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Ways to travel at nearly the speed of light (phys.org)
134 points by dnetesn 7 months ago | hide | past | web | favorite | 71 comments

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler [1].

This neglects the expansion of the universe, the health and safety effects of traveling near to the speed of light, and the fact that we do not know how to do this (you would not be able to bring your own fuel). Nevertheless we do not know why it should be physically impossible.

[1] https://en.m.wikipedia.org/wiki/Space_travel_using_constant_...

This a key fact that a lot of people seem to misunderstand, even a lot of popular science journalists trying to explain relativity. But to me it really illuminates the connection between space and time and the inviolability of c. From your own perspective you can travel arbitrarily fast in terms of how much you age inside your spaceship... but as you move faster in space, time in the the Universe outside your ship appears to be moving faster as well such that always more than a year will have passed outside for every lightyear you've traveled, even if it was only days, hours, or minutes for you.

Vernor Vinge does a great job with this in A Fire Upon the Deep and A Deepness in the Sky. I do wish that he had written more about Pham Numen and the Qeng Ho, back in their Slow Zone heyday.

Alastair Reynolds also does a great job with this in his Revelation Space trilogy, with his lighthuggers.

One of the most interesting parts to me of Pham Nuwen's character was his relationship to his distant descendants.

Humans just aren't wired to consider family members beyond 3 or 4 generations up and down.

Perhaps I'd feel the same general apathy as Pham Nuwen 20 generations down. Then again, he had other familial issues that played a part.

One started with him hiding in a nursing home, after faking his death. I don't recall specifics, but I suppose that it's possible that all of the other characters are his descendants. And they had spent a long time looking for him.

In the other, he'd been resurrected by some godlike AI as a meatspace agent. Millennia after the Qeng Ho era, I think.

I absolutely recommend all of these. And of course Reynolds's "House of Suns", which sort of takes the "lighthugger" concept up to eleven.

I have yet to read Vinge's books, but Joe Haldeman's The Forever War is an excellent book that also makes good use of this effect.

the way i put it is that relativity imposes no limit on how soon you can get from point A to B; it only limits how soon anyone else can do it.

Isn't it the other way around? From the perspective of the spaceship rider, time outside the ship appears to be moving more slowly. That is, as the spaceship rider flies by earth, she will observe earth-bound watches ticking more slowly than her wristwatch.

It's both actually. But that gives an apparent contradiction - how can both observers see that the other party has slower clocks? Who is 'right'?

The answer is that the 'moving' traveler needs to flip the ship around at some point and thrust back towards the starting point. Whoever turns back around will find that it's their clocks that passed more slowly.

And if the universe is spherical?

I think the question you are asking is what if the universe is toroidal. What if you traveled so far you came back around.

You come back around on a sphere too, e.g. traveling by the equator.

In that case you would be traveling a curved path so there would be acceleration even if your speed remained constant.

Isnt a torus curved? What's the argument?

If you use toroidal boundary conditions then you could remain in an inertial reference frame and return back to the same pont.

See https://en.m.wikipedia.org/wiki/Periodic_boundary_conditions

My instinct is that as you fly toward Earth at near the speed of light, earth-bound stopwatches will appear to be going nearly double their normal speed. As you fly away from Earth, the stopwatches will slow to a crawl approaching standstill.

This illustrates a fundamental misunderstanding of time dilation. Direction has no impact, and this example that is often used is terrible for illustrating what is being proposed re. time dilation, because it just has to do with the perception of time as it relates to light reflecting off of a clock within certain frames of reference.

I might be misunderstanding your comment, but you seem to be saying that the change in time is only actually thebvissible perception?

If so, that is not correct. Atomic clocks have measured actual, lasting time differences.

I was saying the opposite. The claim of time dilation has nothing to do with the visual perception of clocks, which is a common (flawed) example that is used. The visual perception of clocks differing by heading towards or away from a clock is just a phenomenon akin to the Doppler effect.

If you correct for this effect caused by changing distance between the parties, you will still observe a difference in passage of time in the two different inertial frames of reference (each observing the other). In both cases the party taking the measurement will observe that time is passing more slowly in the frame of reference that the measurer sees as moving with respect to the measurer's frame, regardless of whether that motion is toward or away from the measurer.

