window to a clock in the ship). From the perspective of the astronaut nothing has changed but if he looks out his window at you he will notice your clocks moving very fast. Remember, each observer thinks everything is normal in his reference frame and its the other frame that’s all screwy.

Now you might ask, if you’re the astronaut, how it is you fly thousands of light years to (say) the center of the galaxy in a few minutes. The trick here is going fast not only slows your clocks down it shortens your rulers. At near the speed of light everything will seem MUCH closer to you. Instead of having to travel 30,000+ LY to the center of the galaxy you now have to travel only a few miles (or less) from your perspective. This is one of the reasons travelling at the speed of light is problematical. The universe would shrink, from your perspective, to the size of a point and you’d essentially be everywhere at once.

Akatsukami has done the math already but here is a link and a tidbit from something else that illustrates what Akatsukami is talking about:

Remember though that in the example above it seems to take a traveller on the Oh My God Particle 3 seconds to reach the center of the galaxy (and 3 seconds back) something like 75,000 years would pass on earth. Earth would be a very different place from the one the traveller left.

22

bryanmcc

I don’t see any way of reconciling the cause and effect paradoxes that FTL travel invokes.

It would take some fundamental changes in theory to allow violations of causality.

You have Bell’s theorem saying that Quantum mechanics, local causility, and objective reality leads to a contradiction. Therefore, at least one of these must be abandoned. Local causality is so dear that scientists are willing to give up objective reality in order to keep it.

I suppose it is possible that some future theory will find some loophole, but I’m not going to book a two week vacation to Vulcan.

Not exactly; the situation is symetrical. It is the true that an outside observer would say your clocks are moving slowly, but you would say that the “stationary” observer’s clocks are running slow, not fast. Remember there is no absolute time. If you go past me at .9c, I am going past you at .9c.

23

This is OT but if that is the case (and I believe it is a good catch on DrMatrix’s part) then how do the clocks get reconciled when our traveller returns to earth? To the traveller his clock is “normal” and he isn’t moving…everything around him is whizzing by at some significant portion of the speed of light. He sees all of their clock moving slooowwww.

So, he travels to the center of the galaxy, turns aound and comes home. While looking out his window all clocks on earth move slow. This is true throughout his trip (assume he makes a looping turn and comes back without decelerating and re-accelerating). However, when he comes to a screeching halt on earth and steps off his spaceship the earth is some x-thousands of years past the day he left.

In short, he made a trip to the center of the galaxy in (say) 10 seconds, everyone else’s clock was moving slower than his (from his perspective) and yet he steps out of his ship some thousands of years after he left.

What am I missing? (I feel the twin paradix lurking in this one but I never did understand that one very well)

24

Apart from that perennial sport, “fun with special relativity”, the answer to the OP was probably best given by friedo. Exploration of our own solar system can be practically done at speeds far, far under c, and will keep us occupied in terms of practical challenge for a long time yet. The percieved futility of trying to reach the stars doesn’t enter into the equation. Yet. Hold that thought.

The percieved lack of benefit to space exploration does. That’s the bottom line answer to why we landed on the moon over thirty years ago, and haven’t been back since (anybody who believes the moon landing was hoaxed will please go to another thread to discuss it). Come up with a way for space exploration to MAKE money instead of spend it, and it will happen, and happen FAST. Figure out a way for it to alleviate some social or political problem, and it will happen, too. Pure research doesn’t rate very high on the totem pole with most segments of human society, I’m afraid.

Now, back to that thought I was holding. There are many scenarios which could make it attractive for humanity to reach the stars without finding a way to break the speed limit, so to speak.

Imagine that we have developed far enough to be able to engineer artificial environments on a planetary scale, and are really capable of such projects as terraforming Mars, or constructing Dyson spheres out of our solar system. This doesn’t require any violation of physics as we know it, just plain old engineering. LOTS of it, on a scale that beggars anything we can currently accomplish.

