Celestia Forums

Just another question toying around with fast travel in the galaxy

The kinetic energy of any object going close to light speed is very high and the time onboard is slowed down, if seen from outside.
Question one: What happens to the mass of the moving object? Is it also increasing, or staying the same?

Question two: What would happen to this object if it would have a very close flyby on a black hole at let's say a speed between 0.9 to 0.95 of light?

If this object was manned, and even adequately shielded against the radiation that occurs when you go blasting around at insane speeds, what would happed to them?

Have some idea for a SciFi story, but I don't want to violate physics more than necessary

Oh, another question: If a Bussard ramscoop engine would be possible, and an energetic cone of some sort to collect the hydrogen from space would be used, this cone would also produce some sort of "drag" since the particles collide with it have the speed of light wouldn't it?
Or would these collisions be infrequent enough to be neglected?

Regards,

Guckytos

Guckytos,

If you haven't already, you should read "Tau Zero" by Poul Anderson. Although it doesn't include black hole interactions, it does a reasonably good job of describing many of the effects encountered by a highly relativistic Bussard ram jet. Anything you write about them will be compared to that book. Larry Niven has written some short stories about Bussard ramscoops, too.

Don't forget that energy = mass. So yes, the mass increases as you approach light speed.

Sorry, I don't know enough (any) general relativity, so I can't answer your second question. Cham probably could comment.

Yes, friction with the intersteller medium will cause drag. Remember that's what's being used for fuel, so some of it'll have to be accelerated to the velocity of the spacecraft.

There are many pages on the Web which discuss the pros and cons of various types of interstellar ramjets.

Flyby near a black hole will gives some totally weird and deformed images of space around it. I can't possibly resume things here, as there are too much things to say. Just run to this web site, which shows many very interesting visual aspects of black holes :

http://casa.colorado.edu/~ajsh/

Without black holes (which will just complicate things a lot), flying at near light speed will also distort the field of view. You'll see about everything around you concentrated in a small area ahead of you, all in blue and possibly ultraviolet, x-rays, etc (Doppler shift). You'll see also all objects behind you concentrated in a small area behind, all in red (or infrared, radio, etc, depending of your velocity).

If there's a large conic collector on the front of your ship, you'll get a huge drag effect. If you move at very high velocity, collisions with the interestellar medium may be, well, catastrophic ! Even without your collector, you will interact with the Cosmic radiation background, which will be very Doppler shifted. Actually, there is a theoretical upper limit on the kinetic energy of moving particles in our universe, because of that field and Compton scatering. I think it's called the Zeldovich effect.

By the way, it is actually false that mass is increasing with velocity. This is another misconception about relativity theory. It's an interesting question directly related to the question "What is mass and what is energy ?". Here's what I may say here :

In relativity, we have

-Relativistic energy of a free particle : E = gamma m c^2,

-Kinetic energy : K = (gamma - 1) m c^2

-Proper mass energy (or rest energy) : E0 = m c^2.


Here, gamma is the relativistic factor.

We can also write E = E0 + K.

Mass is actually an intrinsic property of matter and is defined by the PROPER mass of the object, as measured by a comoving observer. It is simply m and is a constant, whatever the velocity. In most books, we write m0. Sometimes, we can write also

m = gamma m0,

but this is only a matter of convention and is not physically mesurable. I hate that notation, because it is the source of the misconception. Here, I use only m == m0.

It is true that energy is equal to mass, but NOT the kinetic energy part ! It is the "proper mass energy" E0 which is equal to mass. We must be carrefull to say what kind of energy we are talking about.

Actually a Bussard Ramscoop experiences more drag than it can possibly produce as extra thrust, according to my sources; so they can't be used as described by Poul Anderson and others.

Cham wrote:By the way, it is actually false that mass is increasing with velocity.

Yes, this seems to be the source of endless confusion ... every now and then someone comes up with the idea that if you move anything fast enough it will become so massive it will turn into a black hole. There's at least one SF novel in which this crops up as the explanation for mysteriously vanishing spacecraft.

Grant

granthutchison wrote:

Cham wrote:By the way, it is actually false that mass is increasing with velocity.

Yes, this seems to be the source of endless confusion ... every now and then someone comes up with the idea that if you move anything fast enough it will become so massive it will turn into a black hole. There's at least one SF novel in which this crops up as the explanation for mysteriously vanishing spacecraft.

Grant

LOL ! I never heard of that funny stuff ! Indeed, this is bad Sci-Fi.

I'm a physicist, but this doesn't make me hate Sci-Fi. On the contrary ! I'm an hardcore Sci-Fi fan, even if, most of the times, Sci-Fi is completely crazy. Isn't the only presence of our own universe crazy ? This is the real Sc-Fi ! Geez, there is something and we are there to talk about it. This IS the most crazy thing imaginable. How can it be ? That's why I wanted to study physics and mathematics. The truth is out there.

IMHO, people interested in Pop-Science and hard Sci-Fi must undestand some basics of relativity. It MUST be clear that mass doesn't increase with velocity, but Kinetic energy does increase with velocity (of course). All the confusion is coming from that %$#@!*& formula E = m c^2, where m == gamma m0. The "relativistic mass" m == gama m0 doesn't represent anything, physically. It's just a stupid convention. Only the m0 constant (proper mass) is the real mesurable mass.

Thanks guys,

Mass is actually an intrinsic property of matter and is defined by the PROPER mass of the object, as measured by a comoving observer. It is simply m and is a constant, whatever the velocity. In most books, we write m0. Sometimes, we can write also

m = gamma m0,

but this is only a matter of convention and is not physically mesurable. I hate that notation, because it is the source of the misconception. Here, I use only m == m0.

yep that was the culprit that led me to my question about the mass. It has been quite some time since my last relativistic physics class, so i was a bit confused.

@selden:
Nope haven't yet read that book of Poul Anderson, but will do. Niven's novels are quite good and i like most of them.


Well at the moment i have other things to do, so this story is going to be on the back burner anyways. Just writing on it when my head bursts with ideas

But now i have my english good enough together to refine my original question.
If that theoretical spaceship would nearly graze (hope it's the right word) the event horizon of a black hole with its theoretical EM ramscoop cone, wouldn't there be an enourmous amout of radiation emitted, that would most surely fry anyone onboard?

Anyways, thanks so far. I will now go off and hunt some of those websites Selden mentionend.
BTW: Selden you wouldn't happen to have ready for me?

Regards,

Guckytos