Two-fer about near-lightspeed travel

[beowulff]

@wolfpup

The comet example isn’t really relevant. Comet nuclei are composed of ice and other volatiles that outgas as they get close to the sun and if they come close enough can get completely vaporized by the heat. The article cites an unusual comet that came within 2.2 million miles of the sun and survived intact.

That’s not what we’re talking about here. To an object traveling at 0.999…-whatever of the speed of light, a collision with any matter at all – no matter how tenuous – would release colossal amounts of energy (ETA: in fact, the energy given in post #17). Just how far into a star do you imagine such a spaceship would penetrate intact?

Any ship that can travel at .99C will have some way of dealing with such collisions.

[Just_Asking_Questions]

So, no earth-shattering kaboom. More like a foomf as the ship becomes one with the star.

Fascinating.

And Terrell Rotation answers the main question. I would have never thought of it, but it makes sense.

This all came from me reading Poul Anderson’s Tau Zero. In the book, the ship is moving so fast, and time compression so great, that the ship passed through entire galaxies in seconds of ship time. So, I wondered what the ship looked like to non-relativistic people. It still takes 200K years to pass through a galaxy. So anybody near to the ship’s path had lots* of time to study the ship as it went by. They just couldn’t interact with it.

It’s like the ultimate in inscrutableness. You know aliens exist, because there’s their ship, but that’s all you can do is look at it. Can’t study it in any more detail at all. I wondered if a computer program could “unfold” the image and give an accurate picture of the ship.

The second question was what happened if the ship randomly hit something big. It’s a good thing space is mostly empty space.
*eta: lots being a relative (heh) term. Helps if they have FTL so they can more or less keep up with it. Otherwise, you get about 20 hours.

[wolfpup]

@rat_avatar

To add some context to just how dense the core of the sun is. It can easily take 100,000 years for a single photon to travel from the core of the Sun to get to the surface but it only takes 8 minutes to reach the earth once it does. (liberal use of the term single photon but note my avatar)

Note that fusion in stars does not happen due to pressure as much as being due to the fact that matter is so densely packed that due to quantum effects the changes of a particle will end up inside another particle is high enough that it happens enough to sustain the reaction. On earth we have to use a lot more energy to accomplish fusion because we don’t have this “closeness” which allows it to happen with much less energy.

Indeed, the density of the sun’s core, as stated earlier, is about 150 g/cm[sup]3[/sup]. The nominal density of water is of course 1 g/cm[sup]3[/sup], while lead is 11.34 g/cm[sup]3[/sup] and gold 19.32 g/cm[sup]3[/sup].

It’s also interesting that the sun’s power density – energy production per unit volume – is remarkably low. From Wikipedia: Theoretical models of the Sun’s interior indicate a power density, or energy production, of approximately 276.5 watts per cubic metre, which is about the same rate of power production as takes place in reptile metabolism or a compost pile. This is not how we normally think of nuclear reactions! The sun produces so much power only because it’s so huge.

[wolfpup]

@beowulff

Any ship that can travel at .99C will have some way of dealing with such collisions.

Ah, so now we invoke magic!

[Buck_Godot]

@Buck_Godot

Getting to your second question, its all about energy. There is no way you are surviving, but one might ask how big a boom you leave when you hit.

Going at 0.99999c will give you a Kenetic energy of mc^2*(1/sqrt(1-0.99999)-1) or m2.8310^19 (meters/second)^2

For arguments sake, lets say that your spaceship is a souped up Delorean that with you as a single passenger tips the scales at around 1350kg. In which case the total energy 3.83*10^22 J. Or the equivalent of a 9.2 Teraton bomb. Observable solar flares vary from between 10^20 J to 10^25 J. So solar scientists on earth would probably record your passing but alas the rest of the earth probably won’t notice.

ETA: didn’t notice the mass of the ship in the OP, so take markn+'s answer.

I want to change my answer in a big way. First, to incorporate markn+'s weight estimations, and second to recognize that unlike a solar flare, this energy would be released in milliseconds if not microseconds. 2x10^26 is about the same as the total output of the sun for half a second. using the bomb analogy it would be a 90,000 Teraton bomb. Now the sun is about 1.5*10^8 km from us, and the energy disipates as the square of the distnace. So if my calculations are correct and if we assume that the time of energy release is about equal to an atomic bomb, it would be as bright as the Nagasaki bomb viewed from a distance of 100KM.

[CookingWithGas]

@wolfpup

Ah, so now we invoke magic!

“Any sufficiently advanced technology is indistinguishable from magic.”

[wolfpup]

I did a quick calculation in which I may have forgotten to carry the 1 or miscounted decimal points, but I wanted to try to estimate the energy released if our hypothetical spaceship traveling at .99999c hits a golf ball at that relative velocity.

A regulation golf ball is supposed to have a mass of 45.93 grams, or 0.04593 kg. Running the numbers, it appears that smashing into this golf ball at .99999c would release about 1.3 x 10[sup]18[/sup] Joules of energy, equivalent to a 310.7 megaton nuclear explosion.

[DPRK]

Colliding with balls near the speed of light will not leave your ship intact. Much less stars.

[ftg]

I like mulling over the ladder paradox. You have a ladder longer than a barn rapidly zipping through the open doors at opposite ends of the barn. Can you close the barn doors with the ladder fitting inside at just the right moment?

But the real question is what happens when the front of the ladder hits the now closed barn door.

[Just_Asking_Questions]

@ftg

But the real question is what happens when the front of the ladder hits the now closed barn door.

An earth-shattering kaboom, I suspect. With a gamma ray burst.

[Quercus]

@markn_1

I’ll guess your 500 foot spaceship has a mass of about 40,000 tons, like a medium sized aircraft carrier. Then traveling at .99999c, it has a relativistic kinetic energy of about 2 x 10[sup]26[/sup] J (according to this calculator).
[…]
The Earth’s gravitational binding energy is 2.2 x 10[sup]32[/sup] J. The ship’s kinetic energy is still a factor of a million lower than that.

Coming within a factor of a million of the Earth’s gravitational binding energy wouldn’t completely vaporize the Earth into a new asteroid belt, sure, but it wouldn’t exactly go unnoticed… it would be like ten to one hundred dino-extincting comets hitting the Earth all at once. [Cite]

So it wouldn’t vaporize Earth, but might sterilize it.