If we reach that point, I would submit that, lacking an FTL breakthrough, humans will get to the stars on generation ships, or maybe just rove around between them if we can construct livable habitats that will be viable for centuries and don’t have to be tied to a star system. Maybe we’ll use the solar system itself as the generation ship at this point - as Niven imagined the Puppeteers doing with their worlds to escape the galaxy blowing up. And yes, I’ve read the Heinlein story about the inhabitants of the generation ship that have forgotten they’re aboard a ship at all.

Or maybe you can imagine that society becomes stable enough that you can figure on it existing for millenia, or we succeed in being able to live for thousands of years. Under these conditions, extremely long journeys to other stars, or subjective time displacement from near light speeds become acceptable. Reference Joe Haldeman’s “Forever Wars” for an imaginative treatment, or Ursula K. LeGuin’s universe pre-ansible.

BTW, another calculation you might do is to simply calculate the kinetic energy of your vessel using its relativistic mass, and realize you have to get that energy from somewhere to reach that speed. Imagine that you could convert mass to energy to do so. You can then calculate how much of a vessel’s original mass would have to be converted to energy to attain a given velocity for the remaining mass, assuming 100% efficiency. At near light speeds it’s disappointing.

25

Jeff_42:

Even a “looping turn” requires acceleration. If he wasn’t accelerating, he would just continue on straight forever. There will be three periods of acceleration for our traveler: at the beginning of his trip, at the turn-around point, and when he stops back at home. These accelerations are what syncs brings the clocks into the expected relative positions (and solves the Twin Paradox). This is the distinction (I believe) between Special and General Relativity: Special does not consider accelerating bodies (and therefore introduces the Twin Paradox) while General does.

Hopefully someone like DrMatrix or Chronos will be along shortly to explain this better.

Oh, and DrMatrix:

Lousy causality! There go my vacation plans! So, if I understand you, this means that for anything to travel through an Einstein-Rosen bridge, all information contained within must be destroyed, correct? I.e., whatever you chuck into this black hole becomes nothing but energy and fundamental particles emerging from the other end. I think I’ll pass on that mode of travel, no matter what Jodi Foster says.

-b

26

Um, ignore the word “syncs” in that post, willya?

-b

27

Thanks, bryanmcc… I was just about to mention that. Acceleration is the key: For the two observers to ever meet again, at least one of them is going to have to accelerate, and that’s going to introduce an asymetry.

As for causality: It’s not quite so precious to all physicists. Going back to Bell’s Inequality, there are some who would argue that once you give up the notion of objective reality, you’ve stopped doing physics. This is, of course, a question of philosophy, not physics, since either view will tell you what numbers will appear on your measuring instruments, so it really belongs more in GD than GQ. It’s possible, too, that paradoces themselves are prohibited, without necessarily forbidding FTL/time travel (which are equivalent). Maybe I’m allowed to go back in time and meet my grandfather, and shake his hand and such, but I’m just not allowed to kill him. Maybe if I try, I miss, or I have a heart attack just before I pull the trigger, r am otherwise prevented, or maybe folks disposed towards attempting such things just never get ahold of time machines. Causality would still be preserved, albeit in a rather convoluted way.

Finally, as for calculations for astronaut’s time to reach a given destination, given some certain acceleration: You just ignore relativity altogether, and you’ll get the right answer. The effects from the ship moving slower than Newton would predict are exactly countered by the relativistic effects on the astronaut’s time. At a constant acceleration a (with turnaround halfway there), to travel a rest distance of x, the time t experienced by the astronaut will be t = 2*sqrt(x/a). Plugging in x = 4 Ly and a = 9.8 m/s/s (to keep our astronauts comfortable and unsquished) you get t = 3.94 years. Just don’t ask me how you manage to maintain a constant one G of acceleration for four years straight.

28

You just contradicted yourself. If subjective time is all there is, how can you say it’s “slowing” down? Slowing down compared to what? While “subjective” and “objective” are perhaps not the best terms, there are definitely two different types of times, proper time and “normal” time.

