Why Can’t You Reach the Speed of Light?

A friend from high school had a good question that I wanted to share:
I have a science question!!! Why can’t we travel the speed of light? We know what it is, and that its constant. We’ve even seen footage of it moving along a path (it was a video clip I saw somewhere [Edit to add: there are now two different experiments that have done this. One that requires multiple repeats of the light pulse and a newer technique that can work with just one). So, what is keeping us from moving at that speed? Is it simply an issue of materials not being able to withstand those speeds, or is it that we can’t even propel ourselves or any object fast enough to reach those speeds? And if its the latter, is it an issue of available space/distance required is unattainable, or is it an issue of the payload needed to propel us is simply too high to calculate/unfeasable (is that even a word?) for the project? Does my question even make sense? I got a strange look when I asked someone else…
 This question makes a lot of sense actually, because when we talk about space travel, people often use light-years to discuss vast distances involved and point out how slow our own methods are in comparison. But it actually turns out the road block is fundamental, not just practical. We can’t reach the speed of light, at least in our current understanding of physics, because relativity says this is impossible.

To put it simply, anything with mass can’t reach the speed of light. This is because E=mc2 works in both directions. This equation means that the energy of something is its mass times the speed of light squared. In chemistry (or a more advanced physics class), you may have talked about the mass defect of some radioactive compounds. The mass defect is the difference in mass before and after certain nuclear reactions, which was actually converted into energy. (This energy is what is exploited in nuclear power and nuclear weapons. Multiplying by the speed of light square means even a little mass equals a lot of energy. The Little Boy bomb dropped on Hiroshima had 140 pounds of uranium, and no more than two pounds of that are believed to have undergone fission to produce the nearly 16 kiloton blast.)

But it also turns out that as something with mass goes faster, its kinetic energy also turns into extra mass. This “relativistic mass” greatly increases as you approach the speed of light. So the faster something gets, the heavier it becomes and the more energy you need to accelerate it. It’s worth pointing out that the accelerating object hasn’t actually gained material – if your spaceship was initially say 20 moles of unobtanium, it is still 20 moles of material even at 99% the speed of light. Instead, the increase in “mass” is due to the geometry of spacetime as the object moves through it. In fact, this is why some physicists don’t like using the term “relativistic mass” and would prefer to focus on the relativistic descriptions of energy and momentum. What’s also really interesting is that the math underlying this in special relativity also implies that anything that doesn’t have mass HAS to travel at the speed of light.

A graph with X-axis showing speed relative to light and Y-axis showing energy. A line representing the kinetic energy the object expoentially increases it approach light speed.

The kinetic energy of a 1 kg object at various fractions of the speed of light. For reference, 10^18 J is about a tenth of United States’ annual electrical energy consumption.

The graph above represents  the (relativistically corrected) kinetic energy of an 1 kilogram (2.2 pound) object at different speeds. You can basically think of it as representing how much energy you need to impart into the object to reach that speed. In the graph, I started at one ten thousandth the speed of light, which is about twice the speed the New Horizons probe was launched at. I ended it at 99.99% of the speed of light. Just to get to 99.999% of the speed of light would have brought the maximum up another order of magnitude.
Edit to add (9/12/2017): A good video from Fermilab argues against relativistic mass, but concedes it helps introduce relativity to more people.
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Where are all the engineering blogs?

I was browsing through Dynamic Ecology recently on my reader to catch up on end-of-2015 posts and was intrigued by one of the author’s comments on why there isn’t really an ecology blogosphere. And though I’ve pondered it before, this makes me wonder where the engineering blogosphere is. I don’t have much evidence to back up the loading of that question, but I’ve been in grad school for engineering for 3.5 years now, and it’s worth noting that I still haven’t heard of any major engineering blogs people follow. And the sheer randomness of Blogmetric’s ranking of engineering blogs seems to corroborate this: only the top 2 of the ranked engineering blogs are tracked to have over 100 visitors a month. A Github list of engineering blogs (which is currently the first result for Googling “engineering blogs”) seems incredibly focused on tech company blogs and IT/programming/development.

Engineering.com’s blog seems to have ended without even a goodbye at the end of 2014. Engineer Blogs has been radio silent since September of 2012. And the American Chemical Society’s magazine, Chemical & Engineering News, closed up nearly all of its blogs in mid-2014, with an explanation implying this was because they were viewed as a drain on resources that could be more productively used for other tasks. Chemical Engineering World (which as far as I can tell, is a personal blog and not affiliated to the Indian publication of the same name) seems to have just came back after a hiatus.

The Dynamic Ecology post’s second point on ecology not being very news-driven sounds compelling to me as a reason that could easily cross-apply to engineering, especially if you’re trying to move away from just tech company gossip. Having something well-known to react to can make it easier to post content that’ll actually engage readers because they start searching for it. Point 1 of the Neuroecology post’s on neuroscience lacking a blogosphere because neuroscience bloggers focus more on outreach to general audiences than technical exchanges with each other also seems valid. What’s interesting is the comparison I’m making. As Jeremy from Dynamic Ecology points out, the general science blogosphere is pretty vibrant. He and the Neuroecologist are focusing more on the lack of interacting blogging communities in specific disciplines. Engineering seems to lack this at both levels. I also wonder about some specific issues in engineering that can contribute to this.

  • Is engineering too broad to have a meaningful blogosphere? I see two distinct forces here.
    • First, is the breadth of engineering disciplines. I could see it being hard for there to be a lot of substantive discussion between, say, a chemical engineer and a computer scientist on a broad range of topics.
    • Second, there’s the huge influence of a lot of engineering actually being done in industry. I’m not going to say academic and corporate engineers don’t talk to each other, but it would also be dumb to pretend they have the same interests in how they approach outreach.
  • Is engineering too tied into the science blogosphere? (I wondered a similar thing last time I posted about engineers and outreach) Interested scientists and science writers can (and do) do a good job of explaining concepts and results from related engineering fields. For instance, Dot Physics is written by a physicist who routinely covers topics that are related to technology and engineering. On the opposite end, I clearly try to cover science topics that I think I can explain, even if I’m not experts in them. Randall Munroe straddles the border a lot in What If? and Thing Explainer.
  • You might think I’m treading around one obvious potential component of an engineering blogosphere, and that’s tech blogs. But engineering isn’t just “tech companies”, which in modern parlance seems to really just mean computer and Internet companies. (I’ve somewhat ranted about this before in the last two paragraphs of this post.) A lot of stuff also goes on in physical infrastructure that engineers could talk about. And in an era where the Internet seems increasingly interested in discussion of how system shapes our lives, it seems like we’re missing out if the people who help shape physical systems don’t share their voices.

Edit to add: I also realize I didn’t include any discussion about Twitter here, mainly because I’m still a novice there. But I still haven’t seen very long discussions on specific engineering issues on Twitter, though I assume tech is the exception again.