What happens if both parties are moving? They start back to back, fly a distance, both turn around and meet.

Each should see the other as dilated, no?

There is no difference between you moving with velocity X and the other party moving with velocity -X; or you moving with velocity 2X and the other party being stationary; or you being stationary and the other party moving with velocity -2X. There is no experiment you can perform that will tell the difference.

You missed the point that both parties accelerate, we are not talking about constant velocity here.

In science-fiction, engines that can accelerate at a constant 1g (more or less) are called torchships. Interestingly enough, there are theoretical designs that achieve that and give us huge amounts of ΔV and still large amounts of thrust.

An example is the "Plasma Jet Magneto Inertial" [1]

The main issue that may not be solved by eventual iterative progress is that these kinds of engines create way too much waste heat.

[1]: http://www.projectrho.com/public_html/rocket/enginelist2.php...

This is fascinating! I had no idea! Thanks for sharing this, I'm inspired.

From the point of view of conventional, reductionist models of human biology, the only basic physical health effects, given a reasonably humane spaceship life, would be the risks of collision.

Otherwise, I'd think 1 g would actually make a space journey more livable!

(Velocity is entirely relative, so traveling near the speed of light really doesn't mean much of anything in terms of basic physics/chemistry in the traveling frame. Physics undergrads are told that: light/physics under 1g of acceleration, by the theory of general relativity, is going to behave in exactly the same way as light/physics under 1g of gravity from mass.)

Now, as far as psychological/spiritual/emotional health are concerned, that's another matter. Relativistic effects would extract you from the context of time that gives meaning to all of your relationships with other Earth life, and with Earth itself. It'd be very deep.

Okay but now I'm remembering the article recently shared about Posner molecules and the possibility of meaningful quantum biological organizations in the body.

If our bodies physically depend on, in some yet to be articulated way, quantum information being exchanged/entangled ecologically, and if we depend on the synchronicity of these exchanges (say for example to inform some deeper biological sense of meaningful time)... Well then moving close to the speed of light would take those processes to a different tempo. To what degree are our physical bodies in temporally dependent relationships with their environment or ecologies? How critical are these relationships to basic functioning?

Now I have to agree that we don't know what effects on physical health might be!

No, the physical health effects are that you would be fried to a crisp by particles you hit en route, or assuming empty space, the blue-shifted cosmic background radiation when you get close enough to c. The general problem with going fast is that you are heading into a high-energy particle beam in the direction of travel, and will be cooked alive in far less time than it takes to philosophize about the meaning of life.

That's a really good point! In some way my question is related, but it's about the "redshifting" of processes (in the sense of dilating their frequency spectrum) that very deep parts of us may be dependent on to keep sense of time.

I might be wrong, but doesn’t theory of relativity prove that we won’t feel a difference?

> less time than it takes to philosophize

You will have had enough time for that before you start moving really fast. (Also, philosophizing is usually a very slow process anyway.)

According to the relativity theory, it’s physically not possible to determine if you’re on Earth, or in a spaceship accelerating at 1 g, just by observing physical processes.

I think you're confusing special and general relativity here. The equivalence principle in accelerated systems only holds true for infinitesimal regions of spacetime.

In the sense that an infinitesimal region of spacetime is a "perfect" reference frame, sure. But for the scales we're talking about, the equivalence principle gives a very strong approximation of the situation. Remember, we are extremely small!

(Some people don't even believe the Earth is round. They think that their local tangent space is all there is too it! That's how small we are.)

The equivalence principle is the opening from special to general relativity, I don't think ajconway was confusing the two at all!

Absolutely. That was my first intuition as well.

But then I got to wondering if our bodies aren't critically entangled with our Earth ecologies in various time sensitive ways, and realized that I don't know the answer to this question.

(I.e. https://arxiv.org/abs/quant-ph/0004105 )

So my second question is treating the human body as being meaningfuly physically entangled with it's environment and as being in some sense unified with it. Accelerating a body away from Earth at 1g might be akin to cutting off a limb and expecting it to survive on its own. I don't know if this is the case, but it seems possible.

The halo drive concept is interesting. It uses a beam of light to extract gravity energy from a black hole and use to to maintain acceleration of your ship.

Almost no fuel needed because its really just a kind of extreme gravity assist that we’ve been doing since what, the 1970s?