29

That would be Earl Grey…HOT!? Aaah it’s all coming back. Thanks for the erm… friendly reminder.

30

[QUOTE]
*Originally posted by Akatsukami *
**tsunamisurfer asks:

Pharaonic Egypt? Qin China? Medieval Catholic Europe?

Forced labor?

*An exploratory mission first requires tremendous R&D outlays, which itself requires long-term public financing. Again, zero results for generations.

Not impossible, but a hard sell, politically.*

31

I intuit that the direction that the direction that I’ve taken in the discussion with tsunamisurfer is rapidly moving in a GD direction.

Rather than suffer the humiliation of having manhattan move this whole thread to GD (and put him the servers to that trouble), I’ll create a sub-thread there.

32

That is not true. In GR, the concept of simultaneous space is indeterminate. Einstein’s field equations give only a locally deterministic simultaneous spacelike surface. The entire light cone causality structure however is indeterminate and c is not globally invariant. If you have traversed a wormhole you cut all connections to your past.

33

This thread has very interesting participants.

34

No contradiction. It is arbitrary which frame you designate as stationary. Time slows down relative to the stationary frame no matter which frame is considered stationary. There is no absolute time frame to compare to.

I agree that the temporal relationship between an event, say your entry into a wormhole, and an event outside the light cone, say your arrival at your destination is indeterminate. This is not the same as undefined. This means that different reference frames will disagree as to which event is before the other. But once you decide on a reference frame, you have defined the temporal order. The causility structure is indeterminate precisely because you cannot transmit a signal faster than light. If you can travel faster than light, you have established a causility structure.

If you allow faster than light transmission of information (leaving your light cone), you allow time travel and a temporal paradox can be constructed.

35

I could be wrong about this but I don’t think that in GR you can have a global reference frame. The only reference frames that are viable are local frames where spacetime is flat and SR governs.

For instance as an observer’s light cone crosses the event horizon of a BH the cone tips and it’s time axis becomes it’s spatial axis, and it is totally cut off from the rest of the universe.

This tipping of light cones isn’t limited to Black Holes however, it happens wherever space is curved. When two light cones are tipped wrt each other, c as an invariant quantity loses its meaning and SR no longer applies. What this means is that causality loses it’s meaning - the future light cones either don’t intersect or if they do it happens far enough in the future that causality is again established.

While it was pretty much established by Bell’s inequality that entangled pairs communicate superluminally this does not affect macro causality, as no communication via this phenomenon is possible.

36

You certainly can have global reference frames. We can talk about the size of the event horizon of a black hole, for example. However, you must give up rectilinear coordinates.

What are you saying here? Entangled pairs communicate superluminally, but no communication via this phenomenom is possible? So, they communicate, but they don’t. :confused:

As I understand the implications of Bell’s inequality, it is not local causility that is sacrificed, but rather objective reality. That is, it is not meaningful to say a particle has properties like spin unless they are actually observed.

37

What about space debris, those tiny particles of dust that’s everywhere in space. Would anyone care to calculate how fast a ship with a one-inch thick steal hull could travel before such particles penetrate it.

And what about larger objects? If we could see them in time we could miss them but at the speed of light how do we see ahead?

We need to develop the ability of tapping into the nervous system and having it control electronic devices. Then we could take human brains, place them in fluids for life support and send them out to explore the universe at their leisure.

38

No. If i am travelling at .9c and i go past you at .9c, then you are stationary. From what i’ve learned, the speed of light is a constant, and time slows down for objects that approach that speed. Therefore, if i were travelling at .9c, then your clocks would seem to be moving faster, not slower. If the time slow-down were relative, it would create an impossible paradox.

39

Wrong-o, Monocracy. Here’s some free advice: don’t argue with DrMatrix. He knows his shit.

-b

40

I realise that DrMatrix is very knowledgeable on this subject, but he cannot possibly be correct because it would create paradoxes that would make Einstein’s theories laughable.