> you would not be able to bring your own fuel

I love that you point this out. People are so used to even "hard" science fiction hand-waving this away they often don't realize the magnitude of the problem.

For example, if you use a fusion drive (I_sp = .12 c) and start at 99% fuel (9:1 fuel ratio for initial acceleration and then again for deceleration at the end of the journey, so m_0/m_1 = 10 both times) then you can only travel at .27 c which means time dilation is nearly negligible, only about 4%. Similarly, if you start with 99.99% fuel, you still only achieve 0.5 c, 99.9999% (the payload is now one-millionth of the ship) gives you .68 c, one part per trillion gives you .93 c, etc. We assumed perfect engineering, used a trillion tons of fuel for one ton of payload, and we're still only at Lorentz factor of 2.7. If someone invents an antimatter rocket we can improve the .12 to 1, but that's only a one-order-of-magnitude constant difference. The tyranny of the (relativistic) rocket equation is brutal. This wikipedia article contains the equations used for the above calculations:


Now, you might not have to take all your own fuel with you: Bussard ramjet style designs gather their own fuel as they go once they get up to speed, thereby breaking free of the tyranny of the rocket equation. These are in the realm of science fiction at the moment, but at least they are within the known laws of physics.


Realistic projects like Breakthrough Starshot usually assume (a) an external power source (such as lasers pushing a mirrored object), (b) no deceleration at the end (an order of magnitude savings), (c) very small payloads, and (d) only moderate velocities.


I've been moderately critical of Starshot for other reasons (how can a useful camera or other sensor fit into such a small device? How could it have a directional antenna large enough to focus over 4 light years? How could it contain a power source large enough the send a signal back?) but in terms of just getting a payload a certain distance, it seems workable and is perhaps the best design we have.

So yes, bringing your own fuel is not a good idea if you are trying to get close to light speed!

For acceleration, the best idea I've seen is using a laser on Earth, a mirror on the ship, and another mirror on Earth. The more bounces, the higher efficiency.

> (b) no deceleration at the end

That kills it for me.

Lithobraking is always an option.

When you’re going nearly the speed of light?

I didn't claim it's a good option...

And is also a way to send a signal back...

In this case (Breakthrough Starshot), the core of the probe weighs grams. It would never make it to the ground.

I was more thinking about the flash when something of relativistic speed hits something else. An atmosphere for instance.

Good point!

Interestingly if we can find a way to accelerate at a gentle constant 1g it is possible to take a round trip to the edge of the universe in the lifetime of the traveler

But what kind of specific impulse are we talking about? It has to be quite high, or you're talking about unphysical amounts of reaction mass. Even with photon rockets, we're talking about godlike amounts of energy. Or, we could find clever ways to circumvent the rocket equation.

Right we don't know how to do it but it's not unphysical either. You almost surely couldn't push your own fuel but a scoop is at least not physically impossible.

The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft.

You almost surely couldn't push your own fuel but a scoop is at least not physically impossible.

As far as we know, you always wind up with more drag than you can overcome by using what you scoop as fusion fuel.

The most problematic part is likely the fantastic particle and radiation flux that would impinge on the front of the craft.

Exploiting time dilation would probably be rendered impossible by this.

take a round trip to the edge of the universe

The observable universe. That one word makes a huge difference.

If you could harness the wave-particle interactions mentioned in the article, you wouldn't need to carry your own fuel.

If we could accelerate at 1g for just a few days, and reach just a fraction of a percent of the speed of light, the solar system would be our playground. We could go to the moon for lunch, visit Mars for the weekend, and take a trip to Neptune every year.

The round trip is going to take 30 billion years (assuming no expansion). By the time you got back, Earth would be long gone.

No need to return makes it even easier.

What I always found interesting was you can go as fast as you want - to you, going from here to alpha centauri could take just weeks for you if you had enough energy - it's just that time would speed up around you so no matter what everyone else would see you get there in 4.5 - 5 years.

And if I'm wrong, I apologize and would gladly hear the correct answer.

Isn’t it the opposite - time slows down for you when you move at relativistic speeds, and hence you end up getting to alpha Centauri in 5years w.r.t clocks at Centauri or earth?

An object's relative speed to itself is always 0 (by definition) and so the passage of time is always constant for every object. As you can guess by the name of the theory there is also no "universal clock" or "0 speed" in the universe to compare against, it's only possible to measure speed and the flow of time relatively between objects.

For these reasons "time slows down for you when you move at relativistic speeds" is really a bad way to think of it (and not how the math of the theory actually works). It's much more proper to say things in the form "To <observer a> it appeared as if the clock of <observer b> was <sped up|slowed down>". E.g.:

To the traveler it appeared as if the clock of Alpha Centauri was sped up. To Alpha Centauri it appeared as if the clock of the traveler was slowed down.

It's pretty confusing but essentially, as you travel faster, your clock slows down arbitrarily. In the most extreme case, at the speed of light, your clock stops. For a photon, it emerges and appears at its final location instantly, even if those distances are millions of lightyears apart.

So if you travel to Centauri from Earth, the fastest you get can get there is 5 years as seen from Earth but to the actual spaceship, it could perceive any arbitrarily small change in time.

> Yet all across space, from black holes to our near-Earth environment, particles are, in fact, being accelerated to incredible speeds, some even reaching 99.9% the speed of light.

Or that, but with a dozen more 9s in it: https://en.wikipedia.org/wiki/Oh-My-God_particle

Shielding at the speed of light is an interesting challenge.

The beginning of Alastair Reynolds's short story "Weather" (in "Galactic North") addresses this.

The easy way to achieve this is get the other party to travel toward you at .99c. Slightly more seriously, it's always interesting to think about how you can have the universe expanding faster than c, and so you can't see it all. You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c).

>You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c).

Not true. At relativistic speeds, relative velocity is not a simple addition: https://en.wikipedia.org/wiki/Velocity-addition_formula#Spec...

In your example where both parties are moving at 0.6c towards each other, each will perceive the other to be moving at ~0.88c.

But what about a third observer? We very much make electrons collide...

Yes, but when they collide their relative velocities to each other do not exceed c.

>You can also approach someone else at faster closing speed than c if both parties are traveling where the sum of their velocities toward each other is greater than c (like both are going at .6c).

In neither your reference frame or the other person's will it appear to be true that your closing speed is faster than c.

However due to the speed of light delay you may perceive the other person as going much faster than c, but this is strictly an illusion. For example, if I were watching a near-light speed ship approaching earth in a telescope, I might see it begin its journey and then see it arrive a few seconds later and my monkey brain would conclude it made the whole trip in the time I was watching it. In reality the light from when it started the trip is a few years old.

Here's an example with numbers to illustrate what is happening:

Let us say that a ship starts at a distance of 100 ly (lightyears) and moves with 0.95c (speed of light) for all of the distance toward the observer through empty space.

Light will need 100 years to cover the distance. The ship will need ~ 105.26 years (= 100ly/0.95c). The observer can not know that the ship launched before the 100 years passed that it took for light (or any other signal!) to arrive. So it looks like that only 5.26 years later the ship arrives, making it appear that it only took 5.26 years for a distance of 100 ly which appears like a speed of 19 times the speed of light (= 100 ly / 5.26 years).

Would you expect a 0.99c spaceship to be "brighter" in the telescope than a 0.01c? I mean, the average power of the bouncing light on it should be higher, right? Since it's gonna be years of light in a shorter time.

There would be a doppler blueshift out of the visible spectrum i believe, but quantity wouldn’t change much IIRC from this Myhrthbusters episode https://youtu.be/HtbJbi6Sswg

Note that Mythbusters video was revisited later in the series and the claims reversed: https://mythresults.com/episode38

It's not an equivalent question either, the question is intensity not sum.

Very cool, need to check that out.

However, isn't intensity equal to sum when it comes to light? At least in the visible spectrum, more energy just shifts the wavelength...perceived intensity comes from photon count.

Photons sent on a 5 year near c journey would arrive over e.g. 5 days instead of 5 years, hence the intensity. I.e. it doesn't take 5 years for the light you emitted 5 light days ago to arrive but the light you emitted 5 years ago may only be 5 light day ahead.

Yes due to the reasons you are thinking but also yes for a more interesting reason, relativistic beaming results in more of the light being aimed in the direction of motion meaning more total light will also be delivered.

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